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A third party has asked me to setup a new subdomain and point it too sub.theirdomain.com. Apparently I can do this with a single CNAME record. Although I set it up over 24 hours ago, it doesn't seem to be resolving. Here's the entry I made:

sub.mydomain.com. IN CNAME sub.theirdomain.com.

Does that look OK?

The host is Webmasters, and I asked their helpdesk who thought it was fine.

The only way I know that it isn't resolving is that when I surf to sub.mydomain.com I get a server not found error. What tools should I be using to diagnose further?

Any you can see, I'm no DNS expert, so any assistance is much appreciated!

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Did you update the serial number and, if this is using BIND, did you reload the file after making changes? – John Gardeniers Apr 12 '11 at 3:04
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2 Answers

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An HTTP server not found error doesn't mean that the DNS is not working. Try doing the following and see if it resolves correctly. It should say sub.mydomain.com cnames to sub.theirdomain.com

nslookup sub.mydomain.com

If it resolves then telnet to port 80 to see if its open.

telnet sub.mydomain.com 80
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Thanks. The nslookup says it can't find sub.mydomain.com. Next step? – Kim Apr 12 '11 at 2:54
Check your zone file. You can check the zone file is error free using named-checkzone command which is included with Bind. – Sameer Apr 12 '11 at 3:00
Also ensure that you have updated the zone serial (as John suggested in his comment above) and look at your named logs to see if there were any errors reloading the zone. – voretaq7♦ Apr 12 '11 at 14:34
Thanks All, particularly @Sameer. Turns out that it was just a propogation delay. I did learn a lot by asking the question, though. – Kim Apr 13 '11 at 1:08
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up vote 2 down vote

The format of the CNAME itself looks fine.

Depending on what kind of checking the input on your zone editor does, it's possible that the problem is some added nesting. Do you get the CNAME result for 'sub.mydomain.com.mydomain.com'?

Usually the best way to check DNS issues is with tools like 'dig' (typically on *nix systems) or 'nslookup' (built into Windows).

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nslookup is available on *nix too. – Sameer Apr 12 '11 at 3:03
*nix may have replaced nslookup with host. – BillThor Apr 12 '11 at 3:20
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