On Wed, Apr 1, 2020 at 8:36 AM Tony Finch <d...@dotat.at> wrote: > > This error behaviour is mostly specified by the UPDATE protocol (RFC > 2136). It's worth reading the RFC becasue (as you have found) some of the > behaviour is a bit surprising. For instance, adding a record that already > exists is not an error because multiple copies of the same record > traditionally get silently de-duplicated in the DNS. (I can't explain the > lack of error in the CNAME case...) >
This was a bit unexpected for me too, until I saw that RFC2136 does explicitly cover the CNAME case. From Section 3.4.2.2 (the "vice versa" covers the original poster's case): "In the case of a CNAME Update RR and a non-CNAME Zone RRset or vice versa, ignore the CNAME Update RR, otherwise replace the CNAME Zone RR with the CNAME Update RR." The implication is that "ignore" also means set the response code to NOERROR. Although, I suppose CNAME related UPDATE processing could have been special cased to return an error code like YXRRSET (even without a specified prerequisite clause). Shumon.
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