Re: [ietf-smtp] CNAME considered harmful, was MTS-STS validation when MX host points to a CNAME

John R Levine <johnl@taugh.com> Sun, 04 April 2021 17:00 UTC

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Date: 4 Apr 2021 13:00:43 -0400
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From: "John R Levine" <johnl@taugh.com>
To: "John C Klensin" <john-ietf@jck.com>, ietf-smtp@ietf.org
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Subject: Re: [ietf-smtp] CNAME considered harmful, was MTS-STS validation when MX host points to a CNAME
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> No, and my apologies if parts of what follow sounds like a rant.

Thanks, I'd forgotten about the RFC 1123 language.

> Once upon a time, we used to try to design protocols so that the
> functionality that was needed was available but that the number
> of different ways to do something was minimized, more or less on
> the assumption that two or three ways to do the same thing
> created opportunities for errors, ...

Yes indeed.  In retrospect, CNAME was a mistake.  If you look at RFC 
1034, you can see that the motivation for CNAME was to provide short 
local versions of names and temporary forwarding when a host name changes. 
But now it's mostly used to transfer the management of a name to someone 
else.

The normal way to do that is with a zone cut, and I think that most 
applications of CNAME would better be done with NS.  There are two 
differences: a zone cut needs to know what name is pointing at it and a 
zone cut covers all names below the redirected one while a CNAME doesn't, 
but in my experience, the situations where that matters generally have 
other problems.

Regards,
John Levine, johnl@taugh.com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly