I just got bitten by this ... not sure if it is a bug in
ares or an error in some DNS setup around here, but here
is some info for the archives:

I have a DNS server that accepts queries on one IP-address,
but sends the answers with a different source-IP. A network
trace says (IP's and domain-names changed):

 10.29.31.155 -> 10.29.37.21 DNS C dns01a.foo.com. Internet Addr ?
  10.29.10.5 -> 10.29.31.155 DNS R dns01a.foo.com. Internet Addr 10.29.37.21
 
Note that the request is sent to 10.29.37.21, but the answer
originates from 10.29.10.5.

The "standard" DNS lookup tools and libraries (nslookup, dig,
libresolv) happily accept this and completes the name lookup.
Ares doesn't - it appears to just hang waiting for a response
(it doesn't even timeout, from what I can see).

I'll try to figure out tomorrow why our dns server is acting
up like that - it doesn't seem right, it certainly breaks
lookups that go through a stateful firewall - but I thought
it would be good at least to have it documented somewhere.

-- 
Henrik Storner <henrik_at_hswn.dk> 
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