[Pdns-users] pdns server fails every morning when it starts getting used.

sthaug at nethelp.no sthaug at nethelp.no
Thu Jan 8 09:40:57 UTC 2015

> > I've got a pdns server at one site that is causing me massive headaches.  
> > Every morning when the staff come in and start using it (and were not talking 
> > large numbers) it fails to serve external dns and has to be restarted 
> > (usually) or rebooted.
> Which version of PowerDNS do you run? 
> The recursor you specify is in fact an open recursor. This means it is
> likely participating on DNS reflection attacks, and might therefore be slow
> at times.  From here (in europe at least) is is very slow, so the timeout
> may actually be real.

Recursors that are being used for attacks often have problems - this
is well known. We have been monitoring our PowerDNS recursors for
quite a while with a very simple script which logs, via crontab every
minute, the number of open sockets for the recursor:

#! /bin/sh
# Find approximate queue length for pdns_recursor, based on number of open sockets
d=`date +'%Y%m%d %H:%M'` ; echo -n $d ""
fstat -p `cat /var/run/pdns_recursor.pid` | wc -l

During <random>.domain attacks we often see the number of open sockets
increase dramatically - and this correlates well with slow(er) replies
to clients. Monitoring this, and lately also via the PowerDNS "Graphing
as a service" offer,


has significantly improved our ability to handle these attacks.

Steinar Haug, AS 2116

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