High recursive client counts

[Jason Brandt]

We recently migrated to BIND for our internal resolvers, and since the
migration, we are experiencing periods of high recursive client counts,
which will at times cause the BIND server to quit responding.  As a
workaround, I've been able to point the BIND server to a forwarder,
bypassing the root hints, to restore stability, but this morning even with
the forwarder, our count spiked.

We are using Ubuntu 12.04 LTS, BIND version 9.8.1-P1.  The server is
configured strictly as a resolver, and is not authoritative for any domains.

We have approximately 15-20k client devices on campus.  Our average
recursive client count is between 10 and 50.  When the spikes occur, counts
will get upwards of 3-4k (this morning: recursive clients:
2358/9900/10000).

What are possible causes of high recursive client count?  What can be done
to prevent this or tune around it?  Obviously raising the max clients
doesn't solve the problem, and the forwarder seemed to help, but apparently
is still susceptible to the issue.

Any suggestions would be greatly appreciated.

-- 
Jason K. Brandt
Systems Administrator
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