strange problem with query being dropped/ignored by the BIND process

Ben Croswell ben.croswell at
Wed Jun 28 14:37:26 UTC 2017

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>


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 !
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