path: root/doc/rfc/rfc9091.txt

Internet Engineering Task Force (IETF)                      S. Kitterman
Request for Comments: 9091                        fTLD Registry Services
Category: Experimental                                  T. Wicinski, Ed.
ISSN: 2070-1721                                                July 2021


    Experimental Domain-Based Message Authentication, Reporting, and
        Conformance (DMARC) Extension for Public Suffix Domains

Abstract

   Domain-based Message Authentication, Reporting, and Conformance
   (DMARC), defined in RFC 7489, permits a domain-controlling
   organization to express domain-level policies and preferences for
   message validation, disposition, and reporting, which a mail-
   receiving organization can use to improve mail handling.

   DMARC distinguishes the portion of a name that is a Public Suffix
   Domain (PSD), below which Organizational Domain names are created.
   The basic DMARC capability allows Organizational Domains to specify
   policies that apply to their subdomains, but it does not give that
   capability to PSDs.  This document describes an extension to DMARC to
   fully enable DMARC functionality for PSDs.

   Some implementations of DMARC consider a PSD to be ineligible for
   DMARC enforcement.  This specification addresses that case.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9091.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Example
     1.2.  Discussion
   2.  Terminology and Definitions
     2.1.  Conventions Used in This Document
     2.2.  Public Suffix Domain (PSD)
     2.3.  Organizational Domain
     2.4.  Longest PSD
     2.5.  Public Suffix Operator (PSO)
     2.6.  PSO-Controlled Domain Names
     2.7.  Non-existent Domains
   3.  PSD DMARC Updates to DMARC Requirements
     3.1.  General Updates
     3.2.  Changes in Section 6.3 ("General Record Format")
     3.3.  Changes in Section 6.4 ("Formal Definition")
     3.4.  Changes in Section 6.5 ("Domain Owner Actions")
     3.5.  Changes in Section 6.6.1 ("Extract Author Domain")
     3.6.  Changes in Section 6.6.3 ("Policy Discovery")
     3.7.  Changes in Section 7 ("DMARC Feedback")
   4.  Privacy Considerations
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  PSD DMARC Privacy Concern Mitigation Experiment
   Appendix B.  DMARC PSD Registry Examples
     B.1.  DMARC PSD DNS Query Service
     B.2.  DMARC PSD Registry
     B.3.  DMARC PSD PSL Extension
   Appendix C.  Implementations
     C.1.  Authheaders Module
     C.2.  Zdkimfilter Module
   Acknowledgements
   Authors' Addresses

1.  Introduction

   DMARC [RFC7489] provides a mechanism for publishing organizational
   policy information to email receivers.  DMARC allows policy to be
   specified for both individual domains and for Organizational Domains
   and their subdomains within a single organization.

   To determine the Organizational Domain for a message under
   evaluation, and thus where to look for a policy statement, DMARC
   makes use of a public suffix list.  The process for doing this can be
   found in Section 3.2 of the DMARC specification [RFC7489].
   Currently, the most common public suffix list being used is the one
   maintained by the Mozilla Foundation and made public at
   <https://publicsuffix.org>.

   In the basic DMARC model, Public Suffix Domains (PSDs) are not
   Organizational Domains and are thus not subject to DMARC processing.
   In DMARC, domains fall into one of three categories: Organizational
   Domains, subdomains of Organizational Domains, or PSDs.  A PSD can
   only publish DMARC policy for itself and not for any subdomains under
   it.  In some cases, this limitation allows for the abuse of non-
   existent organizational-level domains and hampers identification of
   domain abuse in email.

   This document specifies experimental updates to the DMARC
   specification [RFC7489] in an attempt to mitigate this abuse.

1.1.  Example

   As an example, imagine a Top-Level Domain (TLD), ".example", that has
   public subdomains for government and commercial use (".gov.example"
   and ".com.example").  The maintainer of a list of such a PSD
   structure would include entries for both of these subdomains, thereby
   indicating that they are PSDs, below which Organizational Domains can
   be registered.  Suppose further that there exists a legitimate domain
   called "tax.gov.example", registered within ".gov.example".

