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CNAME handling in “5.1. Locating the Target Host” #92

Open aamelnikov opened 7 months ago

aamelnikov commented 7 months ago

Alexey Melnikov wrote:

In 5.1: The lookup first attempts to locate an MX record associated with the name. If a CNAME record is found, the resulting name is processed as if it were the initial name.

Is CNAME expansion only done once? Or multiple times? If multiple times, what the minimal expected limit?

aamelnikov commented 7 months ago

John Klensin wrote:

RFC 1123 Section 5.2.2 prohibits CNAMEs in MAIL or RCPT commands. At some point we changed that and 2821 contains the text above, with no hints about iteration or recursion. I vaguely recall the change being made in some document between 1123 and 2821, but that is a dozen or so very eventful years.

I think being more specific requires involving the WG and recognizing that whatever we say is likely to render some existing implementation non-conforming. How would you feel about a statement in the A/S pointing out that long CNAME chains are a source of trouble, that, when possible, one should stick to 1123-style canonical names, and when that is not possible, to try to keep the chains short.