Why can't a domain's root be a CNAME?

Question

I've just read this question which essentially says that when I set up DNS for example.com, the root record can't be a CNAME, it has to be an A record.

My question is, why?

I'm sure the clever people who designed DNS didn't make arbitary restrictions for no reason, but I don't see what we gain by requring root domains to be A records. I would love to just point my example.com domain to someserver.somewebhost.example and forget about it, but I can't.

Please enlighten me, billpg.

Answer 1

Firstly, the underlying reason is not that you must use an A record, but that you cannot use a CNAME record because those cannot coexist with other normal resource record types.

The reason for that restriction is in §3.6.2 of RFC 1034:

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. This rule also insures that a cached CNAME can be used without checking with an authoritative server for other RR types.

As the root of a domain must have an SOA and NS records the rule above kicks in, preventing use of CNAMEs too.