Domain Name System – How to Set Up MX Record for Subdomains

Unfortunately, what you're running into is a limitation of the DNS specification. Having an MX record for the same hostname as is defined as a CNAME record will fail in most DNS server implementations. Some older DNS servers will allow this, but they have been mostly phased out in favor of newer, more secure implementations.

Instead of using CNAME records, you will need to use 'A' records with the IP addresses of the customer sites directly instead of aliasing the names.

Yes, that's an appropriate use of CNAMEs. In the discussions I've been part of, the arguments tend to go like this:

Against CNAMEs:

• There is a (tiny) performance penalty, as the downstream DNS caches need to perform 2 DNS lookups, one for the CNAME and one for the A-Record the CNAME points to.
• Vague, bogus arguments about CNAMEs having less "authority" or compatibility issues.

In favor of CNAMEs:

• They provide a clean abstraction between hardware (physical servers) and services.
• They simplify DNS management -- when a server moves, you only need to change one record.

After trying a couple of different ways to do this, I now have a personal favorite style. It is:

• One A Record for each physical server; with a fairly low TTL (perhaps 30 minutes); giving the server a human-friendly name.
• One CNAME for each service; with a high TTL (perhaps 24 hours); pointing to the above server names.
• As the sole exeption to the rules above, the domain root is an A-Record, pointing to the webserver / web load balancer. (The @ is required to be an A-record.)

I find that this setup works well. It keeps extra DNS lookups for the CNAMES down; and if a server crashes I can still change public DNS around fairly fast.

Here's a (improvised) example in BIND syntax:

;name     ttl   class rr     value 
server01  30m   IN    A      192.168.0.3
server02  30m   IN    A      192.168.0.4

webmail   24h   IN    CNAME  server01
extranet  24h   IN    CNAME  server02
ftp       24h   IN    CNAME  server02