> We allow DNS over UDP only and have not had any problems.
Since this only affects large queries, you would probably not notice the
failures. It is, however, wrong.

> Don, can you give us a reference/RFC for these large DNS requests over
> TCP?
RFC 1035.

http://www.faqs.org/rfcs/rfc1035.html

Section 4.2

> I find nothing
> about it, it is my understanding that only zone transfers use TCP.
This is a very common misconception. UDP is the standard transport, but
not the required one. You could perform every request with a TCP
connection, however a TCP connection is usually only used after a UDP
connection has failed to fit within the 512 byte window resulting in the
TC bit being set in the header.

-Don

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