[dnsdist] Marking downstream Unexplained

[Aleš Rygl]

> I realized the retries option was not documented, it's now fixed. It
> only applies to the number of times dnsdist tries to reconnect to a
> backend over TCP before giving up.

Thanks. Much better now.

> 
> I have now added a maxCheckFailures option to be able to configure the
> number of health check failures required to consider a backend down.
> 
> > Under normal conditions we should not see the servers being down at all
> > (unless there is something really wrong with them) so logging the
> > healthcheck retries could be a good warning that the resposivnes of the
> > resolver is getting worse.
> 
> I've just added a lot of log messages to be able to debug health check
> failures, they are only enabled in verbose mode _and_ if
> setVerboseHealthChecks(true) is used.

How nice!

> 
> > I have noticed that even if everything is fine and there is no DDoS
> > running my resolvers (unbound) are dropping the healthcheck query. It
> > occures with a rate of 1e-07 (once per day or less)
> 
> If you take some time to debug with the new log messages, let me know
> what is happening and if we can improve something on dnsdist's side.

No problem, I will come back with some results. 

Regards
Ales