Path: utzoo!utgpu!watserv1!watmath!att!linac!uwm.edu!ux1.cso.uiuc.edu!mp.cs.niu.edu!rickert
From: rickert@mp.cs.niu.edu (Neil Rickert)
Newsgroups: comp.protocols.tcp-ip.domains
Subject: Re: Domain names in resource records (was: PTR records ...)
Message-ID: <1991Jan17.210309.15170@mp.cs.niu.edu>
Date: 17 Jan 91 21:03:09 GMT
References: <1120@nikhefh.nikhef.nl> <20520@yunexus.YorkU.CA>
Organization: Northern Illinois University
Lines: 30

In article <20520@yunexus.YorkU.CA> davecb@yunexus.YorkU.CA (David Collier-Brown) writes:
>e07@nikhefh.nikhef.nl (Eric Wassenaar) writes:
>| RFC1034, 3.6.2 Aliases and canonical names, page 15, states:
>| "Domain names in RRs which point to another name should always
>| point at the primary name and not the alias."
>
>  I've requested the exact opposite of our network manager in at least one
>case:  mailhub.yorku.ca (a CNAME) is the target of an MX record
>
>  I wonder if the community can comment:
>	1) do MX->CNAME->A record chains break any application
>		using MX records?

 Sendmail, and perhaps other mailers, break on this under certain circumstances.
The problem occurs when the MX record is not the best preference.  Assume that
the host named in the A-record has mail for the MX address.  It attempts to
send to the best preference MX record, but fails because the host is
down.  It next tries to send to the second preference.  Before doing so, it
compares the name with its own host name to be sure it is not sending to
itself.  Because the name on the MX is the name of the CNAME record, and not
of the A record, it does not recognize the name as its own.  It therefore
tries sending to itself, discovers the problem, aborts and bounces the mail.

 I believe the problem is not in sendmail, but in the MX -> CNAME.

-- 
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  Neil W. Rickert, Computer Science               <rickert@cs.niu.edu>
  Northern Illinois Univ.
  DeKalb, IL 60115                                   +1-815-753-6940