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

Juri Haberland <juri@sapienti-sat.org> Thu, 23 April 2020 07:38 UTC

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Date: Thu, 23 Apr 2020 09:38:18 +0200
From: Juri Haberland <juri@sapienti-sat.org>
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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.

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


   Juri

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