mailing list archives
Re: Question for DNS pros
From: Paul Schmehl <pauls () utdallas edu>
Date: Fri, 23 Jul 2004 17:11:10 -0500
--On Friday, July 23, 2004 09:50:44 PM +0200 Oliver () greyhat de wrote:
hm... you could also try reverse lookups for all existing ip-adresses in
the world :)
Well, no, because that wouldn't solve the problem.
A host on our network is being queried quite regularly on udp/53 by other
hosts. A review of the packets reveals that these other hosts believe that
our host is a dns server. (AAMOF the IP address isn't even in use at the
Now, if you do a reverse lookup for that IP, *our* DNS servers, which are
authoritative for our network will tell you what the hostname is. But that
isn't what I want to know. Obviously, a simple dig -x IP will tell me that.
What I want to know is *why* do these "foreign" hosts think an IP on my
network is serving DNS when there's not even a host at that address.
I can think of two possibilities:
1) At some time in the past, a host *was* serving DNS at that address and
some "foreign" hosts have cached the address.
2) Someone somewhere has registered a domain and used our IP address for
one of their "nameservers" in the registration.
(If anyone can think of other explanations, please let me know.)
Now how is a reverse lookup going to help you with that? It would be
trivial to write a perl script that did reverse lookups for every IP on the
Internet and wrote the responses to a comma delimited file, but the
resulting file would be useless to solve the problem that I'm trying to
And for those who were thinking "just do a tcpdump", here's what *that*
looks like - no domain info there -
17:01:44.646943 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 48072 NS? .
17:01:45.386919 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 48073 NS? .
17:01:46.153402 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 48074 NS? .
17:01:47.657898 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 1084 PTR?
17:01:48.399150 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 1085 PTR?
17:01:49.144398 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain: 1086 PTR?
The best suggestion yet has been to set up a name server at that address
with verbose logging. That's probably what I will do next week.
Paul Schmehl (pauls () utdallas edu)
Adjunct Information Security Officer
The University of Texas at Dallas
AVIEN Founding Member
Full-Disclosure - We believe in it.
Re: Question for DNS pros Roberto Navarro (Jul 24)
Re: Question for DNS pros Nils Ketelsen (Jul 25)