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Re: What is the history of 2821 and implict MX?

2008-04-07 12:46:09

Pete Resnick wrote:

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.

You lost me Pete. While the actual usage of the term "implicit MX" is not used in 974, and personally, I think it is a BAD term, the description in 974 which you repeated above is effectively the same result that 2821 described as the implicit MX record.

But I don't see how your last statement applies because you are assuming the the IPv4 client is aware of AAAA record and more specifically IPv6 level 2 TCP protocol layers.

Regardless of 974 or 2821 NO MX records behavior (which to me is the same), AAAA results would be meaningless to a IPv4 only client.

I just don't see the different here and in my opinion, it really does not matter what 974 said differently because I doubly very highly ANYONE would be operating using its recommendations - even though I think its the same 2821 inherited and simply provided an official term for it.

2. 2821 changed the rule and made explicit claims about looking for fallback A records.

Semantics.  That is basically what 974 is describing when it says:

   It is possible that the list of MXs in the response to the query will
   be empty.  This is a special case.  If the list is empty, mailers
   should treat it as if it contained one RR, an MX RR with a preference
   value of 0, and a host name of REMOTE.  (I.e., REMOTE is its only
   MX).  In addition, the mailer should do no further processing on the
   list, but should attempt to deliver the message to REMOTE.  The idea
   here is that if a domain fails to advertise any information about a
   particular name we will give it the benefit of the doubt and attempt

This is exactly what 2821 provides with the implicit MX concept. I fail to see the difference.

In fact, I think 2821BIS should reintroduce the last "The idea here is" sentence. Its perfect and will give people a better understanding as to why it exist.

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.

Probably because there are basically the same?

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.

+1.  Adding that "The idea here is" line would be wonderful. :-)


Hector Santos, CTO