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Report Number: C4-040R-02

Router Security Configuration Guide
Principles and guidance for secure configuration of IP routers, with detailed instructions for Cisco Systems routers

Router Security Guidance Activity of the System and Network Attack Center (SNAC)
Authors: Vanessa Antoine Raymond Bongiorni Anthony Borza Patricia Bosmajian Daniel Duesterhaus Michael Dransfield Brian Eppinger Kevin Gallicchio Stephen Hamilton James Houser Andrew Kim Phyllis Lee Brian McNamara Tom Miller David Opitz Florence Richburg Michael Wiacek Mark Wilson Neal Ziring December 15, 2005 Version: 1.1c

National Security Agency 9800 Savage Rd. Suite 6704 Ft. Meade, MD 20755-6704

Router Security Configuration Guide

This document is only a guide to recommended security settings for Internet Protocol (IP) routers, particularly routers running Cisco Systems Internet Operating System (IOS) versions 11.3 through 12.4. It cannot replace well-designed policy or sound judgment. This guide does not address site-specific configuration issues. Care must be taken when implementing the security steps specified in this guide. Ensure that all security steps and procedures chosen from this guide are thoroughly tested and reviewed prior to imposing them on an operational network.

This document is current as of October, 2005. The most recent version of this document may always be obtained through

The authors would like to acknowledge Daniel Duesterhaus, author of the original NSA “Cisco Router Security Configuration Guide,” and the management and staff of the Applications and Architectures division for their patience and assistance with the initial development of this guide. Special thanks also go to Ray Bongiorni for quality assurance and editorial work, and to Julie Martz and Kathy Jones for proof-reading assistance. Additional contributors to the guide effort include Andrew Dorsett, Charles Hall, Scott McKay, and Jeffrey Thomas. Thanks must also be given to the dozens of professionals outside NSA who made suggestions for the improvement of this document, especially George Jones, John Stewart, and Joshua Wright.

Trademark Information
Cisco, IOS, and CiscoSecure are registered trademarks of Cisco Systems, Inc. in the USA and other countries. Windows 2000 and Windows XP are registered trademarks of Microsoft Corporation in the USA and other countries. All other names are trademarks or registered trademarks of their respective companies.

Revision History
1.0 1.0b 1.0f 1.0g 1.0h 1.0j 1.0k 1.1 1.1b 1.1c Sep 2000 Oct 2000 Mar 2001 Apr 2001 Aug 2001 Nov 2001 Mar 2002 Sep 2002 Dec 2003 Dec 2005 First complete draft, extensive internal review. Revised after review by Ray Bongiorni Second release version: second pre-pub review Third release version: incorporated external feedback. Fourth release version; another QA review. Fifth release version. Last release of 1.0, another pre-pub review. Major revision and expansion, another pre-pub review Minor revision, corrections, additions, fixed links Updated, fixed inconsistencies, checked links


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Preface 1. Introduction
1.1. 1.2. 1.3. 1.4.

5 7

The Roles of Routers in Modern Networks .....................................................................7 Motivations for Providing Router Security Guidance......................................................9 Typographic and Diagrammatic Conventions Used in this Guide .................................10 Structural Overview .......................................................................................................12

2. Background and Review
2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7.


Review of TCP/IP Networking ......................................................................................15 TCP/IP and the OSI Model ............................................................................................17 Review of IP Routing and IP Architectures ...................................................................19 Basic Router Functional Architecture ............................................................................24 Review of Router-Relevant Protocols and Layers .........................................................27 Quick “Review” of Attacks on Routers .........................................................................29 References......................................................................................................................30

3. Router Security Principles and Goals
3.1. 3.2. 3.3. 3.4. 3.5.


Protecting the Router Itself ............................................................................................33 Protecting the Network with the Router.........................................................................35 Managing the Router......................................................................................................43 Security Policy for Routers ............................................................................................46 References......................................................................................................................51

4. Implementing Security on Cisco Routers
4.1. 4.2. 4.3. 4.4. 4.5. 4.6. 4.7.


Router Access Security ..................................................................................................55 Router Network Service Security...................................................................................70 Access Control Lists, Filtering, and Rate Limiting........................................................83 Routing and Routing Protocols ....................................................................................102 Audit and Management................................................................................................139 Security for Router Network Access Services .............................................................175 Collected References....................................................................................................202

5. Advanced Security Services
5.1. 5.2. 5.3. 5.4. 5.5. 5.6.


Role of the Router in Inter-Network Security ..............................................................204 IP Network Security.....................................................................................................205 Using SSH for Remote Administration Security .........................................................227 Using a Cisco Router as a Firewall ..............................................................................232 Cisco IOS Intrusion Detection .....................................................................................241 References....................................................................................................................247

6. Testing and Security Validation


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Router Security Configuration Guide

6.1. 6.2. 6.3. 6.4. 6.5.

Principles for Router Security Testing .........................................................................250 Testing Tools................................................................................................................250 Testing and Security Analysis Techniques ..................................................................251 Using the Router Audit Tool........................................................................................258 References....................................................................................................................261

7. Additional Issues in Router Security
7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8. 7.9.


Routing and Switching.................................................................................................263 IPv6 ..............................................................................................................................265 ATM and IP Routing....................................................................................................266 Multi-Protocol Label Switching (MPLS).....................................................................267 IPSec and Dynamic Virtual Private Networks .............................................................268 Tunneling Protocols and Virtual Network Applications ..............................................269 IP Quality of Service (QoS) and RSVP........................................................................270 Secure DNS..................................................................................................................271 References....................................................................................................................272

8. Appendices
8.1. 8.2. 8.3. 8.4.


Top Ways to Quickly Improve the Security of a Cisco Router....................................274 Application to Ethernet Switches and Related Non-Router Network Hardware..........280 Overview of Cisco IOS Versions and Releases ...........................................................283 Glossary of Router Security-related Terms..................................................................289

9. Additional Resources
9.1. 9.2. 9.3.


Bibliography.................................................................................................................295 Web Site References ....................................................................................................298 Tool References ...........................................................................................................300




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Routers direct and control much of the data flowing across computer networks. This guide provides technical guidance intended to help network administrators and security officers improve the security of their networks. Using the information presented here, you can configure your routers to control access, resist attacks, shield other network components, and protect the integrity and confidentiality of network traffic. This guide was developed in response to numerous questions and requests for assistance received by the NSA System and Network Attack Center (SNAC). The topics covered in the guide were selected on the basis of customer interest, community concensus, and the SNAC’s background in securing networks. The goal for this guide is a simple one: improve the security provided by routers in US Government operational networks.

Who Should Use This Guide
Network administrators and network security officers are the primary audience for this configuration guide, throughout the text the familiar pronoun “you” is used for guidance directed specifically to them. Most network administrators are responsible for managing the connections within their networks, and between their network and various other networks. Network security officers are usually responsible for selecting and deploying the assurance measures applied to their networks. For this audience, this guide provides security goals and guidance, along with specific examples of configuring Cisco routers to meet those goals. Firewall administrators are another intended audience for this guide. Often, firewalls are employed in conjunction with filtering routers; the overall perimeter security of an enclave benefits when the configurations of the firewall and router are complementary. While this guide does not discuss general firewall topics in any depth, it does provide information that firewall administrators need to configure their routers to actively support their perimeter security policies. Section 5 includes information on using the firewall features of the Cisco Integrated Security facility. Information System Security Engineers (ISSEs) may also find this guide useful. Using it, an ISSE can gain greater familiarity with security services that routers can provide, and use that knowledge to incorporate routers more effectively into the secure network configurations that they design. Sections 4, 5, and 6 of this guide are designed for use with routers made by Cisco Systems, and running Cisco’s IOS software. The descriptions and examples in those sections were written with the assumption that the reader is familiar with basic Cisco router operations and command syntax.

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Router Security Configuration Guide

This guide was created by a team of individuals in the System and Network Attack Center (SNAC), which is part of the NSA Information Assurance Directorate. The editor was Neal Ziring. Comments and feedback about this guide may be directed to the SNAC (Attn: Neal Ziring), Suite 6704, National Security Agency, Ft. Meade, MD, 20755-6704, or via e-mail to


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3 for a quick review of TCP/IP and IP addressing. Directing data between portions of a network is the primary purpose of a router. In larger. Therefore. it creates a packet with address 14.6. An Ethernet local area network (LAN) is essentially a broadcast network.20. Consulting its internal route table.9.1.2. it is feasible to use simple broadcast or sequential mechanisms for moving data from point to point.9. most modern networks cannot function.20. Figure 1-1. data must be directed specifically to the intended destination. Router 1 forwards the packet to Router 2.2. See Section 2. The Roles of Routers in Modern Networks On a very small computer network.2. Without robust routing.1c 7 . Router 2 sends the packet over LAN 3 to the File Server. Introduction 1.2. In practice.Introduction 1. more complex networks. and sends the packet over LAN 1 to its gateway. the operation of any large network depends on the route tables in all of its constituent routers. Version 1. Routers direct network data messages. LAN 1 190.2. illustrates the primary function of a router in a small IP network.0 Router 2 LAN 3 File Server 14. or packets. Consulting its own route table. below. based on internal addresses and tables of routes. or known destinations that serve certain addresses.0 User Host 190. Most large computer networks use the TCP/IP protocol suite. Router 1.10 Figure 1-1 – A Simple Network with Two Routers If the user host (top left) needs to send a message to the file server (bottom right). the security of routers and their configuration settings is vital to network operation.10.12 Router 1 Wide Area Network LAN 2 14.

it allows them to protect computers and other network components from illegitimate or hostile traffic. and 5. For more information.Router Security Configuration Guide In addition to directing packets. allowing some data packets to pass and rejecting others. a router may be responsible for filtering traffic.1c . Filtering is a very important responsibility for routers. consult Sections 3. 4. 8 Version 1.

and available software. Compromise of a router can lead to various security problems on the network served by that router.1c 9 . Security policy enforced at a router is difficult for negligent or malicious end-users to circumvent. secure operation of the networks they serve. • On the other hand. with pertinent references at the end of each section identifying the most useful vendor documentation. well-configured secure routers can greatly improve the overall security posture of a network. For example.2. ensure data integrity. thus avoiding a very serious potential source of security problems. denial of network communication services. • Compromise of a router’s access control can result in exposure of network configuration details or denial of service. standards. or even other networks with which that router communicates. and facilitate secure cooperation between independent enclaves. Cisco offers extensive on-line documentation and printed books about the security features supported by their products. • A poor router filtering configuration can reduce the overall security of an entire enclave. and can facilitate attacks against other network components. expose internal network components to scans and attacks. and make it easier for attackers to avoid detection. but they are not sufficient. Most vendor-supplied router security documents are focused on documenting all of the security features offered by the router. This guide also provides pointers to related books. Version 1. • Compromise of a router’s route tables can result in reduced performance. These books and papers are valuable. and do not always supply security rationale for selecting and applying those features. Motivations for Providing Router Security Guidance Routers provide services that are essential to the correct. There are substantial security resources available from router vendors. vendor documents. and exposure of sensitive data.Introduction 1. This guide attempts to provide security rationale and concrete security direction. proper use of router cryptographic security features can help protect sensitive data. In general.

using Courier typeface.0. Elision of long output is denoted by two dots.150/24 has a netmask of 255. the bare address will be used. usually a remark about the line that follows it.” Command arguments are shown in Courier italics: “syntax for a simple IP access list rule is access-list number permit host address.255. user input and comments are shown in Courier bold typeface. Central> enable Password: Central# ! list interfaces in concise format Central# show ip interface brief Interface IP Address OK? Method Ethernet 0/0 14. this guide uses many abbreviations commonly employed in the Cisco documentation and literature.250 YES Manual . or as A. In some cases. .2.2.C. most of the sections include extensive instructions and examples. to make it narrower. 14. 10 Version 1.C.3.B. Central# exit • IP addresses will be shown in the text and in diagrams as A.6 logging buffered 16000 • Transcripts of router sessions are shown separately from the text.Router Security Configuration Guide 1.B. output that would be too wide to fit on the page is shown with some white space removed. For example. • Specific router and host commands are identified in the text using Courier bold typeface: “to list the current routing table. Typographic and Diagrammatic Conventions Used in this Guide To help make this guide more practical.15.9. For commands that are typed very frequently.255. The exclamation point begins a comment line. ! set the log host IP address and buffer size logging 14. Input in the transcript is distinguished from output.9.D/N. use the command show ip route. unambiguous abbreviation for any command or keyword.D.2. For example.” • Sequences of commands to be used in a configuration are shown separately from the text. where N is the number of set bits in the IP netmask. The following typographic conventions are used as part of presenting the examples. this classless netmask notation will be used where a netmask is relevant.250 YES NVRAM Ethernet 0/1 14.9.) • Cisco IOS accepts the shortest unique.1c .2. (In general. the interface name ethernet is commonly abbreviated “eth” and the command configure terminal is commonly abbreviated “config t”. Otherwise. using Courier typeface.

such as an Ethernet. This icon represents a router. commands shown in examples are too long to fit on one line. they are shown broken across several lines. Such networks normally include other routers. is represented by a horizontal or vertical bus. and may include bridges. and other network hardware.56. This guide uses the following set of icons in all of its diagrams. link encrypters. Workstation Server Router2 Small LAN 12. This icon represents a LAN or a wide-area network over which routers communicate.0/24 A local-area network (LAN) segment. when attempting to apply these examples.1c 11 . you will need to type the long command on one line. The IOS command line interface will not permit this. Computers on the network are represented with one of these two icons.34. Each line connected to a router icon represents a network interface on that router. with several connections. Network Version 1. Each router is presumed to have an administrative console line connection.Introduction • In a few cases. Discussions of network structure and security frequently depend on network diagrams. which is not shown. switches.

along with a discussion of relationships between router security and overall network security. and describes some simple network security threats. • Section 5 describes advanced security services that some routers can provide.Router Security Configuration Guide 1. and defines general goals and mechanisms for securing routers. References are included at the end of each section.4. with a focus on Cisco routers’ capabilities. controlling traffic and filtering using a router. Structural Overview The various parts of this guide are designed to be fairly independent. • Section 6 presents testing and troubleshooting techniques for router security.1c . security management for routers. It is divided into six main parts: • • • • • • securing access to the router itself. Secure Shell (SSH). this section describes some areas that a router security policy should address. and a router security glossary. • Section 3 presents a security model for routers. and network access control for routers. • Section 4 details the methods and commands for applying security to Cisco routers. configuring routing protocols security. The list below describes the major sections. It is essential for good security that any router security configuration undergoes testing. and using a Cisco router as a simple firewall and Intrusion Detection System (IDS). collected from all the sections of the guide. and this section presents both vendorindependent and Cisco-specific testing techniques. readers may want to skip directly to the sections most immediately useful to them. Security mechanisms must be applied in support of security policy. but which may become important in the near future. The main topics of this section are IP security (IPSec). securing router network services. using recent versions of the Cisco IOS software. 12 Version 1. • Section 9 provides a list of resources. • Section 8 consists of four diverse appendices: • • • • tips for quickly improving the security of a router how to apply parts of this guide to LAN switches overview of the Cisco IOS software family and versions. • Section 7 previews some security topics that are not yet crucial for router configuration. including pointers to web sites and security tools. • Section 2 reviews some background information about TCP/IP networking and network security.

4 – for routing protocol security instructions (unless the routers are using static routes exclusively) Version 1. and router operational concepts Section 3 – for general router security principles Section 4. Read the sections listed below if your role is network administrator.1 through 4.1c 13 . the detailed instructions for locking down a router are the most important part of this guide.1 – for quick tips that will greatly improve router security Section 4. if necessary Section 3 – for the security principles behind the advice in Section 4 Section 4 – for detailed instructions on configuring Cisco routers Section 5. The paragraphs below offer roadmaps for using this guide for several different network security roles. Read the sections listed below if your goal is to quickly lock down a router.3 – for a quick guide to using SSH for Cisco administration Section 8. network. For network security planners and system security designers. • • • • Section 8. Read the sections listed below if your role is security planner or security designer. • • • • • • • • • Section 2 – for a review.1 – for advice for quickly securing a Cisco router Section 8. firewall.3 – for information on Cisco IOS versions and upgrades Section 9 – for an overview of recommended references and tools For network security analysts or administrators trying to improve the security posture of a network as quickly as possible.2 – for instructions on configuring IPSec on Cisco routers Section 5.Introduction How to Use This Guide Several different roles are involved in securing a network. and each may need some information about router security.1 – for explicit directions on router access security Section 4. this guide offers detailed advice and direction.3 – for an idea of what Cisco routers can do for network security Section 5 – for information about Cisco router VPN. • • • • • Section 2 – for a review of TCP/IP. the high-level view of router security is more important than the details of Cisco router commands.2 – for instructions on applying this guide to LAN switches Section 8.1. 5. and other advanced security capabilities Section 7 – for a preview of potential future issues For network administrators involved in the daily operation of a network with Cisco routers.3 – for advice and guidance on setting up filtering Section 4.

and applying configuration changes to a router can instantly affect large numbers of hosts. and standards that you can use to obtain more information or greater detail. it is not practical for this document to include full background and technical details. be sure to test it thoroughly in a lab or testbed network. This guide provides security guidance for a large number of topics. Operational networks are complex. Every section includes references to books.Router Security Configuration Guide Before applying any of the guidance in this guide to operational routers. 14 Version 1. web sites. In most cases.1c .

1.5 is the MAC standard for Token Ring. To compensate for that briefness. router hardware architecture. data must be directed more carefully. router software architecture. a typical office LAN. why not humor the author and read on. while 802. addressing is done using IP addresses. As one would expect. using 100BaseT Ethernet. large networks are actually composed of a collection of LANs that are interconnected or “internetworked”.1. Token based LANs avoid collisions by only allowing one host at time to transmit (the host that currently has the token may transmit).Background and Review 2. In order to keep this section brief. it is feasible to use simple broadcast or sequential (token) mechanisms for moving data from point to point. An Ethernet local area network (LAN) is essentially a (logical) bus based broadcast network. it glosses over a lot of issues. either by preventing them or detecting them and taking appropriate action. If you are lost at this point. If you are still lost at the end of Section 2.” consider that a 10BaseT Ethernet (10 megabit per second using twisted pair cabling) has a usual maximum of 1024 stations over a maximum cable distance of 2500 meters. “Relatively small” is the important phrase here. might have 100 computers (and printers) attached to a switch or set of hubs.2) standard and above that it the High Level Interface (802.1c 15 . A local area network is composed of a relatively small number of hosts connected over a relatively small physical area.1) standard. To give some meaning to the term “relatively. Within a LAN.1. though the physical implementation may use hubs (with a physical star topology). addressing is done with a MAC address. Purpose of a Router In larger. Between LANs using TCP/IP. broadcast LANs must deal with collisions.3 is the Media Access Control (MAC) standard for CSMA/CD (the Ethernet standard). Review of TCP/IP Networking As mentioned in Section 1. and network security. more complex computer networks. 2. In almost all cases. For instance.1. Readers with a good grasp of network and router fundamentals may want to skip this section.x series. the reference list at the end of the section includes a long list of other useful sources of background information. keep reading because much of this will be explained below. then consider reading parts of some of the books and/or web pages listed at the end of the section. This is where routers come in. Background and Review This section reviews some background information about TCP/IP networking. Routers take Version 1. on a small computer network. but since it is relatively brief. 802. Just above the MAC level is the Logical Link Control (802. For instance. Standards that relate to LANs are primarily the IEEE 802. 2.

2.1c . allowing some packets to pass through and rejecting others. The goal is almost always to get these packets to another LAN and ultimately to the correct host on that LAN. see Section 3 and Section 4. Most large computer networks use the TCP/IP protocol suite. There are a number of schemes or routing protocols for routers to acquire and share route table information. a router may be responsible for filtering traffic.1.4. Other routers will generally only look at a packet’s header information. not at the contents or data in the packet. Routers also make decisions about where to send these packets. that a message passes through. In addition to directing packets. Updating these routing tables and forwarding data packets between portions of a network are two of the primary tasks of a router. For more information about filtering. In some sense this is the lingua franca of the Internet. Building packets and unwrapping packets are additional router functions performed by the first and last routers. based on: the addresses contained within the packet headers and a table of routes maintained within the router. There is no reason that routers cannot be used to send messages between hosts (as shown in Figure 1-1) but more typically routers are used to connect LANs to each other or to connect a LAN to a WAN. respectively. it allows them to help protect computers and other network components. 2.Router Security Configuration Guide network data messages from a LAN and convert them into packets suitable for transmission beyond the LAN on a wide area network (WAN). See Section 2.2 for a quick review of TCP/IP and IP addressing. In the past these tables were built and updated by hand and this is referred to as static routing. 16 Version 1. the router learns about where various addresses are relative to itself and builds up route tables based on this information. Filtering can be a very important function of routers. there is a substantial discussion of routing protocols is in Section 4. Route Tables As mentioned. Part of the “conversion” process is to add a packet header. In dynamic routing. It is also possible that at the destination end a router may have to break large packets up to accommodate the size limits of the destination LAN. one of tasks of a router is to maintain route tables which are used to decide where a packet is to go and thus which interface it should be sent out. While a thorough treatment of the details is beyond the scope of this document.

so in the interest of space they are merely enumerated here.3. Layer 4: Transport Layer deals with end to end error recovery and delivery of complete messages. 2. Note that the word “Defense” has been deleted and added back over time.2. Layer 1: Physical Layer deals with transmission of a bit stream and definition of physical link.Background and Review 2. Layer 6: Presentation Layer deals with formatting. Also. Layer 3: Network Layer deals with transmission of packets and establishing connections. The definitive source for information on TCP/IP are the RFCs. Network Layers and Standards. Layer 7: Application Layer deals with services such as email and file transfer. Origin of TCP/IP The Transmission Control Protocol (TCP) and Internet Protocol (IP) comprise what is often seen written as TCP/IP. and compression of data. for a visual mapping of TCP/IP to the Version 1. These seven layers are described in almost every reference. ARPA and DARPA are one and the same organization. or “Request for Comments” issued by the Internet Engineering Task Force (IETF) as described in Section 2. TCP/IP and the OSI Model 2. See Figure 2-1. Layer 5: Session Layer deals with setup and management of sessions between applications. Since the development of TCP/IP preceded the ISO OSI seven layer model.2. The OSI Model After TCP/IP was well-established and other networking protocols. some routers can be used to “translate” between different protocols running on either side of themselves. As stated above.2.2. the International Standardization Organization (ISO) developed the Open Systems Interconnection (OSI) seven layer reference model.1. The National Science Foundation (NSF) also contributed to the foundation of the Internet by taking the DARPA technology and making it available to universities. The Defense Advanced Research Projects Agency (DARPA) originated TCP/IP. the Internet essentially runs on TCP/IP protocols. such as DECnet and Novell’s IPX were operational.7. Layer 2: Data Link Layer deals with transmission of packets on one given physical link.1c 17 . encryption. the “mapping” of TCP and IP to the seven layer model is only an approximation. Note that in addition to TCP/IP there are other protocols such as Novell’s IPX (Internetwork Packet eXchange) that can be used with routers.

This involves such details as signal voltage and duration. 18 Version 1. or optical signaling details for fiber. However. It also covers the mechanical aspects of connectors and cables.2 802. the Physical Layer is at a level of detail well below the concerns of this document. It may also cover some low level error control.3 (Ethernet) Application Transport Network TCP or UDP IP Data link { Logical Link Control Media Access Control Physical Figure 2-1: Network Layers and Standards Routing occurs at layer three. It is concerned with the transmission of an unstructured bit stream over a physical link.Router Security Configuration Guide OSI model.1c . the Network Layer. and some of this is discussed in the following sections. A collection of various compatible protocol layers is referred to as a stack. To fully understand routing it is useful to appreciate some of what goes on beneath it at the Data Link Layer.1 802. Layer 7 6 5 4 3 2 1 ISO/OSI Model IEEE Standards TCP/IP Model Application Presentation Session Transport Network Data Link Physical 802.

If this seems confusing. but still within the OSI Data Link Layer. What is important to keep in mind is that MAC addresses are used within a LAN. Figure 2-2: Moving Data through Protocol Stacks Version 1. Each device on the LAN listens for packets with its address. or IP addresses..4 is token bus. Above the MAC standards. Note that the data may pass through many intermediate devices on its way from the sending host to the ultimate recipient.5 is token ring.1c 19 . IEEE 802. such as 100BaseTx. data begins at an application relatively high up in a protocol stack and works its way down the stack to the physical layer. How far up the stack it goes depends on whether that device is the ultimate recipient of the data or merely an intermediate device. Review of IP Routing and IP Architectures If one is dealing only with a local area network (LAN). is the IEEE 802.1 High Level Interface standard corresponds to part of the OSI Network Layer.3. Each device on the LAN will have a something like a network interface card (NIC) which has a unique MAC address. The MAC address is appended to the front of the data before it is placed on the LAN.. on an Ethernet LAN each device has an appropriate Ethernet card which complies with a particular link layer standard. there is generally no need for routing. most protocols are defined by Institute of Electrical and Electronics Engineers (IEEE) standards. For instance.3 is the Ethernet (CSMA/CD) standard. For example. Thus. At this point it is transferred to another device and works its way up the protocol stack at that point. Figure 2-2 illustrates this process. and 802.Background and Review 2. in reality bits can only move from one device to another at the Physical Layer.2 Logical Link Control standard. The IEEE 802. it’s not essential to an understanding of routers. While one can envision logical connections at various layers in a protocol stack. routers. At this level. Sending Host Intermediate Network Infrastructure Devices Receiving Host Router 1 Router n . it must use an IP address. and which was configured with a MAC address. 802. do not worry about it. Within a LAN everything will be handled by Media Access Control (MAC) addresses and by a LAN protocol such as Ethernet. TCP/IP. Once a message is destined to leave one LAN bound for a trip across a wide area network (WAN) to another LAN.

IP addresses used a simple fixed hierarchy based on classes. a new header that contains a MAC address will be added in front of the IP header. At the network layer. due to mergers and acquisitions the manufacturer identification is not as “clean” as it once was. 2.) As stated previously. Still. all network interface devices have globally unique addresses unless their PROMs have been rewritten. the IP header added will contain the destination IP address (in addition to other information). a header will be removed at each layer. Some manufacturers have had more than one identification number (more than one block of serial numbers). Up until the mid-1990s. IP Addresses Under the current IP version 4 standard. Traditionally. IP addresses are hierarchical.2. They are assigned by the device (or interface card) manufacturer. Each address is unique and fixed to a particular piece of hardware. 20 Version 1. (On some newer devices it is possible to change them but normally this should not be done. 24 bits uniquely identify the manufacturer and 24 bits act as a serial number to uniquely identify the unit.1c . Application Layer View Application Data Application Byte Stream Transport Layer View TCP Header bytes TCP (or UDP) Packet Network Layer View IP Header bytes IP Packet Media Access Layer View Media Header bytes Media Trailer optional Ethernet Packet (or other media format message) Figure 2-3: Wrapping Lower Level Headers around Data 2. They are used by layer three devices such as routers. Ethernet MAC addresses) are 48 bits long. IP addresses are 32 bits long. The header is named for the protocol layer that adds it. MAC addresses are used within a LAN by layer two (data link) protocols.3. On the way up the stack. Both systems are explained below. MAC Addresses MAC addresses (specifically.1. Unlike MAC addresses. each layer adds a relevant header to the packet.3. Figure 2-3 should help you visualize how headers are added. Also. today all IP address allocation on the Internet is done using masks and aggregation. also sometimes called the Media Access layer.Router Security Configuration Guide On the way down the stack. Each new header is added in front of all higher layer headers. under a scheme called “Classless Inter-Domain Routing” (CIDR). At the data link layer.

and allocating a block of addresses using a netmask of the smallest size that satisfies the needs of the recipient. (Another way to think of this is that CIDR allocates several contiguous Class C addresses to a network. One was address depletion. and D. very few have enough to “fill” a Class B address range (65. B. Classless Inter-Domain Routing (CIDR) and IP Addressing As the Internet grew over the 1980s and early 1990s. and any beginning with bits 1110 is class D. the remaining bits are the host (or subnet) portion. it is also possible to take the host portion of the address and further divide that range into two fields. there is a good bit of information in an IP header. C. This is done by specifying a parameter called a subnet mask.1c 21 .4. While many networks have more hosts than a single Class C address can accommodate (255 hosts). Moreover the second 4 bits specify the length of the header. referred to as Classes A. see Stevens’ book [10]. The class determines how much of the 32 bit address is used to specify the network address and how much is used to specify the host within that network. there are four “classes” of IP addresses.3. The number of contiguous Class C addresses allocated is a function of the size of the network. Any address beginning with bits 110 is Class C. In addition there a number of special addresses. an organization that needed 1000 addresses would be assigned a netmask of 22 bits. For a detailed explanation of TCP/IP packet header formats. For any class. Any address beginning with bits 10 is a Class B address. In addition to both source and destination addresses. most notably the exhaustion of Class B networks. Thus it is quite feasible to introduce longer IP addresses. under CIDR.1.Background and Review 2. Classful IP Addressing Under the original IP standards.7. CIDR avoids this problem by eliminating the notion of a ‘class’. Any address beginning with a zero bit is a Class A address. The netmask simply specifies the number of bits in the assigned address that designate the network portion. Special addresses are used for such things as to broadcast to all hosts on a network or to specify a loopback packet which will never leave the host. 2. it encountered two problems related to the expanding number of networks and hosts. these addresses should not be used on the Internet or other wide-area networks (see Section 4.3).3.535 hosts). It should be noted that the first 4 bits of an IP header contain a version number so new versions of the protocol can be implemented. There are also a set of IP addresses that are reserved for experimental or private networks.) Version 1. For example. For a fuller discussion of subnetting see Albritton’s book [1] or one of the other references listed in Section 2. The class is determined by the first one to four bits of the address. which specify a subnet address and a host address respectively.3. and the other was increased route table sizes. Allocating an entire Class B network to an organization that only needed 1000 addresses would be (and was) terribly wasteful.

2.255.14. In general. which are composed of several related networks which are collectively advertised as only one aggregated path to that supernet.0 .3.24. A major allocation authority might obtain a block of addresses with a netmask of 8 bits (16777216 addresses).24.3. They might allocate part of that large space as a block with netmask of 13 bits (524288 addresses) to a large ISP. Aggregation To avoid explosive growth in the size of routing tables as the Internet grows it is desirable to somehow group or aggregate related network addresses together so that they form only one routing table entry. This is illustrated below. This essentially forms “supernets”.0. how routers exchange this information.0.31) Figure 2-4: Hierarchical IP Address Range Assignment under CIDR Internet Routing and Aggregation As alluded to in Section 2. then they might be assigned a netmask of 28 bits. and how they decide where to forward any given packet.255) Customer X 14.24. For these topics. The addresses of customers X and Y would still be within the large block ‘owned’ by the major allocation authority. any meaningful discussion of routing protocols in general and BGP routing in particular is beyond the scope of this Background and Review section.2.1c .4 of this guide covers security issues for routing protocols. and smaller customer Y a block with netmask of 28 bits.0/13 (14.63.0 . For example. The ISP might give big customer X a block with netmask of 18 bits.255) 16384 addresses 16 addresses Customer Y 14.16/28 (14.0.0 . or consult RFCs 1771 and 1772. adding a bit more complexity to routing.0.0.3. there is a detailed treatment in Huitema’s book [12].Router Security Configuration Guide CIDR also permits address allocation authorities to allocate blocks of addresses smaller than a Class C network. The network aggregation required by CIDR required the development of a revised routing protocol for the Internet: BGP version 4 (BGP-4).16-14. Another important aspect of CIDR is that it is hierarchical. if an organization required only 10 addresses. Regional Authority 14. the definitions of standard routing protocols specify many of the details of: how routers keep track of available routes (route tables). The prefixes which specify networks under CIDR vary in length.0/8 (14.0.0/18 (14.255) 16777216 addresses 524288 addresses Major ISP 14.0. This reduces the number of entries required in the 22 Version 1.0. Section

with its longer addresses. so that near and far do not necessarily relate to physical distances.1c 23 . Beyond CIDR Until IPv6.Background and Review route tables of routers which are “far” from a given network. CIDR has enabled the Internet community to sidestep the Class B exhaustion problem. is put into common use these problems will continue. As traffic gets to the routers “near” a given supernet more detailed routing information becomes available. Version 1. In the meantime. CIDR and BGP-4 have helped to mitigate the problem of route table size explosion. The aggregation strategy may be based on regions (geography) or providers (network topology).

various kinds of memory. but hardware other than Cisco would run similar software.4. First. CPU speed and memory size are important considerations for both performance and capabilities (e. the term IOS will be used in this guide to refer the router’s operating system and associated software. a special purpose device will probably be a lower cost solution for a given level of functionality. Typically. and can also be used to partition networks and prohibit or restrict access to certain networks or services. and what justifies the bother of learning yet another system? The answer. Non-volatile RAM (NVRAM) is used to store the startup configuration that the IOS reads when the router boots. ease of reprogramming. access speed. although it may have other kinds of removable storage such as Flash memory cards. Second. because it can be designed to support more interface card slots. concerns performance: a special purpose router can have much higher performance than a general purpose computer with routing functionality tacked onto it. or CD-ROM drive. floppy drive.4. the mere fact that it is different means that an attacker has one more thing to learn. routing protocols can advertising selected network routes to neighbors. and sophisticated security services usually require substantial computation). Thus. in part. The mix of types and the amount of each type are determined on the basis of: volatility.) This means that it is potentially less vulnerable. EEPROM). similar to any other computer. and connections to other devices. ROM is used to store a router’s bootstrap software. purpose-built router rather than a general purpose machine with a “standard” operating system (OS)? What justifies this expense. Conceptual Model for Typical Router Hardware A router is essentially just another computer. like Cisco’s Internetwork Operating System (IOS) can be smaller. Why Have a Special Purpose Router? What are some of the motivations for using a dedicated. a router does not have a hard disk. (For brevity. Flash. consolidating network routing and related functions on a dedicated devices restricts access to and limits the exposure of those critical functions. and ROM (PROM. These are listed roughly in order of volatility.1c . But there are also a number of security benefits to a special purpose router. a specialized router operating system. and more thoroughly tested than a general purpose OS. So. Flash 24 Version 1. some Cisco IOS features require more than the default amount of memory.4.Router Security Configuration Guide 2. it has a central processor unit (CPU).1.g. Using filtering. and other factors.2. 2. Also. Filtering is useful as a “firewall” technique. thus helping protect parts of your network. and that known vulnerabilities in other systems are of little help to the router attacker. NVRAM. cost. Finally. one can potentially add more network connections to a machine designed for that purpose. specialized routing software enables a fuller and more robust implementation of filtering. in general. Basic Router Functional Architecture 2. better understood. There are typically a number of types of memory in a router possibly including: RAM.

In addition to a number of interfaces. most routers have an auxiliary port. FDDI. Interface types include Ethernet. fast Ethernet. provides high-level services for management. and control. and an external cable of the appropriate type is connected to the card. Figure 2-5 shows a simple representation of a notional router’s hardware structure. This can be important. fast serial. Also. logging. port 23 for Telnet). Only management and control traffic for the router travel to or from the CPU. Interface cards fit into slots in a router. and if there is enough flash it may store more than one version of IOS.] Version 1. [Do not confuse the hardware serial ports with the concept of network protocol port numbers (e. low-speed serial. instead. etc. Network n Interface 0 Interface 1 . it usually does not touch the CPU. the packets travel across the routing fabric from the incoming interface to the appropriate destination interface. The CPU.Background and Review memory stores the IOS (or other router OS). It loads the configuration of the router during boot-up. routing. and manages the operation of the interfaces.g. Routing Fabric Interface n CPU Configuration Router Console Figure 2-5: A Notional Router’s Hardware Structure Interfaces provide the physical connections from a router to networks.. HSSI. which is frequently used for connecting a modem for router management. Each interface is named and numbered.1c 25 .. gigibit-Ethernet. also called the central processor or route processor. almost all routers have a console port providing an asynchronous serial connection (RS-232). When traffic is being forwarded from one network to another. Network 0 Network 1 . because the bandwidth of the channel to the CPU may be far smaller than the bandwidth of the interfaces. ISDN BRI. token ring...

Unneeded services and facilities should be disabled to avoid wasting CPU and memory resources. they run Cisco’s Internetwork Operating System (IOS). a router will run a control program or operating system (OS). In the case of Cisco routers. If changes to a configuration are written to the startup configuration. Description of Typical Router Software Similar to any other computer. Each router should have a unique name to identify it. An operational router will have a large number of processes executing to support the services and protocols that the router must support.Router Security Configuration Guide 2. The startup or backup configuration is stored in NVRAM. Each router vendor supplies their own router OS.1c . then they will also take effect on reboot. and each interface should have unique network addresses associated with it.3. These kinds of considerations are discussed in more detail later in this guide. It is executed when the router boots. All routers support a variety of commands that display information about what processes are running and what resources. Basic security settings should be established on any router before it is connected to an operational network. 26 Version 1. Changes made only to the running configuration will be lost upon reboot. such as CPU time and memory. It is the IOS that interprets the access control list (ACL) settings and other commands to the router. As part of the boot process a copy of this configuration is loaded into RAM. and to avoid giving attackers additional potential avenues for attack. Changes made to a running configuration are usually made only in RAM and generally take effect immediately.4. they are consuming.

The protocols are grouped according the OSI layer to which they correspond.1c 27 . best effort delivery of those packets. ARP – Hosts use the Address Resolution Protocol (ARP) to acquire the MAC address of other hosts.4.Background and Review 2.1. Transport Layer 4 TCP – the Transmission Control Protocol (TCP) is a connection-oriented. An area is a logical collection of routers and networks. RIP – Routing Information Protocol is a dynamic routing protocol that allows routers to share network information with each other. the ping command relies on ICMP. 2. reliable protocol. For more details see Section 4. UDP – the User Datagram Protocol (UDP) is a connectionless. ICMP – the Internet Control Message Protocol (ICMP) provides the mechanisms for hosts and routers to report network conditions and errors to other hosts and routers. the physical layer is defined by IEEE standards or similar standards that define what are primarily physical and electrical characteristics. It is a distance vector protocol that Version 1. Data Link Layer 2 The IEEE and other standards that apply at this layer have also been discussed previously. When we speak of TCP/IP we are usually implicitly including UDP.) OSPF – Open Shortest Path First is a relatively complex. 2. connectionless. (For example.5. Physical Layer 1 As previously discussed.3. “Routing and Routing Protocols”. Network Layer 3 IP – the Internet Protocol (IP) provides a specification for packet formatting and an unreliable. Review of Router-Relevant Protocols and Layers The following sections are not inclusive of all protocols that might be of interest but are representative. best effort protocol with no guarantee of delivery or confirmation of delivery. Before transmitting data a connection must be established and after data transmission is complete the connection must be closed. 2.5. 2. fast-converging routing protocol. It is an interior gateway protocol that uses a link state routing algorithm and requires that a hierarchy of areas be designed.5. It has lower overhead than TCP.

FTP – File Transfer Protocol (TCP) enables transfers of files between hosts. SSH – Secure Shell (TCP) provides cryptographic security for remote login sessions and other stream-oriented protocols. TFTP – the Trivial File Transfer Protocol (UDP) provides file transfers without any authentication or security. It is used as an interior gateway protocol. Presentation Layer 6. 2. HTTP – the Hypertext Transfer Protocol (TCP) is used for retrieving web pages and many related tasks. SMTP – Simple Mail Transport Protocol (TCP) is the Internet standard protocol for transmitting e-mail messages. NTP – the Network Time Protocol (UDP) is the Internet standard protocol for synchronizing time between network hosts and authoritative time sources. and Application Layer 7 These protocols are labeled (TCP) or (UDP) depending on which layer 4 protocol they are based upon. 28 Version 1. Telnet – (TCP) Enables terminal oriented processes to communicate.1c . DNS – Domain Name System (both TCP and UDP) performs naming resolution service by translating host names into IP addresses and vice versa. SNMP – Simple Network Management Protocol (UDP) enables a management station to trap certain information messages from network devices.5. it is used for remote login.Router Security Configuration Guide allows routers to only share information with their nearest neighbors. Session Layer 5.5.

possibly manipulated. redirect (address) attacks. In addition to threats to a router from the network. and an explanation of the tradeoffs involved is the substance of later sections of this document. Quick “Review” of Attacks on Routers General threats include but are not limited to: unauthorized access.Background and Review 2. eavesdropping. by blocking spoofed source addresses. Masquerades can be used to gain unauthorized access or to inject bogus data into a network. Masquerade attacks occur when an attacker manipulates IP packets to falsify IP addresses. These kinds of attacks are sometimes called “route injection” attacks. and information theft. sequence number prediction and alteration. SNMP attacks. Attack techniques include: password guessing. Target resources may be bandwidth. Careful router configuration can help prevent a (compromised) site from being used as part of a Distributed Denial of Service (DDoS) attack. Denial of Service (DoS). Resource starvation attacks usually involve flooding the router with traffic or requests designed to consume all of some limited resource. Version 1. masquerading.1c 29 .6. routing protocol attacks. Session hijacking may occur if an attacker can insert falsified IP packets after session establishment via IP spoofing. or even computation. Session replay attacks use a sequence of packets or application commands that can be recorded. An enumeration of steps to take to improve router security. Rerouting attacks can include manipulating router updates to cause traffic to flow to unauthorized destinations. session hijacking. rerouting. and then replayed to cause an unauthorized action or gain access. and circular redirect – for denial of service. memory. DDoS attacks use a number of compromised sites to flood a target site with sufficient traffic or service requests to render it useless to legitimate users. or other methods. dial up access to a router exposes it to further threats. IP fragmentation attacks – to bypass filtering.

A. well written. [6] Sacket.Router Security Configuration Guide 2.. A “classic”. Prentice-Hall. this book provides detailed information about a wide variety of topics and features. References 2. A practical introduction to the concepts and practices for using Cisco routers. This book offers good explanations of all the underlying concepts. This book is primarily oriented toward network application programmers. good background reading. 30 Version 1. Computer Networks.. Cisco Press. For administrators already comfortable with basic operation of a router. 1992. (ed.. 1998. [4] Chappell. 1998..7. This thick book provides a lot of detail on the architecture of Cisco routers and their operational concepts. McGraw-Hill. Managing IP Networks with Cisco Routers. If you need more introductory information than this section provides. G. [3] Chappell. [5] Perlman. L.R.M.1. For the network administrator who already has basic familiarity with Cisco IOS. Unix Network Programming. J. Cisco IOS Essentials.. an excellent source for understanding all the concepts behind networks. 1999. 1999. L.) Advanced Cisco Router Configuration. 1999. but it also provides a great deal of technical background information. R. Interconnections: Bridges and Routers. Books [1] Albritton. [8] Tannenbaum. and Hundley. Cisco Press. [2] Ballew. with an emphasis on Cisco IOS. Introduction to Cisco Router Configuration. K.. A good book for learning the basics. [9] Stevens. with explanations of many of the concepts. An excellent introduction to basic IOS operations. W. and TCP/IP. [7] Held. 1998.7. McGraw-Hill. 2nd edition. routers. 1997.1c . 1999. this book provides concepts and practical advice for using a router securely. G. O’Reilly Associates. Cisco Router Handbook. Prentice-Hall. McGraw-Hill. this book is a good source. with no vendor emphasis. Cisco Security Architectures. S. McGraw-Hill.

ietf. 1994. RFC 1812.2. [12] Huitema. protocols. [21] Malkin. (ed. “Requirements for IP Version 4 Routers”. Routing in the Internet. and Parker T.). Available at: http://www. C. “User Datagram Protocol (UDP)”. Cisco Systems.7.html [15] “TCP/IP”. Cisco Systems.3. “A TCP/IP Tutorial”.. and how routing works in the Internet. W.R. 1981. 1996. The Protocols..7. 1999.. [18] Postel. J.0 Configuration Fundamentals.html . technical. “Internet Protocol (IP)”. [16] Postel. too. 1999. RFC 768. This book provides a valuable reference for all the basic operation and configuration features. C. TCP/IP Illustrated – Volume 1. RFC 793. Cisco Systems Brochure. with a great deal of background information... RFC 1180.1c 31 . [14] “OSI: The Network Layer”. A series of introductory-level papers by RFC 791. For really deep. RFC 1392. 1999. these are the definitive sources for information about the protocols and architecture of the Internet. Available at: Prentice-Hall. As the official documents of the Internet Engineering Task 1981. this book is the best source. 1997. Available at: http://www.html 2. Cisco Press.L. Cisco Product Overview. [19] Baker.. RFCs RFC stands for Request for Comments. bit-by-bit analysis of the TCP/IP includes coverage of all the topics discussed in this section. T. 2nd Edition.. Papers [13] “Internetworking Technology Overview”. A deep and detailed textbook about IP routing technologies. 1991. 1997.Background and Review [10] Stevens. 2. [20] Socolofsky. Version 1. “Transmission Control Protocol (TCP)”. F. “Internet User’s Glossary”. [17] Postel. Cisco Systems. 1980. [11] Cisco IOS 12. All RFCs may be downloaded from http://www. J. J. and Kale.

. RFC 1518. J. T.Router Security Configuration Guide [22] Rekhter. RFC 1519..1c . and Yu. 32 Version 1.. “Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy”. 1993.. Varadhan K. and Li. “An Architecture of IP Address Allocation with CIDR”. [23] Fuller. V. T. Y. 1993.. Li.

an uninterruptible power supply (UPS) should be installed and spare Version 1.1c 33 . The data or forwarding plane handles the packets transiting the router among the networks it serves. It should have controls for temperature and humidity.1. The last part of this section gives some guidance about router security policy. or interference with router operation. Threats to the data plane usually concern violations of network security for the networks that the router supports. Physical Security There are a number of ways to provide physical security for a router. Conceptually. This section describes general principles for protecting a router itself. Protecting the Router Itself 3.Router Security Principles and Goals 3. route table updates.1. Router Security Principles and Goals Routers can play a role in securing networks. we must consider the possible threats to each plane. configuration commands software updates authentication configuration backups routing information cryptographic negotiations audit logs network data network data Figure 3-1: Three-Plane Conceptual Model for a Router To secure a router.1. and generally the persistent state of the router. a router operates on three distinct domains or planes. protecting a network with a router. Threats to the management and control planes mostly concern unauthorized access to the router. If continuous operation of the router is critical. The room that contains the router should be free of electrostatic or magnetic interference. The management plane handles administration. 3. and generally the dynamic operation of the router. and managing a router securely. The control plane covers monitoring. configuration.

on the grounds that memory costs money and therefore a router should be purchased with the minimum amount of memory it needs to supports its task. * Some readers might balk at this recommendation. In the Internet Service Provider community. the router should be configured with the maximum amount of memory possible. and to allow it to support the widest range of security services.2 discusses management of updates to the router configuration.1. However. Section 3. To help protect against some denial of service attacks. for example.. Unnecessary services should be disabled in the router configuration. * Also. PC cards.1c . modems) used to connect to the router require storage protection. Many of these services are unnecessary and may be used by an attacker for information gathering or for exploitation.Router Security Configuration Guide components kept on hand. adding memory to an operational router requires taking that router out of service.g. and Sections 4 and 8 include information on Cisco’s IOS operating system. Physical devices (e. and the added performance and flexibility that the extra memory will provide is almost always worthwhile when amortized over the number of users and services that depend on the router for connectivity over its service lifetime. This is a false savings.2 discusses the management of updates to the operating system. Decide what features the network needs.2. One should use the latest stable release of the operating system that meets the feature requirements. Configuration Hardening A router is similar to many computers in that it has many services enabled by default. it is considered an industry best practice to equip every operational router with as much memory as it can hold.1.3. the very latest version of any operating system tends not to be the most reliable due to its limited exposure in a wide range of network environments. Also.3. 34 Version 1.3. and use the feature list to select the version of the operating system. Operating System The operating system for the router is a crucial component. The incremental cost of extra memory is usually small compared to the total cost of a fully configured router. Section 3. 3. 3. the router should be placed in a locked room accessible only to authorized personnel.

1c 35 . The traffic between the different networks that make up the Internet is directed by backbone routers. Version 1.2. Interior Routers An interior router forwards traffic between two or more local networks within an organization or enterprise.Router Security Principles and Goals 3.1. but for this discussion we will examine three fundamental ways in which routers are employed. The networks connected by an interior router often share the same security policy. If an enterprise has many interior routers. they will usually employ an Interior Gateway Protocol to manage routes. The primary security goals for a backbone router are to ensure that the management and operation of the router are conducted only by authorized parties. Most of the directions in this guide are useful for interior routers.2. and to protect the integrity of the routing information it uses to forward traffic. without imposing any restrictions on it. Internal LAN 1 Internal LAN 2 Router Internal LAN 3 Figure 3-2: An Interior Router Connects an Organization’s Internal Networks Backbone Routers A backbone or exterior router is one that forwards traffic between different enterprises (sometimes called different ‘autonomous systems’). Configuring backbone routers is a very specialized task. The level of trust between the networks connected by a backbone router is usually very low. Backbone routers typically employ Exterior Gateway Protocols to manage routes. Protecting the Network with the Router 3. but may need to be modified or adapted to specific applications. Interior routers may impose some restrictions on the traffic they forward between networks. Typically. backbone routers are designed and configured to forward traffic as quickly as possible. and the level of trust between them is usually high. Roles in Network Operations and Security Routers perform many different jobs in modern networks. Most of the techniques described in this guide are applicable to backbone routers.

Using a firewall and a router together can offer better security than either one alone.g. The firewall may also perform user authentication. or it may depend entirely on static routes. In the Figure 3-5. and untrusted external networks (e. many enterprises also employ a firewall to enforce fine-grained security policy. Internet Router Internal Networks Figure 3-4: A Border Router Connects Internal Networks to an External Network Typically. the Internet).Router Security Configuration Guide Router Router Router Router Figure 3-3: Backbone Routers Connect Many Networks Border Routers A border router forwards traffic between an enterprise and exterior networks. the border router acts as the first line of defense and is known as a screening router. 36 Version 1. The firewall provides additional access control over connections and network traffic.1c . A border router may employ routing protocols. a border router is not the only component at the boundary. It contains a static route that passes all connections intended for the protected network to the firewall. It can help to secure the perimeter of an enterprise network by enforcing restrictions on the traffic that it controls. The key aspect of a border router is that it forms part of the boundary between the trusted internal networks of an enterprise.

destination IP address. Internet Router Premises or Gateway router Router Internal or Local net router Internal Network Firewall Figure 3-6: A Two-Router Firewall Configuration for a Network Boundary All of the directions in this guide are suitable for border routers. For example. and protocol type. in Figure 3-1. destination port. Packet Filters for TCP/IP A packet filter for TCP/IP services provides control of the data transfer between networks based on addresses and protocols. and then another router between the firewall and the trusted internal networks. while others have filters that apply only in one direction. 3. Packet filters are especially important for routers that act as the gateway between trusted and untrusted networks. the router can enforce security policy. some routers need two filters to handle bi-directional services.1c 37 . source port. a user on System A telnets to System B.2. Some routers can even filter on any bit or any pattern of bits in the IP header.Router Security Principles and Goals Internet Router Firewall Protected Network Figure 3-5: A Simple One-Router Firewall Configuration for a Network Boundary Another approach is to position one router at the connection between the external networks. (Many services are bi-directional. This configuration offers two points at which policy can be enforced. Routers can apply filters in different ways.) Most routers can filter on one or more of the following: source IP address. For example. The DMZ is often used for servers that must be accessible from the Internet or other external network. often called the de-militarized zone (DMZ) between the two routers. So. the router should enforce the constraint that packets sent Version 1. Some routers have filters that apply to network services in both inbound and outbound directions. However. FTP file name). routers typically do not have the capability to filter on the content of services (e. Filters are also important for their ability to enforce addressing constraints. rejecting protocols and restricting ports according to the policies of the trusted network.2.g. and System B sends some type of response back to System A. In that role. It also offers an intermediate area.

If you have a router made by a company other than Cisco Systems. When the router analyzes a packet against a filter the packet is effectively compared to each filter rule in sequential order. If possible. You must carefully create filter rules in the proper order so that all packets are treated according to the intended security policy. the length and ordering of a packet filter rule set can affect the router’s performance. Two key characteristics of TCP/IP packet filters are length and ordering. use the following guideline for creating filters: those services that are not explicitly permitted are prohibited. Unless you have a specific operational need to 38 Version 1. If a match is found then the packet is either permitted or denied and the rest of the filter is ignored. add a filtering rule that permits that service but only for the specific host addresses or address ranges. The first table lists those services that should be completely blocked by a typical border router. Make a list of the services and protocols that must cross the router. the router should enforce the constraint that packets arriving from the Internet must bear a source address outside the range valid for the protected network. most other kinds of routers. the network firewall host might be the only address authorized to initiate web connections (TCP port 80) through the router. and prohibits all other traffic. Applying Packet Filters: Reject Risky Protocols and Services Sometimes. Similarly. For example. This guideline is especially important for border routers. The order of the rules in a packet filter is critical. (Note: This discussion is applicable to the packet filtering facilities of Cisco routers. as the length grows the filter becomes more complex and more difficult to troubleshoot. In cases where only certain hosts or networks need access to particular services. The number of rules in a filter determines its length. fall back to prohibiting services that are commonly not needed. This is sometimes called egress filtering.3. and those that the router itself needs for its operation. or are known to be popular vehicles for security compromise. In that case. This is a form of ingress filtering. and most packet filtering firewalls. with each rule either accepting or denying a certain set of packets. One method of ordering involves placing those rules that will handle the bulk of the traffic as close to the beginning of the filter as possible. If no match is found then the packet is denied due to the implicit deny rule at the end of the filter. Applying Packet Filters: Permit Only Required Protocols and Services Carefully consider what network services will be allowed through the router (outbound and inbound) and to the router. Consequently. it is not possible to follow the strict security guideline discussed above. The following two tables present common services to restrict because they can be used to gather information about the protected network or they have weaknesses that can be exploited against the protected network. consult its documentation for details). Generally. Create a set of filtering rules that permit the traffic identified on the list. Cisco filtering is discussed in detail in Section 4. A filter consists of one or more rules.1c .Router Security Configuration Guide from the internal or protected network (right to left) must bear a source address within a particular range.

Router Security Principles and Goals support them. 5000 (TCP & UDP) 2049 (UDP) 6000 .1c 39 .6063 (TCP) 6667 (TCP) 12345-6 (TCP) 31337 (TCP & UDP) Service tcpmux echo discard systat daytime netstat chargen time whois bootp tftp supdup sunrpc loc-srv netbios-ns netbios-dgm netbios-ssn xdmcp netbios (ds) rexec lpr talk ntalk uucp Microsoft SQL Server Microsoft UPnP SSDP NFS X Window System IRC NetBus Back Orifice Table 3-2 lists some services on the internal network or on the router itself that should not be accessible to connections from the external networks. Table 3-1: Services to Block Completely at a Border Router Port (Transport) 1 (TCP & UDP) 7 (TCP & UDP) 9 (TCP & UDP) 11 (TCP) 13 (TCP & UDP) 15 (TCP) 19 (TCP & UDP) 37 (TCP & UDP) 43 (TCP) 67 (UDP) 69 (UDP) 95 (TCP & UDP) 111 (TCP & UDP) 135 (TCP & UDP) 137 (TCP & UDP) 138 (TCP & UDP) 139 (TCP & UDP) 177 (UDP) 445 (TCP) 512 (TCP) 515 (TCP) 517 (UDP) 518 (UDP) 540 (TCP) 1434 (UDP) 1900. Version 1. the protocols listed in Table 3-1 should not be allowed across the router in either direction.

) • Reject all traffic from the external networks that bears a source address belonging to the internal networks. or illegal address range. In most cases filtering rules should apply both ingress and egress filtering. • Reject all traffic from the internal networks that bears a source IP address which does not belong to the internal networks.Router Security Configuration Guide Table 3-2: Some Services to Block at the Router from External Clients Port (Transport) 79 (TCP) 161 (TCP & UDP) 162 (TCP & UDP) 513 (TCP) 513 (UDP) 514 (TCP) 514 (UDP) 550 (TCP & UDP) Service finger snmp snmp trap rlogin who rsh. and is almost certainly erroneous or malicious in nature. Traffic bearing such spoofed addresses is often part of an attack. and the Defense Information System Agency (DISA) is attempting to manage the creation of a standard list for the entire DOD. unroutable. and rejecting all others.1c . rdist. any other traffic is attempting to claim a bogus source address. Various organization in the US DOD maintain such lists. rcp. and should be dropped by a border router. especially on border routers. traffic sent from the external networks should always bear a source address from some range other than those assigned to the internal networks. allowing only those ports and protocols mandated by the standard list. 40 Version 1. For networks that are subject to such lists. including blocking reserved addresses. Address Filtering Router filters should also be used to protect against IP address spoofing.) • Reject all traffic with a source or destination address belonging to any reserved. rdump syslog new who Standard Ports and Protocols Some organizations maintain a list of standard ports and protocols that should be allowed or supported on their networks. (Assuming that addresses are assigned correctly. it is best to take the first approach. (Legitimate traffic generated by sources on the internal networks will always bear a source address within the range or ranges assigned to the internal networks. The principles to apply on border routers are listed below.

There are several ways to filter out these improperly-addressed packets. router services that support attacks or attack amplification should be disabled (see Section 4.3. Black hole routing can also be useful.3). Denial of Service (DoS) refers to willful attempts to cause such disruptions. network devices. you should log occurences of bad packets. Also. it is important for every network administrator to help reduce the number of DoS attack launch platforms. Most Cisco routers support a facility called Unicast Reverse-Path Forwarding Verification that uses the route table to detect and drop improperlyaddressed packets (see Section 4. invalid addresses are used in SYN flood attacks to ensure that the TCP handshake on the target host times out waiting for a response (see Section 6. To be effective. There are measures that network administrators can take to protect networks from DoS attacks and lessen their effects. Access control lists are a general filtering facility available on all routers (see Section 4. and works on all routers (see Section Though DoS attacks can be viewed as tolerable annoyances. 3.3. These measures require some cooperative effort between those who administer hosts.2).6). they can have serious consequences if they occur during a time of crisis. For example. logging these violations can help identify compromised hosts that need to be removed from your network. Do not let your network be the origin point for a DoS attack. At the enterprise level there are three primary strategies for combatting DoS attacks.2). on Cisco routers this facility is called Version 1. as long as the resources of a network are limited and openly available they will be vulnerable to attack.4. Of course. detection will depend on reviewing the router logs on a regular basis. Many of these attacks require use of invalid or spoofed source addresses. There are several mechanisms available on routers to thwart certain kinds of DoS attacks. Mitigating Denial of Service Attacks Loss of service or severely degraded network performance can result from a variety of causes. Where possible. and provider access. Configure and deploy local protective measures. You can defend against some individual DoS attacks locally by rejecting packets with invalid source addresses as they arrive at a border router (see Section 4.1c 41 . these measures must be planned and in place before an attack occurs. Some routers and firewalls offer specialized facilities to mitigate TCP SYN flood attacks. There is no complete solution to the DoS problem. Prevent malicious traffic from entering the common network from the enterprise network.3. keep hosts secure and eliminate compromised hosts from the network immediately. Coordinate protective measures against distributed DoS attacks with network access providers and/or backbone administrators. First.Router Security Principles and Goals 3. Invalid or otherwise untraceable source addresses are often used to hide the actual source of an attack. at both border and interior routers. described in detail below. 2.7). 1.

In some cases. then the network being attacked will need to actively request its access provider filter out offending traffic.3. they can take steps to block and trace back bad traffic as the attack ramps up. DoS mitigation necessarily requires cooperative action “upstream. but cautious design. Connections and data transfers are starved and eventually time out or hang because they are unable to resynchronize. (possibly from) the transport provider.1c . There is no set of methods that can completely counter all known DoS attacks. If your access provider performs statistical monitoring of traffic. contingency planning. router traffic rate control or quality of service facilities can be used to protect critical services from the full effects of DoS attacks (see Section 4. as the packets of an ICMP flood converge at the uplink. For example. and cooperation among network administrators are also necessary.6).e. It is still prudent to be prepared to handle well-known DoS attacks using facilities already available. A border router cannot control the type or overall volume of traffic that is sent to it. legitimate traffic is crowded out by bogus traffic and packets are lost to traffic flow control. from the access provider.” i. the source point access provider.3.3). and certainly there will be novel kinds of DoS attacks discovered in the future. If no such quality of service monitoring exists.Router Security Configuration Guide TCP Intercept (see Section 4. Router facilities may also be supplemented by commercial anti-DoS products that provide finer-grained filtering and attack detection. Routers are a part of the solution. or even from the administrators of the attacking hosts. 42 Version 1.

