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What is the history of 2821 and implict MX?

2008-04-07 11:57:50

On 4/7/08 at 9:54 AM -0700, Dave Crocker wrote:

John C Klensin wrote:

Someone would need to check with Craig Partridge...

A side discussion about history might be interesting, but I'll suggest that it should not really be a factor for the current discussion.

So, I want to disagree with Dave and wish to consider some history, though Dave may have only been referring to the history leading up to 974. I think the history leading up to 2821 might be important:

1. 974 did *not* have an "A fallback" rule at all. 974 is quite clear that the rule is, "If you can't find an MX, you should treat that as if you *did* find an MX where the preference was 0 and the name in the response was identical to the name queried. (That is, if you get back no MXs for, then it should be equivalent to getting back a single MX of " IN MX 0".) If this rule were in effect today, you *would* connect to a mail host's IPv6 address even if it had no MX records.

2. 2821 changed the rule and made explicit claims about looking for fallback A records. The question is, was that intentional? Did DRUMS intend to change the 974 rule in this regard, or was it trying to explain what the 974 rule entailed by using examples? So far, I can find nothing in the DRUMS archive to support the position that 2821 intentionally changed the 974 rules.

If 2821 had preserved the 974 MX rule, we would not be having this discussion at all. The rule would be clear that the lack of the MX got you an implicit MX, and then you'd look up A and AAAA records as normal.

This entire argument reduces (for me) to answering this question: Did 2821 intend to preserve the 974 rule? If DRUMS did *not* intend to change the 974 rule, then 2821's current text is an errant interpretation of 974 and should be fixed back to the 974 rule. If DRUMS *did* intend to change the rule to make it clear that 974 was all about backwards-compatibility for hosts that didn't have MX records, then there is a reasonable argument to be made for not doing so for AAAA.

My current feeling is that 2821bis should clarify that it intended the 974 rule. Yes, that gets us implicit MXs that point to AAAA hosts. Such is life.

Pete Resnick <>
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