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