[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-kitterman-dmarc-psd) 00 01 02 03 04 05 06 07

Network Working Group                                       S. Kitterman
Internet-Draft                                    fTLD Registry Services
Intended status: Experimental                           October 14, 2019
Expires: April 16, 2020


DMARC (Domain-based Message Authentication, Reporting, and Conformance)
               Extension For PSDs (Public Suffix Domains)
                        draft-ietf-dmarc-psd-07

Abstract

   DMARC (Domain-based Message Authentication, Reporting, and
   Conformance) is a scalable mechanism by which a mail-originating
   organization can express domain-level policies and preferences for
   message validation, disposition, and reporting, that a mail-receiving
   organization can use to improve mail handling.  The design of DMARC
   presumes that domain names represent either nodes in the tree below
   which registrations occur, or nodes where registrations have
   occurred; it does not permit a domain name to have both of these
   properties simultaneously.  Since its deployment in 2015, use of
   DMARC has shown a clear need for the ability to express policy for
   these domains as well.

   Domains at which registrations can occur are referred to as Public
   Suffix Domains (PSDs).  This document describes an extension to DMARC
   to enable DMARC functionality for PSDs.

   This document also seeks to address implementations that consider a
   domain on a public Suffix list to be ineligible for DMARC
   enforcement.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 16, 2020.



Kitterman                Expires April 16, 2020                 [Page 1]


Internet-Draft                  PSD DMARC                   October 2019


Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   5
     2.1.  Conventions Used in This Document . . . . . . . . . . . .   5
     2.2.  Public Suffix Domain (PSD)  . . . . . . . . . . . . . . .   5
     2.3.  Longest PSD . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Public Suffix Operator (PSO)  . . . . . . . . . . . . . .   5
     2.5.  PSO Controlled Domain Names . . . . . . . . . . . . . . .   6
     2.6.  Non-existent Domains  . . . . . . . . . . . . . . . . . .   6
   3.  PSD DMARC Updates to DMARC Requirements . . . . . . . . . . .   6
     3.1.  General Updates . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Section 6.3 General Record Format . . . . . . . . . . . .   6
     3.3.  Section 6.5.  Domain Owner Actions  . . . . . . . . . . .   7
     3.4.  Section 6.6.1.  Extract Author Domain . . . . . . . . . .   7
     3.5.  Section 6.6.3.  Policy Discovery  . . . . . . . . . . . .   7
     3.6.  Section 7.  DMARC Feedback  . . . . . . . . . . . . . . .   8
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Feedback leakage  . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Subdomain Policy Tag  . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  The Experiment . . . . . . . . . . . . . . . . . . .  11
     A.1.  PSD DMARC Privacy Concern Mitigation  . . . . . . . . . .  11
     A.2.  Non-Existent Subdomain Policy . . . . . . . . . . . . . .  12
   Appendix B.  DMARC PSD Registry Examples  . . . . . . . . . . . .  12
     B.1.  DMARC PSD DNS Query Service . . . . . . . . . . . . . . .  13
     B.2.  DMARC Public Suffix Domain (PSD) Registry . . . . . . . .  13
     B.3.  DMARC PSD PSL Extension . . . . . . . . . . . . . . . . .  13
   Appendix C.  Implementations  . . . . . . . . . . . . . . . . . .  14



Kitterman                Expires April 16, 2020                 [Page 2]


Internet-Draft                  PSD DMARC                   October 2019


     C.1.  Authheaders Module  . . . . . . . . . . . . . . . . . . .  14
     C.2.  Zdkimfilter Module  . . . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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 sub-domains within a single organization.  DMARC leverages
   public suffix lists to determine which domains are organizational
   domains.  It presumes that public suffix list listed domains are not
   organizational domains and not subject to DMARC processing; domains
   are either organizational domains, sub-domains of organizational
   domains, or listed on a public suffix list.  For domains listed in a
   public suffix list, i.e. TLDs and domains that exist between TLDs and
   organization level domains, policy can only be published for the
   exact domain.  No method is available for these domains to express
   policy or receive feedback reporting for sub-domains.  This missing
   method allows for the abuse of non-existent organizational-level
   domains and prevents identification of domain abuse in email.

   As an example, imagine a country code TLD (ccTLD) which has public
   subdomains for government and commercial use (.gov.example and
   .com.example).  Suppose there exists a registered domain
   "tax.gov.example" that is responsible for taxation in this imagined
   country.  However, 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".  These domains are not registered.  Within the
   ".gov.example" public suffix, use of DMARC has been mandated, so
   "gov.example" publishes the following DMARC record:

   "v=DMARC1; p=reject; rua=mailto:dmarc@dmarc.service.gov.example"

   at

   _dmarc.gov.example.

   This DMARC record provides policy and a reporting destination for
   mail sent from @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 @tax.gov.example
   does not and cannot fall under a DMARC policy.

