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Re: [dmarc-ietf] Composition Kills: A Case Study of Email Sender Authentication

ned+dmarc@mrochek.com Thu, 23 April 2020 14:40 UTC

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From: ned+dmarc@mrochek.com
Cc: dmarc@ietf.org
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Date: Thu, 23 Apr 2020 07:34:06 -0700
In-reply-to: "Your message dated Thu, 23 Apr 2020 09:38:18 +0200" <74a807f52a59fa87053fcc76aaa28a73@sapienti-sat.org>
References: <20200423002020.BAB07181BB09@ary.qy> <74a807f52a59fa87053fcc76aaa28a73@sapienti-sat.org>
To: Juri Haberland <juri@sapienti-sat.org>
Archived-At: <https://mailarchive.ietf.org/arch/msg/dmarc/VcgSrqcgOAFhFwdW1Fm1_-auWg4>
Subject: Re: [dmarc-ietf] Composition Kills: A Case Study of Email Sender Authentication
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> On 2020-04-23 02:20, John Levine wrote:
> > In article <51be5654-94c4-38c6-8f6b-dca403d6680a@dcrocker.net> you
> > write:

> >> The paper asserts that AR is used as DMARC input.  I suspect that is
> >> rarely, if ever, true.  Yes? No?
> >
> > I'd say never.  To do DMARC rejects you have to do all of the
> > validation in the SMTP daemon, which is before anything has a chance
> > to create an A-R header.

> Of course this is possible with the Milter protocol introduced by
> Sendmail and used also by Postfix. The mail traverses during the SMTP
> phase through different milters, e.g. a SPF milter, followed by a DKIM
> milter, and every milter injects an AR header with its results. The last
> milter is a DMARC milter that processes the AR headers and signals the
> SMTP daemon do either accept or reject the message. This is how OpenDKIM
> & OpenDMARC work together.

This is done all the time, and not just by Sendmail and Postfix.

> Other projects do it all in one milter (rspamd?)...

Yes, but when you do it all in one go it's more difficult to customize.

				Ned

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