draft-ietf-dmarc-dmarcbis-06.txt

```




DMARC                                                       T. Herr (ed)
Internet-Draft                                                  Valimail
Obsoletes: 7489 (if approved)                             J. Levine (ed)
Intended status: Standards Track                           Standcore LLC
Expires: 22 September 2022                                 21 March 2022


Domain-based Message Authentication, Reporting, and Conformance (DMARC)
                      draft-ietf-dmarc-dmarcbis-06

Abstract

   This document describes the Domain-based Message Authentication,
   Reporting, and Conformance (DMARC) protocol.

   DMARC permits the owner of an email author's domain name to enable
   verification of the domain's use, to indicate the Domain Owner's or
   Public Suffix Operator's message handling preference regarding failed
   verification, and to request reports about use of the domain name.
   Mail receiving organizations can use this information when evaluating
   handling choices for incoming mail.

   This document obsoletes RFC 7489.

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 22 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  High-Level Goals  . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Anti-Phishing . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Scalability . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Out of Scope  . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Terminology and Definitions . . . . . . . . . . . . . . . . .   7
     3.1.  Conventions Used in This Document . . . . . . . . . . . .   7
     3.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Authenticated Identifiers . . . . . . . . . . . . . .   8
       3.2.2.  Author Domain . . . . . . . . . . . . . . . . . . . .   8
       3.2.3.  Domain Owner  . . . . . . . . . . . . . . . . . . . .   8
       3.2.4.  Identifier Alignment  . . . . . . . . . . . . . . . .   8
       3.2.5.  Mail Receiver . . . . . . . . . . . . . . . . . . . .   8
       3.2.6.  Non-existent Domains  . . . . . . . . . . . . . . . .   9
       3.2.7.  Organizational Domain . . . . . . . . . . . . . . . .   9
       3.2.8.  Public Suffix Domain (PSD)  . . . . . . . . . . . . .   9
       3.2.9.  Public Suffix Operator (PSO)  . . . . . . . . . . . .   9
       3.2.10. PSO Controlled Domain Names . . . . . . . . . . . . .   9
       3.2.11. Report Consumer . . . . . . . . . . . . . . . . . . .  10
   4.  Overview and Key Concepts . . . . . . . . . . . . . . . . . .  10
     4.1.  DMARC Basics  . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Use of RFC5322.From . . . . . . . . . . . . . . . . . . .  11
     4.3.  Authentication Mechanisms . . . . . . . . . . . . . . . .  12
     4.4.  Identifier Alignment Explained  . . . . . . . . . . . . .  12
       4.4.1.  DKIM-Authenticated Identifiers  . . . . . . . . . . .  13
       4.4.2.  SPF-Authenticated Identifiers . . . . . . . . . . . .  14
       4.4.3.  Alignment and Extension Technologies  . . . . . . . .  14
     4.5.  Flow Diagram  . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  DNS Tree Walk . . . . . . . . . . . . . . . . . . . . . .  16
     4.7.  DMARC Policy Discovery  . . . . . . . . . . . . . . . . .  17
     4.8.  Organizational Domain Discovery . . . . . . . . . . . . .  18
   5.  Policy  . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     5.1.  DMARC Policy Record . . . . . . . . . . . . . . . . . . .  21
     5.2.  DMARC URIs  . . . . . . . . . . . . . . . . . . . . . . .  21
     5.3.  General Record Format . . . . . . . . . . . . . . . . . .  21
     5.4.  Formal Definition . . . . . . . . . . . . . . . . . . . .  25



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     5.5.  Domain Owner Actions  . . . . . . . . . . . . . . . . . .  27
       5.5.1.  Publish an SPF Policy for an Aligned Domain . . . . .  27
       5.5.2.  Configure Sending System for DKIM Signing Using an
               Aligned Domain  . . . . . . . . . . . . . . . . . . .  27
       5.5.3.  Setup a Mailbox to Receive Aggregate Reports  . . . .  27
       5.5.4.  Publish a DMARC Policy for the Author Domain  . . . .  28
       5.5.5.  Collect and Analyze Reports . . . . . . . . . . . . .  28
       5.5.6.  Decide If and When to Update DMARC Policy . . . . . .  28
     5.6.  PSO Actions . . . . . . . . . . . . . . . . . . . . . . .  28
     5.7.  Mail Receiver Actions . . . . . . . . . . . . . . . . . .  28
       5.7.1.  Extract Author Domain . . . . . . . . . . . . . . . .  29
       5.7.2.  Determine Handling Policy . . . . . . . . . . . . . .  29
       5.7.3.  Store Results of DMARC Processing . . . . . . . . . .  30
       5.7.4.  Send Aggregate Reports  . . . . . . . . . . . . . . .  30
     5.8.  Policy Enforcement Considerations . . . . . . . . . . . .  31
   6.  DMARC Feedback  . . . . . . . . . . . . . . . . . . . . . . .  32
   7.  Other Topics  . . . . . . . . . . . . . . . . . . . . . . . .  32
     7.1.  Issues Specific to SPF  . . . . . . . . . . . . . . . . .  32
     7.2.  DNS Load and Caching  . . . . . . . . . . . . . . . . . .  33
     7.3.  Rejecting Messages  . . . . . . . . . . . . . . . . . . .  33
     7.4.  Identifier Alignment Considerations . . . . . . . . . . .  34
     7.5.  Interoperability Issues . . . . . . . . . . . . . . . . .  35
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
     8.1.  Authentication-Results Method Registry Update . . . . . .  35
     8.2.  Authentication-Results Result Registry Update . . . . . .  36
     8.3.  Feedback Report Header Fields Registry Update . . . . . .  37
     8.4.  DMARC Tag Registry  . . . . . . . . . . . . . . . . . . .  38
     8.5.  DMARC Report Format Registry  . . . . . . . . . . . . . .  39
     8.6.  Underscored and Globally Scoped DNS Node Names
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  40
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  40
     9.1.  Authentication Methods  . . . . . . . . . . . . . . . . .  40
     9.2.  Attacks on Reporting URIs . . . . . . . . . . . . . . . .  41
     9.3.  DNS Security  . . . . . . . . . . . . . . . . . . . . . .  41
     9.4.  Display Name Attacks  . . . . . . . . . . . . . . . . . .  42
     9.5.  External Reporting Addresses  . . . . . . . . . . . . . .  42
     9.6.  Secure Protocols  . . . . . . . . . . . . . . . . . . . .  43
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  43
   11. Informative References  . . . . . . . . . . . . . . . . . . .  45
   Appendix A.  Technology Considerations  . . . . . . . . . . . . .  46
     A.1.  S/MIME  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     A.2.  Method Exclusion  . . . . . . . . . . . . . . . . . . . .  47
     A.3.  Sender Header Field . . . . . . . . . . . . . . . . . . .  48
     A.4.  Domain Existence Test . . . . . . . . . . . . . . . . . .  48
     A.5.  Issues with ADSP in Operation . . . . . . . . . . . . . .  49
     A.6.  Organizational Domain Discovery Issues  . . . . . . . . .  49
     A.7.  Removal of the "pct" Tag  . . . . . . . . . . . . . . . .  51
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  52



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     B.1.  Identifier Alignment Examples . . . . . . . . . . . . . .  52
       B.1.1.  SPF . . . . . . . . . . . . . . . . . . . . . . . . .  52
       B.1.2.  DKIM  . . . . . . . . . . . . . . . . . . . . . . . .  53
     B.2.  Domain Owner Example  . . . . . . . . . . . . . . . . . .  54
       B.2.1.  Entire Domain, Monitoring Only  . . . . . . . . . . .  54
       B.2.2.  Entire Domain, Monitoring Only, Per-Message
               Reports . . . . . . . . . . . . . . . . . . . . . . .  55
       B.2.3.  Per-Message Failure Reports Directed to Third
               Party . . . . . . . . . . . . . . . . . . . . . . . .  56
       B.2.4.  Subdomain, Testing, and Multiple Aggregate Report
               URIs  . . . . . . . . . . . . . . . . . . . . . . . .  57
     B.3.  Mail Receiver Example . . . . . . . . . . . . . . . . . .  59
       B.3.1.  SMTP Session Example  . . . . . . . . . . . . . . . .  59
     B.4.  Utilization of Aggregate Feedback: Example  . . . . . . .  61
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  61
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  62

1.  Introduction

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
   The source for this draft is maintained in GitHub at:
   https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis
   (https://github.com/ietf-wg-dmarc/draft-ietf-dmarc-dmarcbis)

   Abusive email often includes unauthorized and deceptive use of a
   domain name in the RFC5322.From header field.  The domain typically
   belongs to an organization expected to be known to - and presumably
   trusted by - the recipient.  The Sender Policy Framework (SPF)
   [RFC7208] and DomainKeys Identified Mail (DKIM) [RFC6376] protocols
   provide domain-level authentication but are not directly associated
   with the RFC5322.From domain.  DMARC leverages these two protocols,
   providing a method for Domain Owners to publish a DNS record
   describing the email authentication policies for the RFC5322.From
   domain and to request specific handling for messages using that
   domain that fail authentication checks.

   As with SPF and DKIM, DMARC classes results as "pass" or "fail".  In
   order to get a DMARC result of "pass", a pass from either SPF or DKIM
   is required.  In addition, the passed domain must be "aligned" with
   the RFC5322.From domain in one of two modes - "relaxed" or "strict".
   The mode is expressed in the domain's DMARC policy record.  Domains
   are said to be "in relaxed alignment" if they have the same
   "Organizational Domain", which is the domain at the top of the domain
   hierarchy for the RFC5322.From domain while having the same
   administrative authority as the RFC5322.From domain.  Domains are "in
   strict alignment" if and only if they are identical.





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   A DMARC pass indicates only that the RFC5322.From domain has been
   authenticated for that message.  Authentication does not carry an
   explicit or implicit value assertion about that message or about the
   Domain Owner.  Furthermore, a mail-receiving organization that
   performs DMARC verification can choose to honor the Domain Owner's
   requested message handling for authentication failures, but it is
   under no obligation to do so; it might choose different actions
   entirely.

   For a mail-receiving organization supporting DMARC, a message that
   passes verification is part of a message stream that is reliably
   associated with the RFC5322.From field Domain Owner.  Therefore,
   reputation assessment of that stream by the mail-receiving
   organization is not encumbered by accounting for unauthorized use of
   that domain in the RFC5322.From field.  A message that fails this
   verification is not necessarily associated with the Domain Owner's
   domain and its reputation.

   DMARC policy records can also cover non-existent sub-domains, below
   the "Organizational Domain", as well as domains at the top of the
   name hierarchy, controlled by Public Suffix Operators (PSOs).

   DMARC, in the associated [DMARC-Aggregate-Reporting] and
   [DMARC-Failure-Reporting] documents, also specifies a reporting
   framework.  Using it, a mail-receiving domain can generate regular
   reports about messages that claim to be from a domain publishing
   DMARC policies, sending those reports to the address(es) specified by
   the Domain Owner in the latter's DMARC policy record.  Domain Owners
   can use these reports, especially the aggregate reports, to identify
   not only sources of mail attempting to fraudulently use their domain,
   but also (and perhaps more importantly) gaps in their own
   authentication practices.  However, as with honoring the Domain
   Owner's stated mail handling preference, a mail-receiving
   organization supporting DMARC is under no obligation to send
   requested reports, although it is recommended that they do send
   aggregate reports.

   Use of DMARC creates some interoperability challenges that require
   due consideration before deployment, particularly with configurations
   that can cause mail to be rejected.  These are discussed in
   Section 7.

2.  Requirements

   Specification of DMARC is guided by the following high-level goals,
   security dependencies, detailed requirements, and items that are
   documented as out of scope.




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2.1.  High-Level Goals

   DMARC has the following high-level goals:

   *  Allow Domain Owners and PSOs to assert their desired message
      handling for authentication failures for messages purporting to
      have authorship within the domain.

   *  Allow Domain Owners and PSOs to verify their authentication
      deployment.

   *  Minimize implementation complexity for both senders and receivers,
      as well as the impact on handling and delivery of legitimate
      messages.

   *  Reduce the amount of successfully delivered spoofed email.

   *  Work at Internet scale.

2.2.  Anti-Phishing

   DMARC is designed to prevent bad actors from sending mail that claims
   to come from legitimate senders, particularly senders of
   transactional email (official mail that is about business
   transactions).  One of the primary uses of this kind of spoofed mail
   is phishing (enticing users to provide information by pretending to
   be the legitimate service requesting the information).  Thus, DMARC
   is significantly informed by ongoing efforts to enact large-scale,
   Internet-wide anti-phishing measures.

   Although DMARC can only be used to combat specific forms of exact-
   domain spoofing directly, the DMARC mechanism has been found to be
   useful in the creation of reliable and defensible message streams.

   DMARC does not attempt to solve all problems with spoofed or
   otherwise fraudulent email.  In particular, it does not address the
   use of visually similar domain names ("cousin domains") or abuse of
   the RFC5322.From human-readable <display-name>.

2.3.  Scalability

   Scalability is a major issue for systems that need to operate in a
   system as widely deployed as current SMTP email.  For this reason,
   DMARC seeks to avoid the need for third parties or pre-sending
   agreements between senders and receivers.  This preserves the
   positive aspects of the current email infrastructure.





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   Although DMARC does not introduce third-party senders (namely
   external agents authorized to send on behalf of an operator) to the
   email-handling flow, it also does not preclude them.  Such third
   parties are free to provide services in conjunction with DMARC.

