


DMARC.org                                              M. Kucherawy, Ed.
                                                               Cloudmark
                                                          March 30, 2012


 Domain-based Message Authentication, Reporting and Conformance (DMARC)

Abstract

   The email ecosystem currently lacks a cohesive mechanism through
   which email senders and receivers can make use of multiple
   authentication protocols in an attempt to establish reliable domain
   identifiers.  This lack of cohesion prevents receivers from providing
   domain-specific feedback to senders regarding the accuracy of
   authentication deployments.  Inaccurate authentication deployments
   preclude receivers from safely taking deterministic action against
   email that fails authentication checks.  Finally, email senders do
   not have the ability to publish policies specifying actions that
   should be taken against email that fails multiple authentication
   checks.

   This memo presents a proposal for a scalable mechanism by which an
   organization can express, using the Domain Name System, domain-level
   policies and preferences for message validation, disposition, and
   reporting with predictable and accurate results.

   The enclosed proposal is not intended to introduce mechanisms that
   provide elevated delivery privilege of authenticated email.  The
   proposal presents a mechanism for policy distribution that enables a
   continuum of increasingly strict handling of messages that fail
   multiple authentication checks, from no action, through silent
   reporting, up to message rejection.



















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Table of Contents

   1.  License  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.   Scalability . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.   Anti-Phishing . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.   Towards An Authenticated Future . . . . . . . . . . . . .  7
     2.4.   Experiment Team . . . . . . . . . . . . . . . . . . . . .  8
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.   High-Level Requirements . . . . . . . . . . . . . . . . .  8
     3.2.   Security Dependencies . . . . . . . . . . . . . . . . . .  9
     3.3.   DMARC Discovery Requirements  . . . . . . . . . . . . . .  9
     3.4.   Detailed Requirements . . . . . . . . . . . . . . . . . . 10
     3.5.   Out Of Scope  . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Terminology and Definitions  . . . . . . . . . . . . . . . . . 13
     4.1.   Summary . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.2.   Identifier Alignment  . . . . . . . . . . . . . . . . . . 15
       4.2.1.   DKIM-authenticated Identifiers  . . . . . . . . . . . 16
       4.2.2.   SPF-authenticated Identifiers . . . . . . . . . . . . 16
       4.2.3.   Alignment and Extension Technologies  . . . . . . . . 17
   5.  Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  DMARC Policy Record  . . . . . . . . . . . . . . . . . . . . . 18
     6.1.   DMARC URIs  . . . . . . . . . . . . . . . . . . . . . . . 18
     6.2.   General Record Format . . . . . . . . . . . . . . . . . . 19
     6.3.   Formal Definition . . . . . . . . . . . . . . . . . . . . 21
   7.  Policy Enforcement Considerations  . . . . . . . . . . . . . . 23
   8.  DMARC Feedback . . . . . . . . . . . . . . . . . . . . . . . . 23
     8.1.   Feedback Considerations . . . . . . . . . . . . . . . . . 23
     8.2.   Verifying External Destinations . . . . . . . . . . . . . 24
     8.3.   Aggregate Reports . . . . . . . . . . . . . . . . . . . . 25
     8.4.   Failure Reports . . . . . . . . . . . . . . . . . . . . . 26
   9.  Policy Discovery . . . . . . . . . . . . . . . . . . . . . . . 26
   10. Domain Owner Actions . . . . . . . . . . . . . . . . . . . . . 27
   11. Mail Receiver Actions  . . . . . . . . . . . . . . . . . . . . 29
     11.1.  Extract Author Domain . . . . . . . . . . . . . . . . . . 29
     11.2.  Determine Handling Policy . . . . . . . . . . . . . . . . 29
     11.3.  Message Sampling  . . . . . . . . . . . . . . . . . . . . 30
     11.4.  Store Results of DMARC Processing . . . . . . . . . . . . 30
   12. Feedback Mechanism . . . . . . . . . . . . . . . . . . . . . . 31
     12.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . 31
     12.2.  Transport . . . . . . . . . . . . . . . . . . . . . . . . 31
       12.2.1.  Email . . . . . . . . . . . . . . . . . . . . . . . . 32
       12.2.2.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . 33
       12.2.3.  Other Methods . . . . . . . . . . . . . . . . . . . . 33
       12.2.4.  Error Reports . . . . . . . . . . . . . . . . . . . . 33
   13. Minimum Implementations  . . . . . . . . . . . . . . . . . . . 34
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35
     14.1.  Authentication-Results Method Registry Update . . . . . . 35



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     14.2.  Authentication-Results Result Registry Update . . . . . . 35
     14.3.  DMARC Tag Registry  . . . . . . . . . . . . . . . . . . . 36
     14.4.  DMARC Report Format Registry  . . . . . . . . . . . . . . 37
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 38
     15.1.  Use of RFC5322.From . . . . . . . . . . . . . . . . . . . 38
     15.2.  Display Name Attacks  . . . . . . . . . . . . . . . . . . 39
     15.3.  Attacks on Reporting URIs . . . . . . . . . . . . . . . . 39
     15.4.  Issues Specific to SPF  . . . . . . . . . . . . . . . . . 40
     15.5.  DNS Load  . . . . . . . . . . . . . . . . . . . . . . . . 40
     15.6.  External Reporting Addresses  . . . . . . . . . . . . . . 41
     15.7.  Feedback Loops  . . . . . . . . . . . . . . . . . . . . . 41
     15.8.  Rejecting Messages  . . . . . . . . . . . . . . . . . . . 42
     15.9.  Capacity Planning . . . . . . . . . . . . . . . . . . . . 42
     15.10. Privacy Considerations  . . . . . . . . . . . . . . . . . 43
       15.10.1. Data Exposure Considerations  . . . . . . . . . . . . 43
       15.10.2. Report Recipients . . . . . . . . . . . . . . . . . . 44
       15.10.3. Report Generators . . . . . . . . . . . . . . . . . . 44
       15.10.4. Secure Protocols  . . . . . . . . . . . . . . . . . . 44
     15.11. Identifier Alignment Considerations . . . . . . . . . . . 44
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
     16.1.  Normative References  . . . . . . . . . . . . . . . . . . 45
     16.2.  Informative References  . . . . . . . . . . . . . . . . . 46
   Appendix A.  Technology Considerations . . . . . . . . . . . . . . 47
     A.1.   S/MIME  . . . . . . . . . . . . . . . . . . . . . . . . . 47
     A.2.   Method Exclusion  . . . . . . . . . . . . . . . . . . . . 48
     A.3.   Sender Header Field . . . . . . . . . . . . . . . . . . . 48
     A.4.   Domain Existence Test . . . . . . . . . . . . . . . . . . 49
     A.5.   Issues With ADSP In Operation . . . . . . . . . . . . . . 49
     A.6.   Organizational Domain Discovery Issues  . . . . . . . . . 50
       A.6.1.   Public Suffix Lists . . . . . . . . . . . . . . . . . 51
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 51
     B.1.   Identifier Alignment examples . . . . . . . . . . . . . . 51
       B.1.1.   SPF . . . . . . . . . . . . . . . . . . . . . . . . . 51
       B.1.2.   DKIM  . . . . . . . . . . . . . . . . . . . . . . . . 52
     B.2.   Domain Owner example  . . . . . . . . . . . . . . . . . . 53
       B.2.1.   Entire Domain, Monitoring Only  . . . . . . . . . . . 53
       B.2.2.   Entire Domain, Monitoring Only, Per-Message
                Reports . . . . . . . . . . . . . . . . . . . . . . . 54
       B.2.3.   Per-Message Forensic Reports Directed to Third
                Party . . . . . . . . . . . . . . . . . . . . . . . . 55
       B.2.4.   Sub-Domain, Sampling, and Multiple Aggregate
                Report URIs . . . . . . . . . . . . . . . . . . . . . 56
       B.2.5.   Third Party Sender and Identifier Alignment . . . . . 58
       B.2.6.   Sub-Domain Policy, Reporting Interval . . . . . . . . 59
     B.3.   Mail Receiver Example . . . . . . . . . . . . . . . . . . 60
       B.3.1.   SMTP-time Processing  . . . . . . . . . . . . . . . . 60
       B.3.2.   Real-time Feedback Processing . . . . . . . . . . . . 62
     B.4.   Utilization of Aggregate Feedback example . . . . . . . . 62



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     B.5.   mailto Transport example  . . . . . . . . . . . . . . . . 62
     B.6.   https Transport example . . . . . . . . . . . . . . . . . 63
   Appendix C.  DMARC XML Schema  . . . . . . . . . . . . . . . . . . 64
   Appendix D.  Public Discussion . . . . . . . . . . . . . . . . . . 69
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 70














































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1.  License

   As of the date shown at the top right of this page, the Contributors
   have made this Specification available under the Open Web Foundation
   Contributor License Agreement Version 1.0, which is available at:
   http://www.dmarc.org/cla.html

   You can review the signed copies of the Open Web Foundation
   Contributor License Agreement Version 1.0 for this Specification at:
   http://www.dmarc.org/CLAs/

   The current list of Contributors can be found at:
   http://www.dmarc.org/contributors.html

   Your use of this Specification may be subject to other third party
   rights.  THIS SPECIFICATION IS PROVIDED "AS IS".  The contributors
   expressly disclaim any warranties (express, implied, or otherwise),
   including implied warranties of merchantability, non-infringement,
   fitness for a particular purpose, or title, related to the
   Specification.  The entire risk as to implementing or otherwise using
   the Specification is assumed by the Specification implementer and
   user.  IN NO EVENT WILL ANY PARTY BE LIABLE TO ANY OTHER PARTY FOR
   LOST PROFITS OR ANY FORM OF INDIRECT, SPECIAL, INCIDENTAL, OR
   CONSEQUENTIAL DAMAGES OF ANY CHARACTER FROM ANY CAUSES OF ACTION OF
   ANY KIND WITH RESPECT TO THIS SPECIFICATION OR ITS GOVERNING
   AGREEMENT, WHETHER BASED ON BREACH OF CONTRACT, TORT (INCLUDING
   NEGLIGENCE), OR OTHERWISE, AND WHETHER OR NOT THE OTHER PARTY HAS
   BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


2.  Introduction

   For years, various receivers have tried to protect senders who are
   known to authenticate their outbound email from phishing by using
   [DKIM] and/or [SPF] results to detect and block unauthorized email.
   At the same time, senders have leveraged SPF-authorized and DKIM-
   signed messages to achieve domain-level email authentication.
   However, a broadly accepted mechanism to assert domain-specific
   message-disposition policies, or to request reporting of same, has
   been lacking.

   The fundamental idea behind these approaches is that if a sender
   authenticates all legitimate outbound mail using the authentication
   protocols SPF and DKIM, then receivers can quarantine or reject
   unauthenticated mail purporting to be from that sender.  Over time,
   one-on-one relationships were established between select senders and
   receivers with privately communicated means to assert policy and
   receive message traffic and authentication disposition reporting.



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   Although these ad hoc practices have been generally successful, they
   require significant manual coordination between parties.

   This memo defines Domain-based Message Authentication, Reporting and
   Compliance (DMARC), a mechanism by which email operators leverage
   existing authentication and policy advertisement technologies to
   enable both message-stream feedback and enforcement of policies
   against unauthenticated email.

   DMARC encourages senders and receivers to collaborate by monitoring
   message authentication and disposition, building confidence in the
   coverage and accuracy of email authentication, and moving -- one
   domain at a time -- towards the goal of deploying the strongest
   possible message handling policies.

   For the purpose of discussion, this document defines the word
   "authentication" to be inclusive of techniques used to verify message
   integrity and/or sending-entity authorization.  Exceptions to this
   convention are expressly noted.

