Path: utzoo!attcan!uunet!mcsun!hp4nl!charon!piet
From: piet@cwi.nl (Piet Beertema)
Newsgroups: comp.mail.sendmail
Subject: Re: Local Configuration Error
Message-ID: <2234@charon.cwi.nl>
Date: 25 Sep 90 13:34:37 GMT
References: <6460@jhunix.HCF.JHU.EDU> <wayWTiX0BwwORNyX8k@transarc.com> <1990Sep21.161726.18266@mp.cs.niu.edu>
Sender: news@cwi.nl
Organization: CWI, Amsterdam
Lines: 40


	But what do you do when host A can only communicate on to its
	local network. Perhaps it has bad networking code. Perhaps there
	is a security concern. However B can talk to the local network
	and the world.

	  The possible solutions (using only SMTP) are:
	
	   1.  Have two MX records for host A.  Preference 0 delivers to A, and
	       preference 10 to B.  Anyone sending email from outside the local
	       network will timeout on trying to send to A, and will (we hope)
	       then send to B from where it will be forwarded.  I claim this is
	       a poor choice, for it makes the whole Internet suffer the need to
	       first time out on the primary MX record.
	
	   2.  Have MX records for A pointing only to B (and to similarly capable
	       hosts).   Make sure sendmail doesn't declare a configuration error
	       when the best MX preference is for the local (i.e. B) host, but
	       instead looks for an A-record.  This method works very simply and
	       transparently.  Sendmail-5.64, as distributed by Berkeley already
	       does this IF there is only a single MX record, but fumbles it if there
	       are two or more MX records.  (This is what started the discussion).
	
	   ......

That would all be fine iff DNS RR's would only represent hosts.
But they don't, they represent domains. Take this example:
cwi.nl.		MX 10	charon.cwi.nl.
		MX 20	piring.cwi.nl.
Here cwi.nl is not a host and thus it doesn't have an A record.
(The same would be true if cwi.nl would be a domain in the uucp
world and the MX records would point to a gateway).
If charon.cwi.nl wouldn't be able to deal with user@cwi.nl, that
would really be a local configuration error. Attempts to find an
A record would fail and lead to "550 Host unknown". That's what
I consider as unwanted because the MX record is there precisely
to avoid that. Besides, the error is less meaningful.

--
	Piet Beertema, CWI, Amsterdam	(piet@cwi.nl)