Re: general question about failed DNS request when looking up MXs
Ned Freed <Ned.Freed@innosoft.com> Tue, 14 January 1997 01:47 UTC
Received: from cnri by ietf.org id aa06055; 13 Jan 97 20:47 EST
Received: from mail.proper.com by CNRI.Reston.VA.US id aa27238; 13 Jan 97 20:47 EST
Received: (from majordomo@localhost) by mail.proper.com (8.8.4/8.7.3) id RAA16489 for ietf-smtp-bks; Mon, 13 Jan 1997 17:22:27 -0800 (PST)
X-Authentication-Warning: mail.proper.com: majordomo set sender to owner-ietf-smtp@imc.imc.org using -f
Received: from THOR.INNOSOFT.COM (THOR.INNOSOFT.COM [192.160.253.66]) by mail.proper.com (8.8.4/8.7.3) with ESMTP id RAA16484 for <ietf-smtp@imc.org>; Mon, 13 Jan 1997 17:22:24 -0800 (PST)
Received: from INNOSOFT.COM by INNOSOFT.COM (PMDF V5.1-6 #8694) id <01IE62J9I2SGA8DRXP@INNOSOFT.COM> for ietf-smtp@imc.org; Mon, 13 Jan 1997 17:22:06 PST
Date: Mon, 13 Jan 1997 16:47:27 -0800
From: Ned Freed <Ned.Freed@innosoft.com>
Subject: Re: general question about failed DNS request when looking up MXs
In-reply-to: "Your message dated Mon, 13 Jan 1997 19:19:43 -0500" <3.0.32.19970113191942.00f39a90@lacroix>
To: Jack De Winter <jack@wildbear.on.ca>
Cc: ietf-smtp@imc.org
Message-id: <01IE67O1A1PSA8DRXP@INNOSOFT.COM>
MIME-version: 1.0
Content-type: text/plain; charset="us-ascii"
Sender: owner-ietf-smtp@imc.org
Precedence: bulk
> Okay, here's a general question for the group at large: > You are doing an MX request to resolve an address that > you need to send to. You get back a Server Error (RCODE=2) > or an illegal return code (RCODE=6..15). What is the best > thing to do to try and resolve this? You have to abort the delivery attempt and try again later (temporary error). No other course of action is acceptable, really -- there is nothing that says that trying the host directly (even assuming you can get its A record when you can't get its MX record list) will produce correct results. Even if you're willing to enter a special mode where you treat any errors you get under these circumstances as temporary, it won't work in general. There are cases where the host in user@host being an MX pointing to several other systems ends up resolving to an entirely different mailbox than user@host where host is treated as simply an A record. There are also cases where direct delivery ignoring MX leads to loops. (Yes, such setups are problematic given the fact that not everything on the Internet is consistent in its support of MX, but such setups exist nevertheless and have to be dealt with.) The bottom line is that RFC974 says that when an the DNS returns a non-empty MX list the client is supposed to try all the hosts on that list and then stop, and setups have been built and are actively being used that take advantage of every nuance of this described behavior. So if you cannot get the MX list you don't know what you should or should not do, so your only course of action is to not do anything. Ned
- general question about failed DNS request when lo… Jack De Winter
- Re: general question about failed DNS request whe… Ned Freed