   The DMARC method involves evaluation of messages during an SMTP
   session on entry to a specific receiving Administrative Management
   Domain (ADMD; see [EMAIL-ARCH]).  DMARC is thus applied by message
   transport software and not by user agents or their respective
   protocols such as POP or IMAP.

   DMARC operates as a policy layer atop implementations of DKIM and
   SPF.  These technologies are the building blocks of DMARC as each
   technology is widely deployed, supported by mature tools, and is
   readily available to both senders and receivers.  They are also
   complementary, as each is resilient to many of the failure modes of
   the other.  Furthermore, neither of these require direct user
   interaction to be successful, nor are they burdened by heavy
   considerations such as public key infrastructure, which have
   inhibited the uptake of other message signing and encryption
   protocols.  (For further discussion, see Section 1 of [DKIM].)  In
   addition, DMARC can operate even if a message author has chosen to
   deploy only one of these.

   DMARC differs from previous approaches to policy advertisement (e.g.,
   [SPF] and [ADSP]) in that:

   o  Authentication technologies are:

      1.  decoupled from any technology-specific policy mechanisms; and

      2.  used solely to establish reliable per-message domain-level
          identifiers.



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   o  Multiple authentication technologies are utilized to:

      1.  reduce the impact of transient authentication errors; and

      2.  create authenticated message streams that are resilient to
          site-specific configuration errors and deployment gaps.

   o  Receiver-generated feedback is employed to establish confidence in
      authentication practices, enabling widespread, safe enforcement of
      strong message disposition policy.

   o  The domain name extracted from a message's RFC5322.From field is
      the primary identifier in the DMARC mechanism.  This identifier is
      used in conjunction with the results of the underlying
      authentication technologies to evaluate results under DMARC.

2.1.  Scalability

   DMARC is designed to support the extreme scalability requirements
   that characterize the systemic problem of identifying the origination
   and legitimacy of email.  DMARC seeks to preserve the positive
   aspects of the current email infrastructure, such as the ability for
   anyone to communicate with anyone else without introduction.

   The DMARC mechanism specifically does not introduce third-party
   policy publishers or feedback consumers.  Third parties are not
   prevented, however, from using these mechanisms in private and/or
   public contexts.

2.2.  Anti-Phishing

   This document is significantly informed by ongoing efforts to enact
   large-scale, Internet-wide, anti-phishing measures.  Whereas DMARC
   can only be used to combat specific forms of exact-domain phishing
   directly, the DMARC mechanism is viewed more importantly as a
   substantial step forward in terms of creating reliable and defensible
   message streams.

   Specifically, DMARC does not attempt to solve problems related to use
   of Cousin Domains or abuse of the RFC5322.From "display name".

2.3.  Towards An Authenticated Future

   The DMARC mechanism is designed to enable highly accurate Internet-
   scale deployments of email authentication technologies.  Anti-
   phishing measures are a compelling instance of what widely-deployed
   authenticated messaging streams can enable.  As email authentication
   deployments continue to mature, additional authentication-enabled



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   services are expected to be developed.

2.4.  Experiment Team

   [NOTE TO RFC EDITOR: Remove this section prior to publication.]

   The contributors to DMARC share the view that layering security on
   top of Internet Mail requires a partnership between those who send
   mail (who sign messages with DKIM, authorize email servers with SPF,
   and consume feedback to achieve highly-accurate deployments) and
   those who receive mail (who test the authenticity assertions from
   senders and report authentication results back to senders to enable
   authentication accuracy and domain-usage intelligence).  The team
   that produced this specification acknowledges that this new security
   layer is optional for the Internet community in general, though of
   increasing value to our peers due to the urgent need to respond to
   the persistent threats of phishing and malware distribution.

   If this first public informational draft addresses your use cases for
   improved messaging security, please contact the authors expressing
   your interest to work with us on implementation testing and rolling
   implementation experience back into future versions of DMARC.  It is
   the intention of the contributors to submit DMARC into a new IETF
   Working Group on a formal standards track, but only after gaining
   significant implementation experience.  Please join us in making
   Internet messaging more secure.


3.  Requirements

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

3.1.  High-Level Requirements

   At a high level, DMARC is designed to satisfy the following
   requirements:

   o  Minimize false positives.

   o  Provide robust authentication reporting.

   o  Allow senders to assert policy for consumption by receivers.

   o  Reduce the amount of successfully delivered phish.





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   o  Work at Internet scale.

   o  Minimize complexity.

   o  Produce an RFC draft -- supported by real-world operational
      experience -- that can be submitted to the IETF for publication as
      a proposed Internet Standard.

3.2.  Security Dependencies

   Security issues DMARC observes:

   o  The security of DMARC and its underlying technologies (SPF, DKIM)
      depend on the security of the DNS.

   o  DMARC depends upon DKIM, and thus security of the private keys
      used for signing messages must be assured.

   o  DMARC depends upon SPF, and thus the listing of authorized servers
      in the author domain's SPF record must be accurately maintained.

   o  In addition to the above, authors must ensure that their outbound
      mail servers are not sending unauthorized mail (e.g., are not
      infected by spam bots or malware, or relaying messages from
      systems so afflicted).

   o  DMARC relies on the concept of message quarantining as a valid
      message disposition, and thus relies on the various components of
      the recipient's mailbox service provider and the user interface to
      make that facility available.

3.3.  DMARC Discovery Requirements

   Contributors to DMARC have also compiled a list of requirements that
   have informed the design of how DMARC policy is determined:

   1.  Simple to implement, especially for the feedback generator.

   2.  Minimize DNS queries in the discovery phase.

   3.  Resilient to abuse of the report consumer.  The ability of
       abusers to publish feedback addresses on wildcarded domains to
       create a lot of meaningless work for the generator is to be
       avoided.  In recognition that DMARC can be used to perform "joe-
       job" attacks, the feedback destination URI should be within the
       same organizational domain.  If it is not, the feedback
       generators need to make a best-effort attempt not to joe-job the
       apparent feedback consumer.



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   4.  Support for multiple report consumers.  Multiple consumers should
       be able to receive feedback reports in parallel.

   5.  Transport layer security as an option.

   6.  Feedback generator verification.  Posting a URI in DNS to which
       anyone can upload large amounts of data is always dangerous.  The
       feedback consumer has to have a way to prevent denial of service
       attacks by dropping or blocking unwanted data.

3.4.  Detailed Requirements

   DMARC's specification requirements, in detail:

   1.   The RFC5322.From domain is the identifier used for all message
        validation operations, as it is the single identifier in the
        message likely to be visible to the user.

   2.   Senders can specify a "strict" or "relaxed" mode in terms of
        enforcing identifier checks (see Section 4.2).  In "strict"
        mode, all identifiers from authentication systems upon which
        DMARC is predicated must match the RFC5322.From domain.  In
        "relaxed" mode, the organizational domains (see Section 4) must
        match.  The "relaxed" mode shall be the default.

   3.   A sender's policy must be discoverable via DNS queries.

   4.   It must be possible to specify reject or quarantine policies
        when none of the underlying authentication systems succeed.

   5.   It must be possible to specify a "no action" policy in order to
        collect authentication statistics without impacting delivery.

   6.   Senders can specify a policy that is in effect for subdomains of
        its organizational domain that is different for the policy of
        the organizational domain itself.

   7.   Message disposition requests via DMARC override those requested
        by any other public mechanism.

   8.   Senders should be able to specify a percentage of their messages
        to which their policies should be applied, with the rest
        unaffected, in order to experiment and to understand and
        minimize deployment risk.

   9.   Receivers should endeavour to reject or quarantine email if the
        RFC5322.From purports to be from a domain that appears to be
        either non-existent or incapable of receiving mail.



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   10.  Reporting configuration in DMARC should override any such
        options specified by DKIM or SPF or extensions to them.

   11.  The sender must be able to to specify independent reporting
        addresses for failed message reporting and aggregate data feeds.

   12.  The aggregate report must contain enough information for the
        report consumer to re-calculate DMARC disposition based on the
        published policy, message dispositon, and SPF, DKIM, and
        identifier alignment results.

   13.  The aggregate report must still contain data for each sender
        subdomain separately from mail from the sender's organizational
        domain, even if no subdomain policy is applied.  The report must
        indicate any policy applied to subdomains.

   14.  It must be possible to specify a minimum reporting interval.
        Reporting sites should make a best effort to accommodate that
        request.

   15.  The sender can specify a time-to-live for policy records.

   16.  The sender can indicate which domains under its control never
        send email, either by omitting them from the DNS entirely or by
        declaring specific use of DKIM and SPF that no email will ever
        fulfill.

   17.  The sending and receiving domains should be included in the
        aggregate report.

   18.  The policy request and the one applied (if different) should be
        included in the aggregate report.

   19.  The number of successful authentications should be included in
        the aggregate report.

   20.  The report should be generated based on all messages even if
        filtering agents such as anti-virus or anti-spam engines
        ultimately block delivery.

   21.  For real-time reporting of failed messages, including a [URI] to
        identify phishing sites and diagnostics on DKIM and SPF failures
        will be recommended.

   22.  Static conformance requirements shall be documented to enable
        testing programs to help ensure consitency of results.  (This
        will be done in a separate Best Current Practices document.)




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   23.  Aggregate reports should communicate DMARC message disposition
        regardless of any subsequent action that affects message
        disposition or delivery.

   24.  The mechanism overall should be flexible enough to swap in or
        out any authentication technology.

   Tags throughout the specification part of this document indicate
   conformance to the above requirements.  For example "{R1}" indicates
   a component of the protocol that addresses requirement #1 in the list
   above.

3.5.  Out Of Scope

   Items specifically not in scope for this work include:

   o  DMARC shall not be required to protect against any attacks against
      components listed in Security Dependencies (i.e.  DNS attacks,
      bugs in DKIM verification, malware on the end-user machine or in
      the sender's system).  Compromised components at or near the
      sender can cause false positives in terms of DKIM and SPF results;
      while compromised components at the receiver can cause false
      positives to be rendered to the user or interefere with the
      sender-requested actions.

   o  DMARC will not make a distinction between absence of DKIM
      signature and failed DKIM signature.

   o  DMARC (at least, the base version) will not provide the ability to
      publish a policy for message disposition results other than "all
      authentication tests failed".

   o  DMARC will not allow for use of header fields other than the
      RFC5322.From to perform identifier alignment checks.

   o  DMARC has no "short-circuit" provision, such as specifying that a
      pass from one authentication test allows one to skip the other(s).
      All are required for reporting.

   o  This first version of DMARC supports only a single reporting
      format.

   o  DMARC makes no attempt to accommodate discovery of policy outside
      of the DNS.  Such policy communications may be accomplished out-
      of-band, but not within the mechanisms described here.

   o  DMARC provides no advice about handling of malformed messages that
      might seek to exploit message processing weaknesses.  There are



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      other specifications and operational documents that cover these
      issues.

   o  DMARC reports only on the last-hop IP address, and does not
      provide for reporting of the originating IP.

   o  DMARC does not address attacks that provide false information in
      the "display name" portion of the RFC5322.From field.


4.  Terminology and Definitions

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

   For the purpose of establishing context, readers are encouraged to be
   familiar with the contents of [EMAIL-ARCH].  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 some third party.  This memo does not address the distinctions
   among such roles; the reader is encouraged to become familiar with
   that material before continuing.

   The following terms are also used:

   Authenticated Identifiers:  Authentication technologies allow
      evaluation agents to associate email with domain-level
      identifiers.  Domain-level identifiers that are established using
      authentication technologies are referred to as "Authenticated
      Identifiers".  The following Authenticated Identifiers are
      considered in this document:

      *  [DKIM] provides a domain-level identifier in the content of the
         "d=" tag of a validated signature.

      *  [SPF] can authenticate the domain found in an [SMTP] MAIL
         command.

