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Re: [dmarc-ietf] ARC questions

Brandon Long <blong@google.com> Mon, 23 November 2020 19:34 UTC

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From: Brandon Long <blong@google.com>
Date: Mon, 23 Nov 2020 11:34:25 -0800
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To: John R Levine <johnl@taugh.com>
Cc: Michael Thomas <mike@mtcc.com>, "dmarc@ietf.org" <dmarc@ietf.org>
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Subject: Re: [dmarc-ietf] ARC questions
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On Mon, Nov 23, 2020 at 11:28 AM John R Levine <johnl@taugh.com> wrote:

> > From what I can tell, the main thing that ARC is doing is binding an
> auth-res
> > to a dkim signature-like thing. But as I recall -- it's been a long time
> --
> > there were ordering requirements ala received headers for where new
> > dkim-signatures and auth-res go in the header. Assuming my memory is
> correct,
> > that means you can reconstruct "what this looked like before i messed
> with
> > it" already by signing the incoming auth-res as part of the new DKIM
> > signature.
> >
> > Is there something more going on here?
>
> Not really.  There are ordering rules but mail systems do not follow them
> reliably, DKIM signatures in practice are not ordered.  Also, A-R can be
> deleted in some situations, so ARC makes copies of them to be more robust
> in transit.
>

Right, A-R is explicitly called out to be deleted in the RFC on inbound if
the admd
matches (so someone can't fake an A-R to you).  Even if it's not fake, a
message
can go through multiple hops through the same admd and each hop have a
different
A-R result, but there is no mechanism for multiple A-Rs for the same admd
having
any ordering or such.

>From the other direction, one could say that ARC is a superset of A-R and
DKIM with
different purpose, and you might be able to subsume them into ARC, but you
couldn't
build ARC out of the originals.

Brandon

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