DNS load balancing: UDP or TCP ?

Alan Clegg alan at clegg.com
Wed Feb 20 15:22:25 UTC 2019

On 2/20/19 7:55 AM, Roberto Carna wrote:

> DNS clients send a UDP query to a DNS server, if no response is received
> until some seconds, then they try with UDP.
> You tell me this is not true, just clients try with UDP is the response
> is truncated.

Tony is correct, the first paragraph above IS NOT TRUE.

Truncation is a situation in which the server responding to a client
provides a message that won't fit in the specified packet size that the
specification (and possibly the client, but I won't get into that here)
has set for the response, thus providing a response that does not
contain the entire response and sets the header bit TC=1.

This has nothing to do with TCP vs. UDP in the initial query.  There is
no fallback from UDP to TCP when the initial UDP query times out.

Please read up on `dnsdist` and give it a try.


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