[E] Re: strange problem with query being dropped/ignored by the BIND process

[Marc Richter]

Hi Ben,

thanks for the answer.

Yeah, I think you are right. I see a lot of udpInOverflows on the system,
which suggest that the receive buffer is too small indeed.

Is there any kind of recommendation or best-practice advice what the
buffers should ideally be set to on Solaris ?
I did search the ISC Knowledge Base, but didn't find any useful advice.

Regards
arc

On 06/28/17 14:37, Ben Croswell wrote:
> Have you checked deeper at the OS level? I have seen on Linux DNS servers
> silent drops of queries on very busy servers that were exhausting UDP
> receive buffers.
> 
> On Jun 28, 2017 10:26 AM, "Marc Richter" <marc.richter at de.verizon.com
> <mailto:marc.richter at de.verizon.com>> wrote:
> 
>     Hi,
> 
>     we have a setup here consisting of a recursive DNS server and two
>     monitoring servers. The monitoring servers sent a test query to the DNS
>     server once every two minutes to check if it is answering properly.
> 
>     We now have the problems that these test queries are timing out from time
>     to time, (correctly) resulting in alarms in our monitoring system.
> 
>     I have checked this now and noticed that each time we see that alarm, the
>     query sent by the monitoring server is not being answered at all.
>     To debug that I ran tcpdump on both the monitoring server and the recursive
>     DNS server. I see the query being sent out on the monitoring server and I
>     also see the query being received on the DNS server, however there is no
>     response sent to this query at all.
>     Looking at the query log, which I enabled temporarily, the query is also
>     not logged there so it looks like BIND is ignoring that query somewhere,
>     although it is properly received by the IP stack of the server.
> 
>     Do you have any suggestions how to debug this further, to hopefully find
>     out where these queries are stuck/dropped/ignored, as I have run out of
>     ideas ?
> 
>     The environment is:
>     BIND 9.9.9-P5 (Extended Support Version) <id:1ab232a>
>     running on SunOS sun4v 5.11 11.3
> 
> 
>     Thanks !
>     Marc
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-- 
Marc Richter
Engr III Cslt-Ntwk Eng&Ops

Sebrathweg 20
44149 Dortmund
Germany

O +49 231 972 1293
F +49 231 972 2587
E marc.richter at de.verizon.com