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1 FNS 1.0—3-1

Module 3
ACLs and CBAC

© 2003 Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-2

Learning Objectives
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• Describe the process of creating, applying, editing and troubleshooting ACLs • Understand the types of ACLs • Understand how CBAC works • Configure, apply and test CBAC

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FNS 1.0—3-3

Overview
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This module will discuss, in greater detail, how routers are utilized to secure a network through the use of Access Control Lists (ACLs) and Context-based Access Control (CBAC).

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FNS 1.0—3-4

Key terms
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• CBAC • Turbo ACLs • Lock and Key ACLs • Authentication Proxy • PAM

FNS 1.0—3-5

Access Control Lists

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FNS 1.0—3-6

Identifying Access Lists
Cisco routers can identify access lists using two methods:
• Access list number (All IOS versions)—The number of the access list determines what protocol it is filtering:
– (1-99) and (1300-1399)—Standard IP access lists. – (100-199) and (2000-2699)—Extended IP access lists. – (800-899)—Standard IPX access lists.
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• Access list name (IOS versions > 11.2)—You provide the name of the access list:
– Names contain alphanumeric characters. – Names cannot contain spaces or punctuation and must begin with a alphabetic character.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-7

Basic Types of IP Access Lists
Cisco routers support two basic types of IP access lists: • Standard—Filter IP packets based on the source address only. • Extended—Filter IP packets based on several attributes, including:
– Protocol type. – Source and destination IP addresses. – Source and destination TCP/UDP ports. – ICMP and IGMP message types.
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FNS 1.0—3-8

Standard Numbered Access List Format
Router(config)#
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access-list access-list-number {deny | permit} source [source-wildcard] Austin2(config)# access-list 2 permit 36.48.0.3 Austin2(config)# access-list 2 deny 36.48.0.0 0.0.255.255 Austin2(config)# access-list 2 permit 36.0.0.0 0.255.255.255 Austin2(config)# interface e0/1 Austin2(config-if)# ip access-group 2 in

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FNS 1.0—3-9

Standard Named Access List Format
Router(config)#
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ip access-list standard access-list-name
Router(config-std-nacl)#

{deny | permit} source [source-wildcard] Austin2(config)# ip access-list standard protect Austin2(config-std-nacl)# deny 36.48.0.0 0.0.255.255 Austin2(config-std-nacl)# permit 36.0.0.0 0.255.255.255 Austin2(config)# exit

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-10

Extended Numbered Access List Format
Miami Internet e0/0 128.88.3.0 128.88.1.0 SMTP host

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128.88.1.2

Router(config)#

access-list access-list-number {deny | permit} {protocol-number | protocol-keyword}{source source-wildcard | any | host} {source-port} {destination destination-wildcard | any | host} {destination-port} [established][log | log-input] Miami(config)# access-list 103 permit tcp any 128.88.0.0 0.0.255.255 established Miami(config)# access-list 103 permit tcp any host 128.88.1.2 eq smtp Miami(config)# interface e0/0 Miami(config-if)# ip access-group 103 in

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FNS 1.0—3-11

Extended Named Access List Format
Router(config)#

ip access-list extended access-list-name
Router(config-ext-nacl)#

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{deny | permit} {protocol-number | protocolkeyword} {source source-wildcard | any | host} {source-port} {destination destination-wildcard | any | host} {destination-port} [established][log | log-input] Miami(config)# ip access-list extended mailblock Miami(config-ext-nacl)# permit tcp any 128.88.0.0 0.0.255.255 established Miami(config-ext-nacl)# permit tcp any host 128.88.1.2 eq smtp Miami(config-ext-nacl)# exit
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-12

Commenting IP Access-List Entries
Router(config)#
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remark message Miami(config)# access-list 102 remark Allow traffic to file server Miami(config)# access-list 102 permit ip any host 128.88.1.6

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FNS 1.0—3-13

Basic Rules for Developing Access Lists
Here are some basic rules you should follow when developing access lists: • Rule #1—Write it out!
– Get a piece of paper and write out what you want this access list to accomplish. – This is the time to think about potential problems.
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• Rule #2—Setup a development system.
– Allows you to copy and paste statements easily. – Allows you to develop a library of access lists. – Store the files as ASCII text files.

