i believe that the hard part of the traversal for www.flickr.com is:
; <<>> DiG 9.4.1-P1 <<>> @ns3.yahoo.com www.flickr.vip.mud.yahoo.com
; (1 server found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41226
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 5
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;www.flickr.vip.mud.yahoo.com. IN A
;; ANSWER SECTION:
www.flickr.vip.mud.yahoo.com. 900 IN A 68.142.214.24
;; AUTHORITY SECTION:
mud.yahoo.com. 172800 IN NS ns1.yahoo.com.
mud.yahoo.com. 172800 IN NS ns2.yahoo.com.
mud.yahoo.com. 172800 IN NS ns3.yahoo.com.
mud.yahoo.com. 172800 IN NS ns4.yahoo.com.
mud.yahoo.com. 172800 IN NS ns5.yahoo.com.
;; ADDITIONAL SECTION:
ns1.yahoo.com. 172800 IN A 66.218.71.63
ns2.yahoo.com. 172800 IN A 68.142.255.16
ns3.yahoo.com. 172800 IN A 217.12.4.104
ns4.yahoo.com. 172800 IN A 68.142.196.63
ns5.yahoo.com. 1800 IN A 119.160.247.124
;; Query time: 153 msec
;; SERVER: 217.12.4.104#53(217.12.4.104)
;; WHEN: Wed Sep 10 16:58:43 2008
;; MSG SIZE rcvd: 232
because this is a yahoo.com nameserver which is simultaneously answering
and delegating. this is a sensible thing for it to do since it's
authoritative for both yahoo.com and mud.yahoo.com, but it's also an
insensible thing for it to do since the downward referral trumps the
non-empty answer section. (it would also trump a non-empty answer
section which would otherwise be seen as a NODATA response.) i'm not
throwing stones, since this is ambiguous in the spec, and for all i know
it's what BIND9 would do. but my own toy traversal tool spake thusly:
response from 217.12.4.104 (ns3.yahoo.com) is NOERROR (1 1 5 5) (AA)
down-referral
downward referral trumps nonempty ANSWER
cache modified by AUTHORITY
cache unmodified by ADDITIONAL
upstream transaction complete (tryagain)
requires iteration (#3)
and the complexity thus revealed may behoove yahoo to put the mud.yahoo.com
zone separate nameservers (or separate views) from the yahoo.com zone.
--
Paul Vixie