2.4.  Out of Scope

   Several topics and issues are specifically out of scope for this
   work.  These include the following:

   *  Different treatment of messages that are not authenticated versus
      those that fail authentication;

   *  Evaluation of anything other than RFC5322.From header field;

   *  Multiple reporting formats;

   *  Publishing policy other than via the DNS;

   *  Reporting or otherwise evaluating other than the last-hop IP
      address;

   *  Attacks in the RFC5322.From header field, also known as "display
      name" attacks;

   *  Authentication of entities other than domains, since DMARC is
      built upon SPF and DKIM, which authenticate domains; and

   *  Content analysis.

3.  Terminology and Definitions

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

3.1.  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] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are encouraged to be familiar with the contents of [RFC5598].
   In particular, that document defines various roles in the messaging
   infrastructure that can appear the same or separate in various
   contexts.  For example, a Domain Owner could, via the messaging
   security mechanisms on which DMARC is based, delegate the ability to
   send mail as the Domain Owner to a third party with another role.



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   This document does not address the distinctions among such roles; the
   reader is encouraged to become familiar with that material before
   continuing.

3.2.  Definitions

   The following sections define terms used in this document.

3.2.1.  Authenticated Identifiers

   Domain-level identifiers that are verified using authentication
   technologies are referred to as "Authenticated Identifiers".  See
   Section 4.3 for details about the supported mechanisms.

3.2.2.  Author Domain

   The domain name of the apparent author, as extracted from the
   RFC5322.From header field.

3.2.3.  Domain Owner

   An entity or organization that owns a DNS domain.  The term "owns"
   here indicates that the entity or organization being referenced holds
   the registration of that DNS domain.  Domain Owners range from
   complex, globally distributed organizations, to service providers
   working on behalf of non-technical clients, to individuals
   responsible for maintaining personal domains.  This specification
   uses this term as analogous to an Administrative Management Domain as
   defined in [RFC5598].  It can also refer to delegates, such as Report
   Consumers, when those are outside of their immediate management
   domain.

3.2.4.  Identifier Alignment

   When the domain in the address in the RFC5322.From header field has
   the same Organizational Domain as a domain verified by an
   authenticated identifier, it has Identifier Alignment. (see
   Section 3.2.7)

3.2.5.  Mail Receiver

   The entity or organization that receives and processes email.  Mail
   Receivers operate one or more Internet-facing Mail Transport Agents
   (MTAs).







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3.2.6.  Non-existent Domains

   For DMARC purposes, a non-existent domain is consistent with the
   meaning of the term as described in [RFC8020].  That is, if the
   response code received for a query for a domain name is NXDOMAIN,
   then the domain name and all the names under it do not exist.

3.2.7.  Organizational Domain

   The Organizational Domain is typically a domain that was registered
   with a domain name registrar.  More formally, it is any Public Suffix
   Domain plus one label.  The Organizational Domain for the domain in
   the RFC5322.From domain is determined by applying the algorithm found
   in Section 4.8.

3.2.8.  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 of
   these points 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.

3.2.9.  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.

3.2.10.  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.







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3.2.11.  Report Consumer

   An operator that receives reports from another operator implementing
   the reporting mechanisms described in this document and/or the
   documents [DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting].
   Such an operator might be receiving reports about messages related to
   a domain for which it is the Domain Owner or PSO, or reports about
   messages related to another operator's domain.  This term applies
   collectively to the system components that receive and process these
   reports and the organizations that operate them.

4.  Overview and Key Concepts

   This section provides a general overview of the design and operation
   of the DMARC environment.

4.1.  DMARC Basics

   DMARC permits a Domain Owner or PSO to enable verification of a
   domain's use in an email message, to indicate the Domain Owner's or
   PSO's message handling preference regarding failed verification, and
   to request reports about use of the domain name.  All information
   about a Domain Owner's or PSO's DMARC policy is published and
   retrieved via the DNS.

   DMARC's verification function is based on whether the RFC5322.From
   domain is aligned with a domain name used in a supported
   authentication mechanism, as described in Section 4.3.  When a DMARC
   policy exists for the domain name found in the RFC5322.From header
   field, and that domain name is not verified through an aligned
   supported authentication mechanism, the handling of that message can
   be affected based on the DMARC policy when delivered to a
   participating Mail Receiver.

   A message satisfies the DMARC checks if at least one of the supported
   authentication mechanisms:

   1.  produces a "pass" result, and

   2.  produces that result based on an identifier that is in alignment,
       as described in Section 4.4.

   It is important to note that the authentication mechanisms employed
   by DMARC authenticate only a DNS domain and do not authenticate the
   local-part of any email address identifier found in a message, nor do
   they validate the legitimacy of message content.





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   DMARC's feedback component involves the collection of information
   about received messages claiming to be from the Author Domain for
   periodic aggregate reports to the Domain Owner or PSO.  The
   parameters and format for such reports are discussed in
   [DMARC-Aggregate-Reporting]

   A DMARC-enabled Mail Receiver might also generate per-message reports
   that contain information related to individual messages that fail
   authentication checks.  Per-message failure reports are a useful
   source of information when debugging deployments (if messages can be
   determined to be legitimate even though failing authentication) or in
   analyzing attacks.  The capability for such services is enabled by
   DMARC but defined in other referenced material such as [RFC6591] and
   [DMARC-Failure-Reporting]

4.2.  Use of RFC5322.From

   One of the most obvious points of security scrutiny for DMARC is the
   choice to focus on an identifier, namely the RFC5322.From address,
   which is part of a body of data that has been trivially forged
   throughout the history of email.  This field is the one used by end
   users to identify the source of the message, and so it has always
   been a prime target for abuse through such forgery and other means.

   Several points suggest that it is the most correct and safest thing
   to do in this context:

   *  Of all the identifiers that are part of the message itself, this
      is the only one guaranteed to be present.

   *  It seems the best choice of an identifier on which to focus, as
      most MUAs display some or all of the contents of that field in a
      manner strongly suggesting those data as reflective of the true
      originator of the message.

   *  Many high-profile email sources, such as email service providers,
      require that the sending agent have authenticated before email can
      be generated.  Thus, for these mailboxes, the mechanism described
      in this document provides recipient end users with strong evidence
      that the message was indeed originated by the agent they associate
      with that mailbox, if the end user knows that these various
      protections have been provided.

   *  The absence of a single, properly formed RFC5322.From header field
      renders the message invalid.  Handling of such a message is
      outside of the scope of this specification.





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   Since the sorts of mail typically protected by DMARC participants
   tend to only have single Authors, DMARC participants generally
   operate under a slightly restricted profile of RFC5322 with respect
   to the expected syntax of this field.  See Section 5.7 for details.

4.3.  Authentication Mechanisms

   The following mechanisms for determining Authenticated Identifiers
   are supported in this version of DMARC:

   *  DKIM, [RFC6376], which provides a domain-level identifier in the
      content of the "d=" tag of a verified DKIM-Signature header field.

   *  SPF, [RFC7208], which can authenticate both the domain found in an
      SMTP [RFC5321] HELO/EHLO command (the HELO identity) and the
      domain found in an SMTP MAIL command (the MAIL FROM identity).  As
      noted earlier, however, DMARC relies solely on SPF authentication
      of the domain found in SMTP MAIL FROM command.  Section 2.4 of
      [RFC7208] describes MAIL FROM processing for cases in which the
      MAIL command has a null path.

4.4.  Identifier Alignment Explained

   Email authentication technologies authenticate various (and
   disparate) aspects of an individual message.  For example, DKIM
   [RFC6376] authenticates the domain that affixed a signature to the
   message, while SPF [RFC7208] can authenticate either the domain that
   appears in the RFC5321.MailFrom (MAIL FROM) portion of an SMTP
   [RFC5321] conversation or the RFC5321.EHLO/HELO domain, or both.
   These may be different domains, and they are typically not visible to
   the end user.

   DMARC authenticates use of the RFC5322.From domain by requiring
   either that it have the same Organizational Domain as an
   Authenticated Identifier (a condition known as "relaxed alignment")
   or that it be identical to the domain of the Authenticated Identifier
   (a condition known as "strict alignment").  The choice of relaxed or
   strict alignment is left to the Domain Owner and is expressed in the
   domain's DMARC policy record.  Domain names in this context are to be
   compared in a case-insensitive manner, per [RFC4343].











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   It is important to note that Identifier Alignment cannot occur with a
   message that is not valid per [RFC5322], particularly one with a
   malformed, absent, or repeated RFC5322.From header field, since in
   that case there is no reliable way to determine a DMARC policy that
   applies to the message.  Accordingly, DMARC operation is predicated
   on the input being a valid RFC5322 message object, and handling of
   such non-compliant cases is outside of the scope of this
   specification.  Further discussion of this can be found in
   Section 5.7.1.

   Each of the underlying authentication technologies that DMARC takes
   as input yields authenticated domains as their outputs when they
   succeed.

4.4.1.  DKIM-Authenticated Identifiers

   DMARC requires Identifier Alignment based on the result of a DKIM
   authentication because a message can bear a valid signature from any
   domain, including domains used by a mailing list or even a bad actor.
   Therefore, merely bearing a valid signature is not enough to infer
   authenticity of the Author Domain.

   DMARC permits Identifier Alignment based on the result of a DKIM
   authentication to be strict or relaxed.  (Note that these terms are
   not related to DKIM's "simple" and "relaxed" canonicalization modes.)

   In relaxed mode, the Organizational Domains of both the DKIM-
   authenticated signing domain (taken from the value of the d= tag in
   the signature) and that of the RFC5322.From domain must be equal if
   the identifiers are to be considered to be aligned.  In strict mode,
   only an exact match between both Fully Qualified Domain Names (FQDNs)
   is considered to produce Identifier Alignment.

   To illustrate, in relaxed mode, if a verified DKIM signature
   successfully verifies with a "d=" domain of "example.com", and the
   RFC5322.From address is "alerts@news.example.com", the DKIM "d="
   domain and the RFC5322.From domain are considered to be "in
   alignment", because both domains have the same Organizational Domain
   of "example.com".  In strict mode, this test would fail because the
   d= domain does not exactly match the RFC5322.From domain.

   However, a DKIM signature bearing a value of "d=com" would never
   allow an "in alignment" result, as "com" should be identified as a
   PSD and therefore cannot be an Organizational Domain.

   Note that a single email can contain multiple DKIM signatures, and it
   is considered to produce a DMARC "pass" result if any DKIM signature
   is aligned and verifies.



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4.4.2.  SPF-Authenticated Identifiers

   DMARC permits Identifier Alignment based on the result of an SPF
   authentication.  As with DKIM, Identifier Alignement can be either
   strict or relaxed.

   In relaxed mode, the Organizational Domains of the SPF-authenticated
   domain and RFC5322.From domain must be equal if the identifiers are
   to be considered to be aligned.  In strict mode, the two FQDNs must
   match exactly in order for them to be considered to be aligned.

   For example, in relaxed mode, 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 header field contains
   "payments@example.com", the Authenticated RFC5321.MailFrom domain
   identifier and the RFC5322.From domain are considered to be "in
   alignment" because they have the same Organizational Domain
   ("example.com").  In strict mode, this test would fail because the
   two domains are not 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.

4.4.3.  Alignment and Extension Technologies

   If in the future DMARC is extended to include the use of other
   authentication mechanisms, the extensions will need to allow for
   domain identifier extraction so that alignment with the RFC5322.From
   domain can be verified.

4.5.  Flow Diagram


















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    +---------------+                             +--------------------+
    | Author Domain |< . . . . . . . . . . . .    | Return-Path Domain |
    +---------------+                        .    +--------------------+
        |                                    .               ^
        V                                    V               .
    +-----------+     +--------+       +----------+          v
    |   MSA     |<***>|  DKIM  |       |   DMARC  |     +----------+
    |  Service  |     | Signer |       | Verifier |<***>|    SPF   |
    +-----------+     +--------+       +----------+  *  | Verifier |
        |                                    ^       *  +----------+
        |                                    *       *
        V                                    v       *
     +------+        (~~~~~~~~~~~~)      +------+    *  +----------+
     | sMTA |------->( other MTAs )----->| rMTA |    **>|   DKIM   |
     +------+        (~~~~~~~~~~~~)      +------+       | Verifier |
                                            |           +----------+
                                            |                ^
                                            V                .
                                     +-----------+           .
                       +---------+   |    MDA    |           v
                       |  User   |<--| Filtering |      +-----------+
                       | Mailbox |   |  Engine   |      |   DKIM    |
                       +---------+   +-----------+      |  Signing  |
                                                        | Domain(s) |
                                                        +-----------+

     MSA = Mail Submission Agent
     MDA = Mail Delivery Agent

   The above diagram shows a simple flow of messages through a DMARC-
   aware system.  Solid lines denote the actual message flow, dotted
   lines involve DNS queries used to retrieve message policy related to
   the supported message authentication schemes, and asterisk lines
   indicate data exchange between message-handling modules and message
   authentication modules.  "sMTA" is the sending MTA, and "rMTA" is the
   receiving MTA.

   Put simply, when a message reaches a DMARC-aware rMTA, a DNS query
   will be initiated to determine if a DMARC policy exists that applies
   to the author domain.  If a policy is found, the rMTA will use the
   results of SPF and DKIM verification checks to determine the ultimate
   DMARC authentication status.  The DMARC status can then factor into
   the message handling decision made by the recipient's mail sytsem.

   More details on specific actions for the parties involved can be
   found in Section 5.5 and Section 5.7.





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4.6.  DNS Tree Walk

   The DMARC protocol defines a method for communicating information
   through the publishing of records in DNS.  Both the content of the
   records and their location in the DNS hierarchy are used for two
   purposes: policy discovery (see Section 4.7) and Organizational
   Domain determination (see Section 4.8).

