Routing, UDP, and the DNS

Tom Kelliher, CS43

Mar. 7, 1996

Note on the DNS homework problem: See named(1). You may also need to refer to RFCs mentioned in the man page. Here are two URLs for RFC files repositories.

• ftp://ftp.internic.net/rfc
• ftp://nic.ddn.mil/rfc

Routing

Schematic view:

Host vs. router

• Routing mechanism --- search routing table to determine which interface to send a packet out
• Routing policy --- set of rules for deciding which routes are entered into the routing table

The mechanism:

1. Match host address
2. Match network address
3. Use default entry (if possible)

ICMP redirect messages

Dynamic Routing

Static routing --- routing table set at boot time

Ok for hosts which:

• on small network
• have a single connection point to other networks
• have no redundant routes

Autonomous systems:

• NSFNET backbone
• PrepNet
• Westminster

Interior Gateway Protocols (IGP):

• Routing Information Protocol (RIP)
• Open Shortest Path First (OSPF)

Exterior Gateway Protocols (EGP):

• EGP
• Border Gateway Protocol (BGP)

Current size of routing tables: 10,000 entries

Could be reduced to 200 entries (CIDR)

RIP

Routing metric: hop count

Example:

Problems with downed routers, partitioned networks:

1. Routing loops
2. Convergence

OSPF

• RIP --- distance-vector protocol
• OSPF --- link-state protocol

UDP

IP with port numbers and an optional checksum:

1. Re-check:
   • Source, destination IP addresses
   • Protocol field
2. Silently discards checksum errors
3. Fragmentation
4. UDP input queue full drops (source quench ignores)
5. UDP output queue drops on miss in ARP cache

The DNS

The /etc/hosts crisis.

A distributed database:

1. Zones, zone files
2. Primaries, secondaries; the SOA record
3. Resolver routines, server
4. Caching
5. Address, pointer queries
6. Hostname spoof check