   Defensively registering all variants of "tax" is obviously not a
   scalable strategy.  The intent of this specification, therefore, is



Kitterman                Expires April 16, 2020                 [Page 3]


Internet-Draft                  PSD DMARC                   October 2019


   to enhance the DMARC algorithm 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 algorithm.

   This document provides a simple extension to DMARC [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, extends the DMARC policy query
   functionality to detect and process such a policy, describes receiver
   feedback for such policies, and provides controls to mitigate
   potential privacy considerations associated with this extension.

   This document also provides a new DMARC [RFC7489] tag to indicate
   requested handling policy for non-existent subdommains.  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 DMARC [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 had 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:

   o  Branded PSDs (e.g., ".google"): These domains are effectively
      Organizational Domains as discussed in DMARC [RFC7489].  They
      control all subdomains of the tree.  These are effectively private
      domains, but listed in the 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.

   o  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 [RFC7489] and the nature of the Internet
   email architecture [RFC5598], there are interoperability issues



Kitterman                Expires April 16, 2020                 [Page 4]


Internet-Draft                  PSD DMARC                   October 2019


   associated with DMARC [RFC7489] deployment.  These are discussed in
   Interoperability Issues between 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 Requests for Comment (RFC).  It defines a tree of names
   starting with root, ".", immediately below which are Top Level Domain
   names such as ".com" and ".us".  They are not available for private
   registration.  In many cases the public portion of the DNS tree is
   more than one level deep.  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.  Longest PSD

   The longest PSD is the Organizational Domain with one label removed.

2.4.  Public Suffix Operator (PSO)

   A Public Suffix Operator manages operations within its PSD.






Kitterman                Expires April 16, 2020                 [Page 5]


Internet-Draft                  PSD DMARC                   October 2019


2.5.  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 (the
   term Organizational Domains is defined in DMARC [RFC7489]
   Section 3.2).  Depending on PSD policy, these will have one (e.g.,
   ".com") or more (e.g., ".co.uk") name components.

2.6.  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 NXDOMAIN [RFC8020].

3.  PSD DMARC Updates to DMARC Requirements

   This document updates DMARC [RFC7489] as follows:

3.1.  General Updates

   References to "Domain Owners" also apply to PSOs.

3.2.  Section 6.3 General Record Format

   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 not
      present, 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 DMARC [RFC7489]
      Section 6.6.3.

   The following tag definitions from DMARC [RFC7489] 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.'




Kitterman                Expires April 16, 2020                 [Page 6]


Internet-Draft                  PSD DMARC                   October 2019


   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.  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.  The mechanism for doing so is one of the
   experimental elements of this document.  See the experiment
   description (Appendix A).

3.4.  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) 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 DMARC [RFC7489]:

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

3.5.  Section 6.6.3.  Policy Discovery

   A new step between step 3 and 4 is added:

   3A.  If the set is now empty and the longest PSD (Section 2.3) of the
      Organizational Domain is one that the receiver has determined is
      acceptable for PSD DMARC (discussed in the experiment description
      (Appendix A)), the Mail Receiver MUST query the DNS for a DMARC
      TXT record at the DNS domain matching the longest PSD
      (Section 2.3) 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
   (Section 2.3).  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



Kitterman                Expires April 16, 2020                 [Page 7]


Internet-Draft                  PSD DMARC                   October 2019


   not a mechanism to provide feedback addresses (RUA/RUF) when an
   Organizational Domain has declined to do so.

3.6.  Section 7.  DMARC Feedback

   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 this document for discussion of Privacy
   Considerations.

4.  Privacy Considerations

   These privacy considerations are developed based on the requirements
   of [RFC6973].  The Privacy Considerations of [RFC7489] apply to this
   document.

4.1.  Feedback leakage

   Providing feedback reporting to PSOs can, in some cases, create
   leakage of information outside 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:

   o  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.

   o  Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
      PSD DMARC based reports 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 feedback leakage risk 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.

   o  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) vice opt-in, which would be the more
      desirable characteristic.  This means that any non-DMARC



Kitterman                Expires April 16, 2020                 [Page 8]


Internet-Draft                  PSD DMARC                   October 2019


      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 is best 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 particpate in DMARC, any Feedback Reporting related to multi-
   organizational PSDs ought to be limited to non-existent domains
   except in cases where the reporter knows that PSO requires use of
   DMARC.

5.  Security Considerations

   This document does not change the Security Considerations of
   [RFC7489] and [RFC7960].

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

   The risks of the issues identified in [RFC7489], Section 12.5,
   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

   This section describes actions requested to be completed by IANA.

6.1.  Subdomain Policy Tag

   IANA is requested to add a new tag to DMARC Tag Registry in the
   Domain-based Message Authentication, Reporting, and Conformance
   (DMARC) Parameters Registry.

