DNS problem

[Stephane Bortzmeyer]

On Tue, Mar 02, 2004 at 08:19:11PM +0100,
 HuMPie <humpie at grunn.org> wrote 
 a message of 60 lines which said:

> The only port's the DNS traffic's use is:
> - UDP port 53 for resolving
> - TCP port 53 for zone transfers

This is a common misconception but it is wrong nevertheless.

RFC 1035 :

4.2. Transport

The DNS assumes that messages will be transmitted as datagrams or in a
byte stream carried by a virtual circuit.  While virtual circuits can be
used for any DNS activity, datagrams are preferred for queries due to
their lower overhead and better performance.  Zone refresh activities
must use virtual circuits because of the need for reliable transfer.

The Internet supports name server access using TCP [RFC-793] on server
port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP
port 53 (decimal).

(End of quote)

Do note that TCP is mandatory for zone transfer but it is allowed for
other activities (typically, when the reply is too big to fit in the
UDP packet).