Re: TCP vs UDP for dns RE: Really odd problem

[Jason Dixon <jason_(_at_)_dixongroup_(_dot_)_net>]

On Sep 29, 2004, at 11:39 AM, Philip Guenther wrote:

> Jason Dixon <jason_(_at_)_dixongroup_(_dot_)_net> writes:
> ...
> > Not to say you're full of it, but... well maybe.  I explicitly block
> > all tcp/udp outbound from my LAN, allowing only "good" ports (udp/53
> > being one of them, tcp/53 not).  I have no problems running nslookup
> > queries on Windows or Linux.
>
> By default, if a DNS resolver receives a truncated answer to a query
> sent via UDP, it will retry the query using TCP.  If you never make
> queries that get truncated answers and never use AXFR or IXFR, then
> you won't see any errors from blocking tcp/53.  'Course, it's a bit
> difficult to guarantee that you'll never get a truncated answer...

Yes, that assumes no axfr.  Sorry, I completely forgot about truncated. 
 That's what I get for having a reliable DNS pool and not having to 
query it for large zones.  ;-)


--
Jason Dixon, RHCE
DixonGroup Consulting
http://www.dixongroup.net