A famous issue when setting up DNS records for domains is that you can't combine a CNAME record with any other type, such as a MX record or a SOA (which is required at the top level of a domain). One modern reason that you would want such a CNAME record is that you're hosting your domain's web site at some provider and the provider wants to be able to change what IP addresses it uses for this, so from the provider's perspective they want you to CNAME your 'web site' name to 'something.provider.com'.

The obvious reason for 'no CNAME and anything else' is 'because the RFCs say so', but this is unsatisfying. Recently I wondered why the RFCs couldn't have said that when a CNAME is combined with other records, you return the other records when asked for them but provide the CNAME otherwise (or maybe you return the CNAME only when asked for the IP address if there are other records). But when I thought about it more, I realized the answer, the short version of which is caching resolvers.

If you're the authoritative DNS server for a zone, you know for sure what DNS records are and aren't present. This means that if someone asks you for an MX record and the zone has a CNAME, a SOA, and an MX, you can give them the MX record, and if someone asks for the A record, you can give them the CNAME, and everything works fine. But a DNS server that is a caching resolver doesn't have this full knowledge of the zone; it only knows what's in its cache. If such a DNS server has a CNAME for a domain in its cache (perhaps because someone asked for the A record) and it's now asked for the MX records of that domain, what is it supposed to do? The correct answer could be either the CNAME record the DNS server has or the MX records it would have to query an authoritative server for. At a minimum combining CNAME plus other records this way would require caching resolvers to query the upstream DNS server and then remember that they got a CNAME answer for a specific query.

In theory this could have been written into DNS originally, at the cost of complicating caching DNS servers and causing them to make more queries to upstream DNS servers (which is to say, making their caching less effective). Once DNS existed with the CNAME behavior such that caching DNS resolvers could cache CNAME responses and serve them, the CNAME behavior was fixed.

(This is probably obvious to experienced DNS people, but since I had to work it out in my head I'm going to write it down.)

Sidebar: The pseudo-CNAME behavior offered by some DNS providers

Some DNS providers and DNS servers offer an 'ANAME' or 'ALIAS' record type. This isn't really a DNS record; instead it's a processing instruction to the provider's DNS software that it should look up the A and AAAA records of the target name and insert them into your zone in place of the ANAME/ALIAS record (and redo the lookup every so often in case the target name's IP addresses change). In theory any changes in the A or AAAA records should trigger a change in the zone serial number; in practice I don't know if providers actually do this.

(If your DNS provider doesn't have ANAME/ALIAS 'records' but does have an API, you can build this functionality yourself.)