tcp versus udp

[Matt Baxter]

On May 4, 2009, at 7:28 PM, Martin McCormick wrote:

> 	When are tcp dns queries necessary?
>
> 	It was my understanding that clients could user tcp or
> udp.


When a response can not fit in a single UDP packet the server will  
mark the truncated flag (and respond with all the data it can inside  
the UDP packet).  That should trigger a client to resubmit the query  
via TCP.  Zone transfers are the most common use for TCP, but it can  
be required for normal queries, although that is far from normal.



--
Matt Baxter
mjb at fatpipe.org