   By exploiting the typically unauthenticated nature of email, there
   are regular malicious campaigns to impersonate this organization that
   use similar-looking ("cousin") domains such as "t4x.gov.example".
   Such domains are not registered.

   Within the ".gov.example" public suffix, use of DMARC has been
   mandated, so "gov.example" publishes the following DMARC DNS record:

   _dmarc.gov.example. IN TXT ( "v=DMARC1; p=reject;"
                                "rua=mailto:dmc@dmarc.svc.gov.example" )

   This DMARC record provides policy and a reporting destination for
   mail sent from @gov.example.  Similarly, "tax.gov.example" will have
   a DMARC record that specifies policy for mail sent from addresses
   @tax.gov.example.  However, due to DMARC's current method of
   discovering and applying policy at the Organizational Domain level,
   the non-existent Organizational Domain of @t4x.gov.example does not
   and cannot fall under a DMARC policy.

   Defensively registering all variants of "tax" is not a scalable
   strategy.  The intent of this specification, therefore, is to enhance
   the DMARC discovery method by enabling an agent receiving such a
   message to be able to determine that a relevant policy is present at
   "gov.example", which is precluded by the current DMARC specification.

1.2.  Discussion

   This document provides a simple extension to [RFC7489] to allow
   operators of Public Suffix Domains (PSDs) to:

   *  Express policy at the level of the PSD that covers all
      Organizational Domains that do not explicitly publish DMARC
      records

   *  Extend the DMARC policy query functionality to detect and process
      such a policy

   *  Describe receiver feedback for such policies

   *  Provide controls to mitigate potential privacy considerations
      associated with this extension

   This document also provides a new DMARC tag to indicate requested
   handling policy for non-existent subdomains.  This is provided
   specifically to support phased deployment of PSD DMARC but is
   expected to be useful more generally.  Undesired rejection risks for
   mail purporting to be from domains that do not exist are
   substantially lower than for those that do, so the operational risk
   of requesting harsh policy treatment (e.g., reject) is lower.

   As an additional benefit, the PSD DMARC extension clarifies existing
   requirements.  Based on the requirements of [RFC7489], DMARC should
   function above the organizational level for exact domain matches
   (i.e., if a DMARC record were published for "example", then mail from
   example@example should be subject to DMARC processing).  Testing has
   revealed that this is not consistently applied in different
   implementations.

   There are two types of Public Suffix Operators (PSOs) for which this
   extension would be useful and appropriate:

   Branded PSDs (e.g., ".google"):
      These domains are effectively Organizational Domains as discussed
      in [RFC7489].  They control all subdomains of the tree.  These are
      effectively private domains but listed in the current public
      suffix list.  They are treated as public for DMARC purposes.  They
      require the same protections as DMARC Organizational Domains but
      are currently unable to benefit from DMARC.

   Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
      Because existing Organizational Domains using this PSD have their
      own DMARC policy, the applicability of this extension is for non-
      existent domains.  The extension allows the brand protection
      benefits of DMARC to extend to the entire PSD, including cousin
      domains of registered organizations.

   Due to the design of DMARC and the nature of the Internet email
   architecture [RFC5598], there are interoperability issues associated
   with DMARC deployment.  These are discussed in "Interoperability
   Issues between Domain-based Message Authentication, Reporting, and
   Conformance (DMARC) and Indirect Email Flows" [RFC7960].  These
   issues are not typically applicable to PSDs since they (e.g., the
   ".gov.example" used above) do not typically send mail.

2.  Terminology and Definitions

   This section defines terms used in the rest of the document.

