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Versions: (draft-dmarc-interoperability) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 7960

DMARC Working Group                                       F. Martin, Ed.
Internet-Draft                                                  LinkedIn
Intended status: Informational                              E. Lear, Ed.
Expires: November 23, 2015                            Cisco Systems GmbH
                                                         T. Draegen, Ed.
                                              Eudaemonic Development LLC
                                                          E. Zwicky, Ed.
                                                                   Yahoo
                                                            May 22, 2015


     Interoperability Issues Between DMARC and Indirect Email Flows
                  draft-ietf-dmarc-interoperability-03

Abstract

   DMARC introduces a mechanism for expressing domain-level policies and
   preferences for email message validation, disposition, and reporting.
   The DMARC mechanism can encounter interoperability issues when
   messages do not flow directly from the author's administrative domain
   to the final recipients.  Collectively these email flows are referred
   to as indirect email flows.  This document describes interoperability
   issues between DMARC and indirect email flows.  Possible methods for
   addressing interoperability issues are presented.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 23, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Conventions  . . . . . . . . . . . . . . . . . .   3
   2.  Causes of Interoperability Issues . . . . . . . . . . . . . .   4
     2.1.  Originator vs Receiver Perspective  . . . . . . . . . . .   4
     2.2.  Identifier Alignment  . . . . . . . . . . . . . . . . . .   4
     2.3.  Message Forwarding  . . . . . . . . . . . . . . . . . . .   5
     2.4.  Message Modification  . . . . . . . . . . . . . . . . . .   6
   3.  Internet Mail Architecture, DMARC, and Indirect Email Flows .   6
     3.1.  Message Handling System . . . . . . . . . . . . . . . . .   6
       3.1.1.  Message Submission Agents . . . . . . . . . . . . . .   7
       3.1.2.  Message Transfer Agents . . . . . . . . . . . . . . .   8
         3.1.2.1.  Message Encoding  . . . . . . . . . . . . . . . .   8
         3.1.2.2.  Header Standardization  . . . . . . . . . . . . .   8
         3.1.2.3.  Email Address Internationalization  . . . . . . .   8
       3.1.3.  Message Delivery Agents . . . . . . . . . . . . . . .   9
     3.2.  Mediators . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Alias . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  ReSenders . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.3.  Mailing Lists . . . . . . . . . . . . . . . . . . . .  10
       3.2.4.  Gateways  . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.5.  Boundary Filters  . . . . . . . . . . . . . . . . . .  12
     3.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Possible Mitigations of Interoperability Issues . . . . . . .  13
     4.1.  Mitigations in Current Use  . . . . . . . . . . . . . . .  13
       4.1.1.  Mitigations for Senders . . . . . . . . . . . . . . .  13
         4.1.1.1.  Identifier Alignment  . . . . . . . . . . . . . .  13
         4.1.1.2.  Message Modification  . . . . . . . . . . . . . .  14
       4.1.2.  Mitigations for Receivers . . . . . . . . . . . . . .  14
         4.1.2.1.  Identifier Alignment  . . . . . . . . . . . . . .  14
         4.1.2.2.  Policy Override . . . . . . . . . . . . . . . . .  14
         4.1.2.3.  Email Address Internationalization  . . . . . . .  15
       4.1.3.  Mitigations for ReSenders . . . . . . . . . . . . . .  15
         4.1.3.1.  Changes to the RFC5322.From . . . . . . . . . . .  15
         4.1.3.2.  Avoiding Message Modification . . . . . . . . . .  15
         4.1.3.3.  Mailing Lists . . . . . . . . . . . . . . . . . .  16
     4.2.  Proposed and In-Progress Mitigations  . . . . . . . . . .  17



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       4.2.1.  Getting More Radical: Requiring New Communication
               Paths Between MUAs  . . . . . . . . . . . . . . . . .  18
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   DMARC [RFC7489] introduces a mechanism for expressing domain-level
   policies and preferences for message validation, disposition, and
   reporting.  The DMARC mechanism can encounter several different types
   of interoperability issues due to third-party message sourcing,
   message transformation or rerouting.