   The relevant DMARC record for these purposes is not necessarily the
   DMARC policy record found in DNS at the same level as the name label
   for the domain in question.  Instead, some domains will inherit their
   DMARC policy records from parent domains one level or more above them
   in the DNS hierarchy.  Similarly, the Organizational Domain may be
   found at a higher level in the DNS hierarchy.

   These records are discovered through the technique described here,
   known colloquially as the "DNS Tree Walk".  The target of any DNS
   Tree Walk is a valid DMARC policy record, but the rules defining
   required content for that record depend on the reason for performing
   the Tree Walk.

   To prevent possible abuse of the DNS, a shortcut is built into the
   process so that domains that have more than five labels do not result
   in more than five DNS queries.

   The generic steps for a DNS Tree Walk are as follows:

   1.  Query the DNS for a DMARC TXT record at the DNS domain matching
       the one found in the domain(s) described above.  A possibly empty
       set of records is returned.

   2.  Records that do not start with a "v=" tag that identifies the
       current version of DMARC are discarded.  If multiple DMARC
       records are returned, they are all discarded.

   3.  If the set is now empty, then determine the target for additional
       queries, using steps 4 through 8 below.

   4.  Break the subject DNS domain name into a set of "n" ordered
       labels.  Number these labels from right to left; e.g., for
       "a.mail.example.com", "com" would be label 1, "example" would be
       label 2, "mail.example.com" would be label 3, and so forth.









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   5.  Count the number of labels found in the subject DNS domain.  Let
       that number be "x".  If x < 5, remove the left-most (highest-
       numbered) label from the subject domain.  If x >= 5, remove the
       left-most (highest-numbered) labels from the subject domain until
       4 labels remain.  The resulting DNS domain name is the new target
       for subsequent lookups.

   6.  Query the DNS for a DMARC TXT record at the DNS domain matching
       this new target in place of the RFC5322.From domain in the
       message.  A possibly empty set of records is returned.

   7.  Records that do not start with a "v=" tag that identifies the
       current version of DMARC are discarded.  If multiple DMARC
       records are returned, they are all discarded.

   8.  If the set is now empty, determine the target for additional
       queries by removing a single label from the target domain as
       described in step 5 and repeating steps 6 and 7 until there are
       no more labels remaining.

   To illustrate, for a message with the arbitrary RFC5322.From domain
   of "a.b.c.d.e.mail.example.com", a full DNS Tree Walk would require
   the following five queries, in order to locate the policy or
   Organizational Domain:

   *  _dmarc.a.b.c.d.e.mail.example.com

   *  _dmarc.e.mail.example.com

   *  _dmarc.mail.example.com

   *  _dmarc.example.com

   *  _dmarc.com

4.7.  DMARC Policy Discovery

   For policy discovery, a DNS Tree Walk starts at the point in the DNS
   hierarchy that matches the domain in the RFC5322.From header of the
   message.  The DMARC policy to be applied to the message will be the
   record found at one of these three locations:

   *  The RFC5322.From domain

   *  The Organizational Domain (as determined by a separate DNS Tree
      Walk) of the RFC5322.From domain

   *  The Public Suffix Domain of the RFC5322.From domain



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   If a retrieved policy record does not contain a valid "p" tag, or
   contains an "sp" tag that is not valid, then:

   *  If a "rua" tag is present and contains at least one syntactically
      valid reporting URI, the Mail Receiver SHOULD act as if a record
      containing a valid "v" tag and "p=none" was retrieved, and
      continue processing;

   *  Otherwise, the Mail Receiver applies no DMARC processing to this
      message.

   If the set produced by the DNS Tree Walk contains no DMARC policy
   record (i.e., any indication that there is no such record as opposed
   to a transient DNS error), Mail Receivers MUST NOT apply the DMARC
   mechanism to the message.

   Handling of DNS errors when querying for the DMARC policy record is
   left to the discretion of the Mail Receiver.  For example, to ensure
   minimal disruption of mail flow, transient errors could result in
   delivery of the message ("fail open"), or they could result in the
   message being temporarily rejected (i.e., an SMTP 4yx reply), which
   invites the sending MTA to try again after the condition has possibly
   cleared, allowing a definite DMARC conclusion to be reached ("fail
   closed").

   Note: 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.

4.8.  Organizational Domain Discovery

   For Organizational Domain discovery, it may be necessary to perform
   multiple DNS Tree Walks in order to determine if any two domains are
   in alignment.  This means that a DNS Tree Walk to discover an
   Organizational Domain might start at any of the following locations:

   *  The domain in the RFC5322.From header of the message.

   *  The RFC5321.MailFrom domain if there is an SPF pass result for the
      message.

   *  Any DKIM d= domain if there is a DKIM pass result for the message
      for that domain.

   Note: There is no need to perform Tree Walk searches for
   Organizational Domains under any of the following conditions:




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   *  No applicable DMARC policy is discovered for the RFC5322.From
      domain during the first tree walk.  In this case, the DMARC
      mechanism does not apply to the message in question.

   *  There is no SPF pass result and no DKIM pass result for the
      message.  In this case, there can be no DMARC pass result, and so
      the Organizational Domain of any domain is not required to be
      discoverd.

   *  The record for the RFC5322.From domain indicates strict alignment.
      In this case, a simple string compare between the RFC5322.From
      domain and the RFC5321.MailFrom domain (if SPF authenticated),
      and/or the DKIM d= domain (if present and authenticated) is all
      that is required.

   To discover the Organizational Domain for a domain, perform the DNS
   Tree Walk described in Section 4.6 as needed for any of the domains
   in question.

   Select the Organizational Domain from the domains for which valid
   DMARC records were retrieved in the following sequence:

   1.  If a valid DMARC record explicitly contains the psd= tag set to
       'n' (psd=n), this is the Organizational Domain and the selection
       process is complete.

   2.  From the DMARC records that contain the psd= tag set to 'y'
       (psd=y), select the record for the domain with the largest number
       of labels that is also not the starting point for this Tree Walk,
       if such domain exists.  The Organizational Domain is the domain
       one label below this one in the DNS hierarchy, and the selection
       process is complete.

   3.  From the DMARC records that do not contain the psd= tag set to
       'y' (psd=y), select the record for the domain with the smallest
       number of labels.  This is the Organizational Domain and the
       selection process is complete.

   If this process does not determine the Organizational Domain, then
   the initial target domain is the Organizational Domain.

   For example, given the starting domain "a.mail.example.com", a search
   for the Organizational Domain would require a series of DNS queries
   for DMARC records starting with "_dmarc.a.mail.example.com" and
   finishing with "_dmarc.com".  If there are DMARC records for
   "_dmarc.mail.example.com" and "_dmarc.example.com", but not for
   "_dmarc.a.mail.example.com" or "_dmarc.com", then the Organizational
   Domain for this domain would be "example.com".



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   As another example, given the starting domain "a.mail.example.com",
   if a search for the Organizational Domain only yields a DMARC record
   at "_dmarc.com" and that record contains the tag psd=y, then the
   Organizational Domain for this domain would be "example.com".

   Note: Since 'n' is the default value for the psd tag, it might be
   argued that all DMARC policy records that do not contain psd=y then
   necessarily contain psd=n.  To be clear, step 1 from the above three-
   step sequence of determining the Organizational Domain refers only to
   those DMARC policy records that actually contain the tag-value pair
   'psd=n', to cover the very unusual case of a parent PSD publishing a
   DMARC record without the requisite 'psd=y' tag.

5.  Policy

   A Domain Owner or PSO advertises DMARC participation of one or more
   of its domains by adding a DNS TXT record (described in Section 5.1)
   to those domains.  In doing so, Domain Owners and PSOs indicate their
   handling preference regarding failed authentication for email
   messages making use of their domain in the RFC5322.From header field
   as well as their desire for feedback about those messages.  Mail
   Receivers in turn can take into account the Domain Owner's stated
   preference when making handling decisions about email messages that
   fail DMARC authentication checks.

   A Domain Owner or PSO may choose not to participate in DMARC
   evaluation by Mail Receivers simply by not publishing an appropriate
   DNS TXT record for its domain(s).  A Domain Owner can also choose to
   not have some underlying authentication technologies apply to DMARC
   evaluation of its domain(s).  In this case, the Domain Owner simply
   declines to advertise participation in those schemes.  For example,
   if the results of path authorization checks ought not be considered
   as part of the overall DMARC result for a given Author Domain, then
   the Domain Owner does not publish an SPF policy record that can
   produce an SPF pass result.

   A Mail Receiver implementing the DMARC mechanism SHOULD make a best-
   effort attempt to adhere to the Domain Owner's or PSO's published
   DMARC Domain Owner Assessment Policy when a message fails the DMARC
   test.  Since email streams can be complicated (due to forwarding,
   existing RFC5322.From domain-spoofing services, etc.), Mail Receivers
   MAY deviate from a published Domain Owner Assessment Policy during
   message processing and SHOULD make available the fact of and reason
   for the deviation to the Domain Owner via feedback reporting,
   specifically using the "PolicyOverride" feature of the aggregate
   report defined in [DMARC-Aggregate-Reporting]





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5.1.  DMARC Policy Record

   Domain Owner and PSO DMARC preferences are stored as DNS TXT records
   in subdomains named "_dmarc".  For example, the Domain Owner of
   "example.com" would post DMARC preferences in a TXT record at
   "_dmarc.example.com".  Similarly, a Mail Receiver wishing to query
   for DMARC preferences regarding mail with an RFC5322.From domain of
   "example.com" would issue a TXT query to the DNS for the subdomain of
   "_dmarc.example.com".  The DNS-located DMARC preference data will
   hereafter be called the "DMARC record".

   DMARC's use of the Domain Name Service is driven by DMARC's use of
   domain names and the nature of the query it performs.  The query
   requirement matches with the DNS, for obtaining simple parametric
   information.  It uses an established method of storing the
   information, associated with the target domain name, namely an
   isolated TXT record that is restricted to the DMARC context.  Use of
   the DNS as the query service has the benefit of reusing an extremely
   well-established operations, administration, and management
   infrastructure, rather than creating a new one.

   Per [RFC1035], a TXT record can comprise several "character-string"
   objects.  Where this is the case, the module performing DMARC
   evaluation MUST concatenate these strings by joining together the
   objects in order and parsing the result as a single string.

5.2.  DMARC URIs

   [RFC3986] defines a generic syntax for identifying a resource.  The
   DMARC mechanism uses this as the format by which a Domain Owner or
   PSO specifies the destination for the two report types that are
   supported.

   The place such URIs are specified (see Section 5.3) allows a list of
   these to be provided.  The list of URIs is separated by commas (ASCII
   0x2c).  A report SHOULD be sent to each listed URI provided in the
   DMARC record.

   A formal definition is provided in Section 5.4.

5.3.  General Record Format

   DMARC records follow the extensible "tag-value" syntax for DNS-based
   key records defined in DKIM [RFC6376].

   Section 8 creates a registry for known DMARC tags and registers the
   initial set defined in this document.  Only tags defined in that
   registry are to be processed; unknown tags MUST be ignored.



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   The following tags are valid DMARC tags:

   adkim:  (plain-text; OPTIONAL; default is "r".)  Indicates whether
      strict or relaxed DKIM Identifier Alignment mode is required by
      the Domain Owner.  See Section 4.4.1 for details.  Valid values
      are as follows:

      r:  relaxed mode

      s:  strict mode

   aspf:  (plain-text; OPTIONAL; default is "r".)  Indicates whether
      strict or relaxed SPF Identifier Alignment mode is required by the
      Domain Owner.  See Section 4.4.2 for details.  Valid values are as
      follows:

      r:  relaxed mode

      s:  strict mode

   fo:  Failure reporting options (plain-text; OPTIONAL; default is "0")
      Provides requested options for generation of failure reports.
      Report generators MAY choose to adhere to the requested options.
      This tag's content MUST be ignored if a "ruf" tag (below) is not
      also specified.  Failure reporting options are shown below.  The
      value of this tag is either "0", "1", or a colon-separated list of
      the options represented by alphabetic characters.  The valid
      values and their meanings are:

      0:  Generate a DMARC failure report if all underlying
         authentication mechanisms fail to produce an aligned "pass"
         result.

      1:  Generate a DMARC failure report if any underlying
         authentication mechanism produced something other than an
         aligned "pass" result.

      d:  Generate a DKIM failure report if the message had a signature
         that failed evaluation, regardless of its alignment.  DKIM-
         specific reporting is described in [RFC6651].

      s:  Generate an SPF failure report if the message failed SPF
         evaluation, regardless of its alignment.  SPF-specific
         reporting is described in [RFC6652].

   np:  Domain Owner Assessment Policy for non-existent subdomains
      (plain-text; OPTIONAL).  Indicates the message handling preference
      that the Domain Owner or PSO has for mail using non-existent



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      subdomains of the domain queried.  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 Section 4.7.

   p:  Domain Owner Assessment Policy (plain-text; RECOMMENDED for
      policy records).  Indicates the message handling preference the
      Domain Owner or PSO has for mail using its domain but not passing
      DMARC verification.  Policy applies to the domain queried and to
      subdomains, unless subdomain policy is explicitly described using
      the "sp" or "np" tags.  If this tag is not present in an otherwise
      syntactically valid DMARC record, then the record is treated as if
      it included "p=none" (see Section 4.7).  This tag is not
      applicable for third-party reporting records (see
      [DMARC-Aggregate-Reporting] and [DMARC-Failure-Reporting])
      Possible values are as follows:

      none:  The Domain Owner offers no expression of preference.

