DNS load balancing: UDP or TCP ?

Tony Finch dot at dotat.at
Tue Feb 19 16:24:45 UTC 2019

Roberto Carna <robertocarna36 at gmail.com> wrote:

> Dear, I have to balance two DNS servers for a special reason.


> The DNS clients are a mix of Windows, Cisco and Linux machines, so I
> think they ask for a FQDN using UDP and after that -if there is no
> response-, they ask the same FQDN using TCP, and so the load balancing
> will be succesful.

No, fallback to TCP relies on receiving a truncated UDP response. You
never want a DNS client to be waiting around for a response that will
not arrive.

f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
Rockall, Malin: Southeast veering southwest 6 to gale 8, occasionally 5 later.
Rough or very rough. Rain. Moderate or poor.

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