3 shows how to set up a Cisco router to support SSH encryption.2 shows an example of setting up IPSec encryption with a Cisco router and Windows 2000. For example. Access Mechanisms for Administrators Controlling access to a router by administrators is an important issue.3. (Section 5. However. Another method is to encrypt all traffic between the administrator’s computer and the router. Figure 3-7 shows an example of this. Remote access typically involves allowing telnet or SNMP connections to the router from some computer on the same subnet or a different subnet. Another role might be “operators”.1.) In either case. There are two types of access: local and remote.1c 43 . Establish a dedicated management network. and administrators authorized to assume that role may be able to view and modify the configuration settings and interface parameters. there are some options if remote access is required. In addition to how administrators access the router. Local access usually involves a direct connection to a console port on the router with a dumb terminal or a laptop computer. packet filters can be configured to permit only the identified administration hosts management access to the router. or more than one administrative role.3. Managing the Router 3. 1.Router Security Principles and Goals 3. Section 5. there may be a need to have more than one level of administrator. LAN 1 Internet Router Firewall Router LAN 2 Management LAN Administration Host Logging Host Figure 3-7: Using a Management LAN for Administration 2. The management network should include only identified administration hosts and a spare interface on each router. If an attacker can collect network traffic during remote access then he can capture passwords or community strings. administrators authorized to assume that role Version 1. It is recommended to only allow local access because during remote access all telnet passwords or SNMP community strings are sent in the clear to the router. Define clearly the capabilities of each level or role in the router security policy. one role might be “network manager”.

it can show what types of probes or attacks are being attempted against the router or the protected network.1c . Set up and test file transfer capability between the administrator’s host and the router. the administrator should follow procedures similar to the following. By checking over the logs periodically. 3. The most important thing to remember about logging is that logs must be reviewed regularly. Perform tests to confirm that the update works properly. Two areas that should be logged are (1) matches to filter rules that deny access. Instead. or an isolated and dedicated router interface. The log host should be connected to a trusted or protected network.2. you can gain a feeling for the normal behavior of your network. direct the router to at least two different reliable time servers (via NTP) to ensure accurate and reliable 44 Version 1. but this is usually not sufficient.3. If the tests are successful then restore or reconnect the interfaces on the router. to improve performance or support new features (perhaps some that allow more advanced security policies). Configuring logging on the router should be done carefully. Using the information in a log. In some cases. Back up the current operating system and the current configuration file to the administrator’s computer. Schedule the required router and network downtime.Router Security Configuration Guide might be authorized only to clear connections and counters. In general. to perform the update. Load the update for either the operating system or for the configuration file. which is a separate computer whose only job is to accept and store logs. Determine the memory required for the update. If the tests are not successful then back out the update. Logging Logging a router’s activities and status offers several benefits. All routers are capable of maintaining their own time-of-day. it is best to keep the number of fully privileged administrators to a minimum. usually after regular business hours. 3. the administrator can tell whether the router is working properly or whether it has been compromised. These updates are necessary for one or more of the following reasons: to fix known security vulnerabilities.3.3. The logging level may need to be modified based on how much of the log information is useful. and (2) changes to the router configuration. Harden the log host by removing all unnecessary services and accounts. Before updating. Set the level of logging on the router to meet the needs of your security policy. A sound understanding of normal operation and its reflection in the logs will help you to identify abnormal or attack conditions. After obtaining an update from the router vendor (and verifying its integrity). and if necessary install additional memory. Shut down or disconnect the interfaces on the router. Send the router logs to a designated log host. and expect to modify the log settings as the network evolves. Accurate timestamps are important to logging. Updating the Router Periodically the router will require updates to be loaded for either the operating system or the configuration file. the administrator should complete the following checks.

it is highly desirable to preserve the evidence. This will allow you to trace network attacks more credibly. Version 1. and then exercise the plan periodically so that all the participants understand their roles.g. Another important aspect of lifetime security is preparing for problems.Router Security Principles and Goals of time information.4. Include a timestamp in each log message. so that it can be used in a forensic investigation or even prosecution. Operational Security Management Maintaining the security of a router over its operational lifetime requires regular assessment. Plan your recovery actions. 3. and correction. Your recovery plan must be coordinated with your security policy (see next section). Include the steps for capturing the compromised state of a router in your recovery plan. Direct the logging host to reliable time servers. compromise of the log host). consider also sending the logs to write-once media or a dedicated printer to deal with worst case scenarios (e.3. Keeping up to date backups of router configurations and installed IOS releases is essential for quick and reliable recovery from security compromises or simple hardware failures. Finally. write down the procedures.1c 45 . In the case of a security compromise. testing.

and their security is a vital part of the overall security for the networks they serve. physical access must be controlled to provide a solid foundation for the overall security of the router. The route tables themselves are the most obvious part of this. and audit logs. particularly the stored configuration. and the access controls for direct access to the router’s command interface. Security policy usually includes strict rules about access to this layer. shows a layered view of the security of a router. ARP tables. What does it mean for a router to be secure? One simple way to define the security of a router is this: does the operation. Some important aspects of the stored configuration are the interface addresses. below. The next innermost zone of the diagram is the stored software and configuration state of the router itself. The next outermost zone of the diagram is the dynamic configuration of the router. and management of the router satisfy your security policy? 3. usually called ‘Console’ or ‘Control’ ports. A Conceptual Basis for Router Security Policy Figure 3-8. Router security policy should define rules for where and how these ports may be used. are also very important.1. Any router can be compromised by an attacker with full physical access. such as interface status. in terms of both administrative roles and network mechanisms. therefore.4. The security of each layer depends on the security of the layers inside it. Security Policy for Routers Routers are an important part of a network. Router Security Layers Physical Integrity of the Router Core Static Configuration of the Router Dynamic Configuration and Status of the Router Network Traffic through the Router Corresponding Access • Physical access • Electrical access • Administrative access • Software updates • Routing protocols • Access to the network that the router serves. these ports usually provide special mechanisms for controlling the router. If an attacker can compromise either of these. configuration. If an attacker can compromise the dynamic configuration of a 46 Version 1. Figure 3-8: Layered View of Router Security The innermost zone is the physical security of the router. then he will also gain control of the outer two layers.Router Security Configuration Guide 3.1c . Most routers offer one or more direct connections. the user names and passwords. Other pieces of dynamic information.4.

and probably in the router security policy. operator. The router policy might specify the particular rules to be enforced by the router to prevent unauthorized remote administration. For example. for example. he can compromise the outermost layer as well. and the router policy must reflect this. the network that a router serves will have a security policy. identifying permitted protocols and services. the router enforces network policy. The overall network security policy may include rules about this. Each of the roles would be granted privileges in the router policy that permit them to fulfill their responsibilities as outlined in the network policy.1c 47 . although it is sometimes overlooked. The outer zone of the diagram represents the intra-network and inter-network traffic that the router manages. the policy is more portable across router software versions and between different kinds of routers.3. might be held responsible by the network security policy for periodic review of the audit logs.2. not particular commands or mechanisms – When the policy specifies the security results to be achieved. access mechanisms. The router security policy might include only two: administrator and operator. • Specify policy for all the zones identified in the figure above – Begin with physical security. Security policy for a router should include rules about access to this layer. In other regards. 3.Router Security Principles and Goals router. and administrative roles. For example. 3.4. the router policy will involve far more detail than the network policy. In some cases.4. Router Security Policy and Overall Network Security Policy Typically. and for traffic flow. The roles defined in the router security policy will usually be a subset of those in the network policy. The policy for a router must fit into the overall framework. defining roles. and responsibilities. Version 1. rules of conduct. The router security policy might grant the operator login privileges to the router so that they can access the router logs. and work outwards to security for the static configuration. rather than a particular command or mechanism. permissions. and user. the dynamic configuration. a network security policy might define three roles: administrator. Creating a Security Policy for a Router There are several important tips to remember when creating the security policy for a router: • Specify security objectives. The high-level requirements of the network security policy must be reflected in the configuration of the router. the network security policy might forbid administration of the router from anywhere but the local LAN. The rules of conduct for administering the router should clarify the application of the network rules to the router. The operator.

the policy should clearly state any limitations or restrictions that can be enforced. concentrate on services and protocols that have been identified as explicitly needed for network operation. to ensure that it meets all the requirements.1c . Maintaining policy awareness is crucial for policy compliance. due to performance concerns or differences in the security policies of the different networks served. Router Security Policy Checklist The checklist below is designed as an aid for creating router security policy. explicitly permit those. a new VPN) or new network components (e. examine the router security policy and revise it as needed whenever any of the following events occur.4.Router Security Configuration Guide • Services and protocols that are not explicitly permitted should be denied – When representing the network policy in the router policy. Update the router policy to reflect changes in the network policy. avoid specifying the particular mechanisms in the policy. After drafting a policy.g. In these kinds of cases. or whenever the security objectives for the router change. some organizations have high-level policies that impose specific requirements on the contents of individual network security policies. it may not be practical to identify and list all the services and protocols that the router will explicitly permit. In particular. A security policy must be a living document. procedures. When drafting a policy. Carefully check your router’s security policy against any applicable high-level policy.g. keep most of the directives and objectives high-level. 3. A backbone router that must route traffic to many other networks cannot always enforce highly tailored policies on the traffic flowing through it. It may be necessary to revise the router security policy whenever there is a major change in the network architecture or organizational structure of network administration. • New connections made between the local network and outside networks • Major changes to administrative practices. notify all individuals authorized to administer the router and all individuals authorized for physical access to it. a new firewall) • Detection of an attack or serious compromise When the router security policy undergoes a revision. and deny everything else. 48 Version 1. or staff • Major changes to the overall network security policy • Deployment of substantial new capabilities (e. In some cases. Finally. step through the list and check that each item is addressed in your policy.4. Make it part of the security practices of your organization to regularly review the network security policy and the router security policy.

Designates protocols. de-install. SNMP) Outlines response procedures or guidelines for detection of an attack against the router itself. Defines recovery procedures for the event of physical damage to the router. Defines procedures and practices for making changes to the router static configuration (e. • • • Static Configuration Security • Designates who is authorized to log in directly to the router via the console or other direct access port connections. procedures. Defines the recovery procedures and identifies individuals responsible for recovery. Defines the audit log policy for the router.Router Security Principles and Goals Physical Security • • Designates who is authorized to install.g lifetime. NTP. • • • • • • • • • • • Dynamic Configuration Security • Identifies the dynamic configuration services permitted on the router. and move the router.1c 49 . and SNMP. staff changes. Defines controls on placement and use of console and other direct access port connections. Version 1. change recording. such as those for routing protocols. and networks permitted for logging in to the router remotely. Designates who is authorized to perform hardware maintenance and to change the physical configuration of the router. Designates who is authorized to assume administrative privileges on the router. log book. and the networks permitted to access those services. Defines the key management policy for long-term cryptographic keys (if any). RADIUS.g. Designates who is authorized to make physical connections to the router. including outlining log management practices and procedures and log review responsibilities.g. TACACS+. Include a list of conditions that require passwords to be changed (e. in the case of compromise of the router’s static configuration. or evidence of tampering with the router. review procedures) Defines the password policy for user/login passwords. compromise) Designates who is authorized to log in to the router remotely. Defines the management policy and update intervals for long-term secrets. Designates procedures and limits on use of automated remote management and monitoring facilities (e. and for administrative or privilege passwords.

and services to be permitted or filtered by the router. and objectives for response after a successful attack against the network.Router Security Configuration Guide • Identifies the routing protocols to be used. inbound and outbound). manual setting. authorities. • • 50 Version 1. ports.g. and identifies procedures and authorities for authorizing them. Designates mechanisms and policies for setting or automating maintenance of the router’s clock (e. including provision for preserving evidence and for notification of law enforcement. and the security features to be employed on each. • Compromise Response • Enumerates individuals or organizations to be notified in the event of a network compromise. Identifies key agreement and cryptographic algorithms authorized for use in establishing VPN tunnels with other networks (if any). Defines response procedures. Describes security procedures and roles for interactions with external service providers and maintenance technicians. • • Network Service Security • Enumerates protocols.g. Identifies relevant configuration information to be captured and retained.1c . for each interface or connection (e. NTP).

1c 51 . Since that time. 1994. References 3. [4] Akin.. and Hundley. A seminal overview of network boundary security concerns and techniques. the Registered Ports. [2] Held. This revised edition includes all the sound background of the original. T.iana.. and Zwicky.Router Security Principles and Goals 3.. O’Reilly & Associates. Types of numbers range from unique port assignments to the registration of character sets. TCP/IP Illustrated. It is also included on every Cisco documentation CD. 2nd IANA houses the many unique parameters and protocol values necessary for the operation of the Internet and its future development. S. The port numbers are divided into three ranges: the Well Known Ports. A pragmatic and detailed guide to securing Cisco routers.B. [6] IANA Port and Protocol Number Assignments http://www. 2002. these numbers were documented through the RFC document series. E.2.. K. Books and Manuals [1] Chapman. Hardening Cisco Routers. McGraw-Hill. great technical background for any network This site contains a set of excellent technical overviews for a wide variety of networking technologies. 2000. O’Reilly & Associates..iana. 3. with extensive updates for newer technologies.1. the last of these documents was RFC 1700. The overview “Routing Basics” is a fine introduction to IP includes a good section on physical security. W. 1999. G.5.R. In the past. This book includes excellent general advice about router and router-related network security.D. [3] Stevens.. which is also now outdated. The most comprehensive and readable guide to the TCP/IP protocol suite. Web Sites and On-Line Resources [5] Cisco Internetworking Technology Overview http://www. and the Dynamic and/or Private http://www. Cooper. the assignments have been listed in this directory as living documents. constantly updated and revised when new information is available and new assignments are made. Version 1.5. Addison-Wesley. The ‘port-numbers’ file contains the listing of all registered port numbers. Building Internet Firewalls. Volume 1. in addition to its Cisco-specific material. Cisco Security Architectures. D.

K. 52 Version Security Consensus Operational Readiness “Cisco Checklist”. available under: http://www. SANS. This RFC provides extensive information about network and system security policies. Cisco Systems. Cisco White Paper. A detailed checklist of security and operational conditions to check for in the audit of a http://www. (ed.sans. and related ones. This white paper. 2000. Cisco White Paper. and presents a network architecture designed to protect against it. September 1997. RFC This is the main site for looking up Internet RFCs. This detailed white paper describes a threat model for enterprise networks. 2000. 2000. are available under the SAFE web page: http://www.) “Site Security Handbook”. Cisco Systems. especially for systems connected to the [8] “Network Security Policy: Best Practices White Paper”.Router Security Configuration Guide [7] The RFC Editor Site http://www. [9] Naidu.1c . [10] “Cisco SAFE: A Security Blueprint for Enterprise Networks”. [11] Fraser.html A complex and highly detailed architecture and practices document for setting up enterprise networks.rfc-editor. The retrieval service supports a variety of keyword B. as well as straight by-number lookup.

Router Security Principles and Goals Version 1.1c 53 . Mail Server 14.20 Internet 7. eth 0/0 14.0/16 Remote Host South User Host 14.10.250/24 DNS Server eth 0/0 modem LAN 1 14.6/24 FTP & Web Server 14.2. Implementing Security on Cisco Routers The diagram below shows a simple network configuration.2/24 LAN 2 14.1c .9.2/24 Figure 4-1: Example Network Architecture Figure 4-1 is simply a vehicle for presenting security guidance about routers. 5.0/24 User Host 14.6.250/16 Authentication Server 14.6.64/24 Protected Enclave However.250/16 Central eth 0/1 14. The structures and addresses illustrated here are used for all of the examples in Sections and 6.0/24 eth 0/0 14.0/24 User Host eth 0/1 (firewall) modem North (Perimeter router) eth 0/1 14.250/24 Telephone Network Public Web Server 14.6/24 14.20 Remote Remote Network 7.0.2/16 Facility Network Admin Server 14.18/24 eth 0 East net access eth 1 14.2.2. 54 Version 1.1.10.Router Security Configuration Guide it is not a design for a secure network. this architecture reasonably reflects the kinds of networks found in many organizations.

but a knowledgeable attacker with access can never be completely defeated. consider the password recovery procedure for Cisco routers. This problem is not only confined to network devices but is also true of computers and any other electrical or mechanical device.1. Network infrastructure components. this area should only be accessible by personnel with administrative responsibilities for the router. An administrator (or an attacker) can simply connect a terminal or computer to the console port and follow the procedure below (taken from “Password Recovery Process” in [1]). “Step 1 Configure the router to boot up without reading the configuration memory (NVRAM). especially routers and switches. use encryption to protect the confidentiality and integrity of the remote administration connection (see Sections 5. and configuration issues. where applicable. remote administration concerns. or electronic monitoring. Router Access Security This section discusses the various mechanisms used to protect the router itself. When thinking about the security of your network it is important to consider these issues for all your systems. Using this procedure. If possible. If remote administration is used to configure and control routers. user account protection. This area should be under some sort of supervision 24 hours a day and 7 days a week. If possible. but always includes the following basic steps. system personnel.2 and 5. then consider ways of protecting the machines used for remote administration. To illustrate one reason why physical security is critical to overall router security. In practice. Use access lists to limit remote administration access to hosts that enjoy reasonable physical security.Advanced Security Services 4. an individual with physical access can gain full privileged (enable) access to a Cisco router without using a password. This can be accomplished through the use of guards. should be located in a limited access area. physical security mechanisms and policies must not make access too difficult for authorized personnel. like routers. Version 1.3). only slowed down. as well as for your routers. or they may find ways to circumvent the physical security precautions. and the networks they use to communicate with the router.1.1c 55 . The details of the procedure varies between router models. These include physical access. There are things that can be done to make this more difficult.1. Network equipment. It is always a matter of time and effort. are especially important because they are often used to protect segments of the network and can also be used for launching attacks against other network segments. One of the best additions to the security features of a computer network is to limit access. Physical Security Once an individual has physical access to a piece of networking equipment there is no way to stop him from modifying the system. This is sometimes called the test system mode. 4. software protection.

Permitting direct dial-in to any vital piece of network infrastructure is potentially very risky. do not neglect to restore the system boot settings after regaining access to the router. 56 Version 1. the procedure takes only a couple of minutes.” Anyone with experience or training using Cisco routers can parley physical access into full privileged administrative access. In many cases. Routers equipped with these kinds of slots will give preference to memory installed in a slot over memory installed in the chassis. and should be set up only when timely access by other means is not feasible. (Note: Step 5 is very important.1.1c . routers are sensitive to extreme temperature and humidity.Router Security Configuration Guide Step 2 Reboot the system. Step 4 View or change the password. if you need to use the password recovery procedure for any reason. It is important to apply these measures before connecting a new router to any potentially hostile network. Step 6 Reboot the system. An operational security concern closely related to physical security is physical operating environment. The area should also be controlled for temperature and humidity. If done carefully and well. or replace an old one. They could then boot the router with their flash. The primary difference between the two ports is that the password recovery mechanism can be used on the console port only. Like most networking equipment. thus causing the router to run their IOS version and configuration. the auxiliary port should be disabled (see Section 4. Most Cisco routers have both a console and an auxiliary port. A room where routers are located should be free of electrostatic and magnetic interference. or erase the configuration. The console (con) and auxiliary (aux) ports on Cisco routers are used for serial connections to the router. This is also a personnel safety issue. If at all possible. If a router is not located in an environmentally friendly area then it may operate in unexpected ways and degrade its security. all routers should be placed on an Uninterruptible Power Supply (UPS). Some administrators connect a modem to the auxiliary port to facilitate remote administration via dial-up. because a short power outage can leave some network equipment in undetermined states. Failure to do so will usually result in the router coming up in an insecure state on subsequent reboots.3). In general. this kind of attack can be very difficult to detect. Step 3 Access enable mode (which can be done without a password if you are in test system mode).) A second reason for controlling physical access to the router involves flash memory cards. Step 5 Reconfigure the router to boot up and read the NVRAM as it normally does. Many Cisco router models offer PC-Card slots or CompactFlash slots that can hold additional flash memory. An attacker with physical access to a router on your network can install a flash memory card. The rest of this section describes configuration steps that you should take to lock down your routers. some of the smallest models have only a console port. the auxiliary port is unused. The best defense against it is good physical security.

but not cutting-edge. Router Configuration and Commands (IOS) After connecting to a router and initially logging in. and logging router functions. Router Software Versions Cisco issues new IOS versions and upgrades fairly frequently.3. Cisco IOS releases. Access-list mode is used for building named access lists. and accounting (AAA). auxiliary port and virtual terminal lines.2. the system is in user mode also known as EXEC mode.e. later versions (i. Newer versions of IOS fix bugs and vulnerabilities that existed in the older versions. authentication systems. The list below describes some of the sub-modes. 12. the so-called ‘new model’. For more details on IOS upgrades. routing protocols.1 and later offer a comprehensive model for authentication. or Vlan2. Ethernet 0/1. authorization. Privileged EXEC mode is sometimes called ‘enable mode’. • line (config-line) is used to set up the console port. To enter the global configuration mode (config) type the command configure terminal . access lists. The recommended newest release would be the most recent “GD” version that is at least a month old (at the time of this writing.6 for more details.0. • route (config-route) is where specific parameters can be set and modified for a selected routing protocol.0. Cisco IOS 11. There are several configuration modes on a Cisco router. authorization. is reserved for the privileged EXEC mode.5 and 8. lines. EXEC mode gives limited access to the command set of the router. logging.Advanced Security Services 4. making it a significant administrative burden to keep all the routers on a large network up to date. 12.1. log in and enter the command show version. This will allow others to find the bugs in the newer versions (and get them fixed). In addition to the standard authentication. Access to all the router commands.1.0.1. while 12. extended (config-ext-n) and standard (config-std-n). routing protocols. see Sections 4. etc. There are sub-modes which are used to configure specific settings for interfaces. To check your IOS version.9 was a mature version of Release 12).1c 57 . • interface (config-if) is used to configure aspects of a particular interface like FastEthernet0. Keep your IOS as up to date as is practical. See Section 4. and much more. which can be used instead of numbered lists. In the global configuration mode a wide variety of overall router features and settings can be changed: banners.1 was an early version of IOS Release 12. Version 1. and add new security features. A good approach to this problem is to maintain operational routers with recent. Typing the enable command at an EXEC mode prompt will give access to the privileged EXEC mode. A second problem is that the early versions of new IOS releases can be less robust than more mature. commonly abbreviated “config t”. including the ability to change the configuration. 4.1.6 for a brief description and Section 4.3. • access-list: There are two types of IP named access lists.21). The recommended minimum IOS release is IOS 12.

Router Network Traffic and the Loopback Interface The pimary job of a router is forwarding traffic between networks. not the external network. consult [5]. It means that IP packets generated by the router will have the loopback interface’s address as their source address. Passwords. Logins. Commands to set source interface bindings are given with the discussion of each service in the rest of the guide. 4. Central(config)# interface loopback0 Central(config-if)# description Main loopback interface Central(config-if)# ip address 14.) To create a loopback interface.1c . typically a router will select a source address from one of the addresses bound to one of its network interfaces.1.4.250 255. because the address of the loopback interface is fixed. but routers also generate some network traffic.11. and Accounts Logins and Banners A login banner. a statement about 58 Version 1. Cisco IOS routers have the ability to define internal virtual interfaces. to define one loopback interface. hiding administrative aspects of your network from potential attackers is usually good practice.255.2. simply assign it an IP address. should be set up on each operational router. In addition to physical interfaces. Routers and other network devices communicate using various management protocols. When a router is configured to use the loopback interface for services. This can be problematic in several ways. Adopting this practice yields several benefits for the overall stability and security management of a network. Central# config t Enter configuration commands. router network services that can be bound to the loopback interface should be. which includes a legal notice. in configuring Cisco routers. It is considered best practice. one per line. nor can it be part of the same network as any other interface. NTP. we say that the service is bound to that interface. that connection must have some source address.255. such as routing protocols. (When a service is configured to use the loopback interface as its source.1. SNMP. and designate it as the source interface for most traffic generated by the router itself. the loopback interface’s address does not appear in any route-based network maps.255 Central(config-if)# end Central# In general. called loopback interfaces. the loopback’s address usually should be in the range of the internal or DMZ network. End with CNTL/Z. A legal notice usually includes a ‘no trespassing’ warning. Note that the loopback address cannot be the same as the address of any other interface.5. Also. Privileges. a statement that all use of the device must be authorized by the owning organization. mainly because the source address for some services can vary. For further discussion of loopback interfaces. it is possible to configure the security of other devices in the network more tightly.Router Security Configuration Guide 4. and TFTP. When the router initiates a network connection. For a border router.

should be disabled. The delimiter can be any single character. so that if an administrator forgets to log out. and protected from unauthorized access. Central(config)# username brian privilege 1 password g00d+pa55w0rd Central(config)# line con 0 Central(config-line)# login local Central(config-line)# end Central# The auxiliary port. one per line. otherwise you will be locked out of the console.1c 59 . has no need for the aux port. A proper legal notice protects the ability of the owning organization to pursue legal remedies against an attacker. The connection to the console port should not be left logged in. Consult your organization’s legal staff or general counsel for suitable language to use in your legal notice. see the Accounts sub-section below). one per line. Router Central.0 and earlier only). The example below shows how to set up the console line to enforce a five-minute timeout. Central(config)# line con 0 Central(config-line)# transport input none Central(config-line)# exec-timeout 5 0 Central(config-line)# exit Central(config)# End with CNTL/Z. Apply the command login local to the console line to enforce user log.Advanced Security Services the device being subject to monitoring. Version 1. Configure the console line to time out EXEC sessions. Be careful not to provide any information in the banner message that should not be shared with the general public. The console (con) port is the default location for performing router management and configuration. Do not include any network architecture or device information in the banner message. and perhaps a statement threatening prosecution. If you do not already have users accounts set up. the command transport input none prevents remote access to the console port via reverse-telnet (on IOS 12. Router model and location information should never be included. in the sample network diagram (Figure 4-1). The example below shows how to disable login on the auxiliary port (login to enable mode first): Central# config t Enter configuration commands. but that terminal (or computer) should be standalone. Central# config t Enter configuration commands. then create at least one before setting the console to use local login. See also [7] for more discussion of banners and their content. The example below shows how to create an account with a password and set console login. The syntax for creating a local user is username name privilege level password string. the router will log him or her out automatically. Each authorized user should log in using their own account (for more details. Note that you must create at least one user account. To set the router's message-of-the-day banner use the command banner motd delimiter message delimiter. It is okay to leave a connection to the console port attached all the time. End with CNTL/Z. if at all possible.

see Section 4. Login on the virtual terminal lines should be disabled if remote administration is not absolutely necessary. one per line.1c . If the auxiliary port is required for a second local serial connection then configure it as shown below.Router Security Configuration Guide Central(config)# line Central(config-line)# Central(config-line)# Central(config-line)# Central(config-line)# Central(config-line)# aux 0 transport input none login local exec-timeout 0 1 no exec exit Section 4. [Note: perform these commands only when connected to the aux or console port. and Sections 5. and 60 Version 1. as shown in the example below. these connections are called virtual terminal lines. or even more. or use the command transport input none. It is important to know how many virtual terminals your IOS version has.1. Most versions of IOS have five virtual terminals.1.] South# config t Enter configuration commands.2 and 5.5 discusses configuration of the auxiliary port if it is required for a modem. Central(config)# line Central(config-line)# Central(config-line)# Central(config-line)# Central(config-line)# Central(config-line)# Central# aux 0 exec-timeout 5 0 login local transport input none exec end VTYs and Remote Administration One primary mechanism for remote administration of Cisco routers is logging in via Telnet or SSH. create an access list and apply it to the virtual terminal lines. do not perform them while logged into the router via Telnet. Some IOS versions (including the versions designated “Enterprise”) may have 15. numbered 0 through 4. To disable network virtual terminal connections to the router.6 for details on configuring remote administration. South(config)# no access-list 90 South(config)# access-list 90 deny any log South(config)# line vty 0 4 South(config-line)# access-class 90 in South(config-line)# transport input none South(config-line)# login local South(config-line)# exec-timeout 0 1 South(config-line)# no exec South(config-line)# end South# End with CNTL/Z.3 for cryptographic mechanisms for protecting the remote administration connections. Remote administration without encryption is inherently dangerous because anyone with a network sniffer on the right LAN segment can acquire the router passwords and would then be able to take control of the router. If remote administration is necessary. 64.

(Note: on most IOS versions. one per line. then disable the extra ones. you cannot delete VTYs 0 through 4. If the router is configured with aaa newmodel then local or remote AAA can be used for user authorization (see Section 4. If the router has more vtys than you need.6 for more details). two more than the default complement of five. South# show line vty 0 ? <1-935> Last Line range summary Quick line status summary | Output modifiers <cr> South# show line vty 0 935 Tty Typ Tx/Rx A Modem Roty AccO AccI 66 VTY 67 VTY 68 VTY 69 VTY 70 VTY 71 VTY 72 VTY South# Uses Noise Overruns Int 0 0 0/0 0 0 0/0 0 0 0/0 0 0 0/0 0 0 0/0 0 0 0/0 0 0 0/0 - The seven lines of output from the ‘show line’ command indicate that the router South has seven virtual terminals. Normally. South(config)# no line vty 5 South(config)# exit South# show line vty 0 935 Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int 66 VTY 0 0 0/0 67 VTY 0 0 0/0 68 VTY 0 0 0/0 69 VTY 0 0 0/0 70 VTY 0 0 0/0 South# Privileges Cisco IOS provides for 16 different privilege levels ranging from 0 to 15. you can list them using the command show line vty in the manner shown below. By default Cisco provides EXEC (level 1) with a few commands which may. The transcript below shows how to delete the extra two vtys on the router South . or delete them with the configuration mode command no line vty. If you do not know how many vtys your router supports.Advanced Security Services to explicitly configure all of them securely. User EXEC mode runs at privilege level 1 and “enabled” mode (privileged EXEC mode) runs at level 15. you would configure all of the vtys on the router identically.simply delete 5. and both 5 and 6 will disappear. The next example Version 1.1c 61 . make more sense being at a higher privilege level. End with CNTL/Z.) South# config t Enter configuration commands. Cisco IOS comes with 2 predefined user levels. Every IOS command is pre-assigned to either level 1 or level 15. in terms of security.

Central(config)# Central(config)# Central(config)# Central(config)# Central(config)# Central(config)# Central(config)# Central(config)# Central(config)# privilege exec level 15 connect privilege exec level 15 telnet privilege exec level 15 rlogin privilege exec level 15 show ip access-lists privilege exec level 15 show access-lists privilege exec level 15 show logging ! if SSH is supported. Cisco recommends using Type 5 encryption instead of Type 7 where possible (see “Configuring Passwords and Privileges” in the IOS 12 Security Configuration Guide). and then assigning particular commands to that privilege level. First.Router Security Configuration Guide shows how to move the commands to the privileged mode. This could be done by assigning a password to an intermediate level. once a user has write access they could leverage this to acquire greater access. For example. an organization might want to set up more than the two levels of administrative access on their routers. South# config t Enter configuration commands. this could cause unexpected security holes in the system. do not use the enable password command. South(config)# enable secret 2-mAny-rOUtEs South(config)# no enable password South(config)# end South# 62 Version 1. Type 7 encryption is used by the enable password.1c . do not use the username command to set up accounts above level 1. End with CNTL/Z. it will not be used and may give away a system password. privilege exec level 15 ssh privilege exec level 1 show ip The last line is required to move the show command back down to level 1. one per line. like 5 or 10. • To protect the privileged EXEC level as much as possible. Second. use the enable secret command to set a level password instead (see next sub-section). But. be very careful about moving any part of the configure command down. and line password commands. Even if the enable secret is set do not set the enable password. username. Type 5 uses an iterated MD5 hash which is much stronger. Third. Passwords There are two password protection schemes in Cisco IOS. if an attempt was made to do something like this there are a few things to be very careful about. be very careful about moving too much access down from level 15. Type 7 uses the Ciscodefined encryption algorithm which is known to the commercial security community to be weak. which in most configurations should be protected better. only use the enable secret command. It is also possible to set up intermediate privilege levels.. Deciding which commands to assign to an intermediate privilege level is beyond the scope of this document.

In addition to the above password access mechanisms. Version 1. But user accounts should be created for auditing purposes (see Accounts. • Be aware that there are some secret values that service passwordencryption does not protect. Never set any of these secret values to the same string as any other password.1 and earlier) • NTP authentication keys – for more information about NTP security.3). below). • Avoid more than 4 digits or same-case letters in a row. The username command should be used to create individual user accounts at the EXEC level and then the higher privilege levels should be protected with enable secret passwords. Some of the more important rules are provided in the following list. • RADIUS keys (in 12. and audit users (see Section 4. uppercase letters. no user account should be created above privilege level 1. AAA mechanisms may be used to authenticate. • Avoid dictionary words. • Always include at least one of each of the following: lowercase letters.1 and earlier) • TACACS+ keys (in 12. phone numbers. dates.Advanced Security Services • Because it is not possible to use Type 5 encryption on the default EXEC login or the username command (prior to IOS 12. Then users with a need to work at higher levels would be given the higher privilege level password. addresses.1 and earlier) – for more information about routing protocol authentication see Section 4. • Make all passwords at least eight characters long.1c 63 . and special characters. But if the login local command is used to protect a line then the specified user name/password pair is used.3. authorize. this will keep passersby from reading your passwords when they are displayed on your screen. • SNMP community strings – for more information about SNMP security see Section 4. For access and logging reasons the login local method should be used.5.6 for details).5. see Section 4.4. Good security practice dictates some other rules for passwords. Do not set any user or line password to the same value as any enable secret password. • The privileged EXEC secret password should not match any other user password or any other enable secret password. • If the login command is used to protect a line then the line password command is the only way to set a password on a line. proper names. • Peer router authentication keys (in 12. • Enable service password-encryption. digits.

which describes AAA. In addition. 4. and AAA is used for access control.6. Remote Internal only with AAA – administration can be performed on the router from a trusted internal network only. Remote Access This document will discuss five connection schemes which can be used for router administration. No Remote – administration is performed on the console only. above). The login accounts created with the username command should be assigned privilege level 1 (see Passwords. See Section 4. and remove the local user named ‘brian’. Central# config t Enter configuration commands. the log message that is generated will include the name of the login account which was used.1c . 4. Remote External with AAA – administration can be performed with both internal and external connections and uses AAA for access control. 5. 2.Router Security Configuration Guide See [4] for more detailed guidance on selecting good passwords. one per line. 64 Version 1.1. Accounts First. do not create any user accounts without passwords! When an administrator no longer needs access to the router. The example below shows how to create local user accounts for users named ‘rsmith’ and ‘bjones’. for a preferred user account mechanism. End with CNTL/Z. Central(config)# service password-encryption Central(config)# username rsmith password 3d-zirc0nia Central(config)# username rsmith privilege 1 Central(config)# username bjones password 2B-or-3B Central(config)# username bjones privilege 1 Central(config)# no username brian Central(config)# end Central# Only allow accounts that are required on the router and minimize the number of users with access to configuration mode on the router. When an administrator logs in with a user name and changes the configuration. Note: enable secret and username passwords may be up to 25 characters long including spaces. 1. 3. give each administrator their own login user name for the router.6. remove their account. Remote Internal only – administration can be performed on the router from the internal network only. Remote External – administration can be performed with both internal and external connections.

anyone with a network sniffer and access to the right LAN segment can acquire the router account and password information. Cisco has added support for the Secure Shell (SSH) protocol to many versions of IOS 12. A detailed discussion on setting up modems is beyond the scope of this document.6. When you use remote administration. Consult the Cisco IOS Dial Services guide [6] for information about connecting modems and configuring callback.2. create and apply an access list explicitly listing the hosts or networks from which remote administration will be permitted. Central(config)# ip telnet source-interface loopback0 Central(config)# access-list 99 permit 14. all connections to the router should require authentication (using individual user accounts) for access.4 and later.2. The Auxiliary Port As discussed in Section 4. and nearly all IOS releases in 12.5. To configure the vtys for remote access do the following: bind the telnet service to the loopback interface.1 log Central(config)# access-list 99 permit 14.6 describes remote access with AAA. The five regimes listed above are listed in the order that best protects the router and allows for accounting of router activities. The security of remote administration can be enhanced by using a protocol that provides confidentiality and integrity assurances. because the user’s password will travel the network in clear text form.6 log Central(config)# access-list 99 deny any log Central(config)# line vty 0 4 Central(config-line)# access-class 99 in Central(config-line)# exec-timeout 5 0 Central(config-line)# transport input telnet Central(config-line)# login local Central(config-line)# exec Version 1.1.1c 65 .3T. This is why remote administration security issues center around protecting the paths which the session will use to access the router. Remote access over untrusted networks (e. 12. Only if absolutely required should a modem be connected to the aux port as a backup or remote access method to the router. Network Access Remote network connections use the VTY lines to connect to the router.9. so it is necessary to apply access controls to the aux port. unless the traffic is adequately protected. with or without AAA. Section 4. Section 5. Setting up IPSec for remote administration is covered in Section 5.Advanced Security Services As discussed in Section 4.2.5 the aux port should be disabled.6). and SSH should always be used instead of Telnet whenever possible. IOS callback features should be used. For better security. and set an exec session timeout.0 and later. This can be accomplished by using login local (see next sub-section for example) or AAA (see Section 4. the Internet) should not be used.g.3 describes how to use SSH for secure remote administration. remote administration is inherently dangerous. such as IPSec or SSH. As discussed earlier. Attackers using simple war-dialing software will eventually find the modem.1. This section will discuss remote internal only access without AAA.

and logging of user activities on a router. they can establish further telnet sessions from the router to other devices. it should be disabled as shown below.3. Central(config)# service tcp-keepalives-in Central(config)# service tcp-keepalives-out Central(config)# exit Central# 4.1. like rlogin and web. Authorization. Logistics for Configuration Loading and Maintenance There are two basic approaches for configuration loading and maintenance: online editing and offline editing. if you are going to permit remote administration via Telnet.1. Central(config)# line vty 0 4 Central(config-line)# transport output none Central(config-line)# exit Lastly. Additionally. Online editing provides for syntax checking but provides limited editing capability and no comments.6 contains details on configuring AAA in an example network. privileges. Authentication is the mechanism for identifying users before allowing access to a network component. Offline editing provides the ability to add comments. and Accounting (AAA) This is Cisco’s new access control facility for controlling access.8. Section 4.Router Security Configuration Guide Central(config-line)# end Central# The IP access list 99 limits which hosts may connect to the router through the vty ports. once an administrator or attacker has gained telnet access via a VTY. enable TCP keepalive services.1c . These services will cause the router to generate periodic TCP keepalive messages. Using this service does not remove the need for setting an exec-timeout time as recommended above. This is important because the other supported protocols.7. are less secure and should be avoided. Authorization is the method used to describe what a user has the right to do once he has authenticated to the router. For more details on access lists see Section 4. They each have advantages and disadvantages. Cisco IOS supports outgoing telnet as well as incoming. Finally. Authentication. Unless this capability is important for managing your network. Accounting is the component that allows for logging and tracking of user and traffic activities on the router which can be used later for resource tracking or trouble shooting. the IP addresses which are allowed to connect must be on an internal or trusted network. the transport input telnet command restricts the management interface to telnet only. thus allowing it to detect and drop orphaned (broken) TCP connections to/from remote systems. The login local command requires a username and password be used for access to the router (this command will be different if you are using AAA with an authentication server). 4. allows for the use 66 Version 1.

the system where the configuration files are stored should use the local operating system’s security mechanisms for restricting access to the files. In this situation it is only necessary to manage the startup configuration since the running configuration is identical. but provides no syntax checking. and SCP (available in many releases 12. You can obtain the encrypted string by setting the password manually on the router console. Also. so that if there is a power failure or some other problem the router will restart with the correct configuration. it is best to type the passwords while on-line (using the console) and then copy the encrypted strings to the offline configuration.1. ! set the enable secret password using MD5 encryption enable secret 5 $1$fIFcs$D. described below. The example below shows how an encrypted enable secret setting would appear in an off-line configuration file. The configuration files would be stored on the computer attached to the console and the local machine’s copy/paste buffer can be used for transferring the configuration to the router. In this case.0 and newer). because the saving will not include default values and after an IOS upgrade you may encounter unexpected configuration problems. Cisco configuration save utilities will also not save default values. rcp. Third. Only a few lines should be copied at a time so that you can determine that the entire Version 1. The recommended approach is a hybrid of the two.1c 67 . then displaying the running configuration. and guarantees all settings will be visible.8 Local and Remote Administration Section 4.Advanced Security Services of better editors. First. When saving and loading configurations. FTP (available for IOS 12. With the online editing. It is also important to keep the running configuration and the startup configuration synchronized. tracking the configuration can become very difficult. When managing configuration files offline there are several security issues. the show run command will only show those configuration settings which are different from the IOS defaults. using the terminal is the best choice for loading a new configuration. and then copying and pasting the encrypted string into your offline configuration file. TFTP. then they will be stored in the clear and transferred in the clear. But the offline method will leave passwords in the clear. Only save off the running configuration for an emergency. Instead.3 recommends performing local administration. Second. always use the startup configuration to avoid problems. This is especially true for the enable secret password. maintain the configuration offline by writing it offline (see above). The possible methods for transferring files to a router have increased with newer IOS releases.lgcsUnsgtLaWgskteq. The primary mechanisms available are the console terminal. with the configuration files offline the files must be transferred to the router in the relatively secure method. use configuration management to track changes to your configurations. Because each Cisco IOS release changes the default values for some of the commands.1 later that support SSH). Only authorized router administrators should be given access to the files. if you set passwords in an offline configuration file. Old and alternative configurations should be stored offline.

use a better communication program. End with CNTL/Z.0 then use the FTP protocol to transfer the configuration files to and from the router. The file would again be transferred using the host systems copy/paste buffer to move the text from a file editor to the terminal emulator. If remote administration is allowed and the IOS is newer then version 12. Central(config)# ip ftp username nsmith Central(config)# ip ftp password 1pace-4ward Central(config)# ip ftp source-interface loopback0 Central(config)# exit Central# copy /erase ftp: startup-config Address or name of remote host []? 14. Central# config t Enter configuration commands. On Linux.1 Source filename []? /rtr-backup/central-config Destination filename [startup-config]? Accessing ftp://14.0 serial communication program. Set the source interface for FTP to the loopback interface if you have defined one. [Note: the default Windows NT 4.364 secs Central# 68 Version 1. otherwise use the interface closest to the FTP server.9. Central# copy running-config startup-config Central# config t Enter configuration commands. End with CNTL/Z.2.1c . On Solaris. The following example shows how to save the startup configuration to a file. performs copy/paste very slowly. if you have one available. On Windows NT and 2000. Erasing the nvram filesystem will remove all files! Continue? [confirm] y [OK] Erase of nvram: complete Loading /rtr-backup/central-config ! [OK . such as TeraTerm Pro.. one per line.9.2.1/rtr-backup/central-config..9.352 secs (459 bytes/sec) Central# The next example demonstrates how to load a new configuration file into the startup configuration.2.5516/1024 bytes] 5516 bytes copied in 4.1 Destination filename [startup-config]? /rtr-backup/central-config Writing central-config !! 5516 bytes copied in 12. Hyperterminal. the minicom program is suitable for Cisco local console access. Central(config)# ip ftp username nsmith Central(config)# ip ftp password 1pace-4ward Central(config)# ip ftp source-interface loopback0 Central(config)# exit Central# copy startup-config ftp: Address or name of remote host []? 14. one per line. the tip command can be used.Router Security Configuration Guide configuration file is transferred successfully.] If remote administration is being allowed and the router is running an IOS older than version 12.0 then using the console connection or a telnet connection is the best choice for administration.

available at: http://www. Cisco Press. B. SCP should be used whenver possible. Cisco Press. National Institute of Standards and Technology.0 Configuration Fundamentals. J. References [1] Cisco IOS Release 12.. O’Reilly & Associates.1c 69 . 1st Edition. J. [6] Cisco IOS Dial Services Configuration Guide. Cisco IOS 12. It includes information about configuring logins. 1985. Hardening Cisco Routers. ed. SANS Institute.Advanced Security Services The other protocols.9. This is the reference manual and guide for basic IOS configuration tasks. See Section 4. 1999. Cisco Press.0 Security Configuration Guide. especially for initial set-up of a new router. Cisco IOS Essentials.5. This is the reference manual and guide for major security features in IOS 12. are less secure than FTP and should not be used for loading or saving router configurations. Securing Cisco Routers: Step-by-Step. [7] Akin. J. [4] “Password Usage” Federal Information Processing Standard Publication 112. such as rcp and TFTP.1. Cisco ISP Essentials. and more. Configuring Passwords and Privileges. 2002. An excellent introduction to basic usage and configuration of IOS routers. [2] Buckley. T. A. because it provides integrity and confidentiality protection. [8] Stewart. vtys. and an in-depth treatment of Unicast RPF. 1999.nist. and Wright. Cisco Press. This detailed Cisco guide for Internet Service Providers includes extensive discussion of routing protocols (especially BGP). 2000. Version 1. P.itl. The sections about passwords and warning banners contain very good information. A pragmatic and detailed guide to securing Cisco routers. Relevant sections include: Security Overview. 2002. and Smith.5 for details on using TFTP if required. 4. and dial-in features. [5] Greene.. all with fully worked-out examples. [3] Albritton. modem. Relevant sections include: IOS User Interfaces and File Management. and Traffic Filtering and Firewalls.htm This federal standard includes some good guidelines on choosing passwords that are difficult to guess. This is the reference manual and guide for serial line. 1999. April 2002. A very specific guide to configuring many IOS features securely.

etc. Table 4-1: Overview of IOS Features to Disable or Restrict Feature Cisco Discovery Protocol (CDP) TCP small servers UDP small servers Finger Description Proprietary layer 2 protocol between Cisco devices. general security practice for routers should be to support only traffic and protocols the network needs. and Security Risks The table below lists some of the services offered on Cisco IOS 11. etc. Standard TCP network services: echo. 3.2. Unix user lookup service. As stated in Section 3. If a particular portion of a network needs a service but the rest does not.2: enabled Enabled Recommendation CDP is almost never needed. then the restriction features should be employed to limit the scope of the service. disable it.0. IP source routing is a little-used feature of IP that can be utilized in network attacks.3: disabled 11. discard. Some of these services can be restricted or disabled. For example. This list has been kept short by including only those services and features that are security-relevant and may need to be disabled. and later versions. Typical Services. Some of these services are application layer protocols that allow users and host processes to connect to the router. a router may support a network where the bootp protocol is employed. Required Services. the router’s bootp server should be disabled. but some other host is acting as the bootp server. IP source routing should be disabled. and 7. 12. 4. Standard UDP network services: echo. In many cases. This is a legacy feature. 4. Router Network Service Security Cisco routers support a large number of network services at layers 2. Cisco IOS supports turning a service off entirely. allows remote listing of logged in users.2: enabled 11.3: disabled 11.1c . Unless it is required for the network to operate.Router Security Configuration Guide 4. Others are automatic processes and settings intended to support legacy or specialized configurations but which are detrimental to security. improving security without degrading the operational use of the router. For example. Unauthorized persons don’t need to know this. chargen. disable it.1. disable it explicitly. Turning off a network service on the router itself does not prevent it from supporting a network where that protocol is employed. In this case. Turning off an automatic network feature usually prevents a certain kind of network traffic from being processed by the router or prevents it from traversing the router. Default Enabled 11.3. most of the services listed below are not needed. or restricting access to particular network segments or sets of hosts. disable it explicitly.2. 70 Version 1. This is a legacy feature.

explicitly disable. Disable if not explicitly needed. disable it. disable it if it is not in use.1c 71 . Can be helpful in attacks. disable on interfaces to untrusted networks. This is rarely needed and may open a security hole.Advanced Security Services Feature HTTP server Description Some Cisco IOS devices offer web-based configuration. otherwise restrict access. explicitly disable on interfaces to untrusted networks. Routers can support SNMP remote query and configuration. Packets can identify a target LAN for broadcasts. Routers can perform DNS name resolution. Default Varies by device Enabled Recommendation If not in use. Set the DNS server addresses explicitly. If not in use. Legacy management protocol . Bootp server Configuration auto-loading PAD service IP source routing Proxy ARP Disabled Enabled Enabled Enabled IP directed broadcast IP unreachable notifications IP mask reply Enabled (11. Can aid network mapping. explicitly disable. This is rarely used. Service to allow other routers to boot from this one. Can aid IP address mapping. part of the DECNet protocol suite. or disable DNS lookup. Router will attempt to load its configuration via TFTP. Protocol Enabled Domain Name Service Enabled (broadcast) Version 1. disable it.25 packet assembler service. remove default community strings and explicitly disable. Feature that allows a packet to specify its own route. disable it. Router will send an ICMP redirect message in response to certain routed IP packets. Directed broadcast can be used for attacks. Router will support X. disable on interfaces to untrusted networks. Can aid network mapping. If not in use.3 & earlier) Enabled Disabled IP redirects Enabled Maintenance Operations Protocol (MOP) NTP service Enabled (on Ethernet interfaces) Enabled (if NTP is configured) Simple Network Mgmt. otherwise restrict access. Router will explicitly notify senders of incorrect IP addresses. Disable if not explicitly needed. Router can act as a time server for other devices and hosts. Router will send an interface’s IP address mask in response to an ICMP mask request. Disable this service unless the router is serving as a LAN bridge. Router will act as a proxy for layer 2 address resolution. otherwise restrict access.

2. Central(config)# no cdp run Central(config)# exit Central# show cdp % CDP is not enabled Central# End with CNTL/Z.2.9. and then disable it on each interface where it is not needed using the no cdp enable command in interface configuration mode.2. % Connection refused by remote host Central# 72 Version 1. it is not necessary for routers to support these services.250 closed by foreign host] Central# config t Enter configuration commands. it can be enabled and disabled for each interface. In the unlikely event that CDP is needed for part of a network.. then tcp-small-servers are enabled Central# connect 14.3 and 12. and is considered deleterious to security. In virtually all cases. 13 .9. Open Monday. End with CNTL/Z. How to Disable Unneeded Features and Services Each sub-section below describes how to disable or restrict particular services and features under Cisco IOS 11.Router Security Configuration Guide 4. April 3.250.9. 13 .. one per line. Central# ! if connect succeeds.9.1c .. 2000 11:48:39-EDT [Connection to 14.250 daytime Trying 14. CDP The Cisco Discovery Protocol is a proprietary protocol that Cisco devices use to identify each other on a LAN segment. use the commands shown below in global configuration mode.2. one per line. The example below shows how to test whether the TCP small servers are running. To enable CDP use the cdp run command in global configuration mode.9.2. TCP and UDP Small Servers The TCP and UDP protocol standards include a recommended list of simple services that hosts should provide.2. Central# config t Enter configuration commands..250 daytime Trying 14. Central(config)# no service tcp-small-servers Central(config)# no service udp-small-servers Central(config)# exit Central# connect 14. and how to disable the TCP and UDP small servers.250. It is useful only in specialized situations. and they should be disabled. To turn off CDP entirely.2.

.9.2. then its use should be restricted as follows. 80 . Central(config)# no ip http server Central(config)# exit Central# connect 14.2.250 www Trying 14. Line User Host(s) Idle Location 130 vty 0 14.9.9. Typically. Version 1. access restricted. The example below shows how to test and disable the finger server.2.250 finger Trying 14. Further. one per line. Therefore. On a Cisco router. which is used for querying a host about its logged in users.2.250. While the web access features are fairly rudimentary on most Cisco router IOS releases.9. one per line.250.9.250..250 closed by foreign host] Central# config t Enter configuration commands.1c 73 . Web-based remote administration is useful primarily when intervening routers or firewalls prevent use of Telnet for that purpose.2.2.6 00:00:00 goldfish *131 vty 1 idle 00:00:00 central [Connection to 14.. then it should be disabled as shown below.2. web-based administration imposes the requirement that users log in at full (level 15) privilege. configuring. Central# config t Enter configuration commands. If web-based administration is examined and found necessary for network operations. Central# connect 14. they are a viable mechanism for monitoring. % Connection refused by remote host Central# HTTP Server Newer Cisco IOS releases support web-based remote administration using the HTTP protocol.. Open This is the CENTRAL router.. 79 . However.250 finger Trying 14. the show users command may be used to list the logged in users. End with CNTL/Z.9. it is important to note that both Telnet and web-based remote administration reveal critical passwords in the clear.. % Connection refused by remote host Central# End with CNTL/Z. Central(config)# no ip finger Central(config)# no service finger Central(config)# exit Central# connect 14.9. 79 . users who are not authorized to log in to the router have no need to know who is logged in.9. If web-based remote administration is not needed.Advanced Security Services Finger Server The IOS finger server supports the Unix ‘finger’ protocol. and attacking a router.2. web-based remote administration should be avoided.

Router Security Configuration Guide • Set up usernames and passwords for all administrators. Access lists are described in Section 4.0. If possible.9. use the commands shown below.5. consult the Cisco IOS documentation. then turn on http auth Central(config)# username nzWeb priv 15 password 0 C5-A1rCarg0 Central(config)# ip http auth local Central(config)# ! Create an IP access list for web access Central(config)# no access-list 29 Central(config)# access-list 29 permit host 14. This guide does not provide instructions on configuring SSL.) Bootp Server Bootp is a datagram protocol that is used by some hosts to load their operating system over the network.0 network and the host log Central(config)# access-list 29 deny any log Central(config)# ! Apply the access list then start the server Central(config)# ip http access-class 29 Central(config)# ip http server Central(config)# exit Central# If possible. as discussed in Section 4. bootp is very rarely used. • Configure and enable syslog logging as described in Section 4. Administrators will be allowed to connect from the 14. Cisco routers are capable of acting as bootp servers.18 log Central(config)# access-list 29 permit 14. Central(config)# no ip bootp server Central(config)# exit End with CNTL/Z.2.1. Cisco IOS does not yet support the superior HTTP digest authentication standard).3.1c .18 only. Central(config)# ! Add web admin users. as described in Section 5.0 0. one per line. protect the HTTP traffic by setting up IPSec. • Create and apply an IP access list to limit access to the web server.9. The example below illustrates each of these points. Central# config t Enter configuration commands. End with CNTL/Z. The router’s web server will use HTTP basic authentication to demand a username and password (unfortunately. and offers an attacker the ability to download a copy of a router’s IOS software. This facility is intended to support a deployment strategy where one Cisco router acts as the central repository of IOS software for a collection of such routers.6. but IPSec is preferred. To disable bootp service.6. Central# config t Enter configuration commands.2. use AAA user access control as described in Section 4. one per line.6. (Some recent versions of IOS can support SSL (HTTPS) for protecting administrative HTTP traffic.2. AAA will give more control and better audit.2.2. primarily for other Cisco hardware. 74 Version 1. HTTPS is better than plain HTTP. In practice.

Unless a network depends on source routing. and should be considered only on a network that is wholly trusted (e. Cisco routers normally accept and process source routes. Central(config)# no service pad Central(config)# IP Source Routing Source routing is a feature of IP whereby individual packets can specify routes. Central# config t Enter configuration commands.Advanced Security Services Configuration Auto-Loading Cisco routers are capable of loading their startup configuration from local memory or from the network. it should be disabled on all the net’s routers. PAD Service The packet assembler/disassembler (PAD) service supports X. using the interface configuration command no ip proxy-arp . Disable it on each interface where it is not needed. Cisco routers perform proxy ARP by default on all IP interfaces.25 links. This service is called proxy ARP. Normally. responding to ARP queries on selected interfaces and thus enabling transparent access between multiple LAN segments. This feature is used in several kinds of attacks. ARP transactions are confined to a particular LAN segment. Explicitly disable loading the startup configuration from the network using the commands shown below. effectively extending a LAN at layer 2 across multiple segments. one per line. Because it breaks the LAN security perimeter. proxy ARP should be used only between two LAN segments at the same trust level. A Cisco router can act as intermediary for ARP. This service is on by default. and only when absolutely necessary to support legacy network architectures. Loading from the network is not secure. even on interfaces that are currently idle. Central(config)# no boot network Central(config)# no service config Central(config)# exit Central# End with CNTL/Z.1c 75 . The example below shows how to disable IP source routing. but it is not needed unless your router is using X. Disable it from global configuration mode as shown below. The example below shows how to disable proxy ARP on four Ethernet interfaces. a standalone lab network).g.25. Central(config)# no ip source-route Central(config)# Proxy ARP Network hosts use the Address Resolution Protocol (ARP) to translate network addresses into media addresses. Version 1.

250 YES NVRAM up up Ethernet0/2 unassigned YES unset down down Ethernet0/3 unassigned YES unset down down Central# config t Enter configuration commands.9. This technique was used in some old denialof-service attacks. and ‘Mask Reply’.1. routes. Redirects. Automatic generation of these messages should be disabled on all interfaces. Mask Replies The Internet Control Message Protocol (ICMP) supports IP traffic by relaying information about paths.250 YES NVRAM up up Ethernet0/1 14.1c . Cisco routers automatically send ICMP messages under a wide variety of conditions. Three ICMP messages are commonly used by attackers for network mapping and diagnosis: ‘Host unreachable’. ‘Redirect’.15. one per line. one per line. Explicitly disable directed broadcasts on each interface using the interface configuration command no ip directed-broadcast as shown in the example in the next subsection. especially interfaces that are connected to untrusted networks. Central(config)# interface eth 0/0 Central(config-if)# no ip unreachables Central(config-if)# no ip redirects Central(config-if)# no ip mask-reply Central(config-if)# no ip directed-broadcast Central(config-if)# end Central# 76 Version 1. End with CNTL/Z. IP Unreachables. End with CNTL/Z. and network conditions.2. and the default Cisco IOS configuration is to reject directed broadcasts.Router Security Configuration Guide Central# show ip interface brief Interface IP-Address OK? Method Status Protocol Ethernet0/0 14. Central# config t Enter configuration commands. Central(config)# interface eth 0/0 Central(config-if)# no ip proxy-arp Central(config-if)# exit Central(config)# interface eth 0/1 Central(config-if)# no ip proxy-arp Central(config-if)# exit Central(config)# interface eth 0/2 Central(config-if)# no ip proxy-arp Central(config-if)# exit Central(config)# interface eth 0/3 Central(config-if)# no ip proxy-arp Central(config-if)# end Central# IP Directed Broadcast Directed broadcasts permit a host on one LAN segment to initiate a physical broadcast on a different LAN segment. The example below shows how to turn them off for an interface.

• Disable SNMP system processing. SNMP Services The Simple Network Management Protocol (SNMP) is the standard Internet protocol for automated remote monitoring and administration. If possible.10. configure all routers as part of an NTP hierarchy. Central(config)# interface eth 0/0 Central(config-if)# no mop enabled Central(config-if)# end NTP Service Cisco routers and other hosts use the Network Time Protocol (NTP) to keep their time-of-day clocks accurate and in synchrony.1. one per line. use an access list. Disable it on each Ethernet interface as shown below. In the absence of a deployed SNMP scheme. North# show ip interface brief Interface IP-Address OK? Method Status Protocol Ethernet0/0 14.1.Advanced Security Services MOP The Maintenance Operations Protocol (MOP) was used for system utility services in the DECnet protocol suite.3. North(config)# interface eth 0/0 North(config-if)# ntp disable North(config-if)# exit North(config)# interface eth 1/0 North(config-if)# ntp disable North(config-if)# end North# Disabling NTP on an interface will not prevent NTP messages from traversing the router. To reject all NTP messages at a particular interface. as described in Section 4. as discussed in Section 4. It starts with listing the current configuration to find the SNMP Version 1.250 YES NVRAM up up North# config t Enter configuration commands. all SNMP facilities on all routers should be disabled using these steps: • Explicitly unset (erase) all existing community strings. End with CNTL/Z. If NTP services are not available on the network. The example below shows how to disable SNMP by implementing these recommendations.20 YES NVRAM up up Ethernet1/0 14. It is enabled by default on Ethernet interfaces in some versions of IOS.1c 77 . If a network has a deployed SNMP infrastructure in place for administration. There are several different versions of SNMP. with different security properties.2. • Disable SNMP system shutdown and trap features. then all routers on that network should be configured to securely participate in it.5. then disable NTP as shown below.

DNS name resolution should be enabled on a router only if one or more trustworthy DNS servers are available. you must simply list the entire configuration and inspect it visually).5. The configuration listing is often quite long. so you may want to use IOS output filtering to display only the lines related to SNMP (under IOS 12. For information on setting up and using SNMP securely. When SNMP processing is shut down. If one or more name servers are available on the network.1c .0 and earlier. In general. but they can still be there! The safest way to ensure that SNMP is really unavailable to an attacker. one per line. IOS sends DNS name queries to the broadcast address 255. By default. turn off DNS name resolution as shown below. North(config)# no ip domain-lookup North(config)# end End with CNTL/Z.255. It is also a very good idea to give the router a name.Router Security Configuration Guide community strings.255. using the 78 Version 1. see Section 4. snmp-server community public RO snmp-server community admin RW Central# Central# config t Enter configuration commands. Router Name and DNS Name Resolution Cisco IOS supports looking up host names with DNS. one per line.255. shuts down all SNMP processing on the router. then explicitly set the name server addresses using the global configuration command ip name-server addresses.. note that SNMP must be enabled in order for the SNMP community strings to appear in the configuration listing.. and will remain so. North# config t Enter configuration commands. Central(config)# ! erase old community strings Central(config)# no snmp-server community public RO Central(config)# no snmp-server community admin RW Central(config)# Central(config)# ! disable SNMP trap and system-shutdown features Central(config)# no snmp-server enable traps Central(config)# no snmp-server system-shutdown Central(config)# no snmp-server trap-auth Central(config)# Central(config)# ! disable the SNMP service Central(config)# no snmp-server Central(config)# end The last command in the example. and you want to be able to use names in IOS commands. some SNMP configuration statements will not appear in any listing of the running configuration. Central# show running-config | include snmp Building configuration. If you do not want your router to send queries.3. End with CNTL/Z. no snmp-server. is to list the established SNMP community strings and explictly unset them as shown above.

the name you give to the router will appear in the prompt. use the command shutdown in interface configuration Central(config)# ip domain-name testnet.1. To disable an interface.5. 4. End with CNTL/Z.4. one per line.9. ! ----. see Section 4. use the config command ip domain-name domain as shown below.2. router(config)# hostname Central Central(config)# ip name-server 14. To set a domain name. as described above.2.3). and set up a main and backup DNS server address for the router Central.2. This helps discourage unauthorized use of extra interfaces.testnet. router# config t Enter configuration commands. For more information about NTP and SNMP security configuration. one per line. Configuration Example The configuration listing below shows the configuration commands for disabling typical unneeded services. Central(config)# interface eth0/3 Central(config-if)# shutdown Central(config-if)# end Central# End with CNTL/Z.3. This sample is formatted as it would appear in a configuration text file stored on a host for download to the router Central. and enforces the need for router administration privileges when adding new network connections to a router. Disable Unused Interfaces It is a good idea to explicitly shut down (disable) unused interfaces on your router. The example below shows how to set the router name. which will be used as part of the fullyqualified host name of the router and any unqualified name lookups. Central(config)# ! full name of this router: Central.1 Central(config)# ip domain-lookup Central(config)# You can also set a default DNS domain name.1c 79 .1.2 Central(config)# end Central# 4.Advanced Security Services command hostname. Central# config t Enter configuration commands. Setting a domain name is also necessary for using SSH (see Section 5.IP and network services section no cdp run no ip source-route no service tcp-small-serv no service udp-small-serv no ip finger no service finger no ip bootp server Version 1.

1c .1.0/24 net no ip proxy-arp no ip directed-broadcast no ip unreachable no ip redirect no mop enabled ntp disable interface eth 0/2 description Unused interface no ip proxy-arp no ip directed-broadcast no ip unreachable no ip redirect no mop enabled ntp disable shutdown interface eth 0/3 description Unused interface no ip proxy-arp no ip directed-broadcast no ip unreachable no ip redirect no mop enabled ntp disable shutdown interface loopback0 80 Version 1.Per-interface services section interface eth 0/0 description Outside interface to 14.SNMP Section (for totally disabling SNMP) ! disable SNMP trap and system-shutdown features no snmp-server enable traps no snmp-server system-shutdown no snmp-server trap-auth ! turn off SNMP altogether no snmp-server ! ----.2.Boot control section no boot network no service config ! ----.Router Security Configuration Guide no no no no service pad ip http server ip name-server ip domain-lookup ! ----.9.0.0/16 net no ip proxy-arp no ip directed-broadcast no ip unreachable no ip redirect no mop enabled ntp disable interface eth 0/1 description Inside interface to 14.

available under: http://www. Cisco IOS 12. though. Cisco Security Architectures. 1999. 9 Issue 55.Advanced Security Services description Loopback interface for service bindings no ip proxy-arp no ip directed-broadcast no ip unreachable no ip redirect ntp disable 4.php?p=55&a=10 A concise and readable article with practical advice on setting up a router at a boundary between a trusted and untrusted network.phrack. Short article with some good advice on features to turn off. [6] Held. K. Cisco Systems.0 Security Configuration.3. “Building Bastion Routers Using Cisco IOS.0 Configuration Very helpful article from Cisco. Version 1. 1999. September 1999. The sections on “Performing Basic System Management” and “Monitoring the Router and Network” include valuable advice on how to configure basic features and services. Part 1. Available from Cisco’s web site.” Phrack Magazine. 1998.” National Y2K Information Coordination Center. 1999. References [1] Eldridge. Cisco Internetworking Case Studies. and Hundley.2. G. Good overview of Cisco router and TCP/IP architecture. available at: http://www. New York. Cisco Press. [3] “Increasing Security on IP Networks. [4] Buckley.1c 81 . Can’t seem to find it on the web anymore. Vol. [5] Cisco IOS Network Protocols Configuration Guide.” . includes common-sense measures to take on routers running IOS [2] “Cisco IOS Version 12. plus excellent coverage of access lists. B. McGraw-Hill. Cisco Press. September 1999. The section on “IP Addressing and Services” includes information about several of the IP services described in this section.

The standard for HTTP basic authentication used for access control by Cisco IOS web remote administration.Router Security Configuration Guide [7] Franks. ed. F.. June 1999. RFC 2617. June 1995.1c . 82 Version 1. “HTTP Authentication: Basic and Digest Access Authentication”. J. This comprehensive standard describes the services that routers must or may provide. al. [8] Baker. RFC 1812. et. including several of the ones discussed in this section. “Requirements for IP Version 4 Routers”.

vty lines (for remote access). routing protocols.1c 83 . but do not involve application-level information. Version 1. IPSec. An access list consists of one or more rules. and what kind of access list it is. and many router features. Extended access lists also support selective logging. permit permits access if the condition is matched.3. Filtering. Extended access lists can permit or deny packets based on their protocols. or ICMP or IGMP message types.3. for restricting access to services on the router itself. The condition field. Secure configuration of Cisco routers makes very heavy use of access lists. and for other packet identification tasks.Advanced Security Services 4. Concepts Access lists on Cisco routers provide packet selection and filtering capabilities.1. source or destination TCP/UDP ports. The following is the syntax for a statement (rule) in a standard IP access list: access-list list-number {deny | permit} source [source-wildcard] [log] where list-number is the number of the access list and can be any decimal number from 1 to 99. access-list list-number {deny | permit} condition The access list number tells Cisco IOS which access list the rule should be a part of. source-wildcard is the wildcard bits to be applied to the source. specifies which packets match the rule. Conditions typically involve protocol information and addresses. source is the IP address of the network or host from which the packet is being sent. source or destination IP addresses. with some extensive examples. there are two types of access lists available: standard and extended. and Rate Limiting Cisco IOS uses access lists to separate data traffic into that which it will process (permitted packets) and that which it will not process (denied packets). deny denies access if the condition is matched. and for filtering traffic passing through the router. For IP traffic. This section gives a moderately detailed description of access list syntax. Standard access lists only allow source IP address filtering. which is different for each kind of access list. Only standard IP access lists can be applied to SNMP. Both standard and extended IP access lists can be applied to router interfaces. 4. Access Control Lists. Syntax The basic structure for an access list rule is shown below.

