Re: systemd-resolved switching DNS servers

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Once upon a time, Tom Seewald <tseewald@xxxxxxxxx> said:
> Yeah I'm not very happy that systemd-resolved seemingly does this silently and that I have to just restart the service for it to try again. My server is just a consumer router running OpenWRT which uses Dnsmasq.

So, outside of classic Unix/Linux /etc/resolv.conf... most software does
not treat a list of multiple DNS servers as explicitly "primary" and
"secondary" (and so on).  Some software will start with the first, then
at any error or timeout (which can happen due to errors up the recursive
line, not necessarily with the server itself), go to the second, and
continue using it until there's an error/timeout, when it'll go to the
third (and so on until it starts back at the top).

Some software sends to multiple servers at first and then watches which
one is faster and uses it for a while, checking all again periodically.

Some software rotates through the list for each request.

And really... almost all of these behaviors work out better in practice
than the classic resolv.conf behavior of each program having its own
query list, trying the first server with lots of retries and timeouts,
then the second, etc.  That behavior means that whenever the first
server is down, all kinds of stuff times out, and keeps timing out
because each thing starts a new process (which starts with the first
server again).

A local cache or even basic resolver to manage queries is better
behavior, and what other OSes have used for years.  I'm not a fan of how
systemd-resolved does some things, but having something like that is
long overdue.

As for logging... this is something that has the potential to bounce
around a bunch under some conditions, so I don't think logging it is a
great idea (can easily cause log spam).
-- 
Chris Adams <linux@xxxxxxxxxxx>
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