   DMARC is, as this writing, an Informational RFC, however it has a
   significant deployment within the email community.

   Cases in which email does not flow directly from the author's
   administrative domain to the recipients are collectively referred to
   in this document as indirect email flows.  Due to existing and
   increasing adoption of DMARC, the impact of DMARC-based email
   rejection policies on both direct and indirect email flows can be
   significant.

   Several known causes of interoperability issues are presented,
   followed by a description of components within the Internet Mail
   Architecture [RFC5598] where interoperability issues can arise.

   Lastly, known and possible methods for addressing interoperability
   issues are presented.  There are often multiple ways to address any
   given interoperability issue.  While this document strives to be
   comprehensive in its review, it should not be treated as complete.

1.1.  Document Conventions

   Notation regarding structured fields is taken from [RFC5598].

   Organizational Domain and Authenticated Identifiers are specified in
   DMARC [RFC7489].








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2.  Causes of Interoperability Issues

   Interoperability issues between DMARC and indirect email flows arise
   when conformance to the DMARC specification leads an implementation
   to apply DMARC based policy to messages that are both compliant with
   the architecture as specified in [RFC5598] and viewed as legitimate
   by the intended recipient.  To be clear, this document does not
   address emails considered legitimate by the intended recipient but
   which are not conformant to other email specifications.  The rest of
   this section describes several conceptual causes of interoperability
   issues.

2.1.  Originator vs Receiver Perspective

   Some Receivers are concerned that wanted email messages are received,
   regardless if such email messages are not strictly in conformance to
   any standard or protocol.

   Some Originators, particularly for high value transactional messages,
   want the message discarded if it passes through an intermediary and
   is modified in any way resulting in a failure to validate.  Examples
   of such messages include those related to financial organizations and
   medical establishments.

2.2.  Identifier Alignment

   DMARC relies on DKIM [RFC6376] and SPF [RFC7208] to perform message
   source validation.  The DMARC [RFC7489] mechanism refers to source
   domains that are validated by DKIM or SPF as Authenticated
   Identifiers.  DMARC requires an Authenticated Identifier to be
   relevant to the domain found in the message's RFC5322.From header
   field [RFC5322].  This relevancy is referred to as Identifier
   Alignment.

   Identifier Alignment can be strict, where the domains exactly match
   each others, or relaxed where the domains are part of the same
   Organizational Domain.  There are, in general, the same
   interoperability issues between strict and relaxed alignment, however
   in strict mode the possible solutions are more constrained when
   possible.  This Document mainly implies relaxed Identifier Alignment.

   DKIM provides a cryptographic means for a domain to be associated
   with a particular message.  DKIM does not make any constraints on
   what domains may or must present this association.  However, for a
   DKIM identifier to align in DMARC, the signing domain of a valid
   signature must be part of the same Organizational Domain as the
   domain in the RFC5322.From header field [RFC5322].




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   In addition, DKIM allows for the possibility of multiple valid
   signatures.  The DMARC mechanism will process Authenticated
   Identifiers that are based on DKIM signatures until an aligned
   Authenticated Identifier is found (if any).  However, operational
   experience has shown that some implementations have difficulty
   processing multiple signatures.  The impact on DMARC processing is
   clear: implementations that cannot process multiple DKIM signatures
   may erroneously apply DMARC based policy to otherwise legitimate
   messages.

   SPF can provide two Authenticated Identifiers based on two different
   SPF identities: RFC7208.HELO [RFC7208] and RFC7208.MAILFROM
   [RFC7208].  DMARC uses only the RFC7208.MAILFROM identifier for
   alignment.  The SPF validated domain in RFC7208.MAILFROM must be part
   of the same Organizational Domain as the domain in the RFC5322.From
   header field to be aligned.  It is important to note that even when
   an SPF record exists for the domain in RFC5322.From, SPF will not
   authenticate it unless it is also the domain in RFC7208.MAILFROM,
   furthermore, RFC7208.MAILFROM definition is different from
   RFC5321.MailFrom [RFC5321] definition.

