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We've got a farm of centos5 boxes, which were configured by default to do DNS lookup via our host's internal DNS servers.

As of today, these lookups are failing about 10% of the time -- the default 5 second DNS lookup expires (sometimes up to 4 times in a row), and then we finally get a response (so operations that require a DNS lookup take 5, 10, 15, or 20 seconds to respond).

My question is, is there some way to count successful/timed out DNS lookups in centos/linux? I'd love to monitor this, so I'm not taken by surprise again when things start taking 5 seconds longer than they should.

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Do you already have an internal monitoring system? What are you using already? Would a check from your monitoring system against the internal DNS servers work for you? Nagios has a nice dns check that can send a request and check that the results were valid and returned quickly. – Zoredache Aug 23 '12 at 21:50
Using nagios. It's looking like we will probably just use something like check_dns to run test queries. I worry, though, that this won't give us much visibility into situations like the current one, where 10% of requests fail. – Frank Farmer Aug 23 '12 at 21:56
Apparently our host rate-limits DNS requests. It's a bit alarming that a simple 1-request-per-second test was able to trip this limit. This lends itself to answering both of my problems however: we may install our own DNS proxy which we can then (1) monitor and (2) receive reliable service from – Frank Farmer Aug 24 '12 at 18:28

Another approach would be to modify your /etc/resolv.conf to:

  • Lower the timeout period (add options timeout 1) or...

  • Perform round-robin lookups (add options rotate) to distribute the DNS queries.

Either option is better than waiting 5 for seconds per resolver timeout.

Of course, those are only stopgap measures. Why is your host's DNS failing at such a high rate?

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