full-disclosure - Question for DNS pros

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From: pauls at utdallas.edu (Paul Schmehl)
Subject: Question for DNS pros

--On Friday, July 23, 2004 09:50:44 PM +0200 Oliver@...yhat.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 
present time.)

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 
solve.

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)
17:01:45.386919 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  48073 NS? . 
(17)
17:01:46.153402 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  48074 NS? . 
(17)
17:01:47.657898 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  1084 PTR? 
63.37.110.129.in-addr.arpa. (44)
17:01:48.399150 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  1085 PTR? 
63.37.110.129.in-addr.arpa. (44)
17:01:49.144398 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  1086 PTR? 
63.37.110.129.in-addr.arpa. (44)

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@...allas.edu)
Adjunct Information Security Officer
The University of Texas at Dallas
AVIEN Founding Member
http://www.utdallas.edu/ir/security/