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RE: TCP DNS requests
From: "Douglas K. Fischer" <fischerdk () purefm net>
Date: Thu, 31 Oct 2002 18:04:16 -0500

It is a common misconception that TCP DNS is only for zone transfers. The DNS specification calls for the use of TCP whenever the response exceeds the size of a UDP packet (512 bytes). Zone transfers happen to fall within this category of large responses; however, some client requests can result in responses > 512 bytes, hence the use of TCP DNS.

Blocking clients from using TCP DNS will result in lookup failures whenever the server response exceeds 512 bytes.

Cheers,

Doug

At 02:42 PM 10/30/2002, Raghu Chinthoju wrote:
TCP/DNS(53) is used for zone transfer. To be simple, TCP/DNS(53) is used
between the name servers to exchange/update there name databases where as
UDP/DNS(53) is used for querying.

I see two possibilities for having generated TCP based DNS requests in your
network.
1. You must have another DNS server in that network trying to do zone
transfer with your server
2. Some one is explicitly requesting your name server for zone information.
This could be done by in many ways. For example, "ls" command of nslookup
does it.

Cheers,
Raghu.

Wilco International Systems
Hyderabad.


-----Original Message-----
From: Carl R Diliberto [mailto:cdiliberto () hotmail com]
Sent: Wednesday, October 30, 2002 7:16 PM
To: security-basics
Subject: TCP DNS requests

We are reporting TCP based DNS requests to one of our DNS servers coming
from internal, client IP addresses.  My manager would like to block the TCP
packets.  What or why would their be random TCP packets?  We monitored
several clients and it appears it only needs UDP.

Thanks
Carl


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