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IPv4 Overview

Cyber Security Spring 2006

• • • • Review Layered Network Architecture Network Layer protocols Transport Layer Protocols Application Layer Protocols

Reading Material
• Many texts on IP networking
– Computer Networks, Andrew Tannenbaum – Data and Computer Communications, William Stallings – Internetworking with TCP/IP Vol 1, Douglas Comer

• Plus all the originals from the Internet Engineering Task Force (IETF)

. SMTP. e.OSI Reference Model • The layers – – – – – – – 7: Application..g. e. FTP 6: Presentation 5: Session 4: Transport. UDP 3: Network. e. TCP. IP. ATM cells 1: Physical. e..g. HTTP. Ethernet media.g.g. e. Ethernet frames.g. ATM media • Standard software engineering reasons for thinking about a layered design . IPX 2: Data link.

Layers Limit Need for Intelligence • Intermediate devices only need to process the packet headers up to the level they understand Ether Hdr IP Hdr TCP Hdr HTTP Hdr Data .

From the outside world make changes at levels 2 (in transparent mode) or 3 (in routing mode) • Routers – Operate at Level 3 (network) • Switches or Hubs – Operate at level 2 (data link) • Gateways – Operate at level 2 Data Http Hdr TCP Hdr IP Hdr Ether Hdr .Various network devices • Hosts and servers – Operate at Level 7 (application) • Proxies – Operate at level 7 • Firewalls – Operate between levels 2 and 7.

168.g.1.0 Broadcast address is the last address in the subnetwork. e. 192.1.g.0 255.255..0/24 or 192.0 Host address is the first address in subnetwork.255.g. e.168. e. 192. e.168..IPv4 • 32 bit Addressing scheme – – – – Host address.1..168.1 Network address. 192.1.255 Version IHL Type of service DF MF Protocol Source address Destination Address 0 or more words of options Total length Frag Offset Header checksum Identification Time to live .168.1.g. 192.

Address spoofing • Sender can put any source address in packets he sends: – Can be used to send unwelcome return traffic to the spoofed address – Can be used to bypass filters to get unwelcome traffic to the destination • Reverse Path verification can be used by routers to broadly catch some spoofers .

Fragmentation • May need to fragment an IP packet if one data link along the way cannot handle the packet size – Perhaps path is a mix of different HW – Perhaps unexpected encapsulation makes the packet larger than the source expected – Hosts try to understand Maximum Transmission Unit (MTU) to avoid the need for fragmentation (which causes a performance hit) • Any device along the way can fragment – Identification field identifies all elements of the same fragment – Fragmentation stored in the MF (more fragments) and fragment offset fields – Devices can reassemble too – But generally the destination does the reassembly .

g. • Resource attacks on re-assemblers – Send all but one fragment for many packets .g.Fragmentation Flaws • Split packet to fool simple firewall and IDS – Intermediate content observers must do reassembly • Overlapping fragments – Can be used to trick IDS by hiding. a “get /etc/password” request – Different clients reassemble overlapping fragments differently – Just drop overlapping fragments • Bad fragment offsets exploit poor stack implementations – E. Teardrop attack. e. negative offsets or overlarge offsets cause buffer overflows – Firewalls can check for well formed packets.

Address Resolution Protocol (ARP) • Used to discover mapping of neighboring ethernet MAC to IP addresses.168.3 which is in your interfaces subnetwork – Broadcast an ARP request on the link – Hopefully receive an ARP reply giving the correct MAC – The device stores this information in an ARP cache or ARP table . – Need to find MAC for 192.1. Poison. http://www. • Solutions – Encrypt all traffic – Monitoring programs like arpwatch to detect mapping changes • Which might be valid due to DHCP .php • Classic Man-in-the-middle attack – Send arp reply messages to device so they think your machine is someone else – Better than simple sniffing because packets will get to your regardless of sniffing. Anyone can send an ARP reply – The Ingredients to ARP Poison.ARP cache poisoning • Bootstrap problem with respect to security.

so you use the next hop’s data-link address. Next hop address.g. Routing table consists of entries of • Network. take the destination address and find the best match network in the table. interface May have routing table per incoming interface To route a packet.Basic IPv4 Routing • – – – Static routing. 225 maximum . ethernet MAC address – Decrement “time to live” field in IP header at each hop. In case of a tie look at the metric • • Use the corresponding next hop address and interface to send the packet on. Drop packet when it reaches 0 • • Attempt to avoid routing loops As internet got bigger. The next hop address is on the same link as this device. e. TTL fields got set bigger. metric. Used by hosts and some firewalls and routers.

168.5.0. 1.1.56 on inside interface Local routing table for inside interface 1. dmz 192. outside 192.0.0/24.5. But metric for 3 is better Entries 1 and 2 are for directly connected networks .2.3.0/24.0. 192.3.0/29. 192.0/30. 3. 5. 192. 4. 1.0. 3. outside 0. 2.0. 1.1.168.Routing example • • Receive a packet destined to 192. dmz 192. 1. outside • • Entries 3 and 4 tie.168. 192.

can specify a source route – Was conceived of as a way to ensure some traffic could be delivered even if the routing table was completely screwed up.Source Based Routing • In the IP Options field. • Can be used by the bad guy to avoid security enforcing devices – Most folks configure routers to drop packets with source routes set .

there are security Options – Used for DNSIX. a MLS network encryption scheme . IP Option mechanisms never really took off • In addition source routing.IP Options in General • Originally envisioned as a means to add more features to IP later • Most routers drop packets with IP options set – Stance of not passing traffic you don’t understand – Therefore.

g.Dynamic Routing Protocols • For scaling. e. so it avoids counts to infinity. Domain X will only carry traffic of paying customers • Receives full paths from neighbors. Can encode nontechnical transit constraints. . discover topology and routing rather than statically constructing routing tables – Open Shortest Path First (OSPF): Used for routing within an administrative domain – RIP: not used much anymore – Border Gateway Protocol (BGP): Used for routing between administrative domains.

