Do MX Records interfere with wildcard CNAME records?

Question

I've got a wildcard CNAME record defined on my domain (*.mydomain.com)

So, a user at foo.mydomain.com would resolve correctly.

I then went and defined a specific MX record: foo.mydomain.com and pointed that to a mail server

This seems to have caused the wildcard CNAME to break for that domain and not resolve. I would assume that CNAME wildcards are isolated from MX Records, but my test appears to show otherwise.

Do MX Records cause wildcard CNAME queries with the same subdomain to not get resolved?

Answer 1

The record types involved actually don't matter here. What does matter is that if a name explicitly exists, a wildcard record will not be looked for. One way, and by far the most common way, for a name to explicitly exist is for it to have a record of some kind. An MX record, for example. Once that record is there, any queries for that name will either get a response with the MX record (possibly with accompanying DNSSEC records, but let's ignore that for now) in question, or a response saying "The name exists, but it doesn't have a record of the type you asked for".

A wildcard entry is only used for names that do not exist at all. What you want here, to have a wildcard record "fill in" records in existing names, is not ordinarily possible in DNS. It wouldn't be impossible to write a specific name server software that did it, of course, but the resulting semantics would be rather hideous.