2.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Public Suffix Domain (PSD)

   The global Internet Domain Name System (DNS) is documented in
   numerous RFCs.  It defines a tree of names starting with root, ".",
   immediately below which are Top-Level Domain names such as ".com" and
   ".us".  The domain name structure consists of a tree of names, each
   of which is made of a sequence of words ("labels") separated by
   period characters.  The root of the tree is simply called ".".  The
   Internet community at large, through processes and policies external
   to this work, selects points in this tree at which to register domain
   names "owned" by independent organizations.  Real-world examples are
   ".com", ".org", ".us", and ".gov.uk".  Names at which such
   registrations occur are called "Public Suffix Domains (PSDs)", and a
   registration consists of a label selected by the registrant to which
   a desirable PSD is appended.  For example, "ietf.org" is a registered
   domain name, and ".org" is its PSD.

2.3.  Organizational Domain

   The term "Organizational Domain" is defined in Section 3.2 of
   [RFC7489].

2.4.  Longest PSD

   The longest PSD is the Organizational Domain with one label removed.
   It names the immediate parent node of the Organizational Domain in
   the DNS namespace tree.

2.5.  Public Suffix Operator (PSO)

   A Public Suffix Operator is an organization that manages operations
   within a PSD, particularly the DNS records published for names at and
   under that domain name.

2.6.  PSO-Controlled Domain Names

   PSO-Controlled Domain Names are names in the DNS that are managed by
   a PSO and are not available for use as Organizational Domains.  PSO-
   Controlled Domain Names may have one (e.g., ".com") or more (e.g.,
   ".co.uk") name components, depending on PSD policy.

2.7.  Non-existent Domains

   For DMARC purposes, a non-existent domain is a domain for which there
   is an NXDOMAIN or NODATA response for A, AAAA, and MX records.  This
   is a broader definition than that in [RFC8020].

3.  PSD DMARC Updates to DMARC Requirements

   To participate in this experiment, implementations should interpret
   [RFC7489] as described in the following subsections.

3.1.  General Updates

   References to "Domain Owners" also apply to PSOs.

3.2.  Changes in Section 6.3 ("General Record Format")

   The following paragraph is added to this section.  A new tag is added
   after "fo":

   |  np:  Requested Mail Receiver policy for non-existent subdomains
   |     (plain-text; OPTIONAL).  Indicates the policy to be enacted by
   |     the Receiver at the request of the Domain Owner.  It applies
   |     only to non-existent subdomains of the domain queried and not
   |     to either existing subdomains or the domain itself.  Its syntax
   |     is identical to that of the "p" tag defined below.  If the "np"
   |     tag is absent, the policy specified by the "sp" tag (if the
   |     "sp" tag is present) or the policy specified by the "p" tag (if
   |     the "sp" tag is absent) MUST be applied for non-existent
   |     subdomains.  Note that "np" will be ignored for DMARC records
   |     published on subdomains of Organizational Domains and PSDs due
   |     to the effect of the DMARC policy discovery mechanism described
   |     in Section 6.6.3 of [RFC7489].

   The following tag definitions from DMARC are updated:

   p:  The sentence 'Policy applies to the domain queried and to
      subdomains, unless subdomain policy is explicitly described using
      the "sp" tag' is updated to read 'Policy applies to the domain
      queried and to subdomains, unless subdomain policy is explicitly
      described using the "sp" or "np" tags.'

   sp:  The sentence 'If absent, the policy specified by the "p" tag
      MUST be applied for subdomains' is updated to read 'If both the
      "sp" tag is absent and the "np" tag is either absent or not
      applicable, the policy specified by the "p" tag MUST be applied
      for subdomains.'

