[dnsdist] Marking downstream Unexplained

[Remi Gacogne]

Hi Alejandro,

> Can I enable verbose for loggin?? 

Unfortunately we are not logging more information regarding health check
failures, even at verbose level. That's mostly because it might fill up
your logs very quickly otherwise.

Based on the messages you posted, only one health check fails and the
next ones are fine. So basically, dnsdist sent a UDP query and
either did not get any response at all in less than one second, or the
response was invalid:
- complete garbage or
- response ID not matching the query ID or
- QR flag not set or
- ServFail or
- NXDomain or Refused (only considered an error if you have set
mustResolve to true for this backend).

If you don't have a lot of traffic you might be able to see what happen
by using tcpdump.

Perhaps we should add an option to consider a backend as down only after
several checks failed in a row. Maybe some counters for health check
failures might help too.

-- 
Remi Gacogne
PowerDNS.COM BV - https://www.powerdns.com/

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