Re: Question for DNS pros

[Steffen Schumacher <ssch () wheel dk>]

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..

/Steffen 

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