Or it can be an integer in the range 0 to 255 representing an IP protocol number. including port numbers and other protocol-specific information. After the list is defined by name. destination-wildcard is the IP address wildcard bits to be applied to the destination. including port numbers and other protocol-specific information. deny denies access if the condition is matched.Router Security Configuration Guide The optional keyword log may be applied to log matches to the rule. you can add statements beginning with either the permit or deny 84 Version 1. nos. It can be one of the following keywords: eigrp. destination-qualifiers are optional details on the packet destination. It also provides a convenient way to build lists on-line. if present. and log-input causes a message that includes the interface (logging is described in Section 4. causes a message about the packet that matches the statement to be logged. ipinip. source-wildcard is the wildcard bits to be applied to the source. This feature allows you to refer to an access list by a descriptive name instead of by number. ip.) source is the IP address of the network or host from which the packet is being sent. igmp. The syntax for defining an IP access list by name is shown below. gre. ospf. (Some protocols allow further qualifiers: source or destination ports can be specified for tcp or udp. protocol is the name or number of an IP-related protocol. destination is the IP address of the network or host to which the packet is being sent.5. log. The keyword any can be used in place of source and source-wildcard. and message types can be specified for icmp or igmp.1c . source-qualifiers are optional details on the packet source.0 and later. icmp. igrp. The following is simplified syntax for a statement in an extended IP access list: access-list list-number {deny | permit} protocol source source-wildcard source-qualifiers destination destination-wildcard destination-qualifiers [ log | log-input] where list-number is the number of the access list and can be any decimal number from 100 to 199. The keyword any can be used in place of destination and destination-wildcard.1). permit permits access if the condition is matched. tcp or udp. Note that logging for IP standard access lists is supported only in IOS 12. Cisco has also created an alternative called named IP access lists for both standard and extended lists.

ip access-list {standard | extended} name where standard specifies a standard IP access list.g. In each access list there must be at least one permit statement. Otherwise.2. some UDP traffic) the access list will only affect approximately half the packets.1. Syntax Examples The example below shows how to create a small extended IP access list that permits DNS traffic to the address 14.0. The second table lists those services on the internal network or on the router that should not be accessible by external clients. For any connection that requires two-way interaction (e.0.2 eq 53 North(config-ext-nacl)# deny udp any any log North(config-ext-nacl)# permit tcp any 14. The first table lists those services that should be completely blocked at the router.1. North(config)# North(config)# North(config)# North(config)# access-list access-list access-list access-list 140 140 140 140 permit deny permit deny udp udp tcp ip any any any any host 14.255 North(config-ext-nacl)# deny ip any any log North(config-ext-nacl)# exit General Recommendations Refer to the two tables in Section that present common services to restrict because they can be used to gather information about an internal network or they have weaknesses that can be exploited. The diagram below shows how access lists work when applied to router interfaces. After the permit or deny keyword the syntax is the same as defined above for either the standard list or the extended list.2.0 0.Advanced Security Services keyword. using the router East as an example.0. Version 1.0. name is the name of the access list.0.255. and any TCP traffic from the Internet to all hosts in the 14. an access list with no permit statements will block all network traffic wherever it is applied.1.1. North(config)# ip access-list extended border-filter-14 North(config-ext-nacl)# permit udp any host 14.0/16 network.2 eq 53 any log 14. all TCP traffic.1. It is possible however to apply two access lists (one for each direction) for router interfaces. Note that an access list is applied to packets traveling in one direction only.1c 85 ..255.0 0.1. they should not be allowed across the router in either direction or to the router. extended specifies an extended IP access list.255 any log The example below shows the same list as a named IP access list. The name cannot contain spaces or punctuation and must begin with an alphabetic character. vty lines and routing protocols.1.

0 network Inbound Access List deny permit Inbound Access List deny 14. whenever you need to change an access list. Thus. Since the original access list is 86 Version 1.Router Security Configuration Guide 14. due to limited editing capability on the Cisco router. then use the qualifier log-input instead of log.1.6.0 0.2.0. Access list log messages always include the access list number.6. as shown in the example below.255 any East(config)# access-list 102 deny ip any any log-input Add the following statements at the end of each extended IP access list to deny and to log any packets that are not permitted.20 East Eth1 14.2.0. which is usually sufficient to identify the provenance of the traffic. East(config)# access-list 102 permit ip 14. it is best to build it offline on a separate computer.1.0 network Trash Trash permit Outbound Access List Routing Fabric Outbound Access List permit Figure 4-2: Conceptual Model for Access Lists on Interfaces Use the log keyword at the end of each deny statement in each extended access list.0/16 Eth0 14. These statements include the entire port ranges for TCP and UDP explicitly. you cannot easily modify access lists.6. East(config)# access-list 100 deny tcp East(config)# access-list 100 deny udp East(config)# access-list 100 deny ip any any any any any range 0 range 0 range 0 range 0 any log 65535 65535 log 65535 65535 log Finally.6.0/24 Interface Eth0 Interface Eth1 permit . If you might apply the same access list to more than one interface. When the access list is ready you can cut and paste the access list via a connection to the router.1. Logs of denied packets can be useful for detection and analysis of probes and attacks against a network. This will guarantee that the router will log the values for the source and destination ports for TCP and UDP traffic. Log messages generated by access lists are at log level 6 ‘Informational’. This feature will provide valuable information about what types of packets are being denied. 14.

This simple IP access list allows the hosts with IP addresses 14. and more.6.2. Thus. 4. SNMP servers and routing protocols. It is safest to build an access list completely first. East(config)# no access-list 100 Be careful when clearing and rebuilding an access list that is in use.18 any eq 23 log East(config)# access-list 105 deny ip any any log East(config)# line vty 0 4 East(config-line)# access-class 105 in East(config-line)# end SNMP Service A Cisco router can be configured to act as a client for SNMP. possibly dropping traffic or even leaving the router vulnerable. Versions 1 and 2 of SNMP are not considered secure due to the lack of strong authentication. The list denies all other connections.2.2. then apply it to the interface or service where you need it. Typically. For more information about services on the router. a router administrator telnets to one of the vty lines. more reliable. SNMP should be used only on internal or protected networks. it is typically easier.6. Version 1. East(config)# access-list 75 permit host any eq 23 log East(config)# access-list 105 permit tcp host 14.5.3. see Section 4. When SNMP service is enabled on a router. Remote Login (Telnet) Service There are a number of methods to filter access to the router itself: vty lines.6 East(config)# access-list 75 deny any log East(config)# snmp-server community N3T-manag3m3nt ro 75 For more information about SNMP configuration. traffic load.6. network management tools can use it to gather information about the router configuration.6 and 14. The list denies all other connections.2. your changes will take place instantly.2 and 4. you must purge it before adding the updated access list.2.2.2. The following example shows the configuration of an extended IP access list that is applied to the vty lines.1c 87 . The following example shows the configuration of a standard IP access list that is applied to a snmp server. Filtering Traffic to the Router Itself Access lists are used in a variety of ways to control access to services on the router itself.Advanced Security Services still on the router. and more efficient to use the specialized facilities that IOS makes available to apply access controls directly to the services themselves. see Sections 4. Below is an example of how to clear an access list.6 to gather SNMP information from the router. and how to disable unneeded ones.2.18 to connect to the router East via Telnet. It also logs all successful and unsuccessful connections. route table. Most of the time.2. This access list allows the host with IP address 14. East(config)# access-list 105 permit tcp host 14. While it is possible to incorporate access controls for these services into the access lists placed on interfaces.6.3. The vty lines are used for remote access to the router.

Router Security Configuration Guide

Routing Service
Communications between routers for routing table updates involve routing protocols. These updates provide directions to a router on which way traffic should be routed. You can use access lists to restrict what routes the router will accept (in) or advertise (out) via some routing protocols. The distribute-list acl-num out command is used to restrict routes that get distributed in routing updates, while the distribute-list acl-num in command may be used used to filter routes that will be accepted from incoming routing updates. The following example shows the configuration of a standard IP access list applied with the EIGRP routing protocol. With the access list applied, router South will not advertise routes to the network.
South(config)# access-list 10 deny South(config)# access-list 10 permit any South(config)# router eigrp 100 South(config-router)# distribute-list 10 out South(config-router)# end South#

Access lists can be used for general filtering of routing updates with distance-vector routing protocols like RIP, EIGRP, and BGP. With link-state routing protocols like OSPF, access lists can be used only for some specialized kinds of filtering. For more information about this topic, see Section 4.4.

4.3.3. Filtering Traffic through the Router
The following examples illustrate methods to protect the router or the internal network from attacks. Note: these separate examples should not be combined into one access list because the result would contain contradictions. In the next section an example configuration file is presented that shows one way to combine these methods into access lists. Refer to the network diagram in Figure 4-1 to understand the example interfaces, their IP addresses and the corresponding access lists.

IP Address Spoof Protection
The filtering recommendations in this sub-section are applicable to border routers, and most interior routers. With backbone routers, it is not always feasible to define ‘inbound’ or ‘outbound’. Additional guidance and recommandations maybe found in [6] and [11].

Inbound Traffic
Do not allow any inbound IP packet that contains an IP address from the internal network (e.g.,, any local host address (, the link-local DHCP default network (, the documentation/test network (, or any reserved private addresses (refer to RFC 1918) in the source field. Also, if your network does not need multicast traffic, then block the IP multicast address range


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( Apply this access list to the external interface of the router, as shown in the transcript below.
East(config)# no access-list 100 East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip any East(config)# access-list 100 deny ip host any East(config)# access-list 100 permit ip any East(config)# access-list 100 deny ip any any log East(config)# interface eth0 East(config-if)# description External interface to net East(config-if)# ip address East(config-if)# ip access-group 100 in East(config-if)# exit East(config)# interface eth1 East(config-if)# description Internal interface to net East(config-if)# ip address East(config-if)# end log log log log log log log log log log

Outbound Traffic
Do not allow any outbound IP packet that contains an IP address other than a valid internal one in the source field. Apply this access list to the internal interface of the router. See example rules below.
East(config)# no access-list 102 East(config)# access-list 102 permit ip any East(config)# access-list 102 deny ip any any log East(config)# interface eth 0/1 East(config-if)# description "internal interface" East(config-if)# ip address East(config-if)# ip access-group 102 in

On most Cisco routers, IOS 12 offers another mechanism for IP address spoof protection: IP unicast reverse-path forwarding verification. Though specialized, and not suitable for all networks, this facility offers good performance and ease of maintenance. Section 4.4.7 shows how to set up reverse-path forwarding verification on routers that support it.

Exploits Protection
This sub-section describes how to use access lists to defeat or discourage several common attacks using IOS traffic filtering capabilities.

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TCP SYN Attack
The TCP SYN Attack involves transmitting a volume of connections that cannot be completed at the destination. This attack causes the connection queues to fill up, thereby denying service to legitimate TCP users. The following discussion shows two different approaches. External Access Blocked The access list rules shown below will block packets from an external network that have only the SYN flag set. Thus, it allows traffic from TCP connections that were established from the internal network, and it denies anyone coming from any external network from starting any TCP connection.
East(config)# access-list 106 permit tcp any established East(config)# access-list 106 deny ip any any log East(config)# interface eth 0/0 East(config-if)# description External interface East(config-if)# ip access-group 106 in

Limiting External Access with TCP Intercept The access list rules shown below will block packets from unreachable hosts using the TCP intercept feature; thus, it only allows reachable external hosts to initiate connections to a host on the internal network. In intercept mode the router intercepts each TCP connection establishment, and determines if the address from which the connection is being initiated is reachable. If the host is reachable, the router allows the connection to be established; otherwise, it prevents the connection.
East(config)# ip tcp intercept list 107 East(config)# access-list 107 permit tcp any East(config)# access-list 107 deny ip any any log East(config)# interface eth0 East(config-if)# description External ethernet interface to net East(config-if)# ip access-group 107 in East(config-if)# exit

TCP intercept is a very effective mechanism for protecting hosts on a network from outside TCP SYN attacks, for extensive details consult the Cisco IOS 12 Security Configuration Guide [5]. The TCP intercept feature is available in most, but not all, Cisco IOS version 12.0 and later releases. Note that TCP intercept, while it can be very useful, can also impose significant overhead on router operations. Examine and test the performance burden imposed by TCP intercept before using it on an operational network.

Land Attack
The Land Attack involves sending a packet to the router with the same IP address in the source and destination address fields and with the same port number in the source


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port and destination port fields. This attack may cause denial of service or degrade the performance of the router. The example below shows how to prevent this attack.
East(config)# access-list 103 deny ip host host log East(config)# access-list 103 permit ip any any East(config)# interface eth0 East(config-if)# description External interface to East(config-if)# ip address East(config-if)# ip access-group 103 in East(config-if)# exit

Smurf Attack
The Smurf Attack involves sending a large amount of ICMP Echo packets to a subnet's broadcast address with a spoofed source IP address from that subnet. If a router is positioned to forward broadcast requests to other routers on the protected network, then the router should be configured to prevent this forwarding from occurring. This blocking can be achieved by denying any packets destined for broadcast addresses. The example statements below block all IP traffic from any outside host to the possible broadcast addresses ( and for the subnet.
East(config)# access-list 110 deny ip any host log East(config)# access-list 110 deny ip any host log East(config)# interface interface eth0 East(config-if)# ip access-group 110 in East(config-if)# exit

ICMP Message Types and Traceroute
There are a variety of ICMP message types. Some are associated with programs. For example, the ping program works with message types Echo and Echo Reply. Others are used for network management and are automatically generated and interpreted by network devices. For inbound ICMP traffic, block the message types Echo and Redirect. With Echo packets an attacker can create a map of the subnets and hosts behind the router. Also, he can perform a denial of service attack by flooding the router or internal hosts with Echo packets. With ICMP Redirect packets the attacker can cause changes to a host’s routing tables. Otherwise, the other ICMP message types should be allowed inbound. See the example below for inbound ICMP traffic.
East(config)# East(config)# East(config)# East(config)# access-list access-list access-list access-list 100 100 100 100 deny icmp any any echo log deny icmp any any redirect log deny icmp any any mask-request log permit icmp any

For outbound ICMP traffic, one should allow the message types Echo, Parameter Problem, Packet Too Big, and Source Quench and block all other message types. With Echo packets users will be able to ping external hosts. Parameter Problem packets and Source Quench packets improve connections by informing about problems with packet headers and by slowing down traffic when it is necessary. Packet Too Big is necessary for Path MTU discovery. The example below shows a set of filter rules for outbound ICMP traffic that permit these message types.

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East(config)# East(config)# East(config)# East(config)# East(config)#

access-list access-list access-list access-list access-list

102 102 102 102 102

permit permit permit permit deny

icmp icmp icmp icmp icmp

any any any any any

any any any any any

echo parameter-problem packet-too-big source-quench log

Another program that deals with certain ICMP message types is traceroute. Traceroute is a utility that prints the IP addresses of the routers that handle a packet as the packet hops along the network from source to destination. On Unix and Linux operating systems, traceroute uses UDP packets and causes routers along the path to generate ICMP message types ‘Time Exceeded’ and ‘Unreachable’. An attacker can use traceroute response to create a map of the subnets and hosts behind the router, just as they could do with ping’s ICMP Echo Reply messages. Therefore, block naïve inbound traceroute by including a rule in the inbound interface access list, as shown in the example below (ports 33400 through 34400 are the UDP ports commonly used for traceroute).
East(config)# access-list 100 deny udp any any range 33400 34400 log

A router may be configured to allow outbound traceroute by adding a rule to the outbound interface access list, as shown in the example below.
East(config)# access-list 102 permit udp any any range 33400 34400 log

Distributed Denial of Service (DDoS) Attacks
Several high-profile DDoS attacks have been observed on the Internet. While routers cannot prevent DDoS attacks in general, it is usually sound security practice to discourage the activities of specific DDoS agents (a.k.a. zombies) by adding access list rules that block their particular ports. The example below shows access list rules for blocking several popular DDoS attack tools. [Note that these rules might also impose a slight impact on normal users, because they block high-numbered ports that legitimate network clients may randomly select. You may choose to apply these rules only when an attack has been detected. Otherwise, they would be applied to traffic in both directions between an trusted network and an untrusted network.]
! the TRINOO DDoS systems access-list 170 deny tcp any any eq 27665 log access-list 170 deny udp any any eq 31335 log access-list 170 deny udp any any eq 27444 log ! the Stacheldraht DDoS system access-list 170 deny tcp any any eq 16660 log access-list 170 deny tcp any any eq 65000 log ! the TrinityV3 system access-list 170 deny tcp any any eq 33270 log access-list 170 deny tcp any any eq 39168 log ! the Subseven DDoS system and some variants access-list 170 deny tcp any any range 6711 6712 log access-list 170 deny tcp any any eq 6776 log access-list 170 deny tcp any any eq 6669 log access-list 170 deny tcp any any eq 2222 log access-list 170 deny tcp any any eq 7000 log


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The Tribe Flood Network (TFN) DDoS system uses ICMP Echo Reply messages, which are problematic to block because they are the heart of the ping program. Follow the directions in the ICMP sub-section, above, to prevent at least one direction of TFN communication.

4.3.4. Example Configuration File
The configuration file shown below is not a complete configuration file. Rather, it provides an example for using access lists on a Cisco router. The diagram below shows the topology that this file is based on. The security policy implemented with the access lists allows most traffic from the internal network to the external network. The policy restricts most traffic from the external network to the internal network.
Protected network

Other networks
Interface eth0

Interface eth1

hostname East ! interface Ethernet0 description Outside interface to the network ip address ip access-group 100 in ! interface Ethernet1 description Inside interface to the network ip address ip access-group 102 in ! ! access-list 75 applies to hosts allowed to gather SNMP info ! from this router no access-list 75 access-list 75 permit host access-list 75 permit host ! ! access-list 100 applies to traffic from external networks ! to the internal network or to the router no access-list 100 access-list 100 deny ip any log access-list 100 deny ip host host log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any log access-list 100 deny ip any host log access-list 100 deny ip any host log access-list 100 permit tcp any established

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0 0.0 host 0.0 eq 23 log access-list 150 permit tcp host 14.0.0 eq 23 log access-list 150 deny ip any any log ! snmp-server community N3T-manag3m3nt ro 75 ! line vty 0 4 access-class 150 in password 7 123456789012345678901234 login transport input telnet 94 Version 1.0.0 gt 1023 any lt 1024 access-list 102 permit udp 14.255 gt 1023 access-list 100 deny tcp any range 0 65535 any range 0 65535 log access-list 100 deny udp any range 0 65535 any range 0 65535 log access-list 100 deny ip any any log ! ! access-list 102 applies to traffic from the internal network ! to external networks or to the router itself no access-list 102 access-list 102 deny ip host any echo access-list 102 permit icmp host 14.255 gt 1023 access-list 100 deny udp any any eq 2049 log access-list 100 deny udp any any eq 31337 log access-list 100 deny udp any any range 33400 34400 log access-list 100 permit udp any eq 53 any source-quench access-list 102 deny tcp any any range 1 19 log access-list 102 deny tcp any any eq 43 log access-list 102 deny tcp any any eq 93 log access-list 102 deny tcp any any range 135 139 log access-list 102 deny tcp any any eq 445 log access-list 102 deny tcp any any range 512 518 log access-list 102 deny tcp any any eq 540 log access-list 102 permit tcp 14.2.255 gt 1023 any eq 53 access-list 102 permit udp 14.0 0.12 host 0.0 any packet-too-big access-list 102 permit icmp 14.0 eq 23 log access-list 150 permit tcp host host 0.0 host 14.0.0 0.6.0 0.6.0 0.20 access-list 100 deny tcp any any range 6000 6063 log access-list 100 deny tcp any any eq 6667 log access-list 100 deny tcp any any range 12345 12346 log access-list 100 deny tcp any any eq 31337 log access-list 100 permit tcp any eq 20 14.1.Router Security Configuration Guide access-list 100 deny icmp any any echo log access-list 100 deny icmp any any redirect log access-list 100 deny icmp any any mask-request log access-list 100 permit icmp any access-list 100 permit ospf 14.255 any parameter-problem access-list 102 permit icmp log access-list 102 permit icmp any range 33400 34400 log access-list 102 deny tcp any range 0 65535 any range 0 65535 log access-list 102 deny udp any range 0 65535 any range 0 65535 log access-list 102 deny ip any any log ! ! access-list 150 applies to admin access from specific hosts no access-list 150 access-list 150 permit tcp host 14.1c .0.0.

3. For more information on CAR commands. then compiled ACLs can give improved performance. Once you have enabled turbo access lists.6. simply type the rule in interface configuration mode. data passing through the router can be manipulated to preserve fragile traffic.5. ordered set of rules for the in-bound (receiving) and out-bound (sending) directions. (If your IOS does not support compiled access lists. and limit spoofed traffic. If you use access lists with six or more rules on high-speed interfaces. In the latter case. Using compiled access control lists can greatly reduce the performance impact of long lists. however. You can use CAR to reserve a portion of a link’s bandwidth for vital traffic. the command will generate a harmless error message. rate-limit {input | output} [access-group [rate-limit] acl] token-bit-rate burst-normal-size burst-excess-size conform-action action exceed-action action To add a rule to an interface. CAR Command Syntax Configuring CAR requires you to apply rate limiting rules to each interface where you enforce constraints on traffic or bandwidth usage. eliminate excessive traffic. somewhat simplified. To remove a rule. you can view statistics about them using the command show access-list compiled. it may not be necessary to keep CAR rules in place at all times. use the command show interface rate-limit. but to be ready to apply them quickly when you detect an attack in progress. consult the “IOS Quality of Service Solutions Command Reference” section of the IOS documentation. and a few simple examples. A sample of the output is included in the first example below. the most important task that CAR can perform is to mitigate the paralyzing effects of DoS attacks and flash crowds. 4.1(6). enter it again adding the keyword no to the front. The general syntax for a CAR rule is shown below. To view the CAR rules on all the interfaces. IOS will automatically compile all suitable access lists into fast lookup tables while preserving their matching semantics.3. Turbo Access Control Lists Some Cisco router models support compiled access control lists. Rate Limiting with Committed Access Rate Committed Access Rate (CAR) is a router service that gives administrators some control over the general cross-section of traffic entering and leaving a router. use the configuration mode command access-list compiled. and later.1c 95 .Advanced Security Services 4. Version 1. This short section gives an overview of CAR. Each interface can have a separate. called “Turbo ACLs”. The output of the command will show both the rules and some traffic statistics about the rate limiting. To enable turbo access lists on a router.) Once this facility is enabled. as shown in the examples below. or to limit the amount of bandwidth consumed by a particular kind of attack. By allocating a specific amount of bandwidth to defined traffic aggregates. in IOS 12.

and the second how to handle packets when the aggregate exceeds its bandwidth allocation. the most commonly used four are described below. such as ICMP or DNS. but that is outside the scope of this brief discussion. For a more detailed description of the token bucket model. given in bytes. If the rule is meant to apply to packets entering the router. Transmit or forward the packet.] If the keyword rate-limit appears. The CAR facility uses a token bucket model to allocate or limit bandwidth of traffic. Apply the next rate-limit rule. otherwise the access list should be a standard or extended IP access list. and the traffic burst exccess size. It generally describes the allowed rate for the aggregate. the token bucket parameters. it would simply be the size of a typical message. For simple protocols. then the CAR rule will apply only to traffic that is permitted by or matches that access list. is generally the size of a typical traffic transaction in a single direction. 96 Version 1. The token bucket model needs three parameters for configuration: the token bit rate. the traffic burst normal size (in bytes).Router Security Configuration Guide Defining Rules Each rate limit rule is made up of 3 parts: the aggregate definition. This model gives you a flexible method to stipulate bounds of traffic behavior for an aggregate. • The second part of the rate-limit command is comprised of the three token bucket parameters. for packets leaving the router use the output keyword. if you supply no access-group clause then the rule applies to all traffic. consult [9]. and the action specifications. The burst normal size. it indicates that the aggregate is defined by a rate-limit access list. Rate-limit access lists define aggregates based on IP precedence or MAC addresses. before the aggregate uses up its allocated bandwidth. The token bit rate parameter must be specified in bits per second (bps). [It is also possible to apply CAR rules to packets by QoS header and other criteria.1c . The aggregate definition must include the traffic direction. • The aggregate definition section of a rule defines the kind of traffic (or “packet aggregate”) to which the rule applies. there may be as many as nine possible actions. • The last section of a rule consists of the two action specifications. The first action instructs the router on how to handle packets when the aggregate conforms to bandwidth allocation. CAR Action Syntax drop transmit continue Action Performed Discard the packet. If the aggregate definition includes an access-group clause. and must be greater than 8000. The burst excess size denotes the upper bound or maximum size expected for traffic bursts. Depending on your IOS version. use the input keyword. and may also include fine-grained traffic selection specified with an access control list.

CAR is being used to throttle a TCP SYN flood attack. conformed 2000 bps. CAR is used to reserve 10% of a 10Mb Ethernet link for vital outgoing SMTP traffic.Advanced Security Services CAR Action Syntax set-prec-transmit prec Action Performed Set the IP precedence to prec and transmit or forward the packet. In practice. North(config)# North(config)# North(config)# North(config)# no access-list 160 access-list 160 deny tcp any any established access-list 160 permit tcp any any syn interface eth0/0 Version 1. action: transmit exceeded 0 packets. 112000 limit. The rest of the link’s bandwidth will be usable by excess SMTP traffic and all other IP traffic. 0 bytes. 25000 limit. 225000 extended limit conformed 346 packets. 27074 bytes. 24990 bytes. 50000 extended limit conformed 12 packets. 2500 limit. 11699 bytes. current burst: 0 bytes last cleared 00:02:32 ago. exceeded 0 bps North# In this second example. exceeded 990 bps matches: all traffic params: 9000000 bps. current burst: 2434 bytes last cleared 00:02:04 ago. and to limit outgoing ICMP ‘ping’ traffic to less than 1% of the link. action: transmit exceeded 0 packets. current burst: 0 bytes last cleared 00:01:40 ago. you might want to impose both outbound and inbound rate limiting to protect the vital SMTP traffic. conformed 0 bps. action: continue last packet: 2668ms ago. 2500 extended limit conformed 130 packets. action: drop last packet: 7140ms ago. 12740 bytes. 0 bytes. conformed 0 bps. CAR Examples In the first example. action: drop last packet: 7120ms ago. exceeded 0 bps matches: access-group 131 params: 16000 bps.1c 97 . North(config)# no access-list 130 North(config)# access-list 130 permit tcp any any eq smtp North(config)# no access-list 131 North(config)# access-list 131 permit icmp any any echo North(config)# access-list 131 permit icmp any any echo-reply North(config)# interface eth0/0 North(config-if)# rate-limit output access-group 130 1000000 25000 50000 conform-action transmit exceed-action continue North(config-if)# rate-limit output access-group 131 16000 8000 8000 conform-action continue exceed-action drop North(config-if)# rate-limit output 9000000 112000 225000 conform-action transmit exceed-action drop North(config-if)# end North# show interface rate-limit Ethernet0/0 Output matches: access-group 130 params: 1000000 bps. action: continue exceeded 255 packets.

3. Before attempting to configure CPP. 4. 1. For another example of using CAR to combat a DoS attack. consult [10].” and “default” classes). is easy to accidentally restrict the wrong traffic and disrupt management or control plane services.g.7.” “malicious. 3.) 2. identify the classes you wish to handle.” “normal. The management plane consists of traffic for configuring and monitoring router operations. In this case. do not create an access list for it. Create access lists that match (permit) the traffic from members of each class. the “critical. signaling and link management protocols. using the class-map command. To implement a CPP policy. The control plane consists of the routing. all traffic destined for the control plane of a router must be categorized into network administrator-defined groups or classes (e. Detailed information about CPP may be found in a Cisco white paper [12]. Take care when defining and applying CPP policy -. control. CPP protects the central processor via policies that filter or rate limit traffic directed to the processor. Timely and reliable operation of the management and control planes are essential for maintaining the flow of traffic through the forwarding plane.1c . If you knew the general source of the attack (perhaps an IP address range) then you could make the defense more selective by incorporating the address range into the aggregate definition access list. and rough traffic rate limits for each of them. Define a named class map for each of the access lists you created in step 1.Router Security Configuration Guide North(config-if)# rate-limit input access-group 160 64000 8000 8000 conform-action transmit exceed-action drop North(config-if)# end North# The CAR rule in this example simply discards excessive TCP SYN packets. legitimate traffic would also be affected. or rate limited. Control Plane Policing (CPP) Conceptually. Control Plane Policing (CPP) is a Cisco IOS feature that you can employ to counter resource starvation-based DoS attacks that target the central processor of a router (control plane and management plane). use the class map-name command to define rate-limiting policy for each 98 Version 1. Then service policies should be created and applied that cause traffic classes destined for the route processor to be accepted. In the map. The forwarding plane (also called the “data” plane) forwards user data packets through the router. Once you have defined your classes. (If you have a ‘default’ class. and management. setting up control plane policing on IOS requires four steps. Create a policy map using the policy-map command. router operations can be abstracted into three planes: forwarding.

Traffic from the malicious host will be dropped entirely. and the bandwidth required to maintain router operations.255 any North(config)# access-list 151 permit ip host 7. a malicious class for a known hostile host.Advanced Security Services named class.20 any North(config)# ! define ACL for known hostile host North(config)# access-list 152 permit ip host 1.12. 4. When planning your CPP rate limits. Define a default rate-limiting policy using the command class class-default.255.0.0. North(config)# ! define ACL for CPP trusted hosts North(config)# access-list 151 permit ip 14. consider the bandwidth from possibly hostile sites. and a default class for all other addresses.4 any North(config)# ! define a class mapping for trusted host North(config)# class-map match-any cpp-trusted North(config-cmap)# match access-group 151 North(config-cmap)# exit North(config)# ! define a class mapping for the malicious host North(config)# class-map match-any cpp-malicious North(config-cmap)# match access-group 152 North(config-cmap)# exit North(config)# ! define our CPP policy map North(config)# policy-map cpp-policy North(config-pmap)# class cpp-trusted North(config-pmap-c)# ! no action here.2. North# config t Enter configuration commands. allow any rate North(config-pmap-c)# exit North(config-pmap)# class cpp-malicious North(config-pmap-c)# ! drop all traffic in this class North(config-pmap-c)# police rate 10 pps North(config-pmap-c-police)# conform-action drop North(config-pmap-c-police)# exceed-action drop North(config-pmap-c-police)# exit North(config-pmap-c)# exit North(config-pmap)# class class-default North(config-pmap-c)# ! rate-limit all other traffic North(config-pmap-c)# police rate 150 pps North(config-pmap-c-police)# conform-action transmit North(config-pmap-c-police)# exceed-action drop North(config-pmap-c-police)# exit North(config-pmap-c)# exit North(config-pmap)# exit North(config)# ! apply the policy map for CPP North(config)# control-plane North(config-cp)# service-policy input cpp-policy North(config-cp)# end North# Version 1. Traffic from all other hosts will be rate-limited to 150 packets per second.0. one per line.255 any North(config)# access-list 151 permit ip 14. The example below shows how to configure CPP with three different classes: a trusted class for internal and specific external hosts. Traffic from hosts in the trusted class will have no rate limits.0.1c 99 .255. End with CNTL/Z.2.0 0. Apply your policy map to the control plane using the commands control-plane and service-policy.1.1.0 0.3.

0.0.3.. References [1] Chapman.255.. North(config)# control-plane North(config-cp)# no service-policy input cpp-policy North(config-cp)# end North# 4. This book offers detailed information about access control lists and many examples of list syntax and usage.16. use the command no service-policy command as shown below. D.g. Telnet.. 1999. D. use the command show policy-map control-plane.255. 2000. and dynamic access lists. etc. Cisco Press. February 1996. Building Internet Firewalls. It includes information on TCP Intercept. This Internet ‘Best Current Practice’ RFC gives a good overview of source address filtering. [6] Ferguson. O’Reilly Associates.168. SMTP. 1995.31.10. and 192. and Hundley. 100 Version 1. and a good primer on access lists [5] Cisco IOS Release 12. Moskowitz. [2] Karrenberg.8. McGraw-Hill. D. This RFC describes the IP address allocation for private intranets. e. 172. “Address Allocation for Private Internets”. Brent and Zwicky. FTP. RFC 1918.0. Cisco Access List Field Guide.255. G.0.168. This text provides valuable information on how to packet filter many of the commonly used services. Y. 1999 This book includes a good introduction to router security. B.192. P.0 ..255. G. [3] Held.0 . McGraw-Hill.255. reflexive access lists. and Senie. K. [4] Held. K..255.0.0 .172. and Rekhter. This is the reference manual and guide for major security features in IOS 12. Elizabeth D. and Hundley. “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing”. RFC 2827.255.1c . To remove a CPP policy.0 Security Configuration Guide. 1999.. The Internet Assigned Numbers Authority has reserved the following three blocks of the IP address space for private intranets: 10. Cisco Security Architectures.Router Security Configuration Guide To view the current CPP policy and traffic statistics..

March 2004. Cisco Systems. 2005. gives guidance on how to select usable values. [10] “Using CAR During DOS Attacks”.html Describes the CAR token bucket model and burst size parameters in some depth. A detailed guide to access lists. This detailed Cisco guide for Internet Service Providers includes extensive discussion of routing protocols (especially BGP). Cisco white paper. F. [8] Sedayao. Cisco Systems. P.. [12] “Deploying Control Plane Policing”.pdf Walks through a detailed CAR example related to ICMP flooding. Cisco ISP Essentials. Cisco Tech Note. 1st Edition. including coverage of using access lists with routing protocols. and Smith. B. Version ps6642/prod_white_papers_list. available under: and Savola. [9] “Selecting Burst and Extended Burst Values for Class-based Policing”. 2001. Detailed directions for doing RFC 2827-compliant filtering on networks connected to multiple providers.html This white paper explains the motivations for CPP and provides detailed instructions on how to configure it. available at: http://www. Cisco Systems. [11] Cisco Press. Cisco IOS Access Lists.. O’Reilly Associates.Advanced Security Services [7] Greene. Feb all with fully worked-out examples.1c 101 . Cisco Tech Note.. P. and an in-depth treatment of Unicast RPF. “Ingress Filtering for Multihomed Networks”. It also lists the Cisco IOS releases that support CPP. available at: http://www. April 2002. RFC 3704.

with a focus on the latter. The routed protocol allows the router to correctly interpret the logical network. efficient. instead it only needs to know the address of one or a small number of routers. it is the basis for the worldwide Internet.”[7] These protocols support routed protocols and are used to maintain routing tables. This section will not provide an in-depth discussion of this protocol. its foundation is the Internet Protocol (IP). These routers are responsible for directing each IP packet to its intended destination. a connected host anywhere on a network can communicate with any other. Each IP host does not need to know a path through the network to every other host. which takes place exclusively at the network layer. Today. Routing and Routing Protocols “A protocol is a formal description of a set of rules and conventions that govern how devices on a network exchange information. connecting every router to every other would be prohibitively expensive. Instead.1c . BGP. they use one of two transport-layer protocols built on top of IP: the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP).Router Security Configuration Guide 4. AppleTalk. In practice. each router can simply be connected directly to every other router. For a detailed introduction to the concepts of routing. Some examples of routed protocols are IP. and the flexible and open nature of the IP network architecture.4. IPX. and secure operation of any large IP network depends on the integrity of its route tables. each router maintains a route table with information about how to forward packets to their destination addresses. to reach those networks. IP is designed for use on large networks. consult [6] for a detailed introduction. and EIGRP. Use of TCP or UDP is immaterial to routing. as that is far beyond the scope of this guide. For larger networks. and DECnet. Some examples of routing protocols are OSPF. consult [16]. • Routing protocols – “A routing protocol gathers information about available networks and the distance. The two types of protocols are: • Routed protocols – These are protocols that can be routed by a router. IS-IS. Instead. All of the examples in this section are based on the sample network architecture shown in Figure 4-1. ARPA sponsored the development of IP over twenty-five years ago under the ARPANET project. of course. 102 Version 1. In a small network. or cost.”[5] This section will discuss two basic types of protocols. host applications almost never use raw IP to communicate. using IP. Its growth and popularity can be attributed to IP’s ability to connect different networks regardless of physical environment. Correct. RIP. Routed Protocols The most commonly used routed network protocol suite is the TCP/IP suite.

Version 1. depending on the contents of the table and its routing algorithm. each router needs a route table.0/24. and exterior gateway protocols on backbone routers. although not universal. IS-IS. RIP. Routers that depend on a single default gateway usually do not use routing protocols. Direct connection: Any LAN segment to which the router is directly connected is automatically added to the route table.1c 103 . It is typical. This method usually takes precedence over any other method of routing. 4. This section focuses on a small number of widely used routing protocols: RIP. OSPF. An interior gateway protocol (IGP) is used for exchanging routing information between gateways within an autonomous system. BGP. As network administrator. and the last. For example. Default routing. or both. Static routing. This method is most useful for border routers and routers that serve as the sole connection between a small LAN and a large network like the Internet.9. they can be divided into two groups: interior and exterior gateway protocols. 2. An exterior gateway protocol (EGP) is used between autonomous systems. 3. is vendor-defined. Border Gateway Protocol version 4 (BGP-4) is the exterior gateway protocol used for conveying route information between autonomous systems on the Internet. Each router builds its table based on information from the network and from the network administrators. This method is the most flexible because it can automatically adapt to changes in the network. depending on the network architecture in which they are found. The first three are IETF standards. Uses a manually entered route to a specific ‘gateway of last resort’ when route is not known by any other routing mechanism. The routing algorithm associated with the particular routing protocol determines the optimal path to a particular destination.Advanced Security Services Route Tables and Routing Protocols A router’s primary responsibility is to send a packet of data to the intended destination. EIGRP.2. An autonomous system is a group of networking components under one administrative domain. you can manually instruct a router to use a given route to a particular destination. IS-IS is an ISO standard. Although many different dynamic routing protocols exist. The gateways within the autonomous system use the route information conveyed by the IGP messages to direct IP traffic. Border routers might use either. is an example of a distance vector based interior gateway protocol. The router then uses a set of metrics. the Routing Information Protocol. OSPF. to compare routes and to determine the ‘best’ path to a destination. To accomplish this. Routers use four primary mechanisms for building their route tables: 1. Dynamic routing. the router Central is connected to the LAN segment 14. that interior gateway protocols are employed on interior routers. and updates the route table. Uses router update messages from other routers to create routes. and EIGRP.

OSPF is suitable for large networks. expense and error. Link state protocol: uses a link speed-based metric to determine paths to other networks. and IS-IS. the number of routers a packet must traverse to reach its destination. and one which is outside the scope of this guide. Common routing hazards A question that is often overlooked is “Why do we need to concern ourselves with security of the network?” A better question to ask would be “What kind of damage 104 Version 1. and Cisco IOS offers several security services for routing. A distance vector exterior gateway protocol that employs a sophisticated series of rules to maintain paths to other networks. the Enhanced Interior Gateway Routing Protocol. the Border Gateway Protocol. consult the Cisco IOS documentation. Each update is a full route table. Routers establish and maintain neighbor adjacencies every 10 seconds by default. Each router maintains a simplified map of the entire network. IS-IS is suitable for large networks. Configuring routing in IP networks can be a very complex task. or [3]. EIGRP is suitable for large networks. and are sent only when the network configuration changes. version 4. This is usually called the rate of convergence. Optional metrics are delay. Broadcasts updates every 90 seconds to all EIGRP neighbors. Updates are sent via multicast.1. Routing does raise several security issues. this section discusses some of these security issues and describes several of the security services in moderate detail. Each update only includes changes to the network. A complete link state database is broadcast by a designated router every 10 seconds by default to synchronize neighbor route tables.4. is the IETF standard exterior gateway protocol. are examples of link state interior gateway protocols. Cisco IOS supports only the cost based metric. Intermediate-System to Intermediate-System. Updates are sent over TCP connections between specifically identified peers. BGP-4 employs route aggregation to support extremely large networks (e. Distance vector protocol: maintains a complex set of metrics for the distance to other networks. Each update includes only changes to the network. BGP-4. OSPF IS-IS EIGRP BGP Another important aspect of a routing protocol scheme is the amount of time it takes for network architecture or connectivity changes to be reflected in the route tables of all affected routers.1c . EIGRP. partly because the maximum distance is 15 hops.g. in a large network OSPF offers much faster convergence than RIP. Broadcasts updates every 30 seconds to neighboring RIP routers to maintain integrity. Table 4-2 – Five Popular IP Routing Protocols RIP Distance vector protocol: maintains a list of distances to other networks measured in hops. RIP is suitable only for small networks. The table below provides a short comparison.and incorporates some features of link state protocols. For general guidance on routing protocols. 4. is a proprietary Cisco IGP that is sometimes used in all-Cisco networks.Router Security Configuration Guide Open Shortest Path First. the Internet). Link state protocol: uses a cost-based metric by default to determine paths to other networks. For example.

2. the attacker can re-route network traffic in whatever manner he desires. network administrators can deter attacks based on unauthorized routing changes.4. an attacker who sends false routing update packets to an unprotected router can easily corrupt its route table. Proxy ARP is a method of routing packets using the Ethernet MAC address instead of the IP address to determine the final destination of a packet. bogus messages are automatically discarded. • prevent network failures and interruptions in service. Anyone on a LAN segment can modify an entry in the ARP cache of Version 1. 4. This can be accomplished in many different ways. Authenticated router updates ensure that the update messages came from legitimate sources. To resist denial of service attacks. and recover from them quickly. There are two basic approaches available for protecting route table integrity: 1. The fundamental security weakness of ARP is that it was not designed to use any form of authentication. 2. For example.Advanced Security Services could an adversary do to our network?” Section 3 presents some motivations for overall router security. • protect mission information from unauthorized exposure and modification. This section focuses on security issues related to routing and routing protocols. or ARP. Another form of attack an adversary might attempt against a router is a denial of service attack. For example. Routing security should be a top priority for network administrators who want to: • prevent unauthorized access to resources on the network. However. An unprotected router or routing domain makes an easy target for any network-savvy adversary. ARP and LANs Address Resolution Protocol. routers need rapid convergence and backup routes. By doing this. Use only static routes – This may work in small networks. is the protocol used to map IP addresses to a particular MAC or Ethernet address. preventing router update messages from being sent or received will result in bringing down parts of a network. but is unsuitable for large networks because it increases administrative overhead and requires administrative response to any failures. For a detailed description of Proxy ARP. neither does Proxy ARP.1c 105 . Authenticate route table updates – By using routing protocols with authentication. ARP is described in more detail in RFC 826 and Parkhurst [2]. consult RFC 1027. The key to preventing such an attack is to protect the route tables from unauthorized and malicious changes. because ARP offers no security. A detailed analysis of routing protocol threats and countermeasures may be found in a Cisco SAFE white paper [45].

Each router accomplishes authentication by the possession of an authentication key. and it should be disabled. Plaintext authentication uses a shared secret key known to all the routers on the network segment. Central# config t Enter configuration commands. Router Neighbor Authentication The primary purpose of router neighbor authentication is to protect the integrity of a routing domain. authentication in RIP version 2 and EIGRP work in a similar fashion. BGP-4.4. Authentication ensures that the receiving router incorporates into its tables only the route information that the trusted sending router really intended to send. This sub-section describes the implementation of router neighbor authentication in OSPF. The main focus of this section is using peer router authentication with interior gateway protocols.Router Security Configuration Guide a router that serves the segment. Each sending router then uses this key to ‘sign’ each route table update message. if a host on the network does not use default gateways. In this case. Central(config)# interface ethernet0/0 Central(config-if)# no ip proxy-arp Central(config-if)# exit Central(config)# interface ethernet0/1 Central(config-if)# no ip proxy-arp Central(config-if)# end Central# End with CNTL/Z. but instead uses Proxy ARP to handle the routing. OSPF uses two types of neighbor authentication: plaintext and message digest (MD5). 4. Section 4.3. Proxy ARP is generally not used anymore. a denial of service. one per line. authentication occurs when two neighboring routers exchange routing information. when applied correctly.1c . routers connected to the same network segment all use a shared secret key. malicious. can prevent many routing attacks. That is. it signs the 106 Version 1. it is susceptible to bad or malicious routes. Such a compromise might lead to re-routing of traffic. and routing protocols This section describes how to protect routers from some common routing hazards. or corrupted routing updates that would compromise the security or availability of a network. OSPF Authentication Router neighbor authentication is a mechanism that. In any case.4. Route tables. Therefore. The following example illustrates how to do just that. static routes. The receiving router checks the shared secret to determine whether the message should be accepted. because it is a good illustration of the basic principle. or simply giving access to certain packets of data to an unauthorized person.5 gives some security guidance for one exterior gateway protocol. It prevents a legitimate router from accepting and then employing unauthorized. When a sending router builds an OSPF packet.

the router rejects the packet.2.1.6. Using this method reveals the secret key to any attacker using a network sniffer on the right LAN segments.15.9.1. r0utes-4-all.0.Advanced Security Services packet by placing the key as plaintext in the OSPF header.2. and more secure method. The receiving router then compares the received key against the key in memory.2. East. then the router accepts the packet.0/24 Figure 4-3: An Example Routing Architecture In this example.2.2.0/24 Central eth0/1 Autonomous System Border Router (ASBR) 14.250/24 eth0/0 14. The second. Otherwise. they can pose as a trusted router.2.2. Figure 4-3 shows our example network with its routing protocols.1.10.0/16 eth1 eth0 14.0/24 eth0/0 14.1.1. whose cryptographic Version 1.250/16 14.64/24 South eth0/1 14.9. is message digest authentication. If the keys match.250/24 RIP Second Floor 14. with a Key ID of 1. Internet eth0/0 North eth0/1 14. and Central all share the same secret key. This method does not provide much security because the key is in plaintext in the packet.6. Each of these routers authenticates to each other using the MD5 message digest authentication method.10.1. Once an attacker captures the key.64/24 14.20/16 East 14.9.250/16 OSPF Area 0 Facility Network 14. routers North.1c 107 .

use the superior MD5 method.Router Security Configuration Guide authentication type is denoted by a value of 2. The output. is placed in the header. The receiving router. 1. then the North trusts the packet. In this case. If the regenerated hash matches the hash that was sent from East. The corresponding Key ID. against the OSPF packet. is written over the secret that was appended to the packet. both header and body. East also places a 32-bit sequence number in the header. looks at the Key ID to determine which key was used to generate the hash. MD5. or signature. 108 Version 1. East first builds an OSPF packet. The sequence number is incremented with every new packet. the secret key is appended to the packet. Finally. This sequence number protects against replay attacks so that no two OSPF packets will have the same hash value. Otherwise. the key is r0utes-4-all. Figure 4-4 shows how East authenticates to North. It then picks a primary key to use on the network segment. East runs the cryptographic hash algorithm. North.1c . OSPF Version OSPF pkt type OSPF Version OSPF pkt type OSPF length Source OSPF Router ID OSPF length Source OSPF Router ID OSPF Area ID 0 (no checksum) 2 (cryptographic auth type) 0 1 (Key ID) 16 (MD5 len) MD5 hash algorithm OSPF Area ID 0 (no checksum) 2 (cryptographic auth type) 0 1 (Key ID) 16 (MD5 len) Cryptographic sequence number OSPF packet body Cryptographic sequence number OSPF packet body 16-byte secret MD5 cryptographic hash Figure 4-4: OSPF Calculation of an MD5 Authentication Signature (from [4]) OSPF Plaintext Authentication This method is not recommended. The router then uses its own key to regenerate the hash on the received packet in the same manner as the sending router. 16 bytes. it rejects the packet as being invalid.

receiving the key my-supersecret-key. The receiving router then uses the shared secret to check the hash and determine whether the message should be accepted.1. RIP MD5 Authentication The example below illustrates an example of setting up MD5 for RIP router neighbor authentication.0.255 area 0 East(config-router)# exit East(config)# int eth0 East(config-if)# ip ospf message-digest-key 1 md5 r0utes-4-all East(config-if)# end East# RIP Authentication The RIP routing protocol also supports authentication to prevent routing attacks. End with CNTL/Z. Using the example network shown in Figure 4-1. The neighboring RIP routers use shared secret keys. using the same key.6.0.0 0. one per line.0 0. North(config)# router ospf 1 North(config-router)# network 14.255. In practice.0. use the superior MD5 method.0. The example transcripts below show routers from Figure 4-3.2. Each sending router uses these keys to generate the cryptographic hash incorporated into each RIP update message. North# config t Enter configuration commands. although the IOS commands are somewhat different.Advanced Security Services OSPF MD5 Authentication The example below illustrates an example of setting up MD5 for OSPF router neighbor authentication. router Central would also have to be configured with MD5 authentication and the same shared key as shown below.255 area 0 East(config-router)# network 14. below. Central and South. one per line.255 area 0 North(config-router)# area 0 authentication message-digest North(config-router)# exit North(config)# int eth0/1 North(config-if)# ip ospf message-digest-key 1 md5 r0utes-4-all North(config-if)# end North# East# config t Enter configuration commands. East(config)# router ospf 1 East(config-router)# area 0 authentication message-digest East(config-router)# network 14.1c 109 . End with CNTL/Z. The example transcripts below show routers North and East receiving the key r0utes-4-all.1. contained in their respective Version 1.255. RIP Plaintext Authentication This method is not recommended. all the routers participating in a given network should be configured in the same way.0 0. RIP’s method of authentication is very similar to that of OSPF.0.0.

110 Version 1. However. and uses the first valid key that is encountered. one per line. all the routes connected to a given network must be configured in the same way. both routers will only use the first valid key. Central(config)# key chain CENTRAL-KC Central(config-keychain)# key 1 Central(config-keychain-key)# key-string my-supersecret-key Central(config-keychain-key)# exit Central(config-keychain)# key 2 Central(config-keychain-key)# key-string my-othersecret-key Central(config-keychain-key)# end Central# South# config t Enter configuration commands. The router examines the key numbers in order from lowest to highest. each neighboring router must have a shared secret key.0. End with CNTL/Z. Central# config t Enter configuration commands. one per line. Central# config t Enter configuration commands. Central and South have key chains named CENTRAL-KC and SOUTH-KC.Router Security Configuration Guide key chains. all of them must possess the same shared key(s). End with CNTL/Z. only one authentication packet is sent. one per line.0. The example below shows how to enable version 2 of RIP. Prior to enabling RIP MD5 authentication. Each RIP router must first be configured to use version 2 in order to enable authentication during routing updates. In the example below.1c . Central(config)# router rip Central(config-router)# version 2 Central(config-router)# network 14. In practice. End with CNTL/Z. End with CNTL/Z. Both key chains share the keys my-supersecret-key and my-othersecretkey. This was a feature that was included in RIP version 2. Multiple keys with different lifetimes can exist. However. one per line. The other key is usually used when migrating to different keys. South(config)# key chain SOUTH-KC South(config-keychain)# key 1 South(config-keychain-key)# key-string my-supersecret-key South(config-keychain-key)# exit South(config-keychain)# key 2 South(config-keychain-key)# key-string my-othersecret-key South(config-keychain-key)# end South# RIP version 1 did not support authentication. That is. RIP manages authentication keys by the use of key chains. A key chain is a container that holds multiple keys with the associated key IDs and key lifetimes.0 Central(config-router)# end Central# South# config t Enter configuration commands.

4. 2.0. End with CNTL/Z. Enable authentication for EIGRP messages. Router North’s key chain is defined as northkc and router East’s key chain is named eastkc. key number. Specify the key chain. Central will be using the key chain CENTRAL-KC that was created earlier and the MD5 method of authentication. the example below shows how to enable authentication for RIP. Select the MD5 authentication mode. EIGRP is configured on both routers for the 14. and key string to be used.0. The key chain name is locally significant and neighboring routers do not have to be configured with the same name. In the example below. Central# config t Enter configuration commands. In this example. and key string. one per line. South(config)# int ethernet0/0 South(config-if)# ip rip authentication key-chain SOUTH-KC South(config-if)# ip rip authentication mode md5 South(config-if)# end South# EIGRP Authentication EIGRP route authentication is provided through the use of a keyed Message Digest 5 (MD5) hash. MD5 authentication is enabled within autonomous system 100 and linked to a particular key chain.1c 111 . Authentication for RIP is enabled on the interfaces. To configure EIGRP authentication: 1. Initially. key number. End with CNTL/Z. The example below details the steps necessary to configure MD5 authentication on two EIGRP peers.0 end Finally. This insures the integrity of routing messages accepted from neighboring routers. Finally. Router North has associated key number 1 with the Version 1. Configure key management (optional). North and East. 3.0/16 network.1. Central(config)# int ethernet0/1 Central(config-if)# ip rip authentication key-chain CENTRAL-KC Central(config-if)# ip rip authentication mode md5 Central(config-if)# end Central# South# config t Enter configuration commands.Advanced Security Services South(config)# router South(config-router)# South(config-router)# South(config-router)# South# rip version 2 network 14.0. one per line. Proceeding into the interface configuration mode. the key chain is defined within key chain configuration mode consisting of a key name.

the key-string “secret-key” is associated with key number 1.255.255. In this case.0.0 255.0.0 North(config-router)# exit North(config)# interface eth 0/1 North(config-if)# ip authentication mode eigrp 100 md5 North(config-if)# ip authentication key-chain eigrp 100 NORTH-KC North(config-if)# exit North(config)# key chain NORTH-KC North(config-keychain)# key 1 North(config-keychain-key)# key-string my-secret-key North(config-keychain-key)# send-lifetime 00:00:00 Oct 1 2003 00:00:00 Jan 1 2004 North(config-keychain-key)# accept-lifetime 00:00:00 Oct 1 2003 00:00:00 Jan 7 2004 North(config-keychain-key)# end North# East# config t Enter configuration commands. Key management is optionally configured with the acceptlifetime and send-lifetime commands. 2003 until January 1. North(config)# router eigrp 100 North(config-router)# network 14.1c .Router Security Configuration Guide key-string ‘secret-key’.6.1.0 East(config-router)# network 14.255. East(config)# router eigrp 100 East(config-router)# network 14. The examples below show how to configure EIGRP authentication and keys.1. it will accept updates with that key until January 7. one per line.0. North# config t Enter configuration commands.0 255.255.0 255. but only one authentication packet is sent. In the example above.2. 112 Version 1. End with CNTL/Z. 2004. Multiple keys and key-strings can be configured on a router.0 East(config-router)# passive-interface eth1 East(config-router)# exit East(config)# interface eth 0 East(config-if)# ip authentication mode eigrp 100 md5 East(config-if)# ip authentication key-chain eigrp 100 EAST-KC East(config-if)# exit East(config)# key chain EAST-KC East(config-keychain)# key 1 East(config-keychain-key)# key-string my-secret-key East(config-keychain-key)# send-lifetime 00:00:00 Oct 1 2003 00:00:00 Jan 1 2004 East(config-keychain-key)# accept-lifetime 00:00:00 Oct 1 2003 00:00:00 Jan 7 2004 East(config-keychain-key)# end East# It is important to note that each key string is associated with a specific key number. the routers will send updates authenticated with the key ‘my-secret-key’ from October 1. The router chooses the first valid key while examining the key numbers from lowest to highest. one per line. 2004.0.End with CNTL/Z.

The IS-IS HMAC-MD5 authentication can be applied to all five types of PDU. Once an attacker captures the key. The IS-IS HMAC-MD5 authentication feature adds an HMAC-MD5 digest to each IS-IS protocol data unit (PDU) packet before transmitting the packet across the network. The message digest is created using a shared secret key and a message. preventing it from being read while it is being transmitted. an IS-IS routing protocol instance. Serial Hello. CNSP and PSNP) can be enabled with authentication on different interfaces. Each sending router uses its keys to generate the cryptographic hash incorporated into IS-IS messages. The digest is used by the receiving router(s) to authenticate each PDU. The receiving router then uses its identical set of keys to check the IS-IS authentication. Serial Hello CNSP and PSNP. LAN Hello. you must configure the following three things: 1. This method sends a “message digest” instead of the authenticating key itself. The authentication can be enabled on the two different IS-IS Levels (Level 1 and Level 2) independently. North# config t Enter configuration commands. and 3.1c 113 . 2.” [26] In order to use IS-IS HMAC-MD5 authentication. with different levels and different passwords. in their respective key chains. they can pose as a trusted router. HMAC-MD5 authentication. a key chain. one per line.Advanced Security Services IS-IS Authentication IS-IS provides three methods of authentication to prevent routing attacks: Plaintext (or clear text). and recommended security practice is to use HMAC-MD5 authentication. This method does not provide much security because the key is in plaintext in the packet. The final. North(config)# key chain ISIS-KC North(config-keychain)# key 1 North(config-keychain-key)# key-string my-secret-key North(config-keychain-key)# end North# Version 1. Using this method reveals the secret key to any attacker using a network sniffer on the associated LAN segments. The keys on the key chain must be identical on neighboring routers. End with CNTL/Z. being configured with the key my-secret-key. This process prevents unauthorized PDUs from being accepted and used in the IS-IS routing domain. Enhanced Clear Text authentication functions like Plaintext authentication with the exception that the authenticating key is encrypted only within the configuration of the router. IS-IS has five PDU packet types: LSP. North and East. Plaintext authentication uses a shared secret key known to all the routers on the network segment. “The interface-related PDUs (LAN Hello. Enhanced Clear Text and Hashed Message Authentication Code Message Digest 5 (HMAC-MD5). The transcripts below show routers from Figure 4-5. but the key itself is not sent.

1.e. one per line.1.0001.0/24 Central Figure 4-5: An Example Routing Architecture Next.0.6. East(config)# key chain ISIS-KC East(config-keychain)# key 1 East(config-keychain-key)# key-string my-secret-key East(config-keychain-key)# end East# Internet eth0/0 North eth0/1 NET = 49.0001. The following scripts implement this security policy.00 eth1 eth0 IS-IS Area 49.1.0001.0001 Facility Network 14.250 255.250/16 14.0001.250/16 NET = 49. Level 1: LSP.0001.Router Security Configuration Guide East# config t Enter configuration commands. Notice that IS-IS HMAC-MD5 authentication is applied to the North router’s ethernet interface 0/1 and on the East router’s ethernet interface 0 to authenticate neighbor adjacencies (i. and PSNP packets): North# config t Enter configuration commands. End with CNTL/Z.0/16 14.1c .20/16 East 14.2.0000. Then select the IS-IS PDU types to authenticate and enable IS-IS HMAC-MD5 authentication on the routers. CNSP. one per line.0003.250/24 eth0/0 14.00 North(config-router)# is-type level-1 North(config-router)# authentication mode md5 level-1 North(config-router)# authentication key-chain ISIS-KC level-1 North(config-router)# exit North(config)# interface ethernet 0/1 North(config-if)# ip address 14.0000.0000.e.255.1.6. North(config)# router isis secure-network North(config-router)# net 49.0000.1. End with CNTL/Z.2. Also notice that IS-IS HMAC-MD5 authentication is applied to the IS-IS instance to authenticate routing database updates (i.0000. ensure the IS-IS routing protocol process is running on the routers.0.0000.00 14. LAN Level 1: Hello packets).1.0 North(config-if)# ip router isis secure-network 114 Version 1. a site’s security policy may require that routers authenticate to establish neighbor adjacencies within an area and perhaps it requires that routers authenticate before accepting routing database updates within an area.15. For example.1.1.

Advanced Security Services North(config-if)# isis authentication mode md5 level-1 North(config-if)# isis authentication key-chain ISIS-KC level-1 North(config-if)# end North# East# config t Enter configuration commands. Key lifetime is also important. RIP. OSPF and BGP. whether one key should be used for the entire routing domain. The other issue with maintaining secrecy is the question of “How many keys should be used in the routing domain?” That is.0.0000.0000.00 East(config-router)# is-type level-1 East(config-router)# authentication mode md5 level-1 East(config-router)# authentication key-chain ISIS-KC level-1 East(config-router)# exit East(config)# interface ethernet 0 East(config-if)# ip address 14. East(config)# router isis secure-network East(config-router)# net 49. so using a common key for the entire routing domain is recommended. However. OSPF uses single keys. A key’s secrecy is intact only if it is known by the trusted routers but hidden from any attacker. End with CNTL/Z. Version 1. and EIGRP use Cisco IOS key chains. one per line. IS-IS. depends on two factors: the secrecy of the keys and the quality of the keys. IS-IS offers a unique security advantage compared to other IP routing protocols. Therefore an attacker has to be physically attached to a router in the IS-IS network to maliciously disrupt the IS-IS routing environment.0001. maintaining the secrecy of the key becomes much more important. or a separate key for each router neighbor-to-neighbor connection. such as RIP. and EIGRP routing update authentication. which makes them susceptible to remote access by intrusive applications.20 255.0003. OSPF.1.” [27] Key Management The strength of these methods. which offer substantial control over key lifetime. The best method for distributing keys to trusted routers is to do it manually.1. “Other routing protocols. IS-IS. Using a separate key for each router neighbor-to-neighbor connection can become an administrative nightmare. an administrator must manually change the keys when their lifetimes expire. RIP. ISIS packets are encapsulated over the data link and are not carried in IP packets.0 East(config-if)# ip router isis secure-network East(config-if)# isis authentication mode md5 level-1 East(config-if)# isis authentication key-chain ISIS-KC level-1 East(config-if)# end East# Authentication on Level 2 packets is accomplished by changing the level-1 parameter in the example above to level-2.255. because failure to do so can compromise the entire network. are susceptible to attacks from remote IP networks through the Internet because routing protocol packets are ultimately embedded in IP packets.1c 115 .

if an administrative distance is specified. For more information on the internal workings of static routes. These commands apply to keys in a keychain. EIGRP. Static Routes Static routes are manually configured on the router as the only path to a given destination. such as link failures.1. static routes take precedence over their dynamic counterparts. Central(config)# ip route 14.1c . can correctly re-route traffic in the case of a router or link failure. Also. but not OSPF authentication. then that static route can be overridden by dynamic information.5. The following example illustrates how to create a static route with a higher administrative distance than OSPF. See Section 4. The rules for generating good passwords apply to generating good keys as well. consult [7].2). then your router security policy should define the key management procedures and responsibilities. and RIPv2 authentication. Both of these are configured within the key chain configuration mode.0 255.1 for a detailed description. Static routes have a default administrative distance of 1.255. one per line. However. configuring a large network to use only static routes can make the availability of large pieces of the network subject to single points of failure. The number of different keys and the key validity periods should be defined in the router security policy.Router Security Configuration Guide Management of key lifetime is accomplished optionally through the use of the keychain accept-lifetime and send-lifetime commands.0 14. For example. Central# config t Enter configuration commands.6. The other factor that authentication relies upon is the quality of the keys. and specify the start-time and end-time to accept and send individual keys.255. such as OSPF. The router must be configured to maintain correct time (see Section 4. Thus a static route must have an administrative distance greater than 110 if the OSPF derived route is to have precedence over the static route. A dynamic routing protocol. Using static routes exclusively can make network administration very difficult. If you use routing update authentication. OSPF-derived routes have a default administrative distance of 110. Static routes cannot easily handle events that change the network topology. In one sense. They are not vulnerable to spoofing attacks because they do not deal with router update packets.1. so they can be used for IS-IS.20 120 Central(config)# end Central# 116 Version 1.2. End with CNTL/Z. static routes are very secure. These routes typically take precedence over routes chosen by dynamic routing protocols. In most cases.

Wait 40. . unless the attacker continues to send routing updates. which is 40 seconds in the example.Advanced Security Services The simplest approach for discarding traffic with black-hole routes is to set up static routes. then Central starts an SPF calculation after receiving a topology change. If this value is 0. Timer intervals configured. constantly sending routing updates will likely expose the identity of the infiltrator. As a cautionary note. is the time hello packets must not have Version 1.168. as discussed in Section 4. Retransmit 5 Hello due in 00:00:05 . The default settings have been selected to provide optimal performance. Hello 10. Area 1 Transmit Delay is 1 sec. . Central# show ip ospf interface ethernet0/0 Ethernet0/0 is up. The timers for OSPF can be viewed by using the show ip ospf pid command and the show ip ospf interface interface command. . The example below illustrates how to reduce convergence on an OSPF and RIP network. then reducing the convergence time will cause that false route to die quickly. then two consecutive SPF calculations can be done without any waiting period. Hold time between two SPFs 10 secs . especially when using RIP. line protocol is up Internet Address 192. .6. Convergence Reducing the convergence time (the time it takes for all routers to learn of a change in the network) can improve the level of security. In either case.150/24. It will also wait 10 seconds between two consecutive SPF calculations. SPF schedule delay 5 secs. Central# show ip ospf 1 .4.20. If this value is 0.1c 117 . Dead 40. The output of the show ip ospf pid command shows that OSPF on Central will perform an SPF (Shortest Path First) calculation 5 seconds after it receives a topology change. different aspects of network security will be addressed. If an attacker creates a spoofed route to redirect traffic. will increase network load. Reducing both of these timers causes routing to switch to an alternate path more quickly in the event of a failure. The Dead interval. reducing convergence time. The output of the show ip ospf interface interface command shows that the time between Hello packets on interface ethernet0/0 is 10 seconds. However. State DROTHER. Priority 1 .