3.3.  Changes in Section 6.4 ("Formal Definition")

   The ABNF [RFC5234] for DMARC is updated to include a new definition,
   "dmarc-nprequest":

               dmarc-nprequest =  "np" *WSP "=" *WSP
                   ( "none" / "quarantine" / "reject" )

   The "dmarc-record" definition is also updated to include the
   following:

   dmarc-record    = dmarc-version dmarc-sep
                 [dmarc-request]
                 [dmarc-sep dmarc-srequest]
                 [dmarc-sep dmarc-auri]
                 [dmarc-sep dmarc-furi]
                 [dmarc-sep dmarc-adkim]
                 [dmarc-sep dmarc-aspf]
                 [dmarc-sep dmarc-ainterval]
                 [dmarc-sep dmarc-fo]
                 [dmarc-sep dmarc-rfmt]
                 [dmarc-sep dmarc-percent]
                 [dmarc-sep]
                 [dmarc-sep dmarc-nprequest]
                 ; components other than dmarc-version and
                 ; dmarc-request may appear in any order

3.4.  Changes in Section 6.5 ("Domain Owner Actions")

   In addition to the DMARC Domain Owner actions, PSOs that require use
   of DMARC and participate in PSD DMARC ought to make that information
   available to receivers.  This document is an experimental mechanism
   for doing so; see the description in Appendix B.

3.5.  Changes in Section 6.6.1 ("Extract Author Domain")

   Experience with DMARC has shown that some implementations short-
   circuit messages, bypassing DMARC policy application, when the domain
   name extracted by the receiver (from the RFC5322.From domain) is on
   the public suffix list used by the receiver.  This negates the
   capability being created by this specification.  Therefore, the
   following paragraph is appended to Section 6.6.1 of the DMARC
   specification [RFC7489]:

   |  Note that domain names that appear on a public suffix list are not
   |  exempt from DMARC policy application and reporting.

3.6.  Changes in Section 6.6.3 ("Policy Discovery")

   A new step is added between steps 3 and 4:

   |  3A.  If the set is now empty and the longest PSD ([RFC9091],
   |     Section 2.4) of the Organizational Domain is one that the
   |     receiver has determined is acceptable for PSD DMARC (based on
   |     the data in one of the DMARC PSD Registry Examples described in
   |     Appendix B of [RFC9091]), the Mail Receiver MUST query the DNS
   |     for a DMARC TXT record at the DNS domain matching the longest
   |     PSD in place of the RFC5322.From domain in the message (if
   |     different).  A possibly empty set of records is returned.

   As an example, for a message with the Organizational Domain of
   "example.compute.cloudcompany.com.example", the query for PSD DMARC
   would use "compute.cloudcompany.com.example" as the longest PSD.  The
   receiver would check to see if that PSD is listed in the DMARC PSD
   Registry, and if so, perform the policy lookup at
   "_dmarc.compute.cloudcompany.com.example".

      Note: Because the PSD policy query comes after the Organizational
      Domain policy query, PSD policy is not used for Organizational
      Domains that have published a DMARC policy.  Specifically, this is
      not a mechanism to provide feedback addresses (RUA/RUF) when an
      Organizational Domain has declined to do so.

3.7.  Changes in Section 7 ("DMARC Feedback")

   The following paragraph is added to this section:

   |  Operational note for PSD DMARC: For PSOs, feedback for non-
   |  existent domains is desirable and useful, just as it is for org-
   |  level DMARC operators.  See Section 4 of [RFC9091] for discussion
   |  of privacy considerations for PSD DMARC.

4.  Privacy Considerations

   These privacy considerations are developed based on the requirements
   of [RFC6973].  Additionally, the privacy considerations of [RFC7489]
   apply to the mechanisms described by this document.  To participate
   in this experiment, implementations should be aware of the privacy
   considerations described in this section.  If this experiment is
   successful, this section should be incorporated into the "Privacy
   Considerations" section as "Feedback Leakage".

   Providing feedback reporting to PSOs can, in some cases, cause
   information to leak out of an organization to the PSO.  This leakage
   could potentially be utilized as part of a program of pervasive
   surveillance (see [RFC7624]).  There are roughly three cases to
   consider:

   Single Organization PSDs (e.g., ".google"):
      RUA and RUF reports based on PSD DMARC have the potential to
      contain information about emails related to entities managed by
      the organization.  Since both the PSO and the Organizational
      Domain Owners are common, there is no additional privacy risk for
      either normal or non-existent domain reporting due to PSD DMARC.

   Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
      Reports based on PSD DMARC will only be generated for domains that
      do not publish a DMARC policy at the organizational or host level.
      For domains that do publish the required DMARC policy records, the
      feedback reporting addresses (RUA and RUF) of the organization (or
      hosts) will be used.  The only direct risk of feedback leakage for
      these PSDs are for Organizational Domains that are out of
      compliance with PSD policy.  Data on non-existent cousin domains
      would be sent to the PSO.

   Multi-organization PSDs (e.g., ".com") that do not mandate DMARC
   usage:
      Privacy risks for Organizational Domains that have not deployed
      DMARC within such PSDs are significant.  For non-DMARC
      Organizational Domains, all DMARC feedback will be directed to the
      PSO.  PSD DMARC is opt out (by publishing a DMARC record at the
      Organizational Domain level) instead of opt in, which would be the
      more desirable characteristic.  This means that any non-DMARC
      Organizational Domain would have its Feedback Reports redirected
      to the PSO.  The content of such reports, particularly for
      existing domains, is privacy sensitive.

   PSOs will receive feedback on non-existent domains, which may be
   similar to existing Organizational Domains.  Feedback related to such
   cousin domains have a small risk of carrying information related to
   an actual Organizational Domain.  To minimize this potential concern,
   PSD DMARC feedback MUST be limited to Aggregate Reports.  Feedback
   Reports carry more detailed information and present a greater risk.

   Due to the inherent privacy and security risks associated with PSD
   DMARC for Organizational Domains in multi-organization PSDs that do
   not participate in DMARC, any feedback reporting related to multi-
   organizational PSDs MUST be limited to non-existent domains except in
   cases where the reporter knows that PSO requires use of DMARC (by
   checking the DMARC PSD Registry).

5.  Security Considerations

   This document does not change the security considerations of
   [RFC7489] and [RFC7960].

   The risks of the issues identified in Section 12.3 of [RFC7489] ("DNS
   Security") are amplified by PSD DMARC.  In particular, consequences
   of DNS cache poisoning (or name chaining) are increased because a
   successful attack would potentially have a much wider scope (see
   [RFC3833] for details).

   The risks of the issues identified in Section 12.5 of [RFC7489]
   ("External Reporting Addresses") are amplified by PSD DMARC.  By
   design, PSD DMARC causes unrequested reporting of feedback to
   entities external to the Organizational Domain.  This is discussed in
   more detail in Section 4.

6.  IANA Considerations

   IANA has added a new tag to the "DMARC Tag Registry" in the "Domain-
   based Message Authentication, Reporting, and Conformance (DMARC)
   Parameters" registry.  The "Status" column is defined in Section 11.4
   of [RFC7489].

   The new entry is as follows:

     +==========+===========+=========+=============================+
     | Tag Name | Reference | Status  | Description                 |
     +==========+===========+=========+=============================+
     | np       | RFC 9091  | current | Requested handling policy   |
     |          |           |         | for non-existent subdomains |
     +----------+-----------+---------+-----------------------------+

                                 Table 1


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [PSD-DMARC]
              "Public Suffix Domain DMARC", <https://psddmarc.org/>.

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August
              2004, <https://www.rfc-editor.org/info/rfc3833>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC7960]  Martin, F., Ed., Lear, E., Ed., Draegen, T., Ed., Zwicky,
              E., Ed., and K. Andersen, Ed., "Interoperability Issues
              between Domain-based Message Authentication, Reporting,
              and Conformance (DMARC) and Indirect Email Flows",
              RFC 7960, DOI 10.17487/RFC7960, September 2016,
              <https://www.rfc-editor.org/info/rfc7960>.

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <https://www.rfc-editor.org/info/rfc8020>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  PSD DMARC Privacy Concern Mitigation Experiment

   The experiment being performed has three different questions that are
   looking to be addressed in this document.