   Cousin Domain:  A DNS domain that, when rendered by a Mail User Agent
      (MUA), looks similar to, or can lead users to believe the domain
      is associated with, another name.  For instance, "vendor5.example"
      would be a Cousin Domain of "vendors.example".  This is a common
      tool in a homograph attack.



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   Domain Owner:  The 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.
      Entities that are 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
      [EMAIL-ARCH].

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

   Organizational Domain:  For the purposes of this document, an
      Organizational Domain is the domain that was registered with a
      domain name registrar.  Heuristics are used to determine this
      given an arbitrary domain name, since it is not always the case
      that the registered domain name is simply a top-level DNS domain
      plus one component (e.g., "example.com", where "com" is a top-
      level domain).  The Organizational Domain is determined by
      applying the following algorithm:

      1.  Acquire a "public suffix" list, i.e., a list of DNS domain
          names reserved for registrations.  Some country 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 public suffix list is the union of all of these.
          Appendix A.6.1 contains some discussion about obtaining a
          public suffix list.

      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



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      "example.com".

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

4.1.  Summary

   DMARC's filtering component is based on whether or not SPF or DKIM
   can provide an authenticated -- and relevant -- identifier for any
   given message.  Messages that purport to be from a Domain Owner's
   domain and arrive from servers that are not authorized by SPF and do
   not contain an appropriate DKIM signature can be affected by DMARC
   policies.

   DMARC's feedback component involves the collection of information
   pertaining to received messages, in the aggregate, for periodic
   reporting back to the Domain Owner.  The parameters and format for
   such reports are discussed in later sections of this document.

   A DMARC-enabled Mail Receiver might also generate per-message reports
   that contain information related to individual messages which fail
   SPF and/or DKIM.  Per-message reports are useful for forensic use in
   debugging deployments (if messages can be determined to be legitimate
   albeit failing authentication) or in analyzing attacks.  The
   capability for such services is enabled by DMARC but defined in other
   referenced material.

   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.

4.2.  Identifier Alignment

   Email authentication technologies authenticate various (and
   disparate) aspects of an individual message.  For example, [DKIM]
   authenticates the domain that affixed a signature to the message,
   while [SPF] authenticates the domain that appears in the
   RFC5321.MailFrom portion of [SMTP].  The DMARC mechanism introduces
   the concept of Identifier Alignment to address the possible
   discrepancy of Authenticated Identifiers supplied by underlying
   authentication technologies.

   DMARC uses the RFC5322.From domain to tie together Authenticated
   Identifiers {R1}.  The selection of the RFC5322.From domain as the
   central identity of the DMARC mechanism is due to the ubiquity of
   this identity and the behavior of most MUAs to represent the
   RFC5322.From field as the originator of the message and to render
   some or all of this header's content to end users.



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   To be considered "in alignment" for the purposes of the DMARC
   mechanism, implementors MUST observe the considerations described in
   the following sections.  Domain names in this context are to be
   compared in a case-insensitive manner, per [DNS-CASE].

4.2.1.  DKIM-authenticated Identifiers

   DMARC provides the option of applying DKIM in a strict mode or a
   relaxed mode {R2}.

   In relaxed mode, the Organizational Domain of 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.  In
   strict mode, only an exact match is considered to produce identifier
   alignment.

   To illustrate, in relaxed mode, if a validated DKIM signature
   successfully verifies with a "d=" domain of "example.com", and the
   RFC5322.From domain is "alerts@news.example.com", the DKIM "d="
   domain and the RFC5322.From domain are considered to be "in
   alignment".  In strict mode, this test would fail.

   However, a DKIM signature bearing a value of "d=com" would never
   allow an "in alignment" result as "com" should appear on all public
   suffix lists, and therefore cannot be an Organizational Domain.

   Identifier alignment is required to prevent abuse by phishers that
   send DKIM-signed email using an arbitrary "d=" domain (such as a
   Cousin Domain) to pass authentication checks.

   Note that a single email can contain multiple DKIM signatures,
   raising the possibility of processing multiple signatures in an
   attempt to establish an "in alignment" result.

4.2.2.  SPF-authenticated Identifiers

   DMARC provides the option of applying SPF in a strict mode or a
   relaxed mode {R2}.

   In relaxed mode, the [SPF]-authenticated RFC5321.MailFrom (commonly
   called the "envelope sender") domain and RFC5322.From domain must
   have the same Organizational Domain.  In strict mode, only an exact
   DNS domain match is considered to produce identifier alignment.

   For example, if a message passes an SPF check with an
   RFC5321.MailFrom domain of "cbg.bounces.example.com", and the address
   portion of the RFC5322.From field contains "payments@example.com",
   the Authenticated RFC5321.MailFrom domain identifier and the



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   RFC5322.From domain are considered to be "in alignment" in relaxed
   mode, but not in strict mode.

4.2.3.  Alignment and Extension Technologies

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


5.  Policy

   DMARC policies are published by Domain Owners and applied by Mail
   Receivers.

   A Domain Owner advertises DMARC participation by adding a DNS TXT
   record (described in Section 6) {R3, R15, R16} to one or more sending
   domains under its direct or indirect control (e.g. operated by a
   delegate by agreement with the Domain Owner).  In doing so, senders
   make specific requests of Mail Receivers regarding the disposition
   of, and feedback on, messages purporting to be from one of the Domain
   Owner's domains.

   A Mail Receiver MUST consider an arriving message to pass the DMARC
   test if and only if one or more of the underlying message
   authentication mechanisms pass with proper identifier alignment.

   A Domain Owner that does not advertise an SPF policy or sign with
   DKIM is making an implicit statement that the use cases those
   protocols satisfy are not to be considered when determining whether
   or not the message under evaluation is valid.  For example, not
   publishing an SPF policy is an implicit message from Domain Owners to
   Mail Receivers that successful path authorization is not to be taken
   as sufficient evidence that the Domain Owner authorized the message.

   A Mail Receiver implementing the DMARC mechanism MUST make a best-
   effort to adhere to the Domain Owner's published DMARC policy when a
   message fails the DMARC test.  Recognizing that email streams can be
   complicated (due to forwarding, existing RFC5322.From domain-spoofing
   services, etc.), Mail Receivers MAY deviate from a Domain Owner's
   published policy during message processing and SHOULD make available
   the fact and reason of the deviation to the Domain Owner via feedback
   reporting.







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6.  DMARC Policy Record

   Domain Owner 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 records are stored in the DNS for three key engineering
   reasons:

   Wildcarding:  DNS natively supports wildcarding, enabling child
      domains to inherit parent domain policy easily.

   Overrides:  DMARC records published at child domains explicitly
      override extant parent policy.

   Efficiency:  DNS caching is a common practice, reducing operational
      overhead of a new DNS-based mechanism.

   Per [DNS], 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.

6.1.  DMARC URIs

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

   The place such URIs are specified (see Section 6.2) allows a list of
   these to be provided, to be tried in order until one of them
   successfully receives the report.  The list of URIs is separated by
   commas (ASCII 0x2C).

   Each URI can have associated with it a maximum report size that may
   be sent to it.  This is accomplished by appending an exclamation
   point (ASCII 0x21), followed by a maximum size indication, before a
   separating comma or terminating semi-colon.

   Thus, a DMARC URI is a URI within which any commas or exclamation
   points are percent-encoded per [URI], followed by an OPTIONAL
   exclamation point and a maximum size specification, and, if there are



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   additional reporting URIs in the list, a comma and the next URI.

   For example, the URI "mailto:reports@example.com!50m" would request a
   report be sent via email to "reports@example.com" so long as the
   report payload does not exceed 50 megabytes.

   A formal definition is provided in Section 6.3.

6.2.  General Record Format

   DMARC records follow the extensible "tag-value" syntax for DNS-based
   key records defined in [DKIM]. {R24}

   Section 14 creates a registry for known DMARC tags and registers the
   initial set defined in this memo.  Only tags defined in this memo or
   in later extensions, and thus added to that registry, are to be
   processed; unknown tags MUST be ignored.  To avoid version
   compatibility issues, tags added to the DMARC specification SHOULD
   NOT change the semantics of existing records when processed by
   implementations conforming to prior specifications.

   The following tags are introduced as the initial valid DMARC tags:

   adkim:  (plain-text; OPTIONAL, default is "r".)  Indicates whether or
      not strict DKIM identifier alignment is required by the Domain
      Owner.  If and only if the value of the string is "s", strict mode
      is in use.  See Section 4.2.1 for details.

   aspf:  (plain-text; OPTIONAL, default is "r".)  Indicates whether or
      not strict SPF identifier alignment is required by the Domain
      Owner.  If and only if the value of the string is "s", strict mode
      is in use.  See Section 4.2.2 for details.

   p: Requested Mail Receiver policy (plain-text; REQUIRED).  Indicates
      the policy to be enacted by the Receiver at the request of the
      Domain Owner.  Policy applies to the domain queried and to sub-
      domains unless sub-domain policy is explicitly described using the
      "sp" tag.  Possible values are as follows:

      none:  {R5} The Domain Owner requests no specific action be taken
         regarding delivery of messages.

      quarantine:  {R4} The Domain Owner wishes to have email that fails
         the DMARC mechanism check to be treated by Mail Receivers as
         suspicious.  Depending on the capabilities of the Mail
         Receiver, this can mean "place into spam folder", "scrutinize
         with additional intensity", and/or "flag as suspicious".




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      reject:  {R4} The Domain Owner wishes for Mail Receivers to reject
         email that fails the DMARC mechanism check.  Rejection SHOULD
         occur during the SMTP transaction.  See Section 15.8 for some
         discussion of SMTP rejection methods and their implications.

   pct:  (plain-text integer between 0 and 100, inclusive; OPTIONAL;
      default is 100). {R8} Percentage of messages from the DNS domain's
      mail stream to which the DMARC mechanism is to be applied.
      However, this MUST NOT be applied to the DMARC-generated reports,
      all of which must be sent and received unhindered.  The purpose of
      the "pct" tag is to allow Domain Owners to slowly roll out
      enforcement of the DMARC mechanism.  The prospect of "all or
      nothing" is recognized as preventing many organizations from
      experimenting with strong authentication-based mechanisms.

   rf:  Format to be used for message-specific forensic information
      reports (comma-separated plain-text list of values; OPTIONAL;
      default "afrf").  The value of this tag is a list of one or more
      report formats as requested by the Domain Owner to be used when a
      message fails both [SPF] and [DKIM] tests to report details of the
      individual failure.  The values MUST be present in the registry of
      reporting formats defined in Section 14; a Mail Receiver observing
      a different value SHOULD ignore it, or MAY ignore the entire DMARC
      record.  Initial default values are "afrf" (defined in [AFRF]) and
      "iodef" (defined in [IODEF]).

   ri:  Interval requested between aggregate reports (plain-text, 32-bit
      unsigned integer; OPTIONAL; default 86400). {R14} Indicates a
      request to Receivers to generate aggregate reports separated by no
      more than the requested number of seconds.  DMARC implementations
      MUST be able to provide daily reports and SHOULD be able to
      provide hourly reports when requested.  However, anything other
      than a daily report is understood to be accommodated on a best-
      effort basis.

   rua:  Addresses to which aggregate feedback is to be sent (comma-
      separated plain-text list of DMARC URIs; OPTIONAL). {R11} A comma
      or exclamation point that is part of such a DMARC URI MUST be
      encoded per Section 2.1 of [URI] so as to distinguish it from the
      list delimiter or an OPTIONAL size limit.  Section 8.2 discusses
      considerations that apply when the domain name of a URI differs
      from that of the domain advertising the policy.  See Section 15.6
      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 not
      provided, Mail Receivers MUST NOT generate aggregate feedback
      reports.  URIs not supported by Mail Receivers MUST be ignored.
      The aggregate feedback report format is described in Section 8.3.



