Subject: Re: [yam] [Technical Errata Reported] RFC6409 (3995)

From: Tony Finch <[email-masked]>
Date: Tue, 3 Jun 2014 17:40:02 +0100

John C Klensin <[email-masked]> wrote:
>
> Actually, something else just occurred to me.  I don't think it
> changes the "verified" answer and I can't remember why Randy and
> I left the prohibition there when it was removed from SMTP.

The text in section 8.7 that I reported the problem with is new in RFC
6409 - it was not present in RFC 4409. It isn't something you "left". It
looks like it was added to provide more rationale for the MAY provision in
that section.

The MAY is there (I think) because Sendmail does the rewrite that the MAY
permits, and most other MTAs/MSAs do not. In RFC 4409 the rationale simply
discourages MSAs from rewriting, which is consistent with the change in
DRUMS several years earlier to remove CNAME canonicalization, and which
agrees with the argument you laid out in the message I am replying to.

The error is that RFC 6409's expanded rationale is completely backwards.
It makes it sound like the MAY is giving MSAs permission not to rewrite
when it would otherwise be required by RFC 5321. However there is no such
requirement in RFC 5321, and the MAY is actually giving MSAs permission to
rewrite even though it is not necessary and can be harmful.

So the point of my suggested replacement text is to explain the historical
background which has led to some MSAs doing a rewrite which we now
consider to be misguided. Yes it is suboptimal - it could indeed be
improved by expanding pronouns as Randall suggested, and I should not have
left out the perfectly correct sentences that came from RFC 4409.

Tony.
-- 
f.anthony.n.finch  <[email-masked]>  http://dotat.at/
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