DNS problems to RoadRunner - tcp vs udp

Kevin Oberman oberman at es.net
Fri Jun 13 18:26:28 UTC 2008

> Date: Fri, 13 Jun 2008 14:14:55 -0400
> From: Jon Kibler <Jon.Kibler at aset.com>
> Hash: SHA1
> Mark Price wrote:
> <SNIP>
> >>From what I have read, public DNS servers should support both UDP and
> > TCP queries.  TCP queries are often used when a UDP query fails, or if
> > the answer is over a certain length.
> > 
> UDP is used for queries.


> TCP is used for zone transfers.

> If my server responded to TCP queries from anyone other than a secondary
> server, I would be VERY concerned.

If it does not, you should be very concerned. The RFCs (several, but
I'll point first to good old 1122) allow either TCP or UDP to be used
for any operation that will fit in a 512 byte transfer. (EDNS0 allows
larger UDP.) 

TCP is to be used any time a truncated bit is set in a replay. If you
ever send a large reply that won't fit in 512 bytes, the request will
be repeated using a TCP connection. If you ignore these, your DNS is
broken. It is even allowed under the spec to start out with TCP, as AXFR
queries typically do.

Yes, I realize that this is fairly common and it does not break much,
but, should DNSSEC catch on, you might just find the breakage a bit
worse than it is today and there is no reason to have even the slight
breakage that is there now.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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