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Re: Question for DNS pros
From: Steffen Schumacher <ssch () wheel dk>
Date: Sat, 24 Jul 2004 12:02:04 +0200

On 23.07.2004 17:11:10 +0000, Paul Schmehl wrote:
--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 
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.

DHCP telling the hosts to use that DNS server? 
Do you use DHCP? If so, check the config, if it is in the clear, there may be 
a rouge DHCP server popping up once in a while. To check for this you should 
check your DHCP logs.

Just a suggestion..


(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? (44)
17:01:48.399150 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  1085 PTR? (44)
17:01:49.144398 x.x.x.x.17388 > xxxxxx.utdallas.edu.domain:  1086 PTR? (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 () utdallas edu)
Adjunct Information Security Officer
The University of Texas at Dallas
AVIEN Founding Member

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