   *  Section 3.2 modifies policy discovery to add an additional DNS
      lookup.  To determine if this lookup is useful, PSDs will add
      additional DMARC records in place and will analyze the DMARC
      reports.  Success will be determined if a consensus of PSDs that
      publish DMARC records are able to collect useful data.

   *  Section 3.2 adds the "np" tag for non-existent subdomains (DNS
      NXDOMAIN).  PSOs wishing to test this will add this flag to their
      DMARC record and will analyze DMARC reports for deployment.
      Success will be determined if organizations find explicitly
      blocking non-existent subdomains desirable and that doing so
      provides added value.

   *  Section 4 discusses three cases where providing feedback could
      cause information to leak out of an organization.  This experiment
      will analyze the Feedback Reports generated for each case to
      determine if there is information leakage.

Appendix B.  DMARC PSD Registry Examples

   To facilitate experimentation around mitigation of data leakage,
   samples of the DNS-based and IANA-like registries are available at
   [PSD-DMARC].

B.1.  DMARC PSD DNS Query Service

   A sample stand-alone DNS query service is available at [PSD-DMARC].
   It was developed based on the contents suggested for an IANA registry
   in an earlier draft version of this document.  Usage of the service
   is described at [PSD-DMARC].

B.2.  DMARC PSD Registry

   [PSD-DMARC] provides an IANA-like DMARC Public Suffix Domain (PSD)
   Registry as a stand-alone DNS query service.  It follows the contents
   and structure described below.  There is a Comma-Separated Value
   (CSV) version of the listed PSDs that is suitable for use in build
   updates for PSD DMARC-capable software.

   PSDs that are deploying DMARC and are participating in PSD DMARC must
   register their public suffix domain in this new registry.  The
   requirement has to be documented in a manner that satisfies the terms
   of Expert Review, per [RFC8126].  The Designated Expert needs to
   confirm that provided documentation adequately describes PSD policy
   to require Domain Owners to use DMARC or that all Domain Owners are
   part of a single organization with the PSO.

   The authoritative registry can be found here: <https://psddmarc.org>

B.3.  DMARC PSD PSL Extension

   [PSD-DMARC] provides a file formatted like the Public Suffix List
   (PSL) in order to facilitate identification of PSD DMARC
   participants.  Contents are functionally identical to the IANA-like
   registry but presented in a different format.

   When using this approach, the input domain of the extension lookup is
   supposed to be the output domain of the regular PSL lookup, i.e., the
   Organizational Domain.  This alternative data approach is potentially
   useful since DMARC implementations already need to be able to parse
   the data format, so it should be easier to implement.

Appendix C.  Implementations

   There are two known implementations of PSD DMARC available for
   testing.

C.1.  Authheaders Module

   The authheaders Python module and command line tool is available for
   download or installation from Pypi (Python Packaging Index).

   It supports both use of the DNS-based query service and download of
   the CSV registry file from [PSD-DMARC].

C.2.  Zdkimfilter Module

   The zdkimfilter module is a separately available add-on to Courier-
   MTA.

   Mostly used for DomainKeys Identified Mail (DKIM) signing, it can be
   configured to also verify, apply DMARC policies, and send Aggregate
   Reports.  For PSD DMARC, it uses the PSL extension list approach,
   which is available from [PSD-DMARC].

Acknowledgements

   Thanks to the following individuals for their contributions (both
   public and private) to improving this document: Kurt Andersen, Seth
   Blank, Dave Crocker, Heather Diaz, Tim Draegen, Zeke Hendrickson,
   Andrew Kennedy, John Levine, Dr. Ian Levy, Craig Schwartz, Alessandro
   Vesely, and Tim Wicinski.

   A special mention to Dave Crocker for coming up with the name.

Authors' Addresses

   Scott Kitterman
   fTLD Registry Services
   Suite 400
   600 13th Street, NW
   Washington, DC 20005
   United States of America

   Phone: +1 301 325-5475
   Email: scott@kitterman.com


   Tim Wicinski (editor)
   Elkins, WV 26241
   United States of America

   Email: tjw.ietf@gmail.com