The second modification sets the Hello-interval to 5 seconds. which is 5 seconds. then all other OSPF routers on that network must be changed as well. Area 1 Transmit Delay is 6 sec. The example below shows how to modify OSPF timers. the Retransmit-interval to 8 seconds. Otherwise. The first modification sets the SPF calculation delay to 1 second and the delay between two consecutive SPF calculations to 4 seconds. State DR. The Retransmit interval is the time between LSA (Link State Advertisement packets sent by OSPF) retransmissions. must be greater than the expected round trip between Central and any other router on the same network. Wait 20.Router Security Configuration Guide been seen before Central declares its neighbor dead. . line protocol is up Internet Address 192. That is. and the Transmit-delay to 6 seconds. . Central# config t Enter configuration commands. Dead 20. The Transmit Delay is the time in seconds that Central will take to transmit a link-state update packet. Timer intervals configured. End with CNTL/Z. the Dead-interval to 20 seconds. . one per line. If the Hello-interval and Dead-interval are modified on a router. Retransmit 8 Hello due in 00:00:02 .20. the timers for RIP can be viewed by using the show ip protocols command. Hold time between two SPFs 4 secs . Priority 1 .168.1c . the routers will be sending needless LSA packets. Central(config)# interface ethernet0/0 Central(config-if)# ip ospf hello-interval 5 Central(config-if)# ip ospf dead-interval 20 Central(config-if)# ip ospf retransmit-interval 8 Central(config-if)# ip ospf transmit-delay 6 Central(config-if)# end Central# show ip ospf interface ethernet0/0 Ethernet0/0 is up. Central# config t Central(config)# router ospf 1 Central(config-router)# timers spf 1 4 Central(config-router)# end Central# show ip ospf . This time.150/24. Hello 5. . Similarly. all routers on that network must have the same Hello-interval and Dead-interval. SPF schedule delay 1 secs. Central# show ip protocols . 118 Version 1.

OSPF and IS-IS are preferable to RIP.Advanced Security Services . Router1# show config . is the amount of time. The example below illustrates such a case.4. It is typically used when the wildcard specification on the network router configuration command configures more interfaces than desirable. one per line.0 ! Version 1. next due in 22 seconds Invalid after 180 seconds. If no update is received within 180 seconds. . then the route is declared invalid. hold down 180. which is not shown. .1c 119 . next due in 6 seconds Invalid after 120 seconds. Disabling unneeded routing-related services Passive Interfaces The passive-interface command is used to prevent other routers on the network from learning about routes dynamically. In its current configuration. hold down 150. It can also be used to keep any unnecessary parties from learning about the existence of certain routes or routing protocols used. Central(config)# router rip Central(config-router)# timers basic 20 120 150 230 3 Central(config-router)# end Central# show ip protocols . 4. measured in milliseconds. Central# In general. End with CNTL/Z. The example shows how to modify the RIP timers.250 255. an update will be delayed before transmission.255. . The hold down time is the time that a route will remain in the routing table before a new route is accepted. Central# config t Enter configuration commands.1. and this is preferable to running RIP on an entire large network. consult Chapter 13 of [2].0. Routing Protocol is "rip" Sending updates every 20 seconds. flushed after 230 . . For details on this topic. Routing Protocol is "rip" Sending updates every 30 seconds.4. interface ethernet0 description Active routing interface for 14.15. It is also possible to redistribute OSPF or IS-IS routes over RIP. flushed after 240 .1. RIP routing updates are sent every 30 seconds. The flush time is the amount of time that a route will remain in the routing table before it is removed if no update to that route is received.0 net ip address 14.0. The sleep time.

0.2.0 ! interface ethernet2 ip address 14.0.50 255.13. this command blocks routing updates from being sent or received on an interface.2. In the example above. RIP broadcasts will be sent through that interface. this command stops routing updates from being sent out on an interface.255.255.255. interface ethernet0 ip address 14.255.0. The reason for this is that both interfaces appear to have the same Class A internet address.1c . 255.0 ! router ospf 1 network 14. The example below illustrates how to remedy that problem. area 0 . .0.0 ! router ospf 1 network 14.e.255 area 0 passive-interface ethernet2 .0 0. enabling RIP on Central will cause RIP broadcasts to be sent out of interfaces ethernet0/0 and ethernet0/1. as shown below. by designating ethernet2 as a passive interface. Router1# This command functions slightly differently on RIP. i.255. .255.0.255 area 0 network 14. OSPF has been enabled to run on all subnets of 14. In Figure 4-3. 120 Version 1. This command is especially important when using RIP version 1.0 net ip address 14.0. because that version only uses major network numbers. Thus.3.x. OSPF will run only on interfaces ethernet0 and ethernet1. An alternative method to this is to simply not enable OSPF on certain interfaces.90.0. although ethernet0/0 is part of an OSPF network.15.1.0 ! interface ethernet2 description Passive interface on the 0.Router Security Configuration Guide interface ethernet1 description Active routing interface for 14.0 net ip address 14.0 but routing updates will still be received and processed. Router1# show config . Router1# When used on OSPF.0.x. When used on RIP. .50 255.2.0 ! interface ethernet1 ip address 14. However. 14.150 255.

With this setting North and East would not see a route to the 14.Advanced Security Services Central# config t Enter configuration commands. End with CNTL/Z.0 0.2.10. you must first create an access list.2. use the distribute-list out command . Thus all downstream routers will learn about the networks that were supposed to be filtered in these LSAs.10. routes from a different autonomous system (AS). this restriction only applies to external routes. use the distribute-list in command. For more information about how to create access lists. Each command behaves differently with respect to the routing protocol used. To apply this command to a routing protocol. Central(config)# router ospf 1 Central(config-router)# passive-interface ethernet0/1 Central(config-router)# end Central# Using filters to block routing updates The distribute-list command is used to apply access lists on routing protocols.0 network. This command has two primary functions. the following example is for illustration purposes only. advise against using distribute-list in for OSPF.2. End with CNTL/Z. End with CNTL/Z. one per line. Central(config)# router rip Central(config-router)# passive-interface ethernet0/0 Central(config-router)# end Central# The syntax for using this command on OSPF is nearly identical.1c 121 . an access list with rules filtering out 14.255 Central(config)# access-list 55 permit any Central(config)# end Central# The OSPF distribute-list in configuration command prevents routes from being inserted into the routing table. Central(config)# access-list 55 deny 14. this step is unnecessary.0 network from the RIP routing domain into the OSPF routing domain. The following example shows how to prevent Central from advertising the 14. Version 1. Central# config t Enter configuration commands.0.0. that is. Central# config t Enter configuration commands. For illustration purposes. however.2. To suppress advertisements of particular networks in updates. see Section 4. since OSPF is not enabled on the interface to the RIP network. However.3. but it does not stop routes from being sent out in the link-state advertisements (LSAs). one per line. including Parkhurst [2]. Some authors.10.10. The example below illustrates that. The distribute-list out command in OSPF configuration mode stops routes from being advertised in updates.0/24 will be used. one per line. Therefore. To filter the receipt of network updates.

0 network can be made with hosts on the 14.0 network if the hosts on the latter network use South.10. Subsequently. End with CNTL/Z. as their default gateway. However. South# config t Enter configuration commands.0 network from communicating with hosts on any other subnets is to simply turn off interface Ethernet0/1 on South.10. East and Central will not see routes to the 14. one per line. South still maintains a route to the 14. all updates sent from that router will not advertise the deleted route. Connections from hosts on the 14.10. The following example shows Central deleting the route to 14.) 122 Version 1.0. Central(config)# router rip Central(config-router)# distribute-list 55 in Central(config-router)# end Central# The RIP distribute-list out command prevents routes from being advertised in updates.0 0.10. thereby preventing North from learning that route.0 network can communicate with hosts on the 14.2. Using the first filter. Thus. instead of Central. Central(config)# router ospf 1 Central(config-router)# distribute-list 55 out Central(config-router)# end Central# The RIP distribute-list in command deletes routes from incoming RIP updates. Central# config t Enter configuration commands. one per line. South(config)# access-list 55 deny 14.0. hosts on the 14. the three different filters also have unusual side effects. a similar problem arises. Therefore. This is because.10.1.0 network. However.Router Security Configuration Guide Central# config t Enter configuration commands.2.0 network are prevented from reaching the Internet. (Of course. as their default gateway.2.0 network because it is directly connected to it. the easiest way to prevent hosts on the 14.0. North and East will not see a route to 14. Thus.0 network. South(config)# access-list 55 permit any South(config)# router rip South(config-router)# distribute-list 55 out South(config-router)# end South# The examples above essentially accomplish the same task.2. but that is a drastic approach.2. since Central no longer has a route to network 14. End with CNTL/Z. End with CNTL/Z. hosts from the 14.10. that is. In either case.10.10. while Central is not advertising a route to the 14. it will not advertise this network to other routers.10. the effect of applying the same filter used in the previous examples to South is that North. This is because either Central is filtering the routes it receives (second filter) or South filters the routes it advertises (third filter). The second and third filters each fix the problem that was evident with the first filter. instead of North.1c .0 network as it comes in from a RIP update from South. Central still has the route in its table.0 network if the hosts on the latter network use Central. one per line.

OSPF is the superior routing protocol. Migration procedures are beyond the scope of this document.Advanced Security Services Migrating from RIP to OSPF: Security issues and concerns Although RIP has withstood the test of time and proven itself to be a reliable routing protocol. while the service provider gateway router. Both protocols are supported by virtually every routing vendor.7. Exterior Gateway Routing Protocol Security Configuring an exterior gateway protocol can be very complex. but an attacker could then take advantage of RIP and insert a malicious route into the routing table. MD5 authentication. Central(config)# no router rip Central(config)# After disabling RIP. but OSPF offers better scaling and faster convergence. However. The examples in this sub-section show how to configure the indicated BGP security mechanisms. While both protocols support authentication. and [30] through [43]. If support for RIP is not an essential requirement. [21].3. a brief discussion of CPP appears in Section 4. AS number 26625. Remember to turn off RIP on all the routers after migration. In this particular case. prefix list filtering. consult [15]. Version 1. This sub-section presents four security mechanisms that you should when using BGP-4: the Generalized Time-to-Live (TTL) Security Mechanism. For more information on BGP and BGP Security. and route flap damping. see [2] for detailed directions. and the service provider that serves as its connection to the Internet. check the configured protocols using the command show ip protocols. [17]. 4. an important step to remember is to remove RIP after OSPF has been enabled.4. our example network constitutes a single autonomous system. is part of AS 27701.5. [18]. named ISPCust7.1c 123 . Control Plane Policing (CPP) can also help protect BGP operations. OSPF offers better convergence times. Figure 4-6 shows the relationship between our example network. The example below shows how to turn off RIP. then migrating to OSPF is the recommended solution. but they do not show how to configure BGP routers in a large network. Failure to do so will not cause a routing failure. and is outside the scope of this guide. and using OSPF reduces the likelihood of accidentally sending out routing update packets on an unintended interface. [23].

2.0(27)S and 12.3(7)T. 255 minus an administrator defined hop count). consult [15]. Otherwise.e. utilizes the Time-to-Live (TTL) field of the IP header to protect exterior BGP (eBGP) peering sessions from remote attacks. Since remote attacks originate multiple router hops away from an intended target. and [31].2. The following example shows how to implement the GTSM between routers North and ISPCust7 in Figure 4-6. [21]. otherwise the BGP session will terminate.250 124 Version 1.2.250 Type escape sequence to abort Tracing the route to 14.2.20 North ISP Network ISPCust7 AS 27701 Local Network Other autonomous systems of the Internet AS 26625 Figure 4-6: BGP Neighbors and their Autonomous System Numbers This sub-section shows how to set up BGP authentication and other security measures. If the received IP packet contains a TTL value greater than or equal to the expected TTL value (i. Before enabling GTSM. coordinate the implementation of GTSM between eBGP peers so that the command sequence is issued on both peers during a time interval less than the BGP hold time. then the packet is processed.0.0. North# trace ip 14. limiting the hop count to the actual number of hops between eBGP peers will help prevent attacks initiated on any network that does not lie between the peers. the packet is silently discarded.Router Security Configuration Guide 14.1c .250 14. Additional information on GTSM may be found in [33]. use the trace command to verify that the BGP peers are indeed separated by the expected number of hops. This mechanism uses the TTL value in a received packet and compares it to an administrator defined hop count. Also. it does not address operational issues that arise when deploying BGP infrastructure in a large network.0.0. documented in RFC 3682 [32] and introduced in Cisco IOS 12. For more information on BGP operations. [17]. Generalized TTL Security Mechanism (GTSM) The Generalized TTL Security Mechanism (GTSM).

The commands below would be performed on North by the local administrator.Advanced Security Services 1 14.250 ttl-security hops 1 North(config-router)# end North# ISPCust7# trace ip 14.0.250 0 msec * 0 msec North# config t Enter configuration commands. This security mechanism operates between two BGP peers and requires the configuration of a shared key on each of these peers.2.20 0 msec * 0 msec ISPCust7# config t Enter configuration commands. BGP shared keys should follow the password guidance specified in section 4.0.250 password r0utes4All North(config-router)# end North# The commands below would have to be performed by the network administrators of the ISP router to which North has a BGP peering connection.20 1 14. one per line. North# config t Enter configuration commands.0. The shared key is used to create and verify the MD5 signature (i. one-way hash) for each segment transmitted and received during the BGP session respectively. End with CNTL/Z. End with CNTL/Z.2. The shared key takes the form of a password configured on each peer router.2. ISPCust7(config)# router bgp 27701 Version 1.1. End with CNTL/Z. It shows how the network administrators of North in AS 26625 and the network administrators of ISPCust7 in AS 27701 would use the shared key “r0utes4All” when configuring MD5 authentication for their peering session.0. one per line. and man-in-the-middle attacks by implementing RFC 2385.1c 125 . one per line.2.250 remote-as 27701 North(config-router)# neighbor 14. North (config)# router bgp 26625 North(config-router)# neighbor 14.2. one per line.0.2. ISPCust7(config)# router bgp 27701 ISPCust7(config-router)# neighbor 14.2.2. “Protection of BGP Sessions via the TCP MD5 Signature Option” [34].0. spoofing.e.20 ttl-security hops 1 ISPCust7(config-router)# end ISPCust7# BGP and MD5 Authentication BGP peers can be protected from DoS.20 Type escape sequence to abort Tracing the route to 14. North(config)# router bgp 26625 North(config-router)# neighbor 14.5 and in the relevant IOS documentation The following two-part example pertains to Figure 4-6. ISPCust7# config t Enter configuration commands.0. End with CNTL/Z.0.

Cisco. ISP egress prefix lists should filter what is advertised to other ISP peers. 5. prefix list filtering is an important technique employed by AS network administrators to filter BGP route advertisements. (Prefix filtering may be used to prevent an ISP from transiting traffic through a multihomed customer to another ISP. ISP ingress prefix lists applied to advertisements from a customer should permit only those prefixes that have been assigned or allocated to the customer. address space that IANA has not yet allocated).20 password r0utes4All ISPCust7(config-router)# end ISPCust7# Prefix List Filtering Prefix list filtering is a common technique used to prevent damage such as DoS. 3.0. 2.20 remote-as 26625 ISPCust7(config-router)# neighbor 14.e. and others maintain bogon address lists. Although the use of prefix list filters is not restricted to BGP. AS ingress prefix lists should prevent internal prefixes from being advertised to the AS by some other AS.1c . ISP ingress prefix lists should deny advertised prefixes more specific than /24. AS ingress and egress prefix lists should filter martian address space (i. At a minimum. and prefix hijacking caused by malicious prefix advertisements. ISP ingress prefix lists applied to advertisements from peer ISPs should prefixes that are more specific than those assigned by Internet registries (see [39] – [41]).0. 4. address space that should never be propagated through the Internet).2. traffic redirection.2. These filters should only permit advertisement of routes to networks that have been allocated or assigned to the ISP and its customers. including both RFC 1918 special use addresses [44] and bogon addresses [38] (i.) Prefix list filters can be used to filter both inbound and outbound route updates on a per-peer basis. 6. Use prefix list filtering to enforce routing policy by disregarding advertisements and withdrawals of specific prefixes. Team Cymru. examples appear in [18] and [42]. There are several general rules that you should consider when developing prefix lists and filters for your BGP routers: 1.Router Security Configuration Guide ISPCust7(config-router)# neighbor 14. 126 Version 1. ISP egress prefix lists should prevent ISP core network prefixes from being advertised to any other AS.e.

1c 127 .0. It shows how to configure and apply prefix list filters to prevent a subset of martian addresses.e. Version 1.0/16 ISPCust7(config)# ip prefix-list NO-MARTIANS seq 100000 permit 0.0. ISPCust7(config)# ip prefix-list NO-MARTIANS seq 60 deny 10. The sufficiently high sequence number 100000 is chosen so that many new entries can be added to the prefix list sequentially prior to the explicit permit all entry.0/0 le 32 ISPCust7(config)# router bgp 27701 ISPCust7(config-router)# neighbor 14.0/12 North(config)# ip prefix-list NO-MARTIANS seq 80 deny 192.0.16. refer to [35] and [42].168. Damping controls the effect of route flapping which occurs when a route constantly transitions from an up-to-down or down-to-up state.0.0.20 prefix-list NO-MARTIANS in ISPCust7(config-router)# end ISPCust7# Route Flap Damping Route flap damping is a method that may be used to provide router CPU and network stability while BGP routes are converging.Advanced Security Services The following example pertains to Figure 4-6. End with CNTL/Z.0/16 North(config)# ip prefix-list NO-MARTIANS seq 100000 permit 0. from being advertised by router North and accepted by router ISPCust7.0.0. The explicit permit all prefix list entry with sequence number 100000 is used so that after the deny filters are applied. notably private addresses (i.0.0.0/12. one per line.2.0/0 le 32 North(config)# router bgp 26625 North(config-router)# neighbor 14.2.e.0/16). 172.0/8.0.0. and 192.16.0/12 ISPCust7(config)# ip prefix-list NO-MARTIANS seq 80 deny 192.0. These transitions cause excessive BGP route update messages (i. End with CNTL/Z.0. add/withdraw routes) to propagate through the network. no other prefixes will be affected by the implicit deny of the prefix list filter mechanism. one per line.0/8 North(config)# ip prefix-list NO-MARTIANS seq 70 deny ISPCust7(config)# ip prefix-list NO-MARTIANS seq 70 deny 172.250 prefix-list NO-MARTIANS out North(config-router)# end North# ISPCust7# config t Enter configuration commands. North# config t Enter configuration commands.0. North(config)# ip prefix-list NO-MARTIANS seq 60 deny 10.0.0. For more information on prefix filtering. ISPs and other backbone providers may configure BGP damping to mitigate route flapping.

4. default is 15 minutes. The following example shows the network administrator of ISPCust7 in AS 27701 enabling route flap damping with default parameter values.range is 1-225 minutes.2 and earlier ! 12.1c . • max-suppress-time . To display the dampened routes with the corresponding suppression time remaining. If you choose to use route flap damping. The list of dampened paths is useful in determining which remote networks are having instability problems.Router Security Configuration Guide The syntax for the bgp dampening command permits several optional parameters. North# show ip bgp dampened-paths ! 12.range is 1-45 minutes. default is 2000. [37].6. Cisco default parameters are shown below. use one of the two commands shown below. ISPCust7(config)# router ISPCust7(config-router)# ISPCust7(config-router)# ISPCust7(config-router)# ISPCust7# bgp 27701 neighbor 14. access lists would allow an administrator to block only Telnet (TCP port 23) traffic from exiting their network. default is 4 times the value of the half-life parameter (e. and monitor your network carefully to determine the appropriate parameters for your network environment. default is 750.g.3 or North# show ip bgp dampening dampened-paths 4. Access lists provide the administrator with a high degree of precision in selectively permitting and denying traffic. • reuse .range is 1-20000.2.0. • suppress-limit . Consult the following references before implementing route flap damping: [23]. Opinions differ on the usefulness of route flap damping and the correct values for damping parameters. Using Black-Hole Routing Many administrators configure their routers to filter connections and drop packets using basic and extended access lists.20 remote-as 26625 bgp dampening end The selection of BGP damping parameters for use in the Internet is not trivial and has been the topic of debate for several years. depending on the network 128 Version 1. [36]. 60 minutes). bgp dampening [half-life] [reuse] [suppress-limit] [maxsuppress-time] • half-life . Operational experience has shown that vendor defaults may be too aggressive. For example.range is 1-20000. begin by using the defaults. The fine granuality access lists provide can impose significant administrative and performance burdens.

It is important to turn off the generation of ICMP unreachable messages on the null0 interface. or null routing. packets sent there will never reach their intended destination.0/8 network. in particular. When you disable ICMP unreachable messages.0. it imposes little or no performance burden on normal packet flow. Null routing sacrifices the fine selectivity of access lists. There is no simple way to specify which protocols or types of traffic may or may not pass. using ip route statements as shown above. Because the null0 interface is a packet sink.1c 129 . and traffic load. On a Cisco router. it can be used only to impose a ban on all traffic sharing a specific destination address or network. router configuration. the router would send a message back to the host originating the attack. this would cause the router trying to block the attack to ultimately flood its own upstream with ICMP unreachable messages. ALL traffic sent to it will immediately be discarded. It is also well-suited for discarding data directed to unassigned or reserved addresses.0. This can compound the damage of the initial attack. the following configuration would work: Central# config t Central(config)# interface null0 Central(config-if)# no ip unreachables Central(config-if)# exit Central(config)# ip route 10. It is possible to use a selective BGP or OSPF neighbor router to distribute Version 1. To null route additional IP addresses in the future.0. If an administrator was utilizing null routing to block a denial of service attack. End with CNTL/Z. Backbone routers. you would simply add additional static routes. one per line. are often too heavily utilized to permit heavy use of access lists. Because this type of filtering is done as part of normal routing.0. the offending packets will be dropped silently. It is important to note that null routing can only discard traffic based on its addresses (usually only the destination).0. More sophisticated filtering with null routing is possible with more advanced techniques. If an address or network is null routed.0 255. For every packet that was filtered. An alternative to access lists for traffic control is a technique known as black hole routing.Advanced Security Services architecture. to block packets with a destination address in the reserved range of 10. Configuring Null Routing The simple way to configure null routing is to set up a null interface and create a static route that directs the undesirable packets to it.0 null0 Central(config)# exit Central# Enter configuration commands. This makes it well-suited to mitigating attack situations where ‘bad’ traffic into your network is all directed to one or a small number of address ranges.0. For example. the default behavior when a packet cannot be delivered to its intended destination is to send the source address an ICMP unreachable message.

4.D was received is the one that the router would use to send a packet to A. otherwise it is ‘bad’. This is a match. which is the interface on which packet 1 has arrived. in the routing table.B.7. unicast RPF verification prevents most forms of IP address spoofing. Unicast reverse-path verification supports rejecting such packets. Good packets are forwarded normally. However. for a detailed look at the topic. the router looks up its source address. For packet 1.1. How Does Unicast Reverse-Path Verification Work? All routers maintain a routing table that lets them decide how to forward packets. and the router discards the packet. When this feature is enabled on an interface. As noted in Section 4. the router looks up its source address.C. Eth0/1. and in some cases it can offer significant advantages over using access lists for that purpose. the router uses its routing tables to decide whether to accept or drop individual packets arriving on the interface. It finds interface Eth 0/0. Figure 4-7 shows two packets arriving at the router Central on its ‘inside’ interface. which is not the interface on which packet 2 has arrived. Because the packet has arrived on the wrong interface. For packet 2. so the router forwards packet 1 normally out interface Eth 0/0.Router Security Configuration Guide null routes throughout a network. it is from a host behind the South router. It finds the interface Eth 0/1.C. 7. it might have been generated by a piece of malicious software secretly installed somewhere on LAN 2. Additional practices for null routing are described in [15]. the configuration of such advanced null routing is beyond the scope of this guide. you must explicitly apply it to each interface where you want verification to be done. and in situations where it applies. as described in [24].D. Null routing can also be combined with filtering to support traceback of some types of DoS attacks.20. 130 Version 1. 4. The first packet bears a proper source address.2. It is also possible to set up automatically triggered null routing in an entire network. 14. bad packets are discarded.10.B. Unicast reverse-path verification uses the routing table to decide whether a packet with a particular source address is valid: if the interface on which the packet with address A. Setting up such a capability is beyond the scope of this guide. then the packet is considered ‘good’. Unicast reversepath verification is not enabled by default. consult [45].2.1c . in the route table. Unicast Reverse-Path Forwarding Verification Most Cisco routers running IOS 12.0 and later support a routing-based filtering feature called IP unicast reverse-path forward (Unicast RPF) verification. The second packet bears a spoofed source address. Used correctly. it is good security practice to reject a packet with a spoofed source address.12. it fails unicast reverse-path verification.3.

0/0 -14. use access lists instead. - dest=10.1. Version 1. while more broadly applicable. also require more maintenance.1.0/24 14.250 Central Eth0/1 Eth0/0 131 . then you cannot use unicast RPF. • Router uses asymmetric routes – if any of the interfaces on the router participate in asymmetric routes (one interface for sending.250 14. Access lists. Cisco has stated that future IOS versions will perform unicast RPF correctly in these cases [11]. It will incorrectly reject packets arriving on the receive leg of the asymmetric route. and a different one for receiving).Advanced Security Services 14.9.2. unicast reverse-path verification depends on Cisco Express Forwarding.0/24 Pa c ke t 2 Interface Eth0/0 Trash Interface Eth0/1 Packet 1 src=14.0/16 Packet 1 Destination Gateway Interface 14. • Router does not support CEF – according to the Cisco documentation.250 Eth 0/1 Eth 0/1 Eth 0/0 Eth 0/0 Eth 0/0 Routing Table Figure 4-7: IP Unicast RFP Verification Because unicast RPF verification uses the route table.2. If your router does not or cannot support CEF.1.1.0/24 14.0.20 dest=7.12. then simple unicast RPF verification may not be used. Avoid unicast RPF verification if any of the following conditions apply. When to Avoid Unicast Reverse-Path Verification This facility can be very useful for rejecting packets with improper IP source addresses.6. but only when the network architecture permits it to be used.2.9.64 14. it automatically adjusts to most changes in network structure.0/24 14.1.9 Packet 2 src=7.9.

the access list allows you to create exceptions to unicast RPF’s usual functioning. The transcript below shows how to enable verification on the router Central. to use unicast RPF. The example below shows how the output will look when unicast RPF is enabled in IOS 12. Therefore. access lists may be applied to RPF.15. To check whether unicast RPF is enabled on a particular interface. When a packet fails reverse-path verification.250/16 Broadcast address is 255. Central# config t Central(config)# ip cef Central(config)# interface eth 0/0 Central(config-if)# ip verify unicast reverse-path Central(config-if)# end Central# Cisco routers equipped with Versatile Interface Processors (VIPs) may require you to enable CEF with the command ip cef distributed instead of the simple version shown above. first enable CEF.2 and earlier. In particular. allow default 0 verification drops 0 suppressed verification drops Central# To disable unicast RPF.3 and later). enter interface configuration mode and use the command no ip verify unicast reverse-path (12. IP verify source reachable-via RX. Unicast Reverse-Path Verification and Access Lists Cisco IOS version 12. and then enable verification on the desired interfaces. Note that you must not turn off CEF while unicast RPF is enabled. use show ip interface interface-name.3.Router Security Configuration Guide Unicast RPF verification is best suited for routers that act as part of the security boundary between two networks (e. . 132 Version 1.255. If the access list denies the packet. it can provide better performance than an access list for ingress and egress address filtering.255.1c . then it is forwarded. line protocol is up Internet address is 14.0 through 12. consult [10]. Under 12. Configuring Unicast Reverse-Path Verification Unicast RPF verification depends on a particular routing mode called Cisco Express Forwarding (CEF). Used properly. a filtering router between a LAN and the Internet).255 . If the access list permits the packet. then it is dropped. Central# show ip interface eth0/0 Ethernet0/0 is up. For more details on how and where to apply unicast RPF verification.1.g. Also.1 and later include significant enhancements to unicast RPF. only the lines about drops may appear.2) or no ip verify unicast (12. or to view statistics about dropped packets. then the access lists are applied. Consult [10] for details about CEF requirements. Thus.

I. 1999. Cisco IOS Essentials.4. great technical background for any network analyst. The most comprehensive and readable guide to the TCP/IP protocol suite.1) then a log message is generated. This book offers a good overview of IP routing and related topics. too.. McGraw-Hill. [2] Parkhurst. IP Routing Protocols. 1999. Prentice Hall. 1998..1c 133 . Addison-Wesley. 2000. including routing and routing protocols. 1998. Comprehensive and practical guide to OSPF use.. [4] Moy. An excellent introduction to basic Cisco IOS tasks.Advanced Security Services if the access list rule that denies a packet includes the log qualifier (see Section 4. consult [11] and [15].R.T. Detailed analysis of OSPF and MOSPF. Cisco Router OSPF . and also explains how to configure Cisco routers for OSPF in a variety of situations.. 4. Laura. A very good survey of routing protocols and the technologies behind them.R. Coriolis Group..3. 1998. T..8. J. Version 1. 1999. A very practical and pragmatic guide to setting up routing protocols. Portions of this book that are particularly relevant to Routing Protocols are Chapters 2 and 7. Cisco Routers for IP Routing: Little Black Book. Advanced Cisco Router Configuration. [8] Rudenko. 1994.M. McGraw-Hill. Cisco Press. Cisco Press. J. Addison-Wesley. security. References [1] Albritton. Includes discussion of design issues. Includes a good section on troubleshooting. implementation.Design and Implementation Guide. [7] Chappell. Editor. [6] Stevens. Volume 1. W. A great reference book for a variety of Cisco configuration topics. [3] Black.. with lots of practical advice. U. [5] Thomas. For more information about advanced unicast RPF features. W. OSPF – Anatomy of an Internet Routing Protocol. TCP/IP Illustrated. OSPF Network Design Solutions. and deployment.

“Using ARP to Implement Transparent Subnet Gateways”. Cisco ISP Essentials. RFC Cisco Systems. The IETF specification for Proxy ARP. 1st Edition. “An Ethernet Address Resolution Protocol of Converting Network Protocol Addresses to 48-bit Ethernet Address for Transmission on Ethernet Hardware”. Cisco Internetworking Case Studies.1c . B.htm As a general overview of routing concepts and terminology. Cisco Systems. and Smith. Cisco Systems. 2000. [16] “Routing Basics”. Cisco Press.Router Security Configuration Guide [9] Cisco Systems. 134 Version 1. 2000.. RFC 826. Cisco Internetworking Technology Overview. This pragmatic volume offers good advice for diagnosing and correcting problems with routing and routing protocols. all with fully worked-out software/ios120/12cgcr/secur_c/scprt6/scrpf. This detailed Cisco guide for Internet Service Providers includes extensive discussion of routing protocols (especially BGP). available under http://www. available at: http://www. P.htm Documentation for new Unicast RPF features that are being integrated into IOS 12. 2000. this document gives a broad. C-M. [12] Plummer. and an in-depth treatment of Unicast [10] “Configuring Unicast Reverse Path Forwarding”.0 Security Configuration Guide.pdf Basic IOS documentation on unicast reverse-path forwarding ito_doc/routing.1 releases. performance-oriented view of IP routing. [15] [11] “Unicast Reverse Path Forwarding Enhancements”.. available at: http://www. Cisco Systems. 2000. Cisco Router Troubleshooting Handbook. The IETF specification for ARP. P. Cisco IOS 12. [13] Smoot. 1987. software/ios121/121newft/121t/121t2/rpf_plus. and Quarterman. includes a good explanation of the concepts. “RIP and OSPF Redistribution”. Cisco IOS 12. 2002. April 2002. available at: [14] Rybaczyk. M&T Books.1(2)T Release Notes. D.

available at: http://www.R. October 2005..Volume 1. [21] Parkhurst. and includes links to examples for Cisco IOS. 2001. J. Routing TCP/IP . Version 2. Provides a basic understanding of routers and routing protocols through a thorough inspection of IP interior gateway routing protocols. and access control lists to trace back the sources of some DoS attacks. Cisco Systems. Version 1. Cisco Press.ripe.. Cisco Internetworking Technology Overview. [23] Panigl.secsup. October 2001. The command guide provides very good configuration. 1998.Inter-Domain Routing in the Internet. [18] Thomas. Cisco BGP-4 Command and Configuration [24] Morrow. [22] Cisco IOS IP Configuration Guide. This volume of the Cisco IOS documentation offers extensive information on configuring all of the routing protocols discussed in this section. [19] This terse and technical note describes a technique for using null routing.html This note describes rationale for BGP route flap dampening.W. Cisco Press. an excellent reference for network engineers responsible for enterprise design. A white paper that describes the features and operation of EIGRP.. BGP4 . troubleshooting and verification guidance.Advanced Security Services [17] Stewart. Addison-Wesley. Smith. 2002. available at: http://www. 2002. A clear. concise resource for Cisco IOS software BGP-4 commands.0. “RIPE Routing Working Group Recommendations for Coordinated Route-flap Damping Parameters”.2. Cisco Press. The bookemphasizes techniques for designing efficient networks.1”. and Vistoli. 2001. [20] “Enhanced IGRP”.1c 135 . IOS 12.. RIPE-229. RIPE. J. R. “BlackHole Route Server and Tracking Traffic on an IP Network”. available at: http://www. Provides a good overview of BGP and practical advice on using it. C. W. Schmitz. “Secure BGP Template Version 4.html This short but highly prescriptive document gives a detailed example of a locked-down configuration for a backbone or border router using BGP.

[30] Halabi.2(25)S. [34] Heffernan. and tsd_technology_support_protocol_home.0(22)S. Heasley. IS-IS Network Design Solutions. 136 Version 1. A discussion of advanced BGP topics applicable to both enterprises and ISPs. issues. Cisco Press. A. 2002. The IETF specification for GTSM. The definitive IS-IS reference and design guide. containing command syntax and a detailed configuration procedure. A. “The Generalized TTL Security Mechanism (GTSM). Cisco Press. Cisco IOS Release Notes for 12. [32] Gill. Cisco Systems. Cisco Systems. organized by protocol. R.pdf Provides a good overview of IS-IS authentication with an emphasis on plaintext authentication. Good reference describing the IOS command set for HMAC-MD5 authentication. Cisco F.. available at: http://www. [31] Zhang. and 12. A reference for IOS configuration of the GTSM. Cisco Technology White Paper.Second Edition.0(27)S. “Protection of BGP Sessions via the TCP MD5 Signature Option. 2003. February 2004. 2001. and scenarios.0(21)ST and 12. M. [28] “Configuring IS-IS Authentication”.. Cisco Press. 2002..” RFC Troubleshooting IP Routing Protocols.3(7)T. Cisco Systems... 2001.Router Security Configuration Guide [25] “IP Routing Protocol Groups”. A thorough presentation of inter-domain routing concepts. Cisco Technologies Documentation.1c . Internet Routing Architectures . including guidance on multiprotocol This page offers access to a large number of technical documents and examples about IOS-supported routing protocols. [27] Martey. [26] “IS-IS HMAC-MD5 Authentication and Enhanced Clear Text Authentication” IOS Release Notes for 12. BGP Design and Implementation. and Bartell. [29] Shamin.” RFC 2385. A comprehensive hands-on guide for resolving IP routing problems. Cisco Press. 2003. August 1998. 12. [33] “BGP Support for TTL Security Check”. 2004. available under: http://www. S.

for use in prefix list filtering. available at: Ingress-Prefix-Filter-Templates/Prefix-List-Filtering-Mar-03. “Route Flap Damping: Harmful?” NANOG [40] Vegoda. with links to detailed lists and historical data. available at: http://www. B. Version 1. 2002. 2005. P. ARIN.pdf A good overview of prefix filtering with examples and An excellent resource about bogons and bogon filtering. “Flap Damping 2002. Cisco Systems. Asia Pacific Network Information Also includes some explanatory notes. available at: http://www.Advanced Security Services The IETF specification for MD5 protection of BGP sessions. [35] Greene. RIPE.html A list of minimum practical allocations by RIPE. available at: A list of minimum practical allocations by APNIC. 2005. available at: http://www.html#cidr An overview of address space allocated by ARIN. This page also provides a well-maintained list of bogon address ranges in a variety of presentations/ripe50-plenary-wed-flap-damping.. A conference presentation that questions the value of route flap damping. ISP Security Boot Camp.1c 137 . RIPE 349.. [39] “ARIN 2005 Statistics”.pdf A presentation of flap damping issues and a call for a response to deal with those issues. 2005. [36] Bush. available at: http://www.” Proceedings of the RIPE50 Conference. for use in prefix list filtering.Where to now?.nanog. “Address Space Managed by the RIPE NCC”. Griffin. [37] Smith. [41] “Allocation sizes within APNIC Address Ranges”. available at: ftp://ftp-eng.ripe. Cisco course notes. 2005. [38] “The Team Cymru Bogon Reference Page”. Team Cymru. and Mao.arin. L.ripe.

February 1996. July 2005. 2004. available under: ftp://ftp-eng.Router Security Configuration Guide [42] Greene.pdf A recent and detailed paper about threats to routing protocols and means to mitigate those threats. Cisco Systems.. Moskowitz.pdf An in-depth description of how to set up centrally-triggered null routing using your internal BGP mesh. Cisco prodlit/sfblp_wp. “Address Allocation for Private Internets”. and Rekhter. Cisco Systems. [43] Karrenberg. D. RFC 1918. Cisco white .4. Ingress-Prefix-Filter-Templates/ IOS command templates for ingress prefix filter lists. available at: http://www. This RFC describes the reserved IP address allocations for private intranets. [45] “Remotely Triggered Black Hole Filtering – Destination Based and Source Based”. Maintained as part of the bogon project [38]. B. “Ingress Prefix Filter Templates”. to be used for filtering docs/blackhole. 138 Version 1. version 1. B. [44] “SAFE: Best Practices for Securing Routing Protocols”. Cisco white available at: http://www.

1c 139 .5.5. and react to probes and scans of your network. Audit and Management 4. and later. The sub-sections below describe recommended configurations for good security.0. The RMON features depend on SNMP.1. SNMP must be disabled or carefully configured for good security.5. improve security. Version 1. • Network Management – The standard protocol for distributed management of network components is the Simple Network Management Protocol (SNMP). Installing new Cisco IOS software in a router is not especially difficult.0. Concepts and Mechanisms Routers are a critical part of network operations and network security. Careful management and diligent audit of router operations can reduce network downtime. This section describes the logging. troubleshoot service disruptions. Cisco routers have the ability to log a great deal of their status. monitoring.Advanced Security Services 4. and presents some configuration examples.3. • Logging – Cisco routers support both on-board and remote logs. 4. Complete details on the commands and features discussed may be found in the Cisco IOS documentation. Configuring Logging and Time Services Logging is a critical part of router security. • Debugging and Diagnostics – Troubleshooting router problems requires proficiency with Cisco’s diagnostic commands and debugging features. good logs can help you find configuration errors. and aid in the analysis of suspected security breaches. NTP. especially the Cisco IOS Configuration Fundamentals Command Reference documents for IOS 12. and must also be disabled or carefully configured. Cisco routers and Cisco IOS are designed to support centralized audit and management. • Network Monitoring – Cisco routers support basic facilities for Remote Network Monitoring (RMON).2. this section explains the different logging facilities. management. understand past intrusions. because new releases include fixes for security vulnerabilities. • Software Maintenance – Keeping up with new major software releases is important. • Time – Accurate time is important for good audit and management. Cisco routers support the standard time synchronization protocol. 12. describes the logging configuration commands. and update facilities offered in Cisco IOS 11.

Router Security Configuration Guide Keeping the correct time on a router is also important for accurate logs. Console logging – Log messages are sent to the console line (see Section 4. changes in network and interface status. and it is cleared when the router boots. The most valuable forms of logging are forms that are persistent. This form of logging is useful. and stores them in files or prints them according to a simple configuration file. Configuration guidance for NTP appears at the end of this sub-section. 3.1c . Overview and Motivations for Logging Cisco routers can log system errors. and saving a new configuration to the NVRAM.2). changing the configuration via SNMP. The buffered data is available only from a router exec or enabled exec session. but are otherwise of little value unless some other device or piece of software preserves the output. • Recording router configuration changes and reboots • Recording receipt of traffic that violates access lists (see Section 4. Four such events are: changing EXEC privilege level. Syslog logging – Cisco routers can send their log messages to a Unix-style syslog logging service. and many more kinds of events. Console logging is handy for operators when they use the console. login failures. A syslog service simply accepts messages. Messages can be sent to all five.3) • Recording changes in interface and network status • Recording router cryptographic security violations (see Section 5. as discussed below. Log messages can be directed in five different ways. that can be preserved over time. access list matches. This form of logging is not persistent. or any combination. changing a password. This form of 140 Version 1. 4. Buffered logging – Cisco routers can store log messages in a memory buffer. can be configured to receive log messages. Turning on line logging is useful only for the operator using that line. 1. Some motivations for keeping router logs are listed below.1. which is used on the Internet and on all major US DOD networks to distribute accurate time. Cisco routers fully support the standard Network Time Protocol (NTP). This form of logging is not persistent. 2. on any line.2) There are some events that can be important to security but which Cisco routers cannot log. Terminal Line logging – Any enabled exec session. messages printed to the console are not stored by the router. but does not offer enough long-term protection for the logs.

It has a severity level of 5. The lower the severity level number. Table 4-3 – Cisco Log Message Severity Levels Level 0 1 2 3 4 5 6 7 Level Name emergencies alerts critical errors warnings notifications informational debugging Description Router becoming unusable Immediate action needed Critical condition Error condition Warning condition Normal but important event Information message Debug message Example IOS could not load Temperature too high Unable to allocate memory Invalid memory size Crypto operation failed Interface changed state.0 you may use the name or the number. you must use the level name. Note that.2. when you are using logging levels in commands in IOS 11. 5. the message below appears in the log when a user changes the running configuration. Cisco IOS messages are categorized by severity level. because it can provide protected long-term storage for logs.3 and earlier.1c 141 . Cisco routers can generate Simple Network Management Protocol (SNMP) trap messages.9. in IOS 12. This facility allows routers to be monitored as part of an overall SNMP-based network management infrastructure. SNMP trap logging – For some kinds of events.6) Message text Message name and severity level Time that message was generated Figure 4-8: Format of a Cisco IOS Log Message Version 1. The severity levels are described in the table below. the more critical the message is. Mar 31 9:00:16 EST: %SYS-5-CONFIG_I: Configured from console by vty0 (14. up to down. as shown by the numeric field “-5-” in the message name. or down to up Packet denied by an access list on an interface Appears only when debugging is enabled For example.Advanced Security Services logging is the best available for Cisco routers.

enable SNMP trap logging also. Central# config t Enter configuration commands. In general. and consider use of console logging. Central(config)# logging on End with CNTL/Z Setting up Console and Buffered Logging To turn on console logging. notifications. the logging level at the console should be set to display lots of messages only when the console is in use or its output is being displayed or captured. For buffered and other forms of persistent logs. if your router has more than 16 Mbytes of RAM.4 presents RMON configuration issues. use the commands shown below. End with CNTL/Z Central(config)# ! Set a 16K log buffer at information level Central(config)# logging buffered 16000 information Central(config)# ! turn on time/date stamps in log messages Central(config)# service timestamp log date msec local show-timezo Central(config)# exit Central# show logging 142 Version 1. which means that important messages will appear on the console. as shown below. and view the buffered log. As a rule of thumb. but be aware that this can place a burden on the router and should be used sparingly. turn on logging services.3. enable time stamps. Use logging console debug to see ALL messages on the console. consult the “Troubleshooting Commands” section of the IOS Configuration Fundamentals Command Reference. In a network where SNMP management is already deployed. If you are not using the console. one per line.1c . one per line. below. This example sets the logging level for the console to level 5. recording the time and date of the logged message is very important. (SNMP is discussed in sub-section 4. set the console logging level to 2 using the configuration command logging console critical. Use the command logging console info to see all non-debug messages including access list log messages. Central# config t Enter configuration commands. The example below shows how to turn on buffered logging.Router Security Configuration Guide For best security. set up syslog logging. Central(config)# ! set console logging to level 5 (notify) Central(config)# logging console notification Central(config)# exit This example sets the console message level to 5. First. but access list log messages will not. your log buffer size should be about 16 Kbytes. but it must be turned on explicitly. then you can set the log size to 32 or 64 Kbytes. sub-section 4.5. buffered logging.) The descriptions below recommend logging configuration settings.5. RMON is a monitoring facility based on SNMP. for more information about Cisco logging command and facilities. Cisco routers have the ability to timestamp their messages.

Central# Setting up Terminal Line Logging Any terminal or virtual terminal line can act as a log monitor.2. It offers the network administrator the ability to send log messages from all of the routers (and other Cisco equipment) on a network to a central host for examination and storage.0 overruns) Console logging: level critical.6) . 332 message lines logged Logging to 14.3. For more information consult the tools list in Section 9. this needs to be done only once. First.6 Central# config t Enter configuration commands. set the severity level for terminal line monitor log messages.9.1 flushes.6. There are two parts to setting up terminal monitor logging. and several free and commercial syslog servers are available for Windows NT/2000/XP. End with CNTL/Z.9.1c 143 . changed state to administratively down Setting up Syslog Logging Syslog logging is the most useful form of logging offered by Cisco routers. The example below shows how to set up terminal line monitoring for informational severity (level 6) on a telnet session virtual terminal line. Second. Version 1.9. while using a particular line. 302 message lines logged Log Buffer (16000 bytes): Mar 28 11:31:22 EST: %SYS-5-CONFIG_I: Configured from console by vty0 (14. this needs to be done once per session. level 5 Central(config-if)# shutdown Central(config-if)# Mar 28 15:55:29 EST: %LINK-5-CHANGED: Interface Ethernet0/1.2. Central(config)# ! set monitor logging level to level 6 Central(config)# logging monitor information Central(config)# exit Central# ! make this session receive log messages Central# terminal monitor Central# config t Enter configuration commands.2. End with CNTL/Z. one per line.Advanced Security Services Syslog logging: enabled (0 messages dropped. declare it to be a monitor. All Unix and Linux operating system configurations include syslog servers. Central# show users Line User Host(s) Idle Location *130 vty 0 bob idle 00:00:00 14. 0 messages logged Buffer logging: level informational. . 1 messages logged Trap logging: level debugging. one per line. Central(config)# interface eth 0/1 Central(config-if)# ! shutdown will log a message.

The severity level for syslog messages is usually the same as that for buffered log messages. The syslog facility is simply the name you’ll use to configure storage of your messages on the syslog server. but typically only one or two are needed (see below). A syslog client is a host that generates messages. a DNS name.2. The destination host may be specified with host name.1. The diagram below shows a typical configuration with syslog in use. the levels have the same meanings as the Cisco severity levels listed in Table 4-3 above.2.6 South Eth 0/1 Syslog server host Log message storage LAN 3 Figure 4-9: A Small Syslog Configuration There are four things that you must set for syslog logging: the destination host or hosts. use the loopback interface if you have defined one. LAN 1 Eth 0/0 14.10. the log severity level. Set the severity level limit for messages sent to syslog using the logging trap command. Any number of syslog hosts may be specified.conf(4) manual page or other syslog documentation on the server host. but the ones used for routers are typically local0 through local7.2.64 Central log messages log messages 14. for more information consult any Unix syslog.15.250 Eth 0/1 LAN 2 14.2. 144 Version 1. or an IP address.1c .9. the syslog facility .9.250 14. There are several dozen valid syslog facility names. and the source interface for the messages.64 Eth 0/0 14.9.Router Security Configuration Guide Review of Syslog Concepts A syslog server is a network host that accepts messages and processes them. otherwise use the network interface closest to the syslog server. The source interface is the network connection from which the syslog messages will be sent. Syslog servers also support the notion of severity levels.

one per line. Central(config)# logging trap information Central(config)# logging 14. An example access list entry for the router Central is shown below (note: it is usually better to set up your access lists to permit explicitly required ports and protocols and deny all else. Syslog uses UDP port 514.0. 28 message lines logged .9. For more information on access lists.6 Central(config)# logging facility local6 Central(config)# logging source-interface loopback0 Central(config)# exit Central# show logging Syslog logging: enabled (0 messages dropped.6.0. The syslog configuration lines below illustrate this. it is best to designate two independent syslog servers. local5 for interior routers.255 eq syslog access-list 120 deny udp any 14. 0 overruns) Console logging: level notifications. set up access control lists to reject syslog traffic from the outside network. to load informational severity and above (level 6) messages to the syslog server.Advanced Security Services The example below shows how to configure the router Central.9.2. 35 messages logged Buffer logging: level informational.debug /var/log/routers.10.conf. and otherwise save messages for each facility into a separate file.log Additional Issues for Syslog Logging For a router whose security is critical. End with CNTL/Z.0 0. On a border router.0.255 eq syslog In a situation where a sizable set of routers and other devices are sending messages to the same syslog server. 35 messages logged Monitor logging: level debugging.9.2.2. Central# It is important to configure the syslog server to store router messages in their own file. 31 messages logged Logging to 14. consult Section 4.0. Configuration file syntax for syslog servers is uniform for all Unix and Linux syslog servers.0 0.3. . Version 1. Save all messages of critical (level 2) severity and above to a single special file. The example below shows the syslog configuration line for saving Central’s messages into a file. For example. 11 flushes. rather than denying specific ports as shown here). Central# Central# config t Enter configuration commands. using syslog facility local6 and the loopback interface. At least one of the two syslog servers’ logs should be backed up to permanent storage (CD-R or tape). shown in the figure above.1c 145 . separate the devices into 2-5 populations with similar duties.2. and local4 for LAN switches and other network hardware. such as a border router on the Internet. access-list 120 deny udp any 14. Use a separate syslog facility name for each population. local6 for border routers. # Save router messages to routers. the configuration file is almost always /etc/syslog.log local6.

debug /var/log/inner-routers. it may be useful in a network that already has SNMP management deployed. Central# config t Enter configuration commands.crit /var/log/net-critical.log local6.1 traps public Central(config)# snmp-server trap-source loopback0 Central(config)# snmp-server enable traps syslog Central(config)# exit Central# Many of the trap messages sent by a Cisco router will not appear as formatted error messages in commercial SNMP viewing tools.log local5.9.log local4. set the SNMP source interface. error prone.2.log SNMP Trap Logging Cisco routers have the ability to report certain events as SNMP traps.crit /var/log/net-critical. While only a small subset of all log messages can be reported this way.crit /var/log/net-critical.debug /var/log/border-routers. Cisco routers fully support automated network time synchronization based on the standard Network Time 146 Version 1.log # All other router and switch messages to their respective files local6. SNMP is described in more detail in the next sub-section. Manual time synchronization is tedious.1c . trap messages about link status changes and other typical network hardware events should be interpretable by commercial SNMP tools. and unreliable. All Cisco routers have a clock that maintains the time and date. although some older Cisco models may lose time when turned off. and may be useful in monitoring the network status. second. Network Time Synchronization and NTP Successful audit of a large network can depend on synchronization of the various logs and records maintained for the hosts on that network. and no router can keep accurate time by itself over weeks and months of operation. select an SNMP logging host. First. and then keep the time synchronized while the router is in operational use.9. There are four parts to setting up SNMP trap logging. It is possible to perform manual network time synchronization. Time Services.debug /var/log/other-net-hw. one per line. third.log local4.log local5. set the trap logging level. However.Router Security Configuration Guide # Critical and higher messages to critical. last.1. End with CNTL/Z. The example below shows how to configure SNMP trap logging for a receiving host 14.2. It is very important to set the time on a router when it is first installed. Central(config)# logging trap information Central(config)# snmp-server host 14. It may be necessary to add Ciscospecific format specifications to the SNMP tools. adjusting the time on each router and host on a network manually on a regular basis. enable SNMP traps for syslog logging.

follow these three steps: first. set the timezone if necessary. one per line. If the detail output reports a time source of NTP. Central(config)# clock timezone GMT 0 Central(config)# exit Central# Review of NTP Concepts NTP is the standard Internet protocol for time synchronization. Because having accurate time can be important for security. Central# config t Enter configuration commands. see the descriptions of NTP below. Otherwise. End with CNTL/Z. especially for intrusion and forensic analysis. and so on. Version 1. NTP should be used to synchronize all the devices and hosts on a network whenever it is available. and last set the time.Advanced Security Services Protocol (NTP). set the time in privileged EXEC mode by using the clock set command. Central# show clock detail 22:26:21. Setting the Time Manually To set the time. then do not set the clock manually. If the timezone is not correct. and it is used on most large operational networks. Central(config)# clock timezone EST -5 Central(config)# interface eth 0/0 Central(config-if)# ntp disable Central(config-if)# end Central# clock set 17:27:30 28 March 2000 Central# show clock 17:27:34.747 UTC Tue Mar 28 2000 Time source is user configuration Central# config t Enter configuration commands. one per line. The sub-sections below give some background information on NTP. then set the time zone using the clock timezone configuration command. US east and west coasts) then you should set the router time zone on all your routers to universal time or GMT. Typical NTP deployment is hierarchical. second. Stratum 2 hosts get their time from stratum 1 servers. check the clock. NTP is designed to make time synchronization automatic and efficient.1c 147 . and turn off NTP on each interface using ntp disable. End with CNTL/Z.495 EST Tue Mar 28 2000 Central# If you manage routers spread across several time zones (e. Examine the clock using the show clock detail command. and explain how to configure it on Cisco IOS. like an atomic clock or GPS.g. as shown in Figure 4-10: one or more stratum 1 servers get their time from an authoritative source.

Commercial stratum 1 radio receivers are available that use a broadcast time source (e. GPS or a time signal from the US Naval Observatory) to offer NTP service. 148 Version 1. For more information about NTP configuration.ntp. a Cisco router may be used as the border router between the Internet and an internal. consult the “Performing Basic System Management” chapter of the Cisco IOS Configuration Fundamentals Configuration Guide. For more information about NTP and Internet NTP servers. For more information on this topic. such as the ntpq tool distributed with the open-source NTP package. and may serve as the NTP server to the hosts on the internal network. protected network which requires time synchronization from a time server on the Internet. see [11]. As shown in the figure. In these cases. Cisco routers (except the old 1000-series) are capable of acting at any stratum in the NTP hierarchy except stratum 1.g. Note that Cisco IOS implements version 3 of the NTP protocol.Router Security Configuration Guide Atomic Clock Stratum 1 Stratum 2 Stratum 3 Stratum 4 Figure 4-10: The NTP Hierarchy If an NTP client is configured with several NTP servers. NTP authentication should also be used (see below). and can be monitored using NTP standard-compliant tools. or to obtain the latest NTP server software and tools for a variety of operating systems. directly or indirectly. If your network has one of If at all possible.1c . it will select among them automatically based on time accuracy and stratum level. This configuration will allow the router to block general NTP traffic at the boundary. the router should be configured as an NTP client to two or more reliable Internet time servers. NTP clients may also have peer associations. visit the main NTP site: http://www. setting up peer associations is beyond the scope of this guide. In some cases. then you can configure all your routers to get their time from it.

A full explanation of NTP access control is outside the scope of this guide. ~configured South# show clock detail 09:30:08. # master (unsynced). designate one or more NTP servers.25 * master (synced).1c 149 . There are two steps to configuring a Cisco router to be a simple NTP client: first.2. check the Cisco IOS documentation for details. then your interface access lists should be configured to permit the NTP protocol (TCP port 123 and UDP port 123) only for designated NTP participants. To add an NTP server use the command ntp server.15. South# config t Enter configuration commands. consult the network administrator. and peers. End with CNTL/Z.2.2.Advanced Security Services Configuring Basic NTP Service To set up a Cisco router to participate in an NTP network. clients.11..141 eq ntp access-list 120 permit udp host 14. simply designate one or more NTP servers. and restrict NTP transactions to that server alone. The NTP source interface is the network connection from which the NTP control messages will be sent. one per line.250 eq ntp host 14.0 -0. South(config)# interface eth0/0 South(config-if)# no ntp disable South(config-if)# exit South(config)# ntp server 14. second. For more information about access lists consult Section 4.170 EST Wed Mar 29 2000 Time source is NTP South# Access restrictions can be imposed on NTP in two ways: interface access lists and NTP access lists. + selected.11.candidate.250 26.9. South# show ntp associations address ref clock st when poll reach delay offset *~14.9.250 eq ntp host 14. Version source loopback0 South(config)# exit South# ! wait one minute or so.9 9 11 512 377 2. use the network interface on the same network as the designated server. access-list 120 permit tcp host 14. Use the source qualifier to bind the NTP service to the loopback interface.3. or the one that is the fewest number of network hops distant from the servers. The example below shows how to set up an NTP server.141 eq ntp NTP access lists can be used to impose fine-grained access control on NTP servers.. To find out the main NTP servers on the wide-area network you plan to join. If you use NTP.9. The example below shows how to configure the router South to use the router Central as its NTP server.203. The example below shows access list entries that permit NTP traffic between router South’s loopback0 interface and a designated address of 14. and how to check that the NTP association is working. set the NTP source interface.2. .

Central(config)# interface eth 0/2 Central(config-if)# ntp disable Central(config-if)# end Central# End with CNTL/Z. does not prevent a router from responding to NTP queries from other network hosts.0 -0.9. Setting up such a community is outside the scope of this guide. 150 Version 1.250.9 9 11 512 377 2. Central# config t Enter configuration commands. South(config)# ntp authenticate South(config)# ntp authentication-key 1 md5 router South(config)# ntp trusted-key 1 South(config)# ntp server 14.250 source loopback0 South(config)# access-list 21 permit host 14. ~configured South# By default.candidate. use an access list.2.9. Configuration Sample The configuration command listing below shows the configuration commands for a router with console logging. buffered logging. and the time server is 14. End with CNTL/Z.Router Security Configuration Guide South# config t Enter configuration commands. To restrict the set of hosts to which your router will provide NTP service. one per line. one per line.2. Configuring NTP Authentication Cisco IOS supports authenticated NTP. as shown here.25 * master (synced). and authenticated network time synchronization. End with CNTL/Z. .2.9.250 South(config)# access-list 21 deny any South(config)# ntp access-group peer 21 South(config)# exit South# show ntp associations address ref clock st when poll reach delay offset *~14.2. you should disable NTP on all external interfaces. The host receiving the log messages is 14.250 26. syslog logging. the description below shows how to set up authentication for an Cisco router so that it can use a designated NTP server that uses authentication.9. # master (unsynced). a Cisco router configured with one or more NTP servers or peers will act as an NTP server. The example below shows how to disable NTP server facilities on an interface.203. Unless your network is responsible for providing time service to other networks. which uses pre-placed keys to establish a trusted community of NTP servers and peers.15.2.9. South(config)# ntp server 14. South# config t Enter configuration commands. + selected. This sample is formatted as it would appear in a configuration text file stored on a host for download to the router South.250 key 1 source loopback0 South(config)# exit Note that configuring NTP authentication. one per line.2.6.1c .9.

Advanced Security Services ! turn on timestamps for log entries service timestamps log datetime msec localtime show-timezone ! setting logging levels and syslog parameters logging console notifications logging monitor debug logging buffered 16000 informational logging facility local6 logging source-interface loopback0 logging 14.6 logging on ! a tiny access list to permit access only for Central access-list 21 permit 14. the agent. tree-like structure used to store a virtual database of network management information. SNMP may also be used by an entity on the network to send alerts indicating Version 1.2. the information on each device is organized in a virtual store called a Management Information Base (MIB). the SNMPv3 agent used in the creation of this section). values for the entire branch are returned. An OID is a unique. in a MIB is referenced by an object identifier (OID).250 key 1 source loopback0 4. SNMP is the transport protocol used to share and change information between MIBs.5.sourceforge. or object. When requesting the value of an object. In the case of Cisco routers. An SNMP agent device maintains information and makes it accessible to managers. A free and usable implementation of an SNMP agent and manager may be obtained from the NETSNMP home page ( . SNMP may be used to query the status of or set the values of network components.2. If the referenced value is not a bottom leaf of the tree. the router is always the agent. numerical name. Security for the Simple Network Management Protocol (SNMP) The Simple Network Management Protocol (SNMP) supports a connection between two entities that communicate with each other: the manager and the managed entity. An in depth discussion of SNMP data organization is outside the scope of this guide. Each piece of data.NET-SNMP is the successor to ucd-snmp.250 access-list 21 deny any ! designate Central as our sole NTP server with authentication ntp authentication-key 1 md5 LTGTR-769015 ntp authenticate ntp trusted-key 1 ntp access-group peer 21 ntp server 14. A software application on a PC or workstation normally acts as the manager.3. dotted. A MIB is a hierarchical.9. one may use the OID or the actual name of each branch (separated by dots). for more information consult [7].1c 151 . where the dots separate branches in a MIB tree.9.2.

If your IOS release is one of the vulnerable ones (and virtually every IOS prior to February 2002 is) then you should either upgrade your IOS (recommended). This string is sent across the network in clear text. As such. However. or take other protective measures. those that do not have a direct impact on security are not covered.3 supports SNMPv1 and SNMPv2c. SNMPv1 and SNMPv2c have weak security. conflict arose between competing proposal advocates and no security standard was agreed upon. Consequently strong security was left out of SNMPv2c. There are currently three versions of SNMP: SNMPv1. However. it became widely used. Unfortunately. it was developed quickly and strong security was not a requirement. SNMPv1 uses a community string to limit access to the MIB. This section will give a brief overview of SNMP security and will detail how to enable SNMP more securely. In the late 1990s. disable SNMP altogether. IOS versions 12. Proposals were put forth to integrate security (as well as more functionality) into later versions of the protocol. SNMP Security When SNMPv1 was developed. consult the Cisco security advisory “Malformed SNMP Message-Handling Vulnerabilities” [9]. 152 Version 1. Table 4-4: SNMPv3 Security Security Level Authentication Username sent in the clear HMAC-MD5 or HMAC-SHA HMAC-MD5 or HMAC-SHA Encryption None None DES (56-bit) SNMPv3 noAuthNoPriv authNoPriv authPriv The Cisco documentation indicates that IOS 12. it was originally intended to be a short-term solution for (remotely) managing networks.0 supports all three security levels. DES 56-bit encryption was not supported in the versions of IOS used for preparation of this section (12. Cisco IOS supports a large number of SNMP-related commands.Router Security Configuration Guide problems. SNMP Vulnerability In early 2002.1c . They are described in the table below.0(5)).0(7) and 12.0 and later support all three versions of SNMP. SNMPv2 relies on the same mechanism for access control to the MIB. IOS version 11. SNMPv2c and SNMPv3. since it was the only network management protocol available at the time. SNMPv3 defines three levels of security. SNMPv3 was developed specifically with strong security in mind. For more information. serious SNMP vulnerabilities were disclosed that affected Cisco routers and many other network devices.

It is a good idea to disable the public community string. 0/10. The rest of this section focuses on SNMPv3.6 traps public East(config)# exit East# show snmp Chassis: east Contact: John Doe Location: Headquarters 0 SNMP packets input 0 Bad SNMP version errors 0 Unknown community name 0 Illegal operation for community name supplied 0 Encoding errors 0 Number of requested variables 0 Number of altered variables 0 Get-request PDUs 0 Get-next PDUs 0 Set-request PDUs 0 SNMP packets output 0 Too big errors (Maximum packet size 2048) 0 No such name errors 0 Bad values errors 0 General errors 0 Response PDUs 0 Trap PDUs SNMP logging: enabled Logging to 14.3) needs to be created to limit machine access to the router Version 1. East# config t Enter configuration commands. there are some basic commands you must run to enable SNMP. SNMPv3 has been implemented and provides more security features. there is no other alternative when enabling SNMP.Advanced Security Services Configuring SNMP .0.1). on Cisco IOS version 11.6. these commands have been disabled.3 (which implements SNMPv1 and SNMPv2c). Then an access control list (see Section 4.5.2. Unfortunately. While there is some mention of enhanced security options (for SNMPv2c) in the Cisco documentation. 0 dropped. one per line.6.2. East# Running these basic commands by themselves is not very secure. 0 sent. SNMPv3 A Cisco router capable of running SNMPv3 allows for more security measures to be applied.162. In order to enable SNMP a default community string must be set. This string is stored on the router in clear text and will be sent across the network in the clear. in version 12. End with CNTL/Z East(config)# snmp-server community publicstring East(config)# snmp-server host 14. So. as shown below. However. anybody who knows this community string has access to essentially the entire MIB. It is a good idea to run the show snmp command to display the SNMP status and statistics.6.1c 153 .Getting Started In both IOS versions 11 and 12. SNMP logging must also be enabled (see section 4.

