Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!bionet!agate!ucbvax!SH.CS.NET!jcurran
From: jcurran@SH.CS.NET (John Curran)
Newsgroups: comp.protocols.tcp-ip.domains
Subject: Re: Proposed extensions to MX records.
Message-ID: <9103290704.AA13621@ucbvax.Berkeley.EDU>
Date: 28 Mar 91 23:24:24 GMT
References: <9103281829.AA24020@mp.cs.niu.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The Internet
Lines: 36

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] The greatest risk is that there are hosts which don't treat the
] preference as "unsigned" 16 bit values.  My guess would be that if there
] are such hosts they probably have 'int' defined as 16 bit, and probably
] have lots of other problems with networking anyway.

If you were to ignore the high bit, you could avoid the signed/unsigned
existing interpetation problem by using the second MSB for "restricted MX" 
and thus limit the standard MX's values to 0-8192.  (ugly, but..)

Another way to approach the "restricted record" problem would be on a general
(radical) basis with an additional attribute for every record giving the 
applicable domain:

ANYHOST.FOO.BAR.  *.FOO.BAR. IN  MX   100   ANYHOST.FOO.BAR.         
                  *.FOO.BAR. IN  MX   200   OTHERHOST.FOO.BAR.
                             IN  MX   100   GW.FOO.BAR.

The magnitude of a change like this would require rolling out new resolver
code many months in advance of the new records, but would then allow for 
division of mail and nameservers by various regions:
 
LARGE.COM.      *.EAST.LARGE.COM.  IN  NS  EAST-NS.LARGE.COM.
LARGE.COM.      *.EAST.LARGE.COM.  IN  MX  EAST-NS.LARGE.COM.
LARGE.COM.      *.WEST.LARGE.COM.  IN  NS  WEST-NS.LARGE.COM.
LARGE.COM.      *.WEST.LARGE.COM.  IN  MX  WEST-NS.LARGE.COM.
LARGE.COM.                    *.   IN  NS  NS1.ANY.NET.
LARGE.COM.                    *.   IN  NS  NS2.ANY.NET.
LARGE.COM.                    *.   IN  MX  MAIL-GW.LARGE.COM.

I'm certainly NOT advocating a change of this magnitude, but I felt that
if we're considering restricting MX record usage in some manner, we ought
to examine the more general model for comparision.  (Imagine subdomain
delagations that only apply to particular hosts..)

/John