Re: [dmarc-ietf] [Ietf-dkim] DKIM-Signature: r=y and MLM

Kurt Andersen <kurta@drkurt.com> Wed, 24 October 2018 21:19 UTC

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From: Kurt Andersen <kurta@drkurt.com>
Date: Wed, 24 Oct 2018 14:18:48 -0700
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To: hsantos=40isdg.net@dmarc.ietf.org
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Subject: Re: [dmarc-ietf] [Ietf-dkim] DKIM-Signature: r=y and MLM
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On Mon, Oct 15, 2018 at 7:30 AM Hector Santos <hsantos=
40isdg.net@dmarc.ietf.org>; wrote:

<elided earlier part of the message>


> The rewrite should be the last thing to consider, and if it does
> rewrite, it should replace the original author domain strong policy
> with its own strong policy.
>
> For example, the ietf.org mailing list has begun to rewrite and it
> replaces the 5322.From with a dmarc.ietf.org domain, adds a new
> X-Original-From header and resigns the message using an ietf.org
> signer domain:
>
>    DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ietf.org;
> s=ietf1;
>       t=1537415189; bh=TJWGUVdPL8OTY+HJnUzpBRd52OaKfWjFqS68Cby0s/M=;
>       h=Date:To:References:In-Reply-To:Subject:List-Id:List-Unsubscribe:
>       List-Archive:List-Post:List-Help:List-Subscribe:From;
>       b=.....
>     X-Original-From: Hector Santos <hsantos@isdg.net>;
>     From: Hector Santos <hsantos=40isdg.net@dmarc.ietf.org>;
>
> What it should do is:
>
>    1) It should use a 1st party signature using d=dmarc.ietf.org to
>       match the new author domain dmarc.ietf.org.
>
>    2) It should has hash bind the X-Original-From header to the
>       signature.  Since DKIM recommends not to bind "X-" headers,
>       a non "X-" header should be used, i.e. "Original-From:".  This
>       means adding the header to the 'h=" field to avoid potential
>       mail resend exploits using different unprotected Original-from:
>       fields.
>
>    3) and finally, the dmarc.ietf.org domain should have its own
>       DMARC p=reject policy to effectively replace the one it
>       circumvented with the submission.
>

I don't understand why it is necessarily a bad thing to fall back to the
org domain (ietf.org) as this example shows.

I also don't understand how your suggestion would work to handle a mixture
of restrictive policies (some quarantine, some reject) with a single _
dmarc.dmarc.ietf.org record unless there is some trick DNS responder magic
going on (and that won't work well for cached responses anyway).

--Kurt