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I've got a strange problem with our Windows Server 2008 DNS answering requests with a NXDOMAIN for valid domain names. After flushing the cache I get the valid answer. As this machine is our primary domain controller I cannot give login access to lots of people if they have problems with the DNS service.

The server is patched, rebooted and up2date ... and Im kind of lost.

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You need more in your question. What are the domain names? How do you know that they are valid? What does your DNS server debug log say that the server is doing when this happens? – JdeBP Jun 21 '11 at 15:53
Im sorry, when I wrote these lines I was in a hurry. Im not quite sure about the domain name, it was something with the .biz tld. I've got a server running bind which could resolve the IP address for the domain. But thanks for the hint with the debug log! – Comradin Jun 22 '11 at 7:44
Ok, today we had the same problem again with debug logging activated. I see the request in the log: "14.07.2011 11:08:00 0580 PACKET 0000000004486330 UDP Rcv ed8d Q [0001 D NOERROR] A (3)www(3)uol(3)com(2)br(0)". Then follows the answer section: "ed8d R Q [8281 DR SERVFAIL] A (3)www(3)uol(3)com(2)br(0)". Ill have to look for a detailed description of the log details. currently I see no real hint for an explanation. I solved the problem by deleting the .br section from the cache and everything worked fine again. – Comradin Jul 14 '11 at 14:21

If these are external domains, and your server is configured to use forwarders, I would suggest using different forwarders or using root hints instead of forwarders.

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Yes, these are external domains and we do not use any forwarders. The root hints are in place and in use. – Comradin Jun 22 '11 at 7:48

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