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   ruf:  Addresses to which message-specific forensic information is to
      be reported (comma-separated plain-text list of DMARC URIs;
      OPTIONAL). {R11} If present, the Domain Owner is requesting Mail
      Receivers to send detailed forensic reports about messages that
      fail [SPF] and/or [DKIM] evaluation.  The format of the message to
      be generated MUST follow that specified in the "rf" tag.
      Section 8.2 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 not provided, Mail Receivers MUST NOT generate forensic
      reports.  See Section 15.6 for additional considerations.

   sp:  {R6} Requested Mail Receiver policy for subdomains (plain-text;
      OPTIONAL).  Indicates the policy to be enacted by the Receiver at
      the request of the Domain Owner.  It applies only to subdomains of
      the domain queried and not to the domain itself.  Its syntax is
      identical to that of the "p" tag defined above.  If absent, the
      policy specified by the "p" tag MUST be applied for subdomains.

   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.

   Note that 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.

6.3.  Formal Definition

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

















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     dmarc-uri = URI [ "!" 1*DIGIT [ "k" / "m" / "g" / "t" ] ]
               ; "URI" is imported from [URI]; commas (ASCII 0x2c)
               ; and exclamation points (ASCII 0x21) MUST be encoded

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

     dmarc-sep = *WSP %3b *WSP

     dmarc-version = %x76 *WSP "=" %x44.4d.41.52.43

     dmarc-request = %x70 *WSP "=" *WSP ( "none" /
                                          "quarantine" / "reject" )

     dmarc-srequest = %x73.70 *WSP "=" *WSP ( "none" /
                                              "quarantine" / "reject" )

     dmarc-auri = %x72.75.61 *WSP "=" *WSP
                   dmarc-uri *(*WSP "," *WSP dmarc-uri)

     dmarc-ainterval = %x72.69 *WSP "=" *WSP

     dmarc-furi = %x72.75.66 *WSP "=" *WSP
                   dmarc-uri *(*WSP "," *WSP dmarc-uri)

     dmarc-rfmt = %x72.66 *WSP "=" *WSP
                  ( "afrf" / "iodef" )

     dmarc-percent = %x70.63.74 *WSP "=" *WSP
                     1*3DIGIT

     dmarc-adkim = %x61.64.6b.69.6d  *WSP "=" *WSP
                   ( "r" / "s" )

     dmarc-aspf = %x61.73.70.66  *WSP "=" *WSP
                  ( "r" / "s" )

   A size limitation in a dmarc-uri, if provided, is interpreted as a



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   count of units followed by an OPTIONAL unit size ("k" for kilobytes,
   "m" for megabytes, "g" for gigabytes, "t" for terabytes).  Without a
   unit, the number is presumed to be a basic byte count.


7.  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".  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 Domain Owner has published a "reject"
   policy.  Mail Receivers SHOULD make a best effort not to increase the
   likelihood of phishing if it chooses not to reject, against policy.

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

   DMARC-compliant Mail Receivers MUST disregard any mail directive
   discovered as part of an authentication mechanism (e.g., ADSP, SPF)
   where a DMARC policy is also discovered. {R7} Mail Receivers SHOULD
   also implement reporting instructions of DMARC in place of any
   extensions to SPF or DKIM that might enable such reporting. {R10}


8.  DMARC Feedback

   The DMARC mechanism requires highly accurate authentication
   deployments to allow Mail Receivers to safely and scalably enforce
   Domain Owner policies.  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.

8.1.  Feedback Considerations

   It is advisable for a site generating reports of either aggregate
   traffic or specific incidents to ensure the validity and safe
   practices of the entity that will receive the reports.  Some
   documents that provide guidance for such work are [ARF], [ARF-BCP]
   and [ARF-AS].




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8.2.  Verifying External Destinations

   It is possible to specify destinations for the different reports that
   are outside the domain making the request.  This is enabled to allow
   domains that do not have mail servers to request reports and have
   them go someplace that is able to receive and process them.

   Without checks, this would allow a bad actor to publish a DMARC
   policy record that requests reports be sent to a victim address, and
   then send a large volume of mail that will fail both DKIM and SPF
   checks to a wide variety of destinations, which will in turn flood
   the victim with unwanted reports.  Therefore, a verification
   mechanism is included.

   When a Mail Receiver discovers a DMARC policy in the DNS, and the
   domain at which that record was discovered is not identical to the
   host part of the authority component of a [URI] specified in the
   "rua" or "ruf" tag, the following verification steps SHOULD be taken:

   1.  Extract the host portion of the authority component of the URI.
       Call this the "destination host".

   2.  Prepend the string "_report._dmarc".

   3.  Prepend the domain name from which the policy was retrieved.

   4.  Query the DNS for a TXT record at the constructed name.  If the
       result of this request is a temporary DNS error of some kind
       (e.g., a timeout), the Mail Receiver MAY elect to temporarily
       fail the delivery so the verification test can be repeated later.

   5.  If the result includes no TXT resource records or multiple TXT
       resource records, a positive determination of the external
       reporting relationship cannot be made; stop.

   6.  Parse the result, if any, as a series of "tag=value" pairs, i.e.,
       the same overall format as the policy record.  In particular, the
       "v=DMARC1" tag is mandatory and MUST appear first in the list.
       If at least that tag is present and the record overall is
       syntactically valid per Section 6.3, then the external reporting
       arrangement was authorized by the destination ADMD.

   7.  If a "rua" or "ruf" tag is thus discovered, replace the
       corresponding value extracted from the domain's DMARC policy
       record with the one found in this record.  This permits the
       report receiver to override the report destination.  However, to
       prevent loops or indirect abuse, the overriding URI MUST use the
       same destination host from the first step.



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   For example, if a DMARC policy query for "blue.example.com" contained
   "rua=mailto:reports@red.example.net", the host extracted from the
   latter ("red.example.net") does not match "blue.example.com", so this
   procedure is enacted.  A TXT query for
   "blue.example.com._report._dmarc.red.example.net" is issued.  If a
   single reply comes back containing a tag of "v=DMARC1", then the
   relationship between the two is confirmed.  Moreover, red.example.net
   has the opportunity to override the report destination requested by
   "blue.example.com" if needed.

   Where the above algorithm fails to confirm that the external
   reporting was authorized by the destination domain, the URI MUST be
   ignored by the Mail Receiver generating the report.  Further, if the
   confirming record includes a URI whose host is again different than
   the domain publishing that override, the Mail Receiver generating the
   report MUST NOT generate a report to either the original or the
   override URI.

   A report receiver MUST publish such a record in its DNS if it wishes
   to receive reports for other domains.

   The ADMD confirming via the DNS that it wishes to receive reports can
   use a wildcard DNS record to confirm that it is willing to receive
   reports for any domain.  For example, a TXT resource record at
   "*._report._dmarc.example.com" containing at least "v=DMARC1"
   confirms that example.com is willing to receive DMARC reports for any
   domain.

   If the destination of the reports is overcome by volume, it can
   simply remove the confirming DNS record.  However, due to positive
   caching, the result could take as long as the time-to-live on the
   record to go into effect.

   A Mail Receiver might decide not to enact this procedure if, for
   example, it relies on a local list of domains for which external
   reporting addresses are permitted.

8.3.  Aggregate Reports

   Visibility comes in the form of daily (or more frequent) Mail
   Receiver-originated feedback reports that contain aggregate data on
   message streams relevant to the Domain Owner.  This information
   includes data about messages that passed DMARC authentication as well
   as those that did not.

   The format for these reports is defined in Appendix C.

   The report SHOULD include the following data:



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   o  Enough information for the report consumer to re-calculate DMARC
      disposition based on the published policy, message dispositon, and
      SPF, DKIM, and identifier alignment results. {R12}

   o  Data for each sender subdomain separately from mail from the
      sender's organizational domain, even if no subdomain policy is
      applied. {R13}

   o  Sending and receiving domains {R17}

   o  The policy requested by the Domain Owner and the policy actually
      applied (if different) {R18}

   o  The number of successful authentications {R19}

   o  The counts of messages based on all messages received even if
      their delivery is ultimately blocked by other filtering agents
      {R20}

8.4.  Failure Reports

   Message-specific authentication-failure-related forensic reporting
   can be used to identify problems with Domain-Owner-controlled
   infrastructure and to investigate the sources and causes of failing
   messages.  They might also be used to aid investigations into the
   sources and objectives of fraudulent messages.

   The format for these reports is defined in either [AFRF] or [IODEF]
   depending on the value found in the "rf" tag of the DMARC record (or
   its default).

   These reports SHOULD include the "call-to-action" URI(s) from inside
   messages that failed to authenticate. {R21}


9.  Policy Discovery

   As stated above, the DMARC mechanism utilizes DNS TXT records to
   advertise policy.  Policy discovery is accomplished similar to the
   way SPF records are discovered.  Important differences are discussed
   below.

   To balance the conflicting requirements of supporting wildcarding,
   subdomain policy overrides, and limiting DNS query load, the
   following DNS lookup scheme is employed:

   1.  Mail Receivers MUST query the DNS for a DMARC TXT record at the
       DNS domain matching the one found in the RFC5322.From domain in



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                                  DMARC                       March 2012


       the message.  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.

   3.  If the set is now empty, the Mail Receiver MUST query the DNS for
       a DMARC TXT record at the DNS domain matching the Organizational
       Domain in place of the RFC5322.From domain in the message (if
       different).  This record can contain policy to be asserted for
       subdomains of the Organizational Domain.

   4.  Records that do not include a "v=" tag that identifies the
       current version of DMARC are discarded.

   5.  If the remaining set contains multiple records, processing
       terminates and the Mail Receiver takes no action.

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

       A.  if an "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;

       B.  otherwise, the Mail Receiver SHOULD take no action.

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

   If the RFC5322.From domain does not exist in the DNS, Mail Receivers
   SHOULD direct the receiving SMTP server to reject the message {R9}.
   The choice of mechanism for such rejection and the implications of
   those choices are discussed in Section 11 and Section 15.8.


10.  Domain Owner Actions

   To implement the DMARC mechanism, Domain Owners perform the actions
   enumerated in this section.  For a trial operation, a Domain Owner
   might at first deploy DMARC to cover only a subdomain.

   1.  Deploy authentication technologies [DKIM] (see also
       [DKIM-OVERVIEW] and [DKIM-DEPLOYMENT]) and [SPF].





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   2.  Align identifiers; i.e., audit internal systems so that mail
       received by external Mail Receivers will observe that
       Authenticated Identifiers within such messages will be in
       alignment according to the alignment mode to be used.  It is
       important to be thorough with this step, considering all possible
       use cases of mail outbound from the ADMD, as failing to align
       identifiers correctly can cause undesirable handling by
       participating Mail Receivers.  For example, the possibility of
       mail to mailing lists, and the side effects of mailing lists,
       needs to be considered.

   3.  Prepare to receive feedback.  Create dedicated email addresses to
       receive and process feedback from the Mail Receivers.  This
       reporting address SHOULD be serviced by an MTA equipped to
       perform both DKIM and SPF checks.

   4.  Publish a DMARC policy of "none" with a feedback reporting
       address to receive aggregate feedback data from Mail Receivers.

   5.  Review and tune authentication deployments.  Use the provided
       feedback data to remediate unauthenticated email streams and
       correct identifier alignment issues.  This is a good opportunity
       to discover email that, for example, passes SPF checks but is
       missing DKIM signatures, since such email will inevitably fail
       authentication when forwarded.

   6.  Increase policy strength.  When confidence of authentication
       accuracy is gained, publish a DMARC policy of "quarantine" with a
       reasonably small value for "pct".  Debug false positives (due to
       missed unsigned mailstreams) while gradually increasing the value
       of "pct" to 100.

   7.  Fully secure mail streams.  When "pct" reaches 100 with no
       observed ill effects, publish a DMARC policy of "reject" with a
       reasonably small value for "pct".  Repeat debugging and
       corrective process as necessary.

