Re: [dmarc-ietf] Sender-supplied decision matrix for passing DMARC

Tobias Herkula <tobias.herkula@1und1.de> Mon, 14 June 2021 17:53 UTC

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From: Tobias Herkula <tobias.herkula@1und1.de>
To: Seth Blank <seth=40valimail.com@dmarc.ietf.org>, "Brotman, Alex" <Alex_Brotman=40comcast.com@dmarc.ietf.org>, "dmarc@ietf.org" <dmarc@ietf.org>
Thread-Topic: [dmarc-ietf] Sender-supplied decision matrix for passing DMARC
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Date: Mon, 14 Jun 2021 17:53:08 +0000
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Subject: Re: [dmarc-ietf] Sender-supplied decision matrix for passing DMARC
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This risks sendability with the fact that there are a lof of receivers that require SPF-RRs. So not providing SPF-RRs also fails with such an requirement. Besides that does SPF not help with any kind of 5322.From spoofing, but this ist he most important identifier for an enduser.

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Von: dmarc <dmarc-bounces@ietf.org> Im Auftrag von Seth Blank
Gesendet: Montag, 14. Juni 2021 19:45
An: Brotman, Alex <Alex_Brotman=40comcast.com@dmarc.ietf.org>rg>; dmarc@ietf.org
Betreff: Re: [dmarc-ietf] Sender-supplied decision matrix for passing DMARC

HUGE cringe ;-) DMARC has an explicit policy that either SPF or DKIM must pass aligned. This proposal breaks that foundationally.

This is suggested quite frequently, but fails to understand just how few senders of email actually send with DKIM. Most email is sent from services that have a core business that's not in email, and when we're lucky, they manage to publish an SPF record for their customers to use. Only large volume sophisticated services tend to do DKIM.

A domain owner that requires everything that sends on its behalf to use DKIM basically shoots itself in the foot, and makes most of the services they'd need to use unavailable to themselves.

The correct answer is what you said: domain owners who want this should only authenticate services using DKIM.

Seth


On Mon, Jun 14, 2021 at 10:10 AM Brotman, Alex <Alex_Brotman=40comcast.com@dmarc.ietf.org<mailto:40comcast.com@dmarc.ietf.org>> wrote:
Hello,

I was talking to some folks about DMARC, and a question came as to suggest as the domain holder that your messages should always pass DKIM.  Effectively, the asker wants to say "I intend to deploy SPF and DKIM, but I will *always* sign my messages with DKIM."  So the obvious answer may be "Just only use DKIM", but I'm not sure that completely answers the question.  While discussing with someone else, "Tell me when DKIM fails, but SPF is fully aligned".  There was recently an incident at a provider where they were allowing any sender to send as any domain (and I'm aware that's not specifically a DMARC issue).  We all know brands that have just dumped in a pile of "include" statements without fully understanding the implications.  In this case, other users could send as other domains, but perhaps they would not have been DKIM signed.  Should there be a method by which a domain holder can say "We want all message to have both, or be treated as a failure", or "We'll provide both, but DKI
 M is a must"?

>From a receiver side, it makes evaluation more complex.  From a sender side, it gives them more control over what is considered pass/fail.

How does this look in practice?  Maybe "v=DMARC1;p=quarantine;rua=...;pm=dkim:must,spf:should;"
(pm=Policy Matrix)

Does this make everyone cringe, or perhaps worth a larger discussion?

--
Alex Brotman
Sr. Engineer, Anti-Abuse & Messaging Policy
Comcast

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