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.

Recall that the format of an rfc file is rfcnnn.txt, where nnn is replaced the the RFC number. For example, RFC 863 is in the file rfc863.txt.


Schematic view:

Host vs. router

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:

Autonomous systems:

Interior Gateway Protocols (IGP):

Exterior Gateway Protocols (EGP):

Current size of routing tables: 10,000 entries

Could be reduced to 200 entries (CIDR)


Routing metric: hop count


Problems with downed routers, partitioned networks:

  1. Routing loops
  2. Convergence



IP with port numbers and an optional checksum:

  1. Re-check:
  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 /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

Thomas P. Kelliher
Thu Mar 7 12:21:59 EST 1996
Tom Kelliher