Subject: regarding MX records

From: Keith Moore <[email-masked]>
Date: Thu, 11 Mar 1999 18:58:54 -0500

My take on MX records:

- They are used by mail domains to advertise how to route incoming
  mail to their domains, in the absence of explicit agreements
  to route mail some other way.   Explicit agreements take precedence,
  but in the absence of an explicit agreement an Internet MTA should
  use MX records to route to domains for which they are not themselves
  primary MXes.

- They are used only to advertise routing for envelope email addresses.
  If you have your MTA or MUA configured to route outgoing mail to
  a particular host, A records (not MX) are used to find the IP
  address of that host.  In the future SRV and/or SLP might also be 
  used.

I believe this is both architecturally sound and consistent with
long-established practice.

Keith

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From: Philip Hazel <[email-masked]>
Date: Fri, 12 Mar 1999 09:24:58 +0000 (BST)

> long-established practice.

I think this covers what a lot of us have been trying to say on this 
subject recently, and I agree entirely. I would only add that CNAME
records are another possibility for finding the address of a specific 
host.

[If you like the idea of using MX for "smarthost" type routing, you can 
think of it as routing by using a different mail domain, rather than to 
a specific host: "route all mail for domain xxxx as if it were addressed 
to domain yyyy". This of course also comes under the head of private
agreement.]

As for whether explicit routing is or is not conforming: I predict that 
we (and those that read 821bis when it is promulgated) will remain
arguing about this for ever, because the document wraps up the "on-the-
wire" protocol with the general route-discovery algorithm in a single 
package. If they were split into to independent definitions, then it 
would be easier to describe what specific hosts or software packages do. 
However, that position seems to have been lost.

How should one describe a host/MTA which is fully-conformant *some of 
the time* - namely, when it is processing an address for which it does 
MX-lookups, but not *all* the time, because for some addresses it uses 
private agreement to determine routing (and both kinds of address may 
appear in a single message)? It's going to take several paragraphs of 
explanation...

-- 
Philip Hazel            University of Cambridge Computing Service,
[email-masked]      Cambridge, England. Phone: +44 1223 334714.

---

From: John C Klensin <[email-masked]>
Date: Wed, 17 Mar 1999 09:43:28 -0500 (Eastern Standard Time)

> appear in a single message)? It's going to take several paragraphs of 

At present, the document (mostly inadvertently and 
for historical reasons) skirts the question of, and is not 
completely consistent about, whether it describes a 
conforming implementation or conforming behavior.  In this 
case, at least, it needs to focus on the latter, with the 
clear understanding that something might be configured into 
conforming behavior in some situations and into private- 
agreement (non-conforming or out of the scope of the 
specification) in others.

FWIW, strict "on the wire" specifications, are, of 
necessity, about conforming behavior and not about 
conforming implementations except insofar as we define a 
conforming implementation as one which is capable of being 
configured to exhibit conforming behavior.  I would claim 
the latter is more or less the approach taken by 1123.

    john

---

From: Brad Knowles <[email-masked]>
Date: Wed, 17 Mar 1999 16:02:35 +0100

>appear in a single message)? It's going to take several paragraphs of 

    Private agreements can take precedence over MX processing.  This does
not necessarily invalidate ones compliance or full conformance.

-- 
  These are my opinions -- not to be taken as official Skynet policy
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