[tor-relays] unbound bogs down strangely, degrading exit relay

[Dhalgren Tor]

Problem came back again while I was working on the exit.

    unbound-control purge_requestlist

does not help but it appears that

   unbound-control purge_infra
   unbound-control purge_requestlist

will clear up the problem without requiring a daemon restart--at least
temporarily.

Also tried setting

   do-tcp: off

but this did not appear to make a difference.


Seems to me a degenerate interaction between tor's 'eventdns'
subsystem and 'unbound' comes into play when this DNS flood/attack
occurs.  Have an 'info' level log with SafeLogging=0 for a few minutes
where the relay was in the bogged-down state and was failing to
service Tor Browser requests.  If developer is interested in taking a
look at this please contact me directly.

This issue is a PIA and if it continues I'll give up on 'unbound' and
follow the previous operator, switching to bind9 despite the lesser
performance.