[Pdns-users] Intermittent SERVFAIL Response

sthaug at nethelp.no sthaug at nethelp.no
Tue Aug 18 15:08:18 UTC 2015

> I'm running PowerDNS Recursor 3.7.3 on a pair of Ubuntu 14.04.3 LTS servers.
> I'm getting intermittent SERVFAIL responses on both servers to queries for a
> particular name. Could someone please help me understand what might be
> causing these failures? Here are examples of the SERVFAIL responses I'm
> seeing on one of the servers:
> $ dig @localhost www.concoursmustang.com any

I've seen similar problems (intermittent SERVFAIL) for various domains.
My conclusion after investigating these is that PowerDNS recursor does
not have a coherent cache - and you might be hitting different threads,
one or more of which answer SERVFAIL. You may be able to clear the
problem with "rec_control wipe-cache <domain>". Restarting also works,
of course - but throws away the cache.

(We're running with threads=4 in recursor.conf and can easily see the
incoherent cache by simply repeating the same query multiple times and
noting the TTL in the answers. With a coherent cache you'd expect the
TTL to always count down (until 0, of course). With PowerDNS Recursor
the TTL is inconsistent - sometimes *increasing*. I assume it is 
perfectly consistent within one thread.)

Steinar Haug, AS 2116

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