• Rule #3—Apply access list to a router and test.
– If at all possible, run your access lists in a test environment before placing them into production.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-14

Access List Directional Filtering
Austin1

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Internet

s0/0

e0/0

e0/1 Inbound Outbound

• Inbound—Data flows toward router interface. • Outbound—Data flows away from router interface.

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FNS 1.0—3-15

Applying Access Lists to Interfaces
Router(config)#

ip access-group {access-list-number | accesslist-name} {in | out} Tulsa(config)# interface e0/1 Tulsa(config-if)# ip access-group 2 in Tulsa(config-if)# exit Tulsa(config)# interface e0/2 Tulsa(config-if)# ip access-group mailblock out

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FNS 1.0—3-16

Displaying Access Lists
Router#

show access-lists {access-list-number | accesslist-name} Miami# show access-lists Extended IP access list 102 permit ip any host 128.88.1.6 Extended IP access list mailblock permit tcp any 128.88.0.0 0.0.255.255 established Miami#

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FNS 1.0—3-17

Enable Turbo ACLs
R2
e0/0 16.1.1.2 Remote access LAN 16.2.1.0/24 e0/1 16.2.1.1

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Router(config)#

access-list compiled
Router#

show access-list compiled
R2(config)# access-list compiled R2(config)# exit R2# show access-list compiled

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-18

Enhanced Access Lists
Cisco routers support several enhanced types of access lists: • Dynamic (Lock and Key)—Create dynamic entries. • Time-Based—Access lists whose statements become active based upon the time of day and/or day of the week. • Reflexive—Create dynamic openings on the untrusted side of a router based on sessions originating from a trusted side of the router. • Context-Based Access Control (CBAC)—Allows for secure handling of multi-channel connections based on upper layer information.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-19

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Types of IP ACLs

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FNS 1.0—3-20

Context-based Access Control (CBAC)

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FNS 1.0—3-21

CBAC
• Packets are inspected entering the firewall by CBAC if they are not specifically denied by an ACL. • CBAC permits or denies specified TCP and UDP traffic through a firewall. • A state table is maintained with session information. • ACLs are dynamically created or deleted. • CBAC protects against DoS attacks.
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TCP
Internet

UDP

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FNS 1.0—3-22

Cisco IOS ACLs
• Provide traffic filtering by – Source and destination IP addresses. – Source and destination ports. • Can be used to implement a filtering firewall – Ports are opened permanently to allow traffic, creating a security vulnerability. – Do not work with applications that negotiate ports dynamically.
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FNS 1.0—3-23

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FNS 1.0—3-24

How CBAC Works

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How CBAC Works (Cont)
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© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-25

Supported Protocols
• • • • • • TCP (single channel) UDP (single channel) RPC FTP TFTP UNIX R-commands (such as rlogin, rexec, and rsh) • SMTP • HTTP (Java blocking) • Java • SQL*Net • RTSP (such as RealNetworks) • H.323 (such as NetMeeting, ProShare, CUSeeMe) • Other multimedia – Microsoft NetShow – StreamWorks – VDOLive
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FNS 1.0—3-26

Alerts and Audit Trails
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• CBAC generates real-time alerts and audit trails. • Audit trail features use Syslog to track all network transactions. • With CBAC inspection rules, you can configure alerts and audit trail information on a per-application protocol basis.

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FNS 1.0—3-27

CBAC Configuration
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• Set audit trails and alerts. • Set global timeouts and thresholds. • Define Port-to-Application Mapping (PAM). • Define inspection rules. • Apply inspection rules and ACLs to interfaces. • Test and verify.

