Re: [ietf-smtp] MTA-STS validation when MX host points to a CNAME, violating RFC 2181 § 10.3

John Levine <johnl@taugh.com> Thu, 01 April 2021 01:44 UTC

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From: "John Levine" <johnl@taugh.com>
To: ietf-smtp@ietf.org
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Subject: Re: [ietf-smtp] =?utf-8?q?MTA-STS_validation_when_MX_host_points_to_?= =?utf-8?q?a_CNAME=2C_violating__RFC_2181_=C2=A7_10=2E3?=
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It appears that Viktor Dukhovni  <ietf-smtp@ietf.org> said:
>Indeed their implementation is flawed.  They should either streadfastly
>refuse to deliver email to all domains where the the MX is a CNAME, and
>so outside the interoperability scope of RFC5321.  Or, if like most
>MTAs, they're pragmatic and allow CNAMEs, the CNAME should not affect
>the logical MX host name used with MTA-STS, just as would be the case
>with web browsers, etc.

It seems to me pragmatic to say "we think we understand what you think you're doing,
but it's still wrong."

R's,
John