      quarantine:  The Domain Owner considers such mail to be
         suspicious.  It is possible the mail is valid, although the
         failure creates a significant concern.

      reject:  The Domain Owner considers all such failures to be a
         clear indication that the use of the domain name is not valid.
         See Section 7.3 for some discussion of SMTP rejection methods
         and their implications.


















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   psd:  A flag indicating whether the domain is a PSD. (plain-text;
      OPTIONAL; default is 'n').  Possible values are:

      y:  Domains on the PSL that publish DMARC policy records SHOULD
         include this tag with a value of 'y' to indicate that the
         domain is a PSD.  If a record containing this tag with a value
         of 'y' is found during policy discovery, this information will
         be used to determine the Organizational Domain applicable to
         the message in question.

      n:  The default, indicating that the DMARC policy record is
         published for a domain that is not a PSD.  There is no need to
         put psd=n in a DMARC record, except in the very unusual case of
         a parent PSD publishing a DMARC record without the requisite
         psd=y tag.

   rua:  Addresses to which aggregate feedback is to be sent (comma-
      separated plain-text list of DMARC URIs; OPTIONAL).
      [DMARC-Aggregate-Reporting] discusses considerations that apply
      when the domain name of a URI differs from that of the domain
      advertising the policy.  See Section 9.5 for additional
      considerations.  Any valid URI can be specified.  A Mail Receiver
      MUST implement support for a "mailto:" URI, i.e., the ability to
      send a DMARC report via electronic mail.  If the tag is not
      provided, Mail Receivers MUST NOT generate aggregate feedback
      reports for the domain.  URIs not supported by Mail Receivers MUST
      be ignored.  The aggregate feedback report format is described in
      [DMARC-Aggregate-Reporting]

   ruf:  Addresses to which message-specific failure information is to
      be reported (comma-separated plain-text list of DMARC URIs;
      OPTIONAL).  If present, the Domain Owner or PSO is requesting Mail
      Receivers to send detailed failure reports about messages that
      fail the DMARC evaluation in specific ways (see the "fo" tag
      above).  The format of the message to be generated MUST follow the
      format specified for the "rf" tag.  [DMARC-Aggregate-Reporting]
      discusses considerations that apply when the domain name of a URI
      differs from that of the domain advertising the policy.  A Mail
      Receiver MUST implement support for a "mailto:" URI, i.e., the
      ability to send a DMARC report via electronic mail.  If the tag is
      not provided, Mail Receivers MUST NOT generate failure reports for
      the domain.  See Section 9.5 for additional considerations.

   sp:  Domain Owner Assessment Policy for all subdomains (plain-text;
      OPTIONAL).  Indicates the message handling preference the Domain
      Owner or PSO has for mail using an existing subdomain of the
      domain queried but not passing DMARC verification.  It applies
      only to subdomains of the domain queried and not to the domain



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      itself.  Its syntax is identical to that of the "p" tag defined
      above.  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.  Note that "sp" will be ignored for
      DMARC records published on subdomains of Organizational Domains
      due to the effect of the DMARC policy discovery mechanism
      described in Section 4.7.

   t:  DMARC policy test mode (plain-text; OPTIONAL; default is 'n').
      For the RFC5322.From domain to which the DMARC record applies, the
      "t" tag serves as a signal to the actor performing DMARC
      verification checks as to whether or not the domain owner wishes
      the assessment policy declared in the "p=", "sp=", and/or "np="
      tags to actually be applied.  This parameter does not affect the
      generation of DMARC reports.  Possible values are as follows:

      y:  A request that the actor performing the DMARC verification
         check not apply the policy, but instead apply any special
         handling rules it might have in place, such as rewriting the
         RFC5322.From header.  The domain owner is currently testing its
         specified DMARC assessment policy.

      n:  The default, a request to apply the policy as specified to any
         message that produces a DMARC "fail" result.

   v:  Version (plain-text; REQUIRED).  Identifies the record retrieved
      as a DMARC record.  It MUST have the value of "DMARC1".  The value
      of this tag MUST match precisely; if it does not or it is absent,
      the entire retrieved record MUST be ignored.  It MUST be the first
      tag in the list.

   A DMARC policy record MUST comply with the formal specification found
   in Section 5.4 in that the "v" tag MUST be present and MUST appear
   first.  Unknown tags MUST be ignored.  Syntax errors in the remainder
   of the record SHOULD be discarded in favor of default values (if any)
   or ignored outright.

   Note that given the rules of the previous paragraph, addition of a
   new tag into the registered list of tags does not itself require a
   new version of DMARC to be generated (with a corresponding change to
   the "v" tag's value), but a change to any existing tags does require
   a new version of DMARC.

5.4.  Formal Definition

   The formal definition of the DMARC format, using [RFC5234], is as
   follows:




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     dmarc-uri       = URI
                       ; "URI" is imported from [RFC3986]; commas (ASCII
                       ; 0x2C) and exclamation points (ASCII 0x21)
                       ; MUST be encoded

     dmarc-record    = dmarc-version dmarc-sep *(dmarc-tag dmarc-sep)

     dmarc-tag       = dmarc-request /
                       dmarc-test /
                       dmarc-psd /
                       dmarc-sprequest /
                       dmarc-nprequest /
                       dmarc-adkim /
                       dmarc-aspf /
                       dmarc-auri /
                       dmarc-furi /
                       dmarc-fo /
                       dmarc-rfmt
                       ; components other than dmarc-version and
                       ; dmarc-request may appear in any order

     dmarc-version   = "v" *WSP "=" *WSP %x44 %x4d %x41 %x52 %x43 %x31

     dmarc-sep       = *WSP %x3b *WSP

     dmarc-request   = "p" *WSP "=" *WSP
                       ( "none" / "quarantine" / "reject" )

     dmarc-test      = "t" *WSP "=" ( "y" / "n" )

     dmarc-psd       = "psd" *WSP "=" ( "y" / "n" )

     dmarc-sprequest = "sp" *WSP "=" *WSP
                       ( "none" / "quarantine" / "reject" )

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

     dmarc-adkim     = "adkim" *WSP "=" *WSP ( "r" / "s" )

     dmarc-aspf      = "aspf" *WSP "=" *WSP ( "r" / "s" )

     dmarc-auri      = "rua" *WSP "=" *WSP
                       dmarc-uri *(*WSP "," *WSP dmarc-uri)

     dmarc-furi      = "ruf" *WSP "=" *WSP
                       dmarc-uri *(*WSP "," *WSP dmarc-uri)




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     dmarc-fo        = "fo" *WSP "=" *WSP
                       ( "0" / "1" / ( "d" / "s" / "d:s" / "s:d" ) )

     dmarc-rfmt      = "rf"  *WSP "=" *WSP Keyword *(*WSP ":" Keyword)
                       ; registered reporting formats only

   "Keyword" is imported from Section 4.1.2 of [RFC5321].

5.5.  Domain Owner Actions

   This section describes Domain Owner actions to implement the DMARC
   mechanism.

5.5.1.  Publish an SPF Policy for an Aligned Domain

   Because DMARC relies on SPF [RFC7208] and DKIM [RFC6376], in order to
   take full advantage of DMARC, a Domain Owner SHOULD first ensure that
   SPF and DKIM authentication are properly configured.  As a first step
   the Domain Owner SHOULD choose a domain to use as the
   RFC5321.MailFrom domain (i.e., the Return-Path domain) for its mail,
   one that aligns with the Author Domain, and then publish an SPF
   policy in DNS for that domain.  The SPF record SHOULD be constructed
   at a minimum to ensure an SPF pass verdict for all known sources of
   mail for the RFC5321.MailFrom domain.

5.5.2.  Configure Sending System for DKIM Signing Using an Aligned
        Domain

   While it is possible to secure a DMARC pass verdict based on only SPF
   or DKIM, it is commonly accepted best practice to ensure that both
   authentication mechanisms are in place in order to guard against
   failure of just one of them.  The Domain Owner SHOULD choose a DKIM-
   Signing domain (i.e., the d= domain in the DKIM-Signature header)
   that aligns with the Author Domain.

5.5.3.  Setup a Mailbox to Receive Aggregate Reports

   Proper consumption and analysis of DMARC aggregate reports is the key
   to any successful DMARC deployment for a Domain Owner.  DMARC
   aggregate reports, which are XML documents and are defined in
   [DMARC-Aggregate-Reporting], contain valuable data for the Domain
   Owner, showing sources of mail using the Author Domain.  Depending on
   how mature the Domain Owner's DMARC rollout is, some of these sources
   could be legitimate ones that were overlooked during the initial
   deployment of SPF and/or DKIM.






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   Because the aggregate reports are XML documents, it is recommended
   that they be machine-parsed, so setting up a mailbox involves more
   than just the physical creation of that mailbox.  Many third-party
   services exist that will process DMARC aggregate reports, or the
   Domain Owner can create its own set of tools.  No matter which method
   is chosen, the ability to parse these reports and consume the data
   contained in them will go a long way to ensuring a successful
   deployment.

5.5.4.  Publish a DMARC Policy for the Author Domain

   Once SPF, DKIM, and the aggregate reports mailbox are all in place,
   it's time to publish a DMARC record.  For best results, Domain Owners
   SHOULD start with "p=none", with the rua tag containg a URI that
   references the mailbox created in the previous step.

5.5.5.  Collect and Analyze Reports

   The reason for starting at "p=none" is to ensure that nothing's been
   missed in the initial SPF and DKIM deployments.  In all but the most
   trivial setups, it is possible for a Domain Owner to overlook a
   server here or be unaware of a third party sending agreeement there.
   Starting at "p=none", therefore, takes advantage of DMARC's aggregate
   reporting function, with the Domain Owner using the reports to audit
   its own mail streams' authentication configurations.

5.5.6.  Decide If and When to Update DMARC Policy

   Once the Domain Owner is satisfied that it is properly authenticating
   all of its mail, then it is time to decide if it is appropriate to
   change the p= value in its DMARC record to p=quarantine or p=reject.
   Depending on its cadence for sending mail, it may take many months of
   consuming DMARC aggregate reports before a Domain Owner reaches the
   point where it is sure that it is properly authenticating all of its
   mail, and the decision on which p= value to use will depend on its
   needs.

5.6.  PSO Actions

   In addition to the DMARC Domain Owner actions, if a PSO publishes a
   DMARC record it MUST include the psd tag (see Section 5.3) with a
   value of 'y' ("psd=y").

5.7.  Mail Receiver Actions

   This section describes receiver actions in the DMARC environment.





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5.7.1.  Extract Author Domain

   The domain in the RFC5322.From header field is extracted as the
   domain to be evaluated by DMARC.  If the domain is encoded with UTF-
   8, the domain name must be converted to an A-label, as described in
   Section 2.3 of [RFC5890], for further processing.

   In order to be processed by DMARC, a message typically needs to
   contain exactly one RFC5322.From domain (a single From: field with a
   single domain in it).  Not all messages meet this requirement, and
   the handling of those that are forbidden under [RFC5322] or that
   contain no meaningful domains is outside the scope of this document.

   The case of a syntactically valid multi-valued RFC5322.From header
   field presents a particular challenge.  When a single RFC5322.From
   header field contains multiple addresses, it is possible that there
   may be multiple domains used in those addresses.  The process in this
   case is to only proceed with DMARC checking if the domain is
   identical for all of the addresses in a multi-valued RFC5322.From
   header field.  Multi-valued RFC5322.From header fields with multiple
   domains MUST be exempt from DMARC checking.

   Note that Public Suffix Domains are not exempt from DMARC policy
   application and reporting.

5.7.2.  Determine Handling Policy

   To arrive at a policy for an individual message, Mail Receivers MUST
   perform the following actions or their semantic equivalents.  Steps
   2-4 MAY be done in parallel, whereas steps 5 and 6 require input from
   previous steps.

   The steps are as follows:

   1.  Extract the RFC5322.From domain from the message (as above).

   2.  Query the DNS for a DMARC policy record.  Continue if one is
       found, or terminate DMARC evaluation otherwise.  See Section 4.7
       for details.

   3.  Perform DKIM signature verification checks.  A single email could
       contain multiple DKIM signatures.  The results of this step are
       passed to the remainder of the algorithm, MUST include "pass" or
       "fail", and if "fail", SHOULD include information about the
       reasons for failure.  The results MUST further include the value
       of the "d=" and "s=" tags from each checked DKIM signature.





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   4.  Perform SPF verification checks.  The results of this step are
       passed to the remainder of the algorithm, MUST include "pass" or
       "fail", and if "fail", SHOULD include information about the
       reasons for failure.  The results MUST further include the domain
       name used to complete the SPF check.

   5.  Conduct Identifier Alignment checks.  With authentication checks
       and policy discovery performed, the Mail Receiver checks to see
       if Authenticated Identifiers fall into alignment as described in
       Section 4.4.  If one or more of the Authenticated Identifiers
       align with the RFC5322.From domain, the message is considered to
       pass the DMARC mechanism check.  All other conditions
       (authentication failures, identifier mismatches) are considered
       to be DMARC mechanism check failures.

   6.  Apply policy, if appropriate.  Emails that fail the DMARC
       mechanism check are handled in accordance with the discovered
       DMARC policy of the Domain Owner and any local policy rules
       enforced by the Mail Receiver.  See Section 5.3 for details.

   DMARC evaluation can only yield a "pass" result after one of the
   underlying authentication mechanisms passes for an aligned
   identifier.  If neither passes and one or both of them fail due to a
   temporary error, the Mail Receiver evaluating the message is unable
   to conclude that the DMARC mechanism had a permanent failure; they
   therefore cannot apply the advertised DMARC policy.  When otherwise
   appropriate, Mail Receivers MAY send feedback reports regarding
   temporary errors.