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FNS 1.0—3-28

Configure CBAC (Task 1 and 2)

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FNS 1.0—3-29

Enable Audit Trail and Alert
Router(config)#
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ip inspect audit-trail
• Enables the Syslog server and turns on logging

Router(config)# logging on Router(config)# logging 10.0.0.3 Router(config)# ip inspect audit-trail
Router(config)# • Alert can be turned off
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[no] ip inspect alert-off

FNS 1.0—3-30

TCP, SYN, and FIN Wait Times
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Router(config)#

ip inspect tcp synwait-time seconds
• Specifies the time the Cisco IOS Firewall waits for a TCP session to reach the established state. Router(config)#

ip inspect tcp finwait-time seconds
• Specifies the time the Cisco IOS Firewall waits for a FIN exchange to complete before quitting the session.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-31

TCP, UDP, and DNS Idle Times
Router(config)#
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ip inspect tcp idle-time seconds
• Specifies the time allowed for a TCP or UDP  session with no activity. Router(config)# • Specifies the time allowed for a DNS session  with no activity.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-32

ip inspect udp idle-time seconds

ip inspect dns-timeout seconds

Global Half-Opened Connection Limits
Router(config)# • Defines the number of existing half­opened sessions  that cause the software to start deleting half­opened  sessions (aggressive mode). Router(config)# • Defines the number of existing half­opened sessions  that cause the software to stop deleting half­opened  sessions.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-33

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ip inspect max-incomplete high number

ip inspect max-incomplete low number

Global Half-Opened Connection Limits (cont.)
Router(config)# • Defines the number of new half­opened  sessions per minute at which they start being  deleted. Router(config)# • Defines the number of new half­opened  sessions per minute at which they stop being  deleted.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-34

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ip inspect one-minute high number

ip inspect one-minute low number

Half-Opened Connection Limits by Host
Router(config)#

ip inspect tcp max-incomplete host number block-time seconds
• Defines the number of half-opened TCP sessions with the same host destination address that can exist at a time before the Cisco IOS Firewall starts deleting half-open sessions to the host. • After the number of half-opened connections is exceeded to a given host, the software deletes half-open sessions on that host in the following manner: – If block-time is 0, the oldest half-opened session is deleted, per new connection request, to allow new connections. – If block-time is greater than 0, all half-opened sessions are deleted, and new connections to the host are not allowed during the specified block time.

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FNS 1.0—3-35

Port-to-Application Mapping (Task 3)

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FNS 1.0—3-36

Port-to-Application Mapping
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• Ability to configure any port number for an application protocol. • CBAC uses PAM to determine the application configured for a port.

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FNS 1.0—3-37

User-Defined Port Mapping
Router(config)#
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ip port-map appl_name port port_num
• Maps a port number to an application. Router(config)#

access-list permit acl_num ip_addr ip port-map appl_name port port_num list acl_num
• Maps a port number to an application for a given host. Router(config)#

access-list permit acl_num ip_addr wildcard_mask ip port-map appl_name port port_num list acl_num
• Maps a port number to an application for a given network.

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FNS 1.0—3-38

Display PAM Configuration
Router#

show ip port-map
• Shows all port mapping information. Router#

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show ip port-map appl_name
• Shows port mapping information for a given application. Router#

show ip port-map port port_num
• Shows port mapping information for a given application on a given port.

Router# sh ip port-map ftp Default mapping: ftp port 21 system defined Host specific: ftp port 1000 in list 10 user
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-39

Define Inspection Rules (Task 4)

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FNS 1.0—3-40

Inspection Rules for Application Protocols
Router(config)#
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ip inspect name inspection-name protocol [alert {on| off}] [audit-trail {on|off}] [timeout seconds]
• Defines the application protocols to inspect. • Will be applied to an interface – Available protocols: tcp, udp, cuseeme, ftp, http, h323, netshow, rcmd, realaudio, rpc, smtp, sqlnet, streamworks, tftp, and vdolive. – alert, audit-trail, and timeout are configurable per protocol and override global settings.

