North American Network Operators Group
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Re: DNS problems to RoadRunner - tcp vs udp

  • From: Florian Weimer
  • Date: Sun Jun 15 04:15:19 2008 
* Jon Kibler:

>>>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.

I've seen such claims countless times.  8-( However, you can perform
zone file transfers over UFP in some cases (using IXFR), and TCP
fallback may happen even if you never respond with packets with the TC
bit set.