   Handling of messages for which SPF and/or DKIM evaluation encounter a
   permanent DNS error is left to the discretion of the Mail Receiver.

5.7.3.  Store Results of DMARC Processing

   The results of Mail Receiver-based DMARC processing should be stored
   for eventual presentation back to the Domain Owner in the form of
   aggregate feedback reports.  Section 5.3 and
   [DMARC-Aggregate-Reporting] discuss aggregate feedback.

5.7.4.  Send Aggregate Reports

   For a Domain Owner, DMARC aggregate reports provide data about all
   mailstreams making use of its domain in email, to include not only
   illegitimate uses but also, and perhaps more importantly, all
   legitimate uses.  Domain Owners can use aggregate reports to ensure
   that all legitimate uses of their domain for sending email are
   properly authenticated, and once they are, express a stricter message
   handling preference in the p= tag in their DMARC policy records from



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   none to quarantine to reject, if appropriate.  In turn, DMARC policy
   records with p= tag values of 'quarantine' or 'reject' are higher
   value signals to Mail Receivers, ones that can assist Mail Receivers
   with handling decisions for a message in ways that p= tag values of
   'none' cannot.

   Given the above, in order to ensure maximum usefulness for DMARC
   across the email ecosystem, Mail Receivers SHOULD generate and send
   aggregate reports with a frequency of at least once every 24 hours.

5.8.  Policy Enforcement Considerations

   Mail Receivers MAY choose to reject or quarantine email even if email
   passes the DMARC mechanism check.  The DMARC mechanism does not
   inform Mail Receivers whether an email stream is "good"; a DMARC
   result of "pass" only means that the domain in the RFC5322.From
   header has been verified by the DMARC mechanism.  Mail Receivers are
   encouraged to maintain anti-abuse technologies to combat the
   possibility of DMARC-enabled criminal campaigns.

   Mail Receivers MAY choose to accept email that fails the DMARC
   mechanism check even if the published Domain Owner Assessment Policy
   is "reject".  Mail Receivers need to make a best effort not to
   increase the likelihood of accepting abusive mail if they choose not
   to honor the published Domain Owner Assessment Policy.  At a minimum,
   addition of the Authentication-Results header field (see [RFC8601])
   is RECOMMENDED when delivery of failing mail is done.  When this is
   done, the DNS domain name thus recorded MUST be encoded as an
   A-label.

   Mail Receivers are only obligated to report reject or quarantine
   policy actions in aggregate feedback reports that are due to
   published DMARC Domain Owner Assessment Policy.  They are not
   required to report reject or quarantine actions that are the result
   of local policy.  If local policy information is exposed, abusers can
   gain insight into the effectiveness and delivery rates of spam
   campaigns.

   Final handling of a message is always a matter of local policy.  An
   operator that wishes to favor DMARC policy over SPF policy, for
   example, will disregard the SPF policy, since enacting an SPF-
   determined rejection prevents evaluation of DKIM; DKIM might
   otherwise pass, satisfying the DMARC evaluation.  There is a trade-
   off to doing so, namely acceptance and processing of the entire
   message body in exchange for the enhanced protection DMARC provides.






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   DMARC-compliant Mail Receivers typically disregard any mail-handling
   directive discovered as part of an authentication mechanism (e.g.,
   Author Domain Signing Practices (ADSP), SPF) where a DMARC record is
   also discovered that specifies a policy other than "none".  Deviating
   from this practice introduces inconsistency among DMARC operators in
   terms of handling of the message.  However, such deviation is not
   proscribed.

   To enable Domain Owners to receive DMARC feedback without impacting
   existing mail processing, discovered policies of "p=none" SHOULD NOT
   modify existing mail handling processes.

   Mail Receivers MUST also implement reporting instructions of DMARC,
   even in the absence of a request for DKIM reporting [RFC6651] or SPF
   reporting [RFC6652].  Furthermore, the presence of such requests
   SHOULD NOT affect DMARC reporting.

6.  DMARC Feedback

   Providing Domain Owners with visibility into how Mail Receivers
   implement and enforce the DMARC mechanism in the form of feedback is
   critical to establishing and maintaining accurate authentication
   deployments.  When Domain Owners can see what effect their policies
   and practices are having, they are better willing and able to use
   quarantine and reject policies.

   The details of this feedback are described in
   [DMARC-Aggregate-Reporting]

   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

7.  Other Topics

   This section discusses some topics regarding choices made in the
   development of DMARC, largely to commit the history to record.

7.1.  Issues Specific to SPF

   Though DMARC does not inherently change the semantics of an SPF
   policy record, historically lax enforcement of such policies has led
   many to publish extremely broad records containing many large network
   ranges.  Domain Owners are strongly encouraged to carefully review
   their SPF records to understand which networks are authorized to send
   on behalf of the Domain Owner before publishing a DMARC record.




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   Some Mail Receiver architectures might implement SPF in advance of
   any DMARC operations.  This means that a "-" prefix on a sender's SPF
   mechanism, such as "-all", could cause that rejection to go into
   effect early in handling, causing message rejection before any DMARC
   processing takes place.  Operators choosing to use "-all" should be
   aware of this.

7.2.  DNS Load and Caching

   DMARC policies are communicated using the DNS and therefore inherit a
   number of considerations related to DNS caching.  The inherent
   conflict between freshness and the impact of caching on the reduction
   of DNS-lookup overhead should be considered from the Mail Receiver's
   point of view.  Should Domain Owners or PSOs publish a DNS record
   with a very short TTL, Mail Receivers can be provoked through the
   injection of large volumes of messages to overwhelm the publisher's
   DNS.  Although this is not a concern specific to DMARC, the
   implications of a very short TTL should be considered when publishing
   DMARC policies.

   Conversely, long TTLs will cause records to be cached for long
   periods of time.  This can cause a critical change to DMARC
   parameters advertised by a Domain Owner or PSO to go unnoticed for
   the length of the TTL (while waiting for DNS caches to expire).
   Avoiding this problem can mean shorter TTLs, with the potential
   problems described above.  A balance should be sought to maintain
   responsiveness of DMARC preference changes while preserving the
   benefits of DNS caching.

7.3.  Rejecting Messages

   This protocol calls for rejection of a message during the SMTP
   session under certain circumstances.  This is preferable to
   generation of a Delivery Status Notification [RFC3464], since
   fraudulent messages caught and rejected using DMARC would then result
   in annoying generation of such failure reports that go back to the
   RFC5321.MailFrom address.

   This synchronous rejection is typically done in one of two ways:

   *  Full rejection, wherein the SMTP server issues a 5xy reply code as
      an indication to the SMTP client that the transaction failed; the
      SMTP client is then responsible for generating notification that
      delivery failed (see Section 4.2.5 of [RFC5321]).







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   *  A "silent discard", wherein the SMTP server returns a 2xy reply
      code implying to the client that delivery (or, at least, relay)
      was successfully completed, but then simply discarding the message
      with no further action.

   Each of these has a cost.  For instance, a silent discard can help to
   prevent backscatter, but it also effectively means that the SMTP
   server has to be programmed to give a false result, which can
   confound external debugging efforts.

   Similarly, the text portion of the SMTP reply may be important to
   consider.  For example, when rejecting a message, revealing the
   reason for the rejection might give an attacker enough information to
   bypass those efforts on a later attempt, though it might also assist
   a legitimate client to determine the source of some local issue that
   caused the rejection.

   In the latter case, when doing an SMTP rejection, providing a clear
   hint can be useful in resolving issues.  A Mail Receiver might
   indicate in plain text the reason for the rejection by using the word
   "DMARC" somewhere in the reply text.  For example:

   550 5.7.1 Email rejected per DMARC policy for example.com

   Many systems are able to scan the SMTP reply text to determine the
   nature of the rejection.  Thus, providing a machine-detectable reason
   for rejection allows the problems causing rejections to be properly
   addressed by automated systems.

   If a Mail Receiver elects to defer delivery due to inability to
   retrieve or apply DMARC policy, this is best done with a 4xy SMTP
   reply code.

7.4.  Identifier Alignment Considerations

   The DMARC mechanism allows both DKIM and SPF-authenticated
   identifiers to authenticate email on behalf of a Domain Owner and,
   possibly, on behalf of different subdomains.  If malicious or unaware
   users can gain control of the SPF record or DKIM selector records for
   a subdomain, the subdomain can be used to generate DMARC-passing
   email on behalf of the Organizational Domain.

   For example, an attacker who controls the SPF record for
   "evil.example.com" can send mail with an RFC5322.From header field
   containing "foo@example.com" that can pass both authentication and
   the DMARC check against "example.com".





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   The Organizational Domain administrator should be careful not to
   delegate control of subdomains if this is an issue, and to consider
   using the "strict" Identifier Alignment option if appropriate.

7.5.  Interoperability Issues

   DMARC limits which end-to-end scenarios can achieve a "pass" result.

   Because DMARC relies on SPF [RFC7208] and/or DKIM [RFC6376] to
   achieve a "pass", their limitations also apply.

   Additional DMARC constraints occur when a message is processed by
   some Mediators, such as mailing lists.  Transiting a Mediator often
   causes either the authentication to fail or Identifier Alignment to
   be lost.  These transformations may conform to standards but will
   still prevent a DMARC "pass".

   In addition to Mediators, mail that is sent by authorized,
   independent third parties might not be sent with Identifier
   Alignment, also preventing a "pass" result.

   Issues specific to the use of policy mechanisms alongside DKIM are
   further discussed in [RFC6377], particularly Section 5.2.

8.  IANA Considerations

   This section describes actions completed by IANA.

8.1.  Authentication-Results Method Registry Update

   IANA has added the following to the "Email Authentication Methods"
   registry:



















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   +------+-----------+------+----------+--------------+------+--------+
   |Method| Defined   |ptype | Property | Value        |Status|Version |
   +======+===========+======+==========+==============+======+========+
   |dmarc | [RFC7489] |header| from     | the domain   |active| 1      |
   |      |           |      |          |portion of the|      |        |
   |      |           |      |          | RFC5322.From |      |        |
   |      |           |      |          | header field |      |        |
   +------+-----------+------+----------+--------------+------+--------+
   |dmarc | [RFC7489] |polrec| p        | the p= value |active| 1      |
   |      |           |      |          |read from the |      |        |
   |      |           |      |          | discovered   |      |        |
   |      |           |      |          |policy record |      |        |
   +------+-----------+------+----------+--------------+------+--------+
   |dmarc | [RFC7489] |polrec| domain   |the domain at |active| 1      |
   |      |           |      |          | which the    |      |        |
   |      |           |      |          |policy record |      |        |
   |      |           |      |          | was          |      |        |
   |      |           |      |          |discovered, if|      |        |
   |      |           |      |          |different from|      |        |
   |      |           |      |          | the          |      |        |
   |      |           |      |          | RFC5322.From |      |        |
   |      |           |      |          | domain       |      |        |
   +------+-----------+------+----------+--------------+------+--------+

          Table 1: "Authentication-Results Method Registry Update"

8.2.  Authentication-Results Result Registry Update

   IANA has added the following in the "Email Authentication Result
   Names" registry:

   +-----------+-----------+-----------+-------+----------------+------+
   | Code      | Existing/ | Defined   | Auth  | Meaning        |Status|
   |           | New Code  |           |Method |                |      |
   +===========+===========+===========+=======+================+======+
   | none      | existing  | [RFC8601] | dmarc |No DMARC policy |active|
   |           |           |           |(added)| record was     |      |
   |           |           |           |       | published for  |      |
   |           |           |           |       | the aligned    |      |
   |           |           |           |       | identifier, or |      |
   |           |           |           |       | no aligned     |      |
   |           |           |           |       |identifier could|      |
   |           |           |           |       | be extracted.  |      |
   +-----------+-----------+-----------+-------+----------------+------+
   | pass      | existing  | [RFC8601] | dmarc | A DMARC policy |active|
   |           |           |           |(added)| record was     |      |
   |           |           |           |       | published for  |      |
   |           |           |           |       | the aligned    |      |



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   |           |           |           |       |identifier, and |      |
   |           |           |           |       |at least one of |      |
   |           |           |           |       | the            |      |
   |           |           |           |       | authentication |      |
   |           |           |           |       | mechanisms     |      |
   |           |           |           |       | passed.        |      |
   +-----------+-----------+-----------+-------+----------------+------+
   | fail      | existing  | [RFC8601] | dmarc | A DMARC policy |active|
   |           |           |           |(added)| record was     |      |
   |           |           |           |       | published for  |      |
   |           |           |           |       | the aligned    |      |
   |           |           |           |       |identifier, and |      |
   |           |           |           |       | none of the    |      |
   |           |           |           |       | authentication |      |
   |           |           |           |       | mechanisms     |      |
   |           |           |           |       | passed.        |      |
   +-----------+-----------+-----------+-------+----------------+------+
   | temperror | existing  | [RFC8601] | dmarc | A temporary    |active|
   |           |           |           |(added)| error occurred |      |
   |           |           |           |       | during DMARC   |      |
   |           |           |           |       | evaluation.  A |      |
   |           |           |           |       | later attempt  |      |
   |           |           |           |       |might produce a |      |
   |           |           |           |       | final result.  |      |
   +-----------+-----------+-----------+-------+----------------+------+
   | permerror | existing  | [RFC8601] | dmarc | A permanent    |active|
   |           |           |           |(added)| error occurred |      |
   |           |           |           |       | during DMARC   |      |
   |           |           |           |       |evaluation, such|      |
   |           |           |           |       |as encountering |      |
   |           |           |           |       |a syntactically |      |
   |           |           |           |       |incorrect DMARC |      |
   |           |           |           |       |record.  A later|      |
   |           |           |           |       | attempt is     |      |
   |           |           |           |       | unlikely to    |      |
   |           |           |           |       |produce a final |      |
   |           |           |           |       | result.        |      |
   +-----------+-----------+-----------+-------+----------------+------+

          Table 2: "Authentication-Results Result Registry Update"

8.3.  Feedback Report Header Fields Registry Update

   The following has been added to the "Feedback Report Header Fields"
   registry:

   Field Name: Identity-Alignment




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   Description:  indicates whether the message about which a report is
      being generated had any identifiers in alignment as defined in RFC
      7489

   Multiple Appearances: No

   Related "Feedback-Type": auth-failure

   Reference: RFC 7489

   Status: current

8.4.  DMARC Tag Registry

   A new registry tree called "Domain-based Message Authentication,
   Reporting, and Conformance (DMARC) Parameters" has been created.