Router(config)# audit-trail on Router(config)# audit-trail on

ip inspect name FWRULE smtp alert on timeout 300 ip inspect name FWRULE ftp alert on timeout 300

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FNS 1.0—3-41

Inspection Rules for Java
Router(config)#
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ip inspect name inspection-name http java-list acl-num [alert {on|off}] [audit-trail {on|off}] [timeout seconds]
• Controls java blocking with a standard ACL.

Router(config)# ip inspect name FWRULE http java-list 10 alert on audit-trail on timeout 300 Router(config)# ip access-list 10 deny 172.26.26.0 0.0.0.255 Router(config)# ip access-list 10 permit 172.27.27.0 0.0.0.255

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FNS 1.0—3-42

Inspection Rules for RPC Applications
Router(config)#
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ip inspect name inspection-name rpc program-number number [wait-time minutes] [alert {on|off}] [audit-trail {on|off}] [timeout seconds]
• Allows given RPC program numbers—wait­time keeps the  connection open for a specified number of minutes.

Router(config)# ip inspect name FWRULE rpc program-number 100022 wait-time 0 alert off audit-trail on

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FNS 1.0—3-43

Inspection Rules for SMTP Applications
Router(config)#
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ip inspect name inspection-name smtp [alert {on|off}] [audit-trail {on|off}] [timeout seconds]

• Allows only the following legal commands in SMTP  applications: DATA, EXPN, HELO, HELP, MAIL, NOOP, QUIT,  RCPT, RSET, SAML, SEND, SOML, and VRFY. • If disabled, all SMTP commands are allowed through the  firewall, and potential mail server vulnerabilities are exposed.

Router(config)# ip inspect name FWRULE smtp

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-44

Inspection Rules for IP Packet Fragmentation
Router(config)#
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ip inspect name inspection-name fragment max number timeout seconds

• Protects hosts from certain DoS attacks involving fragmented  IP packets – max—number of unassembled fragmented IP packets. – timeout—seconds when the unassembled fragmented IP  packets begin to be discarded.

Router(config)# ip inspect name FWRULE fragment max 254 timeout 4

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-45

Inspection Rules and ACLs Applied to Router Interfaces (Task 5)

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FNS 1.0—3-46

Apply an Inspection Rule to an Interface
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Router (config-if)#

ip inspect inspection-name {in | out}
• Applies the named inspection rule to an interface.

Router(config)# interface e0/0 Router(config-if)# ip inspect FWRULE in
• Applies the inspection rule to interface e0/0 in inward direction.

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FNS 1.0—3-47

General Rules for Applying Inspection Rules and ACLs
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• Interface where traffic initiates – Apply ACL on the inward direction that permits only wanted traffic. – Apply rule on the inward direction that inspects wanted traffic. • All other interfaces – Apply ACL on the inward direction that denies all unwanted traffic.

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-48

Example—Two Interface Firewall
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© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-49

Outbound Traffic
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Router(config)# ip inspect name OUTBOUND tcp Router(config)# ip inspect name OUTBOUND udp
• Configure CBAC to inspect TCP and UDP traffic.

Router(config)# access-list 101 permit ip 10.0.0.0 0.0.0.255 any Router(config)# access-list 101 deny ip any any
• Permit inside­initiated traffic from the 10.0.0.0 network.

Router(config)# interface e0/0 Router(config-if)# ip inspect OUTBOUND in Router(config-if)# ip access-group 101 in
• Apply an ACL and inspection rule to the inside interface in an  inward direction.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-50

Inbound Traffic
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Router(config)# host 10.0.0.3 Router(config)# host 10.0.0.3 Router(config)#

access-list 102 permit icmp any access-list 102 permit tcp any eq www access-list 102 deny ip any any

• Permit outside­initiated ICMP and HTTP traffic to host 10.0.0.3.

