Hi Steve!

On Sun 04/Sep/2022 13:38:39 +0200 Stephen J. Turnbull wrote:
Alessandro Vesely writes:

There is a thread about ARC sealing in bind-users[*].

Not sure what you mean by "sealing". Do you mean they're not implementing the rest of the protocol?

They add a complete ARC set. Actually, they add three ARC sets to each message, one at reception (with a full ARC-Authentication-Results), followed by intermediate and final sets.

They're applying ARC signatures, although they run Mailman 2. It doesn't seem difficult to implement.

It's not.  But

1.  It's a bad idea to do it in Mailman.
2. It was implemented in Mailman 3 three or four years ago as a proof of concept during the development of ARC. 3. There is a milter available for Postfix and Sendmail from the Trusted Domain Project https://github.com/trusteddomainproject/OpenARC as is the basic implementation which I presume is adaptable to Exim, qmail, and other MTAs.

This is the preferred approach, as matter of conformance because it should be implemented by the edge MTA(s), and as a practical matter because Mailman *can't* do SPF since it is never an edge MTA. There is also a pure Python implementation on PyPI, I believe (this is the basis for the Mailman 3 plugin, or maybe it was dkimpy).

Thank you for that much needed clarification. I heard someone saying that the IETF was waiting to implement Mailman 3 to use ARC in mailing list...

BTW, there is also a Perl implementation of ARC included in Mail::DKIM.

It requires trusting the users, though.

I don't think so, not any more than any other sign-and-send protocol.

I think you misunderstood my question here.

What it requires is implementation by recipient domains who trust your host, because if they don't it's 2014 all over again for your subscribers if you have any DMARC p=reject posters.

Exactly. I asked bind-users if anyone verifying ARC saw any difference after trusting isc.org. Besides adding ARC sets, bind-users do From: munging, obviously. Nobody saw any difference.

The point is how does Mailman know whether a recipient's MX trusts this particular list. And what does it do when it knows. Some people babble something about DNS records, which looks difficult. Another possibility could be an SMTP extension, difficult to implement as it involves multiple levels.

An easy way would be to ask the subscriber whether to do From: munging or not. I repeat a possible wording for that option, which would be enabled by default:

   *From munging*:

   Set this option to /Disabled/ to receive messages with the original From:
   line intact.  Keep in mind that disabling this option will fail DMARC, so
   keep it enabled unless your MX either doesn't check DMARC or accepts
   overrides trusting our ARC sets.

Then, a user can disable From: munging for the messages destined to her. That's easy for those who run their own MTA. People using Gmail, say, would have to figure out, presumably by trial and error.

If such an option is not given, a mailing list could add the Author: header field defined by RFC9057. Receivers could restore From: after DMARC filtering.

That assuming that someone is willing to do something to avoid munged From:'s, which I'm beginning to doubt.


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