So a friend of mine had a problem with them not being able to access a windows server using a Cname they had created for the computer.

So what is wrong with this picture. Well using a Cname is as bad as using an IP, the AD does not know about this name. There are so many more things that you need to fix.

There are a few simple and simpler solutions.

Using netdom

netdom computername localhost /add alternative.fqdn.net

Reboot the computer to make it all work.

OptionalNames for Server service [SMB]

By altering OptionalNames (You might need to create it) under the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\parameters, you can make the Server service allow other names for the machine. Remember that type of OptionalNames needs to be Multi-String Value.

SPNs (Service Principal Name)

You can also manually edit the SPNs for a server to allow Kerberos to IIS and other services.