2.3.  Message Forwarding

   Message forwarding is a generic concept encapsulating a variety of
   behaviors.  Section 3 describes forwarding behavior as it relates to
   the components of the Internet Mail Architecture.

   All of these behaviors involve email being retransmitted by a new
   SMTP server.  As discussed above, for SPF to cause a DMARC pass, the
   domain of the RFC7208.MAILFROM, must be aligned with that of the
   RFC5322.From header field:

   o  If the RFC5321.MailFrom is not empty and if the forwarder keeps
      the RFC5321.MailFrom, the SPF validation will fail altogether
      unless the forwarder is an authorized part of the originator's
      email sending infrastructure.  On another hand, if the forwarder
      uses its own domain in the RFC5321.MailFrom, SPF passes but the
      alignment with the RFC5322.From header field fails.

   o  If the RFC5321.MailFrom is empty, the RFC5321.Helo of the
      forwarder will likely be in different organizational domain of the
      RFC5322.From.  SPF may pass but the alignment with the
      RFC5322.From header field fails.

   In either case, SPF cannot produce a DMARC pass, and DKIM will be
   required to get DMARC to pass.





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2.4.  Message Modification

   Modification of email content invalidates most DKIM signatures, and
   many message forwarding systems modify email content.  Mailing list
   processors are the most common example of such systems, but other
   forwarding systems also make modifications.  Although DKIM provides a
   length flag so that content can be appended (See Section 8.2 of
   [RFC6376] for additional security considerations), in practice,
   particularly with MIME-encoded [RFC2045] messages, a mailing list
   processor will do more than append (See Section 5.3 of [RFC5598] for
   details).  Furthermore, the use of the length flag is seldom found in
   emails in part because of its security challenges.

   DKIM has two canonicalizations to use for headers and body
   separately: simple and relaxed.  The latter allows some modest in
   transit modifications that do not change the interpretation of the
   content of the email.  The relaxed canonicalization is more computing
   intensive and may not have been preferred in the early deployment of
   DKIM as this may have been more significant than today.

3.  Internet Mail Architecture, DMARC, and Indirect Email Flows

   This section describes components within the Internet Mail
   Architecture [RFC5598] where interoperability issues between DMARC
   and indirect email flows can be found.

3.1.  Message Handling System

   Section 4 of [RFC5598] describes six basic components that make up
   the Message Handling System (MHS):

   o  Message

   o  Message User Agent (MUA)

   o  Message Submission Agent (MSA)

   o  Message Transfer Agent (MTA)

   o  Message Delivery Agent (MDA)

   o  Message Store (MS)

   Of these components MSA, MTA, and MDA are discussed in relation to
   interoperability with DMARC.






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   A Mediator is a special class of MUA that is given special
   consideration in this section due to the unique issues Mediators face
   when attempting to interoperate with DMARC.

3.1.1.  Message Submission Agents

   An MSA accepts messages submitted by a Message User Agent (MUA) and
   enforces the policies of the hosting ADministrative Management Domain
   (ADMD) and the requirements of Internet standards.

   MSAs are split into two sub-components:

   o  Author-focused MSA functions (aMSA)

   o  MHS-focused MSA functions (hMSA)

   MSA interoperability issues with DMARC begin when an aMSA accepts a
   message where the RFC5322.From header field contains a domain that is
   outside of the ADMD of the MSA.  The ADMD will almost certainly not
   be capable of sending email that yields Authenticated Identifiers
   aligned with the domain found in the RFC5322.From header field.
   Examples of this issue include "forward-to-friend" functionality
   commonly found on news/article websites or "send-as" functionality
   present on some MUAs.

