RE: Strange TCP/IP DNS traffic

["Matthew Ploessel" <matthew () ploessel com>]

Shachar,

UDP port 53 is normally used for general dns traffic, however anytime
there is more then 576 bytes of data being transferred the DNS protocol
migrates up to TCP. Common reasons for this is for zone transfers or
overall large server replies. Most likely your bind server or a user
and/or user application is doing some type of resolving which returns a
large reply and thus traggering the use of tcp traffic..  I haven't
looked up the details of the rfc lately, but tcp is part of the dns
protocol, although just like you, many environments block it. If you
still want to, setup tcpdump for afew days and see if you get any
explaination for whats going on.

-Matt



-----Original Message-----
From: full-disclosure-admin () lists netsys com
[mailto:full-disclosure-admin () lists netsys com] On Behalf Of Shachar
Shemesh
Sent: Thursday, June 03, 2004 7:35 AM
To: full-disclosure () netsys com
Subject: [Full-disclosure] Strange TCP/IP DNS traffic


Hi all,

A few days ago I started seeing outbound TCP connection on port 53, 
aimed at the .com NS servers. These were blocked by the firewall. I 
realize that this does not violate any RFC, but it's still unusual.

The outbound traffic is not generated by the local bind installation, 
which was asked to bind to port 53 for outbound traffic. Also, 
/etc/resolv.conf lists 127.0.0.1 as the nameserver, so as far as I 
understand such traffic should not be initiated by user programs.

Anyone has any idea what that may be?

             Shachar

-- 
Shachar Shemesh
Lingnu Open Source Consulting
http://www.lingnu.com/

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