Why can't a domain's root be a CNAME? [duplicate]

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 (delegated) domain must have an SOA and NS records the rule above kicks in, preventing use of CNAMEs too.

Well it's not that the root of a domain can't be a CNAME - It's that a CNAME cannot coexist with other record types for the same domain.

Simply put, it would make no sense for a domain with a CNAME to have any other record type, because the CNAME would ensure that they would never be seen or read.

Let's say we did try to give example.net both a CNAME (pointing to host.example.com) and an A record (pointing to some other IP address).

1. First, the client would request an A record from example.net from example.net's nameservers.

2. example.net's nameservers would inform us that we need to follow a CNAME record in order to proceed. So we do. The CNAME points to host.example.com.

3. The client would request an A record from host.example.com's nameservers.

Notice how we never got the chance to read an A record, or any other record type, from example.net? The CNAME takes precedence.