SNMP is still enabled but no one has access to the MIB because the community string. one per line. East# config t Enter configuration commands. Limited access may be given to the MIB by defining groups. one per line. More than one machine may be added on the access-list. A MIB view defines a portion of the MIB that a user or group may see/modify provided they have the appropriate credentials. users and MIB views. Following is an example that does this. East# config t Enter configuration commands. “jdoe”.Router Security Configuration Guide (through SNMP). A better method to allow access to the MIB is to use strict controls. End with CNTL/Z East(config)# no snmp-server community publicstring East(config)# ! create access list to use later East(config)# access-list 20 permit 14. If this MIB view is not specified the default is to have access to basically the whole MIB. a group must be defined by specifying a group name. This is an example of a noAuthNoPriv security model. The following example defines a non-privileged user. is disabled.1c . which is the “system” branch of the MIB. the version of SNMP and the security model desired. Then a MIB view should be defined to either include specific MIB branches or exclude specific MIB branches. This group has read access to the “sysonly” view. The following example also introduces two new commands used to verify that the new groups and users have been added correctly.included nonvolatile active East# 154 Version 1. as well as the access to that view may also be defined. who is a member of the “publicUser” group. The second step is to add users to the group. instead authentication is based on the user name. No community string is required. which solely defined access to the MIB. End with CNTL/Z East(config)# snmp-server group publicUser v3 noauth read sysonly East(config)# snmp-server user jdoe publicUser v3 East(config)# snmp-server view sysonly system included East(config)# exit East# East# show snmp group groupname: publicUser security model:v3 noauth readview :sysonly writeview: <no writeview specified> notifyview: <no notifyview specified> row status: active East# East# show snmp user User name: jdoe Engine ID: 00000009020000500F033680 storage-type: nonvolatile active East# East# show snmp view sysonly system . First. This branch contains useful information and is beneficial for users to have access to. A specific SNMP MIB view.6.6 East(config)# exit After these commands.2.

the routing tables are not accessible remotely. This view gives access to all parts of the MIB except the branches that display routing information. “root” who uses MD5 for authentication. Then users must be added to the group with a password string.2).Advanced Security Services The more secure model implemented is authNoPriv. users. Access-lists.ipRouteTable excl East(config)# exit The examples above showed some basic rules that should be followed when configuring SNMP on a router. The following example defines a privileged user. While SNMP is helpful because it allows an administrator to remotely configure the router. Otherwise. 4. follow the guidance below. This security model uses MD5 or SHA to hash the community string.5. the routing tables are not permitted to be modified remotely.4. RMON is based on SNMP. defined by the view “adminview”. Overview of RMON Remote Monitoring (RMON). First. User “root” is a member of the “administrator” group. Likewise.1c 155 . members of the administrator group have restricted read and write access. it also provides a potentially dangerous conduit into a network. In this example. A MIB view may be defined by more than one included/excluded statement to restrict the view to the appropriate MIB branches. This makes it harder to compromise the community string. it is always a good idea to use the show commands to verify the new settings. his community string “secret” will be hashed and then sent across the network. Of course. is an extension of SNMP. The RMON standard was originally developed by the Internet Engineering Task Force (IETF) to provide proactive monitoring and analysis of Version 1.ipAddrTable excl East(config)# snmp-server view adminview ip. to the MIB. It provides the capability of monitoring and analyzing traffic – data to and from network devices on distributed network segments. If you are not using RMON. This means that when user “root” tries to access/modify MIB data. it should be disabled. East# config t Enter configuration commands. even if the community string is somehow compromised. So. End with CNTL/Z East(config)# snmp-server group administrator v3 auth read adminview write adminview East(config)# snmp-server user root administrator v3 auth md5 “secret” access 20 East(config)# snmp-server view adminview internet included East(config)# snmp-server view adminview ip. Then the MIB view is defined. This string may be hashed using MD5 or SHA. Security for Remote Monitoring (RMON) This sub-section describes RMON and security issues related to it. while not shown. The steps to support this security model are similar to the steps in supporting the noAuthNoPriv model. one per line. it can be disabled by disabling SNMP (see Section 4. groups and views must be defined to control access to the MIB. a group must be defined.

Statistics. Optivity LAN. a rising threshold with the associated event.1 on up to IOS 12. History. The basic IOS RMON agent supports the Alarm and Event groups. In order to enable RMON on the Cisco routers. logs a message or generates a trap and transmits it to the Network Management station. a read/write community string is highly discouraged (see Section 4. Protocol analyzers or RMON probes add enhanced monitoring capability of RMON agents by passively collecting data packets on the monitored LAN segment. RFC 2021. basically RMON1.0. IOS images ordered with the explicit RMON option.2). The network management station will record details about all configured events triggered on the monitored router. If the agent installed on the router does not include the explicit RMON option. referred to as RMON1 and in the extended version. referred to as RMON2. the RMON agent fires an event.5. RFC 1757. HostTopN. As a security precaution. RMON specifications are defined in the basic RMON standard.1c . Implementation of RMON in Cisco Routers The Cisco IOS versions installed in most Cisco routers. RMON1 is widely implemented in most data communication devices. Since the RMON option is an add-on enhancement to the Cisco router’s IOS. the polling period. However. Alarm. The configured parameters of an alarm identify a SNMP MIB variable to monitor. and the Event Groups. The alarm group periodically samples statistics from variables and compares them to thresholds configured on the agent. implement a small sub-section of the RMON1 agent standard. RMON2 provides traffic-level statistics plus finer granularity of network behavior from the network to the application layers of the OSI model. Filters. If a data sample crosses a defined threshold. if the network architecture includes a deployed SNMP infrastructure and network management station. The configuration of the alarm group is dependent on a previously configured RMON event. or HP OpenView to process and display the RMON results in graphical or report form.2). On the network management station. Packet Capture. RMON1 collects current and historical traffic statistics up to the MAC-layer of the OSI model. collect and log information in all nine groups. a network administrator uses applications such as NetScout Manager Plus. In addition. the RMON agent implements the Alarm and Event groups only.Router Security Configuration Guide traffic data on distributed LAN segments. Some network monitoring probes may require a read/write community string in order to communicate with the agent. The implementation of the rising 156 Version 1. The event fired. The RMON Management Information Base (MIB) defined in RFC 1757 is a standard method for monitoring basic operations of network devices on LAN segments by providing interoperability between SNMP management stations and RMON monitoring agents. The probe communicates the data collected to a Network Management Station via SNMP. Matrix. then enable SNMP traps on the router (see Section 4. this document covers only those features and security concerns applicable to the most common IOS releases. Host. and a falling threshold. a Read Only community string is required when configuring the standard SNMP agent. beginning with IOS 11.

All other elementary data variables should have an instance number of ‘0’. .1 notation. Variables included in the SNMP table format will have an instance number equivalent to the entry number of the table.13. Use the rmon alarm command to designate the actual MIB variable monitored on the Cisco router. assigned to event 0 On startup enable rising or falling alarm Alarm 2 is active. counter. there are several limitations on the type of SNMP variables RMON is capable of monitoring.1 every 30 second(s) Taking delta samples. All monitored objects must include an instance number of the monitored variable. End with CNTL/Z. the following command defines an alarm configured on a member of the MIB II interfaces table.13. For example.1c 157 . Use the rmon event command to provide a description of an event and specifies whether a message is logged or a trap is generated. owned by rscg Monitors ifEntry. Central# Version 1. timeticks. assigned to event 1 Falling threshold is 0.1 30 delta rising-threshold 40 1 falling-threshold 0 owner rscg Central(config)# exit Central# show rmon alarms Alarm 1 is active.Advanced Security Services and the falling thresholds of an alarm are dependent on the previous configuration of an associated event. gauge. However. one per line. An important requirement that is easily overlooked is the instance number of the monitored variable. Alarms may define any SNMP MIB variable that has an elementary data type such as integer. ifTable: Central# config t Enter configuration commands. owned by config . It is acceptable to use the Object Identifier (OID) or the qualified variable name that resolves to its OID. The MIB object monitored must also resolve to an ASN. The basic IOS RMON agent supports the following commands: show rmon alarms show rmon events rmon event number [log] [trap community] [description string] [owner string] Display information on alarms configured Display information on events configured Configure an RMON event rmon alarm number MIB-object Configure an RMON alarm interval {delta | absolute} rising-threshold value [event-number] falling-threshold value [event-number] [owner string] The first two commands display information on configured RMON facilities. RMON alarms provide an excellent tool for monitoring the network interfaces supported by the router. last value was 3 Rising threshold is 40. etc. Central(config)# rmon alarm 1 ifEntry.

RAM size. unknown or unsupported protocols. use the command show version. . input and output errors. A router’s RMON agent can be very useful for monitoring the number of checksum. To compute the total RAM on the router. . and identify the version and memory size as shown below. Version 11.5. then change the sampling period to a longer time (e. etc. 30 seconds to 60 seconds). If the amount of traffic generated by RMON seems to be too high. simply add the two parts of the RAM size rating: this router has 32MB of RAM. Alarm number 1 defines a sampling period of every 30 seconds for the number of discarded packets inbound to the Ethernet interface stored at table entry 1 or instance 1. An upgrade can be beneficial for security. This section also presents information about backing out of an upgrade.g. booted via flash cisco 3640 (R4700) processor with 28672K/4096K bytes of memory. RELEASE (fc1) Copyright (c) 1986-1998 by cisco Systems. Performing Cisco IOS Software Updates This sub-section outlines the motivations and procedures for upgrading the system software on a Cisco router.Router Security Configuration Guide The interface entry.113-4.T1".13.1c . To determine the current software release running on a router. . the number of inbound packets discarded.3(4)T1. Central> The underlined portions of the transcript are the software version. . 158 Version 1. router model. ifEntry. It is important to know the router model and memory sizes before attempting to obtain a software upgrade. It is important to note that most Cisco updates can only be accomplished by replacing the IOS software running on the router. there is no facility for amending or patching installed IOS software. input and output discarded packets. System image file is "flash:c3640-i-mz. respectively.5. Inc. but if done improperly it can leave a router vulnerable. 8192K bytes of processor board System flash (Read/Write) . . The agent monitors increases of forty discarded packets or more starting from the last value sampled. identifies variable ifInDiscards. 4. RMON may be very data intensive depending on the number of monitored variables and the length of the sampling period. and flash memory size. Central> show version IOS(tm) 3600 Software (C3640-I-M).1.

Print the summary web page. depending on router model and other factors. or to enforce your local security policy.1c 159 . release status. Third. the minimum downtime will range from at least a minute to several minutes. To fix known vulnerabilities – when security vulnerabilities are found in Cisco IOS products. be prepared to back out if the performance has suffered. If the performance benefit is greater than the cost of upgrading. either by implementing new features or by reducing available free memory. If you have a maintenance agreement with Cisco. router memory size. then do the upgrade. for the IOS image you’ve Version 1. the router must be out of service for at least a short time during the installation process. to ensure that the new software can fully support the router functions your network needs. which will include the length. Second. see Section 8. then measure the performance before upgrading. they might be broken or simply unsupported. then it makes sense to upgrade. If you need one or more of these features to support your network. During the software selection and download sequence at Cisco’s web site. To improve performance – you might need an upgrade to support new hardware or hardware features. Software updates entail substantial costs. cost. The logistics of purchasing updates is beyond the scope of this document.3.Advanced Security Services Motivations for Updating Router Software Installing an IOS update entails inconvenience and the risk of disruption of service. a new release may degrade performance. one solution may be to upgrade to a later edition of the IOS software. For more information about Cisco IOS release types. you must take many factors into account: feature availability. Deciding which update to pick is a complex topic. It is very important to read the release notes for a new release and test it carefully before installing it for operational use. If the performance of your router is critical. 3. To support new features – Cisco has added new operational and security features to each new IOS release. Obtaining Updates Cisco makes software updates available through a variety of purchase and maintenance mechanisms. it is best to check the length after downloading. some features may not work in a newer release. you can download updates from the Software Center on the Cisco web site. Weigh the benefits of upgrading against the risks before you start. First. 2. and bug history. you will be given the length of the release in bytes. 1. and again afterwards. The list below describes some good reasons for installing an update. Whenever you download Cisco IOS software (often called an IOS “image”).

Check your TFTP. Use the commands show version and show flash to check the amount of memory your router has. When possible. (If the router to be upgraded is running IOS 11. 4. Every Cisco IOS release has minimum memory requirements. and may impose much longer downtime (up to half an hour if things go wrong and you have to back out). Cisco IOS 12. Before You Perform the Update Check all the items below before installing a new IOS image on your router. Be sure that you are familiar with all the IOS commands involved. Cisco routers have two fundamental kinds of memory: RAM and Flash. you will need to have an SSH server running on a host in your network. it is not a secure service. The summary page will also include the MD5 hash value for the IOS image. a major upgrade will require more memory. Review the entire procedure before you start. then disable it again.3 or earlier.) While TFTP is supported by all IOS versions. Router software updates are usually performed using TFTP or FTP. RCP. Make sure that the TFTP or FTP server is correctly set up for both upload and download. Most newer IOS releases support SCP. and inform the user community as needed. below. discard the file and download it again. Schedule your downtime.1c . consult [12]. After downloading the IOS binary file. Do not install an update unless the router to be upgraded satisfies the memory requirements for both RAM and Flash. If possible. because many Cisco routers are configured with just enough memory to run the IOS version pre-installed at the time of purchase.) 2. or FTP configuration.Router Security Configuration Guide selected. If the length or hash of your file differ from the summary page. For more information. it is prudent to configure operational routers with as much memory as they can hold. Copy the new Cisco IOS software into the server’s download directory. check the length against the printed page. use SCP or FTP for performing Cisco upgrades. 160 Version 1. a secure file copy protocol based on SSH. Installing an update imposes a minimum downtime.0 supports FTP. use the md5sum command. or a similar checksum utility. To use SCP. (Often. Ensure that you have enough memory. then FTP will probably not be available. to check the hash value. Enable TFTP only for the update sequence. 1. 3. and should not be running in a secure network. Schedule your upgrade ahead of time. Read the entire upgrade procedure. most earlier releases do not.

The sequence has three phases: backup. and test. involves checking that the new software is running the old configuration successfully. It is best to perform router updates from the system console rather than from a network login. by following it you can avoid mishaps. Central# config t Central(config)# interface eth 0/0 Central(config-if)# shutdown Central(config-if)# end Version 1. The steps are described below. then disable the outside network interfaces using the shutdown command. Log in on the router console. the router will attempt to log in as user “anonymous”). The test phase. abandon the update and check the server configuration before trying again. The install phase. do not proceed. install. make a record of them. Update Procedure This section presents a suggested sequence of steps for installing Cisco IOS software.Advanced Security Services If possible. Shut down external interfaces. The console will show important status messages in the later steps of the installation that would not be visible otherwise. Check the current IOS version number and flash contents with the commands show version and show flash. step 4. 0. If the router to be upgraded is a border router. take advantage of that to perform the update without disrupting service. steps 1-3. The sequence is very conservative. involves copying the running IOS software and configuration onto the TFTP server host for safekeeping. set an FTP username and password that will work on your server (otherwise. it is safest to replace a router and take it offline for update. If this step fails. If a redundant router or a hot spare is available. Copy the router’s current IOS image to the server using the copy command as shown below. steps 56. Enable privileges. followed by a console transcript of a successful update. Central# copy flash: tftp: or Central# copy flash: ftp: You will need to supply a file name and the IP address or host name of the TFTP or FTP server host. For example: Central(config)# ip ftp username uploader Central(config)# ip ftp password Y3ti4ttack! Central(config)# end 1. If you are planning to use FTP. involves loading the new software. The backup phase.1c 161 . 2. and ensure that you can restore your previous IOS version if necessary. confirm the current IOS and boot version. and back up the current IOS software.

the flash will be erased automatically during this step. Central# 162 Version 1. Watch the boot messages on the router console to confirm the new IOS software version and boot image. . enable privileges and confirm the configuration status with show running-config. 4. . you can specify which one to use at boot using the boot system command. Central# show ip interface .1c . . If this step fails. Central# copy tftp: flash: or Central# copy ftp: flash: On some Cisco routers. Central# show running-config . do not proceed. Central# copy startup-config tftp: or Central# copy startup-config ftp: You must supply the IP address or host name of the server host. Central# reload Proceed with reload? [confirm] y 5. Confirm the new IOS version and boot image. if it does not. Back up the current running configuration. If you performed steps 1 through 4 over a network connection. Copy your current startup configuration to your TFTP or FTP server using the copy command as shown below. Version 12. Central# show version Cisco Internetwork Operating System Software IOS(tm) 1600 Software (C1600-SY56I-M). Load the new software. and check that the access lists and static routes are still present. answer yes. On some Cisco routers. Check the status of the interfaces. . your connection will be broken at this point. re-establish the connection at this point and check the IOS version and boot image with show version. your router may automatically reboot. RELEASE SOFTWARE .0(9). . abandon the update and check your server configuration before trying again.Router Security Configuration Guide 3. if asked whether to erase the flash. If you are performing the update over a network connection. Copy the new IOS software from the TFTP or FTP server to the flash memory of the router. Then. Use the copy command as follows. (If you have several IOS images in flash. . then reboot it manually using the command reload. it is possible to store several IOS releases in flash memory and select which one to run.) If this copy succeeds.

Transcript of a Successful Update Procedure The recorded transcript below shows an upgrade of a Cisco 3640 router from IOS 11.T1 [3208612 bytes used. 5179996 available.113-4. 8388608 total] 8192K bytes of processor board System flash (Read/Write) South> enable Password: South# copy flash: tftp System flash directory: File Length Name/status 1 3208548 c3640-i-mz.T1. if necessary. one per line.3(4) to 12.2.9.T1' from Flash to server as 'c3640-i-mz-113-4.T1]? c3640-i-mz-113-4.113-4..T1' (file # 1).113-4. South>show flash System flash directory: File Length Name/status 1 3208548 c3640-i-mz.113-4.1c 163 . Central# config t Central(config)# interface eth 0/0 Central(config)# no shutdown Central(config)# end Depending on network speed and router model. Note that. they should have come back up as part of the reload in step 4.3(4)T1. 5179996 available. . Consult the release notes for the particular router for details. If you shut down your router’s external interfaces in step 2. this procedure may take about 5-20 minutes.0(5). RELEASE SOFTWARE (fc1) .6]? 14. additional hardware-specific steps may be needed. South(config)# interface ethernet0/1 South(config-if)# shutdown South(config-if)# exit South(config)# exit South# Version 1.bak Verifying checksum for 'c3640-i-mz. for some older Cisco router models.bak'? [yes/no]yes !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Upload to server done Flash device copy took 00:00:19 [hh:mm:ss] South# config t Enter configuration commands. If the second command in step 5 showed that they did not come back up.T1. End with CNTL/Z. 8388608 total] Address or name of remote host [14. Version 11.9.Advanced Security Services 6.T1 [3208612 bytes used. OK Copy 'c3640-i-mz. Bring up external interfaces.6 Source file name? c3640-i-mz.113-4.T1 Destination file name [c3640-i-mz.113-4. then bring them back up now using the command no shutdown. South> show version Cisco Internetwork Operating System Software IOS(tm) 3600 Software (C3640-I-M).2..

6? [confirm] Building configuration. Inc.bak Write file south-config.T1 [3208612 bytes used.2. size: 0x74d170 Self decompressing the image : ############################################# [OK] . South> South> show ip interface brief Interface IP-Address OK? Method Status Protocol Ethernet0/0 14.bin' from server as 'c3640-ik2o3s-mz_120-5_T1.255.2...64 YES NVRAM up up Ethernet0/1 14.2. .250 YES NVRAM up up Ethernet0/2 unassigned YES NVRAM administratively down down Ethernet0/3 unassigned YES NVRAM administratively down down South> enable Password: South# show running-config Building configuration.9. Writing south-config.113-4.2.1(19)AA..9.7656076/8388608 bytes] Verifying checksum.bin Accessing file 'c3640-ik2o3s-mz_120-5_T1.6 (via Ethernet0/0): !!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! [OK . 8388608 total] Address or name of remote host [255.2. 5179996 available..Router Security Configuration Guide South# copy running-config tftp Remote host []? 14. C3600 processor with 32768 Kbytes of main memory Main memory is configured to 64 bit mode with parity disabled program load complete.1c .255]? 14. eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee .erased Loading c3640-ik2o3s-mz_120-5_T1.9.bin Destination file name [c3640-ik2o3s-mz_120-5_T1. OK (0xDC3B) Flash device copy took 00:00:50 [hh:mm:ss] South# reload System configuration has been modified.6.bak !! [OK] South# copy tftp flash System flash directory: File Length Name/status 1 3208548 c3640-i-mz.9. EARLY DEPLOYMENT SOFTWARE (fc1) Copyright (c) 1998 by cisco Systems.bin' into Flash WITH erase? [yes/no]yes Erasing device..6 (via Ethernet0/0): ! [OK] Erase flash device before writing? [confirm] Flash contains files..bin from 14..6 Name of configuration file to write [south-confg]? south-config. entry point: 0x80008000.9.. Save? [yes/no]: no Proceed with reload? [confirm] y %SYS-5-RELOAD: Reload requested System Bootstrap.bin' on 14.bak on host 14.10..9. Are you sure you want to erase? [confirm] Copy 'c3640-ik2o3s-mz_120-5_T1...2. 164 Version 1. Loading c3640-ik2o3s-mz_120-5_T1.bin]? c3640-ik2o3s-mz_1205_T1.2.2.6 Source file name? c3640-ik2o3s-mz_120-5_T1.255. Version 11..9.bin from 14.

In step 3. but the simplest is to ensure that the TFTP traffic does not traverse hostile networks.6). . it is critical that you protect the TFTP transaction and server from potential attackers. .g. Note that. you transmit a copy of your router configuration to a TFTP server. restore the backup copy of the configuration that you saved during the upgrade procedure step 1. South# exit Backing Out an Update If functional testing reveals a problem with your router after an upgrade. Central# .1) or to use a centralized authentication server (see Section 4.1c 165 . First.Advanced Security Services Current configuration: ! version 12.0). starting with step 2.2 to 12. do not leave TFTP enabled on your host. . Simply follow the procedure described above. load the backup copy of the old IOS software. your stored configuration might not work correctly when you fall back to the older version. many default settings differ between various IOS releases. . Also. restore old configuration copy tftp running-config Additional Security Concerns There are several security issues surrounding upgrades. Because TFTP provides no security. always turn it off immediately after you finish the installation procedure. copy tftp flash reload ! Optional. In that case. In step 4. The simplest approach to mitigating this risk is to change the enable secret immediately after installation (see Section 4. from 11. you may be exposing your encrypted passwords to disclosure. Also. Central# . There are several approaches to doing this. this section attempts to address them. Central# Central# . whenever you make any kind of backup copy of a router configuration. If the router supports SCP. if you have upgraded from one IOS major version to another (e.3). Some of these settings can affect your router’s security. some newer versions offer services not present in older versions (see Section 8. use a different name than you used during the upgrade procedure. Version 1. if you follow the installation procedure outlined above. Third. Second. always use it instead of TFTP.0 . you may need to return to your old IOS version.

5. and they offer a great deal of visibility into the operation of the router. so use them sparingly. so the first few messages put into the log reflect startup activity. and some is particular to each interface.Router Security Configuration Guide 4.5. • Debugging commands – Virtually every IOS facility and protocol has associated debugging commands. consult Section 4. The output consists of two parts: a summary of the current logging configuration. East# show logging 166 Version 1. These commands can also be used to help verify that security measures are in force.2.1c . There are IOS commands to display these statistics that can be used to detect problems. For more discussion of logging. even on the simplest Cisco routers. and the log messages. Diagnosing and Debugging Router Operation Effective logging and SNMP help an administrator to stay aware of their routers’ status and operational condition. identify sources and causes. • Router throughput and traffic commands – Each interface. an unauthorized attempt to telnet to the router itself has been logged by access list 131. Router Status and Configuration Commands Each of the items below describes a single status query. so recent messages are at the end of the listing. keep input/output statistics. There are literally hundreds of such queries available. 1. use the command show logging. When a problem occurs. This short sub-section describes some of the most useful techniques for IOS 11. and some other facilities. Viewing the current log – To view the current buffered log messages.6. In the example below. The techniques fall into three groups: • Router status and configuration commands – These commands display information about the settings and tables held by the router. Cisco IOS diagnostic and debug facilities can be used to get vital information. and are useful for understanding a Cisco router’s disposition in a typical TCP/IP network. The messages are shown in the order they occurred. These commands typically produce a correspondingly great deal of output.3 and later. see [2] and [7]. some of this information is global to the whole router. The ones listed here are commonly used for simple troubleshooting. for a discussion of some other useful ones. or when a network is under attack. and validate repairs. The buffered log messages are cleared when the router reboots. Testing and validation are covered in Section 6. Techniques for troubleshooting and debugging routers could (and do) fill entire books.

Ethernet0/0 14. Ethernet 0/0 Route codes Destination gateways Figure 4-11: Interpreting a Route Table Listing Version 1. Ethernet0/1 . the destination gateway is simply the next hop on that route.6.250. 2d18h.250 to network 2d19h. 1d01h. Ethernet0/0 14.0. this list may be very large.0.6. Mar 3 12:51:52 EST: %SEC-6-IPACCESSLOGP: list 131 denied tcp 172.9. 1d13h. Ethernet0/0 172. 2d18h.0 [110/10] via 14. 2. Viewing the current route table – To view the current route table.0 [120/1] via O IA O IA O C O E2 C R O*E2 7.0/8 [110/14] via 14.1. In a router using NTP.1.2.1. 56 message lines logged Monitor logging: level debugging.0.0.0/0 [110/3] via or were added by a configured routing protocol (codes R. 33 message lines logged Log Buffer (16000 bytes): 00:00:17: %LINK-3-UPDOWN: Interface Ethernet0.1c 167 . 1 packet East# Note: log messages should always include the time of the event. 56 message lines logged Trap logging: level informational. Each route code identifies how one route joined the table.18.250(47746) -> 0. 00:01:05. O. Ethernet0/0 7. see Section 4.17. because the messages are generated before NTP has synchronized. Ethernet0/1 14.1.4). Note that the route table listing on an operational router will often be much longer than this sample.0.10.1. 32 message lines logged Buffer logging: level debugging. Check the route codes to make sure that all the routes joined the table either directly (code C). the first few log messages will include the time since boot instead of the correct time. 33 message lines logged Logging to 14.1. A very important part of reviewing the route table is checking the route codes and checking the destination gateway.1.0.Advanced Security Services Syslog logging: enabled (0 messages dropped. Figure 4-11 shows how to interpret the output of show ip route.0. changed state to up . [110/11] via 14.1.0(23).0 is directly connected. 0 flushes) Console logging: level debugging. and others.2.1. Ethernet0/0 is directly connected. use the command show ip route.250. Gateway of last resort is 14.0/8 [110/12] via 14. Depending on the size of the network and the kinds of routing protocols used.0.2. or were added as static routes (code S).

and DNS.20 1 FULL/DR 00:00:33 14.2.64 43 Internet 14. Locally defined names take 168 Version 1.127a 0050. and two are logged in over the network. . and unauthorized devices on a network segment can often be detected by their presence in a router’s address resolution (ARP) table. use the command show arp.3680 0010. Central# Hardware Addr 0004.4 for some recommendations. Central# show arp Protocol Address Age(min) Internet To display the ARP table.7bf9.2.2. as in the example below. The command show ip protocol summary gives a quick overview.acd5.1. Viewing the routing protocols in use – The command show ip protocol gives a verbose listing of the route update mechanisms currently used on the router.1c .1.1.20 idle 00:00:00 57 Internet 14. The example below shows the IP routing protocol summary and (abbreviated) output for a useful OSPF status command. there is one user logged in at the console.9.20 Eth0/0 14. see Section 4.1.baa0 Type Interface ARPA Eth0/1 ARPA Eth0/0 ARPA Eth0/1 ARPA Eth0/0 5.f3f6 0010.1.0f03.Router Security Configuration Guide 3. The output is different for each kind of protocol. Viewing the current ARP table – Extraneous devices.2. Central# Line 0 con 130 vty *131 vty Central# show users User 0 jsmith 0 andrew 1 neal Host(s) Idle Location idle 00:00:56 idle 00:01:02 14.250 1 FULL/DR 00:00:38 14.250 53 .2.2. Viewing host name and name lookup information – Cisco IOS uses two mechanisms for mapping between IP addresses and names: locally defined names.6 6. mis-connected devices.250 Eth0/0 Central# 4. In the example output below. All of the individual routing protocols also have extensive status commands.9.2. Central# show ip protocol summary Index Process Name 0 connected 1 static 2 ospf 1 3 rip Central# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 14. Viewing the logged in users – The command show users displays a list of users that are currently logged in.1.20 10 Internet 14.

Viewing line status – Every Cisco router has at least one physical line connection.15.0. the telnet vty lines.6 514 14. but can provide valuable clues to the services that the router is actually providing.1. slightly abbreviated.250 YES Ethernet0/1 14.9.1 36269 14. Viewing interface status and configuration – Use the command show ip interface to view a verbose display of the status and configuration of a router’s network interfaces. OK) Central# Age 4 ** 52 Type IP IP IP Address(es) 14.2. 14.1. The output is a little cryptic.1.4).1. for instance.1. The example below shows the brief output format. and typically five virtual line connections.20 14.1.250 YES Ethernet0/2 unassigned YES Ethernet0/3 unassigned YES Central# Method NVRAM NVRAM unset unset Status up up down down Protocol up up down down 8.9. In all cases.1. the console.15. the listing will include both active and inactive interfaces. To display the full status of a line. Use the command show host to display the DNS configuration and the list of locally defined names.15.2.0 123 123 17 Host Flags east (perm.2.250 161 17 0.0.Advanced Security Services precedence over DNS names. 9.1.1. use show line name number. Central# show host Default domain is not set Name/address lookup uses domain service Name servers are 14.15.0 520 14.15. OK) central (perm. Viewing currently open UDP sockets – Use the command show ip socket to list the currently open UDP network service sockets on the router.1c 169 .0.250 6082 Central# In Out Stat TTY 0 0 1 0 0 0 1 0 0 0 1 0 0 0 10 132 Version 1.1.9. show line aux 0.64 7. Central# show ip interf brief Interface IP-Address OK? Ethernet0/0 14. The example below shows the output for a router running fairly few services. Use the command show line to display a summary of lines available on a router (see Section 4.250 14. Central# show ip sockets Proto Remote Port Local Port 17 0.250 520 17 14. use the command show ip interface brief.2.9. OK) south (perm. For a quick look.

one per line.3 and earlier. The fourth line is the logging client. sending syslog messages to a Unix host (remote port 514). The third line is the network time service (NTP. This can help with problems. 11. such as Unix diff or Windows fc.Router Security Configuration Guide The first line is the RIP route protocol service (local port 520). these two configurations should be very similar. FTP is not supported. to highlight the differences.0. using IOS 12.5. but TFTP can be used for making archive copies in a very similar manner (see Section 4.9.txt !! 5699 bytes copied in 12.5). The output is usually quite long. Normally. End with CNTL/Z. The second line is the SNMP service to a host running an SNMP/RMON management tool (local port 161). port 123). Check for unwanted processes and services. The resulting output will typically be fairly long. Archive a copy of the configuration after any major change. use the command show running. and also shorten downtime if the router loses its stored configuration. Central# config t Enter configuration commands.2. Viewing currently running processes – Many IOS services and facilities run as separate IOS processes. Use the command show process to list the running processes.1c .1 Destination filename [central-confg]? central-config. If the configurations are very large and complex. Central(config)# ip ftp password 0 r0ut3rQQ Central(config)# ip ftp user rscg Central(config)# exit Central# copy running-config ftp Address or name of remote host []? 14. To view the saved startup configuration (in NVRAM) use show startup. it should be used with care and disabled when not in use. Another way to get an archive copy of the running configuration is to use text logging features of Telnet and terminal emulation applications.txt Writing central-config. and review the hardcopy. Because TFTP is insecure. save the command results to a file. or on a monthly basis. The example below shows how to save an archive copy of a configuration to an FTP server. use a file comparison tool. print it.716 secs (474 bytes/sec) Central# In IOS 11. Viewing the current configuration – To view the current running IOS configuration. 10. 170 Version 1. To review a configuration in depth.

This will display a comprehensive report on the traffic through all the interfaces. 0 frame. or component failure. 0 underruns 0 output errors. .1c 171 . . use the command show interface. simply supply that interface name to the command. Viewing the network traffic on a per-interface basis – To view the total traffic for each interface. 0 output buffers swapped out Central# If traffic volume monitoring is important for a particular interface.0 runts. 38371898 bytes. The output is quite long. Most of these commands produce voluminous but clearly formatted output. 991606 packets input. Viewing IP Protocol Statistics – To display a long listing of IP and related protocol traffic statistics. 1. Central# show interface eth 0/0 Ethernet0/0 is up. but can reveal certain classes of attacks.Advanced Security Services Router Throughput and Traffic Commands The commands listed below display various traffic statistics that can be useful in diagnosing router traffic flow. 0 no carrier 0 output buffer failures. The example below shows how to clear the counters for a single interface. 0 ignored 0 input packets with dribble condition detected 480919 packets output. 0 overrun.15. To view the traffic for a single interface. Viewing SNMP Protocol Statistics – To display the SNMP messages statistics and configuration. address is 0050. use the command show ip traffic. 0 no buffer Received 800624 broadcasts. 215 deferred 0 lost carrier. 0 CRC.1.0 throttles 0 input errors. . 103806395 bytes. Last clearing of "show interface" counters 23:20:53 . line protocol is up Hardware is AmdP2. misconfiguration.0 giants. There are hundreds of traffic and processing status commands in Cisco IOS. Central# clear counter Eth 0/0 Clear "show interface" counters on this interface [confirm]y Central# 2. 0 late collision. Clearing the counters sets the traffic volume record back to zero for both input and output. see [7] for more information about some of them. 3. The example below shows the output format for a single Ethernet interface. Understanding normal network and link traffic loads can be critical for identifying anomalous conditions that are indications of attacks. use the very Version 1. then clear the counters on a periodic basis. 78 collisions. .cbe0 Internet address is 14.7357. 1 interface resets 0 babbles.250/24 .

facility. Also. If the output shows any SNMP traffic. The example below shows the output for a router with a very modest amount of SNMP traffic. be sure to configure the buffered log and syslog for level 6 (informational). The number of messages generated by debugging can often be quite large. The example below shows how to configure debugging and turn on debugging messages for ICMP. all activities of that protocol or feature will generate log messages at level 7. When debugging is enabled for a particular protocol or feature. when using the debug messages for interactive troubleshooting.Router Security Configuration Guide simple command show snmp. and the network does not have an SNMP infrastructure deployed. Central# show snmp Chassis: Central Contact: Vanessa & Phyl Location: second floor 73 SNMP packets input 0 Bad SNMP version errors 0 Unknown community name 0 Illegal operation for community name supplied 0 Encoding errors 263 Number of requested variables 0 Number of altered variables 10 Get-request PDUs 63 Get-next PDUs 0 Set-request PDUs 73 SNMP packets output 0 Too big errors (Maximum packet size 1500) 2 No such name errors 0 Bad values errors 0 General errors 73 Response PDUs 0 Trap PDUs SNMP logging: disabled Central# The only way to clear these SNMP statistics is to reset the router.9. are sent to all log sources configured to receive them.1c . Debug messages. Central# show users Line User *130 vty 0 rscg Central# config t Host(s) idle Idle Location 00:00:00 14. then the router may have been subjected to an SNMP sweep by an attacker. Router Debug Commands Cisco IOS offers a very extensive suite of debugging commands. or feature of the router. Each debugging command is associated with a particular service. debugging can impose a substantial computational burden. when generated.2. and should be used sparingly on operational routers. Therefore.6 172 Version 1.

159 EST: %SYS-5-CONFIG_I: Configured from console by rscg on vty0 (14.9. Version 1. O’Reilly Associates. An excellent introduction to basic IOS operations. The section on “File Management” provides extensive information on downloading updates. The specification for NTP version 3. including examining the configuration and operation of a Cisco router. References [1] Albritton.2. 1999. 1995. 1997.7. A practical introduction to the concepts and practices for using Cisco routers. one per line. 1999. Cisco Press.. Managing IP Networks with Cisco Routers.M. src 14.Advanced Security Services Enter configuration commands. D. dst 14.0 Configuration Fundamentals.6 Central# no debug ip icmp ICMP packet debugging is off Central# The Cisco documentation set includes a volume with comprehensive information about the debug facilities and their behavior. 4. Consult this book for information on upgrade paths and compatibility.250.M. [4] Mills. McGraw-Hill. The sections on “Performing Basic System Management” and “Monitoring the Router and Network” include valuable advice on how to configure basic features and services. Cisco Press.9. An amazingly detailed book about IOS versions and the IOS release process. the version supported by IOS 11 and 12. Central(config)# logging console information Central(config)# logging monitor debug Central(config)# logging buffered information Central(config)# logging trap information Central(config)# exit Central# Mar 3 17:01:58. 2000. the Cisco IOS Debug Command Reference.2.5. “Network Time Protocol (version 3)”.9. [5] Coulibaly.2. [3] Ballew. “ping central” was performed on 14.1c 173 . End with CNTL/Z.9. J. Cisco IOS Essentials. S.2.6) Central# terminal monitor Central# debug ip icmp ICMP packet debugging is on Central# ! At this point. Cisco IOS Releases: The Complete Reference. M. RFC 1305.6 Mar 3 17:02:13 EST: ICMP: echo reply sent. [2] Cisco IOS 12.

[7] McGinnis. Cisco Press.networkcomputing. This Cisco guide for Internet Service Providers includes a good discussion of IOS upgrades. available at: http://www.html This note offers guidance on NTP hierarchy design and tracking performance metrics. B. 174 Version D. A very specific guide to configuring many IOS features securely. Network Computing.. April 2002. [10] Shipley. [8] Greene. J. Prentice-Hall. Cisco Systems. 1999.shtml This Cisco security advisory provides detailed information for dealing with the 2002 SNMP vulnerability on Cisco IOS devices. Understanding SNMP MIBs. Prentice Hall. A Practical Guide to SNMPv3 and Network Management. E. P.. “Getting in Sync: A Look at NTP”. J. A detailed exploration of the SNMP management information base. May 2002. and Wright.. including good coverage of the SNMP basics and SNMPv3 security features. January 1999.. with some configuration suggestions. An in-depth study of SNMPv3 and its Cisco ISP Essentials.. available at: http://www. [12] Stewart. especially for initial set-up of a new router. D. and Smith.Router Security Configuration Guide [6] Zeltserman. Feb 2002. available at: http://www. Cisco Technical White Papers. [11] “Cisco Network Time Protocol .Best Practices White Paper”.com/warp/public/126/ntpm. and Perkins. [9] “Malformed SNMP Message-Handling Vulnerabilities”. It also shows how to set up an IOS router as an NTP master. Cisco Security Advisory. including both standard and vendor-specific This magazine article from 1999 gives a clear and readable overview of NTP. Includes guidance on using SCP for updates. G. 2002. SANS Institute. Securing Cisco Routers: Step-by-Step. 1996.1c . Cisco Systems. 1st edition. as well as examples of router status and diagnostic commands.

when flexible authorization capabilities are required. Cisco provides this security with their authentication. selectively. authorization. Security for Router Network Access Services Security for Network Access Services deals primarily with controlling remote users who are accessing local resources. authorization. RADIUS. AAA is designed to allow the administrator to configure its services globally or by line and interface. Basic Concepts. when accounting is required. or 3. Version 1. and many releases also support Kerberos. AAA allows you to employ. When AAA services are enabled on a Cisco router.6. AAA is the mechanism Cisco recommends for access control. except Kerberos does not accept accounting records. Centralized authentication. authorization. when there a large number of routers so that centralized administration becomes advantageous.6. Three conditions make using network security servers a good choice: 1.1c 175 . TACACS+. All Cisco IOS releases support the RADIUS and TACACS+ network security services. An Internet Service Provider would be a good example of this. and provides additional capabilities for auditing of network service usage. But the majority of this section will cover using Cisco’s AAA services for controlling administrative access to a router and the security server protocols. and line passwords. This means that you can no longer access the commands to configure the older protocols (including login local and login commands).1. and Support Mechanisms Cisco’s authentication. and accounting services provide critical security functions necessary for providing remote access to routers and network resources. By using AAA along with a security server you can control access to routers and other network services from a centralized location. both network security services and security information local to the router. and Kerberos security services provide the facilities required for AAA. AAA allows you to base authentication decisions on the router’s local user database. Overview.Advanced Security Services 4. 2. and accounting (AAA) services. This allows for easier management of user accounts and privileges. The sub-section below dealing with dial-in users will give an introduction to controlling remote users accessing network resources. In addition to network security services. Configuration is performed by using method lists as described further below. the older forms of access control are disabled. and accounting are particularly important when your organization has many routers and other network devices to manage. enable. Where the older access control mechanisms dealt almost solely with user authentication. 4. AAA also has the ability to control each user’s access to resources and provides additional accounting capabilities beyond the router’s logging facilities.

local username database. 3.2 demonstrates how to set up AAA authentication.6. AAA authentication provides the means for identifying users through login/password dialogs. enable passwords and none. Kerberos. and supported token technologies.1c . line passwords. Configuring AAA authentication entails four basic steps: 1.1). etc) will not be possible. to use security server protocols or backup authentication methods you must use AAA authentication. 2. and configure local user or line password support only as fallback mechanisms for when the network security services are unavailable. see the sub-section on “Method Lists” below. For more information. Define one or more method lists for AAA authentication. AAA authentication is set up using method lists. The default method list will be automatically applied to all the lines and interfaces for which a named list was not applied. Authentication Authentication is the mechanism for identifying users before allowing access to network components or services. Section 4. TACACS+. authentication controls the ability of a user or another network component to access a network device or service. Although authentication can be configured without using AAA (see Section 4. Named lists must be applied to the appropriate lines and interfaces.Router Security Configuration Guide Note: When using the local user database instead of a network security server. The authentication method list defines the types of authentication to be performed and the sequence in which to apply them. but console access will be allowed. AAA is very limited in its authorization capabilities and provides no mechanism for accounting. In other words. SSH. For AAA authentication the available methods are RADIUS. 4. The following sections will discuss the three main faculties provided by AAA and their supporting concepts.2. challenge/response mechanisms. in this case. if no local usernames are defined then remote administration (via Telnet.6. Enable AAA (new-model). it is best to use capable and well-managed network security services as your primary AAA mechanisms. Configure security server network parameters. This can be done by a combination of named lists and the default list (see the sub-section “Method lists” below for a complete listing). When AAA authentication has not been set up the default will use the local username information. 176 Version 1. Examples in this section will use a subset of the main network diagram as shown in the "Putting It Together" sub-section in 4. Apply the method lists to a particular interface or line (optional). Therefore.

AAA does not protect the confidentiality of the password during the trip from the remote administration host (e. AAA authorization types are: • exec – which controls the users ability to run an EXEC shell. There is a special case for the console line. Authorization lists are specific to the authorization type which is being defined. PPP NCPs. The RADIUS and TACACS+ security servers. and ARA protocols.4. as discussed in Section 5. Authorization includes one-time authorization. if-authenticated. Default method lists are applied to all lines and interfaces for that particular authorization type. are very limited. authorization for each service. and local methods. (The last two methods. Prerequisites for AAA authorization are: enable AAA services. if-authenticated and local.Advanced Security Services NOTE: The AAA network security protocols each include mechanisms (more or less effective) for protecting the confidentiality of passwords during the exchange between the router and the security server.g. • commands <level> – which controls access to all the commands at the specified privilege level. define security servers. then the security server database will be used to control access to various network components and services as defined by the stored attributes. method lists define what authorization protocols will be used and in what order. Authorization method lists can include RADIUS and TACACS+ security protocols along with Kerberos Instance Maps.6. Additionally. • reverse-access – controls access to all reverse access connections like reverse Telnet. must be applied to the interface or line to be invoked. • network – enables authorization for all network related services like: PPP. use attribute-value pairs to define a user's rights. After a user logs in and has been identified by authentication. SLIP.) As with authentication. Authorization is the method used to describe what a user has the right to do once they are authenticated to the router. other than “default”. PC on the administrator’s desk) to the router. If no authorization list is defined for the authorization type then no authorization will occur for that type. Authorization Authorization controls access to system resources. configure AAA authentication (since authorization relies on authentication's output). But named method lists. Version 1. Authorization works by creating a list of attributes which describe what the user is allowed to do. authorization can only be configured using AAA. and define the rights for each user.1c 177 . To prevent passwords from being exposed in the clear you must use a secure remote administration with a protocol like SSH or IPSec. and authorization for each user. as described in Section 4. if a user has been authenticated when logging into the console line then authorization will not be used (even if configured).

Accounting records will include the command. connection. If accounting is turned on for level 15 and user logged in at enable level 15 runs a level 1 exec command no accounting event will be generated. There are several types of accounting which can be enabled and configured separately: exec. Cisco IOS does not support command acccounting with RADIUS. date. you configure AAA accounting by defining a list of accounting methods. • connection accounting – Provides information about all outbound connections made from the network access server. All types are supported by TACACS+. the usage start and stop times. and the user. network. and ARAP protocols. date. Accounting AAA accounting is used for logging and tracking the activities of users (people or other network components) using a network resource.1c . Information in an accounting record includes the user’s identity. 178 Version 1. • network accounting – Provides information for PPP. command. security analysis.Router Security Configuration Guide Section 4. time. The list will define the list of accounting methods for the indicated accounting type. This includes telnet. etc. IP address of access server. If the list was a named list then it must be applied to the appropriate lines and interfaces. SLIP. For an accounting type. The information includes the number of packets and bytes. and reporting. This would include information like system reboots. These logs can be used for network management.6. The information includes the username. AAA accounting can only use the TACACS+ or RADIUS security servers for record logging. Routers send their accounting records to the security server for storage.2 shows an example of configuring AAA authorization. start and stop times. For more detailed information about configuring authorization using AAA. and telephone number the call originated from for dial-in users. rlogin. resource usage tracking. and the command that was executed. if a default list is not defined and a named list is not applied to the line then no accounting will occur for that type on that line. • system – Provides information about system-level events. accounting being turned on or off. but RADIUS does not support command or system. number of packets and bytes. As with authentication and authorization. • EXEC accounting – Provides information about user EXEC sessions on the router. Account records are generated based upon the level of the command not the level of the user. system. • command accounting – This applies to commands which are entered in an EXEC shell. refer to the “Configuring Authorization” chapter in the IOS Security Configuration Guide [1]. This option will apply accounting to all commands issued at the specified privilege level.

security servers are defined. When a method list is specified using the default keyword the list will be applied to all the appropriate interfaces and lines automatically. Method Lists and Server Groups Method lists are used to specify one or more security protocols or mechanisms for AAA. (Note: the local user database is treated as a special case. Each accounting record is comprised of accounting AV pairs and is stored on the access control server. Methods requiring only a password (e. the “line” method) should never be placed ahead of methods requiring a both username and password. AAA accounting requires that AAA is enabled.2 gives an example of configuring accounting. because the user will never be prompted for a username and the mechanism will always fail. This continues until there is a successful communication with a listed method or the list is exhausted. like RADIUS. the first method on the list determines whether the user will be prompted for a username. and Kerberos security servers. Accounting can also be configured such that a user requested action can not occur until an acknowledgement is received from the security server stating that the accounting record has been saved.) Method lists can be given a specific name or can use the keyword “default”.6. This also means that a named method list will have no effect on a interface or line unless it has been applied to it. In the case of accounting the auditing event will not occur. Method lists also specify the sequence in which the security mechanisms should be used. For AAA login authentication. Other mechanisms. will deny access if the username does not exist. Named method lists can then be defined and then applied to particular ports or lines to override the default behavior. Section 4. especially for authentication. if the user supplies a username that does not exist in the local user database then the next method on the list will be attempted. see the IOS Security Configuration Guide [1].Advanced Security Services etc. If the list is exhausted then the operation will fail. If the Cisco IOS software is unable to complete the task due to failure to communicate with the security server or mechanism then the Cisco IOS will try the next method in the list.g. TACACS+. A negative response from a security server will also deny access in the case of authentication and authorization and the next method in the list will not be attempted. It is important to choose the right order for the methods on a method list. For more information about AAA accounting. Note that system accounting will only use the default list. including RADIUS and TACACS+ attributes. or accounting as appropriate. For AAA the Cisco IOS software will use the first method listed to perform the authentication.1c 179 . except for wait-start accounting which will also deny the user access for the service. These lists can be used to provide backup mechanisms for when the primary security method is unavailable. In the case of authentication and authorization the user will be denied access. Version 1. authorization. Cisco’s implementation of RADIUS does not support system accounting. and that a security server is specified for each accounting type which is desired.

3 was used for testing RADIUS and TACACS+ security servers.6. ! AAA method list syntax for IOS 12. as shown below.1 Security Configuration Guide. Section 4. the command for doing this is: north(config)# aaa new-model The remainder of this section will deal with configuring the three AAA services by giving concrete examples (see Figure 4-10 on page 186) and describing the rationale behind the configuration.0.2. and the FreeRadius server was used for additional RADIUS testing. simply insert the keyword “group” in front of the words “radius” or “tacacs+” when attempting to apply any of the examples.1 or later. Authentication The AAA authentication commands can be grouped into two areas which correspond to how they are applied. this section focuses on using AAA for the security of remote administration. authorization. Router Access Control The previous section introduced authentication. if you are using IOS 12. The four authentication commands used for controlling access to a router are: 180 Version 1.1 and later. ! AAA method list syntax for IOS 12. there is a small example in the next sub-section. The rest of this section uses the IOS 12. there is directly controlling authentication to the router and then there are commands for providing information about the authentication process. Section 4. First. in IOS 11.1 and later aaa authentication login remoteauth krb5 group radius local aaa authorization exec default group tacacs+ group radius aaa accounting network default start-stop group tacacs+ The definition and use of server groups is mostly outside the scope of this guide.Router Security Configuration Guide The following example shows the syntax. Use of RADIUS or TACACS+ requires the keyword group.0 aaa authentication login remoteauth krb5 radius local aaa authorization exec default tacacs+ radius aaa accounting network default start-stop tacacs+ radius In IOS 12. for named and default method lists. In order to use Cisco's AAA mechanisms you must first enable AAA services. consult the IOS 12. The name of the first list is “remoteauth”. Cisco's ACS Version 2.3 briefly covers a dial-in user example.4 describes security server protocols in more detail.6. the syntax for method lists changed slightly with the introduction of security server groups. This section will cover details of configuring AAA for controlling access to the router. AAA configuration is a broad subject.6. For detailed information about server groups. 4.3 and 12.0 syntax. and accounting mechanisms and how method lists are used to define the security protocol to use for a service.1c .

to guarantee all lines are protected. and avoid possible default behavior changes in the IOS.1c 181 . In a simple situation only one authentication list is required. This command will also require that all authentication requests to the router include a username as well as a password. • aaa authentication password-prompt text-string changes the password prompt from “Password” to the supplied value of text-string. • aaa authentication local-override is used to override all authentication method lists to look at the local database first. The authentication commands used for defining messages are: • aaa authentication username-prompt text-string changes the username prompt from "Username" to the defined value of text-string. For setting a banner on all terminals use the banner motd command as suggested earlier in Section 4.4. You may choose to include ‘local’ on your method list. Including a local method will guarantee that if the security server(s) is not available.1.) • (line): login authentication {default | list-name} is required to apply a named login authentication method list to a line. Another bad example would be using an informational banner to identify the router as your border router and list the protocols it accepts. This section will concentrate on the four authentication commands for controlling access to the router. administrators will still be able to gain remote access by using a username and password defined locally on the router.5). Version 1. remember to define at least one local user (see Section 4.1. The commands deal with prompts and informational messages.Advanced Security Services • aaa authentication login {default | list-name} method-list is used to specify login authentication method lists. Using these commands in your environment may be a useful thing to do. when specifying why an authentication operation failed with the aaa authentication fail-message command. This list should be the default list. (Use with care. If you use this approach. There is never really a need to use the "default" option but it could be used to be more explicit. it is better to stick to generic responses and allow the administrator to look in the audit records for debugging purposes. Four authentication commands are used for giving messages to the user. • aaa authentication banner delimiter string delimiter replaces any before system login banners with the value of string. There is an important point to remember when setting prompts and messages: do not give away too much information! For example. • aaa authentication fail-message delimiter string delimiter defines a message to be printed when authentication fails. • aaa authentication enable default method-list can be used to control access to enable mode with the authentication mechanism.

Important: when AAA is turned on. The default method list designates RADIUS Central(config)# username joeadmin password 0 G0oD9pa$8 Central(config)# aaa authentication login default radius local One note about method lists for aaa authentication: whatever method is first in the list controls whether the authentication procedure will prompt for a username or not. Do not use the aaa authentication enable default command since the security server pass phrase is stored in the clear and the enable secret is well protected. including VTY lines. make sure you add an administrator account to the local username name database before enabling AAA. The default list should be for the limited set of administrators. When multiple lists are used it would be a good idea if the default list only used the local method and then named lists can be used to override the default list as appropriate. then any additional method which requires a username will automatically fail. For the named method list which will primarily use the security server.Router Security Configuration Guide Here is an example of setting up local username and password and AAA default login authentication parameters. authorization should be used to control the larger set of administrators. 182 Version 1. decide whether to use usernames and passwords (preferred) or to use just a password (highly discouraged). If the first method in the list is line or enable. then by default. The following is a recommended configuration for using a RADIUS security server and the local user database as described above. to allow additional administrators remote access to the router. and should use the local user database.1c . Accounting records can still be sent to the security server but the security server's authorization capabilities can not be used since no authentication records will be sent to the security server. Central(config)# username annadmin password 0 G%oD9pa$8 Central(config)# username joeadmin password 0 3MiaB-JKJ Central(config)# aaa authentication login default local Central(config)# aaa authentication login remotelist radius local Central(config)# line vty 0 4 Central(config-line)# login authentication remotelist Central(config-line)# exit Central(config)# line aux 0 Central(config-line)# login authentication remotelist Central(config-line)# exit Central(config)# In general the default list should be the most restrictive authorization list. When designing your method lists. To avoid being locked out of your router. first create the default list. The second list should be a named method list and should be applied to the appropriate lines. should apply to the console line only. Use the enable secret password to protect all higher privilege levels. For accounting purposes you should use the methods which allow for usernames and assign each administrator a distinct username. authentication will use the local database on all lines. In a more complex scenario where a more limited set of administrators have access to the console line.

• aaa authorization config-commands tells the router to do authorization on all configuration commands (this is the default mode set by the aaa authorization commands level command). exec is used to control all access to exec shells on the router and commands 15 is used to control access to privilege level 15 for a more restrictive set of administrators. There are two primary scenarios where authorization is useful. First. For scenario 1 there would be additional considerations as described in the Dial-In Users section. the other two (network and reverse-access) primarily deal with dial-in and dial-out access control and apply to interfaces. authorization is useful for controlling who can access network services. The router Version 1. Of the four authorization types. and who can access and configure the router. This section will concentrate on exec and command authorization.1c 183 .6. In scenario 2. is also applied to lines. • (line): authorization {arap | commands level | exec | reverse-access} {default | list-name} applies a specific authorization type to a line (note: arap is part of the network authorization type). authorization can control different administrators who have access to different privilege levels on the router. Scenario 1 – Router with dial-in users. and will not be covered.Advanced Security Services Authorization The commands used for AAA authorization are: • aaa authorization {network | exec | commands level | reverse-access} {default | list-name} method-list turns on AAA authorization for the specified type and designates the order in which authorization methods will be applied. etc. arap. Another network type.3 on Dial-In Users provides an overview of network and reverse-access authorization. AAA authorization is currently of limited use for controlling access to routers beyond the standard authentication mechanisms. exec and command deal with router access control and apply to lines. authorization configuration for controlling access to the router: Central(config)# aaa authorization exec default radius Central(config)# aaa authorization network default radius Scenario 2 – Router with two levels of users (exec and privileged exec) Central(config)# aaa authorization exec default radius Central(config)# aaa authorization commands 15 default radius In both scenarios there was no need to apply the authorization method lists to lines because they are using the default lists. and Section 4. The no form of this command will turn off authorization on configuration commands in the EXEC mode. Second. if the router is used for dial in access.

and sent to the network access server where they are applied to the user's connection. (NULL usernames can occur because of accounting records on a protocol translation) • aaa accounting update {newinfo | periodic number} will allow administrators to specify when accounting records are sent to security servers. • (line): accounting {arap | commands level | connection | exec} [default | list-name] can be used to apply different accounting services and levels to different lines.1). AAA authorization does not apply to the console line. refer to the "RADIUS Attributes" appendix of [1]. • show accounting {system | network | exec | commands level} {start-stop | wait-start | stop-only} tacacs+ command can 184 Version 1. in the examples above if the RADIUS security server is not available no one will be able to get an exec shell and in scenario 2 no one will be able to run privilege level 15 commands. There is one very important exception to this. For both. attributes are defined on the security server.1c . Periodic generates more accounting records than newinfo since it will also include interim reports on actions in progress. Accounting The commands used for AAA accounting are: • aaa accounting {system | network | exec | connection | commands level} {default | list-name} {start-stop | waitstart | stop-only | none} method-list turns on AAA's accounting services for the specified accounting type. • aaa accounting suppress null-username command prevents accounting records from being generated for those users who do not have usernames associated with them. Even if a named method list is created and applied to the console line authorization will be ignored. Also. Important: do not use the username name privilege level command since the password will be weakly protected. For a list of supported TACACS+ A-V pairs. RADIUS and TACACS+. The local database is populated using the username command. For a list of supported RADIUS attributes. But there are no useful parameters to set for access to the router from lines (an exception would be for dial-in access). RADIUS and TACACS+ authorization both define specific rights for users by processing attributes. which are stored in a database on the security server.Router Security Configuration Guide commands turn on the checks to query the security server on the router but the actual user to authorization privilege mapping occurs on the security server. associated with the user. Protect higher levels on the router using the enable secret command (see Section 4. refer to the "TACACS+ Attribute-Value Pairs" appendix of [1].

For command accounting stop-only will provide the necessary coverage and will greatly reduce the number of accounting records. then you must use TACACS+ accounting. In this case the user process can not continue. and will actually be terminated. • wait-start accounting sends an accounting record at the start and stop of each specified type. AAA allows for four levels of accounting as set by the aaa accounting command: • start-stop accounting sends records when the accounting type starts and stops. • stop-only sends an accounting record at the end user process which is of an accountable type. This is all done in the background and the user process will continue regardless of the outcome of the accounting attempt.1c 185 . There are two basic scenarios for accounting depending upon which security server is in use. This is not a configuration command but is worth mention. If the start record is sent and acknowledged the user process can continue and at the end a stop accounting record will also be sent. Do not use wait-start in any accounting method list intended for the console line! A basic recommendation would be to use wait-start for remote users and start-stop for local users.Advanced Security Services be used to show active connection information. As mentioned earlier Cisco's RADIUS implementation does not support system and command accounting. If your security policy calls for keeping a record of every router command. • none specifies that no accounting records will be generated for a particular accounting type. Configuration of TACACS+ accounting: Central(config)# aaa accounting system default start-stop tacacs+ Central(config)# aaa accounting exec default start-stop tacacs+ Central(config)# aaa accounting exec remoteacc wait-start tacacs+ Central(config)# aaa accounting commands 15 cmdacc stop-only tacacs+ Central(config)# aaa accounting connection default start-stop tacacs+ Central(config)# line vty 0 4 Central(config-line)# accounting exec remoteacc Central(config-line)# accounting commands 15 cmdacc Central(config)# line aux 0 Central(config-line)# accounting exec remoteacc Central(config-line)# accounting commands 15 cmdacc Version 1. Important: if wait-start accounting is specified on an interface or line and no security server is available for receiving the accounting record then the user process using that interface or line will be locked out. if the start accounting record can not be recorded.

Router Security Configuration Guide

Configuration of RADIUS accounting:
Central(config)# aaa accounting exec default start-stop radius Central(config)# aaa accounting exec remoteacc wait-start radius Central(config)# aaa accounting connection default start-stop radius Central(config)# line vty 0 4 Central(config-line)# accounting exec remoteacc Central(config)# line aux 0 Central(config-line)# accounting exec remoteacc

Since remote administration is more dangerous than console administration, the configurations above add extra accounting to the remote lines. Part of the extra protection is requiring that before a remote user can get an exec shell an audit record must be recorded into the security server. Note: the aux line configuration is not required if the aux line is disabled as suggested in Section 4.6.2. Also, for information about RADIUS Attributes and TACACS+ AV Pairs for use in accounting, refer to the appendices in the Cisco Security Configuration Guide [1].

Putting It Together
This section will put together the AAA mechanisms from earlier in this section and will apply them to the configuration of the Central and South Routers. The Central router is between the facility backbone and the specific part of the infrastructure. The South router acts as the first layer of defense to a well protected enclave.

Facility Network

eth 0


eth 1

Authentication Server

eth 0/0

eth 0/1

eth 0/0


eth 0/1

Protected Enclave

Figure 4-12: Routers and their Authentication Server

Authorization will not be used in these examples since all the administrators in these examples need configuration access and there is no dial-in access. For a more


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complete example, including authorization and some discussion of dial-in security concerns, see Section 4.6.3. Central Router Configuration (IOS 12.0):
Central(config)# enable secret 3rRsd$y Central(config)# username fredadmin password d$oyTld1 Central(config)# username bethadmin password hs0o3TaG Central(config)# username johnadmin password an0!h3r( Central(config)# service password-encryption Central(config)# banner motd ^T Legal Notice: Access to this device is restricted. . . ^T Central(config)# radius-server host Central(config)# radius-server key i*Ma5in@u9p#s5wD Central(config)# aaa new-model Central(config)# aaa authentication login default radius local Central(config)# aaa accounting exec default start-stop radius Central(config)# aaa accounting exec remoteacc wait-start radius Central(config)# aaa accounting connection default start-stop radius Central(config)# access-list 91 permit log Central(config)# access-list 91 deny any log Central(config)# line con 0 Central(config-line)# transport input none Central(config-line)# exec-timeout 5 0 Central(config-line)# login local Central(config-line)# exit Central(config)# line vty 0 4 Central(config-line)# access-class 91 Central(config-line)# exec-timeout 5 0 Central(config-line)# login local Central(config-line)# transport input telnet Central(config-line)# accounting exec remoteacc Central(config-line)# exit Central(config)# line aux 0 Central(config-line)# transport input none Central(config-line)# login local Central(config-line)# exec-timeout 0 1 Central(config-line)# no exec Central(config-line)# end

The first thing to do when configuring access to a router is to setup the local access. The enable secret command sets the password on the privileged exec level and the username commands setup all the local accounts. Now when AAA is turned on the default authorization will not lock out the console. The message of the day should be used to provide the legal document for controlling access to the device and allowing for monitoring. This message should be generic and hopefully the same on all of your routers, firewalls, servers, workstations, etc.