   When an hMSA takes responsibility for transit of a message containing
   a domain in the RFC5322.From header field that is outside of the
   hMSA's ADMD, the hMSA faces DMARC interoperability issues if the
   domain publishes a DMARC policy of "quarantine" or "reject".  These
   issues are marked by an inherent difficulty in establishing alignment
   with the domain present in a message's RFC5322.From header field.
   Examples of this issue include:

   o  Pseudo-open relays - a residential ISP that allows its customers
      to relay any domains through its infrastructure.

   o  Embedded devices - cable/dsl modems, firewalls, wireless access
      points that send email using hardcoded domains.

   o  Email service providers - ESPs that service customers that are
      using domains that publish a DMARC "reject" policy.

   o  Calendaring software - an invited member of an event modifies the
      event causing calendaring software to emit an update that appears
      to come from the creator of the event.






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3.1.2.  Message Transfer Agents

   MTAs relay a message until the message reaches a destination MDA.

3.1.2.1.  Message Encoding

   An MTA may modify the message encoding, for instance by converting
   8-bit MIME sections to quoted-printable 7-bit sections.  This
   modification is outside the scope of DKIM canonicalization and will
   invalidate DKIM signatures that include message content.

3.1.2.2.  Header Standardization

   An MTA may standardize headers, usually in order to make non-RFC
   compliant headers properly compliant.  For instance, some common MTAs
   will correct comprehensible but non-compliant date formats to
   compliant ones.  This correction is outside the scope of DKIM
   canonicalization and will invalidate DKIM signatures.  This
   correction is also outside the scope of this document in providing
   solutions for non RFC compliant emails.

3.1.2.3.  Email Address Internationalization

   A DMARC related issue may arise in the context of Email Address
   Internationalization [RFC6530].  [RFC6854] allows group syntax in the
   RFC5322.From header field during the transition period to SMTPUTF8.
   If an EAI/SMTPUTF8-aware MTA needs to transmit a message to a non-
   aware MTA, the non aware MTA will reject it.  While an MTA will not
   downgrade a message.  The MUA or MSA may resend the message using the
   group syntax to allow the non-aware MTA to receive the email.

   In another case, a MDA, may use the group syntax to pass a message to
   a non EAI/SMTPUTF8-aware MS (or user email client).  This message
   could then be forwarded to another recipient.

   For an MTA, the group syntax may result in the impossibility of
   finding a domain with a DMARC policy associated with it.  If the
   receiving MTA pays attention to the validity and reputation of
   domains, this may present its own set of delivery problems.  For
   instance an MTA may refuse emails with no valid or emailable domain
   in the RFC5322.From as to avoid simple workarounds against DMARC.

   This case is not an interoperability issue with DMARC, but with other
   email policies an MTA may have to support DMARC.







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3.1.3.  Message Delivery Agents

   The MDA transfers a message from the MHS to a mailbox.  Like the MSA,
   the MDA consists of two sub-components:

   o  MHS-focused MDA functions (hMDA)

   o  Recipient-focused MDA functions (rMDA)

   Both the hMDA and the rMDA can redirect a message to an alternative
   address.  DMARC interoperability issues related to redirecting of
   messages are described in Section 3.2.

   SIEVE [RFC5228] functionality often lives in the rMDA sub-component
   and can cause DMARC interoperability issues.  The SIEVE 'addheader'
   and 'deleteheader' filtering actions can modify messages and
   invalidate DKIM signatures, removing DKIM-supplied Authenticated
   Identifiers as inputs to the DMARC mechanism.  There are also SIEVE
   extensions that modify the body.  SIEVE may only become an issue when
   the email is reintroduced in the transport infrastructure.

3.2.  Mediators

   See [RFC5598] for a complete definition of Mediators.

   Mediators forward messages through a re-posting process.  Mediators
   share some functionality with basic MTA relaying, but have greater
   flexibility in both addressing and content modifications.

   DMARC interoperability issues are prevalent within the context of
   Mediators, which are often used precisely for their ability to modify
   messages.