   Within it, a new sub-registry called the "DMARC Tag Registry" has
   been created.

   Names of DMARC tags must be registered with IANA in this new sub-
   registry.  New entries are assigned only for values that have been
   documented in a manner that satisfies the terms of Specification
   Required, per [RFC8126].  Each registration must include the tag
   name; the specification that defines it; a brief description; and its
   status, which must be one of "current", "experimental", or
   "historic".  The Designated Expert needs to confirm that the provided
   specification adequately describes the new tag and clearly presents
   how it would be used within the DMARC context by Domain Owners and
   Mail Receivers.

   To avoid version compatibility issues, tags added to the DMARC
   specification are to avoid changing the semantics of existing records
   when processed by implementations conforming to prior specifications.

   The initial set of entries in this registry is as follows:
















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   +-------+-----------+----------+-----------------------------+
   | Tag   | Reference | Status   | Description                 |
   | Name  |           |          |                             |
   +=======+===========+==========+=============================+
   | adkim | RFC 7489  | current  | DKIM alignment mode         |
   +-------+-----------+----------+-----------------------------+
   | aspf  | RFC 7489  | current  | SPF alignment mode          |
   +-------+-----------+----------+-----------------------------+
   | fo    | RFC 7489  | current  | Failure reporting options   |
   +-------+-----------+----------+-----------------------------+
   | np    | RFC 7489  | current  | Requested handling policy   |
   |       |           |          | for non-existent subdomains |
   +-------+-----------+----------+-----------------------------+
   | p     | RFC 7489  | current  | Requested handling policy   |
   +-------+-----------+----------+-----------------------------+
   | pct   | RFC 7489  | historic | Sampling rate               |
   +-------+-----------+----------+-----------------------------+
   | psd   | RFC 7489  | current  | Indicates whether policy    |
   |       |           |          | record is published by a    |
   |       |           |          | Public Suffix Domain        |
   +-------+-----------+----------+-----------------------------+
   | rf    | RFC 7489  | historic | Failure reporting format(s) |
   +-------+-----------+----------+-----------------------------+
   | ri    | RFC 7489  | historic | Aggregate Reporting         |
   |       |           |          | interval                    |
   +-------+-----------+----------+-----------------------------+
   | rua   | RFC 7489  | current  | Reporting URI(s) for        |
   |       |           |          | aggregate data              |
   +-------+-----------+----------+-----------------------------+
   | ruf   | RFC 7489  | current  | Reporting URI(s) for        |
   |       |           |          | failure data                |
   +-------+-----------+----------+-----------------------------+
   | sp    | RFC 7489  | current  | Requested handling policy   |
   |       |           |          | for subdomains              |
   +-------+-----------+----------+-----------------------------+
   | t     | RFC 7489  | current  | Test mode for the specified |
   |       |           |          | policy                      |
   +-------+-----------+----------+-----------------------------+
   | v     | RFC 7489  | current  | Specification version       |
   +-------+-----------+----------+-----------------------------+

                   Table 3: "DMARC Tag Registry"

8.5.  DMARC Report Format Registry

   Also within "Domain-based Message Authentication, Reporting, and
   Conformance (DMARC) Parameters", a new sub-registry called "DMARC
   Report Format Registry" has been created.



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   Names of DMARC failure reporting formats must be registered with IANA
   in this registry.  New entries are assigned only for values that
   satisfy the definition of Specification Required, per [RFC8126].  In
   addition to a reference to a permanent specification, each
   registration must include the format name; a brief description; and
   its status, which must be one of "current", "experimental", or
   "historic".  The Designated Expert needs to confirm that the provided
   specification adequately describes the report format and clearly
   presents how it would be used within the DMARC context by Domain
   Owners and Mail Receivers.

   The initial entry in this registry is as follows:

   +--------+-----------+---------+----------------------------------+
   | Format | Reference | Status  | Description                      |
   | Name   |           |         |                                  |
   +========+===========+=========+==================================+
   | afrf   | RFC 7489  | current | Authentication Failure Reporting |
   |        |           |         | Format (see [RFC6591])           |
   +--------+-----------+---------+----------------------------------+

                 Table 4: "DMARC Report Format Registry"

8.6.  Underscored and Globally Scoped DNS Node Names Registry

   Per [RFC8552], please add the following entry to the "Underscored and
   Globally Scoped DNS Node Names" registry:

   +---------+------------+-----------+
   | RR Type | _NODE NAME | Reference |
   +=========+============+===========+
   | TXT     | _dmarc     | RFC 7489  |
   +---------+------------+-----------+

        Table 5: "Underscored and
     Globally Scoped DNS Node Names"
                 registry

9.  Security Considerations

   This section discusses security issues and possible remediations
   (where available) for DMARC.

9.1.  Authentication Methods

   Security considerations from the authentication methods used by DMARC
   are incorporated here by reference.




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9.2.  Attacks on Reporting URIs

   URIs published in DNS TXT records are well-understood possible
   targets for attack.  Specifications such as [RFC1035] and [RFC2142]
   either expose or cause the exposure of email addresses that could be
   flooded by an attacker, for example; MX, NS, and other records found
   in the DNS advertise potential attack destinations; common DNS names
   such as "www" plainly identify the locations at which particular
   services can be found, providing destinations for targeted denial-of-
   service or penetration attacks.

   Thus, Domain Owners will need to harden these addresses against
   various attacks, including but not limited to:

   *  high-volume denial-of-service attacks;

   *  deliberate construction of malformed reports intended to identify
      or exploit parsing or processing vulnerabilities;

   *  deliberate construction of reports containing false claims for the
      Submitter or Reported-Domain fields, including the possibility of
      false data from compromised but known Mail Receivers.

9.3.  DNS Security

   The DMARC mechanism and its underlying technologies (SPF, DKIM)
   depend on the security of the DNS.  Examples of how hostile parties
   can have an adverse impact on DNS traffic include:

   *  If they can snoop on DNS traffic, they can get an idea of who is
      sending mail.

   *  If they can block outgoing or reply DNS messages, they can prevent
      systems from discovering senders' DMARC policies, causing
      recipients to assume p=none by default.

   *  If they can send forged response packets, they can make aligned
      mail appear unaligned or vice-versa.

   None of these threats are unique to DMARC, and they can be addressed
   using a variety of techniques, including, but not limited to:

   *  Signing DNS records with DNSSEC [RFC4033], which enables
      recipients to detect and discard forged responses.

   *  DNS over TLS [RFC7858] or DNS over HTTPS [RFC8484] can mitigate
      snooping and forged responses.




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9.4.  Display Name Attacks

   A common attack in messaging abuse is the presentation of false
   information in the display-name portion of the RFC5322.From header
   field.  For example, it is possible for the email address in that
   field to be an arbitrary address or domain name, while containing a
   well-known name (a person, brand, role, etc.) in the display name,
   intending to fool the end user into believing that the name is used
   legitimately.  The attack is predicated on the notion that most
   common MUAs will show the display name and not the email address when
   both are available.

   Generally, display name attacks are out of scope for DMARC, as
   further exploration of possible defenses against these attacks needs
   to be undertaken.

   There are a few possible mechanisms that attempt mitigation of these
   attacks, such as the following:

   *  If the display name is found to include an email address (as
      specified in [RFC5322]), execute the DMARC mechanism on the domain
      name found there rather than the domain name discovered
      originally.  However, this addresses only a very specific attack
      space, and spoofers can easily circumvent it by simply not using
      an email address in the display name.  There are also known cases
      of legitimate uses of an email address in the display name with a
      domain different from the one in the address portion, e.g.,

      From: "user@example.org via Bug Tracker" support@example.com
      (mailto:support@example.com)

   *  In the MUA, only show the display name if the DMARC mechanism
      succeeds.  This too is easily defeated, as an attacker could
      arrange to pass the DMARC tests while fraudulently using another
      domain name in the display name.

   *  In the MUA, only show the display name if the DMARC mechanism
      passes and the email address thus verified matches one found in
      the receiving user's list of known addresses.

9.5.  External Reporting Addresses

   To avoid abuse by bad actors, reporting addresses generally have to
   be inside the domains about which reports are requested.  In order to
   accommodate special cases such as a need to get reports about domains
   that cannot actually receive mail, Section 3 of
   [DMARC-Aggregate-Reporting] describes a DNS-based mechanism for
   verifying approved external reporting.



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   The obvious consideration here is an increased DNS load against
   domains that are claimed as external recipients.  Negative caching
   will mitigate this problem, but only to a limited extent, mostly
   dependent on the default TTL in the domain's SOA record.

   Where possible, external reporting is best achieved by having the
   report be directed to domains that can receive mail and simply having
   it automatically forwarded to the desired external destination.

   Note that the addresses shown in the "ruf" tag receive more
   information that might be considered private data, since it is
   possible for actual email content to appear in the failure reports.
   The URIs identified there are thus more attractive targets for
   intrusion attempts than those found in the "rua" tag.  Moreover,
   attacking the DNS of the subject domain to cause failure data to be
   routed fraudulently to an attacker's systems may be an attractive
   prospect.  Deployment of [RFC4033] is advisable if this is a concern.

9.6.  Secure Protocols

   This document encourages use of secure transport mechanisms to
   prevent loss of private data to third parties that may be able to
   monitor such transmissions.  Unencrypted mechanisms should be
   avoided.

   In particular, a message that was originally encrypted or otherwise
   secured might appear in a report that is not sent securely, which
   could reveal private information.

10.  Normative References

   [DMARC-Aggregate-Reporting]
              Brotman, A., Ed., "DMARC Aggregate Reporting", February
              2021, <https://datatracker.ietf.org/doc/draft-ietf-dmarc-
              aggregate-reporting/>.

   [DMARC-Failure-Reporting]
              Jones, S.M., Ed. and A. Vesely, Ed., "DMARC Failure
              Reporting", February 2021,
              <https://datatracker.ietf.org/doc/draft-ietf-dmarc-
              failure-reporting/>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.






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   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4343]  Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,
              <https://www.rfc-editor.org/info/rfc4343>.

   [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>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6591]  Fontana, H., "Authentication Failure Reporting Using the
              Abuse Reporting Format", RFC 6591, DOI 10.17487/RFC6591,
              April 2012, <https://www.rfc-editor.org/info/rfc6591>.

   [RFC6651]  Kucherawy, M., "Extensions to DomainKeys Identified Mail
              (DKIM) for Failure Reporting", RFC 6651,
              DOI 10.17487/RFC6651, June 2012,
              <https://www.rfc-editor.org/info/rfc6651>.





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   [RFC6652]  Kitterman, S., "Sender Policy Framework (SPF)
              Authentication Failure Reporting Using the Abuse Reporting
              Format", RFC 6652, DOI 10.17487/RFC6652, June 2012,
              <https://www.rfc-editor.org/info/rfc6652>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [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>.

   [RFC8552]  Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
              <https://www.rfc-editor.org/info/rfc8552>.

   [RFC9091]  Kitterman, S. and T. Wicinski, Ed., "Experimental Domain-
              Based Message Authentication, Reporting, and Conformance
              (DMARC) Extension for Public Suffix Domains", RFC 9091,
              DOI 10.17487/RFC9091, July 2021,
              <https://www.rfc-editor.org/info/rfc9091>.

11.  Informative References

   [RFC2142]  Crocker, D., "Mailbox Names for Common Services, Roles and
              Functions", RFC 2142, DOI 10.17487/RFC2142, May 1997,
              <https://www.rfc-editor.org/info/rfc2142>.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              DOI 10.17487/RFC3464, January 2003,
              <https://www.rfc-editor.org/info/rfc3464>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

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






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   [RFC6377]  Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
              September 2011, <https://www.rfc-editor.org/info/rfc6377>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [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>.

   [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>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 8601,
              DOI 10.17487/RFC8601, May 2019,
              <https://www.rfc-editor.org/info/rfc8601>.

Appendix A.  Technology Considerations

   This section documents some design decisions that were made in the
   development of DMARC.  Specifically addressed here are some
   suggestions that were considered but not included in the design, with
   explanatory text regarding the decision.

A.1.  S/MIME

   S/MIME, or Secure Multipurpose Internet Mail Extensions, is a
   standard for encryption and signing of MIME data in a message.  This
   was suggested and considered as a third security protocol for
   authenticating the source of a message.







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   DMARC is focused on authentication at the domain level (i.e., the
   Domain Owner taking responsibility for the message), while S/MIME is
   really intended for user-to-user authentication and encryption.  This
   alone appears to make it a bad fit for DMARC's goals.