   Many URI schemes involve direct connections to the specified service
   (e.g., http, ftp), but some involve the possibility of intermediate
   handling (e.g. mailto).  A report generator will therefore be able to
   tell right away if submission of a report to the former type of
   service has succeeded or whether an alternate (if available) needs to
   be attempted, but this will not be immediately obvious for the latter
   type of service.  For example, a report submitted by mail may succeed
   at least as far as the local MTA, but could bounce later; however, a
   DMARC report generator will not immediately know about this
   downstream error.




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   Therefore, Domain Owners SHOULD include "mailto" URIs at the end of
   the lists of URIs they publish.


11.  Mail Receiver Actions

   This section describes receiver actions in the DMARC environment.

11.1.  Extract Author Domain

   The domain in the RFC5322.From field is extracted as the domain to be
   evaluated by DMARC.

   A message bearing multiple RFC5322.From identifiers is ambiguous
   under DMARC.  This includes messages with multiple RFC5322.From
   fields (which is also forbidden under [MAIL]) and a message with a
   single RFC5322.From field containing multiple entities.  Such
   messages SHOULD be rejected.  If they are not, they MUST be ignored
   with respect to all DMARC processing, as reliable alignment of
   verified identifiers with what the end user will actually see cannot
   be positively determined.

11.2.  Determine Handling Policy

   To arrive at a policy disposition for an individual message, Mail
   Receivers MUST perform the following actions or their semantic
   equivalents.  The first four steps 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.

   2.  Query the DNS for a DMARC policy record.  Continue if one is
       found, or abort DMARC evaluation otherwise.  See Section 9 for
       details.

   3.  Perform DKIM signature verification checks.  A single email may
       contain multiple DKIM signatures.  The results of this step are
       passed to the remainder of the algorithm and MUST include the
       value of the "d=" tag from all DKIM signatures that successfully
       validated.

   4.  Perform SPF validation checks.  The results of this step are
       passed to the remainder of the algorithm and MUST include the
       domain name from the RFC5321.MailFrom if SPF evaluation returned
       a "pass" result.




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   5.  Conduct identifier alignment checks.  With authentication checks
       and policy discovery performed, the Mail Receiver checks if
       Authenticated Identifiers fall into alignment as decribed in
       Section 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.  Emails that fail the DMARC mechanism check are
       disposed of in accordance with the discovered DMARC policy of the
       Domain Owner.  See Section 6.2 for details.

   Heuristics applied in the absence of use by a Domain Owner of either
   SPF or DKIM (e.g., [Best-Guess-SPF]) SHOULD NOT be used, as it may be
   the case that the Domain Owner wishes a Message Receiver not to
   consider the results of that underlying authentication protocol at
   all.

11.3.  Message Sampling

   Attention must be paid to the possible presence of the "pct" tag in
   the DMARC policy record.  If the tag is present, the Mail Receiver
   MUST NOT enact the requested policy ("p" tag or "sp" tag") on more
   than the stated percent of the totality of affected messages.
   However, regardless of whether or not the "pct" tag is present, the
   Mail Receiver MUST include all relevant message data in any reports
   produced.

   If email is subject to the DMARC policy of "quarantine", the Mail
   Receiver SHOULD quarantine the message.  If the email is not subject
   to the "quarantine" policy (e.g., due to the "pct" tag), the Mail
   Receiver SHOULD apply local spam classification as normal.

   If email is subject to the DMARC policy of "reject", the Mail
   Receiver SHOULD reject the message (see Section 15.8).  If the email
   is not subject to the "reject" policy (due to the "pct" tag), the
   Mail Receiver SHOULD treat the email as though the "quarantine"
   policy applies.  This behavior allows senders to experiment with
   progressively stronger policies without relaxing existing policy.

11.4.  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 6 and Section 12 discuss
   aggregate feedback.




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   See Section 15.9 for a discussion of security matters regarding
   aggregation of such data.


12.  Feedback Mechanism

   The DMARC aggregate feedback report is designed to provide Domain
   Owners with precise insight into authentication results, where
   corrective action needs to be taken by Domain Owners, and the effect
   of Domain Owner DMARC policy on email streams processed by Mail
   Receivers.  The format of the original payload comprising the report
   can be found in Appendix C.

   The availability, publication, and consumption of aggregate DMARC
   feedback provides visibility into real-world email streams that
   Domain Owners need to make informed decisions regarding the
   publication of DMARC policy.  Based on this visibility, Domain Owners
   can publish DMARC policies and be fully cognizant of the resulting
   effect of policy enforcement by Mail Receivers.  This feedback
   mechanism significantly reduces the cost and risk of enforcing
   policies by Mail Receivers.

12.1.  Discovery

   Discovery of a request to receive feedback data is made when a Mail
   Receiver looks up a DMARC policy record.  The presence of the "rua"
   tag specifies where to send feedback.  URI schemes found in "rua" tag
   that are not implemented by a Mail Receiver MUST be ignored.

   For more on the considerations given to DMARC discovery, see
   Section 3.3.

12.2.  Transport

   Where the URI specified in an "rua" tag does not specify otherwise, a
   Mail Receiver generating a feedback report SHOULD apply a secure
   transport mechanism.

   The Mail Receiver, after preparing a report, MUST evaluate the
   provided reporting URIs in the order given.  Any reporting URI that
   included a size limitation exceeded by the generated report (after
   compression and after any encoding required by the particular
   transport mechanism) MUST NOT be used.

   If transport is not possible because the services advertised by the
   published URIs are not able to accept reports (e.g., the URI refers
   to a service that is unreachable, or all provided URIs specify size
   limits exceeded by the generated record), the Mail Receiver SHOULD



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   send a short report (see Section 12.2.4) indicating that a report is
   available but could not be sent.  The Mail Receiver MAY cache that
   data and try again later, or MAY discard data that could not be sent.

12.2.1.  Email

   In the case of a "mailto" URI, the Mail Receiver SHOULD communicate
   reports using the method described in [STARTTLS].

   The message generated by the Mail Receiver must be a [MIME] formatted
   [MAIL] message.  The aggregate report itself MUST be included in one
   of the parts of the message.  A human-readable portion MAY be
   included as a MIME part (such as a text/plain part).

   The aggregate data MUST be an XML file contained within a ZIP file.
   The aggregate data MUST be present using the media type "application/
   zip", and the filenames SHOULD be constructed using the following
   ABNF:

     filename = receiver "!" policy-domain "!" begin-timestamp "!"
                end-timestamp "." extension

     receiver = domain
              ; imported from [MAIL]

     policy-domain = domain

     begin-timestamp = 1*DIGIT
                     ; seconds since 00:00:00 UTC January 1, 1970
                     ; indicating start of the time range contained
                     ; in the report

     end-timestamp = 1*DIGIT
                   ; seconds since 00:00:00 UTC January 1, 1970
                   ; indicating end of the time range contained
                   ; in the report

     extension = "xml" / "zip"

   For the ZIP file itself, the extension MUST be "zip"; for the XML
   report, the extension MUST be "xml".

   Email streams carrying DMARC feedback data MUST conform to the DMARC
   mechanism.  That is, the message comprising the report should be
   DKIM-signed and originate from a source for which an SPF test would
   pass.  This practice minimizes the risk of report consumers
   processing fraudulent reports.




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   The RFC5322.Subject field for individual report submissions SHOULD
   conform to the following ABNF:

     dmarc-subject = %x52.65.70.6f.72.74 1*FWS    ; "Report"
                     %x44.6f.6d.61.69.6e.3a 1*FWS ; "Domain:"
                     domain-name 1*FWS            ; from RFC6376
                     %x53.75.62.6d.69.74.74.65.72.3a ; "Submitter:"
                     1*FWS domain-name 1*FWS
                     %x52.65.70.6f.72.74.2d.49.44.3a ; "Report-ID:"
                     msg-id                       ; from RFC5322

   The first domain-name indicates the DNS domain name about which the
   report was generated.  The second domain-name indicates the DNS
   domain name representing the Mail Receiver generating the report.
   The purpose of the Report-ID: portion of the field is to enable the
   Domain Owner to identify and ignore duplicate reports that might be
   sent by a Mail Receiver.

   This transport mechanism potentially encounters a problem when
   feedback data size exceeds maximum allowable attachment sizes for
   either the generator or the consumer.  See Section 12.2.4 for further
   discussion.

12.2.2.  HTTP

   Where an "http" or "https" method is requested in a Domain Owner's
   URI list, the Mail Receiver MAY encode the data using the
   "application/zip" media type or MAY send the Appendix C data
   uncompressed or unencoded.

   The header portion of the POST or PUT request SHOULD contain a
   Subject field as described in Section 12.2.1.

12.2.3.  Other Methods

   Other registered URI schemes may be explicitly supported in later
   versions.

12.2.4.  Error Reports

   When a Mail Receiver is unable to complete delivery of a report via
   any of the URIs listed by the Domain Owner, the Mail Receiver SHOULD
   generate an error message.  An attempt MUST be made to send this
   report to all listed "mailto" URIs and MAY also be sent to any or all
   other listed URIs.

   The error report MUST be formatted per [MIME].  A text/plain part
   MUST be included that contains field-value pairs such as those found



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   in Section 2 of [DSN].  The fields required, which may appear in any
   order, are:

   Report-Date:  A [MAIL]-formatted date expression indicating when the
      transport failure occurred.

   Report-Domain:  The domain-name about which the failed report was
      generated.

   Report-ID:  The Report-ID: that the report tried to use.

   Report-Size:  The size, in bytes, of the report that was unable to be
      sent.  This MUST represent the number of bytes that the Mail
      Receiver attempted to send.  Where more than one transport system
      was attempted, the sizes may be different; in such cases, separate
      error reports MUST be generated so that this value matches the
      actual attempt that was made.  For example, a "mailto" error
      report would be sent to the "mailto" URIs with one size, while the
      "https" reports might be POSTed to those URIs with a different
      size, as they have different transport and encoding requirements.

   Submitter:  The domain-name representing the Mail Receiver that
      generated, but was unable to submit, the report.

   Submitting-URI:  The URI(s) to which the Mail Receiver tried, but
      failed, to submit the report.

   An additional text/plain part MAY be included that gives a human-
   readable explanation of the above, and MAY also include a URI that
   can be used to seek assistance.

   [NOTE: A more rigorous syntax specification, including ABNF and
   possible registration of a new media type, will be added here when
   more operational experience is acquired.]


13.  Minimum Implementations

   A minimum implementation of DMARC has the following characteristics:

   o  Is able to send and/or receive reports at least daily;

   o  Is able to send and/or receive reports using "mailto" URIs;

   o  Other than in exceptional circumstances such as resource
      exhaustion, can generate or accept a report up to ten megabytes in
      size;




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   o  If acting as a Mail Receiver, fully implements the provisions of
      Section 11.


14.  IANA Considerations

   This section describes actions requested of IANA.

14.1.  Authentication-Results Method Registry Update

   IANA is requested to add the following to the Email Authentication
   Method Name Registry:

   Method:  dmarc

   Defined In:  [this memo]

   ptype:  header

   property:  from

   value:  the domain portion of the RFC5322.From field

14.2.  Authentication-Results Result Registry Update

   IANA has added the following in the Email Authentication Result Name
   Registry:

   Code:  none

   Existing/New Code:  existing

   Defined In:  [AUTH-RESULTS]

   Auth Method:  dmarc (added)

   Meaning:  No DMARC policy record was published for the aligned
      identifier, or no aligned identifier could be extracted.

   Code:  pass

   Existing/New Code:  existing

   Defined In:  [AUTH-RESULTS]







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                                  DMARC                       March 2012


   Auth Method:  dmarc (added)

   Meaning:  A DMARC policy record was published for the aligned
      identifier, and at least one of the authentication mechanisms
      passed.

