Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!samsung!caesar.cs.montana.edu!ogicse!blake!Tomobiki-Cho!mrc
From: mrc@Tomobiki-Cho.CAC.Washington.EDU (Mark Crispin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Host requirements and SMTP
Message-ID: <5238@blake.acs.washington.edu>
Date: 5 Jan 90 22:10:22 GMT
References: <24861.631484091@cs.nott.ac.uk>
Sender: news@blake.acs.washington.edu
Organization: Mendou Zaibatsu, Tomobiki-Cho, Butsumetsu-Shi
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In article <24861.631484091@cs.nott.ac.uk> j.onions@computer-science.nottingham.ac.UK (Julian Onions) writes:
>Section 5.2.17 
>Urgghhhhh! This is awful. Making this a MUST is horrible. Domain
>literals should be stamped out. Parsing a domain literal is ok,
>getting the semantics right is yucky in the extreme, it should at the
>maximum have been a MAY. Next thing we'll need to recognise our own
>ethernet addresses in mail messages.  So to be conformant you HAVE to
>be able to discover all your current IP addresses and to match on
>them. Does this really have to be a MUST? How many current systems can
>accept this syntax? I think this will be my biggest stumbling block to
>conformance.

You must recognize a domain literal that refers to yourself.  There
are reasons for using domain literals, including being able to get
through to a working IP address that can't (for some reason) be
accessed by a name.  There are any number of reasons for this
including DNS failure, inconsistent DNS information, or no DNS!!!

There have been abundant problems because of stupid stupid mailers
that fail to recognize one of their own IP addresses in a domain
literal and eventually get into a loop or bounce the message.

A few years ago, the TOPS-20 mailer was stuck with a DNS resolver
which did not offer reasonable canonicalization or MX access.  The
only way it could get the canonical form of a name (as opposed to a
possible non-fully-qualified name or CNAME) was to look up the
IN-ADDR.ARPA PTR record for that IP address.  It couldn't MX at all.
The problem is that many hosts did not have IN-ADDR records or the
information was inconsistent.  If it could not get an IN-ADDR record,
it substituted the domain literal rather than inflict a name that
someone else may not recognize on the rest of the world.

Today, TOPS-20 systems have a reasonable resolver to talk to that
supports DNS-level canonicalization as well as MX.  So these systems
tend to use domain literals a lot less.  However, they still support
domain literals as an escape mechanism.

Consider the modern case.  I try to mail to you.  However, the DNS
server for you is down and has been down for several days.  I know
your IP address, I can telnet to you, but I can't mail to you.  Even
though my mailer knows that your address is "unresolvable" instead of
"bad" and keeps on trying for a few days, eventually it returns the
message as undeliverable.  Well, damnit, I should be able to mail to
you using your IP address.

Well, I can.  But if you have a stupid mailer that sees the message as
a "mail to IP address a.b.c.d" and passes it on without realizing that
you *ARE* a.b.c.d the message is in a loop.

The correct way to handle domain literals is to consider both a host
name and a domain literal as a single atomic entity with a name
property and an IP address property.  When dealing with mailbox at
this atomic entity, you have to check to see if this entity is
yourself.  So, the name property, when canonicalized, should be the
same as your local name.  You simply have to implement the concept of
"canonicalizing" for a domain literal.  That is, finding out, if
possible, what host the domain literal refers to and substituting that
name.  Presumably, if the domain literal refers to one of your own IP
addresses you should be able to substitute your local name!!!!!!!!

Really, this is pretty important.  Too many mailers implement this
wrong.
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