Date: 2 Aug 1996 13:37:37 -0000
Message-ID: <19960802133737.23886.qmail@koobera.math.uic.edu>
From: djb@koobera.math.uic.edu (D. J. Bernstein)
To: drums@cs.utk.edu
Subject: MX loop example

Here's an example illustrating that the RFC 974 loop-prevention rules
aren't strict enough.

Right now, mail to bytedesigns.com is looping and bouncing. Here's the
MX list:

   50 bdserv.byted.com (unreachable right now)
   75 mailhost.wimsey.com (unreachable right now)
   100 mail.wimsey.com
   200 dewey.mindlink.net

mail.wimsey.com has an A record of 204.191.152.4. So does
mail.vcr.istar.ca. (This is an example of multinaming.)

Under RFC 974, when the host at 204.191.152.4 receives mail for
bytedesigns.com, it has to look for LOCAL in the MX list. What's LOCAL?
It could be either mail.wimsey.com or mail.vcr.istar.ca---neither of
these is an alias, so they're both acceptable choices for LOCAL under
RFC 974.

As it turns out, the host uses mail.vcr.istar.ca as LOCAL. (There's also
a PTR record listing this as the primary name of the host.) It doesn't
notice mail.wimsey.com in the MX list. A loop occurs.

Nothing in this situation violates the RFC 974 recommendation that
aliases not appear in MX lists---mail.wimsey.com is not an alias! It's
not true, as RFC 974 claims, that this prevents loops.

My RFCLOOPS rules, which have been censored by Harald Alvestrand, handle
this case (in fact, all cases) properly, without making any restrictions
on the MX list. See ftp://koobera.math.uic.edu/pub/docs/RFCLOOPS,
section 3.2.

---Dan