recursive-clients : recommended value for a high traffic recursive nameserver

[Niall O'Reilly]

At Sun, 23 Nov 2014 21:00:15 -0800 (PST),
blrmaani wrote:
> 
> Our nameservers take upto 10KQPS (mostly NOERROR type most of the time). 
> 
> Twice or thrice a week, I have seen upto 10% of the queries are
> SERVFAIL and we have started exceeding the default value of 2000 for
> recursive-clients settings in BIND 9.9.x.
> 
> Is there a recommended value for recursive-clients option assuming
> huge number of SERVFAIL queries once in a 2/3 days?
> 
> I'm not convinced to increase it to some arbitrary huge number
> 20,000 or 200,000.
> 
> I am looking for answer like - if your peak SERVFAIL queries are
> 2000/second, then your recursive-clients value should be N.

  I wouldn't expect that such an answer could make sense.

  Exhaustion of the active recursive-clients list and the generation
  of responses marked SERVFAIL are most likely different symptoms of
  the same problem.  I think you'll need to identify this problem and
  then determine what action to take.

  Your resolver seems to be dealing with queries which are
  unanswerable and which are arriving in a quantity sufficient to fill
  the recursive-clients list.  This may be due to rogue clients,
  misconfigured authoritative servers, network problems, or some
  combination of these.  Your logs will help identify which.

  I hope this helps.

  Niall O'Reilly