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Next configure the security server and turn on AAA mechanisms. Since the shared secret to the RADIUS server is stored in the clear do not use the same shared secret for the router with any other device. Since communications to the security server are protected and the connection does not go outside the corporate boundary it is acceptable to allow communications to the server outside the router. With the aaa authentication login command make sure local is in the list as described earlier. Also, notice that the default accounting for exec is set to start-stop and that a named list was created for wait-start. This way by applying the named list to external connections and allowing the default list to automatically apply to console you will not be locked out of the router. Use connection accounting to track outbound connections generated by users logged onto the router, these should be minimal. Create and apply an access-list to the VTYs to limit remote access to internal networks only and if possible limit the remote hosts by actual host IP addresses instead of a network address. Issue the login local command on the console and vtys in case AAA services get turned off. This will continue to allow limited remote access based upon the local database and will be ignored while AAA mechanisms are still running. Also limit remote access to telnet only and limit the connection idle time to 5 minutes. The auxiliary port is disabled in this example. If a TACACS+ server was used in this example instead of the RADIUS server then system accounting would have also been specified. Command level accounting could have been applied as well but would probably not be needed here. South Router Configuration:
South(config)# enable secret rI^3r6Ed South(config)# username bethadmin password hs0o3TaG South(config)# username johnadmin password an0!h3r( South(config)# banner motd ^T . . ^T South(config)# tacacs-server host South(config)# tacacs-server key Ir3@1yh8n#w9@swD South(config)# aaa new-model South(config)# aaa authentication login default tacacs+ local South(config)# aaa accounting exec default start-stop tacacs+ South(config)# aaa accounting exec remoteacc wait-start tacacs+ South(config)# aaa accounting connection default start-stop tacacs+ South(config)# aaa accounting system default start-stop tacacs+ South(config)# aaa accounting commands 15 default stop-only tacacs+ South(config)# access-list 91 permit log South(config)# access-list 91 permit log South(config)# access-list 91 deny any log South(config)# line con 0 South(config-line)# transport input none


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South(config-line)# South(config-line)# South(config-line)# South(config)# line South(config-line)# South(config-line)# South(config-line)# South(config-line)# South(config-line)# South(config-line)# South(config-line)# South(config)# line South(config-line)# South(config-line)# South(config-line)# South(config-line)# South(config-line)#

exec-timeout 5 0 login local exit vty 0 4 access-class 91 exec-timeout 5 0 login local transport input telnet login authentication remotelist accounting exec remoteacc exit aux 0 transport input none login local exec-timeout 0 1 no exec end

As in the first example start by setting up local access to the router. The enable secret command sets the password on the privileged exec level and the username commands setup all the local accounts. In this case there may be fewer local accounts since this router is the first lines of defense to a secure enclave. Again, when AAA is turned on the default authorization will not lock out the console. The Message of the Day should be used to provide the legal document for controlling access to the device and allowing for monitoring. This message should be generic and hopefully the same on all of your routers, firewalls, servers, workstations, etc. Next configure the security server and turn on AAA mechanisms. Since the shared secret to the TACACS+ server is stored in the clear do not use the same shared secret for the router with any other device. Since communications to the security server are protected and the connection does not go outside the corporate boundary it is acceptable to allow communications to the server outside the router. With the aaa authentication login command make sure local is in the list as described earlier. Notice that the default accounting for exec is set to start-stop and that a named list was created for wait-start. This way by applying the named list to external connections and allowing the default list to automatically apply to console you will not be locked out of the router. Use connection accounting to track outbound connections generated by users logged onto the router, these should be minimal. Also, include system and commands 15 accounting since this router is providing protection to a special enclave. As before, create and apply an access-list to the vtys to limit remote access to internal networks only and if possible limit the remote hosts by actual host IP addresses instead of a network address. Issue the login local command on the console and vtys in case AAA services get turned off. This will continue to allow limited remote access based upon the local database and will be ignored while AAA mechanisms are still running. Also limit remote access to telnet only and limit the connection idle time to 5 minutes. The auxiliary port is disabled in this example.

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If a RADIUS server was used in this example instead of the TACACS+ server then system and command accounting would not be specified.

4.6.3. Dial-In Users
AAA services were designed with remote network access in mind. This includes remote access to routers as well as to network services like PPP. AAA using RADIUS is one of the primary means by which this is accomplished by Internet Service Providers (ISP's). Controlling access for dial-in users is similar to controlling access to the router but there are different protocols that are used. Additionally, although it is not shown, it is highly recommended that when dial-in access to the network or router is in use, that AAA services should be used in conjunction with a one-time password or similar token technology. Some important commands for controlling dial-in users are: • aaa authentication ppp {default | list-name} <method-list> is used to specify PPP authentication method lists. • aaa authorization {network | exec | commands level | reverseaccess} {default | list-name} <method-list> turns on AAA authorization for the specified type and designates the order in which authorization methods will be applied. In this case we are particularly interested in turning on network authorization. • aaa accounting {system | network | exec | connection |
commands level } {default | list-name} {start-stop | waitstart | stop-only | none} method-list turns on AAA's accounting

services for the specified accounting type. For dial-in users network needs to be used. • aaa processes number command is used to specify the number of background processes to start to handle concurrent authentication and authorization requests. • (interface): ppp authentication {pap | chap | pap chap | chap
pap} [if-needed] {default | list-name} [call-in] [one-tone]

command is used to enable pap, chap, or both forms of authentication on the selected interface. • (interface): ppp authorization {default | list-name} command is used to apply a ppp authorization list to the selected interface. • (interface): ppp accounting [default | list-name] command is used to apply accounting methods to the PPP service on the selected interface. The example below gives one potential application of AAA services for dealing with dial-in services (Note: this example is not complete). Figure 4-13 shows the relevant portion of the network, and the configuration for East is shown after it.


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eth 0

net access

eth 1

Authentication Server

eth 0/0


User Host

eth 0/1

Telephone Network



Remote Host

Figure 4-13: Router East in the Network
East(config)# enable secret t!tRd-1rZZ East(config)# username fredadmin password d$oyTld1 East(config)# username bethadmin password hs0o3TaG East(config)# banner motd ^T LEGAL NOTICE: Use of this device restricted to authorized persons. This device is subject to monitoring at all times, use of this device constitutes consent to monitoring. ^T East(config)# radius-server host East(config)# radius-server key i3dRc8sRv(@oeU4) East(config)# aaa new-model East(config)# aaa authentication login default radius local East(config)# aaa authorization exec default radius East(config)# aaa authorization network default radius East(config)# aaa accounting exec default start-stop radius East(config)# aaa accounting exec remoteacc wait-start radius East(config)# aaa accounting connection default start-stop radius East(config)# aaa accounting network default wait-start radius East(config)# access-list 91 permit log East(config)# access-list 91 permit log East(config)# access-list 91 deny any log East(config)# line con 0 East(config-line)# transport input none East(config-line)# exec-timeout 5 0 East(config-line)# login local East(config-line)# exit East(config)# line vty 0 4 East(config-line)# access-class 91 East(config-line)# exec-timeout 5 0 East(config-line)# login local East(config-line)# transport input telnet East(config-line)# accounting exec remoteacc

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Configuring dial-in services is far too complex a subject to be dealt with in depth in this guide. RADIUS is a distributed client/server based architecture used to pass security information between access points and a centralized server.6. and Kerberos. for more details. However Cisco's RADIUS 192 Version 1. RADIUS protects the communications using a shared secret. RADIUS Remote Authentication Dial In User Service (RADIUS) is an IETF proposed standard (RFC 2865) for securing network components. Security Server Protocols In Cisco routers and network access servers. 3) the asynchronous interface configuration needs completed (if the aux port is not used as an asynchronous interface disable it see Section 4. In addition. The following descriptions will only discuss items which are different from the Putting It Together examples in the previous section. authorization. Command level accounting could have been applied as well but would probably not be needed here. RADIUS can be used to provide authentication.1c . Consult the Cisco IOS documentation. accounting was added for recording network events. Cisco supported security servers are RADIUS. This section only provides one example for a possible network access server configuration. AAA is the mechanism used to establish communications with security servers. 2) the terminal server lines have not been configured (and will need to have the remoteacc accounting list applied) and. RADIUS was designed with Dial In access control in mind and the accounting features are very flexible along these lines. Also the AAA authorization and accounting default commands for network will also apply to the ppp traffic as it traverses the line. and accounting services.4.1. AAA authorization for exec and network was added to separate the privileges for network users and router administrators. If a TACACS+ server was used in this example instead of the RADIUS server then system accounting would have also been specified. Security servers are important to Cisco network gear when centralized administration is required or when authorization and accounting services are needed.4).Router Security Configuration Guide East(config-line)# exit East(config)# interface async 1 East(config-if)# encapsulation ppp East(config-if)# ppp authentication chap East(config-if)# end In this example there are several items left incomplete: 1) the IPSec tunnel to Central has not been configured (see Section 5.2) to carry remote administrator access to the router (which is required to protect the username and password traveling across the facility backbone in the clear). 4. particularly the “Dial Solutions Configuration Guide”. TACACS+. The asynchronous interface contains the commands necessary for configuring AAA authentication for the ppp protocol.

These groups may be useful for large enterprises.2. Central(config)# ip radius source-interface loopback0 Central(config)# radius-server host 14. To perform authentication and authorization a RADIUS server uses attributes. IOS always uses 1645 unless you specify otherwise. as shown below.18 auth-port 1812 Under IOS 12. This is accomplished using the commands listed: • radius-server host {hostname | ip-address} [auth-port port-number] [acct-port port-number] command specifies the radius server's hostname or IP address and the ports to use for authentication (authorization) and accounting. To define a server group. The shared secret key should be at least 16 characters long and follow the other rules for a good password as described in Section 4.6. users never have to type this string.18 auth-port 1812 Central(config-sg)# server 14. ! RADIUS example .Advanced Security Services client does not support auditing of command or system events on the router or network access server.4. you can define named groups of RADIUS servers. while others use the new standard port of 1812. use the command aaa server group.4.18 key i*Ma5in@u9p#s5wD Central(config-sg)# end Central# Version 1.1. Use the auth-port parameter to cause IOS to send RADIUS requests to the server on that port.a group with one server in it Central(config)# aaa server group radius radGroup1 Central(config-sg)# server 14.2.6. if you have defined it as described in Section 4. Also. As a minimum when setting up a RADIUS server on a Cisco device the host address and shared secret must be configured as well as turning on and configuring AAA on the device. The example below shows how to set up RADIUS on the router Central. East(config)# radius-server host 14. For a complete list of RADIUS router configuration commands see the “RADIUS Commands” section in [1].1 or later.1c 193 .6.18 Central(config)# radius-server key W@t7a8y-2m@K3aKy RADIUS servers are freely available and are in extensive use.2. These attributes can be configured to allow/deny access to various router and network services. • radius-server key string sets the RADIUS server shared encryption key. the RADIUS service should be bound to the loopback0 interface. For more details see the Security Configuration Guide on "Configuring RADIUS" and "RADIUS Attributes" sections for more details.2. where different sets of security servers are used for different groups of users or different purposes. so make it longer than a typical password.6. Some RADIUS servers use the old standard port 1645 for authentication.1. The secrecy and quality of this key is critical to the security of your RADIUS installation.

Under IOS 12.2. For a complete list of TACACS+ router configuration commands see the "TACACS. These attribute-value pairs are configured on the server and used by the router authorization mechanism to control access to network services. TACACS+ was designed with Cisco implementations in mind so it offers a wide range of AAA services including full auditing of Cisco AAA accounting events. For more details on the TACACS+ and attribute-value pairs see the Security Configuration Guide sections “Configuring TACACS+” and “TACACS+ Attribute-Value Pairs”. The shared secret key should be at least 16 characters long and follow all the rules for a good password as described in Section 4.Router Security Configuration Guide To use a server group. you can define named groups of TACACS+ servers.1.18 Central(config)# tacacs-server key W@t7a8y-2m@K3aKy TACACS+ implementations are available through Cisco Secure ACS and Cisco also offers a free implementation as well. Simple example for Central: Central(config)# tacacs-server host 14. so make it longer than a typical login password.1 or later. The key string parmeter sets the secret key for this TACACS+ server host overriding the default but should follow same creation rules as the default.4. TACACS+ protects communications using a shared secret key between the network device and central server. The security of TACACS+ depends on this secret. The [port integer] can be used to specify a new port number. Central(config)# aaa authentication login VTlogin group radGroup1 TACACS+ Terminal Access Controller Access Control System plus (TACACS+) is the most recent Cisco security protocol designed to provide accounting and flexible control of authentication and authorization services. and TACACS+ Commands" section in the "Security Command Reference". • tacacs-server key string command sets the default TACACS+ shared encryption key. The primary commands used for configuring TACACS+ on a Cisco router are: • tacacs-server host {hostname | ip-address} [port portnumber] [key string] command can be used to specify the host. where different sets of security servers are 194 Version 1. TACACS+ is implemented by Cisco using the AAA mechanisms and provides for the centralized validation of users using routers and network services. and users never have to type it. where the TACACS+ server is running. Extended TACACS. TACACS+ uses attribute-value pairs for controlling authentication and authorization services. IP address or DNS name.1c .6. These groups may be useful for large enterprises. name it in a method list instead of the default group ‘radius’.

! TACACS+ Example . After the two parties have been authenticated (in this case.Advanced Security Services used for different groups of users or different purposes. the router and the adminstrator).1 Security Configuration Guide for more information. There are free open sources versions of Kerberos available as well as commercially supported products.6.e. This authentication is performed using a trusted third-party service using conventional (secret key) cryptography. Configuration of Kerberos installations based on MIT Kerberos are already explained in the Cisco IOS documentation. use the command aaa server group. These two topics are outside the scope of the Kerberos coverage in this guide. Some modern operating systems come with Kerberos built in. but more details can be found in the IOS documentation [1]. Host configuration for using MIT Kerberos is not covered in this guide.10. consult [8]. make sure that the IOS installed on your router supports Version 1. as well as in RFC 1510 [5] and in Tung’s book [8]. To define a server group. Kerberos provides strong authentication for client/server applications by using secret-key cryptography. simply use the group name instead of the default name ‘radius’ or ‘tacacs+’.18 key Gx98-vAR1bv*u Central(config-sg)# server 14. This mechanism can verify the identities of two users (i. This section assumes basic familiarity with Kerberos administration and security concepts. Configuration of a Microsoft Windows 2000 Server acting as the Kerberos authentication server is covered below. Kerberos infrastructures are already in wide use. then this form of centralized authentication may be a way to gain excellent security for remote administration. person or network component) on unprotected networks.a group with two servers in it Central(config)# aaa server group tacacs+ myTacGroup Central(config-sg)# server 14. consult the IOS 12. Note that Kerberos only allows for limited authorization capabilities and no accounting. as shown below. Before attempting any of the step below.2. If you already have a Kerberos infrastructure in place. In this system a client would request the credentials of the party they wish to contact from the trusted authentication service.1c 195 . The communications between the router and the Kerberos security server are encrypted. For a good introduction to these topics. Kerberos can provide very effective confidentiality and data integrity services.2. Kerberos can also be used to perform EXEC shell authorization using Kerberos Instance Mapping.39 key t777+08cdcoWW Central(config-sg)# end Central# When you want to include the servers of a particular group in a method list. if your Telnet client supports Kerberos encryption. Central(config)# aaa authentication login VTlogin group myTacGroup Kerberos Kerberos was developed by the Massachusetts Institute of Technology (MIT) and is standardized by the IETF as a network authentication system in RFC 1510.

then “user”.) 4. 196 Version 1. The tools are found in “support\tools\setup.exe”. Update or confirm the DNS entries for the KDC and the router. right click on the “Users” folder.Router Security Configuration Guide Kerberos. (For example. if you get several choices then your IOS supports Kerberos. 2. (Note: this is a Kerberos identity for the router. and select “New”. create the user accounts on the server for administrators that will access the router. 3.) A Windows 2000 Server configured to be a Domain Controller automatically has the Kerberos Key Distribution Center services installed and running on it.1c . not for any user. If necessary. To make it work with a Cisco router. Install the Kerberos support tools from the Windows 2000 installation media. in global config mode type the word kerberos and then type a question mark. Create a user account for the router. perform the following steps on your Windows server: 1. Open up the “Active Directory Users and Computers” tool located in the “Control Panel\Administrative Tools” folder.

Advanced Security Services

5. Check the Kerberos settings for logins; use the settings shown below in the column “Effective Setting”. For more information, consult the NSA Guide to Windows 2000 Kerberos Settings [6].

6. Use the Windows ktpass command, installed in step 1, to create the host’s keytab file, map the router to its account, and set its password.

7. Install the keytab file on the router. This must be done using the IOS kerberos srvtab command, as shown below. Once you are sure that your router supports Kerberos, follow the steps listed below in global config mode. 1. Define the Kerberos realm.
kerberos local-realm kerberos-realm

2. Designate the Kerberos KDC to use in the realm, along with port number.
kerberos server kerberos-realm {hostname | ip-address} [port-number]

3. Map an optional host name or DNS domains to the realm.

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kerberos realm {dns-domain | host} kerberos-realm

4. Define the preauthentication method.
kerberos preauth authentication-method

The recommended method is encrypted-kerberos-timestamp. 5. Generate a local private DES key. The key-password should be 8 randomly-chosen characters.
key config-key 1 key-password

The key will be used to encrypt the Kerberos secret key in the router’s stored configuration. (This key is stored in the router’s NVRAM, but cannot be recovered or extracted.) 6. Load the keytab file from a server, link channel, or local file.
kerberos srvtab remote { URL | host filename }

This command supports a wide variety of means for downloading the srvtab file, including TFTP, FTP, and more. TFTP is the default. 7. Create a login authentication model, specifying Kerberos as the mechanism to use first.
aaa authentication login {default | list-name} krb5 [ {other-mechanisms} ]

The two examples below show two different ways of conveying the Kerberos keytab file, generated by the Windows 2000 ktpass command, over to the router. Neither approach is perfect -- the ideal approach would be to load the keytab file directly over the serial link, but unfortunately that is not supported in any version of IOS that the authors of this guide have been able to test. Kerberos Example 1: Network Download The transcript below shows an example of following the Kerberos setup procedure on router Central. In this case, the Kerberos KDC provides service on IP address, and a TFTP server is on IP address (Note: Windows 2000 installations typically do not include TFTP servers. You will need to use a commercial TFTP server, or distribute the file from a Unix system equipped with a TFTP server.)
Central# config t Enter configuration commands, one per line. End with CNTL/Z. Central(config)# kerberos local-realm KERBEROS.NSA.GOV Central(config)# kerberos server KERBEROS.NSA.GOV Central(config)# kerberos realm KERBEROS.NSA.GOV Central(config)# kerberos realm KERBEROS.NSA.GOV Central(config)# kerberos preauth encrypted-kerberos-timestamp Central(config)# key config-key 1 aW.-8(xZ Central(config)# kerberos srvtab remote Central.keytab Loading Central.keytab from (via Ethernet0/1): ! [OK - 78/4096 bytes] Central(config)#


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The content of the keytab file is very sensitive, because it contains the long-term Kerberos secret key that the router will use to communicate with the KDC. In general, it is not a good idea to transfer the keytab file from your server to the router over TFTP (or FTP, or any other plaintext network protocol). Unless this part of your configuration takes place on an isolated lab or management network, do not use this method to distribute the keytab file. Instead, use (1) the console serial download method described below, or (2) the SCP protocol instead of TFTP (this requires IOS support for SCP, and it requires setting up SSH as described in Section 5.5). Kerberos Example 2: Console Link Download This example shows how to convey the keytab file securely from your server to the router over the console serial link using the YModem protocol.
Central# copy ymodem: flash:central.key **** WARNING **** . . Proceed? [confirm]y Destination filename [central.key]? central.key Erase flash: before copying? [confirm]n Max Retry Count [10]: 15 Perform image validation checks? [confirm]n Ymodem download using crc checksumming with NO image validation Continue? [confirm]y Ready to receive file...........C 4294967295 bytes copied in 23.692 secs (0 bytes/sec) Central# Central# config t Enter configuration commands, one per line. End with CNTL/Z Central(config)# kerberos local-realm KERBEROS.NSA.GOV Central(config)# kerberos server KERBEROS.NSA.GOV Central(config)# kerberos realm KERBEROS.NSA.GOV Central(config)# kerberos realm KERBEROS.NSA.GOV Central(config)# kerberos preauth encrypted-kerberos-timestamp Central(config)# key config-key 1 .XT9+se% Central(config)# kerberos srvtab remote flash:central.key Central(config)# exit Central# ! optional steps: wiping the keytab from flash Central# delete flash:central.key Delete filename [central.key]? central.key Delete flash:central.key? [confirm]y Central# Central# ! the squeeze command may not be supported on all routers Central# squeeze flash: Squeeze operation may take a while. Continue? [confirm]y . . Central#

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Kerberos and AAA Once you have downloaded the keytab file, you can designate Kerberos as your main AAA authentication method.
Central(config)# aaa new-model Central(config)# aaa authentication login default krb5 local Central(config)# exit Central#

This section presents only a very cursory look at Kerberos authentication. For more information about administering Kerberos networks, consult [6], [7], and [8].

4.6.5. References
[1] Cisco Systems, Cisco IOS 12.0 Network Security, Cisco Press, 1999. This book provides a detailed reference for all the security features in Cisco IOS 12. It includes a great deal of information about AAA, including a section on configuring Kerberos. The same information is also available in the on-line documentation, in the Cisco IOS Security Configuration Guide. [2] Cisco System, Cisco IOS 12.0 Dial Solutions, Cisco Press, 1999. This documentation volume provides detailed information on setting up modems and dial-up networking facilities. [3] Rigney C., et. al. “Remote Authentication Dial In User Service (RADIUS)” RFC 2865, June 2000. This is the Internet RFC that defined the core RADIUS protocol. [4] Carrel, D., and Grant, L. “The TACACS+ Protocol Version 1.78”, Cisco Systems, January 1997. available at: This is the draft RFC that would have standardized the TACACS+ protocol. It explains the operation of the protocol in great detail. [5] Kohl, J., “The Kerberos Network Authentication Service (V5)”, RFC 1510, September 1993. This is the Internet RFC that defines the Kerberos authentication protocol. [6] Opitz, D. “Guide to Windows 2000 Kerberos Settings” NSA, July 2001. available under: This guide describes prudent Kerberos security settings for Windows 2000.


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[7] “Step-by-Step Guide to Kerberos 5 Interoperability,” Windows 2000 Step-byStep Guides, Microsoft Corporation, 2002. available at:

This article describes how to use Windows 2000 Kerberos with other Kerberos implementations. [8] Tung, B., Kerberos - A Network Authentication System, Addison-Wesley, 1999. This slim handbook provides a good overview of Kerberos.

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4.7. Collected References
The list below describes the major references and sources of information for the material presented here in Section 4.

4.7.1. Books and Manuals
Cisco Systems, IOS 12.0 Configuration Fundamentals, Cisco Press, 1999. Basic configuration guide for IOS 12, includes good information on using the IOS command interface, basic IOS commands, and much more. Cisco Systems, Cisco IOS Network Security, Cisco Press, 1998. This book is the security configuration manual and command reference for IOS 11.3. It includes extensive coverage of access management, AAA, and related topics. Available on the Cisco Documentation CD as two documents: the “Security Configuration Guide” and the “Security Command Reference”. Cisco Systems, Cisco IOS 12.0 Network Security, Cisco Press, 1999. This book is the security configuration manual and command reference updated for IOS 12.0. It includes extensive coverage of access management, AAA, IPSec, and related topics. Available on the Cisco Documentation CD. Akin, T., Hardening Cisco Routers, O’Reilly & Associates, 2002. A pragmatic and detailed guide to securing Cisco routers; includes detailed examples. Held, G. and Hundley, K., Cisco Security Architectures, McGraw-Hill, 1999. This book includes excellent general advice about router and router-related network security, in addition to its Cisco-specific material. Held, G. and Hundley, K., Cisco Access List Field Guide, McGraw-Hill, 1999. Access lists are critical to most aspects of Cisco IOS security. This book is a detailed, practical guide to creating and understanding access lists. Innokenty, R., Cisco Routers for IP Routing: Little Black Book, Coriolis Group, 1999. This practical little book includes great advice on managing routes and routing protocols, mostly oriented toward IOS 11.2 and 11.3. Chappell, L. editor, Advanced Cisco Router Configuration, Cisco Press, 1999. Good coverage of advanced Cisco configuration issues, including extensive material on access lists and OSPF.


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Addison-Wesley.. “Increasing Security on IP Networks” Cisco Internetworking Case This short but highly prescriptive document distills a great deal of Cisco IOS security practice into an example configuration. “Unicast Reverse Path Forwarding”.cisco.1”. This detailed guide explains a great deal about operational use of Cisco routers in the Internet Service Provider environment. Cisco Press.7. available at: http://www. including good coverage of critical security topics. protocols. and upgrade paths. Cisco Systems.Version 3. McGinnis. A detailed exploration of the SNMP management information base.html A good overview article on tightening up the security on a typical Cisco router running IOS 11.htm Initial documentation on unicast reverse-path forwarding verification. 2nd Edition. B. Cisco ISP Essentials. E. 1999. and Perkins.2. M.. Cisco Press. features in releases. R. Articles and Papers software/ios111/cc111/uni_rpf. 2000. Greene. Routing in the Internet. 1996.. available at: http://www. and how routing works in the Internet. “Improving Security on Cisco Routers”. it includes a good explanation of the concepts. April 2002. and Smith. Includes some coverage of access lists and Cisco IOS 11.Advanced Security Services Coulibaly. available at: http://www. 2002.3 or later. Cisco Security “Secure IOS Template .M. C. 1st edition. the release management process. Understanding SNMP MIBs. available at: http://www. Unbelievably detailed information on Cisco IOS release versions. November 2003. Version 1.1c 203 . available from Cisco Press. 1998. including both standard and vendor-specific structures. 4. A deep and detailed textbook about IP routing technologies. It has also been published as a book..1(CC) Release An old but useful article on using a Cisco router to protect a network boundary.. Huitema. Cisco IOS Release: The Complete Reference. D. P.

Router Security Configuration Guide 5. In 1996. such as the Internet. Thus. This proprietary solution was a stopgap effort for customers until a standards-based solution was in place. Role of the Router in Inter-Network Security When considering the task of joining IP security with IP router functionality. These devices sit between untrusted networks. IPSec tunnels can be constructed between two routers’ interfaces using the IPSec protocol framework. These facilities offer additional security services that can contribute to the secure operation of entire networks or communities. and local trusted networks. this section of the guide will focus on the concept of “packet protection”. 5. Cisco released IOS version 11. etc. While it provided some level of packet protection for Cisco-to-Cisco communications. 204 Version 1. These can then be easily incorporated into current router configurations to help meet specific security requirements.2.g.1. which included the Cisco Encryption Technology (CET).1c .3 and above) and other IP product manufacturers have implemented and offered IPSec solutions for packet protection to their customers. servers. Each packet passing through. To reduce this daunting task to one which is manageable and easily understandable. firewalls. e. the network administrator or security engineer can be overwhelmed. both Cisco (in IOS 11. Advanced Security Services This section describes some Cisco IOS facilities that are not central to the task of securing a router. This standards-based approach allows for interoperability between Cisco routers and other IP security products. It works in conjunction with the standards-based Internet Key Exchange (IKE) protocol to provide the users a very solid IP security foundation. Since the adoption of the IETF IP Security (IPSec) standards. and the specific Cisco IOS commands that implement them. Routers used for supplying packet protection are almost always positioned as gateway or border devices. This framework has been scrutinized by many skilled evaluators in industry and academia. By focusing on this fundamental building block of IP networking. it did not allow Cisco products to interoperate with other IP security products. The vast amount of available literature and the technical jargon can cause an administrator to ignore available security features altogether. non-Cisco routers. or created by the router has source addresses and is carrying data which may need some form of protection. we can devote our energy to providing you with some basic cryptographic concepts.

1. access-list 100 permit 50 host 7. we will only discuss how it uses a jointly held secret value (i. The security guidance of this section does not exhaustively cover all IPSec options.2. Establishing the IPSec Protection Parameters Establishing a Common IKE Authentication Key Prior to establishing an IPSec tunnel between two routers.12. Establishing an IKE Security Policy 3. The example below illustrates the ACL rule syntax for permitting incoming IPSec traffic.0. will be used to help demonstrate the concepts (see Figure 4-1). step-by-step example to help you set up and test IPSec on your network. Building IPSec Tunnels Building IPSec tunnels between two Cisco routers will involve entering three sets of information into each router’s running configuration files. In the basic mode of operation. Establishing a common IKE Authentication Key 2.12. the administrator can update and/or change the current running configuration.0. any access list restrictions on these ports or protocols should be removed or changed to allow the IPSec packets to be transmitted and received by the participating routers. and the Remote router.20 access-list 100 permit 51 host 7. Version 1.0.20 host 14. the external interfaces of the North router.2. Since IPSec utilizes IP protocols 50 and 51. IP Network Security Prior to establishing an IPSec configuration on the router.1. However. For the example in this section. certain network and current router configuration checks should be made to eliminate any router connectivity problems.1. While IKE has several ways it can authenticate the two routers to each other.Advanced Security Services 5.12. 5. and the User Datagram Protocol (UDP) port 500 in its communications. For more information about command modes.2.20 host 14.g. In the example that follows.2.1. the routers may be configured using several different modes of operation. 14. a pre-shared key) to do it. for operational security we HIGHLY recommend that IKE authentication decisions be made using IPSec authentication schemes in conjunction with digital certificates. which algorithms to use) and the appropriate Cisco IOS commands to implement them in an easy-to-follow. This authentication decision is made in the IPSec framework using the IKE protocol.12. anyone with access to the router can view selected information about the current running configuration. see Section 4.1. The sets can be labeled as: 1.2. we assume the routers have two modes of operation: basic mode and privileged EXEC mode.0.20.20 access-list 100 permit udp host 7. Rather.20 eq 500 Also.1.20 host 14.2. In the privileged EXEC mode. it provides a set of options (e. 7.20.e.1c 205 . each router authenticates the peer IP address with which they are building a tunnel.

For this example. numbers. North# config t Enter configuration commands. as shown below. the secret key is “01234abcde”.2. the network administrators for the North and Remote routers (possibly the same person) should enter the key into their routers. End with CNTL/Z. End with CNTL/Z. after entering the new information.0 Security Configuration Guide [2] for details on the other IKE options. Establishing an IKE Security Policy Each router contains a list of IKE security polices.1c . each router must possess the same secret key. To enter the keys.Router Security Configuration Guide Consult the Cisco IOS 12. and punctuation symbols to build operational pre-shared keys. one per line. use the crypto isakmp command in global configuration mode.) To use pre-shared keys for making authentication decisions in IKE. This can be done using the show running-config command in privileged EXEC mode. there must be at least one policy in common between them. We strongly recommend using difficult-to-guess combinations of characters.20 North(config)# exit North# and Remote# config t Enter configuration commands. Each necessary parameter for the policy is listed 206 Version 1. to check and see if the router has received the intended configuration information.0. (Note: the router used for part of this example is named “Remote”. One way to verify that the pre-shared keys were properly entered is to display the router’s running-configuration and look for the preshared key entered above.20 Remote(config)# exit Remote# When entering new configuration information into the router it is always a good idea. These keys should be obtained out-of-band by each of the routers’ administrators. Once the keys are securely held.1. one per line. Do not use a remote administration connection to enter sensitive IPSec parameters – use a local console connection. These policies capture information needed by the IKE protocol to help build a secure IPSec tunnel between the two routers. and that name appears in all the prompts. In order for two routers to be interoperable.12. Remote(config)# crypto isakmp key 01234abcde address 14. The syntax for the crypto isakmp command is: crypto isakmp key key-value address destination-ip-address. North(config)# crypto isakmp key 01234abcde address 7.

to protect information between two peers over a hostile. • hash algorithm – for providing integrity to IKE protocol messages (choices: SHA. 12. however. the Internet). unprotected network (e. one per line. End with CNTL/Z.536.g.0(3)T).g.g. pre-shared keys) • Diffie-Hellman group – used for computing the encryption key (choices: #1 (768 bit modulus).1c 207 . otherwise use group #2 • security association lifetime – lifetime. #2 (1024 bit modulus). group #5 should be used where possible. ! then use the following command to select DES for encryption ! North(crypto-isakmp)# encryption des North(crypto-isakmp)# hash sha North(crypto-isakmp)# authentication pre-share North(crypto-isakmp)# group 2 North(crypto-isakmp)# lifetime 86400 North(crypto-isakmp)# exit North(config)# exit North# and Version 1. RSA encryption. The DES algorithm is not acceptable. #5 (1536 bit modulus)). expressed in seconds or in kilobytes transferred. MD5) • authentication method – for identifying the routers attempting to establish a tunnel (choices: Rivest-Shamir-Adelman (RSA) signature. e. North# North# config t Enter configuration commands. with ! the priority given to lower numbers North(crypto-isakmp)# encryption 3des ! If the user’s version of the IOS only supports the DES ! algorithm. that a tunnel should remain in place before it is automatically rebuilt (default: 86400 (one day)) The administrators for the North and Remote routers should enter the IKE security policy into their routers using the commands shown below. 3DES doesn’t provide the needed performance) always use 3DES. North(config)# crypto isakmp policy 1 ! The policy number may be an integer between 1 and 65. (e. Unless you have a very sound reason to use DES. 3DES in certain IOS versions. and community of interest data separation is needed. so use 3DES for such cases.Advanced Security Services below with a short description of its purpose (the default setting is given first in all lists of choices): • priority number – a positive integer used to uniquely identify the policy when two or more are contained within the routers configuration file (default: none) • encryption algorithm – for protecting the IKE protocol messages (choices: DES.

Creating the appropriate access lists Some administrators will want to create tunnels to protect all protocol data flowing between two routers. no volume limit North# Establishing the IPSec Protection Parameters Using the pre-shared key and the security policy. Others will desire to protect only particular services or a subset of the data flow (e.Router Security Configuration Guide Remote# Remote# config t Enter configuration commands.Data Encryption Standard (56 bit keys) hash algorithm: Secure Hash Standard authentication method: Rivest-Shamir-Adleman Signature Diffie-Hellman group: #1 (768 bit) lifetime: 86400 seconds. The following example displays an access list needed to protect ALL protocol information between the North and 208 Version 1. and http traffic). We now need to give the tunnel its desired characteristics.g. no volume limit Default protection suite encryption algorithm: DES . with ! the priority given to lower numbers Remote(crypto-isakmp)# encryption 3des ! If the user’s version of the IOS only supports DES. then use the ! following command to select DES for encryption ! Remote(crypto-isakmp)# encryption des Remote(crypto-isakmp)# hash sha Remote(crypto-isakmp)# authentication pre-share Remote(crypto-isakmp)# group 2 Remote(crypto-isakmp)# lifetime 86400 Remote(crypto-isakmp)# exit Remote(config)# exit Remote# Using the show crypto isakmp policy command in privileged EXEC mode (on the console of Remote or North) should now display the following information: North# show crypto isakmp policy Protection suite of priority 1 encryption algorithm: 3DES – Triple Data Encryption Standard (168 bit keys) hash algorithm: Secure Hash Standard authentication method: Pre-Shared Key Diffie-Hellman group: #2 (1024 bit) lifetime: 86400 seconds.1c .536. all telnet. End with CNTL/Z. ftp. one per line. Remote(config)# crypto isakmp policy 1 ! The policy number may be an integer between 1 and 65. This parameter set can be built using the following three steps: 1. IKE will determine preliminary information needed to create IPSec tunnels. and ! community of interest data separation is needed.

255.0 0. then choosing either the Authenticated Header (AH) or the Encapsulated Security Payload (ESP) IPSec protocols with either hashing algorithms SHA or MD5 will suffice.1.255 7. End with CNTL/Z.0 access-list access-list-number {deny | permit} protocol source source-wildcard source-options destination destination-wildcard destination-options The network administrator for the North and Remote routers should enter the IPSec access list into their routers using the following commands in privileged EXEC mode: North# config t Enter configuration commands. is shown below. A network administrator could argue that data authentication is not really needed for a protective tunnel between gateway routers since this property is normally obtained by Version 1.1.1c 209 .0.3 to learn more about access lists.2.0.255 North(config)# and Remote# config t Enter configuration commands.0.0.0. somewhat simplified. Security Services Remote routers.255 14.0.0 0.255 0.0 0.0. Configure the appropriate transform set The Cisco transform set identifies the desired protection mechanisms for building the IPSec tunnel.0 0.0.255 North(config)# access-list 161 permit ip 7.0. Choosing the any option for the source and destination also eliminates the need for netmasking in the access list.255.255.255. one per line.0. If the tunnel you are setting up needs data confidentiality protection.0 0.0 North(config)# access-list 161 permit ip 7. Access lists can be used to improve the granularity of the IPSec tunnels. North(config)# access-list 161 permit ip 14.255 7. Using the any option (e. one per line.0 0. access-list 161 below) for both the source and destination in the access list will force all packets to be IPSec protected. Remote(config)# access-list 161 permit ip 7.0 0.255. End with CNTL/Z. see Section 4.0 0.2.255 14.0.255 Remote(config)# Remote(config)# access-list 161 permit ip 0.0.255 Remote(config)# access-list 161 permit ip 14.255 14. The syntax for an access list rule.255.255 14.255. then choose the ESP protocol with either the DES or 3DES encryption algorithms (we highly suggest 3DES). If the tunnel needs data authentication protection.g.0.255 North(config)# access-list 161 permit ip 14.0 0.1.0 Remote(config)# access-list 161 permit ip 7.

but leaves the original IP header intact.1c . the security association lifetime for the tunnel (in kilobytes and/or seconds). and the tunnel modes. but adding it can improve defense in depth. . the ESP protocol is chosen with both data protection and authentication properties applied to all information transmitted between the North and Remote routers. the remote address for the peer end of the IPSec tunnel.Router Security Configuration Guide an application behind the router which is pushing data through the tunnel. Configure the IPSec transform sets using the following commands: North# config t Enter configuration commands. If the IPSec tunnel is used for separating communities of interest over a protected network. the transform-set used to build the tunnel (covered above in section 2). The transform set also specifies what part of each packet is protected by the IPSec tunnel. to which you can give a variety of transform-set related commands. For more information on both the encryption and authentication algorithms. For a hostile network scenario. then the transport mode will be sufficient. consult the Cisco IOS 12. IOS will enter crypto transform set configuration mode. one per line. one per line. Remote(config)#crypto ipsec transform-set set1 esp-3des esp-sha-hmac ! The name set1 is an arbitrary name Remote(cfg-crypto-trans)# mode tunnel Remote(cfg-crypto-trans)# end Remote# 3. This mode protects both the original data portion of the IP packet and the original packet header. This mode protects the original data portion of the IP packet. the preferred mode is tunnel (which is the default). and creates a new IP header using the routers’ IP addresses. Create the necessary crypto map Cisco IOS uses crypto maps to bring together all information needed to create IPSec tunnels. End with CNTL/Z. When you give this command. The command syntax for defining an IPSec transform set is: crypto ipsec transform-set transform-set-name transform1 transform2 .0 Security Configuration Guide [2]. The IPSec standards requires that tunnel mode be used when routers are employed as gateway security devices. transformN. This information includes: the access-list to specify what traffic should be protected (covered above in section 1). In the following example. End with CNTL/Z. and 210 Version 1. This hides potentially sensitive IP protocol information about the networks and applications that are sending data through the tunnel. Use exit to leave transform set configuration mode. . North(config)# crypto ipsec transform-set set1 esp-3des esp-sha-hmac ! The name set1 is an arbitrary name North(cfg-crypto-trans)# mode tunnel North(cfg-crypto-trans)# end North# and Remote# config t Enter configuration commands.

End with CNTL/Z. those with lower the sequence numbers have higher priority. Configure the IPSec crypto maps using the following commands: North# North# config t Enter configuration commands. they limit the length of time and ! number of bytes the tunnel is good for data protection before ! automatic rekeying occurs Remote(config-crypto-map)# set security-assoc lifetime kilo 80000 Remote(config-crypto-map)# set security-assoc lifetime sec 26400 Remote(config-crypto-map)# exit Remote(config)# exit Remote# Version 1. The syntax for the crypto map command is: crypto map map-name seq-num ipsec-isakmp. End with CNTL/Z. Remote(config)# crypto map pipe-1 1 ipsec-isakmp ! The name pipe-1 is an arbitrary name Remote(config-crypto-map)# match address 161 Remote(config-crypto-map)# set peer 14. one per line.Advanced Security Services whether to use the IKE protocol in setting up the tunnel. one per line. The following example shows the construction of a single crypto map for the North and Remote routers.1c 211 .1. Each crypto map is identified by a map-name and a positive integer sequence number (called seq-num below).2.20 Remote(config-crypto-map)# set transform-set set1 ! The following are optional.12.0.0 Security Configuration Guide to learn more about crypto maps. North(config)# crypto map pipe-1 1 ipsec-isakmp ! The name pipe-1 is an arbitrary name North(config-crypto-map)# match address 161 North(config-crypto-map)# set peer 7. while the sequence numbers are used to set the priority for two or more crypto maps with the same name. If two or more crypto maps with the same name are used. The map-name used can represent one or more crypto maps. which combine the previously entered configuration information.20 North(config-crypto-map)# set transform-set set1 ! The following are optional. they limit the length of time and ! number of bytes the tunnel is good for data protection before ! automatic rekeying occurs North(config-crypto-map)# set security-assoc lifetime kilo 80000 North(config-crypto-map)# set security-assoc lifetime sec 26400 North(config-crypto-map)# exit North(config)# exit North# and Remote# Remote# config t Enter configuration commands. See “Configuring IPSec Network Security” in the Cisco IOS 12.

then remove the crypto maps from the interfaces which they were applied. then the following commands should prepare both routers to establish an IPSec tunnel at the first beckoning of an IP packet which matches access lists 161.1. one per line. without an IPSec tunnel). as shown below. we are almost ready to build a tunnel between the North and Remote routers. Assuming the ping was successful. North# config t Enter configuration commands. one per line. in all likelihood. End with CNTL/Z. If both routers are connected to the Internet. A simple ping 7. North# config t Enter configuration commands. using outside interface eth0/0. End with CNTL/Z.12. As a quick check (which could potentially eliminate many headaches) before turning on IPSec. as in Figure 4-1.1.1c .20 on North should.20 set transform-set set1 set security-association lifetime kilobytes 80000 set security-association lifetime seconds 26400 North# Turning on IPSec at the Appropriate Interface Once the previous steps have been completed. one per line. End with CNTL/Z. we are now ready to build a tunnel between our routers.12. Remote(config)# interface ethernet 0/0 Remote(config-if)# crypto map pipe-1 Remote(config-if)# end Remote# If IPSec is no longer needed to protect traffic between two routers.Router Security Configuration Guide The command show crypto map will display the following information on the North router (assuming no other crypto maps have been entered): North# show crypto map Crypto Map "pipe-1" 1 ipsec-isakmp match address 161 peer 7. make sure the two routers are in a state where they can communicate (i. give us this answer.e. North(config)# interface ethernet 0/0 North(config-if)# crypto map pipe-1 North(config-if)# end North# and Remote# config t Enter configuration commands. North(config)# interface ethernet 0/0 North(config-if)# no crypto map pipe-1 North(config-if)# end North# and 212 Version 1.

0. the transform set).12. North# show crypto isakmp sa dst src state 7.2.2. #pkts decrypt: 5. media mtu 1500 current outbound spi: 1B908AE inbound esp sas: spi: 0xEFA038E(251265934) Version 1.: 14.255/0/0) remote ident (addr/mask/prot/port): (7. flags={origin_is_acl. local addr. the first ping requests might time out since the computation time needed for the IKE key exchange / IPSec computations varies depending on the size of the router.20 path mtu 1500. This will cause the routers to use the IKE protocol (including the IKE authentication key and the IKE security policy information) for authenticating the two routers and facilitate the negotiation of the IPSec tunnel’s protection algorithms (i.20/255.20 remote crypto endpt. If the negotiation is successful.0. the access lists will have notified the IOS that an IPSec tunnel has been requested to protect packet data.12. Remote(config)# interface ethernet 0/0 Remote(config-if)# no crypto map pipe-1 Remote(config-if)# end Remote# Testing A quick way to test if our IPSec tunnel has been established between the two routers is to simply execute a ping from one router to the other. If everything has been set up properly.0.20 QM_IDLE conn-id 1 slot 0 North# show crypto ipsec sa interface: Ethernet0 Crypto map tag: pipe-1. #pkts verify 5 #send errors 5.255. the user should be able to review the IPSec tunnel parameters.2.e.2. the tunnel will be established and the ping requests will be protected.1c 213 . These parameters can be seen using the show crypto ipsec security-association and the show crypto isakmp securityassociation commands. End with CNTL/Z.Advanced Security Services Remote# config t Enter configuration commands.20 Depending on the time allotted for a ping echo reply to return to the ping source.255/0/0) current_peer: 17.12.: 7.1. #recv errors 0 local crypto endpt.} #pkts encaps: 5.1. #pkts encrypt: 5.20 local ident (addr/mask/prot/port): ( etc. Once the IPSec tunnel has been established. 14. speed of the network. one per line. #pkts digest 5 #pkts decaps: 5.20 PERMIT.20/255.

} slot: 0. and the global configuration mode command no crypto ipsec sa. several things can be done. the following suggestions will help reset the IPSec relevant router parameters and hopefully allow for a tunnel to be constructed. crypto map: pipe-1 sa timing: remaining key lifetime (k/sec): (4607999/3459) IV size: 8 bytes replay detection support: Y inbound ah sas: outbound esp sas: spi: 0x1B908AE(28903598) transform: esp-3des esp-sha-hmac . then removing them is not a viable option. including Cisco’s.1c .g. will remove the unwanted information. in use settings ={Tunnel. then the information will be removed. If the tunnel fails to be constructed. If the crypto maps are in use by established tunnels. if the crypto maps are removed from the interfaces where they were placed (e. the construction of the IPSec tunnel will not succeed. First. the IOS stores certain information about both of their IPSec configuration files. } slot: 0. this information will reside in IOS memory and hinder future attempts at constructing tunnels between these two peers. in use settings ={Tunnel. Re-initialize the IPSec parameters by removing the IPSec and IKE security associations When an attempt is made to construct an IPSec tunnel between two peers. If your routers do not correctly establish the IPSec tunnels that you need. all tailored to the specific peer devices involved. a few Cisco IOS releases have demonstrated unstable functionality: in some cases packets which should be protected by the tunnel are passed in the clear. And even when a tunnel is established. several commands may be used to collectively remove the unwanted information. Hence. 1. 214 Version 1. conn id: 2. crypto map: pipe-1 sa timing: remaining key lifetime (k/sec): (4607999/3459) IV size: 8 bytes replay detection support: Y outbound ah sas: Troubleshooting Most current IPSec implementations. interface eth0/0 on both North and Remote above). The EXEC mode commands clear crypto sa or clear crypto isa commands. To remove this information and allow the routers to begin a fresh IPSec negotiation of tunnel parameters. conn id: 3. If any one of many parameters are not set properly.Router Security Configuration Guide transform: esp-3des esp-sha-hmac . can be very temperamental.

255 On Remote: access-list 102 permit ip 7. are not mirror images of each other.1.255.2. In our example above.1 0. On the other hand.1/24) with anyone behind the East router (IP address Security Services 2. which can be entered while the router is in privileged EXEC mode. timeout is 2 seconds: .0.0. 100-byte ICMP Echos to M-ID of 405257172 00:19:35: ISAKMP (1): sending packet to 7.0. but both were on while the IPSec tunnel was being constructed. (Note: These debug options were run at different times. Make sure the routers have mirror access lists The Cisco IOS IPSec code can get easily confused when the access lists.!!!! Success rate is 80 percent (4/5).2.1.) North# debug crypto isakmp Crypto ISAKMP debugging is on North# ping 7. Turning on the debug commands to observe the router’s IPSec negotiation It can be very helpful to run both the debug crypto ipsec and the debug crypto isakmp commands.255 7.20/16).1c 215 .0.12.0. (Note: If the routers establishing the IPSec tunnel are not currently operational. Full debugging imposes too great a load to be practical for operational routers.0. round-trip min/avg/max = 32/33/36 ms North# 00:19:35: ISAKMP (1): beginning Quick Mode exchange.1 0. and determine exactly where the negotiations are failing. get protected.0. which are engaged by the crypto maps to determine what packets are protected using the IPSec tunnel. Sending 5. then the following access lists on Remote and North would satisfy the mirror access list requirement and should allow for the tunnel to be constructed between North and Remote. Below is a list of the North router’s output when these two debug commands were turned on. we can see that the access lists used by both North and Remote are mirror images since they both involve using the any option to indicate that all protocol packets.0.20 The debugging messages will allow the network administrator to observe how the local router is processing the remote router’s IPSec packets during the tunnel negotiation.255 14.20 0.29 (I) QM_IDLE 00:19:35: ISAKMP (1): received packet from if we only want to protect packets to/from a LAN behind the Remote router (IP address 7. turning on full debugging using the debug all command supplies even more diagnostic information. with source and destination addresses each behind one of the routers.20 (I) QM_IDLE Version 1.255 3. On North: access-list 101 permit ip 14.20 Type escape sequence to abort.0. dest_proxy= 7. lifedur= 3600s and 4608000kb..1c . (key eng.1. src= 14.2.2. ISAKMP (1): processing NONCE payload.2.12. 4w0d: IPSEC(initialize_sas): .255.0. dest_proxy= 7. 216 Version 1.) dest= 7. msg.1.20 to 7.12.1. message ID = ISAKMP (1): processing ID payload.255/0/0 (type=1).12. message ID = ISAKMP (1): Checking IPSec proposal 1 ISAKMP: transform 1. transform= 3esp-des esp-sha-hmac .20 ) has spi 595658916 and conn_id 5 and flags 4 lifetime of 3600 seconds lifetime of 4608000 kilobytes ISAKMP (1): sending packet to ESP_3DES ISAKMP: attributes in transform: ISAKMP: encaps is 1 ISAKMP: SA life type in seconds ISAKMP: SA life duration (basic) of 3600 ISAKMP: SA life type in kilobytes ISAKMP: SA life duration (VPI) of 0x0 0x46 0x50 ISAKMP: authenticator is HMAC-SHA ISAKMP (1): atts are acceptable.1. src_proxy= protocol= ESP.255.20 (proxy 7.20 (I) QM_IDLE North# no debug all North# debug crypto ipsec Crypto IPSEC debugging is on North# ping 7..20.1.255/0/0 (type=1).255.2.12. lifedur= 0s and 0kb.20 to 7.) dest= 7.20 for prot 3 4w0d: IPSEC(key_engine): got a queue event.1.20 to to 14.20 ) has spi 59056543 and conn_id 4 and flags 4 lifetime of 3600 seconds lifetime of 4608000 kilobytes outbound SA from 14.255. msg. 4w0d: IPSEC(spi_response): getting spi 595658916 for SA from 14. protocol= ESP.20 North# 4w0d: IPSEC(validate_proposal_request): proposal part #1.0. keysize= 0.20/255.20.. spi= 0x0(0). src= 14.255. conn_id= 0.20/ to 14.12. transform= 3esp-des esp-sha-hmac .20 (proxy 14.20/255.255/0/0 (type=1). message ID = ISAKMP (1): Creating IPSec SAs inbound SA from 7.20/255. flags= 0x4 4w0d: IPSEC(key_engine): got a queue event.0. (key eng.255/0/0 (type=1).255..12.Router Security Configuration Guide 00:19:35: 405257172 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 0x0 00:19:35: 00:19:35: 00:19:35: 405257172 00:19:35: 405257172 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: 00:19:35: ISAKMP (1): processing SA payload. src_proxy=

sa_conn_id= 101 North# no debug all 4. perform the following steps: 1. dest_proxy= 7.) dest= 7.2. sa_trans= 3esp-des esp-sha-hmac .Advanced Security Services spi= 0x238108A4(595658916). spi= 0x385219F(59056543). src= 14.20. Enter configuration mode: Version 1.9. conn_id=100. Another use of IPSec would be to use it to protect the administration of a Cisco router. Use an IP packet sniffer to observe the contents of each packet in the IPSec tunnel negotiation This information. a computer on the desk of the administrator is to be used to administer the North router.1. we’ll set up the configuration on the router. First.20/255. 5. sa_prot= 50.250. (sa) sa_dest= 7. src_proxy= 14. sa_spi= 0x385219F(59056543). (sa) sa_dest= 7. sa_conn_id= 100 4w0d: IPSEC(create_sa): sa created.12.0.255/0/0 (type=1).2.255. keysize=0.20/255.flags=0x4 4w0d: IPSEC(initialize_sas): .6. conn_id=101.255. flags=0x4 4w0d: IPSEC(create_sa): sa created.1. Let’s say the computer the administrator uses to configure the router has IP address 14. (key eng. 217 .0. sa_prot= 50. keysize=0.255/0/0 (type=1). and for recovering from failures. or isolating networks within an organization. eliminating the threat of a network sniffer seeing passwords or sensitive configuration information. is invaluable in diagnosing exactly where the tunnel negotiation is failing. lifedur= 3600s and 4608000kb. Using IPSec for Secure Remote Administration The example used throughout the preceding section was to securely connect two networks from their gateways (which were Cisco routers). Configuring a Cisco Router for IPSec Secure Remote Administration On the Cisco router.12. msg. sa_trans= 3esp-des esp-sha-hmac . then examine the configuration sequence for a PC running Microsoft Windows 2000 or XP.2. The IP address of the North router on the interface closest to the administrator is 14. transform= 3esp-des esp-sha-hmac . Since both of these run over IP. sa_spi= 0x238108A4(595658916). protocol= ESP. so we’ll secure a connection to there. like that obtained from running the debug commands on the router.2. This could represent either connecting widely separated networks. which is next to the servers in Figure 41.255.255. IPSec can be used to encrypt this communication. Common ways to perform administration of a Cisco router are to use either telnet (which sends the password in the clear) or SNMP. In this example.2.20.20.

The encryption options are DES and 3DES. we can use transport mode instead of tunnel mode.2. so we will take the default and not enter it. End with CNTL/Z. Again. but again. or anything easily guessed. North(config)# crypto ipsec transform-set 3des-sha-xport esp-3des esp-sha-hmac North(cfg-crypto-trans)# mode transport North(cfg-crypto-trans)# exit North(config)# 218 Version 1. so again we select the stronger option. The transform-set contains the parameters for protecting the actual traffic. we want to use 3DES. North(config)# no access-list 12 North(config)# access-list 12 permit 14. and possibly not even strong enough to protect passwords. certificates just for this probably aren’t warranted. The policy number selected here is 10.1. Create an ISAKMP policy. and punctuation (see Section 4. certificates can be used. it is not forwarding this particular traffic anywhere else). North(config)# crypto North(config-isakmp)# North(config-isakmp)# North(config-isakmp)# North(config-isakmp)# North(config)# isakmp policy 10 authentication pre-share encryption 3des group 2 exit 4. so we will skip this.Router Security Configuration Guide North# config t Enter configuration commands.2. if some form of a public key infrastructure (PKI) is already in place.1c . Enter the authentication password. The key exchange size of group 2 is larger than that for group 1.e. which is just an arbitrary number to set a priority. this does not concern us too much. SHA. However. The other option is the lifetime until a key renegotiation is required. and SHA. Pre-shared keys are passwords – or better yet a passphrase.5 for more guidelines on password quality). North(config)# crypto isakmp key my4pa$$phra$eHere address 14.9. if two or more ISAKMP policies exist on North.9. one per line. include letters. DES has been demonstrated to be weak. is suitable. numbers.6 North(config)# line vty 0 4 North(config-line) access-class 12 in North(config-line) exit North(config)# 3. Since the authentication is only between 2 machines. Please do not use anything in the dictionary. so we recommend 3DES. This choice is also made because currently it is easier to configure IPSec in Windows 2000 to use transport mode. We’ll use access list 12 to list the machines that may to telnet to the router. Enable telnet access to the router for administration from the administrator’s machine. Since we are treating the router as just a host to connect to (i. North(config)# 2. and so pre-shared keys can be used. The default hashing algorithm.6 5.

2. North(config)# access-list 167 permit ip host 14. North(config)# interface ethernet 0/1 North(config-if)# no crypto map North(config-if)# crypto map cisco-admin North(config-if)# exit North(config)# exit North# Configuring Windows 2000/XP for Secure Remote Administration Once the Cisco router has been set up.1. Click the “Add” button and you will see all the possible snap-ins. When doing so. Finally.1. It will ask which computer it should manage. we’ll ensure that no other crypto maps are still in existence before we define this one.9. Version 1.6 host 14. Scroll down until you see one titled “IPSec Policy Management” or “IP Security Policy Management” and select that one.250 host 14.250 7.250 interface is named Ethernet 0/0).1. Any name can be given to this – we use ciscoadmin. That will give you a window containing the currently added list of snap-ins. apply these definitions to the interface (the 14.2.) First.Advanced Security Services 6. Then we exit from configuration mode. This section assume moderate familiarity with Windows 2000 or XP network administration.9. (The screenshots shown below were captured on Windows 2000. so we use the one we entered in the previous step. run Microsoft Management Console (MMC). The IPSec connections must be allowed. but the directions also apply to Windows XP. You can add the snap-in by looking under the “Console” menu and selecting “Add/Remove Snap-in”. initially empty. We number the access list as 167.6 set transform-set 3des-sha-xport match address 167 exit 8.2. and “OK” the one snap-in that you have added. North(config)# crypto map North(config-crypto-map)# North(config-crypto-map)# North(config-crypto-map)# North(config-crypto-map)# North(config)# cisco-admin 10 ipsec-isakmp set peer 14.2. “Add” the IPSec snap-in for the local machine. A crypto map must be created. or by using the “Run” command from the “Start” menu). Priority for this crypto map is set to 10.1. click “Finish”. The match address links the desired access-lists to the crypto map. 167. and IPSec should be running on the Cisco router. A few of the dialogs may look slightly different. and select “Local Computer”. “Close” the list of additional snap-ins. The management console window should now look something like the screenshot below.1c 219 . the Windows 2000 computer on the desktop of the administrator can be prepared. either from a command window prompt.9.6 North(config)# access-list 167 permit ip host 14.1.

Click so that the default response rule is not activated.Router Security Configuration Guide Click right on “IP Security Policies on Local Machine” (either the left or right window will work) and select “Create IP Security Policy”. In this window. but before that. Any name will do. and you aren’t required to fill in a description. and then hit the “Finish” button. for which we will use the Add Wizard. perhaps something like “Admin to Router”. ensure that the “Edit properties” box is selected. Click “Next”.1c . which we will do in a second. The following window should appear. A new rule must be added. click “Next”. Two things must be done in this window. A wizard shows up to assist you on this quest. It asks for a name and description for this new policy. we will configure the key exchange parameters (which were called by the name isakmp in the 220 Version 1. Click “Next”.

not the 768 bit group 1). click the “Methods” button. and any values for when to rekey are acceptable. which is the 1024 bit Diffie-Hellman option. Use the sideways scroll bar to see if a security method exists with the same settings as on the router. In the tabs at the top of this window. under “Protect identities with these security methods”. In this window. To ensure everything is set up the same as on the Cisco. select “General”. Now you will see the following new window. If such a method does not exist. The window that appears contains the title “Key Exchange Settings”. 3DES encryption. Click that. Version 1.1c 221 . and “Medium (2)” for the Diffie-Hellman size (which is Group 2. The others can either be deleted or just left there. and click the “Move up” button until it is the first option on the list. SHA1 Integrity (the hashing algorithm). either modify a currently existing method by highlighting one and hitting the “Edit” button. Those values are IKE negotiation (Cisco calls it ISAKMP. Under that tab at the bottom of the screen is a button for “Key Exchange using these settings” with the word “Advanced” written on the button.Advanced Security Services Cisco configuration). you probably should click on the correct one (which will highlight it). In either case. which is actually the name of a foundational specification on which IKE is built). do not check the “Master key Perfect Forward Secrecy” button. or click the “Add” button to create a new one.

Recall that we previously selected “my4pa$$phra$eHere” as our choice when we configured the Cisco router. You should now be back at the window where you selected the “General” tab. or even better “All network connections” can be used. Click “Next”. choose either LAN connection. which will use a second wizard. The next screen is about which network connections to use. Now select the “Rules” tab and let’s continue. Continue on without specifying a tunnel. When the introduction screen for the wizard shows up. click “Next”. 222 Version 1. but in this case. Now is the time to enter the passphrase. we do not need this tunnel..” button. Since we selected transport mode when configuring the Cisco router. The network type “Remote access” is useful if you are using phone lines to connect remotely.Router Security Configuration Guide Click “OK”. and then click it again on the next window..1c . which will make the following tunnel endpoint window appear. Click the “Add.

Now we need to add a filter. Version 1. click “Add” and the following IP Filter List definition window will appear. it is probably empty. If you use the wizard. From there. or something like that). The IP Filter List window will appear. Name this filter (Cisco Only Filter. Initially. but before you “Add”. This third wizard is not helpful. click “Add” and you should see the following screen. unselect the “Use Add Wizard”. unselect the “Use Add Wizard” option.1c 223 .Advanced Security Services We enter that in the appropriate box. you get several screens in which you will type in the information you can supply to the one screen you see if you do not use the wizard. and click “Next”. So.

You do need to leave the mirrored option on so filters are defined for traffic going in both directions. Double click on “Require Security” to see what options are set. 224 Version 1.255 which permits secure connections only to the one router and leaves all other communications unaffected.255.Router Security Configuration Guide You want it to have the Source address as “My IP Address” and the Destination address as “A specific IP Address” in which you fill in the IP address of the Cisco router. Before selecting the “Require Security” option. Click OK. 14. returning you to the filter list window.” Then select that filter (call it “Cisco Router Filter”) from the list of filters and click “Next”. The next window that appears is the Filter Action window. which you should “Close. “Request Security” and “Require Security”. you will want to examine it in a bit more detail to be sure that it contains the options you need. Use a subnet address of . There are three default filters defined. It should look something like this. “Permit”.2.

if you want to delete all but the one offer that is used. IPSec will be activated automatically. In fact. “Assigned”. For just this connection. and that new policy will appear in the management console window. Look at the third column. Make sure that the policy is actually in effect. Version 1. so any settings for that are acceptable. then right-click on it. and the traffic will be IPSec protected. communication to other places is not affected and so not IPSec protected. When all these settings are correct.1c 225 . After following all these steps." The next time you connect to the Cisco router. typically you must explicitly assign a policy after creating it. We want to select “Negotiate security” here. we want to use security. Choose “Accept unsecured communication. and are not using the AH protocol. but always respond using IPSec”. The lifetime (until keys are renegotiated) is not important. which is mostly used when the initial key exchange is shared. Highlight the “Require Security” button. Perfect Forward Secrecy is a way to do a second key exchange. The reason we don't want to allow unsecured communications is that this IPSec configuration only applies to communication with the router. and select “assign” from the popup menu. click “OK”. This is not the case here. The value in the third column should change to “Yes” and the policy will be imposed. For our example. “Allow unsecured communications with non IPSec aware computer” and “Session key Perfect Forward Secrecy”. you have created an IP Security Policy. If the column contains the word “No”. of the policy listing in the management console window. that would not be bad. The only remaining thing to do is to click "Finish. so we require it. and click “Next”. We do not want to select the final two options. we are using ESP with both 3DES and SHA.Advanced Security Services Click on the security method preference order options and edit them to ensure that at least one of them contains the cryptographic settings for protecting the actual data that was configured in the Cisco.

226 Version 1. The first attempt should fail and report "Negotiating IP Security".Router Security Configuration Guide A quick check to ensure that it is working is to ping the router from the Windows 2000 host. On the router. A network sniffer can be used to verify that communications between the router and host are encrypted. and the router and the Windows 2000 host should have completed their key exchange and the ping should succeed.1c . use the command show crypto ipsec sa to confirm that IPSec is being used. Ping a second time.