   Code:  fail

   Existing/New Code:  existing

   Defined In:  [AUTH-RESULTS]

   Auth Method:  dmarc (added)

   Meaning:  A DMARC policy record was published for the aligned
      identifier, and none of the authentication mechanisms passed.

   Code:  temperror

   Existing/New Code:  existing

   Defined In:  [AUTH-RESULTS]

   Auth Method:  dmarc (added)

   Meaning:  A temporary error occurred during DMARC evaluation.  A
      later attempt might produce a final result.

   Code:  permerror

   Existing/New Code:  existing

   Defined In:  [AUTH-RESULTS]

   Auth Method:  dmarc (added)

   Meaning:  A permanent error occurred during DMARC evaluation, such as
      encountering a syntactically incorrect DMARC record.  A later
      attempt is unlikely to produce a final result.

14.3.  DMARC Tag Registry

   Names of DMARC tags must be registered with IANA.  New entries are
   assigned only for values that have been documented in a published RFC
   that has had IETF Review, per [IANA-CONSIDERATIONS].  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".



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

    +----------+-------------+---------+------------------------------+
    | Tag Name | Defined     | Status  | Description                  |
    +----------+-------------+---------+------------------------------+
    |  adkim   | [THIS MEMO] | current | DKIM alignment mode          |
    +----------+-------------+---------+------------------------------+
    |   aspf   | [THIS MEMO] | current | SPF alignment mode           |
    +----------+-------------+---------+------------------------------+
    |   pct    | [THIS MEMO] | current | Sampling rate                |
    +----------+-------------+---------+------------------------------+
    |    p     | [THIS MEMO] | current | Requested handling policy    |
    +----------+-------------+---------+------------------------------+
    |    rf    | [THIS MEMO] | current | Forensic reporting format(s) |
    +----------+-------------+---------+------------------------------+
    |    ri    | [THIS MEMO] | current | Aggregate Reporting interval |
    +----------+-------------+---------+------------------------------+
    |   rua    | [THIS MEMO] | current | Reporting URI(s) for         |
    |          |             |         | aggregate data               |
    +----------+-------------+---------+------------------------------+
    |   ruf    | [THIS MEMO] | current | Reporting URI(s) for         |
    |          |             |         | forensic data                |
    +----------+-------------+---------+------------------------------+
    |    sp    | [THIS MEMO] | current | Requested handling policy    |
    |          |             |         | for subdomains               |
    +----------+-------------+---------+------------------------------+
    |    v     | [THIS MEMO] | current | Specification version        |
    +----------+-------------+---------+------------------------------+

14.4.  DMARC Report Format Registry

   Names of DMARC forensic reporting formats must be registered with
   IANA.  New entries are assigned only for values that have been
   documented in a published RFC that has had IETF Review, per
   [IANA-CONSIDERATIONS].  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 initial set of entries in this registry is as follows:












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    +--------+-------------+---------+-----------------------------+
    | Format | Defined     | Status  | Description                 |
    |  Name  |             |         |                             |
    +--------+-------------+---------+-----------------------------+
    | afrf   | [THIS MEMO] | current | Authentication Failure      |
    |        |             |         | Reporting Format (see       |
    |        |             |         | [AFRF])                     |
    +--------+-------------+---------+-----------------------------+
    | iodef  | [THIS MEMO] | current | Incident Object Description |
    |        |             |         | Exchange Format (see        |
    |        |             |         | [IODEF])                    |
    +--------+-------------+---------+-----------------------------+


15.  Security Considerations

   This section discusses security-specific issues related to the DMARC
   mechanism.

15.1.  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, which is
   part of a body of data trivially forged throughout the history of
   email.

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

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

   o  It seems the best choice of an identifier on which to focus as
      most Mail User Agents (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.

   o  The focus of email authentication efforts has been to create
      mechanisms by which this field, or at least some field in the
      message, can be deemed genuine.  Thus, this field is not easily
      forged within the context of its use with DMARC.

   o  The DMARC mechanism confers no additional privilege to the message
      without successful authentication of this identifier.







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15.2.  Display Name Attacks

   A common attack in messaging abuse is the presentation of false
   information in the "display name" portion of the RFC5322.From 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 Mail User
   Agents (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:

   o  If the display name is found to include an email address (as
      specified in [MAIL]), 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 is easily circumvented by spoofers simply by 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>

   o  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.

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

15.3.  Attacks on Reporting URIs

   URIs published in DNS TXT records are well-understood possible
   targets for attack.  Specifications such as [DNS] and [ROLES] 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



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   penetration attacks.

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

   o  high-volume denial-of-service attacks;

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

   o  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.

15.4.  Issues Specific to SPF

   SPF results are honored as a backup mechanism, even if DKIM
   verification fails or the signature is absent.  Senders with internal
   policies that require all of their mail to be signed may not express
   a need for this backup mechanism.  However, both senders and
   receivers benefit in significantly reduced support costs if unsigned
   mail-streams are discovered through aggregate feedback reports as
   opposed to rejection of legitimate email that otherwise passes with a
   valid SPF result.

   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.

15.5.  DNS Load

   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 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 Domain Owner'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 to go unnoticed for the
   length of the TTL (while waiting for DNS caches to expire).  Avoiding



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   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.

15.6.  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 8.2 describes a DNS-based
   mechanism for verifying approved external reporting.

   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 time-to-live 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 forensic 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 forensic data to be
   routed fraudulently to an attacker's systems may be an attractive
   prospect.  Deployment of DNSSEC is advisable if this is a concern.

   The verification mechanism presented in Section 8.2 is currently not
   mandatory ("MUST") but strongly recommended ("SHOULD").  It is
   possible that it would be elevated to a "MUST" by later security
   review.

15.7.  Feedback Loops

   Per [ARF-BCP] and [ARF-AS], it is highly advisable to vet the
   destinations of feedback streams to which Mail Receivers will send
   data.  Sending reports to any party that asks invites various
   concerns regarding privacy of the data thus exchanged and the ability
   to keep up with what could be an enormous reporting stream.

   It is also advisable for any operator generating reports to have a
   mechanism by which one can request that no more reports be sent in
   case some entity becomes the unwitting recipient of undesired data in
   high volumes.



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15.8.  Rejecting Messages

   This proposal calls for rejection of a message during the SMTP
   session under certain circumstances.  This is typically done in one
   of two ways:

   o  Full rejection, wherein the SMTP server issues a 5xy reply code
      (550 SHOULD be used) 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
      [SMTP]).

   o  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 may prevent
   "backscatter" (the annoying generation of delivery failure reports,
   which go back to the RFC5321.MailFrom address, about messages that
   were fraudulently generated), but effectively means 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.

   If a Mail Receiver elects to defer delivery due to inability to
   retrieve or apply DMARC policy, this SHOULD be done with a 450 SMTP
   reply code.

15.9.  Capacity Planning

   DMARC participants will need to perform capacity planning to support
   their implementations.  Some factors to consider include:

   Storage:  As Mail Receivers process increasing numbers of messages --
      from increasingly disparate sources -- claiming to be from DMARC-
      enabled domains, additional storage of information must be
      considered to support the generation of feedback reports.  Storage
      needs will also increase as the number of Domain Owners for which
      the Mail Receiver agrees to provide service increases.  Similarly,
      Domain Owners will need to plan based on how long they wish to
      store the data found in received reports.  When Domain Owners



Kucherawy                                                      [Page 42]

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      enter exceptional situations and are unable to accept reports,
      Mail Receivers, as a matter of policy, might discard undelivered
      reports.

   Frequency:  Sending reports more frequently increases processing
      costs at both the Mail Receiver and the Domain Owner, but can
      decrease Mail Receiver storage requirements as data are consumed
      and storage is freed through report generation and transmission.
      At the same time, less frequent report generation may lead to
      somewhat stale feedback.  An appropriate balance should be sought.

   DNS:  DMARC imposes up to two additional DNS queries per arriving
      message, namely the TXT queries to try to locate a policy
      statement.  For Mail Receivers, these are queries sent; for Domain
      Owners, these are queries that must be handled.  Both sides will
      need to plan for the additional DNS load.

15.10.  Privacy Considerations

   This section discusses security issues specific to private data that
   may be included in the interactions that are part of DMARC.

15.10.1.  Data Exposure Considerations

   Aggregate reports are limited in scope to DMARC policy and
   disposition results, to information pertaining to the underlying
   authentication mechanisms, and to the identifiers involved in DMARC
   validation.

   Failed message reporting provides message-specific details pertaining
   to authentication failures.  Individual reports can contain message
   content as well as trace header fields.  Domain Owners are able to
   analyze individual reports and attempt to determine root causes of
   authentication mechanism failures, gain insight into
   misconfigurations or other problems with email and network
   infrastructure, or inspect messages for insight into abusive
   practices.

   Both report types may expose sender and recipient identifiers (e.g.,
   RFC5322.From fields), and although the [AFRF] format used for failed
   message reporting supports redaction, it is capable of exposing the
   entire message to the report recipient.

   Domain Owners requesting reports will receive information about mail
   claiming to be from them, which includes mail that was not, in fact,
   from them.  Information about the final destination of mail where it
   might otherwise be obscured by intermediate systems will therefore be
   exposed.



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15.10.2.  Report Recipients

   A DMARC record can specify for reports to be sent to an intermediary
   operating on behalf of the Domain Owner.  This is done when the
   Domain Owner contracts with an entity to monitor mail-streams for
   abuse and performance issues.  Receipt by third parties of such data
   may or may not be permitted by the Mail Receiver's privacy policy,
   terms of use, or other similar governing document.  Domain Owners and
   Mail Receivers should both review and understand if their own
   internal policies constrain the use and transmission of DMARC
   reporting.

15.10.3.  Report Generators

   The entity (e.g., mailbox provider, Internet service provider)
   receiving emails is typically responsible for generating DMARC
   reports.  Such entities are typically charged with protecting
   accidental disclosure of their users' data.  In this case, disclosure
   is being requested by the entity generating the email in the first
   place, i.e., the Domain Owner, so this may not fit squarely within
   existing privacy policy provisions.  For some providers, aggregate
   and failed message reporting are viewed as a function similar to
   complaint reporting about spamming or phishing, and treated similarly
   under the privacy policy.  Report generators (i.e., Mail Receivers)
   are encouraged to review their reporting limitations under such
   policies before enabling DMARC reporting.

15.10.4.  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.  Open transport mechanisms should be
   avoided.

15.11.  Identifier Alignment Considerations

   The DMARC mechanism allows both DKIM and SPF-authenticated
   identifiers to authenticate email on behalf of a Domain Owner, and,
   in the case of SPF, on behalf of different subdomains.  If malicious
   or unaware users can gain control of the SPF record or signing
   practices for a sub-domain, the sub-domain 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 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 cede
   control of sub-domains if this is an issue, and to consider using the
   "strict" Identifier Alignment option if appropriate.


16.  References

16.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.

   [AFRF]     Fontana, H., "Authentication Failure Reporting using the
              Abuse Report Format",
              I-D draft-ietf-marf-authfailure-report, September 2011.

   [DKIM]     Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 6376,
              September 2011.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [DNS-CASE]
              Eastlake, D., "Domain Name System (DNS) Case Insensitivity
              Clarification", RFC 4343, January 2006.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MAIL]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [MIME]     Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [STARTTLS]
              Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.



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                                  DMARC                       March 2012


   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              January 2005.

16.2.  Informative References

   [ADSP]     Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, August 2009.

   [ARF]      Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              August 2010.

   [ARF-AS]   Falk, J. and M. Kucherawy, Ed., "Creation and Use of Email
              Feedback Reports: An Applicability Statement for the Abuse
              Reporting Format (ARF)", draft-ietf-marf-as (work in
              progress), March 2012.

