Usage of TCP/53

Barry Margolin barmar at
Thu Sep 28 23:50:03 UTC 2006

In article <efh5ui$2dk2$1 at>,
 Peter Dambier <peter at> wrote:

> All queries can be either UDP or TCP.

Not quite.  The standards say that except for zone transfers, the client 
MUST try UDP first, and only switch to TCP if the UDP response is 
truncated.  So if all your responses fit in 500 bytes, TCP should never 
be needed for non-transfer queries.

>  Sometimes servers, routers or
> firewalls are broken and TCP is your only chance.

Never heard of this case.  The usual problem is that TCP/53 is blocked 
at the firewall, not UDP/53.  I've never heard of any common client 
implementations automatically trying TCP when UDP times out, so if your 
network only allows TCP then I'd expect 99% of queries to fail 

Barry Margolin, barmar at
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

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