IOS versions that do not support IPSec usually do not support SSH either.Advanced Security Services 5. Cisco IOS 12. passwords and other sensitive information are not exposed in the clear between the administrator’s host and the router.2.9.1. First.250) to run the SSH server and accept incoming connections.1c 227 . 5. Only certain Cisco IOS versions are shipped with the SSH feature set. you must establish usernames for people that will be attempting to connect to the router. SSH also prevents session hijacking and many other kinds of network attacks. and rcp services.1. SSH was originally intended to be a secure replacement for classic telnet. there are two important prerequisites to address. There are two main versions of the SSH protocol in widespread use. make sure that the router has a local hostname and domain name set.1 log North(config)# line vty 0 4 North(config-line)# access-class 12 in North(config-line)# exit Version 1. consult [13]. See Section 4. Because the connection is encrypted. SSH versions 1 and 2. With SSH. Before you can configure SSH.6 for detailed information on how to define usernames.9. Versions after and including 12. In this scenario the administrator will use an SSH client on the host 14. commonly called SSH. perform the following steps: 1.1 to connect to the router for administrative purposes. The detailed example below shows how to configure the router North (14.0(5)S with IPSec include support for SSH. see Section 4. Instructions for this can be found in section 4. see Section 8.3. End with CNTL/Z. It utilizes RSA public key cryptography to establish a secure connection between a client and a server.1. North(config)# no access-list 12 North(config)# access-list 12 permit host 14.2. For more information about access lists. Configure an access list permitting access from the administrative host. North(config)# 2.3. For a thorough discussion of SSH.0 through 12. For more information about IOS versions.2. This example uses standard IP access list 12 to identify the hosts that may start SSH sessions into router North. IOS 12. Configuring a Router for Secure Remote Administration with SSH While in IOS enable mode. rlogin. Using SSH for Remote Administration Security An alternative to setting up IPSec for secure remote administration is to configure your router to use the secure shell service.3 are currently capable of supporting only SSH version 1. one per line.3.2.4 can support SSH version 2. rsh.3. Enter configuration mode: North# config t Enter configuration commands.

which is also the maximum allowed value. If you need to remove an old key pair. To generate a new key pair. Note: check carefully before deleting a key pair. because there is no way to recover a private key that has been deleted. (Note: you must assign a domain name before creating a key.2 for direction.. generate a new RSA key pair for this router. then you may delete them using the command crypto key zeroize Choose the size of the key modulus in the range of 360 to 2048 for your General Purpose Keys. North(config)# ip ssh time-out 90 North(config)# 228 Version 1. do the following. the SSH service will be present on the router whenever an RSA key pair exists.dod. Set up a username that is permitted to connect to the router. Choosing a key modulus greater than 512 may take a few minutes. use the command crypto key generate as shown below. If you delete the router’s RSA key pair. If you have already created user accounts (with or without AAA).) Cisco suggests a minimum modulus size of 1024 bits. the SSH server is enabled and running. If the router already has a key pair. Once the connection is established. North(config)# username joeadmin password 0 1-g00d-pa$$word North(config)# line vty 0 4 North(config-line)# login local North(config-line)# exit North(config)# To act as an SSH server.. The default authentication timeout is 120 seconds. This is the number of seconds the server will wait for a client to respond with a password. North(config)# crypto key generate rsa The name for the keys will be: North. then the SSH server will stop. the router must possess an RSA key pair.2. as detailed below. see Section 4. By default. you may skip this step. then you may use it for SSH. • Configure an authentication timeout. The recommended value is 90.1c . [OK] North(config)# If this command succeeds. but it will not be used until you configure it.6. To change this from the default. standard vty timeout settings apply. Below are some useful commands for further configuring the new SSH server. and you are absolutely sure that the keys are not being used. Otherwise. How many bits in the modulus [512]: 2048 Generating RSA Keys . perhaps generated as part of its IPSec configuration. as specified in section 4.Router Security Configuration Guide 3.

the example below shows how to set the router to drop the connection at the second failure. North(config)# line vty 0 4 North(config-line)# transport input ssh North(config-line)# exit North(config)# 5. these commands vary between IOS releases. which is recommended.2 To verify that SSH has been successfully enabled. The two subsections below describe the commands in detail. IOS Version 12. If your session is secure then the output should resemble that shown below. execute the following command. There may or may not be any current connections. the maximum value is 5. which is a sound choice. Authentication retries: 2 North# To verify that SSH has been successfully enabled and check that your session is actually using SSH. North# show ssh Connection Version 0 1. connect to the router using your SSH client and type the command show ssh. North(config)# ip ssh authentication-retries 2 North(config)# To disable telnet and require SSH.3. simply leave off use the transport input command as shown below. but if an IOS router supports SSH then Telnet should not be used. North# show ip ssh Version 1. The default value is 3 attempts.2. and verify your output states that SSH is enabled. and verify your output is similar to the following. It is also possible to accept both SSH and Telnet. North# show ip ssh SSH Enabled – version 1. However. Do not set the value higher than three.5 North# Encryption State 3DES Session Started Username joeadmin IOS Version 12.1 To verify that SSH has been successfully enabled.1c 229 . depending on how you are connected to your router.Advanced Security Services • The number of incorrect login attempts that are permitted before the router will drop a remote access connection is also configurable.5 Authentication timeout: 90 secs. Advanced SSH Commands There are a number of commands that will allow you to verify that the SSH server is now operational. execute the following command.

This example disconnects session 0.1.5 Encryption 3DES State 4 Username joeadmin Diagnosing and Managing SSH In the unlikely event that a connection has not properly closed.2.250 joeadmin@14. or that a connection must be forcefully closed.5 3DES Session started joeadmin North# exit Connection to 14. then use of the commands below to disconnect the session. North# disconnect ssh 0 North# .1. there are two ways to go about this. which is in use by user joeadmin.250 closed.250’s password: Warning: Remote host denied X11 forwarding North> enable Password: North# show ip ssh SSH Enabled – version 1. % 230 Version 1.Router Security Configuration Guide Connection 0 North# Version 1.1. Obtain the connection number using one of the two methods shown above.1.1. It is very important to disable debug messages when you are finished using them.1c . % ssh –l joeadmin 14.5 Authentication timeout: 90 secs. Authentication retries: 2 North# show ssh Connection Version Encryption State Username 1 1. North# ! enable debug messages from the SSH service North# debug ip ssh North# ! disable debug messages from the SSH service North# no debug ip ssh A Sample SSH Session The sample session below shows how to connect from a Unix host to a Cisco router using the OpenSSH client.or North# clear line vty 0 North# You can use the IOS command debug ip ssh to diagnose SSH operation.

the host must support an SSH client. and most Unix and Linux distributions include an SSH client by default.3. Authentication retries: 2 North# Version 1. Information on where to download these applications can be found in the Tools listing. Some Available SSH Clients To employ SSH between an administrative host and the router. Even though SSH version 1 may be subject to network man-in-the-middle attacks in some circumstance.3. North(config)# ip ssh version ? <1-2> Protocol version North(config)# ip ssh version 2 North(config)# exit North# show ip ssh SSH Enabled . configure the router to use it exclusively as shown below. Section 9. Unix / Linux • OpenSSH (freeware) • SSH Secure Shell (commercial) Windows • PuTTY (freeware) • TTSSH Plugin for TeraTerm Pro (freeware) • SecureCRT (commercial) • SSH Techtia client (commercial) MacOS • NiftyTelnet 1.3.0 Authentication timeout: 90 secs. consult [12].4. it is still a much more secure choice for remote administration than unprotected Telnet. Below are some useful clients for common host operating systems.version 2.Advanced Security Services 5. For more information about vulnerabilities in SSH and Cisco IOS.1c 231 .3. When your IOS release supports SSH version 2.1 (freeware) 5. Security of SSH Versions There are several known security weaknesses with the SSH version 1 protocol. Attacks that exploit these weaknesses are complex and non-trivial to execute. but tools that implement some of them do exist. There are usable clients available for almost every host platform (even PDAs).

2. Current product catalogs and web pages use the new name. Basic Concepts A network firewall is a network device that connects a protected internal network to some other untrusted. it offers you another facility for enforcing security policy at the boundary between different networks.4.) 5. many router vendors. Using a Cisco Router as a Firewall This section describes how to use a Cisco router as a modest firewall.” Much of the documentation still uses the old name. Configured together with good access lists.1. To reach even a moderate level of effectiveness as a firewall. CBAC can provide modest firewall protection for a network without extra hardware.3 describes access lists in detail. and that is what we will use below. Stateful inspection firewalls do this by inspecting each packet for compliance with the specified security policy. Using NAT. (Note: in mid-2000. by transparently translating all IP addresses and coalescing distinct IP addresses into a single one. it can effectively enforce a number of network security capabilities. provide a rudimentary firewall capability in their routers. including Cisco.) 5. Cisco renamed the IOS Firewall to “Cisco Secure Integrated Software. (Another important feature for firewalls is hiding network addresses and structure. but to offer no such services in the other direction.Router Security Configuration Guide 5. Cisco IOS provides full support for Network Address Translation (NAT). a router can hide the structure of the trusted network. Potential applications for using a Cisco router as a firewall include: a modest Internet firewall. Section 4. consult the Cisco IOS documentation for information about IOS NAT features. Note that CBAC is intended mainly for border routers. a firewall between two different communities of interest. external individuals from gaining access to your internal network.4. As long as all traffic between the trusted and the untrusted network pass through the firewall. and a firewall between a main network and a restricted enclave. Because routers connect networks together.1c . The security policy for this setup is to permit users to take advantage of certain network services on the untrusted network. possibly hostile network. the router configuration must include good access lists. while at the same time allowing authorized users on the trusted network (the ‘inside’) access to services on the untrusted network (the ‘outside’).4. Configuring Cisco IOS Content Based Access Control The Cisco IOS Firewall feature set is designed to prevent unauthorized. This guide does not describe NAT. and to block attacks on your network. if it is running a version of IOS that has firewall capabilities. The figure below shows the basic structure for a CBAC-based firewall setup. The Cisco IOS Firewall feature set Content-Based Access Control (CBAC) facility allows a router to act as a rudimentary stateful inspection firewall. 232 Version 1.

there must be at least one access list in place on the path from the untrusted network to the trusted network.Advanced Security Services Untrusted Network 14.64 Router 14.6.20 port 80.2. CBAC inspects the initial TCP packet of the connection. untrusted network 1. and adds a rule to the inbound access list.10.10. but also examines the application layer protocol information (such as FTP information) to learn about the state of TCP and UDP connections.2.10. 2.2. permitting data from 7.1. CBAC maintains connection state information for individual connections.64 Trusted Network 14. 3.2.20 (port 80) on the untrusted network.9. or an outbound list on the inside interface. passes access list. The heart of CBAC is the ability to inspect outgoing IP traffic in real-time. Response comes back from the web server. either an inbound list on the outside interface.2. maintain state information. inside interface inspect outside interface 2. Because CBAC works by modifying access lists.1c 233 .6 User Host 14.7 Figure 5-1: A Simple Router Firewall CBAC examines not only network layer and transport layer information. and use that information to make access decisions. 1.1. The figure below illustrates this. Host initiates a web connection to web server 7.6. Outbound request Router Inbound response trusted network Host Figure 5-2: CBAC Overview Version 1. CBAC adjust access list 3. The decisions are enacted when CBAC dynamically adds rules to interface access lists to pass permitted traffic.10.0/24 User Host 14.

Testing for CBAC Support on the Router Examine the router IOS installation to ensure it has the firewall feature set. Check that the router supports CBAC. The easiest way to check is to execute a CBAC-related command. 5. 8. 7. POP3. 6. This is the access list that CBAC will be modifying on the fly. usually the outside interface of a border router. Set up an inbound access list on the outside interface. RealAudio (RTSP) 3. 234 Version 1.Router Security Configuration Guide Note that CBAC handles only TCP and UDP protocols.0(9) with Firewall Feature Set 2.1c . SMTP. South. Determine the list of services that users or hosts on the trusted network need from the untrusted network. Example: FTP. Step 1. Other IP protocols and services. Test the configuration from a host on the trusted network by running services. or IPSec. Web (HTTP). There is no simple. then install an IOS version that does (see Section 4. and test it from the untrusted network by running a network scanner (see Section 6). prohibiting all traffic that should not leave the trusted network but allowing traffic on the desired services list (see Section 4. Set the CBAC global timeouts. Central. 4. such as ICMP. Call this list the desired services list. direct way to check whether a router has CBAC capability. Example: IOS 12.5. OSPF. must be separately permitted by the interface access lists if you need them. if the command fails. like H. if it does not. Steps in Setting Up a Cisco Router Firewall To set up a simple firewall using CBAC. follow these steps: 1. The two examples below show a router without CBAC. Create a CBAC inspection ruleset supporting the desired services list. Apply the CBAC inspection ruleset to an interface. permitting traffic that the router must process.323 and FTP. These timeout values determine the duration of window of accessibility opened back through the firewall in response to a request from the trusted network. then CBAC is not supported. values that are too long can leave the trusted network vulnerable.5). and a router with CBAC. Set up an outbound access list on the outside interface.3). but prohibiting other TCP and UDP traffic including the desired services list. It also includes some special case handling for multi-port application protocols.

FTP. Service Basic TCP Protocols Definition Generic connected TCP protocols. Determine the Application Services to Support Decide which application-layer protocols to permit using CBAC. generic UDP must also be configured to use it. H. Central# versus South# show ip inspect all Session audit trail is disabled Session alert is enabled . It also prevents use of non-standard ports for FTP data. a reasonable list of desired services for many installations is: DNS.323 (NetMeeting) RealAudio (RTSP) For web traffic (HTTP). HTTP. Control connection on TCP port 21. select ones to support by permitting them through the access list set up in Step 3. Telnet. Because NetMeeting uses additional non-standard ports. Remarks CBAC will support any of these. CBAC automatically tracks the RealAudio port assignments. Other UDP FTP Mail (SMTP) H. etc. SSL. CBAC in IOS 12. etc. Version 1. Connect TCP protocol on port 25. data on TCP port >1024. POP3. Because the Java blocking capability is very weak. plus SMTP and POP3 to the mail server only. SNMP. NTP. and watches the FTP authentication exchange. Real-Time Streaming Protocol over UDP or TCP. CBAC has some ability to block Java applets. CBAC permits only RFC 821 standard SMTP commands. NTP. For example. such as HTTP. such as DNS.323 video conference protocol over UDP. CBAC will support any of these. .1c 235 . TFTP. CBAC has special support for FTP. select ones to support by permitting them through the access list set up in Step 3. South# Step 2. it is not typically employed.0 supports about a dozen application service types. and Telnet. the most commonly used ones are listed below. IKE.Advanced Security Services Central# show ip inspect all ^ % Invalid input detected at ‘^’ marker. Generic UDP services. Best practice on a router is deny all protocols except those identified as needed.

255 host 14.255 any eq domain South(config-ext-nacl)# permit tcp 0.3 eq smtp South(config-ext-nacl)# permit tcp 14.0. and permits the RIP routing protocol (see Section 4. The source address for each rule in the access list should be a network address or address range valid for the trusted network. Set up an Inbound Access List CBAC works by modifying inbound access lists: it can work with an access list applied to the interface on the trusted or untrusted networks. or even both.0 0.255 any South(config-ext-nacl)# permit udp 14.10. In this example.2.255 any eq telnet South(config-ext-nacl)# permit tcp 14.0. in general.2. the access list is applied to the outside interface.2. This access list must permit the protocols on the desired services list.3). For more information about access lists. Set up an Outbound Access List Before CBAC can do its work. Security Configuration Guide Step 3. permits a modest set of useful ICMP messages.0.0. this is a safe choice.10.2.0 0.1c .0.10.0 0.0 0.255 host 14.10.0 0.255 any eq ftp South(config-ext-nacl)# permit tcp 14. South(config)# ! create inbound access list for CBAC to work on South(config)# no access-list 111 South(config)# ip access-list extended 111 South(config-ext-nacl)# permit icmp any any echo-reply 236 Version 1. see Section 4.255 any eq ntp South(config-ext-nacl)# permit udp in the outbound direction. even those on the desired services list. Also. The example access list below blocks TCP and UDP traffic effectively.2.2.255 any eq www South(config-ext-nacl)# permit tcp 14. The example below shows an access list for our desired services list. An inbound access list intended for use with a simple CBAC firewall scheme should block all TCP and UDP services. South(config)# ! Create the access list South(config)# no access-list 110 South(config)# ip access-list extended 110 South(config-ext-nacl)# permit icmp 14.3 eq pop3 South(config-ext-nacl)# deny ip any any log South(config-ext-nacl)# exit South(config)# ! Apply the access list to the outside interface South(config)# interface eth 0/0 South(config-if)# ip access-group 110 out South(config-if)# exit South(config)# Step 0. the destination address can be the catch-all any.0 0. this access list must be an extended IP access list.0.9. there must be an access list applied to traffic from the trusted net to the untrusted net.

and POP3 email services. using the command ip inspect audit-trail. audit trails could only be turned on globally. The syntax is shown below. and the second rule supports web. the audit-trail option controls whether use of that protocol causes a log message to be generated. use the command ip inspect name. (In older versions of CBAC. Create a CBAC Ruleset To create a CBAC ruleset. Telnet. • The connection closes down (TCP only). This rule gets removed when one of the following conditions are satisfied: • The response does not arrive within the allotted timeout time. Adjust the CBAC Global Parameters When CBAC detects a connection attempt by a client on the trusted network. Version 1. it adds a rule to the inbound access list to permit the expected response. similarly. The name of the ruleset is “fw1. ip inspect name ruleset-name protocol [alert on/off] [audit-trail on/off] [timeout override-timeout] The alert option controls whether use of that protocol causes a console alert message to be generated.) The example ruleset below supports the example desired service list.Advanced Security Services South(config-ext-nacl)# permit icmp any any unreachable South(config-ext-nacl)# permit icmp any any ttl-exceeded South(config-ext-nacl)# permit icmp any any packet-too-big South(config-ext-nacl)# permit udp any any eq rip South(config-ext-nacl)# deny ip any any log South(config-ext-nacl)# exit South(config)# ! apply the access list to the outside interface South(config)# interface eth 0/0 South(config-if)# ip access-group 111 in South(config-if)# exit South(config)# Step 5. South(config)# South(config)# South(config)# South(config)# South(config)# ip ip ip ip inspect inspect inspect inspect name name name name fw1 fw1 fw1 fw1 udp audit-trail on tcp audit-trail on ftp audit-trail on smtp audit-trail on Step 6. • The connection is idle for longer than an allotted idle time.” Its first rule supports DNS and NTP. beyond those generated by interface access lists.1c 237 . Enable the alert and audit-trail features to get additional log messages.

a modem) or on a highly congested network. The table below describes the parameters to change. these values might need to be increased for a very slow connection (e. South(config)# ip inspect tcp South(config)# ip inspect tcp South(config)# ip inspect tcp South(config)# ip inspect udp South(config)# exit South# one per line. one per line.Router Security Configuration Guide The default timeout and idle times in Cisco IOS 12.0 are longer than necessary. 238 Version 1. End with CNTL/Z. but these can be left at their default values. Apply the CBAC Ruleset to the Interface CBAC is not in force until a ruleset has been applied to at least one interface. South(config)# interface eth0/0 South(config-if)# ip inspect fw1 out South(config-if)# end South# show ip inspect interface Interface Configuration Interface Ethernet0/0 .) 15 seconds Of course. Length of time that CBAC continues to manage a TCP session after it has been closed down by a FIN exchange. Timeout Name Synwait-time Finwait-time Description Length of time CBAC waits for a new TCP session to reach established state. synwait-time 15 finwait-time 1 idle-time 1800 idle-time 15 Step 7.1c . . Use the interface configuration command ip inspect name to apply a ruleset. The example below applies the ruleset from step 5 to the outside (untrusted network) interface. South# End with CNTL/Z. Default 30 seconds 5 seconds Suggested 15 seconds 1 second TCP idle-time UDP idle-time 1 hour 30 seconds 30 minutes (1800 sec. Length of time CBAC continues to manage a TCP session with no activity. The example below shows how to set the global timeout parameters. South# config t Enter configuration commands. There are also global CBAC parameters related to half-open TCP session. Length of time that CBAC continues to manage a UDP ‘session’ with no activity.g. South# config t Enter configuration commands.

2. The test shown here has two parts: first. Escape character is '^]'. starting a telnet session from a host on the trusted network to a host on the untrusted network. Access is limited to authorized administrators only! Username: nziring Password: Central> While the Telnet session is active.10. It should show the telnet session.250 Trying 14. Connected to 14. South# show ip inspect sessions Established Sessions Session 6187B230 (14.9. see Section 6.4. confirming that CBAC is managing the session.2. If the command gives no output.2.250.9. The desired service list for this firewall is: DNS.2. Test the CBAC Configuration Perform some simple tests from a host on the trusted network. NTP. HTTP..2.0. Telnet. save the router configuration to NVRAM at this point with the command copy running startup.6) to a host on the untrusted network (14.0.250). This sample is formatted as it would appear in a configuration text file stored on a host for download to the router South..9.10. as illustrated in the example below. The example below shows a Telnet session from a host on the trusted network (14.250. $ telnet 14. and POP3 (to a single host).2.255 any permit udp 14.10. FTP.2.250:23) tcp SIS_OPEN South# If the CBAC configuration seems to be working. check the CBAC session status on the router using the command show ip inspect sessions.9.1c 239 . no access-list 110 ip access-list extended 110 permit icmp 14. and second.0 0.0 0.2. to see that CBAC is working. This is the CENTRAL router. CBAC should be running on the router. Step 8.Advanced Security Services After this step. then CBAC is not working.0.2. For more detailed testing information.10.255 any eq ntp Version 1.189:3175)=>(14. SMTP (to a single host). Configuration Sample The configuration command listing below shows the configuration commands for a firewall router with a simple CBAC configuration.

9.0.0 permit tcp 14.0 permit tcp eq pop3 no access-list 111 ip access-list extended 111 deny ip 14.1c .0 permit tcp 14.0 permit tcp 14.9.0 0.10.255 any eq domain any eq www any eq ftp any eq telnet host permit tcp 14.2.3 eq smtp host 14.255 0.255 any log ! permit routing updates permit udp any any eq rip ! permit useful ICMP message types permit icmp any any echo-reply permit icmp any any unreachable permit icmp any any ttl-exceeded permit icmp any any packet-too-big deny ip any any log exit ip ip ip ip ip ip ip ip inspect inspect inspect inspect inspect inspect inspect inspect name name name name tcp tcp tcp udp fw1 fw1 fw1 fw1 udp audit-trail on tcp audit-trail on ftp audit-trail on smtp audit-trail on synwait-time 15 finwait-time 1 idle-time 1800 idle-time 15 interface eth 0/0 ip access-group 110 out ip access-group 111 in ip inspect fw1 out end 240 Version 1.0.0.Router Security Configuration Guide permit udp 0.0 deny ip any any exit

especially a border router. A unique benefit of implementing an IDS on a router. only the first one found is reported by the IDS. The two categories of information or attack signatures are also divided into two other categories. The Cisco IOS IDS is an in-line intrusion detection package that can monitor all packets and sessions flowing through the router. The network administrator can then specify a particular action to take when an event signature is detected.5. you can apply access lists to filter addresses and protocols from being subject to analysis by any particular signature. different signatures are detected. information-gathering scans. is that all network traffic flows through it and may be examined. The signatures were selected from a broad cross-section of intrusion detection signatures. These signatures are used to match and detect patterns of security violations of the most common network attacks. the IDS feature should not be configured as a standalone protection device.5. Cisco routers. The 59 signatures are divided into two different categories: information signatures (“info”) and attack signatures (“attack”). such as port scans and echo requests. Attack signatures detect intrusions or attacks into the protected network.Advanced Security Services 5. it is important to note that a packet is subject to intrusion detection only if the router actually attempts to forward it. which could be a series of probes or access attempts distributed across multiple hosts over a random time period. Cisco IOS Intrusion Detection The Cisco IOS Firewall Intrusion Detection System (IDS) is a real-time IDS designed to enhance border router security by detecting. would not be scanned. but not all. Also. Although the individual signatures cannot be modified. and terminating unauthorized activity. Each packet is scanned against the Version 1. For adequate security. for example. When using the IDS facility with other IOS security features.1. Atomic signatures detect simple specific access attempts such as an attempt to access a specific port on a specific host. It is possible to enable them independently on separate interfaces. such as denial-ofservice attempts or execution of illegal commands during an FTP session. reporting. Information signatures detect many information-gathering types of activity. The Intrusion Detection System on the router is a part of the IOS Firewall (CBAC) facility (Section 5. IOS IDS Basic Concepts Cisco has identified and incorporated into the IDS software 59 signatures of the most common actual and potential network attacks. Compound signatures detect more complex patterns. 5. This facility is available in IOS releases for many.4). which are either atomic or compound signature types. or misuse in network traffic. Packets dropped by an in-bound access list. This feature is helpful in reducing false positives.1c 241 . Both the firewall and IDS features should be enabled together for the best security. if a packet is scanned and multiple.

it may be possible for you to upgrade your router to a release that does. After you’ve configured everything. the IDS logging facility. initialize the Post Office. When an attack is detected.Initialization You must initialize the IDS facility before configuring it. Once you have determined that a particular router supports the IDS facilities. drop the packet. First. follow the three steps outlined below to configure them. for more details. and the second the response from an IOS without IDS support (Central).2.3 for information on IOS versions.5 for information on loading IOS upgrades. One way to initialize the facility is to set a parameter on one of the IDS signatures. Configuring the IOS Intrusion Detection System Only those IOS releases marked “Firewall/IDS” support the IDS features described in this section. South# show ip audit all Event notification through syslog is enabled Event notification through Net Director is disabled . . See Section 4. the IDS will log an alarm to the syslog server or a Cisco Netranger Director. South(config)# ip audit smtp spam 200 End with CNTL/Z. and/or reset a TCP session. make sure that your router supports them by attempting to execute a simple IDS command.1c . initialize the IDS facility. South# config t Enter configuration commands. Step 1 . South# versus Central# show ip audit all ^ % Invalid input detected at '^' marker. and Section 8. This section presents only a brief overview of the IDS facility.5. The IOS documentation recommends using the command below.Router Security Configuration Guide signature list in order to detect possible attacks or suspicious activity. one per line. 5. 242 Version 1. Third. consult the “Traffic Filtering and Firewalls” section of the Cisco IOS 12. it is good practice to confirm that the IDS facility is working. Central# If your router does not support the Firewall IDS facility. Second.2 Security Configuration Guide (in the IOS documentation). configure and apply the audit rules. depending on how the IDS was configured. The first example below shows the response to an IDS command from a version of IOS with IDS support (South). Before attempting to configure the IDS features.

ip audit po local hostid host-id orgid org-id This command sets the host and organization IDs for the Post Office.10.10. and the org-id value should be the same as in command 2.15 localaddress 14. ip audit po remote hostid host-id orgid org-id rmtaddress dir-ip-address localaddress local-ip-address This rather complex command specifies the address and settings for sending alerts to the IDS Director. 3.Advanced Security Services The value of 200 in this example is the maximum number of recipients that can be in an email message before the IDS will designate it as undesirable ‘spam’. Setting up the IDS facility to send alerts to an IDS Director requires three commands: 1. while the local-ip-address should be the IP address of the router interface closest to the Director. Next. South# config t Enter configuration commands.10.g. End with CNTL/Z. the host-id must be a unique value for this router. then configure the router to send the alerts to it.64 South(config)# Version 1. You can also log the alerts to a syslog server (see Section 4. ip audit notify nr-director This command simply tells the router to use an IDS Director. South(config)# ip audit notify nr-director South(config)# ip audit po local hostid 141 orgid 2 South(config)# ip audit po remote hostid 1 orgid 2 rmtaddress 14.5 for information on syslog configuration). one per line. set the size of the event queue for the Post Office. which is generally a reasonable setting.15.Configure the Post Office When the IDS facility detects a match to one of its intrusion signatures. This command can accept additional parameters.2. above. South(config)# ip audit po max-events 50 South(config)# Step 2 . see the IOS documentation for details. The example below shows how to configure the router South to send alerts to an IDS Director at 14. The dir-ip-address should be the IP address of the IDS Director. it throws an alert. Each alert must be logged if it is to be useful. The host-id value must be the host ID of the Director. and the org-id must be shared by the Director and all Cisco IDS sensors that send alerts to it. The default queue size is 100.1c 243 . less than 32MB) then you should lower the value to 50. directing alerts is the job of the Post Office. 2. If your network supports a Cisco Secure IDS Director.2. If the router has very little RAM (e. also sometimes called a “Netranger director”.2.

discard the packet that triggered the alert • alarm . you must also configure the Director to accept alerts from the router.3. you are ready to define audit rules and apply them to specific interfaces. after you have configured the router to send alerts to an IDS Director. The example below shows how to define a rule named IDR1 containing both info and attack signatures. you can designate any or all of three possible actions to be performed when a signature is matched by traffic. you should configure the IDS facility to send alerts as normal IOS log messages using the command shown below.log an alert to the Post Office and/or syslog • reset . If the network does not have a Cisco IDS Director available. You define an audit rules by choosing a name.Router Security Configuration Guide Note that. South# copy running-config startup-config Building configuration.1c . you must save the running configuration and reboot the router. For each part of an audit rule. South# reload Proceed with reload? [confirm] y . South(config)# ! send IDS alerts to syslog and buffered log South(config)# ip audit notify log After setting or changing any Post Office parameters.cut off the TCP session that matched the signature (TCP only) Use the config commands ip audit name to add items to an audit rule. Using an access list. An item must be based on either the “info” or “attack” signature classes. one per line. you can restrict the detection of an event. If you forget to do this. South# config t Enter configuration commands. Configuring and Applying Audit Rules Once you have initialized the IDS and set up the Post Office.3 for more information on access lists).. . and then adding one or more specific items to the rule.5. the Director will not record the alerts sent by the router. South(config)# ip audit name IDR1 info action alarm South(config)# ip audit name IDR1 attack action alarm drop reset South(config)# It is also possible to limit the application of a rule item with a standard IP access list. Because 244 Version 1. End with CNTL/Z. 5. • drop . although this will raise the performance burden imposed by IDS scanning. and may use standard IP access lists to limit the addresses to which the signatures are applied (see Section 4. or the actions taken.

10.6 to 14. Applying the rule in-bound will yield more complete scanning. Applying the rule outbound will reduce false positives. Oct 10 20:02:19.g. in either the in-bound or outbound directions.0 Oct 10 20:01:15. . you can only restrict scanning by source IP address.1.1c 245 . similar to the one described in Section 6.752 GMT: %IDS-4-ICMP_ECHO_SIG: Sig:2004:ICMP Echo Request .192 GMT: %FW-4-ALERT_ON: getting aggressive. because all traffic received on that interface will be scanned. The example below shows how to disable two signatures related to common ICMP packet types. count (51/500) current 1-min rate: 501 . because only packets which have been permitted by any in-bound ACLs on other interfaces will be scanned.0 Oct 10 20:01:30.1.6 to 14.696 GMT: %IDS-4-TCP_SYN_ATTACK_SIG: Sig:3050:Half-Open Syn Flood .3. a security audit host used to perform test scans).0 network. consult the IOS documentation. This makes the facility most useful for reducing false positives caused by specific trusted hosts (e. for this case. It is also possible to disable and restrict particular IDS signatures.Advanced Security Services only standard IP access lists may be used.1.2. .2. The example below shows how to apply our IDR1 rule for traffic coming into the 14.from 14.2. Note that. South(config)# interface eth0/0 South(config-if)# description External interface. with IDS South(config-if)# ip audit IDR1 in South(config-if)# end South# Detection Sample The transcript below shows the log messages that the IOS Firewall IDS generates and statistics it records. count (0/400) current 1-min rate: 0 South# South# show ip audit statistics Signature audit statistics [process switch:fast switch] signature 2001 packets audited: [3123:6246] signature 2004 packets audited: [4:8] signature 3050 packets audited: [902:0] Interfaces configured for audit 1 Session creations since subsystem startup or last reset 1193 Current session counts (estab/half-open/terminating) [0:0:0] Maxever session counts (estab/half-open/terminating) [0:51:0] Last session created 00:01:50 Last statistic reset never Post Office is not enabled .1.No connections are active South# Version 1. the router IDS was configured to send alarms to normal IOS logging rather than to an IDS Director.from 14. For details.644 GMT: %FW-4-ALERT_OFF: calming down.10. South(config)# South(config)# South(config)# South(config)# South(config)# ! don’t alert on ICMP source-quench ip audit signature 2002 disable ! don’t alert on ICMP time-exceeded ip audit signature 2005 disable You can apply a named audit rule on any interface. This session was captured during a small TCP port scan.10. South# Oct 10 20:01:13.1.

because internal network misuse is not uncommon and potentially as detrimental as an external penetration. 246 Version 1.1) The IOS Firewall IDS facility can help detect and track analysis of your network by remote parties. nor was it designed for this purpose. and attempted root logins. The IDS can report intrusions to a given host's syslog. both require a human to actively monitor them to provide real time information. With common attacks stopped and logged. Security Considerations for Using IOS Firewall IDS Before an intruder can successfully penetrate a network. you cannot use this facility to detect attacks from one host to another on the same LAN.5. Many tools and technique exist to help attackers gain this information (e. which is critical in cases of intrusion. such as distributed scans. nmap. Thus. discussed in Section 6.3. Keep this in mind when planning intrusion detection for your network. buffer overflows. This can be most efficiently accomplished by installing the IOS IDS at a border point with the firewall configured. Recommendations The Cisco IOS Intrusion Detection System does not provide comprehensive intrusion detection as a stand-alone feature. Despite its speed and excellent location (no forwarded packet can avoid being scanned) the small signature database and inability to correlate different events prevent the IDS from being effective against many realistic attacks.Router Security Configuration Guide 5. they must have information about it. and/or a Cisco Secure Director. one or more dedicated IDS should be deployed on internal networks to provide more comprehensive coverage and analysis. It is best used to supplement more complete intrusion detection packages.4. Without the Director. while the console is usually in a secure facility. the router console. By default. The IOS IDS cannot stand alone as a complete network defense package. if a packet does not need to be routed. it is not analyzed. it is difficult to monitor an attack against the network because both the syslog and the router console do not lend themselves to instant reporting . This will provide simple ID at the edge of a protected network and stop simple attacks.1c . and possibly help you understand threats to your network more quickly.the syslog writes a textfile. Cisco IOS IDS cannot monitor internal traffic. that is.g. Special Note: Because it is performed as part of packet routing.

.” RFC 2401. S.B.252735 Contains detailed information about setting up IPSec in Windows ps1018/prod_configuration_examples_list. “Security Architecture for the Internet Detailed information on configuring Cisco VPN client software. Building Internet Firewalls. Intranets. Cisco Press.6. [8] “How to Configure IPSec Tunneling in Windows 2000”.com/default. Version 1. [4] Kent. including IPSec. 1999.D. O’Reilly Associates.html This page offers several CBAC documents and configuration examples. [2] Cisco IOS 12. R. Authoritative source for in-depth descriptions of security-related IOS facilities. and Harkins. 252735. available at: http://www. Cisco Systems.en-us. and related configuration commands.1c 247 . Cooper. includes extensive remarks about VPN architecture. References [1] Chapman. Contains a good overview of IPSec. S. Indianapolis.. CBAC. 2000. available at: http://support. Prentice-Hall. Microsoft Corporation. Microsoft Knowledge Base. [7] “Cisco Secure VPN Client Solutions Guide”. D. A seminal reference for understanding firewalls and the principles for building them. Cisco Systems. available at http://www. E. 2000. [3] Doraswamy. This highly technical book provides detailed explanations and pragmatic advice about IPSec. D. The master document for IPSec. 2nd Edition. IPSec: The New Security Standard for the Internet. Cisco Internetworking Solutions Guides.0 Network Security. [5] Tiller. J. 2004. Cisco Product Support Documentation. Auerbach Publications. and Virtual Private Networks. 2001. [6] “Cisco IOS Firewall Configuration Examples and TechNotes”. A Technical Guide to IPSec Virtual Private Networks. and Zwicky. Article No. 1999.Advanced Security Services 5. N. IN. and Atkinson. ps2138/products_maintenance_guide_book09186a008007da16. plus and technical detail about IKE and VPN design.aspx?scid=kb.

com/ pcgi-bin/Support/browse/psp_view. available at http://www. 1999. and usage.E. Cisco Technical Notes. visit: available at software/ios120/120newft/120t/120t5/iosfw2/ios_ids. [15] “Cisco IOS Firewall Intrusion Detection System”. D. Microsoft Knowledge Base. IOS 12. 2000. O’Reilly Associates. Revision 1.pdf A detailed overview of the IOS Firewall IDS facility. available at: http://www. 231585. Cisco Systems. T. [16] Escamilla.htm A short overview of SSH features in IOS 12.Router Security Configuration Guide [9] “Overview of Secure IP Communication with IPSec in Windows 2000”.com/univercd/cc/td/doc/product/ software/ios121/121newft/121t/121t1/sshv1. Cisco Technical Notes. Cisco Systems. For more documents and examples about IPSec. 248 Version 1. Provides broad and detailed coverage of SSH features. May Article No. and Silverman. IOS 12.J. [10] “An Introduction to IP Security (IPSec) Encryption”.com/warp/public/707/ipsec_debug. November 2001. Cisco Systems. 2000.html An overview of SSH vulnerabilities and IOS versions to which they apply. available at: Cisco .1c . 1998. Wiley.aspx?scid=kb. R.pdf This tech note offers detailed information about Cisco IPSec including a list of the supported information and attack signatures.1 release notes. May 2003.pdf Provides detailed information on using error messages and debug facilities to track down problems in IPSec configuration. Intrusion Detection. [11] “IP Security Troubleshooting − Understanding and Using debug Commands”. [12] “Secure Shell Version 1 Support”.1(1)T. available at: http://support.231585 A good overview of IPSec features in Windows Microsoft Corporation. with examples. [14] SSH The Secure Shell – The Definitive Guide.0(5)T release notes. A good introduction to intrusion detection concepts and techniques. Cisco [13] “Cisco Security Advisory: Multiple SSH Vulnerabilities”.. available at: http://www.

Advanced Security Services Version 1.1c 249 .

Testing Tools There are a variety of tools available for testing purposes. Some of these services are enabled by default whereas other services are frequently enabled by users. Packet sniffer programs are used to monitor traffic passing through the network and steal unencrypted passwords and SNMP community strings. Principles for Router Security Testing The border router is often the first line of defense when protecting against malicious network attack. including some versions of IOS. 6.Router Security Configuration Guide 6. Routers provide many services that can have severe security implications if improperly configured. and whenever major changes have been made to any part of the configuration of a router. Section 9. 250 Version 1. Ideally. Scanners such as Fyodor’s nmap program can be used to scan for open TCP and UDP ports on a router interface.1c . Additional tools are listed in the Tools Reference. this information can then be used to formulate specific attacks against the router. several denial of service (DOS) attacks and the newer distributed denial of service (DDoS) attacks have been highly successful against network devices. Attack scripts are readily available on the Internet for numerous well-known exploits.1. testing should be performed at initial deployment of a router. Testing and Security Validation 6. Security testing provides a means of verifying that security functions are compatible with system operations and that they are configured in a secure manner.2.3.

11. Telnet). 2. there shouldn’t be any ports open.250 Warning: -sU is now UDP scan. The nmap scanning program is a good tool for this purpose. (Note: if IP unreachable messages have been disabled. the default is Interesting ports on (14. For example.) TCP Scan: The following command will perform a TCP scan against router North (IP address 14. The default settings for these services depend upon the IOS version. temporarily re-enable them before performing your UDP port scan by using the interface configuration command ip unreachable. cross-check the ports that nmap reports open against the services that the router is supposed to be running.11.2. DNS must be available for any operation referencing a host by name to succeed (e.1.12 by Fyodor (fyodor@dhp.250): Port State Protocol Service If VTY (Telnet) access is not allowed.2.3. Access lists should be tested thoroughly once assigned to an interface both to be certain that necessary traffic is permitted and unwanted traffic is denied.3.2. The above command would not.250): Port State Protocol Service Version 1.1c 251 .com) Interesting ports on (14.3. some services depend on other services in order to function.3. The examples below show nmap running under Linux. Additionally. run a port scan against the router. Testing and Security Analysis Techniques 6.1. Otherwise. for TCP FIN use -sF Starting nmap v. for IOS v. # nmap –sU -p 1-65535 14. but for IOS v.2. UDP Scan: The following command will perform a UDP scan against router North (14.Testing and Security Validation 6. A serious known problem with Cisco IOS is that some default settings are not displayed as part of the router configuration listing.1.1.250): # nmap –sT 14. as advised in Section 4. use the EXEC mode command show running-config. Testing all allowed services will identify these dependencies. for example.250 –p 1-65535 Starting nmap v.2.g.12 by Fyodor (fyodor@dhp. show the ‘udp-small-servers’ or the ‘tcp-smallservers’ in the configuration. To view the current operational configuration. To verify the entire configuration. Functional Tests Functional testing provides assurance that the implemented configuration is the intended one.2. the default is enabled. 2.

Each packet generates a response from all hosts on the reflector subnet. make sure that all attack testing is coordinated with those responsible for the network and choose a test time when the network usage is likely to be low. Connecting to an outside network exposes the internal network and the perimeter router to many potential risks. 252 Version 1. The source address in these packets is falsified to be the address of the ultimate target. OR EVEN CAUSE THE ROUTER TO CRASH! If the filters are improperly configured. WARNING: RUNNING ATTACK SCRIPTS AGAINST AN OPERATIONAL ROUTER MAY DEGRADE ROUTER PERFORMANCE.1c . remote logins should be disabled. and the Cisco whitepaper “Improving Security on Cisco Routers” [1]. This attack has at least two victims – a target system and one or more reflector systems. For a detailed discussion of the smurf attack. DoS attacks are typically based on high-bandwidth packet floods or other repetitive packet streams. visit: http://www. For a general overview of DoS.html. if The attacker sends a continuous stream of ICMP echo requests (‘pings’) to the broadcast address of a reflector subnet. the network is still at risk of attack. Perform testing in a lab or testbed environment before testing in the operational environment. One popular DoS attack is the ‘smurf’ attack. For a more detailed discussion of Cisco router access security and remote The router should either refuse the request or prompt for a password.2.1..3. To test the remote availability. DO NOT perform attack testing against any network until you have received organizational and legal approval to do so. One of the most important security concerns is access to the router itself. Physical security of the router should provide protection from close-in (non-network) access. visit the CERT site: http://www. Some of the most common attacks on the internet are denial of service (DoS) attacks.Router Security Configuration Guide 6. including recent developments and links to specific DoS advisories. some of these attack tests can have the same effect as a “real” attack from a malicious source. particularly those without the skill to create their own tools. flooding the target and wasting bandwidth for both victims. DO NOT perform attack testing against an operational router without first considering the possible consequences and having a recovery plan. On the network. When you do perform testing on the operational network. i.e. consult Section 4. read Craig Huegen’s paper [9]. Attack Tests Attack testing can provide some assessment of the router’s robustness.cert. The reflector networks receiving these echo requests can block the attack at their routers by using the interface configuration command no ip directed-broadcast (see Section 4.2). or not applied to the interface. For more information on the effects of DoS attacks. how the router will perform under the stress of an attack. telnet to the router. remote access must be limited using authenticated logins or. The easy availability and effectiveness of DoS scripts on the internet make these attacks a favorite among hackers.cert. Once access to the router has been secured.

changing the source IP address to hide the true source of the packet. packets arriving at the router Version 1. The real attacker is able to amplify the DoS flooding while being removed from the attacking machines.11. An integral part of DoS and DDoS attacks is IP spoofing. This feature checks each packet arriving at the router. Another common DoS attack. The attacker continues to send these forged SYN packets at a rapid rate until the victim’s connection queue is filled with half-open requests. There are many such tools in circulation.2 or 11. In a normal TCP connection request.0. the ACK doesn’t arrive and the TCP handshake never completes.7).0). The server responds with a SYN/ACK packet. Access lists should check that no packets arriving from the outside network contain a source address of either the internal network or the well-known. takes advantage of the TCP three-way handshake procedure to deny service to the victim. four historically popular ones are called Tribal Flood Network (TFN). the requesting client sends a SYN packet to the server.e. reserved addresses (defined in RFC1918). file transfer or web traffic to legitimate users. described below. Asymmetric routing will not work with this feature. [4]. and [8]. non-routable. Cisco provides a paper titled “Defining Strategies to Protect Against TCP SYN Denial of Service Attacks” [4]. the queue entry is removed.. The victim responds with a SYN/ACK to the unreachable host and waits for the ACK. arriving packets should not have source addresses of all 0’s or all 1’s or the loopback address (127. Blocking access to the service under attack is usually not feasible and would accomplish precisely what the attacker set out to do. and it is only available in IOS v12. thus emptying the queue of half-open connections more quickly. Tribal Flood Network 2000 (TFN2K) and Stacheldraht. The requester then completes the handshake with an ACK packet.0. filters can be used to prevent IP spoofing in a manner similar to the ip verify unicast reverse-path feature discussed above. For Cisco routers running IOS v. Additionally. For more options. The host could also decrease the wait time for completion of the three-way handshake. an attacker sends a TCP connection request (SYN) packet with an unreachable. More information about DDoS attacks is available from references [3]. which uses a limited number of reflector systems. the timer is reset and the connection is established. i. Cisco routers can help prevent the system behind the router from being an unwitting participant in a DDoS attack by using the ip verify unicast reverse-path interface command (Section 4.4. the packet is dropped.1c 253 .0. the SYN flood. adds an entry in the connection queue and starts a timer. In a SYN flood. These distributed denial of service tools (DDoS) pose a major threat to networked systems and have the potential to severely impact normal business activities. However. victims of a SYN flood do have some options. if the source IP address does not have a route in the CEF tables pointing back to the same interface on which the packet arrived.3. Unlike a “typical” smurf attack. these tools employ many compromised systems to simultaneously attack a single target. tracking the attacker is extremely difficult. requiring the attacker to send more phony packets to flood the service. spoofed source address to an open port on the target. similar protection can be achieved by filtering for IP spoofing. The effect of this attack is to deny TCP services such as e-mail. The host could increase the size of the connection queue. [5]. Trin00.Testing and Security Validation Enhanced denial of service tools have recently become available on the Internet. Also.

This input access list must block traffic destined for UDP port 514 at any of the Cisco IOS device’s own IP addresses.255. The following example shows an access list to block the port 514 UDP traffic. send a series of packets with modified source addresses to the external interface.0. Some vulnerable IOS devices will “hang” and must be manually restarted by reset or power cycle.0 31.0. This vulnerability affects unpatched versions of IOS 11.0 eq 514 access-list 101 deny udp any 14.3AA. To verify the anti-spoofing configuration.0 net access-list 101 deny udp any 14. an attacker might be able to completely disable a Cisco IOS device until the affected device is reconfigured to drop the attack traffic.2.255 0.0.0 eq 514 ! Deny packets addressed to router interface access-list 101 deny udp any host 14. At least one commonly available vulnerability scanner can generate these UDP packets. If a specific interface is not expected to forward legitimate syslog traffic. RFC 2267 discusses network ingress filtering and defeating DoS attacks which employ IP source address spoofing. The packets should test the ability of the router to detect both internal addresses and reserved addresses that should not arrive at an external port. (Note that the internal network may be using one of the RFC1918 reserved addresses with NAT performed at the router. For an in-depth discussion of TCP flooding and IP spoofing. consult [7]. This problem can be prevented by applying the appropriate input access list to all interfaces that might receive these packets.0 0.0. in this case. the router should again drop the packets and log the events. To verify outbound anti-spoofing.255.1c . ! Deny all multicasts and all unspecified broadcasts to port 514 access-list 101 deny udp any 224.255. There is a Cisco syslog vulnerability that may cause the IOS software to crash if an invalid user datagram protocol (UDP) packet is received on the syslog port (port 514).3DB and early (non-GD) releases of 12.0.0. it might require an administrator to physically visit the attacked device to restore service.255 eq 514 ! Deny old-style unspecified net broadcasts access-list 101 deny udp any host 0. an alternative fix would be to deny all syslog traffic arriving on that interface. the access list on the internal interface will recognize such an address as legitimate. as well as at any broadcast or multicast addresses on which the device may be listening.0.0.Router Security Configuration Guide from the internal network should not have a source address that is not one of the legitimate internal addresses. The goal here is to catch packets with a source address of an external network or a reserved address that is not being used by the internal network. By sending such packets continuously. The router should drop these packets at the perimeter and log the events.2.20 eq 514 ! Apply to input interface of router North interface eth0 ip access-group 101 in 254 Version 1.) This check will prevent the internal network from being used as a launch point for a source IP spoofing attack. send packets to the router’s internal interface with source addresses that are not legitimate internal addresses.2.0 eq 514 ! Deny network-specific broadcasts to the 14. 11.

or to touch systems for which you have no legal authority. planning and coordination. web servers. if the router is running a vulnerable version of the IOS software and the access list is not properly configured or not applied. Version 1. CyberCop Scanner performs comprehensive evaluations of intranets. Packetstorm Security has several DoS exploits. Exercise caution when using tools like CyberCop. running DoS attack scripts against the router can have very serious and undesirable consequences. always double-check the addresses to be scanned.Testing and Security Validation This vulnerability can be tested by sending a UDP packet to the router’s port 514. SAINT.3. CyberCop offers monthly updates consisting of new modules and security hotfixes for new and evolving vulnerabilities. or SATAN. visit the IIS web site. Internet Scanner analyzes the scan data and provides reports containing vulnerabilities identified along with recommended corrective actions. and updates containing the latest tests and security checks are available for download daily.iss. WARNING: RUNNING AUTOMATED ATTACK TOOLS ENTAILS SIGNIFICANT RISK! It is easy to accidentally auto-scan more systems than you intended. visit: http://www. key applications and routers for those vulnerabilities frequently used by malicious users to investigate and attack networks. level of risk and suggestions for fixing/mitigating the vulnerability.asp. security concerns. If. Other sites for exploit information and code are listed at the end of this section. For more operating systems. Internet Scanner includes nearly 600 total tests. CyberCop Scanner from Network Associates and Internet Scanner from ISS are two popular commercial products. the attack scripts are readily available from many sources on the internet. and monitor the tools closely while they are operating. The Security Administrator’s Integrated Network Tool (SAINT) and the Security Administrator Tool for Analyzing Networks (SATAN) are publicly available tools. At the time of this writing. the router could crash or hang! As mentioned above.1c 255 . firewalls and screening routers by scanning them and performing extensive tests to identify known vulnerabilities.3. Internet Scanner probes the network’s communication services. the decision is made to go forward with this testing. assessment page: http://www. after careful Internet Scanner is also a network vulnerability analysis and risk assessment available under http://packetstormsecurity. However.01) now contains tests to find hosts infected by DDoS and http://packetstormsecurity. Mechanisms for Automated Testing There are a number of products available to automate the testing process. The latest version of Internet Scanner (6. 6. For more information. CyberCop generates reports from scan results that include information about detected vulnerabilities: a description of the vulnerability.

2 above. If the incoming traffic consists mostly of ICMP echo requests. The vulnerabilities discovered are then reported to the user without actually exploiting them. For each problem found. SATAN was designed to help system administrators responsible for the security posture of their systems. installing vendor hotfixes. consult Section 4.20 of router North: access-list 102 permit access-list 102 permit access-list 102 permit access-list 102 permit access-list 102 permit interface serial 0 ip access-group 102 in icmp any any echo log-input icmp any any echo-reply log-input tcp any any established tcp any any log-input ip any any This access list does not filter out any traffic but does separate the traffic by types. NIS.3. a necessary first step in countering DoS attacks.3. To see the number of matches for each line in the access list. For more information.saintcorporation.1c . SAINT can also be configured to examine trust and dependency relationships in the target network.porcupine. Detecting Attacks As mentioned in section 6. SATAN also provides corrective actions including configuration changes. visit: http://www. Assume the following access list is applied to interface 14. these are the addresses of the reflector sites. SATAN systematically proceeds through a target network probing for common networking-related weaknesses and security problems. it is a tool for investigating the vulnerabilities of remote systems. In normal operations. the attack is probably a smurf attack where North is a reflector. a tutorial and the latest version of SAINT. visit ftp://ftp. An analysis of the packets arriving on the serial interface can identify the specific attack being tftp. that is. For more information about access lists. including a FAQ.4. NFS.2. The third and fourth lines of access list 102 characterize TCP traffic. SATAN offers a tutorial that explains the problem and the potential impact. denial of service attacks are very common on the The initial data collection can then be used to investigate any potential security problems. In a typical smurf attack.Router Security Configuration Guide SAINT gathers information about remote hosts and networks by examining network services such as finger. the TCP SYN packets.0. The keyword established in the third line matches any TCP traffic with the ACK bit set. the source addresses in the echo reply packets are limited to a few networks. use the command show accesslist 102. ftp. this feature exposes the real security implications inherent in network trust and services. rsh commands and other services. or possibly disabling services. The signature of a smurf attack where router North is the ultimate target would show most of the packets as ICMP echo replies. To download a copy of SATAN.3. IOS access lists can be used to characterize the different packet types and to tentatively identify DoS attacks. any TCP traffic that is not a connection request. 6. 256 Version 1. The fourth line therefore matches only packets that are connection requests.

2 and 4. Second. This paper gives an overview of denial of service attacks and a detailed discussion of using access lists to categorize packets. Also.cert.3) will let the administrator determine if such source addresses are and implementing the latest patches from the vendors. Attack Reaction Options It is difficult for the ultimate target of denial of service attacks to stop or even blunt an active attack. Version 1. In a SYN flood. and to only allow services that are needed (see Sections 4. the only defense is to be vigilant about security and to keep up with that latest security news by regularly checking a site such as CERT (www. the upstream provider to the victim may be able to filter out all ICMP echo replies to the target network. this filter should only be in place temporarily and only as a stopgap measure. logging such traffic (as recommended in Section 4. The best advice is to configure the router to check for IP spoofing. these SYN packets typically outnumber other TCP packets many times over. 6. In this case. In a distributed attack. or not sufficient to stop the attack. the only response may be to contact the reflector sites to reconfigure their networks to shut down the attack. if filtering is not possible.1c 257 . First. Still.Testing and Security Validation TCP SYN packets account for a third or less of the total TCP traffic. both inbound and outbound. SYN floods usually contain packets with invalid source addresses. The paper also describes how to trace DoS attacks and the complications inherent in packet tracing [2]. It is almost impossible to protect a network from denial of service attacks.3. An on-going problem is that new attacks can appear so fast on the internet that countermeasures are not immediately available. the ultimate target cannot filter out the attacking addresses. There is a paper available on the Cisco web site titled “Characterizing and Tracing Packet Floods Using Cisco Routers”. if it can be determined that the originators of the attack are limited to a few addresses. it may be possible to apply specific filters at the external interface of the border router.3).5.

As of September 2005. and generates HTML reports detailing which rules the configuration passes and fails. convenient native binaries of RAT are available. To download RAT visit the Internet web site www. First.1c .g. its interfaces. On Windows platforms. If you supply the address of the router. The Center for Internet Security (CIS) maintains RAT. the current version of RAT was 2.6.txt) • An HTML version of the report. routerconf-1.g.html) 258 Version 1. Using this approach.. services. time and logging configuration. you must run the command ncat_config to customize the rules to your particular router(s).org and select the “Cisco IOS Router” link. and a username and passwords. The default rules that RAT applies for Cisco IOS routers are in close agreement with the material presented in Section 4 of this guide. The rat program can take the configuration input in two ways.g. the procedure for running it is fairly simple. rat has the ability to log in to the router via Telnet and obtain the configuration directly. Using the Router Audit Tool The Router Audit Tool (RAT) tests whether a Cisco IOS router configuration complies with a set of community consensus security rules. (e. The procedure for installing RAT varies between platforms. The RAT program can also accept the router configuration as a text file. Ncat_config will ask you a series of questions about the router’s role.ncat_out.cisecurity. consult the RAT documentation. (e. The rat program itself produces four output files for each router configuration tested. although ActiveState’s ActivePerl may also be used. providing the IOS configuration of the router as input. (e. It will run on any Unix or Linux platform supporting Perl 5. consult the download notes at the CIS web site for details. access lists.1 or later. RAT is not difficult to use. via the console port on each router.4. and router enable password on the RAT command line. Second. and requires a Perl installation to run on Unix or Linux. routerconf-1.2P.ncat_report.g. Once you have RAT installed. This option involves supplying an administrator login name. RAT is written in Perl. which should only be done on a completely trusted host and network. RAT uses its own utility called ‘snarf’ to do this.Router Security Configuration Guide 6. password. run the rat command itself.txt) • A formatted text version of the report. For more information. and it provides a means for an administrator to quickly check whether their router meets an IOS security ‘benchmark’ designed by a panel of industry and government security experts. routerconf-1. and sponsors the working group that defines the default RAT rules. or via FTP) and place the files into the directory where the rat command will execute. you gather one or more router configurations by other means (e. • An ASCII file that lists all the raw results for the configuration in a compact text format.

Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/common.ncat_report..html (cisco-ios-benchmark.Testing and Security Validation • A text file that contains IOS command to fix the problems identified in the report. Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/common.ncat_fix. Version 1.txt. routerconf-1.conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/cis-level-2. .%PATH% D:\routeradmin> ncat_config ncat_config: Select configuration type [cisco-ios] ? cisco-ios ncat_config: Applying rules from: ncat_config: c:\CIS\RAT/etc/configs/cisco-ios/common.conf/ Checking: northconf-2aug03 done checking northconf-2aug03.html).g..conf ncat_config: c:\CIS\RAT/etc/configs/cisco-ios/cis-level-2. D:\routeradmin> set PATH=c:\CIS\RAT\bin.conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/local. ncat_config: Check rules and data related to data flow [Yes] ! ncat_config: Apply standard routing protections [Yes] ? yes ncat_config: Forbid directed broadcasts (on IOS 11) [yes] ! ncat_config: Forbid directed broadcasts (on IOS 12) [yes] ! ncat_config: Forbid IP source routing [yes] ! ncat_config: Apply some or all of CIS Level 2 rules [No] ? no D:\routeradmin> rat northconf-2aug02 auditing northconf-2aug03.conf/ ncat_report: writing northconf-2aug03. ncat_report: writing all. D:\routeradmin> Note that this example does not show all of the questions posed by ncat_config.ncat_report.txt.conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/local.txt.conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/cis-level-1.conf ncat_config: c:\CIS\RAT/etc/configs/cisco-ios/cis-level-1. ncat_report: writing northconf-2aug03.ncat_fix.1) run on Windows. ncat_report: writing all.txt. ncat_report: writing rules.conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/cis-level-1.conf ncat_config: Apply some or all of the rules that are selectable [Yes]! ncat_config: Apply some or all of CIS level 1 rules [Yes] ? yes ncat_config: Check rules and data related to system management [Yes] ! ncat_config: Use local authentication [Yes] ? yes . ncat_report: writing all. (e. .ncat_fix.txt) RAT Example The transcript below shows a small RAT session (version 2.html.html.1c 259 .conf/ Parsing: /c:\CIS\RAT/etc/configs/cisco-ios/cis-level-2. ncat_report: writing northconf-2aug03.

do not follow them blindly. In the end.Router Security Configuration Guide RAT Caveats Use the RAT benchmark reports as an aid in securing your routers. your router must meet your local security policy. Examine each rule failure that RAT reports. RAT is simply a useful tool to help you find problems and areas for improvement. treat it as a potential issue. and check the router settings related to the rule carefully.1c . 260 Version 1.

References Web Sites and On-Line Resources [1] “Improving Security on Cisco Routers”.html [5] “Denial of Service Attacks”.com/warp/public/707/ C.html A good summary of basic IOS router security practices.5. [6] “Distributed Denial of Service Tools”.com/warp/public/707/21. available at: http://www.html Detailed guidance on tracing certain kinds of DoS attacks.Testing and Security Validation 6. Software Engineering [4] “Defining Strategies to Protect Against TCP SYN Denial of Service Attacks”.org/advisories/CA-1996-21. 2000. Cisco Technical Tips. available at: http://www. Cisco Systems.html A good overview of DoS attack principles. Cisco Systems.cert. Software Engineering Institute. Cisco White [8] “Distributed Attack Tools”. Cisco [9] Huegens. Cisco White Papers. “The Latest in Denial of Service Attacks: Smurfing”. CERT Advisory CA96. 1996. available at: http://www.21. CERT Coordination Center. available at: http://www. a web-based tool that registered Cisco customers can use for a cursory security analysis of their IOS configurations. CERT Incident Note IN-99-07. 2000. CERT Coordination Center. 2002. available at: http://www.1c 261 . available at: http://www. Cisco Systems.html Version 1. 1999. Securify Inc. CERT Coordination Center. Cisco Technical Tips. 2000. Software Engineering Institute.html [7] “Topic: TCP SYN Flooding and IP Spoofing Attacks”. available at: http://www. available at: http://packetstormsecurity. [2] “Characterizing and Tracing Packet Floods Using Cisco Routers”. Packet Storm. 1999. available at: http://www. [3] “Strategies to Protect Against Distributed Denial of Service (DDoS) Attacks”.org/tech_tips/ also includes a reference to the Cisco IOS Output Interpreter.

com/infocus/1734 Additional Exploit-Related Pages: http://packetstormsecurity.html ISS Internet Scanner http://www.cisecurity. Automated security scanning and testing tool sites: RAT http://www.nessus.Router Security Configuration Guide [10] 262 Version http://www.php SAINT http://packetstormsecurity.html SATAN .1c .nl/spoof/ Additional General Exploit Information Sites: http://www. 2003. “Exploiting Cisco Routers (Part One)” NESSUS . available from: http://www.

Additional Issues in Router Security 7. • Protecting physical access to the switch itself • Controlling virtual access to the switch. Additional Issues in Router Security This section describes a few areas of network technology that will probably have an effect on router and network security in the near future. The list is not comprehensive. Note that some switches can act as routers between their separate VLANs. The security concerns for switched networks and switches correspond directly to those for routed networks and routers. Many Ethernet switches have the ability to maintain one or more separate virtual LANs over the same physical cables and switches. Switches typically have limited or negligible filtering capabilities. each individual host and device must be configured securely rather than relying on protection at their LAN boundaries. For example. The diagram below shows how virtual LANs can be set up to emulate two physical LANs spread across two switches. IP routing will increasingly be replaced by layer 2 switching in high-performance applications. in a network environment that is predominantly based on switching.3). Version 1. 7. while others require a real router.1c 263 . Routing and Switching As network bandwidth demands continue to increase. routers can supply critical protection at network boundaries by filtering traffic (see Section 4.1. including user authentication and authorization • Updating the operating system when necessary to fix known vulnerabilities • Preventing unauthorized modification of the switch configuration • Disabling unneeded services and features Switching imposes new risks. One feature of switched environments that might be usable to improve security is Virtual LAN switching. Therefore. the topics described below are merely a select few of the many technologies that network security administrators will have to incorporate into their security plans and policies in the next few years. while removing the ability to impose some security restrictions.

0/24 Host Y (14.3.4.Router Security Configuration Guide Internet Host A ( Host B (14.0/24 Router LAN 2 Host C ( Virtual View Host B ( Host C (14. 264 Version 1.3) LAN 1 14.2) Host X (14.2.4) Host Z (14.6) Host Y (14.2. but it is clear that VLAN security will grow in importance in the next few years.2.8) Internet Real Construction Router LAN Switch 1 LAN Switch 2 Host A (14.2) Host X (14.2.8) Figure 7-1: Virtual LAN Switching More investigation is needed to determine the security roles and policies for configuring VLANs.2.1c . Host Z (14.

and the IETF initiated efforts to standardize a successor. due use of transition mechanisms and interactions between IPv4 and IPv6 features. and other protocols 576 bytes optional. 8-30 bits 20-60 bytes limited number of small IP options sender or any intermediate router allowed to fragment mixture of non-IP (ARP).gov/ia.nsa.2T. not widely used 128 bits usually fixed. IPv6 The Internet was built using version 4 of the Internet protocol (IPv4). an IPv6 supplement to this guide is in preparation. hundreds of IPv6 address blocks had already been allocated to network carriers and enterprises all over the world. Security best practices for IPv6 are not yet well-established. The US government and US DOD have set ambitious goals to complete substantial migration by 2008. consult any good book on IPv6.3. The Internet community is gradually migrating from IPv4 to v6. At the time of this writing. and should be published by mid-2006. IP version 6 (IPv6) was first standardized in 1995. Defining best practices and configuration guidelines for routers handling IPv4 and IPv6 may be complicated. it is expected that many routers will have to support both IPv4 and IPv6 (so-called “dual stack” operation) as well as supporting transition mechanisms like encapsulation and translation. Property IPv4 IPv6 Address size Network mask size Packet header size Packet-level extensions Fragmentation Control protocols Minimum allowed MTU Path MTU discovery 32 bits varies. 12. By mid-2005.Additional Issues in Router Security 7. In the mid1990s. Version 1. 64 bits 40 bytes (fixed) unlimited number of IPv6 extension headers only sender may fragment all control protocols based on ICMPv6 1280 bytes mandatory Cisco IOS routers support IPv6 in IOS versions 12. During the transition period. ICMP. such as [16].2. although some recommendations exist (see [15]).1c 265 . For more information. it became clear that the Internet would eventually outgrow the address space and capabilities of IPv4. and the standard revised to its current form in 1998. The supplement will be available from www. and later. The table below gives a few comparisons between IPv4 and IPv6.