   S/MIME also suffers from the heavyweight problem of Public Key
   Infrastructure, which means that distribution of keys used to verify
   signatures needs to be incorporated.  In many instances, this alone
   is a showstopper.  There have been consistent promises that PKI
   usability and deployment will improve, but these have yet to
   materialize.  DMARC can revisit this choice after those barriers are
   addressed.

   S/MIME has extensive deployment in specific market segments
   (government, for example) but does not enjoy similar widespread
   deployment over the general Internet, and this shows no signs of
   changing.  DKIM and SPF both are deployed widely over the general
   Internet, and their adoption rates continue to be positive.

   Finally, experiments have shown that including S/MIME support in the
   initial version of DMARC would neither cause nor enable a substantial
   increase in the accuracy of the overall mechanism.

A.2.  Method Exclusion

   It was suggested that DMARC include a mechanism by which a Domain
   Owner could tell Mail Receivers not to attempt verification by one of
   the supported methods (e.g., "check DKIM, but not SPF").

   Specifically, consider a Domain Owner that has deployed one of the
   technologies, and that technology fails for some messages, but such
   failures don't cause enforcement action.  Deploying DMARC would cause
   enforcement action for policies other than "none", which would appear
   to exclude participation by that Domain Owner.

   The DMARC development team evaluated the idea of policy exception
   mechanisms on several occasions and invariably concluded that there
   was not a strong enough use case to include them.  The specific
   target audience for DMARC does not appear to have concerns about the
   failure modes of one or the other being a barrier to DMARC's
   adoption.

   In the scenario described above, the Domain Owner has a few options:

   1.  Tighten up its infrastructure to minimize the failure modes of
       the single deployed technology.





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   2.  Deploy the other supported authentication mechanism, to offset
       the failure modes of the first.

   3.  Deploy DMARC in a reporting-only mode.

A.3.  Sender Header Field

   It has been suggested in several message authentication efforts that
   the Sender header field be checked for an identifier of interest, as
   the standards indicate this as the proper way to indicate a re-
   mailing of content such as through a mailing list.  Most recently, it
   was a protocol-level option for DomainKeys, but on evolution to DKIM,
   this property was removed.

   The DMARC development team considered this and decided not to include
   support for doing so, for the following reasons:

   1.  The main user protection approach is to be concerned with what
       the user sees when a message is rendered.  There is no consistent
       behavior among MUAs regarding what to do with the content of the
       Sender field, if present.  Accordingly, supporting checking of
       the Sender identifier would mean applying policy to an identifier
       the end user might never actually see, which can create a vector
       for attack against end users by simply forging a Sender field
       containing some identifier that DMARC will like.

   2.  Although it is certainly true that this is what the Sender field
       is for, its use in this way is also unreliable, making it a poor
       candidate for inclusion in the DMARC evaluation algorithm.

   3.  Allowing multiple ways to discover policy introduces unacceptable
       ambiguity into the DMARC evaluation algorithm in terms of which
       policy is to be applied and when.

A.4.  Domain Existence Test

   A previous version of this protocol used the test specified in
   [RFC5321] to determine a domain's existence.  This test requires up
   to three DNS lookups for the MX, A, and AAAA RRs for the name in
   question.

   This version of the protocol relies solely on the test for existence
   as defined in [RFC8020].  If a query for a name returns NXDOMAIN,
   then the name does not exist.







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A.5.  Issues with ADSP in Operation

   DMARC has been characterized as a "super-ADSP" of sorts.

   Contributors to DMARC have compiled a list of issues associated with
   ADSP, gained from operational experience, that have influenced the
   direction of DMARC:

   1.  ADSP has no support for subdomains, i.e., the ADSP record for
       example.com does not explicitly or implicitly apply to
       subdomain.example.com.  If wildcarding is not applied, then
       spammers can trivially bypass ADSP by sending from a subdomain
       with no ADSP record.

   2.  Nonexistent subdomains are explicitly out of scope in ADSP.
       There is nothing in ADSP that states Mail Receivers should simply
       reject mail from NXDOMAINs regardless of ADSP policy (which of
       course allows spammers to trivially bypass ADSP by sending email
       from nonexistent subdomains).

   3.  ADSP has no operational advice on when to look up the ADSP
       record.

   4.  ADSP has no support for using SPF as an auxiliary mechanism to
       DKIM.

   5.  ADSP has no support for a slow rollout, i.e., no way to configure
       a percentage of email on which the Mail Receiver should apply the
       policy.  This is important for large-volume senders.

   6.  ADSP has no explicit support for an intermediate phase where the
       Mail Receiver quarantines (e.g., sends to the recipient's "spam"
       folder) rather than rejects the email.

   7.  The binding between the "From" header domain and DKIM is too
       tight for ADSP; they must match exactly.

A.6.  Organizational Domain Discovery Issues

   An earlier informational version of the DMARC protocol [RFC7489]
   noted that the DNS does not provide a method by which the "domain of
   record", or the domain that was actually registered with a domain
   registrar, can be determined given an arbitrary domain name.  That
   version further mentioned suggestions that have been made that
   attempt to glean such information from SOA or NS resource records,
   but these too are not fully reliable, as the partitioning of the DNS
   is not always done at administrative boundaries.




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   That previous version posited that one could "climb the tree" to find
   the Organizational Domain, but expressed concern that an attacker
   could exploit this for a denial-of-service attack through sending a
   high number of messages each with a relatively large number of
   nonsense labels, causing a Mail Receiver to perform a large number of
   DNS queries in search of a policy record.  This version defines a
   method for performing a DNS Tree Walk, described in Section 4.6, and
   further mitigates the risk of the denial-of-service attack by
   expressly limiting the number of DNS queries to execute regardless of
   the number of labels in the domain name.

   As a matter of historical record, the method for finding the
   Organizational Domain described in [RFC7489] is preserved here:

   1.  Acquire a "public suffix" list (PSL), i.e., a list of DNS domain
       names reserved for registrations.  Some country Top-Level Domains
       (TLDs) make specific registration requirements, e.g., the United
       Kingdom places company registrations under ".co.uk"; other TLDs
       such as ".com" appear in the IANA registry of top-level DNS
       domains.  A PSL is the union of all of these.

       A PSL can be obtained from various sources.  The most common one
       is maintained by the Mozilla Foundation and made public at
       http://publicsuffix.org (http://publicsuffix.org).  License terms
       governing the use of that list are available at that URI.

       Note that if operators use a variety of public suffix lists,
       interoperability will be difficult or impossible to guarantee.

   2.  Break the subject DNS domain name into a set of "n" ordered
       labels.  Number these labels from right to left; e.g., for
       "example.com", "com" would be label 1 and "example" would be
       label 2.

   3.  Search the public suffix list for the name that matches the
       largest number of labels found in the subject DNS domain.  Let
       that number be "x".

   4.  Construct a new DNS domain name using the name that matched from
       the public suffix list and prefixing to it the "x+1"th label from
       the subject domain.  This new name is the Organizational Domain.

   Thus, since "com" is an IANA-registered TLD, a subject domain of
   "a.b.c.d.example.com" would have an Organizational Domain of
   "example.com".

   The process of determining a suffix is currently a heuristic one.  No
   list is guaranteed to be accurate or current.



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A.7.  Removal of the "pct" Tag

   An earlier informational version of the DMARC protocol [RFC7489]
   included a "pct" tag and specified all integers from 0 to 100
   inclusive as valid values for the tag.  The intent of the tag was to
   provide domain owners with a method to gradually change their
   preferred assessment policy (the p= tag) from 'none' to 'quarantine'
   or from 'quarantine' to 'reject' by requesting the stricter treatment
   for just a percentage of messages that produced DMARC results of
   "fail".

   Operational experience showed that the pct tag was usually not
   accurately applied, unless the value specified was either "0" or
   "100" (the default), and the inaccuracies with other values varied
   widely from implementation to implementation.  The default value was
   easily implemented, as it required no special processing on the part
   of the Mail Receiver, while the value of "0" took on unintended
   significance as a value used by some intermediaries and mailbox
   providers as an indicator to deviate from standard handling of the
   message, usually by rewriting the RFC5322.From header in an effort to
   avoid DMARC failures downstream.

   These custom actions when the pct= tag was set to "0" proved valuable
   to the email community.  In particular, header rewriting by an
   intermediary meant that a Domain Owner's aggregate reports could
   reveal to the Domain Owner how much of its traffic was routing
   through intermediaries that don't rewrite the RFC5322.From header.
   It required work on the part of the Domain Owner to compare aggregate
   reports from before and after the p= value was changed and pct= was
   included in the DMARC policy record with a value of "0", but the data
   was there.  Consequently, knowing how much mail was subject to
   possible DMARC failure due to lack of RFC5322.From header rewriting
   by intermediaries could assist the Domain Owner in choosing whether
   or not to proceed from an applied policy of p=none to p=quarantine or
   p=reject.  Armed with this knowledge, the Domain Owner could make an
   informed decision regarding subjecting its mail traffic to possible
   DMARC failures based on the Domain Owner's tolerance for such things.

   Because of the value provided by "pct=0" to Domain Owners, it was
   logical to keep this functionality in the protocol; at the same time
   it didn't make sense to support a tag named "pct" that had only two
   valid values.  This version of the DMARC protocol therefore
   introduces the "t" tag as shorthand for "testing", with the valid
   values of "y" and "n", which are meant to be analogous in their
   application by mailbox providers and intermediaries to the "pct" tag
   values "0" and "100", respectively.





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Appendix B.  Examples

   This section illustrates both the Domain Owner side and the Mail
   Receiver side of a DMARC exchange.

B.1.  Identifier Alignment Examples

   The following examples illustrate the DMARC mechanism's use of
   Identifier Alignment.  For brevity's sake, only message headers are
   shown, as message bodies are not considered when conducting DMARC
   checks.

B.1.1.  SPF

   The following SPF examples assume that SPF produces a passing result.
   Alignment cannot exist if SPF does not produce a passing result.

   Example 1: SPF in alignment:

        MAIL FROM: <sender@example.com>

        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample

   In this case, the RFC5321.MailFrom parameter and the RFC5322.From
   header field have identical DNS domains.  Thus, the identifiers are
   in strict alignment.

   Example 2: SPF in alignment (parent):

        MAIL FROM: <sender@child.example.com>

        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample

   In this case, the RFC5322.From header parameter includes a DNS domain
   that is a parent of the RFC5321.MailFrom domain.  Thus, the
   identifiers are in relaxed alignment, because they both have the same
   Organizational Domain (example.com).

   Example 3: SPF not in alignment:






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        MAIL FROM: <sender@example.net>

        From: sender@child.example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample

   In this case, the RFC5321.MailFrom parameter includes a DNS domain
   that is neither the same as, a parent of, nor a child of the
   RFC5322.From domain.  Thus, the identifiers are not in alignment.

B.1.2.  DKIM

   The examples below assume that the DKIM signatures pass verification.
   Alignment cannot exist with a DKIM signature that does not verify.

   Example 1: DKIM in alignment:

        DKIM-Signature: v=1; ...; d=example.com; ...
        From: sender@example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample

   In this case, the DKIM "d=" parameter and the RFC5322.From header
   field have identical DNS domains.  Thus, the identifiers are in
   strict alignment.

   Example 2: DKIM in alignment (parent):

        DKIM-Signature: v=1; ...; d=example.com; ...
        From: sender@child.example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample

   In this case, the DKIM signature's "d=" parameter includes a DNS
   domain that is a parent of the RFC5322.From domain.  Thus, the
   identifiers are in relaxed alignment, as they have the same
   Organizational Domain (example.com).

   Example 3: DKIM not in alignment:

        DKIM-Signature: v=1; ...; d=sample.net; ...
        From: sender@child.example.com
        Date: Fri, Feb 15 2002 16:54:30 -0800
        To: receiver@example.org
        Subject: here's a sample



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   In this case, the DKIM signature's "d=" parameter includes a DNS
   domain that is neither the same as, a parent of, nor a child of the
   RFC5322.From domain.  Thus, the identifiers are not in alignment.

B.2.  Domain Owner Example

   A Domain Owner that wants to use DMARC should have already deployed
   and tested SPF and DKIM.  The next step is to publish a DNS record
   that advertises a DMARC policy for the Domain Owner's Organizational
   Domain.

B.2.1.  Entire Domain, Monitoring Only

   The owner of the domain "example.com" has deployed SPF and DKIM on
   its messaging infrastructure.  The owner wishes to begin using DMARC
   with a policy that will solicit aggregate feedback from Mail
   Receivers without affecting how the messages are processed, in order
   to:

   *  Confirm that its legitimate messages are authenticating correctly

   *  Verify that all authorized message sources have implemented
      authentication measures

   *  Determine how many messages from other sources would be affected
      by a blocking policy

   The Domain Owner accomplishes this by constructing a policy record
   indicating that:

   *  The version of DMARC being used is "DMARC1" ("v=DMARC1;")

   *  Mail Receivers should not alter how they treat these messages
      because of this DMARC policy record ("p=none")

   *  Aggregate feedback reports should be sent via email to the address
      "dmarc-feedback@example.com" ("rua=mailto:dmarc-
      feedback@example.com")

   *  All messages from this Organizational Domain are subject to this
      policy (no "t" tag present, so the default of "n" applies).

   The DMARC policy record might look like this when retrieved using a
   common command-line tool:

     % dig +short TXT _dmarc.example.com.
