The problem though is that you can't put MX and CNAME records at the same point in the DNS hierarchy. So if you want to host a site on an ELB at acme.pagerduty.com, you can't then put in MX records for acme.pagerduty.com, because they'll conflict with the CNAME you need to put there.
This all happens because it creates ambiguity. If you want to look up the MX record of a name that has both MX and CNAME entries (say acme.pagerduty.com), should the name resolver:
a) Grab the MX record at acme.pagerduty.com; OR
b) Do what is usually implied by a CNAME record, and pull the target of acme.pagerduty.com, and search for an MX record at that name?
Because of the potential for conflict, the DNS spec simply forbids CNAME records from existing alongside most other records.
Comments
The problem though is that you can't put MX and CNAME records at the same point in the DNS hierarchy. So if you want to host a site on an ELB at acme.pagerduty.com, you can't then put in MX records for acme.pagerduty.com, because they'll conflict with the CNAME you need to put there.
This all happens because it creates ambiguity. If you want to look up the MX record of a name that has both MX and CNAME entries (say acme.pagerduty.com), should the name resolver:
a) Grab the MX record at acme.pagerduty.com; OR
b) Do what is usually implied by a CNAME record, and pull the target of acme.pagerduty.com, and search for an MX record at that name?
Because of the potential for conflict, the DNS spec simply forbids CNAME records from existing alongside most other records.
Of course. That's a pretty poor oversight on Amazons side.