TCP vs. UDP in query responses?

Chris De Young chd at arizona.edu
Tue Jun 15 00:34:33 UTC 2004


Hi,

My understanding is that DNS queries and responses by default use UDP,
but will switch to TCP if the response record set is large (and TCP is
used for zone transfers).  Am I correct?

If so, what determines when TCP is used vs. UDP?  I have some
recollection that TCP will be used if the response record set is larger
than 512 bytes, but I don't remember where I got that from so I don't
have any confidence that it's right.  :-)

Is this threshold fixed, or will it depend on other factors?

I have a remote user (in Norway, I think) having intermittent problems
resolving a particular name (www.math.arizona.edu, not a large RR
set), and it *seems* tentatively to be the result of firewall rules
that permit DNS over UDP but not TCP -- but I can't prove it yet, and
it doesn't seem intuitive unless there are other factors that I don't
know about determining the use of TCP vs. UDP.

Thanks,
-Chris


More information about the bind-users mailing list