DNS load balancing: UDP or TCP ?

Roberto Carna robertocarna36 at gmail.com
Wed Feb 20 12:55:17 UTC 2019

Dear Tony, thanks for your response.

I've read something I don't know if it's true or not:

DNC 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

Can you confirm thgis is true in 100% of clients???

Thanks again, regards !!

El mar., 19 feb. 2019 a las 13:24, Tony Finch (<dot at dotat.at>) escribió:

> Roberto Carna <robertocarna36 at gmail.com> wrote:
> > Dear, I have to balance two DNS servers for a special reason.
> https://www.powerdns.com/dnsdist.html
> > 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.
> Tony.
> --
> 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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