Subject: [Asrg] RMX and aliases

From: [email-masked]
Date: Tue, 6 May 2003 16:40:28 -0400

RMX and other reverse MX proposals ALMOST meet the "useful to many,
painful to nobody" criteria for success.

Its pairwise agreement between sending MTAs and domain owners is CLOSE
to being mutual and not disruptive to anyone else;

EXCEPT that it interferes with the everyday common use of relaying via
aliases (with no rewriting of SMTP envelope-from), which means that:
	IF my domain starts publishing an RMX record,
	THEN mail from my users can no longer reliably be forwarded
	without an envelope sender re-write,
	SO ALL of my users suddenly can't reach anyone who uses (e.g.)
	a .forward file pointing to any RMX-aware MTA, and they have to
	send out notes to the effect that "sorry, but I can't reach you
	any more unless you adopt something new" but, well, those notes
	can't reach their intended recipients either.

While it may be a "good idea" to abolish aliases and .forward files,
actually doing it is a traumatic transition, and it is one that is
NOT under the control of the RMX publisher or the RMX-obeying MTA, but
depends on the cooperation of the vast unwashed body of RMX-non-adopters.

Isn't this a serious impediment at least short-term?
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From: "Alan DeKok" <[email-masked]>
Date: Tue, 06 May 2003 17:27:21 -0400

> 	can't reach their intended recipients either.

  <claps loudly>

  That's be the best objection to RMX I've seen yet.  Detailed,
quantitative, iterates over possible scenarios, and totally lacking in
misunderstanding of the proposal.

> depends on the cooperation of the vast unwashed body of RMX-non-adopters.

  I agree.

> Isn't this a serious impediment at least short-term?

  It would appear, so long as envelope sender re-writing is not done.

  Now let's look at the problems with the current use of .forward
files.  You receive a message from an MTA in example.com, with the
envelope & body "from" being example-2.com.

  Q1: Is it spam?
  A1: You don't know.

  Q2: Is it non-spam?
  A2: You don't know.

  Q3: Is the user at example-2.com permitted to use the MTA in example.com?
  A3: By implication, it would appear so.

  Q4: How can you tell this situation from open relays, "owned"
     machines, etc?
  A4: You can't.

  In keeping with the charter of this group, I will now discuss
consent.  Within the context of SMTP, RMX allows the originating
domain and MTA do explicitely publish their consentual relationship.
In this case, if envelope sender re-writing is done, we can use RMX to
establish that the domain 'example.com' consented to have that
particular MTA send an email with a body "from" of 'example-2.com'.

  In this case, the answers to questions 1 and 2 stay the same.  The
answer to question 3 now becomes an explicit "yes, consent exists."
The answer to question 4 becomes "the originating MTA is not an open
relay or 'owned' machine, unless the attacker has additionally taken
over the DNS records."

  So the solution to .forward files, is to have the forwarding MTA
explicitely declare it's consent to forwarding messages for another
domain.

  In contrast, the simple act of forwarding messages tells you
nothing about consent, other than that forwarding is occuring.

  Alan DeKok.

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From: Hadmut Danisch <[email-masked]>
Date: Tue, 6 May 2003 23:12:49 +0200

> Isn't this a serious impediment at least short-term?

I have addressed this in my draft. You have to distinguish 
between trusted and untrusted forwarding.

Those common methods of .forward mechanisms without sender rewriting
(in untrusted environment) can't be accepted any longer.

If you permit everybody to forward everything with any given 
sender address, then there is no technical difference between 
forging and forwarding. If you insist on allowing any machines to 
forward without sender rewriting, then every spammer can pretend 
to be a forwarder for the given spam message.

Hadmut

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From: Michael Rubel <[email-masked]>
Date: Tue, 6 May 2003 14:58:20 -0700 (PDT)

> depends on the cooperation of the vast unwashed body of RMX-non-adopters.

Good point--I have added a note about this here:

	http://www.mikerubel.org/computers/rmx_records/#notes_forwarding/

under "What are the implications for .forward?"

Mike

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From: J C Lawrence <[email-masked]>
Date: Tue, 06 May 2003 18:04:43 -0700

On Tue, 06 May 2003 17:27:21 -0400 
Alan DeKok <[email-masked]> wrote:

> Now let's look at the problems with the current use of .forward files.
> You receive a message from an MTA in example.com, with the envelope &
> body "from" being example-2.com.

Forward chained received headers solve the same problem instance, as
well as providing reverse transparency down the full path of the message
(assuming supporting MTAs).

> explicitely declare it's consent to forwarding messages for another

There's a non-starter in the mailing list world.

> In contrast, the simple act of forwarding messages tells you nothing
> about consent, other than that forwarding is occuring.

Unless an embedded consent protocol is involved (cf TMDA).

-- 
J C Lawrence                
---------(*)                Satan, oscillate my metallic sonatas. 
[email-masked]               He lived as a devil, eh?		  
http://www.kanga.nu/~claw/  Evil is a name of a foeman, as I live.
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