Recently we transferred a site onto our servers and had the domain manager (as in person) update the DNS entries to point to our web servers.  It all seemed just fine, until the site owner started to get comments (luckily for her) that her site just wasn't working.  The weird thing was that it was working for us - when we typed in the domain name it came to our server.  We knew the server was serving pages as we tested by overriding our local hosts file.

So how do you find out what the problem is.  We were lucky in a way.  This is how we debugged the problem:

See what the DNS entries say

We went to MXToolbox.  Here you can type in the domain name and it tells you which IP addresses this domain points to.  Unfortunately it only tells you want the MXToolbox DNS entry says (which is just a snapshot of the real entry).  However, this does give you the option of seeing what every DNS server is giving us.  Click on 'DNS lookup' to check.

The 'dns lookup' link changes the command to 'a:' plus the domain name, and then returns the 2-3 DNS records they have.  Usually there are 3 DNS servers (backups in case one is down).  These domain servers are 'authoritative' for this domain - in other words they are the source of the entry and everyone comes back to ask them.

Another useful tool

We then took each of these DNS servers and used this tool.   It enabled us to check each DNS server in turn and see what it was returning.  What we found was that 2 servers were correctly set up and third was still pointing to an old address.  Problem solved.