ATM is a very big topic. Ethernet. Cisco routers support three mechanisms for sending IP traffic over ATM switched networks. It offers a great deal of flexibility.g. MPOA – Multi-Protocol Over ATM (MPOA) is a highly flexible set of mechanisms for transporting IP and other protocols over ATM switched networks. 3. Frame Relay). most of which is out of the scope of this guide. LANE – LAN Emulation (LANE) is a fairly general.1c . 1. but requires a great deal of configuration to deploy. 266 Version 1. Used with LANE.3. Classical IP – This is the oldest technique. Sometimes. and offers very simple configuration at the cost of flexibility and performance. standardized technique for extending an IP LAN over an ATM switched network. The security implications of choosing one of these modes over another are not yet entirely clear. ATM and IP Routing Asynchronous transfer mode (ATM) switched networks are popular for backbones and long-haul high-speed network links.Router Security Configuration Guide 7. 2. MPOA allows routers and other network devices to take advantages of advanced ATM facilities (like ATM quality-of-service). the boundary between switched ATM and routed IP will be a switch or router with one or more ATM interfaces and one or more traditional LAN or WAN interfaces (e.

Packets are forwarded along a label switch path (LSP). the current version is described in RFC 2547 and RFC 3031. Label switching allows routers to make forwarding decisions based on the contents of a simple label.1c 267 . offering traffic engineering (TE) . the LSR removes the existing label and applies a new label which tells the next hop how to forward the packet. MPLS enhances the services that can be provided by IP networks. incoming packets are assigned a label by a label edge router (LER). At each hop. MPLS uses label switch technology to simplify routing and enhance overall network performance. and Virtual Private Networks (VPN) capabilities. MPLS is defined and specified by the IETF. Quality of Service (QOS). Multi-Protocol Label Switching (MPLS) MPLS is an emerging high speed switching protocol typically deployed in the network core of a large enterprise such as an ISP.4. In an MPLS network. Each label switch router (LSR) makes forwarding decisions based only on the contents of the label. rather than by performing a route table lookup based on destination IP address.Additional Issues in Router Security 7. Version 1.

and it must have its own certified public key and private key. There are also trust issues in any large VPN deployment: are all members of the VPN trusted equally? In general. the simple configurations described in Section 5. consult Cisco IOS 12. but not for authorization or access control. • Cryptographic issues – Selection of uniform key sizes and cryptographic algorithms will be a contentious issue in VPN deployment.509 Certificate Revocation Lists (CRLs) and the Online Certificate Status Protocol (OCSP). Currently. and most other VPN systems. For complete information on the IPSec and dynamic VPN capabilities of Cisco IOS 12. • Designating traffic to be encrypted – Cisco routers.0.2 explains some of the basic features of IPSec.5. • PKI enrollment and obtaining certificates – To participate in a dynamic VPN based on Public Key Infrastructure (PKI).Router Security Configuration Guide 7. IPSec and Virtual Private Network (VPN) configuration are complex topics. depending on the installed IOS version and feature set.0 Network Security [2]. removing certified principals from the trusted community is very important. PKI standards define various data formats and protocols for defining revocations and for checking certification status. It may be necessary to configure revocation checking on routers participating in dynamic VPNs. IPSec and Dynamic Virtual Private Networks Section 5. Cisco routers currently support only a small complement of algorithms. a router or any other device must possess a copy of the correct root and authority certificates.2 probably will not scale well enough to satisfy users’ needs. • Certificate revocation – In any large-scale PKI. To achieve scalability. VPNs will need to be dynamic. IPSec is most useful for integrity and confidentiality assurance. support the ability to protect certain traffic based on its protocol and port numbers. there are no uniform guidelines for selecting traffic to protect. However. including X.1c . 268 Version 1. As deployment of VPNs becomes more common. Installing certificates and setting up authorities on Cisco routers is complex but well-documented. employing public keys and public key certificates to set up IPSec-protected links on the fly Security configuration issues are likely to be important in deployment of large dynamic VPNs are listed below.

Version 1.6. over IP networks. This kind of tunneling architecture is called Virtual Private Dial-up Networking (VPDN).19. the primary such protocol used today is called the Layer 2 Tunneling Protocol (L2TP). over IP from a remote router or network access server to a central router. the security for a VPDN service depends on use of IPSec between the two ends of the tunnel: the remote network access server and the central router. Virtual Private Dialup Networking Cisco routers support tunneling dial-up protocols.2. such as Ethernet or PPP. Standard protocols exist for tunneling layer 2 protocols. and actually use their home base LAN address. but it seems possible that small deployments could use static IPSec tunnels as described in Section 5.1.Additional Issues in Router Security 7.9.29 Internet home base router dial-up File Server 14.1c 269 .10 modem Virtual Connection Inside LAN Remote User 14. Use of such tunneling protocols allows remote users to join a LAN. This is an area that needs further study. expect a corresponding increase in mobile users and addressable devices expecting to join home base networks. from a remote part of the network.4.185 Figure 7-2: Overview of Virtual Private Dial-up Networking In general.2. each of which has different security issues. Tunneling Protocols and Virtual Network Applications As VPNs become more popular and widespread.9. 7.6. VPNs. like PPP.9. There are several approaches to doing this. Virtual Private Dial-up Networking configured between these two devices Access Server 126. and it is illustrated in the figure below. and protected networks from remote sites.

270 Version 1. Extensive guidance already exists for integrating IP QoS (RSVP) with ATM QoS. and configuring it securely will be challenging. Quality-of-service support offers the potential for substantial denial-of-service attacks. users will begin to demand quality-of-service assurances. Cisco IOS 12. As bandwidth-hungry network clients.7. all RSVP messages should be denied on external interfaces using IP access-lists. consult Section 4.Router Security Configuration Guide 7. On routers that support RSVP but that do not need to provide any QoS guarantees. For more information about access lists.0 and later support RSVP and QoS features. by providing mechanisms for denying bandwidth to authorized users. Also.1c . While the RSVP protocol itself includes provisions for authentication and authorization. but the security issues involved in that integration have not been explored.1 and later support centralized application of RSVP policies. begin to gain wide acceptance. In general. Many releases of Cisco IOS 12. RSVP configuration will probably be a contentious issue. IP Quality of Service (QoS) and RSVP The Resource reSerVation Protocol (RSVP) is the Internet standard protocol for setting up Quality-of-Service (QoS) parameters for traffic in routed IP networks. such as IP video-conferencing systems. key management and deployment issues for RSVP security have not been resolved. but the security issues associated with this facility have not yet been explored.3.

com) to IP addresses (like 14. The basic DNS protocol offers no authentication or integrity assurance. Cisco routers do not yet support acting as a secure DNS client (in other words. In a network with secure DNS. and pass along only validated results.mydomain. it would be possible to gain some of the security benefits by configuring the router name server (configuration command ip name-server) to be a local secure DNS server. The DNS Security Extensions standard defines comprehensive integrity and authentication facilities for DNS. Secure DNS The Domain Name System (DNS) used on the Internet and other major networks provides the mapping between names (like central. Version 1. It could then perform the security checks on remote DNS requests. The local secure DNS server would have to be configured to perform secure DNS requests on behalf of its non-security capable clients like the router. These security services are supported by the latest versions of the primary Internet DNS server implementation. the mapping between names and addresses is fully authenticated and integrity assured.Additional Issues in Router Security 7. Bind.2.250). it seems unlikely that it will become ubiquitous.8.9. the domain name resolver in Cisco IOS cannot recognize or check DNS security extensions).1c 271 . Given the negligible deployment that secure DNS has enjoyed in the first couple of years that it has been widely available. In a network with secure DNS.

R. “Classical IP and ARP over ATM”. IN. 272 Version 1.1c . Intranets. [5] Kent. NJ. [9] Laubach. defines the protocol structure and intent. The master document for IPSec. The updated standard for secure DNS. Z.0 Network Security. New York.” RFC 2205. [6] Eastlake. [3] Cisco IOS 12. includes extensive remarks about VPN architecture.C.. “RSVP Cryptographic Authentication”. This documentation volume includes extensive configuration information for Cisco ATM switching and LANE. and Harkins. Cisco Press. 1998. [4] Doraswamy. “Domain Name System Security Extensions. also good background reading for understanding the issues of hosting IP over ATM. “Security Architecture for the Internet Protocol. 1999..0 Switching Services. 1999. and Talwar. [2] Cisco IOS 12. and Halpern. RFC 2225. Describes the message authentication service to be used with RSVP. D. 1998. 2000. 2000. Authoritative source for in-depth descriptions of security-related IOS facilities. McGraw-Hill. 1997.” RFC 2401.” RFC 2535. Indianapolis. The definition of Classical IP over ATM. NY. References [1] Sacket. D.9. Prentice-Hall. Lindell. including IPSec and related configuration commands. The basic standard for RSVP. [7] Braden. IN. Cisco Router Handbook. Upper Saddle River. IPSec: The New Security Standard for the Internet. and Atkinson. N. and Virtual Private Networks.Router Security Configuration Guide 7. 1999. Contains a good overview of Cisco ATM facilities. M. includes extensive discussion and examples. Berson. and Jamin. Cisco Press. G. S. H. RFC 2747. “Resource reSerVation Protocol (RSVP) – Version 1 Functional Specification. Contains a good overview and substantial technical detail about IPSec and related topics. J. [8] Baker. 1999. Indianapolis.

D. Internetworking Technologies Handbook. A very detailed overview of remote access and layer 2 tunneling. and comparison with risks posed by IPv4. 2000.pdf A short paper offering an overview of MPLS and a comparison with traditional routing. Prentice-Hall. 2002. Provides detailed instructions for configuring IPv6 functionality on Cisco routers Version 1. with some analysis of its security. MPLS and VPN Architectures: A practical guide to understanding. P. P. and Miller. March 2004.. Cisco Self-Study: Implementing Cisco IPv6 Networks.0. V. available at: http://www. Zorn. 1999. L2TP Implementation and Operation. designing and deploying MPLS and MPLS-enable VPNs.” RFC 2661. Cisco R. An in-depth treatment of L2TP itself.1c 273 . Rubens... available at: http://www. “Layer Two Tunneling Protocol (L2TP). Cisco Systems. A very detailed overview of IPv6. 2003. Cisco Press. Jim and Pepelnjak. ito_doc/mpls_tsw. A highly detailed guide to setting up MPLS networks. “IPv6 and IPv4 Threat Comparison and BestPractice Evaluation”. Ivan. written from a network-oriented viewpoint. It also includes some security guidance for configuring Cisco routers. Addison-Wesley. Definition of the Internet standard tunneling protocol. [13] Cisco System. including discussion of the relationships of IP. [12] Shea.pdf A seminal analysis of network risks posed by including some coverage of security options. 2000. R. [11] Black.. PPP. [16] Desmeules. and L2TP. version 1.Additional Issues in Router Security [10] Townsley. “MPLS/Tag Switching”. [14] documents/v6-v4-threats... PPP and L2TP.. [15] Convery. U. S.

things that aren’t running can’t break.1] Implement access list filters by permitting only those protocols and services that the network users really need. chargen.5. Start by running the show proc command on the router. 8. daytime) based services. • CDP . Some services that should almost always be turned off are listed below. Shut down unneeded services . discard. and save memory and processor slots too. along with some important general principles for maintaining good router security. One of these is the UDP echo which is used in the ‘fraggle’ attack.Router Security Configuration Guide 8. Appendices The sections below offer ancillary material and supplemental guidance for network and security administrators. discard. This is invaluable for diagnosing suspected attacks or problems and recovering from them. and keep all it and all old versions under configuration management. Top Ways to Quickly Improve the Security of a Cisco Router This appendix describes the most important and effective ways to tighten the security of a Cisco router. for more details consult the corresponding parts of Section 4.2] 274 Version 1. [Section 4. Keep the offline copy of each router configuration in sync with the actual configuration running on the router. [Section 4.5. Use the no cdp run command to kill the process and disable CDP globally. [Section 4. Use the commands no service udp-small-servers and no service tcp-small-servers to turn these off.1. apply the command no cdp enable to the appropriate interfaces.1c . References to appropriate parts of Section 4 appear at the end of each recommendation. General Recommendations Comment and organize offline editions of each router configuration file! This sounds fluffy despite being a big security win. 8.3] Specific Recommendations 1.3] Run the latest available General Deployment (GD) IOS version.miscellaneous UDP (echo. [Section 4. Trying to deny just the ‘bad things’ is a losing proposition. CDP sends packets from your router once a minute or so identifying your router. chargen) and TCP (echo. [Sections 4. and explicitly denying everything else. The descriptions here are terse.2] • Small services . then turn off clearly unneeded facilities and services.Cisco Discovery Protocol is used almost exclusively by Cisco RMON. To leave CDP running but disable it for certain network connections.

To prevent this. [Section 4.1c 275 . but include it in your configuration explicitly anyway. use the command no ip directedbroadcast on all interfaces. If NTP is not being employed for time synchronization.2] 2. Log access list port messages properly.the Network Time Protocol. For reasons of efficiency. [Section 4.2] • NTP .2. the attacks will appear to come from you.2] Central(config)# interface eth 0/0 Central(config-if)# no ip directed-broadcast 3.5] • BOOTP – the IP bootp server. a Smurf attack can let an attacker use your network to launch denial of service raids on other sites. Don't be a Smurf buddy! While the Smurf attack doesn't usually attack the router itself.the finger daemon. use the special port range arguments shown below. Use the command no service finger (IOS 11. [Section 4.3 and later).2. 4.1] Central(config)# interface eth 0/3 Central(config-if)# shutdown Central(config-if)# exit Central(config)# line aux 0 Central(config-line)# no exec Central(config-line)# transport input none Central(config-line)# exit 4. To prevent this from happening. If the router has an auxiliary console port (aux port) and it is not in use. NTP can also be disabled for only a specific interface with the ntp disable command. Turn off this little-used server with the command no ip bootp server.6 East(config)# access-list 51 deny any log 5. [Section 4. Check them with the show ip interface brief command. shut it down as shown below.Appendices • Finger .9. instead of the usual logging access list command (such as access-list 106 deny ip any any log).3] East(config)# no access-list 51 East(config)# access-list 51 permit host 14. If packets are rejected by an access list filter for other reasons. This may be the default on some recent versions of IOS. no access-list 106 access-list 106 deny udp any range 0 65535 any range 0 65535 log Version 1. turn it off with no ntp server. the log message will often list the packet as using “port 0”. [Sections 4. Shut down unused interfaces using the shutdown command.2 and earlier) or no ip finger (IOS 11. [Section 4. Always start an access-list definition with the command no accesslist nnn to make sure it starts out clean. Cisco IOS doesn't look at an entire packet header unless it has to.

enforce this using access lists.1c .5. Besides preventing an attacker from using the router to attack other sites.0. use the password command.10.2. 7. consult Sections 4. the source routing facility should be disabled with the command no ip source-route. To control access.1] South(config)# line vty 0 4 South(config-line)# login South(config-line)# password Soda-4-J1MMY South(config-line)# access-class 2 in South(config-line)# transport input telnet South(config-line)# exit South(config)# no access-list 92 South(config)# access-list 92 permit 14. Allow only internal addresses to enter the router from the internal interfaces.3] 6. virtual terminals (telnet) are unprotected.2] Central(config)# no ip source-route 8.0 0.2 and later use the global configuration command no snmp-server enable traps.3] South(config)# no snmp-server enable traps South(config)# no snmp-server 9.0.255 Controlling authentication for login to the router is an extremely important topic. are permitted. use an access list and the access-class command.Router Security Configuration Guide access-list 106 deny tcp any range 0 65535 any range 0 65535 log access-list 106 deny ip any any log The last line is necessary to ensure that rejected packets of protocols other than TCP and UDP are properly logged. [Section 4.1] South(config)# enable secret 2Many-Routes-4-U South(config)# 10. [Section 4. [Section 4. If SNMP is not being used on the router.2. use the transport input command to enable only those methods. turn it off with the command no snmp-server.1 and 4. Turn off SNMP trap authentication to prevent a remote SNMP system shutdown request. [Sections 4. it helps identify mis-configured internal hosts and 276 Version 1. By default. [Section 4. If only specific methods of attaching to the VTY. Unless the network is one of those very rare setups that needs to allow source routed packets.6 for guidance. Block illegal addresses at the outgoing interfaces. Make sure that the router enable password is encrypted using the strong MD5-based algorithm by using the enable secret command rather than the enable password command. 4. Password and access protect the Telnet VTYs. such as Telnet or SSH. To set a password. In IOS 11.

3] East(config)# no access-list 101 East(config)# access-list 101 permit ip 14. These packets cannot be real.2.6 12. This protection should be part of the overall design for traffic filtering at the router interface attached to the external.0 0.0. Turn on the router’s logging capability.2.0.9. Make sure that the router blocks syslog traffic from untrusted networks.5] Central(config)# Central(config)# Central(config)# Central(config)# logging logging logging logging buffered trap info facility local1 14. This approach may not be feasible for very complicated networks. • Block incoming loopback packets (address 127. Incorporate this protection into the access lists applied to interfaces connected to any untrusted networks. • If the network does not need IP multicast then block it. and use it to log errors and blocked packets to an internal (trusted) syslog host.1c 277 . North(config)# North(config)# North(config)# North(config)# North(config)# North(config)# no access-list 107 ! block internal addresses coming from outside access-list 107 deny ip 14.1 0.1).255 any log ! block bogus loopback addresses access-list 107 deny ip 127. [Section 4. This impedes some TCP sequence number guessing attacks and related attacks.1. • Block broadcast packets.0. (A superior but more difficult approach is to permit only necessary ICMP packet types.2.) • A number of remote attacks use ICMP redirects. Block packets coming from the outside (untrusted network) that are obviously fake or are commonly used for attacks. block them.255 any log access-list 107 deny ip 14.255.0. (Note that this may block DHCP and BOOTP services.6.255 any log Version 1. [Section 4.3] • Block packets that claim to have a source address of any internal (trusted) networks. but these services should not be used on external interfaces.) The example below shows how to enforce these rules on router North.Appendices networks. any East(config)# access-list 101 deny udp any range 1 65535 any log East(config)# access-list 101 deny tcp any range 1 65535 any log East(config)# access-list 101 deny ip any any log East(config)# interface eth 1 East(config-if)# ip access-group 101 in East(config-if)# exit East(config)# interface eth 0 East(config-if)# ip access-group 101 out East(config-if)# end 11.0 0.0. [Section 4.0. untrusted network.0 0.

If proxy ARP is not needed.2] Central(config)# interface eth 0/0 Central(config-if)# no proxy-arp 16. a ‘Land’ attack on the router itself). Disable it by applying the command no proxy-arp to each external interface.255. North(config)# interface eth 0/0 North(config-if)# ip access-group 107 in 13. using a rule like the one shown below (part of the configuration file for router East).0 0.2. [Section disable network autoloading with the command no service config. use the global configuration command service timestamps as shown in the example below.255.2] 17.Router Security Configuration Guide North(config)# ! block multicast North(config)# access-list 107 deny ip 224.5] East(config)# service timestamps log date \ msec local show-timezone East(config)# 18. So that the complete date and time are stamped onto entries in the routers buffered log.1] 15. Block incoming packets that claim to have the same destination and source address (i. Unless the router absolutely needs to autoload its startup configuration from a TFTP host.0 any log North(config)# ! block ICMP redirects North(config)# access-list 107 deny icmp any any redirect log .250 255.0 ip access-group 102 in 14.2] 278 Version 1. [Section 4.2. [Section 4.1c .0.0. [Section 4. using the command service tcp-keepalives-in.0. the HTTP server is off by default. To be safe. Turn on TCP keepalive packets for administrative telnet sessions. [Section 4. however. Except on the rarely-seen Cisco 1000 series routers.6.3] no access-list 102 access-list 102 deny ip host 14. [Section 4. disable it on all interfaces.e. Proxy ARP is used to set up routes on the fly for internal hosts or subnets and may reveal internal addresses.250 log access-list 102 permit ip any any interface Eth 0/0 ip address 14. .255 any North(config)# ! block broadcast North(config)# access-list 107 deny ip host 0.0. Incorporate this protection into the access list used to restrict incoming traffic into each interface.250 host 14. include the command no ip http server in all router configurations.6.

1] East(config)# service password-encryption 20. In general. It is not necessary to install each and every new IOS release. see Section 6.Appendices 19. and defending against common attacks. [Section 4. Version 1. and will provide new security features. [Section 4.5] For more information about testing router security. Update your IOS image to the latest General Deployment (GD) release. but it is a good idea to keep your router up to date. newer releases will include fixes for security bugs. This provides some security against casual ‘over-the-shoulder’ attacks. so that regular passwords are stored and displayed in scrambled form. Turn on password encryption.1c 279 . so plan your updates carefully. Installing an update normally imposes some downtime.

it is best to create a security checklist for LAN switches. and other network hardware components that are not IP routers. The security goals for a switch or hub are listed below. If possible. and are therefore subject to compromise over the network. and preventing their use by unauthorized individuals and attackers. hubs. The NSA “Cisco IOS Switch Security Configuration Guide” [2] provides a great deal more information on the secure configuration of Cisco Ethernet switches. and other LAN components have acquired substantial capabilities.nsa. Because they cannot perform network layer or transport layer traffic filtering. managed hubs. local connections. managed hubs. the device must be configured to strictly limit all forms of access: physical. Since the mid-1990s. and remote network connections. but simpler because such a network component does not act as a boundary device between different networks. 280 Version 1.2. Follow the general form of the security checklist given at the end of Section 3. It can be downloaded from http://www.g. and More information is available in [4]. access servers. Security Principles and Goals The general security goals for a switch or smart hub are similar to those for a router.1c . Application to Ethernet Switches and Related Non-Router Network Hardware This appendix identifies specific topical areas and recommendations from the main body of this guide that apply to Ethernet switches.Router Security Configuration Guide 8. Because they are vital to network operations and because they can be used as a staging area for additional attacks. The focus for security for these devices is protecting their own configuration. routers were the only LAN components with sufficient flexibility to need security configuration. switches and hubs cannot generally enforce security policies on network traffic. an Ethernet MAC address). These devices are characterized by participation in the network itself by forwarding and switching traffic based on a media layer address (e. 8. The discussion below focuses mainly on media-level network components: switches.2. • preventing unauthorized examination of device state and configuration • preventing unauthorized changes to the device state and configuration • preventing use of the device for attacking the local network • preventing unauthorized remote management/monitoring of the device To achieve these goals. it is important to configure them securely. switches. Such devices almost always support remote administration and management. access servers.1. Prior to the 1990s. many of them are as flexible and configurable as a router.

Most of the recommendations in this section apply to switches. . remote access restrictions.2 Network service security Version 1. Table 8-1: Router Security Guidance Sections Applicable to IOS-based Switches Section Topic Access security Application to Switches All of this section applies to switches: setting up users and passwords.Appendices 8. 2900 series) and those that do not use IOS (e. but they can be divided into two broad groups: those that use Cisco IOS or a derivative (e. Much of the security guidance given in Section 4 that can be applied to IOS-based Cisco switches. login to the switch and use the show version command to display the operating system name.5(1) NmpSW: 4. . In addition. Version McpSW: 4. Especially important for 2900 switches is restricting access to the HTTP server. . RELEASE SOFTWARE (fc1) .1(2) . but will not generally work on other devices.g. Uptime is 45 days. While the command syntax and command interface structure differ between Cisco IOS-based and other equipment. 4.5(1) . and thus does not need to be configured.1 4. The syntax shown in Section 4 will work for IOS-based switches. Version 11. and even some smart Ethernet hubs. sw20c uptime is 6 days.1c 281 . Application to Cisco IOS-based LAN Equipment Cisco makes several kinds of network switches.2. Before attempting to apply the detailed instructions from Section 4. any network service that is related to routing usually is not supported on a switch. 3 hours. check whether the particular switch is running IOS or some other operating system. 9 minutes .2. the same general principles apply to all of them. . System Bootstrap Version 5.g. Catalyst 5000 series). 3 hours. sw20c# Non-IOS Catalyst 5500 Cat5k# show version WS-C5505 Software. and configuration loading and maintenance. the operating system name and version are underlined in the examples below. If you do not have the switch documentation handy. 51 minutes Cat5k# The table below describes how to apply the guidance in each part of Section 4 to IOS-based LAN switches. all ports should be configured to block traffic to unknown addresses using the port block interface configuration command.2(8)SA. IOS-based Catalyst 2900 sw20c# show version Cisco Internetwork Operating System Software IOS (tm) C2900XL Software (C2900XL-H-M).

June 2004. Catalyst 5000 series Route Switch Modules) can and should be configured using the guidance in Section 4. although some Cisco switches can act as routers. too. “Network Insecurity with Switches”. Duesterhaus. VLAN configuration. available under: http://www. and must have their time set manually.3. References [1] Turner. but do not use them for as many different purposes as a router does. A. Basically. Many of the issues discussed in the paper have since been fixed. This section is not usually applicable to switches.2 and later). It should be configured using Section 4 like any other router. Consult [2] for more detailed guidance and specific examples. if they support AAA (IOS This highly detailed guide covers general switch security. on a switch. and more.] Almost all of this section applies to IOS-based switches. et al. 4. some switch IOS versions do not support NTP. includes several good references. these services should be disabled if not in use.synfin. [Note: some Catalyst 5000 and higher series switches are equipped with a ‘Route Switch Module’.2.1c . 282 Version 1. but not for filtering traffic passing through the switch. or access to them should be restricted. NSA. All of this section is applicable to IOS-based switches.Router Security Configuration Guide Section Topic Access lists Application to Switches IOS-based switches support IP access lists. [2] Borza.pdf An examination of the security (and lack of it) provided by separating traffic with Ethernet switches.6 Access control with AAA Note that Cisco switch-resident routing hardware (e. This module is essentially a 4700-series IOS router attached to the switch.5 Audit and Management 4. access lists are used for limiting access to services on the switch itself.g.. “Cisco IOS Switch Configuration Guide”. but the discussion is still valuable. GIAC Paper. All switches support RMON and SNMP.4 Routing protocols 4. after careful consideration of its role in the network security policy. Most of the security testing guidance given in Section 6 also applies to LAN switches. December 2000.nsa.3 4. available at: http://www. port management.

5T Figure 8-1 – Cisco IOS Release Naming In general.8. the format has changed several times since IOS was first introduced in the mid-1990s.2(9)XA. Often it will not be practical to install all the updates that Cisco makes available. the release “12. release number and release identifiers tell what features could be available.3. critical feature.3T” is IOS version 12. and may also include a number in parentheses. It is intended to help with upgrade strategies and version selection. The current format for a Cisco IOS release name is shown below. especially during the flurry of bug fix releases that tends to follow a major change. Unfortunately. There is a complex naming scheme for early releases that is beyond the scope of this guide. 12 Examples: 12. 11. Some of the suffixes that you might see on special-purpose releases include “XA”.1.Appendices 8. and the release naming scheme. and the revision number tells how many times the release has undergone fixes to correct problems. 8. unless it has some required. Cisco releases may be broadly divided into kinds: regular shipping releases (general or limited) and early releases.3. consult [1] for complete details. Devise a consistent upgrade strategy that matches the needs of your network. operational routers should kept up to date with the newest stable release that provides all the needed features. for example.0 Revision = 3 Release = 11.N.3.3 Release = 12. and then follow it. The ‘T’ identifier designates an early release of new technology features. You might also see maintenance revision numbers in parentheses. A regular release will almost always have a simple number with no release identifier. such as 12.1c 283 .M RR Release identifier Maintenance revision number Minor release number IOS Major release number: 10. VV. Release Levels and Names Cisco follows strict naming schemes for IOS releases. For operational purposes. For example. “HA”. revision 3. 11. This appendix provides an overview of the major release levels.0. In general. “F”.3 Revision = 5 Identifier = T 11. it is usually best to avoid early release software. Overview of Cisco IOS Versions and Releases Cisco provides a very large number of software releases for their routers and other products.0. Version 1. use this appendix and the materials listed in the references.1. to understand what Cisco provides.1. identifier T. An early release will usually include an identifier. usually for ED releases.

so it is generally a good idea to use the simplest feature set that satisfies all of the network’s operational and security needs. 12. Type Description Early Deployment – a pre-shipping release that supports new features. or hardware. Releases.Router Security Configuration Guide Every Cisco IOS release has a release type. It presents the different available releases in a friendly tabular arrangement. 12. Features. to give the greatest latitude for future expansion. Some commercial organizations customarily purchase routers with the maximum memory capacity pre-installed. and allows you to select items of interest (hardware mode. a GD has usually been subject to several rounds of bug fixes since first shipping. Releases at this level are sometimes pre-installed on routers sold by Cisco. DF releases are not available to customers. Remarks This could be considered the ‘beta’ release for an IOS version. The Cisco web site provides a “Software Center” where authorized customers can download software products. The feature sets have names that are roughly evocative of what the features are. The table below describes the types. As an example. look at IOS 12. IOS versions with more features require more memory. it supplies the hardware and memory requirements for that release before permitting you to download it. it supports choosing versions in a very flexible way. and the Cisco IOS Upgrade Planner Every Cisco IOS release is built with a variety of feature sets.1 was ED. LD releases are usually stable. All feature sets support basic IP routing and filtering. including Cisco IOS releases. and 12. The revision numbers for a given release run sequentially.4 was LD.0. The part of the software center that contains the IOS releases is called the “Cisco IOS Upgrade Planner. When you use the IOS Upgrade Planner to select a particular IOS software release.1c . release number) in any order. protocols. two common names are “IP PLUS” and “ENTERPRISE/APPN”. General Deployment – a stable shipping release suitable for general use.0. but have not undergone the extensive customer shakedown and bug fixes of a GD release. ED LD GD DF Deferred Release – a release that was built and named. The most stable type of release. even as the release status moves from ED to GD. Be very careful to check these requirements against the router on 284 Version 1. or telephony.” Registered Cisco customers with software maintenance contracts may download IOS releases via the Upgrade Planner.8 was GD. Most Cisco routers sold come with a GD release pre-installed. feature set.0. Limited Deployment – this is the status of a release when it is first shipped to customers (FCS).0: for the 3640 router. but some also support firewall or IPSec functions (see Section 5) or mainframe protocols. but later retracted.

Ensure that amounts of installed memory meet or exceed the requirements before attempting to load the IOS release. The lists below describe some of the major features introduced into IOS in each of these releases. Some of the important features • RIPv2 (see Section 4. with emphasis on security-relevant features.1. IOS 11. IOS 11. All earlier Cisco IOS releases. and engineering development for it was dropped in 1999.2 The 11. and earlier.5) • The IOS web server and web browser management interface [IOS 11.2.1. 3660). Using this tool [3].1 was first deployed in 1996.2. 12. are now mostly unsupported by Cisco.0.3.5) • Various OSPF and BGP4 enhancements Version 1. 11. freely available on their web site.2. a few of them are listed below. • Named access control lists (See Section 4. A great many new features were added to IOS over the lifetime of 11. see Section 4. Cisco also offers a hardware/software compatibility matrix checker.2 release was the first IOS version to fully implement Cisco’s modular architecture for router software.Glossary which you hope to run the software. IOS 11.5) • Lock-and-Key dynamic access lists IOS 11. 8. 12.g. Major Releases and their Features There are at least five major releases of Cisco IOS software currently in use in operational environments: 11. but is not available for some of the popular newer models (e. although some of them are still available for download.1 is available as a GD release for all older Cisco routers.3) • Network address translation (NAT) • Support for RSVP and IP Quality-of-Service (see Section 7. 7500.3.1(5) and later] • RADIUS support (as part of AAA. you can check what IOS releases are supported on your router model.1 The 11. 1605.6) • RMON support (see Section 4.3. While exceedingly stable and robust. it did not offer extensive security features. 12.1 release was the last IOS release to use the old ‘classic’ or monolithic architecture. and 12.1c 285 .2. 11.

but it was never officially shipped as a GD release.2F only) • Early (pre-IPSec) VPN support • Early versions of the IOS Firewall feature set and CBAC (see Section 5.0T releases brought together a wide variety of features that had previously been available only in selected LD and ED releases of IOS 11. Some of the features introduced in 11.1c .2 is available as a GD release for many older Cisco router models. • Full support for the Firewall feature set and CBAC • Initial version of IOS Intrusion Detection (IDS) • Full support for IPSec • Commented IP access list entries • Full support for the Layer 2 Tunneling Protocol (L2TP) 286 Version 1. 12.Router Security Configuration Guide • Initial support for TCP Intercept (IOS 11.1 through 11.0 was designed to be the basis for future router software releases.0 are listed below.3 was used to introduce a large number of new features into IOS. IOS 11.3T) • Reflexive access lists • TCP Intercept (full availability) • Initial support for VLAN routing • Enhanced IOS filesystem and initial support for FTP • HTTP authentication for the IOS web server IOS 11. Some of the security-relevant features introduced or consolidated in 12.3T) • Cisco Encryption Technology (CET) VPNs • Enhancements to AAA (See Section 4. and to help eliminate the confusion of specialized releases that plagued 11.3 is available for almost all Cisco router models.3 are listed below. • Initial implementations of IPSec (11.4) IOS 11. but only at the ED and LD release levels.7) • Full IOS firewall feature set and CBAC (11.3 11.3.0 and 12.0 The 12. IOS 12.

0.2 release adds some new features to 12.6) • Time-based access lists • General availability of IP unicast reverse-path verification [Section 4. as well as enhancements to some core security features. IOS 12.2 reached GD status in 2002.3 had not yet reached GD status. and also expanded the availability of some features.2 are listed below.1 release is an incremental step forward from 12. IOS 12.5 (SSHv1) supported in nearly all IOS editions • Cisco Auto Secure feature added. • Secure Shell Version 1. IOS 12.0 is available in both LD and GD forms for almost all supported Cisco router platforms.1 are listed below.4] IOS 12.3 release added several substantial new security features.1 The 12. • Enhanced IPSec certificate management and AAA integration • AAA enhancements: server groups. As of summer 2005.1. more accounting features • Unicast reverse path forwarding security enhancements • Initial broad support for Secure Shell (SSH Version 1) server IOS 12.Glossary • SNMP version 3 (See Section 4.2 The 12.3 are listed below. and many other Cisco hardware products. A few of the many enhancements in 12. Some of the security features that appeared in 12. it was the latest release available at GD status for some devices. • Improved support for IP Quality-of-Service and RSVP • Multi-Protocol Label Switching (MPLS) support • Enhancements to SSH support • Enhancements to IPSec and IKE • Turbo Access Lists (some router models) • Better application of service password-encryption IOS 12. Some of the security updates in 12.3 The 12.1c 287 . As of the summer of 2003. simplifies basic secure configuration • New features added for password and authentication controls Version 1.

it only covers up through IOS 12. (Note: may be available to registered users only. available at: http://www. 8. The service is available to registered customers at http://tools.1c .jsp.) [4] Bollapragada. CCIE Professional Development Series. Cisco White Papers. 2004. V. 288 Version • Improvements to protocol inspection for the IOS firewall • Control plane traffic policing (some IOS editions) • Many improvements to PKI and certificate support • Added login password control features: retry delay. and includes a map of releases up through release adds additional security features and many new protocols and functions to IOS.x/Support/HWSWmatrix/ A very detailed examination of IOS. References [1] Coulibaly. focused on packet handling and routing.3.. Cisco Press. Cisco Systems..html This detailed web page explains the IOS release naming scheme. Inside IOS Software Architecture. White. However. This highly specialized book covers the Cisco IOS release system and release history in painstaking Cisco Press. 2001. C.Router Security Configuration Guide • SSL support added for the IOS web server (some IOS editions) • Improvements in PKI and firewall support • Introduced Control Plane Policing (some IOS editions) IOS 12. 2000. Cisco IOS Releases: The Complete Reference. Cisco Systems. retry lockout Finding Features in IOS Releases Cisco’s web site offers a useful service called the ‘Feature Navigator’ that supports searching for features by name or release number.3. 2000. Murphy. • Secure Shell Version 2 (SSHv2) A few of the new security features are listed below.. available at: http://www. part of the Cisco Software Advisor.cgi This interactive web page allows you to find IOS releases compatible with particular router decribes features in IOS 12. M. client and server. R.2. [3] “Hardware/Software Compatibility Matrix”.4 The 12. [2] “Cisco IOS Reference Guide”.

Not commonly used today. CIDR is standardized by RFC 1518. IKE. (See also MAC Address.Glossary 8. (See also RADIUS and TACACS+) Access Control List . Authorization. Cisco Discovery Protocol – a proprietary link layer protocol that Cisco routers use to identify each other on a network. access to router services. BGP version 4 is standardized in RFC 1771. Content-Based Access Control – packet inspection system used for application firewall functionality in Cisco routers. and more. Committed Access Rate – a traffic bandwidth control facility usable for simple quality-of-service and traffic shaping tasks. LAN. the packet format and protocol for IP integrity assurance services.4. or restrict network traffic. CIDR replaced the old class-based IP addressing scheme. LANs. Glossary of Router Security-related Terms AAA Authentication. Classless Inter-Domain Routing . (See also LANE) Border Gateway Protocol – an advanced exterior gateway routing protocol mostly used on backbone routers. usually based on addresses and other information from the packet headers. ACL Access List AH ARP ATM BGP CAR CBAC CDP CEF CIDR Version 1. ESP) Address Resolution Protocol – link-layer protocol used for mapping from IP addresses to MAC addresses. Cisco IOS depends heavily on access lists for traffic filtering. (see IPSec. Proxy-ARP) Asynchronous Transfer Mode – virtual-circuit oriented link layer protocol. and Accounting – The advanced user access control and auditing facility in Cisco IOS 11 and 12. ARP is standardized in RFC 826. and telecommunications facilities.the present standard for network address allocation and network route aggregation on the Internet. used for network backbones. IPSec configuration. Cisco Express Forwarding – a proprietary packet transfer technology used inside most Cisco router models.See Access List A set of rules that identify. permit. Authentication Header – a part of IPSec.1c 289 .

Exterior Gateway Protocol – routing protocol designed for managing route updates between different autonomous systems. Denial of Service – this abbreviation is often used for network attacks that prevent a network component from providing its operational functions. on the fly. DHCP is standardized in RFC 2131. Extended Interior Gateway Routing Protocol – A Cisco proprietary routing protocol that includes peer authentication features. DNS is also the name for the protocol used to transmit and relay name information. ICMP is standardized in RFC 792. A slang expression for a privileged EXEC session on a Cisco IOS router. like IP addresses and gateways. Internet Control Message Protocol – a support protocol used along with IP for control and status message.1c . DNS is standardized in RFCs 1034 and 1035. Domain Name System – hierarchical naming scheme used for host and network names on most IP networks. Typically. The main EGP in use today is BGP version 4. FTP control messages are passed on TCP port 21. ICMP is a network layer protocol that provides error messages and management capabilities in IP networks. the packet format and protocol for IP confidentiality services (see also IPSec. AH) File Transfer Protocol – widely-used TCP-based file transfer and file management protocol. Distributed Denial of Service – This abbreviation is used for DoS attacks that use multiple (usually hundreds or more) coordinated network data sources to attack a single victim. Dynamic Host Configuration Protocol – UDP-based protocol for assigning host network attributes. GTSM is standardized in RFC 3682. Encapsulated Security Payload – a part of IPSec. (see also OSPF). DNS DoS DDoS EGP EIGRP Enable mode ESP FTP GTSM ICMP 290 Version 1. or that crash it. IKE. Generalized TTL Security Mechanism – a simple spoof rejection mechanism that uses the TTL field of the IP header to detect illicit packets. FTP is standardized in RFC 959. including the Internet. derived from the command used to request privileged EXEC mode: enable.Router Security Configuration Guide CPP DHCP Control Plane Policing – a security mechanism that applies rate limiting to traffic into and out of the router’s central processor.

and IS-IS. Internet Operating System – Cisco’s name for the modular software system that runs on their routers and many other network devices. Internet Security Association Key Management Protocol – one of the precursors of IKE (see also IKE. [Note: this guide covers security only for IPv4. IPSec). Kerberos was developed by the Massachusetts Institute of Technology as a network authentication system. Intermediate System to Intermediate System – an OSI standard interior gateway protocol based on a link state model. IS-IS is standardized in ISO-10589 and RFC 1195.] Internet Protocol version 6 – The new network-layer protocol for the future of the Internet. I is standardized in RFC 2460. IETF standards are published by RFC number.] Internet Protocol Security – a set of standards that define confidentiality and integrity protection for IP traffic. IGP IKE IOS IP IPv6 IPSec IS-IS ISAKMP Kerberos LAN LANE Version 1. IPv4 is standardized in RFCs 791 and 1883. the list of current standards (STD 1) is RFC 3700. Kerberos is standardized in RFC 1510 (see also RADIUS and TACACS+). There are two extant versions of IP: IPv4 and IPv6.1c 291 . LAN Emulation – A standard mechanism for routing IP packets over ATM. EIGRP. RIP. IKE is standardized in RFC 2409. and it provides strong authentication for client/server applications by using secretkey cryptography. Internet Protocol version 4 – The network-layer protocol on which the Internet is built. Interior Gateway Protocol – a routing protocol used among the routers in an autonomous system. Local Area Network – general term for a single-segment or switched network of limited physical/organizational extent. IPSec is standardized by a set of RFCs including RFC 2401.Glossary IETF Internet Engineering Task Force – the technical and consultative body that defines standards for the Internet. Internet Key Exchange – the standard security negotiation and key management protocol used with IPSec. Currently popular IGPs include OSPF. [Note: security recommendations for IPv6 will be provided in a separate supplement to this guide.

standardized in RFC 3031. standardized in RFC 1321. especially Ethernet interfaces. (See also RIP. port 123. certificates. Open Shortest Path First – an IP routing protocol that uses a linkstate distance metric. in which packets can be broadcast to particular recipients based on address. Message Digest algorithm 5 – a widely-used cryptographic checksum algorithm.255. addresses from 224.255. MAC Address MD5 MIB MPLS MPOA Multicast NNTP NTP NVRAM OSPF PKI Proxy 292 Version 1. EIGRP) Public Key Infrastructure – mechanisms and components for management of keys. can use UDP or TCP. OSPF is standardized in RFC 2328. and enrollment. but usually uses UDP. In IPv4.Router Security Configuration Guide L2TP Layer 2 Tunnel Protocol – A standard protocol for forwarding lowlevel protocols over IP networks. An Ethernet MAC address is 48 bits long. An application that acts as an intermediary in the network exchanges between applications or services. IGP. NTP is standardized in RFC 1305. L2TP is standardized in RFC 2661.0. Media Access Control address – the link layer address of a network interface.0. (See also SNMP) Multi-Protocol Label Switching – a standard mechanism for transferring packets over backbone networks by tagging them with labels.255 are usually multicast group addresses.0 to 225. Network News Transfer Protocol – a TCP-based application protocol that usually runs on port 119. Non-volatile RAM – device memory that can hold data even when unpowered. Management Information Base – the hierarchical data organization used by SNMP. Network Time Protocol – the standard network time synchronization protocol. Multi-Protocol Over ATM – A proposed standard mechanism for hosting network protocols (such as IP) over ATM.1c . Proxy servers are often employed to moderate exchanges through a firewall. An operational feature of IP. Cisco routers use NVRAM to hold their startup configuration.

SNMP is standardized in RFC 1157 and other RFCs. The Remote Authentication Dial-In User Service (RADIUS) is specified by the IETF RFC 2058. Direction and management of paths through a multi-segment network. based on SNMP.1c 293 . (See also RMON). RSVP is standardized in RFC 2205. SNMP uses UDP ports 161 and 162. (See also OSPF) Remote MONitoring – facilities for remote performance and traffic monitoring of network devices. RADIUS support centralized authentication and accounting. Use SSH instead of Telnet whenever possible. OSPF. Request For Comments – a document describing an Internet standard. (See also RIP. Syslog usually uses UDP port 514. BGP) Resource reSerVation Protocol –standard protocol for requesting quality-of-service guarantees in IP networks. Secure Shell – a remote access protocol that provides end-to-end confidentiality and integrity services. and accounting of users accessing a router or access server. and/or 1812. and 1723. Simple Network Management Protocol – datagram protocol used for monitoring and configuring network devices. 1646. Terminal Access Controller Access Control System Plus – a security protocol to provide centralized authentication. RIP is standardized by RFCs 1088. 1388. TACACS+ is defined by Cisco. authorization. A simple UDP protocol used for logging by Unix systems and Cisco routers. proposed standard. or information related to or supports a standard. SMTP is standardized in RFC 821.Glossary Proxy-ARP A facility offered by some routers where a router responds to ARP queries from a connected LAN on behalf of hosts on other LANs. (see SSH) Simple Mail Transfer Protocol – a TCP-based protocol for sending and relaying e-mail messages. Proxy ARP is rarely used. RADIUS RFC RIP RMON Routing RSVP SCP SMTP SNMP SSH Syslog TACACS+ Version 1. Secure Copy Protocol – a file transfer protocol based on SSH. (See IETF) Router Information Protocol – a simple inter-gateway routing protocol that uses hop count as its distance metric. RADIUS normally uses UDP ports 1645.

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Transmission Control Protocol – connection-oriented data protocol used with IP. TCP supports a large number of application layer network services, including Telnet, web, FTP, and e-mail. A simple TCP-based protocol for remote login, usually on port 23. Also used to refer to client applications that support the protocol. Trivial File Transfer Protocol – simple UDP file transfer protocol, with no authetication features. TFTP normally uses UDP port 69; it is standardized in RFC 1350. User Datagram Protocol – message-oriented data protocol used with IP. UDP is the basis for many core network services, including DNS, RIP, and NTP. UDP is standardized in RFC 768. Virtual LAN – a link layer communication domain that spans several link layer switches; commonly used with Ethernet switches. Virtual Private Dialup Network – an application of VPN technology to secure remote-dialup connections, giving a remote user secure connectivity to their ‘home base’ network. (see also VPN) Virtual Private Network – a closed network of computers or LANs, using the public network as the transport. Usually, traffic between members of the VPN is protected by IPSec during transit over the public network. Virtual TeletYpe – an interface on a host or router that provides the interactive services of a terminal. Cisco routers use VTY lines to host Telnet sessions (see Telnet).

Telnet TFTP





Cisco offers a large glossary of networking terms and acronyms at their web site: Information about a wide variety of protocols may be found at Internet RFCs are available from


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9. Additional Resources
The references below can be useful in designing secure network configurations, and in understanding and maintaining router security.

9.1. Bibliography
The list below consists of books that are useful for router configuration and security, collected from the reference lists throughout this guide. Akin, T. Hardening Cisco Routers, O’Reilly Associates, 2002. A very good prescriptive guide to securing Cisco IOS routers. Albritton, J. Cisco IOS Essentials, McGraw-Hill, 1999. An excellent introduction to basic usage and configuration of IOS routers. Ballew, S.M., Managing IP Networks with Cisco Routers, O’Reilly Associates, 1997. A practical introduction to the concepts and practices for using Cisco routers, with lots of pragmatic examples. Baker, F. ed. “Requirements for IP Version 4 Routers”, RFC 1812, June 1995. A comprehensive introduction to the facilities that an IP router must provide to support the Internet. Black, U. IP Routing Protocols, Prentice Hall, 2000. A very good survey of routing protocols and the technologies behind them, with some discussion of applications. Buckley, A. ed. Cisco IOS 12.0 Configuration Fundamentals, Cisco Press, 1999. This is the reference manual and guide for basic configuration tasks in IOS 12.0. Sections particularly relevant to Router Access Security include: IOS User Interfaces and File Management. Chapman, D.B., Cooper, S., and Zwicky, E.D., Building Internet Firewalls, 2nd Edition, O’Reilly & Associates, 2000. A seminal overview of network boundary security concerns and techniques. This revised edition includes extensive updates for newer technologies. Chappell, Laura, Editor, Advanced Cisco Router Configuration, Cisco Press, 1999. Great reference book for a variety of Cisco configuration topics, including routing and routing protocols.

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Cisco IOS 12.0 Configuration Fundamentals, Cisco Press, 1999. The configuration fundamentals guide and reference in book form; handy to have, but the documentation CD is usually easier to use. Cisco IOS Release 12.0 Security Configuration Guide, Cisco Press, 1999. This is the reference manual and guide for major security features in IOS 12.0, along with many examples. Desmeules, R., Cisco Self-Study: Implementing Cisco IPv6 Networks, Cisco Press, 2003. A sound introduction to IPv6 concepts and protocols, with directions for configuring Cisco IOS routers. Doraswamy, N. and Harkins, D. IPSec: The New Security Standard for the Internet, Intranets, and Virtual Private Networks, Prentice-Hall, 1999. Contains a good overview and substantial technical detail about IPSec and related topics. Doyle. J., Routing TCP/IP - Volume 1, Cisco Press, 1998. Offers a very detailed examination of routing protocols and design of efficient networks. Held, G., and Hundley, K., Cisco Access List Field Guide, McGraw-Hill, 1999. This book offers detailed information and examples on access list syntax and usage. Held, G. and Hundley, K., Cisco Security Architectures, McGraw-Hill, 1999. This book includes excellent general advice about router and router-related network security, in addition to its Cisco-specific material. Huitema, C. Routing in the Internet, 2nd Edition, Addison-Wesley, 1999. A deep and detailed textbook about IP routing technologies, protocols, and how routing works in the Internet Moy, J.T. OSPF – Anatomy of an Internet Routing Protocol, Addison-Wesley, 1998. Detailed analysis of OSPF, with lots of practical advice, too. Includes a good section on troubleshooting. Parkhurst, W.R. Cisco Router OSPF - Design and Implementation Guide, McGraw-Hill, 1998. Comprehensive and practical guide to OSPF use. Includes discussion of design issues, security, implementation, and deployment.


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Rybaczyk, P., Cisco Router Troubleshooting Handbook, M&T Books, 2000 A very practical book, oriented toward finding and correcting problems with router connectivity and routing protocols. Sedayao, J., Cisco IOS Access Lists, O’Reilly Associates, 2001. Provides detailed information on constructing and using access lists. Shamin, F. Troubleshooting IP Routing Protocols, Cisco Press, 2002. A comprehensive hands-on guide for resolving IP routing problems Stevens, W.R., TCP/IP Illustrated, Volume 1, Addison-Wesley, 1994. The most comprehensive and readable guide to the TCP/IP protocol suite; great technical background for any network analyst. Thomas, T.M. OSPF Network Design Solutions, Cisco Press, 1998. This book starts with a good overview of IP routing and related technologies, then goes on to explain how to configure Cisco routers for OSPF in a wide variety of situations.

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9.2. Web Site References
The list below consists of pointers to web sites that provide useful information about routers, network security, and vulnerabilities. CERT

The Carnegie-Mellon University Computer Emergency Response Team (CERT) maintains a web site about network vulnerabilities. Many of the incident reports, advisories, and tips are relevant to router security. Cisco Documentation

This is the root of the Cisco documentation tree. From this page, you can find IOS software documentation, tutorials, case studies, and more. Cisco Press

At the web site of Cisco’s publishing arm, you can order a wide variety of books about Cisco routers and related networking technologies. Cisco Technology Support: Security and VPN tsd_technology_support_category_home.html

This page is the root of Cisco’s security technology support area. From here, you can find papers about a wide variety of security products and protocols. IETF

The IETF is the standards body that defines and maintains the protocol standards for the Internet. Use these sites to look up protocol standards and track emerging technologies that are becoming standards. Microsoft

Microsoft’s site offers extensive information about networking their products, and about product vulnerabilities. This information can often be helpful in configuring routers that protect Microsoft-based networks.


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Packet Storm

This site is a good resource for network security news, vulnerability announcements, and especially testing and attack tools.

This commercial web site offers descriptions and links to information about a very wide range of protocols and telecommunication data formats, as well as a pretty good glossary. Security Focus

Security Focus is a good site for security news and vulnerabilities. Although it doesn’t usually have much information about routers, it sometimes gives advice on how to forestall certain attacks by using your routers.

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Kiwi Syslog http://www. efficient.1c .com/ A syslog server is necessary to capture and preserve log messages from Cisco routers and many other network devices. originally created and distributed by the University of California at Davis. FreeRADIUS Tool References The list below describes some available commercial and non-commercial tools that may be helpful in router administration and improving network security. The Kiwi Syslog is one of several freely available syslog servers for Windows operating systems. earlier versions should not be upgraded.kiwisyslog. Tools like Ethereal are valuable for diagnosing and testing router and network security. NET-SNMP http://net-snmp.ethereal. It was formerly called “ucd-snmp”. a network traffic capture and analysis tool.Router Security Configuration Guide 9.freeradius. and can serve as an effective Cisco router console application on Linux.9.html Minicom is a The Nessus security scanner is a handy tool for getting a quick idea of the security vulnerabilities present on a The FreeRADIUS server is a highly configurable open-source RADIUS server implementation. Minicom http://www.3. Ethereal http://www. effective terminal emulation tool for Linux and Unix.nessus. Ethereal is an effective “sniffer”. Nessus http://www. 300 Version 1. While Nessus is primarily oriented toward scanning host computers. minicom is fast. it may also be used to scan routers.pp.1.clinet. While it has no fancy GUI. The current version is NET-SNMP is a free software toolkit for SNMP.

html The Security Administrator’s Integrated Network Tool (SAINT) is an advanced derivative of was the latest available at the time this guide edition was This is the most widely used port-scanning tool for Linux and Unix systems.Additional Resources Nmap http://www.insecure. RANCID http://www.html The Security Administrator’s Tool for Analyzing Networks (SATAN) is primarily oriented toward network security assessment of traditional host computers. A version is also available for Windows NT/2000 systems. It can perform TCP.html TeraTerm is a freely available terminal emulator and telnet application for Windows operating systems. with benchmark rules specifically targeted to checking Cisco IOS routers. and address scans in a variety of ways.saintcorporation. RAT is provided by the Center for Internet Security. mass application of commands.vector. It can provide valuable security scanning services for hosts. SAINT http://www.cisecurity. routers.1c 301 . and more.shrubbery.openssh.eeye. RAT RAT is a tool for checking network device The OpenSSH project offers a free. UDP. configuration change tracking. OpenSSH TeraTerm Pro Version 1. The included rule sets may be used. Version 2. SATAN This open source tool provides a variety of services for managing operational routers: configuration difference but it can also identify security vulnerabilities of routers and the network boundary protection they provide. or extended with rules that enforce your local security policy. usable implementation of the SSH security protocol for a wide variety of platforms. and networks. and is an invaluable tool for confirming filtering configurations. It makes an effective Cisco router console application.

68. 72 class-map. 117. 145 mirror. 194 BGP. 109 router rip. 237 testing. 157 show running. 193. 271 SSH and. 15 MAC address. 72 CEF. 237 ip name-server. 252 Attacks. for IPSec. 50. 132 CDP. 160 show ip. 22 IP. 28. 132 ip ftp. 211 Audit. 72 service udp-small-servers. 215 named. 72 service-policy. 60 mop enabled. 276 and VTYs. 266 Attack Tests. 132 isis authentication. 73 ip inspect. See Routing. 126 route flap damping. 59 logging. 110. 142–45 login authentication. 161 ip http. 75 service pad. 98 Conventions. 21 Commands aaa accounting. 126 spoofing. 22 AS numbers. 145. 10 Access Control Lists. 239 Abbreviations. 228 debug. 178. 236 compiled. 114 router ospf. 242 ip cef. 187. 110 service config. 278 table. 88 policy-map. See AAA timeout for SSH. 228 ip verify. 127 ip rip. 66. 265 proxy ARP. 193 access-class. 150 passive-interface. 18 Debugging. 252 Land attack. 75. 78 Secure DNS. 206. 112 router isis. 239 viewing current. 124 Black Hole Routing. 206–11. 67 configuring. 60 username. 181 aaa authorization. 196 resolution. 236 global parameters. 154 tacacs-server. 125 transport. 176. 192 server groups. 58 302 Version 1. 99 copy. 83 URPF and. 161 snmp-server. 195 TACACS+. 170 ATM. 95 extended. 109 ip prefix-list. 275 SYN flood. 84 IPSec and. 229 servers. 183 aaa new-model.1c . 25. 95 rmon. 181 login local. 233 ARP. 208 retries. 60 reacting to. 67 show snmp. 184 aaa authentication. 93. 90 password sniffing. 59 Application Layer. 257 smurf Attack. See AAA Auxiliary port. 253 Denial of Service. 168 show line. 98 clock. 253 testing with. 127 Banners. Null routing C CBAC. 228 crypto maps. 59. 75 copying. 96 CBAC and. 57. 239 and access lists. 160. 83 banner. 180. 59 bgp dampening. 37 Domain name service Kerberos and. 227 Authorization. 180 authorization. 252 Console. 79. 131 CIDR. 95 Configurations auto-loading. 62 encryption. 91. 127 TTL Security. 35. 173 distribute-list. 147 config t. 177. 157 router bgp. 142. 25. 97. 212 show flash. 60 access-list. 195–200 method lists. See Logging Authentication for IKE/IPSec. 215 DMZ. 194 terminal. 195. 79 ip ospf. 56. 15 martian. 183 Kerberos. 112 line. 99 show crypto. 111 ip ssh. 79. 209 Logging and. 60 B Backbone Router. 180 aaa server group. 207 exec-timeout. 252. 166 show rmon. 83–93. 161 crypto.Router Security Configuration Guide Index A AAA. 98 Cryptography. 228 D Dampening. 77 ntp. 175–200 accounting. 125 router eigrp. 115 kerberos. 84 standard. 154 show version. 29 DDoS. See also AAA Accounts. 123 prefix filtering. 143 trace ip. 72 control-plane. 128 cdp. 60 hostname. See BGP route flap damping Data Link Layer. 10 CPU. 163 shutdown. 88 enable secret. 61 show logging. 150. 77. 57 connect. 60 CAR rules. 172. See Users Address CIDR allocation. 66 service tcp-small-servers. 184 authentication. 60 logging. 41. 79 ip audit. 193 rate-limit. 68 examples. 105. 98 radius-server. 179 RADIUS. 119 Accounting. 168 Committed Access Rate (CAR). 75 service tcp-keepalives-in. 140 Control plane policing. 198 key chain.

180 and switches. 104 authentication. 160. IOS and. 160 FLASH. 267 3682. 112 Encryption. 97. disabling. 106 migration to. 57 Firewall. 149 disabling. 24 updating. 105 Intrusion Detection (IDS). 63 release numbering. 91. 88. 168 SMTP. 284 versions. 238 IPSec and. 104 authentication for. 111. 20 MAC address. 19 compare to IPv6. See Keys Keys. 234 HTTP. 51 1757. 103 EIGRP. 170 MPLS. 88–93. 109. 58 RFC. 62 encrypting. 283 role of. 217 rules for. 7. 213 IPv6. 124 826. 18 IPSec. 64 SSH for. 79. 158. 85 binding. 58 CBAC and. 46. 235 TACACS+. 63 G Group SNMP. 145 timestamps in. 265 IS-IS. 169 Q Quality of service. 150 P Passwords. 192 3031. 267 N NAT. 18 address. 190 IPSec for. 103 IKE. 115 NTP authentication. 281 virtual. 193 for SSH. 265 Icons. 212 loopback. 277 interface. 166 R RADIUS. 39 exterior gateway. 235 setting username. 120 unused. 141 SNMP trap logging. 61 Protocols by port number. 108 for RADIUS. 279 rules for. See Memory Remote Administration. 142. 38. 129 passive (routing). 192 routing protocols. 73 over SSL. 109 for IKE. 76. 253. 118 Filters. 59 Logging. 160. 17 1027. 105 1700. 139–46 destinations for.323. See SNMP Modes IOS modes. 15 Architecture. 232–40 FTP. 150 configuring. 76 redirects. 144. 275 viewing status. 232 Network Layer. 17 Layers. 204 for IPSec. 205–17 F Filtering. 145 Syslog logging. 40 route advertisements. 43 CET. 204 interior gateway. 123 timers. 88.Index E EGP. 123 IKE. 279.1c 303 . 57 timeout. 113 OSPF. 156 1918. 140 IDS Post Office. 229. 126 Privileges. 76 source routing. 217 testing. 27 NTP authentication. 243 Message severity levels. 283. 33. 63. 56 RAM. See IPSec Interfaces ACLs on. 241–46 Director. 77 IP Addresses. 124 Physical Layer. See CBAC message format. 265 directed broadcast. 125. 11 switches. 122 Version 1. 77 RADIUS. 103. 67 dial-in. 151 viewing logs. 71. 58 null. 18 Physical Security. 24. 104 authentication for. See Access Control List Firewall. 38 dynamic. 15. See AAA RAM. 264 I IANA. 15 icon for. 207 of passwords. 44. See IOS RIP. 11 IGP. 55 Prefix lists. 28. 5. 27 Network Layer. 19 Ethernet. 235 disabling. 207 Memory. 59 O OSI Model. 7. 104 distribute lists. 75 Ethernet. 276 address. 88 2267. 51 ICMP. 68. 254 2865. 161 K Kerberos. 270 Legal notice. 281 command modes. See AAA Key chains. 141 MIB. 268 for administration. 49 and RIP. 74 L LAN. 243 M MD5. 121 strategies. 102. 24 EXEC mode. 227 loopback address. 146 Syslog host configuration. 154 GTSM. 285–88 viewing processes. 103 mop. 205 for OSPF. 16 addresses. 194 key management. 194 H H. 256. 228 for TACACS+. 142 IOS AAA and versions. 15 NVRAM.

76. 163 RMON. 44. 160 Time. 87 log messages on. 28. 161 security concerns. 5. 228 RSVP. 221 SNMP. 287 clients. 276 ACLs on. 238 IKE port. 47. 64 defining local. 106 role of. 171. 251 NTP port. 76 disabling. 59 VTY line. 129 passive interfaces. 73 NTP. 250 Router Audit Tool (RAT). 168 TCP/IP. 97 and CBAC. 24. 68 nmap. 48 Unreachables.Router Security Configuration Guide migration from. 15 Telnet. 145. 204 security policy for. 90 keepalives. 88. 169 Testing. 152 vulnerability in. 251 TFTP. 255 functional. 84 logging port numbers. 86 nmap scan. 147 Timeouts for CBAC. See AAA TCP and CAR. 231 SHA. 27 Turbo ACLs. 251 ntpq. 17 filters for. 46 security testing. 165 transcript of. 92 Routing. 130 viewing. 75 TCP small servers. 149 open sockets. 41 in access lists. 256 SSH clients. 145 using SSH on. 148 RANCID. 60 protecting. 72 timestamps. 169 port numbers. See RMON v3 security levels. 9. 152 Tools CyberCop. See Logging System and Network Attack Center (SNAC). 132 Unix finger protocol. 59 for SSH. 116 Checklist. 72 syslog port. 123 version command. 72 V VLANs. 153 RMON. 27 and CBAC. 210. 86 nmap scan. 60 ACLs on. 151–55 Security Policy. 132 MPLS. 72 SYN attack. 85 intercept. 167 SSH. 70. 68 testing. 77 source routing. 103 null routing. 121 dynamic. 74 CDP. 278 UDP small servers.1. 230 S Scanning. 255 for NTP. See VTY Services bootp. 6 T TACACS+. 126 static routes. 103. 227 availability. 106 unicast RPF and. 256 SATAN. 43 neighbor authentication. 16 Windows IPSec and. 103 distribute lists. 39 small servers. 155–58 Route Table. 231 TeraTerm. 158 procedure for. 251 Users. 119 prefix list filtering. 143 number of. 258 SAINT. 65 reverse telnet. 274 finger. 207. 87 configuring. 66 logging port numbers. 130 Syslog. 1. 250–57 attack. 72 directed broadcast. 73 HTTP. 46. 219 Kerberos and. 196 SSH clients. 235. 166–73 management of. 205 in access lists. 250 Transport Layer. 233 handshake. 37 Review of. 70. 69 Security for the Simple Network Management Protocol (SNMP). 95 Switches. 110 Software Updates. 238 for EXEC. 251 port numbers. 90 RSA. 301 304 Version 1. 59 dial-in. 140. 252 automated. 251 SCP. 280 Switching. 231 diagnosing. 35. 130. 228 W Wide-Area Network (WAN). 190 passwords. 62 viewing logged in. 129. 75 SNMP. 70. 67. 77 pad.1c . 270 Unicast RPF. 253 configuring. 254 traceroute. 300 minicom. 16. 39 small servers. 230 RAT. 258 diagnostic commands. 263 CEF. 267 U UDP. 7. See IPSec VTY. 263 VPN. 106 URPF Verification. 146–50 time zone. 102–30 default. 73 syslog. 276 viewing status. 148 list of.