Router(config)# interface e0/1 Router(config-if)# ip access-group 102 in
• Apply an ACL and inspection rule to outside interface in inward  direction.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-51

Example—Three-Interface Firewall
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© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-52

Outbound Traffic
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Router(config)# ip inspect name OUTBOUND tcp Router(config)# ip inspect name OUTBOUND udp
• Configure CBAC to inspect TCP and UDP traffic.

Router(config)# access-list 101 permit ip 10.0.0.0 0.0.0.255 any Router(config)# access-list 101 deny ip any any
• Permit inside­initiated traffic from 10.0.0.0 network.

Router(config)# interface e0/0 Router(config-if)# ip inspect OUTBOUND in Router(config-if)# ip access-group 101 in
• Apply an ACL and inspection rule to the inside interface in an inward  direction.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-53

Inbound Traffic
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Router(config)# ip inspect name INBOUND tcp
• Configure CBAC to inspect TCP traffic.

Router(config)# 172.16.0.2 Router(config)# 172.16.0.2 eq Router(config)#

access-list 102 permit icmp any host access-list 102 permit tcp any host www access-list 102 deny ip any any

• Permit outside­initiated ICMP and HTTP traffic to host 172.16.0.2.

Router(config)# interface e0/1 Router(config-if)# ip access-group 102 in
• Apply an ACL and inspection rule to the outside interface in an inward  direction.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-54

DMZ-Bound Traffic
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Router(config)# access-list 103 permit icmp host 172.16.0.2 any Router(config)# access-list 103 deny ip any any • Permit only ICMP traffic initiated in the DMZ. Router(config)# access-list 104 permit icmp any host 172.16.0.2 Router(config)# access-list 104 permit tcp any host 172.16.0.2 eq www Router(config)# access-list 104 deny ip any any • Permit only outward ICMP and HTTP traffic to host 172.16.0.2. Router(config)# interface e1/0 Router(config-if)# ip access-group 103 in Router(config-if)# ip access-group 104 out • Apply proper access lists and an inspection rule to the interface.
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-55

Test and Verify (Task 6)

© 2003 Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-56

show Commands
Router#

show show show show show

ip ip ip ip ip

inspect inspect inspect inspect inspect

name inspection-name config interfaces session [detail] all

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• Displays CBAC configurations, interface configurations, and sessions. Router# sh ip inspect session Established Sessions Session 6155930C (10.0.0.3:35009)=>(172.30.0.50:34233) tcp SIS_OPEN Session 6156F0CC (10.0.0.3:35011)=>(172.30.0.50:34234) tcp SIS_OPEN Session 6156AF74 (10.0.0.3:35010)=>(172.30.0.50:5002) tcp SIS_OPEN
© 2003, Cisco Systems, Inc. All rights reserved. FNS 1.0—3-57

debug Commands
Router#

debug debug debug debug debug

ip ip ip ip ip

inspect inspect inspect inspect inspect

function-trace object-creation object-deletion events timers

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• General debug commands. Router(config)# • Protocol­specific debug.

debug ip inspect protocol

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-58

Remove CBAC Configuration
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Router(config)#

no ip inspect
• Removes entire CBAC configuration. • Resets all global timeouts and thresholds  to the defaults. • Deletes all existing sessions. • Removes all associated dynamic ACLs.

© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-59

Summary

© 2003 Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-60

Summary
• ACLs are used to filter and secure network traffic. • While ACLs filter network traffic by controlling whether routed or switched packets are forwarded or blocked at the interface, CBAC is used to create temporary openings in the firewall access lists. • The student should understand the six steps required for configuring CBAC: – Set audit trails and alerts – Set global timeouts and thresholds – Define PAM – Define inspection rules – Apply inspection rules and ACLs to interfaces – Test and verify
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© 2003, Cisco Systems, Inc. All rights reserved.

FNS 1.0—3-61

© 2003, Cisco Systems, Inc. All rights reserved.

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