   [ARF-BCP]  Falk, J., "Message Header Field for Indicating Message
              Authentication Status", RFC 6449, November 2011.

   [AUTH-RESULTS]
              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

   [Best-Guess-SPF]
              Kitterman, S., "Sender Policy Framework: Best guess record
              (FAQ entry)", May 2010,
              <http://www.openspf.org/FAQ/Best_guess_record>.

   [DKIM-DEPLOYMENT]
              Hansen, T., Siegel, E., Crocker, D., and P. Hallam-Baker,
              "DomainKeys Identified Mail (DKIM) Development,
              Deployment, and Operations", RFC 5863, May 2010.

   [DKIM-OVERVIEW]
              Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
              Identified Mail (DKIM) Service Overview", RFC 5585,
              July 2009.

   [DSN]      Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.



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   [IANA-CONSIDERATIONS]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [IODEF]    Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
              Object Description Exchange Format", RFC 5070,
              December 2007.

   [ROLES]    Crocker, D., "Mailbox Names for Common Services, Roles and
              Functions", RFC 2142, May 1997.


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.
   This text is included to explain why they were considered and not
   included in this version.

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.

   DMARC is focused on authentication at the domain level (i.e., the
   ADMD 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 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 chouice 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, experiements have shown that including S/MIME support in the



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   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 Message Receivers not to attempt validation 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 barried 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.

   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 two primary 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
       behaviour among MUAs regarding what to do with the content of the
       Sender field, if present.  Accordingly, supporting checking of



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       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 Sender 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 common practice among MTA operators, and indeed one documented in
   [ADSP], is a test to determine domain existence prior to any more
   expensive processing.  This is typically done by querying the DNS for
   MX, A or AAAA resource records for the name being evaluated, and
   assuming the domain is non-existent if it could be determined that no
   such records were published for that domain name.

   The original pre-standardization version of this protocol included a
   mandatory check of this nature.  It was ultimately removed, as the
   method's error rate was too high without substantial manual tuning
   and heuristic work.  There are indeed use cases this work needs to
   address where such a method would return a negative result about a
   domain for which reporting is desired, such as a registered domain
   name that never sends legitimate mail and thus has none of these
   records present in the DNS.

A.5.  Issues With ADSP In Operation

   DMARC has been chacarterized 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.  Non-existent subdomains are explicitly out of scope in ADSP.
       There is nothing in ADSP that states receivers should simply
       reject mail from NXDOMAINs regardless of ADSP policy (which of



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       course allows spammers to trivially bypass ADSP by sending email
       from non-existent 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 roll-out, i.e., no way to
       configure a percentage of email on which the receiver should
       apply the policy.  This is important for large-volume senders.

   6.  ADSP has no explicit support for an intermediate phase where the
       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

   Although protocols like ADSP are useful for "protecting" a specific
   domain name, they are not helpful at protecting subdomains.  If one
   wished to protect "example.com" by requiring via ADSP that all mail
   bearing an RFC5322.From domain of "example.com" be signed, this would
   "protect" that domain; however, one could then craft an email whose
   RFC5322.From domain is "security.example.com", and ADSP would not
   provide any protection.  One could use a DNS wildcard, but this can
   undesirably interfere with other DNS activity; one could add ADSP
   records as fraudulent domains are discovered, but this solution does
   not scale and is a purely reactive measure against abuse.

   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.  Suggestions 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.

   When seeking domain-specific policy based on an arbitrary domain
   name, one could "climb the tree", dropping labels off the left end of
   the name until the root is reached or a policy is discovered, but
   then one could craft a name that has a large number of nonsense
   labels; this would cause a Mail Receiver to attempt a large number of
   queries in search of a policy record.  Sending many such messages
   constitutes an amplified denial-of-service attack.




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   The Organizational Domain mechanism is a necessary component to the
   goals of DMARC.  The method described in Section 4 is not perfect,
   but serves this purpose reasonably well without adding undue burden
   or semantics to the DNS.

A.6.1.  Public Suffix Lists

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


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.

   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

   SPF In Alignment

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







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   Example 2: SPF in alignment (parent):

        MAIL FROM: <sender@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

   SPF In Alignment (Parent)

   In this case, the RFC5321.MailFrom parameter includes a DNS domain
   that is a parent of the RFC5322.From domain.  Thus, the identifiers
   are in alignment if "relaxed" SPF mode is requested by the Domain
   Owner, and not in alignment if "strict" SPF mode is requested.

   Example 3: SPF not in alignment:

        MAIL FROM: <sender@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

   SPF Not In Alignment

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

B.1.2.  DKIM

   The examples below assume 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

   DKIM In Alignment

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



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   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

   DKIM In Alignment (Parent)

   In this case, the DKIM signature's "d=" parameter includes a DNS
   domain that is a parent of the RFC5322.From domain.  Thus, the
   identfiers are in alignment.

   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

   DKIM Not In Alignment

   In this case, the DKIM signature's "d=" parameter includes a DNS
   domain that is neither the same as nor a parent 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 receivers
   without affecting how the messages are processed, in order to:

   o  Confirm that its legitimate messages are authenticating correctly

   o  Verify that all authorized message sources have implemented
      authentication measures





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   o  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:

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

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

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

   o  All messages from this organizational domain are subject to this
      policy (no "pct" tag present, so the default of 100% 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"

   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 forensic reports when
   authentication failures occur.

   Not all 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 ([AFRF]) meets the Domain Owner's needs in this scenario.

   The Domain Owner accomplishes this by adding the following to its



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   policy record from Appendix B.2):

   o  Per-message forensic 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" )

B.2.3.  Per-Message Forensic 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 receive and process the
   per-message forensic reports.  Again, not all Receivers will honor
   this request, but those that do may implement additional checks to
   validate that the third party wishes to receive the forensic reports
   for this domain.

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

   o  Per message forensic reports should be send 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"




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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
   (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 whichthat this record is published,
   conforming Receivers will implement additional checks as described in
   Section 8.2 of this document.  In order to pass these additional
   checks, the third party will need to publish an additional DNS record
   as follows:

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

   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 forensic reports on behalf of example.com

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

   Intermediaries and other third parties should refer to Section 8.2
   for the full details of this mechanism.

B.2.4.  Sub-Domain, Sampling, and Multiple Aggregate Report URIs

   The Domain Owner has implemented SPF and DKIM in a sub-domain used
   for pre-production testing of messaging services.  It now wishes to
   request that participating receivers act to reject messages from this
   sub-domain that fail to authenticate.



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   As a first step it will ask that a portion (1/4 in this example) of
   failing messages be quarantined, enabling examination of messages
   sent to mailboxes hosted by participating receivers.  Aggregate
   feedback reports will be sent to a mailbox within the Organizational
   Domain, and to a mailbox at a third party selected and authorized to
   receive same by the Domain Owner.

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

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

   o  It is applied only to this sub-domain (record is published at
      "_dmarc.test.example.com" and not "_dmarc.example.com")

   o  Receivers should quarantine messages from this organizational
      domain that fail to authenticate ("p=quarantine")

   o  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")

   o  25% of messages from this Organizational Domain are subject to
      action based on this policy ("pct=25")

   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\; pct=25"

   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 example.com

     _dmarc IN  TXT  ( "v=DMARC1; p=quarantine; "
                       "rua=mailto:dmarc-feedback@example.com,"
                       "mailto:tld-test@thirdparty.example.net; "
                       "pct=25" )







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B.2.5.  Third Party Sender and Identifier Alignment

   The Domain Owner only uses the top-level domain for email, and uses a
   third-party sender for some marketing message traffic.  It has
   implemented SPF and DKIM across its in-house infrastructure and
   required the third-party to do the same.  A monitoring period has
   shown that the Domain Owner and the third-party sender are both
   executing well with respect to email authentication measures.

   The third-party has access to the appropriate DKIM private or signing
   keys for the selectors it will use.  However the third-party uses
   sub-domains like "id1234.bounces.example.com" in the RFC5321.Mailfrom
   address for campaign tracking and troubleshooting purposes.  The sub-
   domain "bounces.example.com" has been delegated to the third-party so
   that it can publish appropriate MX records in the DNS.

   Therefore the Domain Owner wishes to publish a policy that requests
   rejection of messages which fail to authenticate, strict identifier
   alignment for DKIM authentication, and relaxed identifier alignment
   for SPF checks.  Aggregate reports will only be sent to the Domain
   Owner in this example.

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

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

   o  Receivers should reject messages that fail to authenticate
      ("p=reject")

   o  Strict identifier alignment should be applied to DKIM checks
      ("adkim=s")

   o  Relaxed identifier alignment should be applied to SPF checks
      ("aspf=r")

   o  Aggregate feedback reports should be sent via email to the address
      "dmarc-feedback@example.com"
      ("rua=mailto:dmarc-feedback@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=reject\; adkim=s\; aspf=r\;
      rua=mailto:dmarc-feedback@example.com"




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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 example.com
     _dmarc  IN  TXT  ( "v=DMARC1; p=reject; adkim=s; aspf=r; "
                        "rua=mailto:dmarc-feedback@example.com" )

B.2.6.  Sub-Domain Policy, Reporting Interval

   In this example the Domain Owner only uses addresses in the
   Organizational Domain itself ("user@example.com" versus
   "user@sub.example.com").  A business decision has been made that
   messages incorrectly being rejected as false positives during, for
   example, a transient outage are unacceptable.  Therefore, the desired
   policy is that:

   o  Messages from the Organizational Domain that fail authentication
      should be quarantined

   o  Messages from any sub-domain should be rejected

   Furthermore the Domain Owner would like to request that aggregate
   data be sent at four hour intervals to themselves and a third-party
   service for analysis and action.  It recognizes that not all
   Receivers will honor this request, but feels that faster intraday
   analysis of failures and threats make this worthwhile.

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

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

   o  Receivers should quarantine messages from this domain that fail to
      authenticate ("p=quarantine")

   o  Receivers should reject messages from any sub-domains that fail to
      authenticate ("sp=reject")

   o  Aggregate reports should be generated every four hours
      ("ri=14400")

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

   The DMARC policy record might look like this when retrieved using a



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   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=quarantine\; sp=reject\; ri=14400\;
      rua=mailto:dmarc-feedback@example.com,
      mailto:customer-data@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:

     ; DMARC record for the domain example.com
     _dmarc  IN  TXT  ( "v=DMARC1; p=quarantine; sp=reject; "
                        "rua=mailto:dmarc-feedback@example.com,"
                        "mailto:customer-data@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.

B.3.1.  SMTP-time Processing

   An optimal DMARC-enabled Mail Receiver performs authentication and
   identifier alignment checking during the [SMTP] conversation.

   Prior to returning a 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" vs
   "relaxed" options found in the DMARC record).




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

     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 and the DKIM-authenticated
   Identifiers align with the Author Domain.  The Mail Receiver
   considers the above email to pass the DMARC check, avoiding the
   "reject" policy that is 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 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 550 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 450 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.






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B.3.2.  Real-time Feedback Processing

   If the DMARC record for the Author Domain of the message under
   processing requests [AFRF]-based reporting, then the Mail Receiver
   can supply an AFRF report for a message that does not pass all
   underlying DMARC authentication checks.  In other words, if any
   DMARC-supporting authentication checks fail, an AFRF report should be
   generated and sent to the reporting address found in the Author
   Domain's DMARC record.

B.4.  Utilization of Aggregate Feedback example

   Aggregate feedback is consumed by Domain Owners to verify the Domain
   Owners understanding of how the Domain Owner's 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 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 fail to pass,
   the Domain Owner is provided with enough information to either
   directly correct the problem or to 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.  Additionally, the Domain Owner may
   come to an understanding of how its Domain is being misused.

