Re: [dmarc-ietf] Lookup Limitations For Public Suffix Domains

Zeke Hendrickson <> Fri, 30 November 2018 21:40 UTC

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Date: Fri, 30 Nov 2018 21:40:33 +0000
From: Zeke Hendrickson <>
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Subject: Re: [dmarc-ietf] Lookup Limitations For Public Suffix Domains
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On Wed, Nov 21, 2018 at 02:37:19AM -0500, Scott Kitterman wrote:
> While we were discussing making draft-kitterman-dmarc-psd a working group 
> item, the main discussion point was about the use of an IANA registry to 
> identify participating public suffix domains.  I think it would be useful to 
> consider what problems we were trying to solve and see if there are 
> alternative approaches that address those requirements.
> Goals:
> 1.  Minimize additional verifier burden for adding PSD DMARC support.  
> Currently it requires consulting a locally stored, infrequently changing list 
> and one additional DNS lookup only for participating public suffixes when 
> there is no organizational domain DMARC record.
> 2.  Externalize signaling about PSD participation.  As discussed in the 
> Privacy Considerations (section 4.1), we were concerned about the privacy 
> implications of feedback on organizational domain traffic for organizational 
> domains that don't participate in DMARC being inappropriately captured by 
> public suffix operators.  In order to avoid this, we identified criteria for 
> which public suffixes PSD DMARC would be appropriate for and require an 
> external review to ensure those criteria are met.  No solution that's in DNS 
> will address this part of the problem.

I feel that restricting the additional PSD check to nonexistent
organizational domains is the best approach, as it preserves the
opt-in nature of DMARC, limits privacy concerns, remains very
straightforward to implement as a verifier, and does not rely on an
additional list.

draft-ietf-dmarc-psd-00 addresses a slightly broader problem space,
but I feel that adding the ability to get feedback on abuse of
nonexistent domains is the most needed aspect; treating branded PSDs
as organizational domains would be better addressed by improving the
way organizational boundaries are determined.

Zeke Hendrickson (
University of Michigan | Information and Technology Services
Infrastructure | Application Operations | Collaboration Services