This is a Canonical Question about CNAMEs at the apices (or roots) of zones
It's relatively common knowledge that
CNAME records at the apex of a domain are a taboo practice.
example.com. IN CNAME ithurts.example.net.
In a best case scenario nameserver software might refuse to load the configuration, and in the worst case it might accept this configuration and invalidate the configuration for example.com.
Recently I had a webhosting company pass instructions to a business unit that we needed to CNAME the apex of our domain to a new record. Knowing that this would be a suicide config when fed to BIND, I advised them that we would not be able to comply and that this was bunk advice in general. The webhosting company took the stance that it is not outright forbidden by standard defining RFCs and that their software supports it. If we could not CNAME the apex, their advice was to have no apex record at all and they would not provide a redirecting webserver. ...What?
Most of us know that RFC1912 insists that
A CNAME record is not allowed to coexist with any other data., but let's be honest with ourselves here, that RFC is only Informational. The closest I know to verbiage that forbids the practice is from RFC1034:
If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different.
Unfortunately I've been in the industry long enough to know that "should not" is not the same as "must not", and that's enough rope for most software designers to hang themselves with. Knowing that anything short of a concise link to a slam dunk would be a waste of my time, I ended up letting the company get away with a scolding for recommending configurations that could break commonly used software without proper disclosure.
This brings us to the Q&A. For once I'd like us to get really technical about the insanity of apex CNAMEs, and not skirt around the issue like we usually do when someone posts on the subject. RFC1912 is off limits, as are any other Informational RFC applicable here that I didn't think of. Let's shut this baby down.