[Pdns-users] servfail-packets>0 but no data in servfail-queries ring buffer

[Remi Gacogne]

Hi Martin,

On 4/9/20 9:56 AM, Martin Kellermann via Pdns-users wrote:
> I’m facing the fact, that sometimes the servfail-packets counter
> increases and i would debug this and find the reason why PowerDNS sends
> out a „servfail“ packet.
> 
> Actually today the counter increased from 0 to 2 and i don’t know why!
> 
> https://doc.powerdns.com/authoritative/performance.html#servfail-packets
> tells me, that this is due to a database problem.
> 
> https://doc.powerdns.com/authoritative/performance.html#ring-buffers
> tells me that the servfail-queries ring-buffer shows which queries have
> been causing servfails.
> 
> But when i look at this ring-buffer (server:8081/?ring=servfail-queries)
> there are no entries – it’s empty.

At a quick glance the only place in the code where we increase the
'servfail-packets' counter but do not add the offending query to the
'servfail-queries' ring-buffer is when we receive a query for a DNS name
that has an 8-bit byte in it and '8bit-dns' [1] is not set (which is the
default), so I assume that's what you are experiencing.
Note that in that case we also increase the 'corrupt-packets' counter
and place the offending query into the 'remotes-corrupt' ring-buffer, so
you might be able to figure out what query caused this. Be aware however
that there are other cases where we increase 'corrupt-packets' and
insert into 'remotes-corrupt', like a QR or TC bit set in a query, or a
query that we simply could not parse.

[1]: https://docs.powerdns.com/authoritative/settings.html#bit-dns

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

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