– BGP supports peer authentication – BGP blackholing is in fact used as a mechanism to isolate “bad” hosts – Filter out route traffic from unexpected (external) points – OSPF has MD5 authentication. . and can statically configure neighbor routers.Dynamic Routing • Injecting unexpected routes a security concern. rather than discover them.

TTL hit 0 Parameter problem. performance ping • Can use information to help map out a network – Some people block ICMP from outside domain . throttling mechanism rarely used Redirect. feedback on potential bad route Echo Request and Echo reply. bad header field Source quench.Internet Control Message Protocol (ICMP) • Used for diagnostics – – – – – – – Destination unreachable Time exceeded. ping Timestamp request and Timestamp reply.

Smurf Attack • An amplification DoS attack – A relatively small amount of information sent is expanded to a large amount of data • Send ICMP echo request to IP broadcast addresses. Spoof the victim's address as the source • The echo request receivers dutifully send echo replies to the victim overwhelming it • Fraggle is a UDP variant of the same attack .

org/ • Source ports generally dynamically selected – Ports under 1024 are considered well-known ports – Would not expect source ports to come from the well-known range • Scanners probe for listening ports to understand the services running on various machines .iana.Transport layer • UDP and TCP • Transport flows are defined by source and destination ports – A pair of devices can have numerous flows operating simultaneously by communicating between different pairs of ports • Applications are associated with ports (generally just destination ports) – IANA organizes port assignments http://www.

H. e. no guarantee.Datagram Transport • User Datagram Protocol (UDP) – – – – A best-effort delivery..323 also use UDP . no ACK Lower overhead than TCP Good for best-effort traffic like periodic updates No long lived connection overhead on the endpoints • Some folks implement their own reliable protocol over UDP to get “better performance” or “less overhead” than TCP – Such efforts don’t generally pan out • TFTP and DNS protocols use UDP • Data channels of some multimedia protocols.g.

UDP Header Source Port UDP Length Destination Port UDP checksum .

FTP. HTTP.. SSH. ordered stream of traffic – Such guarantees impose overhead – A fair amount of state is required on both ends • Most Internet protocols use TCP.g. H. e.323 control channels .Reliable Streams • Transmission Control Protocol (TCP) – Guarantees reliable.

TCP Header Source Port Sequence Number Acknowledgement number HDR Len U A P R S F R C S S Y I G K H T N N Window Size Urgent Pointer Options (0 or more words) Destination Port Checksum .

2. This leaves B with a bunch of half open (or embryonic) connections that are filling up memory Firewalls adapted by setting limits on the number of such half open connections. Never respond to the second message. . – – – Send many of the first message to B.Syn flood • • A resource DoS attack focused on the TCP three-way handshake Say A wants to set up a TCP connection to B 1. A sends SYN with its sequence number X B replies with its own SYN and sequence number Y and an ACK of A’s sequence number X A sends data with its sequence number X and ACK’s B’s sequence number Y 3.

323 – Have a well known control channel – Negotiate ports and/or addresses on the control channel for subsidiary data channels – Dynamically open the negotiated data channels • Protocol suites. e. SMTP • Dynamic Multi-connection Protocols. Netbios and DNS . e. e. HTTP. FTP and H.Application Protocols • Single connection protocols – Use a single connection.g.g.g.

Spoofing Applications • Often times ridiculously easy • Fake Client – Telnet to an SMTP server and enter mail from whoever you want – Authenticating email servers • Require a password • Require a mail download before server takes send requests • Fake server – Phishing: misdirect user to bogus server .

DHCP • Built on older BOOTP protocol (which was built on even older RARP protocol) – Used by diskless Suns • Enables dynamic allocation of IP address and related information • Runs over UDP • No security considered in the design. • Other solutions – Physically secure networks – Use IPSec . obvious problems – Bogus DHCP servers handing out addresses of attackers choice – Bogus clients grabbing addresses • IETF attempted to add DHCP authentication but rather late in the game to do this.

.uiuc.uiuc. – DNS servers in the chain.Domain Name System (DNS) • Hierarchical service to resolve domain names to IP addresses. one for .edu. and one for . no security designed in – But at least the DNS server is not automatically discovered – Although this information can be dynamically set via DHCP • Queries and responses use UDP. One for • Can have primary and secondary DNS servers per zone. trustworthy servers . – Packet interception attacks – Name chaining attacks – Untrustworthy.cs. – The name space is divided into non-overlapping zones – E. Use TCP based zone transfer to keep up to date • Like DHCP. consider

not confidentiality • DNS Threat Analysis in RFC 3833. .DNSSEC • Seeks to solve the trust issues of DNS • Uses a key hierarchy for verification • Has been under development for a decade and still not really deployed • Provides authentication.

Summary • IPv4 not designed with security in mind • Complexity can be exploited – Poor implementations – Edge cases in standards • Bootstrapping can be exploited – Easy of configuration vs strong trust .