   The entry is as follows:





Kitterman                Expires April 16, 2020                 [Page 9]


Internet-Draft                  PSD DMARC                   October 2019


   +----------+-----------+---------+-------------------------------+
   | Tag Name | Reference | Status  | Description                   |
   +----------+-----------+---------+-------------------------------+
   | np       | this      | current | Requested handling policy for |
   |          | document  |         | non-existent subdomains       |
   +----------+-----------+---------+-------------------------------+

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>.

   [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

   [psddmarc.org]
              multiple, "PSD DMARC Web Site", April 2019,
              <https://psddmarc.org/>.

   [PSL]      multiple, "Public Suffix List", April 2019,
              <https://publicsuffix.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>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

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





Kitterman                Expires April 16, 2020                [Page 10]


Internet-Draft                  PSD DMARC                   October 2019


   [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. Ed., T., 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>.

Appendix A.  The Experiment

   There are two experimental questions addressed in this document: one
   regarding mitigation of PSD related privacy concerns and the other on
   the utility of specifying separate DMARC policies for non-existent
   sub-domains.

   Aditionally, as of the writing of this document operational and
   policy constraints prevent this experiment from being deployed
   globally.  If the experiment shows that PSD DMARC solves a real
   problem and can be used at a large scale, the results could prove to
   be useful in removing constraints outside of the IETF that would
   permit broader deployment.

A.1.  PSD DMARC Privacy Concern Mitigation

   To mitigate the privacy concerns associated with Multi-organization
   PSDs that do not mandate DMARC usage, see Section 4.1, a mechanism to
   indicate which PSDs do not present this privacy risk is appropriate.
   There are multiple approaches that are possible.

   The experiment is to evaluate different possible approaches.  The
   experiment will be complete when there is rough consensus on a




Kitterman                Expires April 16, 2020                [Page 11]


Internet-Draft                  PSD DMARC                   October 2019


   technical approach that is demonstrated to be operationally usable
   and effective at mitigating the privacy concern.

   The mechanism needs the following attributes:

   o  Be reliably, publicly accessible

   o  Be under configuration control based on a public set of criteria

   o  List PSDs that either mandate DMARC for their registrants or for
      which all lower level domains are controlled by the PSO and that
      the relevant PSO has indicated a desire for the PSD to participate
      in PSD DMARC

   o  Have a small operational footprint (e.g. provide a documented,
      lightweight mechanism for developers and operators to retrieve the
      list of PSD DMARC participants)

   o  Not allow PSO to add PSDs to the PSD DMARC participants list
      without third party review

   As of this writing, three approaches have been proposed.  None of
   them are ideal:

   o  An extension to the Public Suffix List at [PSL]

   o  A dedicated registry queried via DNS - an example of such a
      service is described in Appendix B.1 below

   o  An IANA registry

A.2.  Non-Existent Subdomain Policy

   PSOs that plan to implement PSD DMARC have indicated that the ability
   to describe distinct policies for existing and non- existing sub-
   domains would facilitate PSD DMARC deployment.  There are also
   suggestions that it would be more generally useful for DMARC.

   During the period of the experiment, uptake of the new 'np' tag will
   be evaluated to support assessment of the utility of including 'np'
   in a future, non-experimental update.

Appendix B.  DMARC PSD Registry Examples

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




Kitterman                Expires April 16, 2020                [Page 12]


Internet-Draft                  PSD DMARC                   October 2019


B.1.  DMARC PSD DNS Query Service

   A sample stand-alone DNS query service is available at
   [psddmarc.org].  It was developed based on the contents suggested for
   an IANA registry in an earlier revision of this draft.  Usage of the
   service is described on the web site.

B.2.  DMARC Public Suffix Domain (PSD) Registry

   [psddmarc.org] 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 PSD domains which is suitable for use in
   build updates for PSD DMARC capable software.

   Names of PSDs participating in PSD DMARC must be registered this new
   registry.  New entries are assigned only for PSDs that require use of
   DMARC.  The requirement has to be documented in a manner that
   satisfies the terms of Expert Review,per [RFC5226].  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 initial set of entries in this registry is as follows:

   +-------------+---------------+
   |    PSD      | Status        |
   +-------------+---------------+
   | .bank       | current       |
   +-------------+---------------+
   | .insurance  | current       |
   +-------------+---------------+
   | .gov.uk     | current       |
   +-------------+---------------+

B.3.  DMARC PSD PSL Extension

   [psddmarc.org] provides a PSL like file to enable 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.




Kitterman                Expires April 16, 2020                [Page 13]


Internet-Draft                  PSD DMARC                   October 2019


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 [psddmarc.org].

C.2.  Zdkimfilter Module

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

   Mostly used for 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 from
   [psddmarc.org]

Acknowledgements

   Thanks to the following individuals for their contributions (both
   public and private) to improving this document.  Special shout out to
   Dave Crocker for naming the beast.

   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

Author's Address

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

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








Kitterman                Expires April 16, 2020                [Page 14]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/