     "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com"




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   To publish such a record, the DNS administrator for the Domain Owner
   creates an entry like the following in the appropriate zone file
   (following the conventional zone file format):

     ; DMARC record for the domain example.com

     _dmarc  IN TXT ( "v=DMARC1; p=none; "
                      "rua=mailto:dmarc-feedback@example.com" )

B.2.2.  Entire Domain, Monitoring Only, Per-Message Reports

   The Domain Owner from the previous example has used the aggregate
   reporting to discover some messaging systems that had not yet
   implemented DKIM correctly, but they are still seeing periodic
   authentication failures.  In order to diagnose these intermittent
   problems, they wish to request per-message failure reports when
   authentication failures occur.

   Not all Mail Receivers will honor such a request, but the Domain
   Owner feels that any reports it does receive will be helpful enough
   to justify publishing this record.  The default per-message report
   format ([RFC6591]) meets the Domain Owner's needs in this scenario.

   The Domain Owner accomplishes this by adding the following to its
   policy record from Appendix B.2.1:

   *  Per-message failure reports should be sent via email to the
      address "auth-reports@example.com" ("ruf=mailto:auth-
      reports@example.com")

   The DMARC policy record might look like this when retrieved using a
   common command-line tool (the output shown would appear on a single
   line but is wrapped here for publication):

     % dig +short TXT _dmarc.example.com.
     "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
      ruf=mailto:auth-reports@example.com"

   To publish such a record, the DNS administrator for the Domain Owner
   might create an entry like the following in the appropriate zone file
   (following the conventional zone file format):

     ; DMARC record for the domain example.com

     _dmarc  IN TXT ( "v=DMARC1; p=none; "
                       "rua=mailto:dmarc-feedback@example.com; "
                       "ruf=mailto:auth-reports@example.com" )




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B.2.3.  Per-Message Failure Reports Directed to Third Party

   The Domain Owner from the previous example is maintaining the same
   policy but now wishes to have a third party serve as a Report
   Consumer.  Again, not all Mail Receivers will honor this request, but
   those that do may implement additional checks to verify that the
   third party wishes to receive the failure reports for this domain.

   The Domain Owner needs to alter its policy record from Appendix B.2.2
   as follows:

   *  Per-message failure reports should be sent via email to the
      address "auth-reports@thirdparty.example.net" ("ruf=mailto:auth-
      reports@thirdparty.example.net")

   The DMARC policy record might look like this when retrieved using a
   common command-line tool (the output shown would appear on a single
   line but is wrapped here for publication):

     % dig +short TXT _dmarc.example.com.
     "v=DMARC1; p=none; rua=mailto:dmarc-feedback@example.com;
      ruf=mailto:auth-reports@thirdparty.example.net"

   To publish such a record, the DNS administrator for the Domain Owner
   might create an entry like the following in the appropriate zone file
   (following the conventional zone file format):

     ; DMARC record for the domain example.com

     _dmarc IN TXT ( "v=DMARC1; p=none; "
                     "rua=mailto:dmarc-feedback@example.com; "
                     "ruf=mailto:auth-reports@thirdparty.example.net" )

   Because the address used in the "ruf" tag is outside the
   Organizational Domain in which this record is published, conforming
   Mail Receivers will implement additional checks as described in
   Section 3 of [DMARC-Aggregate-Reporting].  In order to pass these
   additional checks, the Report Consumer's Domain Owner will need to
   publish an additional DNS record as follows:

   *  Given the DMARC record published by the Domain Owner at
      "_dmarc.example.com", the DNS administrator for the Report
      Consumer will need to publish a TXT resource record at
      "example.com._report._dmarc.thirdparty.example.net" with the value
      "v=DMARC1;".






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   The resulting DNS record might look like this when retrieved using a
   common command-line tool (the output shown would appear on a single
   line but is wrapped here for publication):

     % dig +short TXT example.com._report._dmarc.thirdparty.example.net
     "v=DMARC1;"

   To publish such a record, the DNS administrator for example.net might
   create an entry like the following in the appropriate zone file
   (following the conventional zone file format):

     ; zone file for thirdparty.example.net
     ; Accept DMARC failure reports on behalf of example.com

     example.com._report._dmarc   IN   TXT    "v=DMARC1;"

   Mediators and other third parties should refer to Section 3 of
   [DMARC-Aggregate-Reporting] for the full details of this mechanism.

B.2.4.  Subdomain, Testing, and Multiple Aggregate Report URIs

   The Domain Owner has implemented SPF and DKIM in a subdomain used for
   pre-production testing of messaging services.  It now wishes to
   express a handling preference for messages from this subdomain that
   fail to authenticate to indicate to participating Mail Receivers that
   use of this domain is not valid.

   As a first step, it will express that it considers to be suspicious
   messages using this subdomain that fail authentication.  The goal
   here will be to enable examination of messages sent to mailboxes
   hosted by participating Mail Receivers as method for troubleshooting
   any existing authentication issues.  Aggregate feedback reports will
   be sent to a mailbox within the Organizational Domain, and to a
   mailbox at a Report Consumer selected and authorized to receive same
   by the Domain Owner.

   The Domain Owner will accomplish this by constructing a policy record
   indicating that:

   *  The version of DMARC being used is "DMARC1" ("v=DMARC1;")

   *  It is applied only to this subdomain (record is published at
      "_dmarc.test.example.com" and not "_dmarc.example.com")

   *  Mail Receivers are advised that the Domain Owner considers
      messages that fail to authenticate to be suspicious
      ("p=quarantine")




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   *  Aggregate feedback reports should be sent via email to the
      addresses "dmarc-feedback@example.com" and "example-tld-
      test@thirdparty.example.net" ("rua=mailto:dmarc-
      feedback@example.com, mailto:tld-test@thirdparty.example.net")

   *  The Domain Owner desires only that an actor performing a DMARC
      verification check apply any special handling rules it might have
      in place, such as rewriting the RFC53322.From header; the Domain
      Owner is testing its setup at this point, and so does not want the
      handling policy to be applied. ("t=y")

   The DMARC policy record might look like this when retrieved using a
   common command-line tool (the output shown would appear on a single
   line but is wrapped here for publication):

     % dig +short TXT _dmarc.test.example.com
     "v=DMARC1; p=quarantine; rua=mailto:dmarc-feedback@example.com,
      mailto:tld-test@thirdparty.example.net; t=y"

   To publish such a record, the DNS administrator for the Domain Owner
   might create an entry like the following in the appropriate zone
   file:

     ; DMARC record for the domain test.example.com

     _dmarc IN  TXT  ( "v=DMARC1; p=quarantine; "
                       "rua=mailto:dmarc-feedback@example.com,"
                       "mailto:tld-test@thirdparty.example.net;"
                       "t=y" )

   Once enough time has passed to allow for collecting enough reports to
   give the Domain Owner confidence that all legitimate email sent using
   the subdomain is properly authenticating and passing DMARC checks,
   then the Domain Owner can update the policy record to indicate that
   it considers authentication failures to be a clear indication that
   use of the subdomain is not valid.  It would do this by altering the
   DNS record to advise Mail Receivers of its position on such messages
   ("p=reject") and removing the testing flag ("t=y").

   After alteration, the DMARC policy record might look like this when
   retrieved using a common command-line tool (the output shown would
   appear on a single line but is wrapped here for publication):

     % dig +short TXT _dmarc.test.example.com
     "v=DMARC1; p=reject; rua=mailto:dmarc-feedback@example.com,
      mailto:tld-test@thirdparty.example.net"





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   To publish such a record, the DNS administrator for the Domain Owner
   might create an entry like the following in the appropriate zone
   file:

     ; DMARC record for the domain test.example.com

     _dmarc IN  TXT  ( "v=DMARC1; p=reject; "
                       "rua=mailto:dmarc-feedback@example.com,"
                       "mailto:tld-test@thirdparty.example.net" )

B.3.  Mail Receiver Example

   A Mail Receiver that wants to use DMARC should already be checking
   SPF and DKIM, and possess the ability to collect relevant information
   from various email-processing stages to provide feedback to Domain
   Owners (possibly via Report Consumers).

B.3.1.  SMTP Session Example

   An optimal DMARC-enabled Mail Receiver performs authentication and
   Identifier Alignment checking during the SMTP [RFC5321] conversation.

   Prior to returning a final reply to the DATA command, the Mail
   Receiver's MTA has performed:

   1.  An SPF check to determine an SPF-authenticated Identifier.

   2.  DKIM checks that yield one or more DKIM-authenticated
       Identifiers.

   3.  A DMARC policy lookup.

   The presence of an Author Domain DMARC record indicates that the Mail
   Receiver should continue with DMARC-specific processing before
   returning a reply to the DATA command.

   Given a DMARC record and the set of Authenticated Identifiers, the
   Mail Receiver checks to see if the Authenticated Identifiers align
   with the Author Domain (taking into consideration any strict versus
   relaxed options found in the DMARC record).

   For example, the following sample data is considered to be from a
   piece of email originating from the Domain Owner of "example.com":








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     Author Domain: example.com
     SPF-authenticated Identifier: mail.example.com
     DKIM-authenticated Identifier: example.com
     DMARC record:
       "v=DMARC1; p=reject; aspf=r;
        rua=mailto:dmarc-feedback@example.com"

   In the above sample, both the SPF-authenticated Identifier and the
   DKIM-authenticated Identifier align with the Author Domain.  The Mail
   Receiver considers the above email to pass the DMARC check, avoiding
   the "reject" policy that is requested to be applied to email that
   fails to pass the DMARC check.

   If no Authenticated Identifiers align with the Author Domain, then
   the Mail Receiver applies the DMARC-record-specified policy.
   However, before this action is taken, the Mail Receiver can consult
   external information to override the Domain Owner's Assessment
   Policy.  For example, if the Mail Receiver knows that this particular
   email came from a known and trusted forwarder (that happens to break
   both SPF and DKIM), then the Mail Receiver may choose to ignore the
   Domain Owner's policy.

   The Mail Receiver is now ready to reply to the DATA command.  If the
   DMARC check yields that the message is to be rejected, then the Mail
   Receiver replies with a 5xy code to inform the sender of failure.  If
   the DMARC check cannot be resolved due to transient network errors,
   then the Mail Receiver replies with a 4xy code to inform the sender
   as to the need to reattempt delivery later.  If the DMARC check
   yields a passing message, then the Mail Receiver continues on with
   email processing, perhaps using the result of the DMARC check as an
   input to additional processing modules such as a domain reputation
   query.

   Before exiting DMARC-specific processing, the Mail Receiver checks to
   see if the Author Domain DMARC record requests AFRF-based reporting.
   If so, then the Mail Receiver can emit an AFRF to the reporting
   address supplied in the DMARC record.

   At the exit of DMARC-specific processing, the Mail Receiver captures
   (through logging or direct insertion into a data store) the result of
   DMARC processing.  Captured information is used to build feedback for
   Domain Owner consumption.  This is not necessary if the Domain Owner
   has not requested aggregate reports, i.e., no "rua" tag was found in
   the policy record.







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B.4.  Utilization of Aggregate Feedback: Example

   Aggregate feedback is consumed by Domain Owners to verify their
   understanding of how a given domain is being processed by the Mail
   Receiver.  Aggregate reporting data on emails that pass all DMARC-
   supporting authentication checks is used by Domain Owners to verify
   that their authentication practices remain accurate.  For example, if
   a third party is sending on behalf of a Domain Owner, the Domain
   Owner can use aggregate report data to verify ongoing authentication
   practices of the third party.

   Data on email that only partially passes underlying authentication
   checks provides visibility into problems that need to be addressed by
   the Domain Owner.  For example, if either SPF or DKIM fails to pass,
   the Domain Owner is provided with enough information to either
   directly correct the problem or understand where authentication-
   breaking changes are being introduced in the email transmission path.
   If authentication-breaking changes due to email transmission path
   cannot be directly corrected, then the Domain Owner at least
   maintains an understanding of the effect of DMARC-based policies upon
   the Domain Owner's email.

   Data on email that fails all underlying authentication checks
   provides baseline visibility on how the Domain Owner's domain is
   being received at the Mail Receiver.  Based on this visibility, the
   Domain Owner can begin deployment of authentication technologies
   across uncovered email sources, if the mail that is failing the
   checks was generated by or on behalf of the Domain Owner.  Data
   regarding failing authentication checks can also allow the Domain
   Owner to come to an understanding of how its domain is being misused.

Acknowledgements

   DMARC and the draft version of this document submitted to the
   Independent Submission Editor were the result of lengthy efforts by
   an informal industry consortium: DMARC.org (see http://dmarc.org
   (http://dmarc.org)).  Participating companies included Agari,
   American Greetings, AOL, Bank of America, Cloudmark, Comcast,
   Facebook, Fidelity Investments, Google, JPMorgan Chase & Company,
   LinkedIn, Microsoft, Netease, PayPal, ReturnPath, The Trusted Domain
   Project, and Yahoo!.  Although the contributors and supporters are
   too numerous to mention, notable individual contributions were made
   by J.  Trent Adams, Michael Adkins, Monica Chew, Dave Crocker, Tim
   Draegen, Steve Jones, Franck Martin, Brett McDowell, and Paul Midgen.
   The contributors would also like to recognize the invaluable input
   and guidance that was provided early on by J.D.  Falk.





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   Additional contributions within the IETF context were made by Kurt
   Anderson, Michael Jack Assels, Les Barstow, Anne Bennett, Jim Fenton,
   J.  Gomez, Mike Jones, Scott Kitterman, Eliot Lear, John Levine, S.
   Moonesamy, Rolf Sonneveld, Henry Timmes, and Stephen J.  Turnbull.

Authors' Addresses

   Todd M. Herr
   Valimail

   Email: todd.herr@valimail.com


   John Levine
   Standcore LLC

   Email: standards@standore.com


































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