Anycast 101

Fri Dec 17 00:59:58 UTC 2004

In message <41C222C3.9020906 at globalstar.com>, Crist Clark writes:
>
>Iljitsch van Beijnum wrote:
>
>> Due to limitations in the DNS protocol, it's not possible 
>> to increase the number of authoritative DNS servers for a zone beyond 
>> around 13.
>
>I believe you misspelled, "Due to people who do not understand the DNS
>protocol being allowed to configure firewalls..."

No, firewalls have nothing to do with it.  Section 4.2.1 of RFC 1035 
says:

   Messages carried by UDP are restricted to 512 bytes (not counting the IP
   or UDP headers).

There's a large installed base of machines that conform to that limit 
and don't understand EDNS0.  I'll leave the packet layout and 
arithmetic as an exercise for the reader (cheaters may want to run 
tcpdump on 'dig ns .' and examine the result), but the net result is 
what Iljitsch said: you can only fit about 13 servers into a response.

		--Steve Bellovin, http://www.research.att.com/~smb