I’m also not quite sure what you mean: English is not my first language
But if I have a CNAME for sub.example.com, my understanding is that nothing should stop me from creating an MX or TXT record for the same name sub.example.com. And this is what doesn’t seem to be possible with Route53, from their documentation:
In addition, if you create a CNAME record for a subdomain, you cannot create any other records for that subdomain. For example, if you create a CNAME for www.example.com, you cannot create any other records for which the value of the Name field is www.example.com.
That is not correct if sub.example.com is a CNAME. Broken software may allow it, but it has never been permissible to create such records. The RFCs have expressly forbidden the existence of such records since the dawn of time, or least the entire history of DNS. Section 2.4 of RFC 1912 explains this. DNSSEC records are an exception to this prohibition.
Thanks for the correction. Cloudflare (and, to the best of my knowledge, every other DNS manager I’ve used in the past) allows it – and it’s always worked… so I assumed that must be the correct implementation of the standard.
Wow! I could see that being allowed in Cloudflare with the Proxy enabled since the CNAME gets flattened, but I’m surprised that no error is thrown when it is set to DNS Only. Of course, as you mentioned, other software has allowed it. I probably even forced BIND to do it over 20 years ago before I knew better.