tcp versus udp

[Sam Wilson]

In article <gttqkl$ot6$1 at sf1.isc.org>,
 Barry Margolin <barmar at alum.mit.edu> wrote:

> In article <gtrqte$2ins$1 at sf1.isc.org>,
>  Sam Wilson <Sam.Wilson at ed.ac.uk> wrote:
> 
> > In article <gtrbsa$296l$1 at sf1.isc.org>, Mark Elkins <mje at posix.co.za> 
> > wrote:
> > 
> > > One place that TCP may make sense - if you are involved in a registry
> > > system and the process involves actually checking the information that
> > > you are given, including nameservers (do they exist, do they serve that
> > > zone - correctly?) - it may make a lot of sense to do TCP Digs for the
> > > information (though that should probably be after a failed UDP dig - as
> > > a number of people do insist on disallowing Port 53 TCP).
> > 
> > If the registry is testing for compliant servers then a failed TCP query 
> > should flag the server as non-working, as would a failed UDP query.
> 
> DNS servers MUST support UDP, and only SHOULD support TCP.  So a failed 
> TCP query should not flag the server as non-working.

You're right of course - my apologies.  A failed TCP query should flag 
the server as not supporting TCP queries, with reference to RFC 1123 
sections 6.1.3.2 and 1.3.2.  The registrar may or may not wish to 
comment on the advisability of that.  

Sam