From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
From: neitzel@gaertner.de (Martin Neitzel)
Message-ID: <HCs2zs.8rH@gaertner.de>
References: <955b04dd781fc8ef726c1f33aef706db@plan9.bell-labs.com>
Subject: Re: [9fans] rewriting From:
Date: Fri,  4 Apr 2003 13:25:49 +0000
Topicbox-Message-UUID: 89cd5b88-eacb-11e9-9e20-41e7f4b1d025

DP> I've noticed over the years that if the domain name in a From: address has
DP> a CNAME, then it gets rewritten by most sendmails.  Anyone know if there
DP> is an RFC that covers the subject?

I'm too lazy to dig through the new versions (e.g. 2821) but
these venerable ones should be good enough:  rfc 821 + 1123.

RFC 821 "SMTP"
	3.7, "Domain Names":
	Whenever domain names are used in SMTP only the official names are
	used, the use of nicknames or aliases is not allowed.

RFC 1123 "Requirements for Internet Hosts" aka STD0003:
	5.2.2  Canonicalization   [...]
	The domain names that a Sender-SMTP sends in MAIL and RCPT
	commands MUST have been  "canonicalized," i.e., they must be
	fully-qualified principal names or domain literals, not
	nicknames or domain abbreviations.  A canonicalized name either
	identifies a host directly or is an MX name; it cannot be a
	CNAME.

They refer, stricly speaking, to the envelope adresses, but this is of
course tightly coupled to header addresses, too.  (Tightly enough to
warrant header rewritings as soon as possible, IMHO).

Closely related:  MXs may not point to CNAMEs, only canonical names.
(Should be in the DNS RFC.)

					Hope this helps, Martin