Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions
Alessandro Vesely <vesely@tana.it> Wed, 02 December 2020 11:11 UTC
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From: Alessandro Vesely <vesely@tana.it>
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Date: Wed, 02 Dec 2020 12:11:31 +0100
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Subject: Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions
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On Wed 02/Dec/2020 03:14:46 +0100 Brandon Long wrote: > On Tue, Dec 1, 2020 at 2:37 AM Alessandro Vesely <vesely@tana.it> wrote: >> On Tue 01/Dec/2020 05:56:46 +0100 Brandon Long wrote: >>> On Thu, Nov 26, 2020 at 12:59 AM Alessandro Vesely <vesely@tana.it> wrote: >>>> On 25/11/2020 20:16, Michael Thomas wrote: >>>>> On 11/25/20 11:11 AM, Alessandro Vesely wrote: >>>>>> On 25/11/2020 19:24, Jesse Thompson wrote: >>>>>>> On 11/25/20 11:30 AM, Alessandro Vesely wrote: >>>>>>>> Without resorting to ARC, it is still possible to validate author >>>>>>>> domain's signatures directly if the MLM just adds a subject tag >>>>>>>> and a footer>>>>> >>>>>>> I agree that ARC isn't really needed to do this (trust the last hop >>>>>>> from the MLM and determine the original authenticity from the MLM's >>>>>>> perspective)>>>> >>>>>> I didn't mean to trust the MLM. I meant remove the subject tag and >>>>>> the footer, then the original DKIM signature verifies. See: >>>>>> https://datatracker.ietf.org/doc/draft-vesely-dmarc-mlm-transform/ >>>>> >>>>> When I was at Cisco, with l= and some subject line heuristics I could get >>>>> probably like 90+% verification rate across the entire company, a company that >>>>> uses external mailing lists a lot. Definitely not 100% though. >>>> >>>> >>>> DKIM itself is not 100%. You always have lines beginning with "From " or >>>> occasional autoconversions. >>>> >>>> l= doesn't cover multipart/alternative nor Content-Transfer-Encoding: >>>> base64. In addition, the DKIM spec discourages its usage and suggests >>>> that "Assessors might wish to ignore signatures that use the tag.">> >>> >>> Right, some of the other dkim-light or diff concepts we discussed would be >>> better than using l= >>> >>> We again got hung up on the 100% solution, though... something that handled >>> subject-prefix and footer in a transport agnostic way might have worked. >> >> I'm not clear about the meaning of "100%". If an author domain puts no >> DKIM signatures, there is no way to verify them. Hence, some compliance of >> the author domain has to be required. >> >> The same holds for conditional signatures. >> >> The same holds for MLM transformations. >> > > Yes, by 100% I meant of messages that were already authenticated and > therefore should continue to be authenticated through the relay. That's ARC. If a message lacks DKIM and was SPF-authenticated, there's no way it can continue to be authenticated through a relay. OTOH, mailing lists and relays are two different beasts. For one thing, it is very unusual for a mailing list to send to another mailing list. Thus, we can safely specify a non-stackable authentication method. > Some of the conditional signatures of the "include a diff you can remove to > validate the original" attempt seemed to fail on the theory that there were > too many things that couldn't be handled. Ie, if your relay removes > attachments, including them back in a diff kind of breaks the whole point of > that... but how common is that (even less now with Yahoo Groups gone, but > possibly still some av/malware relays still do this). Not to mention anonymous lists, which remove the OP identity completely. They are DMARC-proof by themselves, with no additional twists. My draft restricts footers to text/plain MIME type, to overcome the objection to l=. Hence, if a list appends HTML parts (e.g. to use <hr>), it doesn't qualify as DMARC-proof. > I think that one issue we've had is that DMARC is very mechanical and > straight-forward, so anything that's fuzzy in response seems more > complicated. It may seem fuzzy, but it's not. The ietf list (ietf@ietf.org), for example, adds no subject tag and no footer. DKIM signatures should remain valid, then. Yet, if posters sign Sender:, they fail. I wouldn't call that fuzziness. It is the very nature of the spec. If you sign Received:, no relay can hold your signature over. Best Ale --
- [dmarc-ietf] A policy for direct mail flows only,… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Douglas E. Foster
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Jesse Thompson
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… John Levine
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… John Levine
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Brandon Long
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… Brandon Long
- Re: [dmarc-ietf] A policy for direct mail flows o… Brandon Long
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… devel2020
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… Brandon Long
- Re: [dmarc-ietf] A policy for direct mail flows o… Alessandro Vesely
- Re: [dmarc-ietf] A policy for direct mail flows o… John Levine
- Re: [dmarc-ietf] A policy for direct mail flows o… Michael Thomas
- Re: [dmarc-ietf] A policy for direct mail flows o… John Levine
- Re: [dmarc-ietf] A policy for direct mail flows o… Brandon Long