B.5.  mailto Transport example

   A DMARC record can contain a "mailto" reporting address, such as:

     mailto:dmarc-feedback@example.com

   A sample aggregate report from the Mail Receiver at
   mail.receiver.example follows:




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     DKIM-Signature: v=1; ...; d=mail.receiver.example; ...
     From: dmarc-reporting@mail.receiver.example
     Date: Fri, Feb 15 2002 16:54:30 -0800
     To: dmarc-feedback@example.com
     Subject: Report Domain: example.com
         Submitter: mail.receiver.example
         Report-ID: <2002.02.15.1>
     MIME-Version: 1.0
     Content-Type: multipart/alternative;
         boundary="----=_NextPart_000_024E_01CC9B0A.AFE54C00"
     Content-Language: en-us

     This is a multipart message in MIME format.

     ------=_NextPart_000_024E_01CC9B0A.AFE54C00
     Content-Type: text/plain; charset="us-ascii"
     Content-Transfer-Encoding: 7bit

     This is an aggregate report from mail.receiver.example.

     ------=_NextPart_000_024E_01CC9B0A.AFE54C00
     Content-Type: application/zip
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment;
         filename="mail.receiver.example!example.com!
                   1013662812!1013749130.zip"

     <zipped content of report>

     ------=_NextPart_000_024E_01CC9B0A.AFE54C00--

   Not shown in the above example is that the Mail Receiver's feedback
   should be authenticated using SPF.  Also, the value of the "filename"
   MIME parameter is wrapped for printing in this specification but
   would normally appear as one continuous string.

B.6.  https Transport example

   A DMARC record can contain an "https" reporting address, such as:

     https:feedback.example.com/dmarc-feedback-submissions

   A sample aggregate report from the Mail Receiver at
   mail.receiver.example, being posted per the above URL via an
   established secure HTTP (https) connection, might look like this:






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     POST /dmarc-feedback-submissions HTTP/1.1
     Host: feedback.example.com
     Subject: Report Domain: example.com
         Submitter: mail.receiver.example
         Report-ID: <2002.02.15.1>
     Content-Type: application/zip
     Content-Length: 19191

     <zipped content of report here>


Appendix C.  DMARC XML Schema

   The following is the proposed initial schema for producing XML
   formatted aggregate reports as described in this memo:

   <?xml version="1.0"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
     targetNamespace="http://dmarc.org/dmarc-xml/0.1">

   <!-- The time range in UTC covered by messages in this report,
        specified in seconds since epoch. -->
   <xs:complexType name="DateRangeType">
     <xs:all>
       <xs:element name="begin" type="xs:integer"/>
       <xs:element name="end" type="xs:integer"/>
     </xs:all>
   </xs:complexType>

   <!-- Report generator metadata -->
   <xs:complexType name="ReportMetadataType">
     <xs:sequence>
       <xs:element name="org_name" type="xs:string"/>
       <xs:element name="email" type="xs:string"/>
       <xs:element name="extra_contact_info" type="xs:string"
                  minOccurs="0"/>
       <xs:element name="report_id" type="xs:string"/>
       <xs:element name="date_range" type="DateRangeType"/>
       <xs:element name="error" type="xs:string" minOccurs="0"
         maxOccurs="unbounded"/>
     </xs:sequence>
   </xs:complexType>

   <!-- Alignment mode (relaxed or strict) for DKIM and
        SPF. -->
   <xs:simpleType name="AlignmentType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="r"/>



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       <xs:enumeration value="s"/>
     </xs:restriction>
   </xs:simpleType>

   <!-- The policy actions specified by p and sp in the
        DMARC record. -->
   <xs:simpleType name="DispositionType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="none"/>
       <xs:enumeration value="quarantine"/>
       <xs:enumeration value="reject"/>
     </xs:restriction>
   </xs:simpleType>

   <!-- The DMARC policy that applied to the messages in
       this report. -->
   <xs:complexType name="PolicyPublishedType">
     <xs:all>
       <!-- The domain at which the DMARC record was found. -->
       <xs:element name="domain" type="xs:string"/>
       <!-- The DKIM alignment mode. -->
       <xs:element name="adkim" type="AlignmentType"/>
       <!-- The SPF alignment mode. -->
       <xs:element name="aspf" type="AlignmentType"/>
       <!-- The policy to apply to messages from the domain. -->
       <xs:element name="p" type="DispositionType"/>
       <!-- The policy to apply to messages from subdomains. -->
       <xs:element name="sp" type="DispositionType"/>
       <!-- The percent of messages to which policy applies. -->
       <xs:element name="pct" type="xs:integer"/>
     </xs:all>
   </xs:complexType>

   <!-- The DMARC-aligned authentication result. -->
   <xs:simpleType name="DMARCResultType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="pass"/>
       <xs:enumeration value="fail"/>
     </xs:restriction>
   </xs:simpleType>

   <!-- Reasons that may affect DMARC disposition or execution
        thereof. -->
   <xs:simpleType name="PolicyOverrideType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="forwarded"/>
       <xs:enumeration value="sampled_out"/>
       <xs:enumeration value="trusted_forwarder"/>



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       <xs:enumeration value="mailing_list"/>
       <xs:enumeration value="local_policy"/>
       <xs:enumeration value="other"/>
     </xs:restriction>
   </xs:simpleType>

   <!-- How do we allow report generators to include new
        classes of override reasons if they want to be more
        specific than "other"? -->
   <xs:complexType name="PolicyOverrideReason">
     <xs:all>
       <xs:element name="type" type="PolicyOverrideType"/>
       <xs:element name="comment" type="xs:string"
                  minOccurs="0"/>
     </xs:all>
   </xs:complexType>

   <!-- Taking into account everything else in the record,
        the results of applying DMARC. -->
   <xs:complexType name="PolicyEvaluatedType">
     <xs:sequence>
       <xs:element name="disposition" type="DispositionType"/>
       <xs:element name="dkim" type="DMARCResultType"/>
       <xs:element name="spf" type="DMARCResultType"/>
       <xs:element name="reason" type="PolicyOverrideReason"
                     minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
   </xs:complexType>

   <!-- Credit to Roger L. Costello for IPv4 regex
       http://mailman.ic.ac.uk/pipermail/xml-dev/1999-December/
            018018.html -->
   <!-- Credit to java2s.com for IPv6 regex
       http://www.java2s.com/Code/XML/XML-Schema/
            IPv6addressesareeasiertodescribeusingasimpleregex.htm -->
   <xs:simpleType name="IPAddress">
     <xs:restriction base="xs:string">
       <xs:pattern value="((1?[0-9]?[0-9]|2[0-4][0-9]|25[0-5]).){3}
                   (1?[0-9]?[0-9]|2[0-4][0-9]|25[0-5])|
                   ([A-Fa-f0-9]{1,4}:){7}[A-Fa-f0-9]{1,4}"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="RowType">
     <xs:all>
       <!-- The connecting IP. -->
       <xs:element name="source_ip" type="IPAddress"/>
       <!-- The number of matching messages -->



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       <xs:element name="count" type="xs:integer"/>
       <!-- The DMARC disposition applying to matching
            messages. -->
       <xs:element name="policy_evaluated"
                   type="PolicyEvaluatedType"
                   minOccurs="0"/>
     </xs:all>
   </xs:complexType>

   <xs:complexType name="IdentifierType">
     <xs:all>
       <!-- The envelope recipient domain. -->
       <xs:element name="envelope_to" type="xs:string"
                  minOccurs="0"/>
       <!-- The payload From domain. -->
       <xs:element name="header_from" type="xs:string"
                  minOccurs="1"/>
     </xs:all>
   </xs:complexType>

   <!-- DKIM verification result, according to RFC 5451
        Section 2.4.1. -->
   <xs:simpleType name="DKIMResultType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="none"/>
       <xs:enumeration value="pass"/>
       <xs:enumeration value="fail"/>
       <xs:enumeration value="policy"/>
       <xs:enumeration value="neutral"/>
       <xs:enumeration value="temperror"/>
       <xs:enumeration value="permerror"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="DKIMAuthResultType">
     <xs:all>
       <!-- The d= parameter in the signature -->
       <xs:element name="domain" type="xs:string"
                   minOccurs="1"/>
       <!-- The DKIM verification result -->
       <xs:element name="result" type="DKIMResultType"
                   minOccurs="1"/>
       <!-- Any extra information (e.g., from
            Authentication-Results -->
       <xs:element name="human_result" type="xs:string"
                   minOccurs="0"/>
     </xs:all>
   </xs:complexType>



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   <!-- SPF result -->
   <xs:simpleType name="SPFResultType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="none"/>
       <xs:enumeration value="neutral"/>
       <xs:enumeration value="pass"/>
       <xs:enumeration value="fail"/>
       <xs:enumeration value="softfail"/>
       <!-- "TempError" commonly implemented as "unknown" -->
       <xs:enumeration value="temperror"/>
       <!-- "PermError" commonly implemented as "error" -->
       <xs:enumeration value="permerror"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="SPFAuthResultType">
     <xs:all>
       <!-- The envelope From domain. -->
       <xs:element name="domain" type="xs:string" minOccurs="1"/>
       <!-- The SPF verification result -->
       <xs:element name="result" type="SPFResultType"
                   minOccurs="1"/>
     </xs:all>
   </xs:complexType>

   <!-- This element contains DKIM and SPF results, uninterpreted
        with respect to DMARC. -->
   <xs:complexType name="AuthResultType">
     <xs:sequence>
       <!-- There may be no DKIM signatures, or multiple DKIM
            signatures. -->
       <xs:element name="dkim" type="DKIMAuthResultType"
         minOccurs="0" maxOccurs="unbounded"/>
       <!-- There will always be at least one SPF result. -->
       <xs:element name="spf" type="SPFAuthResultType" minOccurs="1"
         maxOccurs="unbounded"/>
     </xs:sequence>
   </xs:complexType>

   <!-- This element contains all the authentication results used
        to evaluate the DMARC disposition for the given set of
        messages. -->
   <xs:complexType name="RecordType">
     <xs:sequence>
       <xs:element name="row" type="RowType"/>
       <xs:element name="identifiers" type="IdentifierType"/>
       <xs:element name="auth_results" type="AuthResultType"/>
     </xs:sequence>



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   </xs:complexType>

   <!-- Parent -->
   <xs:element name="feedback">
     <xs:complexType>
       <xs:sequence>
         <xs:element name="report_metadata"
                    type="ReportMetadataType"/>
         <xs:element name="policy_published"
                    type="PolicyPublishedType"/>
         <xs:element name="record" type="RecordType"
                     maxOccurs="unbounded"/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>
   </xs:schema>

   Descriptions of the PolicyOverrideTypes:

   forwarded:  Message was relayed via a known forwarder, or local
      heuristics identified the message as likely having been forwarded.
      There is no expectation that authentication would pass.

   local_policy:  The Mail Receiver's local policy exempted the message
      from being subjected to the Domain Owner's requested policy
      action.

   mailing_list:  Local heuristics determined that the message arrived
      via a mailing list, and thus authentication of the original
      message was not expected to succeed.

   other:  Some policy exception not covered by the other entries in
      this list occurred.  Additional detail can be found in the
      PolicyOverrideReason's "comment" field.

   sampled_out:  Message was exempted from application of policy by the
      "pct" setting in the DMARC policy record.

   trusted_forwarder:  Message authentication failure was anticipated by
      other evidence linking the message to a locally-maintained list of
      known and trusted forwarders.


Appendix D.  Public Discussion

   Public discussion of the DMARC proposal documents is taking place on
   the dmarc-discuss@dmarc.org mailing list.  Subscription is available
   at http://www.dmarc.org/mailman/listinfo/dmarc-discuss.



Kucherawy                                                      [Page 69]

                                  DMARC                       March 2012


Author's Address

   Murray S. Kucherawy (editor)
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   USA

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com









































Kucherawy                                                      [Page 70]

