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From: matt@oddjob.UChicago.EDU (Matt Crawford)
Newsgroups: comp.mail.misc
Subject: Re: MX records
Message-ID: <13919@oddjob.UChicago.EDU>
Date: Sun, 2-Aug-87 20:33:34 EDT
Article-I.D.: oddjob.13919
Posted: Sun Aug  2 20:33:34 1987
Date-Received: Sun, 2-Aug-87 23:50:59 EDT
References: <801@hao.UCAR.EDU> <2017@aw.sei.cmu.edu>
Reply-To: matt@oddjob.uchicago.edu (I am not a Grook)
Organization: Morse Science High School
Lines: 60

In article <801@hao.UCAR.EDU> woods@hao.UCAR.EDU (Greg Woods) writes:
>>Now that we have a running named(8) name server, and a version of 
>>sendmail(8) capable of querying it, I'd like to make use of it.

And pdb@sei.cmu.edu (Pat Barron) says:
>An MX record simply specifies the name of another site that can accept mail
>addressed to your site, along with a preference value that specifies what
>order to try mail exchangers if there are more than one.

Let me elaborate a little on what Pat said.

Let's say that an MX-using host S is trying to send mail to a
destination D.  It finds all the MX records for D.  If none of these
name D itself as a mail-exchanger for D, then S will not attempt to
connect to D directly.

If S itself is listed as a mail-exchanger for D, with some preference
P, then S will only connect to a mail exchanger whose preference is
strictly less than P.  

If that last rule leaves no eligible mail-exchangers for D (or if
there were no MX records to begin with), then S may (must?) pretend
that there was an MX record listing D as a mail-exchanger with
preference zero.

Not that you don't need to do anything funky in sendmail.cf to use
the MX records.  If ruleset zero resolves to host D, then code in
deliver.c will look up MX records for D.

Now for two named caveat and some cute tricks.  

It is awfully tempting to put a wildcard MX record in the data for
your domain.  This is usually not as useful as you think.  Wildcard
records are only returned to a querant if there is no other record of
any type for the name asked about.

If the mail-exchanger listed for some host is itself a nickname for
something, that is if it is the subject of a CNAME record, you may
create a forwarding loop.

Here at the university of Chicago we have a high speed but unreliable
internet connection, and a lower speed connection.  We list the host
which owns the low speed connection as an MX with preference 100 for
every mail host in our domain.  That host is "dumb" and always
forwards internal mail to another host, gargoyle, so to avoid loops
we must list gargoyle as an MX for every host and give it a lower
preference.  Thus a typical host's MX records look like:

chip.UChicago.EDU	preference = 0, mail exchanger = chip.UChicago.EDU
chip.UChicago.EDU	preference = 10, mail exchanger = sphinx.UChicago.EDU
chip.UChicago.EDU	preference = 99, mail exchanger = gargoyle.UChicago.EDU
chip.UChicago.EDU	preference = 100, mail exchanger = cypress.UChicago.EDU

When our primary link is down, only host cypress.UChicago.EDU is
reachable from off-campus.  Thus mail from MX-using hosts will still
get in.  (Yes, we do have an off-campus secondary server for our
domain!)
________________________________________________________
Matt	     University		matt@oddjob.uchicago.edu
Crawford     of Chicago     {astrovax,ihnp4}!oddjob!matt