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Re: [dmarc-ietf] ARC questions

Brandon Long <blong@google.com> Mon, 23 November 2020 19:59 UTC

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From: Brandon Long <blong@google.com>
Date: Mon, 23 Nov 2020 11:59:02 -0800
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To: Michael Thomas <mike@mtcc.com>
Cc: John R Levine <johnl@taugh.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 11:55 AM Michael Thomas <mike@mtcc.com> wrote:

>
> 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.
>
Which works for one hop.

Brandon

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