Re: [dmarc-ietf] ARC questions

Michael Thomas <mike@mtcc.com> Mon, 23 November 2020 20:45 UTC

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To: John R Levine <johnl@taugh.com>
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From: Michael Thomas <mike@mtcc.com>
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Date: Mon, 23 Nov 2020 12:45:48 -0800
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Subject: Re: [dmarc-ietf] ARC questions
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On 11/23/20 12:29 PM, John R Levine wrote:
>> 1) A mailing list creates an auth-res on the incoming mail to the list
>>
>> 2) It modified the message
>>
>> 3) It resigns the message with DKIM
>>
>> 4) It is then delivered to the subscriber's mail server
>>
>> 5) The destination mail server can look at the incoming message 
>> including the mailing list's auth-res and decide whether to trust it 
>> or not just like ARC.
>>
>> It seems to me this covers the vast majority of cases. What are the 
>> other cases where this is not sufficient and how significant are they 
>> in reality?
>
> Two or more levels of forward are quite common, particularly in large 
> mail systems.  Look at mail coming out of Google and Microsoft's 
> hosted mail and you'll see a lot of ARC headers.
>
> Considering that the ARC RFC was published over a year ago, and it is 
> implemented all over the place, could you explain what the point of 
> this discussion is?  The people who designed ARC are not idiots.  If 
> we could have fixed the mailing list problem with existing DKIM 
> signatures, we would have.
>
Then why is it not standards track? And am I to understand that I'm not 
allowed to comment on an experiment? Perhaps the working group chairs or 
AD can clarify that.

In any case, if this all boils down to whether I trust the intermediary 
who resigned the message, then that is a previously solved problem: you 
can base the reputation check based on the resigned signature. I'm not 
entirely sure what the value of  the previous auth-res is. If I recall 
correctly, the document was sort of short on what the specific utility 
is, but I may have missed it.

Mike