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

John R Levine <johnl@taugh.com> Thu, 01 April 2021 16:02 UTC

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Date: Thu, 01 Apr 2021 12:02:07 -0400
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From: John R Levine <johnl@taugh.com>
To: Kristijonas Lukas Bukauskas <kr@n0.lt>
Cc: ietf-smtp@ietf.org
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References: <20210401003023.713A871BE253@ary.qy> <d8768a088e2e9b73682e3d9bdbb372d9@n0.lt> <b2acacaa-5bed-e332-1855-6ef0183cf42@taugh.com> <464756802f3425cb53c3be6392af4390@n0.lt>
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Subject: Re: [ietf-smtp] MTS-STS validation when MX host points to a CNAME, violating RFC 2181 § 10.3
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>>> {"organization-name":"Microsoft 
>>> Corporation","date-range":{"start-datetime":"2021-03-30T00:00:00Z","end-datetime":"2021-03-30T23:59:59Z"},"contact-info":"tlsrpt-noreply@microsoft.com","report-id":"132616914860181612+n0.lt","policies":[{"policy":{"policy-type":"sts","policy-string":["version: 
>>> STSv1","mode: enforce","mx: mx.n0.lt","max_age: 
>>> 84600"],"policy-domain":"n0.lt"},"summary":{"total-successful-session-count":0,"total-failure-session-count":492},"failure-details":[{"result-type":"certificate-host-mismatch","failed-session-count":492}]}]}
>>> Do they complain about the certificate which includes both n0.lt and 
>>> *.n0.lt anyways?
>> 
>> Even without the CNAME error, why would it do that?  The cert matches
>> the name of the MX.
>
> So certificate-host-mismatch here should be understood that EITHER §4.1 [1] 
> (MX Host Validation) OR §4.2 [2] (Recipient MTA Certificate Validation) 
> failed and not that, according to the report, the certificate hostname 
> mismatched? Or what exactly?

It's probably just giving you the wrong error message.

Here's a radical idea: fix your MX entry and see if the errors go away.

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

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