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I just had a really alarming situation with one of the subdomains.

So the problem was that one of subdomains mail.xxx.pl was unavailable (server request failure, no ping, no dig response), when all other subdomains were fine (the same domain of course).

All subdomains pointing the same server, and are as A entry. Domain itself were good too, name servers are OK, and were responsing good.

So somehow just one subdomain couldn't have been resolved. (it was working all few weeks already, and I haven't change any settings, it just stopped working with no reason).

Is this possible that somehow on the higher level, got wrong? Because I really can't imagine what happened.

I was trying to look logs, and few minutes later it just started to work again.

I would appreciate any clues where to check or what could have happen.

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Everything else on xxx.pl worked fine? Is it all in a single zone on that server, or is that subdomain delegated to your server? –  NickW Jun 12 '13 at 15:13
    
yes its all one server, just one CNAME pointing to the other, but din't check this one. And yes all other domains, services were ok. –  l1em1on1 Jun 12 '13 at 15:15
    
I hope you mean one CNAME pointing to an A, not CNAME to CNAME :)That aside, if you did a dig @ that server, what did it reply? –  NickW Jun 12 '13 at 15:22
    
;; QUESTION SECTION: ;mail.xxx.pl. IN A and no answer –  l1em1on1 Jun 12 '13 at 15:29
    
the CNAME works ok ;) and it could not even be there. Everything was OK, using that all the time. Just for few hours (2-3h) one subdomain wasn't responding. And for now is OK so this is just more to know what happened. So I knew next time. –  l1em1on1 Jun 12 '13 at 15:31

1 Answer 1

If it happens again:

  1. Determine first whether you are dealing with an IP connectivity issue or a DNS resolution issue (i.e. ping mail.xxx.pl. Look at the result -- do you get an "unknown host" error or does it resolve to an IP address but ICMP fails to make the round trip to the server and back?
  2. If you determine that it is an IP connectivity issue, troubleshoot it as such (i.e. traceroute, etc.)
  3. If you determine it is probably a DNS resolution issue (for example because pinging by name gives an "unknown host" error but pinging by IP address works fine) use dig to try to resolve the name in question, beginning with "dig mail.xxx.pl A" (or CNAME if the record for that name is a CNAME.)
  4. Carefully examine the results of dig. Start with the "status" field. Did your query return NOERROR? NXDOMAIN? REFUSED? Each of these return codes should be handled differently.
  5. If dig does not manage to resolve the answer, try asking the question directly to one of the authoritative nameservers for the zone in question, i.e. "dig @ns1.xxx.pl mail.xxx.pl" (substitute the name of one of your nameservers for ns1.xxx.pl.) If the authoritative server answers correctly, proceed to ask each authoritative server for the zone in turn -- one of them might be giving the wrong answer. If it does not answer correctly, troubleshoot the problem as a fault with the authoritative nameservers.
  6. If the authoritative nameservers answer correctly but your client cannot get an answer from its recursive server, troubleshoot the problem as a delegation issue by tracing the delegation chain from the root and seeing where you encounter failure -- dig +trace can be helpful here.
  7. The above presumes that you are not signing your zone, which can introduce additional reasons why you might not get a valid answer when using a validating resolver. If you are using DNSSEC, indicate that in the question.
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