Re: [dmarc-ietf] ARC questions

Michael Thomas <mike@mtcc.com> Mon, 23 November 2020 19:55 UTC

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To: Brandon Long <blong@google.com>
Cc: John R Levine <johnl@taugh.com>, "dmarc@ietf.org" <dmarc@ietf.org>
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From: Michael Thomas <mike@mtcc.com>
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Date: Mon, 23 Nov 2020 11:55:24 -0800
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Subject: Re: [dmarc-ietf] ARC questions
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On 11/23/20 11:49 AM, Brandon Long wrote:
>
>     I imagine that the vast majority of intermediaries that break
>     signatures
>     number exactly one extra domain, so it's not very hard to reconstruct
>     the chain of custody from origin to destination. Assuming the
>     intermediary resigns with the incoming auth-res, the destination can
>     choose to believe that auth-res or not, right? Since this is an
>     experiment, do we have an idea of what the rest of the problem is
>     after
>     the typical mailing list-like signature breakers are excluded?
>
>
> No, as in the RFC says to remove them, so it's a standard part of 
> implementation.
>
> RFC 7601 4.1:
>
>     instances of the header field that appear to originate within the
>     ADMD but
>
>     are actually added by foreign MTAs will be removed before delivery.
>
>
> That's very different than "just maybe it might be removed"
>
The receiving MTA in the next domain doesn't have to discard the 
information before removing it. The act of removing it is so there isn't 
confusion about the ultimate auth-res, especially with MUA's. The 
incoming MTA is free to consider the previous auth-res just like it's 
free to consider the previous arc auth-res.

Mike