Re: [dmarc-ietf] Ticket #1 - SPF alignment

Scott Kitterman <sklist@kitterman.com> Mon, 25 January 2021 21:35 UTC

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From: Scott Kitterman <sklist@kitterman.com>
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Date: Mon, 25 Jan 2021 16:35:09 -0500
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Subject: Re: [dmarc-ietf] Ticket #1 - SPF alignment
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On Monday, January 25, 2021 4:04:33 PM EST Todd Herr wrote:
> On Thu, Jan 21, 2021 at 4:24 AM Alessandro Vesely <vesely@tana.it> wrote
> 
> > I agree that the spec needs some text somewhere to counter the passage in
> > Section 2.3 of RFC 7208.  This, methinks, is the intended semantics of the
> > second paragraph of section 3.1.2 of dmarcbis:
> > 
> > OLD:
> >     Note that the RFC5321.HELO identity is not typically used in the
> >     context of DMARC (except when required to "fake" an otherwise null
> >     reverse-path), even though a "pure SPF" implementation according to
> >     [RFC7208] would check that identifier.
> > 
> > I'd rather replace that paragraph and leave item 4 of Section 6.6.2 as
> > is.  For
> > a possibly less confusing wording:
> > 
> > NEW:
> >     Even tough a "pure SPF" implementation, according to [RFC7208], would
> >     avoid to check the RFC5321.MailFrom identity if the RFC5321.HELO was
> >     conclusively determined to pass, DMARC authentication requires the
> >     authenticated identity to be aligned.
> 
> May I propose that the section labeled "SPF-Authenticated Identifiers" be
> rewritten as follows:
> 
> CURRENT:
> 
>    DMARC permits Identifier Alignment, based on the result of an SPF
>    authentication, to be strict or relaxed.
> 
>    In relaxed mode, the [SPF
> <https://tools.ietf.org/html/rfc7489#ref-SPF>]-authenticated domain
> and RFC5322 <https://tools.ietf.org/html/rfc5322>.From
>    domain must have the same Organizational Domain.  In strict mode,
>    only an exact DNS domain match is considered to produce Identifier
>    Alignment.
> 
>    Note that the RFC5321 <https://tools.ietf.org/html/rfc5321>.HELO
> identity is not typically used in the
>    context of DMARC (except when required to "fake" an otherwise null
>    reverse-path), even though a "pure SPF" implementation according to
>    [SPF <https://tools.ietf.org/html/rfc7489#ref-SPF>] would check
> that identifier.
> 
>    For example, if a message passes an SPF check with an
>    RFC5321 <https://tools.ietf.org/html/rfc5321>.MailFrom domain of
> "cbg.bounces.example.com", and the address
>    portion of the RFC5322 <https://tools.ietf.org/html/rfc5322>.From
> field contains "payments@example.com".com",
>    the Authenticated RFC5321
> <https://tools.ietf.org/html/rfc5321>.MailFrom domain identifier and
> the
>    RFC5322 <https://tools.ietf.org/html/rfc5322>.From domain are
> considered to be "in alignment" in relaxed
> 
>    mode, but not in strict mode.
> 
> 
> 
> NEW:
> 
> DMARC permits Identifier Alignment, based on the result of an SPF
> 
> authentication, to be strict or relaxed.
> 
> 
> In relaxed mode, the [@!RFC3986]-authenticated domain and RFC5322.From
> 
> domain must have the same Organizational Domain.  In strict mode,
> 
> only an exact DNS domain match is considered to produce Identifier
> 
> Alignment.
> 
> 
> For example, if a message passes an SPF check with an
> 
> RFC5321.MailFrom domain of "cbg.bounces.example.com", and the address
> 
> portion of the RFC5322.From field contains "payments@example.com".com",
> 
> the Authenticated RFC5321.MailFrom domain identifier and the
> 
> RFC5322.From domain are considered to be "in alignment" in relaxed
> 
> mode, but not in strict mode. In order for the two identifiers to
> 
> be considered "in alignment" in strict mode, the domain parts would
> 
> have to be identical.
> 
> 
> The reader should note that SPF alignment checks in DMARC rely solely
> 
> on the RFC5321.MailFrom domain. This differs from section 2.3 of
> [@!RFC7208],
> 
> which recommends that SPF checks be done on not only the "MAIL FROM"
> 
> but also on a separate check of the "HELO" identity.

I think this is fine, but there is a subtlety to be aware of.

If you look at RFC 7208 Section 2.4, when Mail From is null, postmaster@HELO 
is the mail from for SPF purposes.  DMARC really can't change that.

As a result, there are cases where Mail From results actually are derived from 
HELO and it's unavoidable.  I believe the proposed text is clear enough about 
not using separate HELO identity results and that's appropriate.

Scott K