DNS MX hacks

In a previous post I talked about mail gateway load balancing by having two MX records in BIND. I mentioned that I'd want the more powerful server chosen a larger percentage of the time.

You would think I could just add another instance of the same host:

MX 10 mta0
MX 10 mta1
MX 10 mta1

but this doesn't work. BIND just ignores the second entry as redundant. I could make mta2 a CNAME for mta1 and then add mta2 as a third MX record. I've done some tests in a test environment and this works. However, this is a hack and having CNAMES for MX records is not theoretically not permitted (RFC 1034 section 3.6.2).