3.2.1.  Alias

   An Alias is a simple re-addressing facility that provides one or more
   new Internet Mail addresses, rather than a single, internal one.  A
   message continues through the transfer service for delivery to one or
   more alternative addresses.

   Aliases can be implemented by mailbox-level forwarding (e.g. through
   "dot-forwarding") or SIEVE-level forwarding (through the SIEVE
   'redirect' action) or other methods.  When an Alias preserves message
   content and does not make significant header changes, DKIM signatures
   may remain valid.  However, Aliases often extend the delivery path
   beyond SPF's ability to grant authorization.

   Examples of Aliasing include:



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   o  Forwarding email between freemail providers to try different
      interfaces while maintaining an original email address.

   o  Consolidating many email addresses into a single acccount to
      centralize processing.

   o  Services that provides "activity based", "role based" , "vanity"
      or "temporary" email addresses such as universities and
      professional associations.  For instance professional or alumni
      institutions may offer to their members an alias for the duration
      of their membership but may not want to deal with the long term
      storage of emails.

   In most cases, the aMSA providing Alias services has no
   administrative relationship to the ADMD of the final recipient, so
   solutions to Alias-related DMARC failure should not assume such a
   relationship.

3.2.2.  ReSenders

   ReSenders "splice" a message's addressing information to connect the
   Author of the original message with the Recipient(s) of the new
   message.  The new Recipient sees the message as being from the
   original Author, even if the Mediator adds commentary.

   ReSenders introduce DMARC interoperability issues as content
   modification invalidates DKIM signatures.  SPF's ability to grant
   authorization via alignment is removed as the new Recipient receives
   the message from the Mediator.

   Without an ability to produce Authenticated Identifiers relevant to
   the Author's RFC5322.From header field domain using either DKIM or
   SPF, the new Recipient has almost no chance of successfully applying
   the DMARC mechanism.

   Examples of ReSenders include MUA-level forwarding by resending a
   message to a new recipient or by forwarding a message "inline" to a
   new recipient (this does not include forwarding a message "as an
   attachment").  An additional example comes in the form of calendaring
   software that allows a meeting attendee (not the meeting organizer)
   to modify the content of an invite causing the invitations to appear
   to be reissued from the meeting organizer.

3.2.3.  Mailing Lists

   A Mailing List receives messages as an explicit addressee and then
   re-posts them to a list of subscribed members.  The Mailing List
   performs a task that can be viewed as an elaboration of the ReSender.



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   Mailing Lists share the same DMARC interoperability issues as
   ReSenders (Section 3.2.2), and very commonly modify headers or
   message content in ways that will cause DKIM to fail, including:

   o  prepending the RFC5322.Subject header field with a tag, to allow
      the receiver to identify visually the mailing list.

   o  adding a footer to the email body to contain administrative
      instructions.

   o  removing some MIME-parts from the email or converting the message
      to text only.

   o  PGP-encrypting or S/MIME encrypting the body to the receiver's
      key.

   o  enforcing community standards by rewriting banned words.

   o  allowing moderators to add arbitrary commentary to messages.

   Any such modifications would invalidate a DKIM signature.

   Mailing Lists may also have the following DMARC interoperability
   issues:

   o  Subscribed members may not receive email from members that post
      using domains that publish a DMARC "p=reject" policy.

   o  Mailing Lists may interpret DMARC-related email rejection as an
      inability to deliver email to the recipients that are checking and
      enforcing DMARC policy.  This processing may cause subscribed
      members to be suspended or removed from the Mailing List.

   These changes are common for many mailing lists and receivers are
   used to them.  Furthermore MUA expects certain mailing list behavior
   in presenting emails to the end users

3.2.4.  Gateways

   A Gateway performs the basic routing and transfer work of message
   relaying, but it also is permitted to modify content, structure,
   address, or attributes as needed to send the message into a messaging
   environment that operates under different standards or potentially
   incompatible policies.

   Gateways share the same DMARC interoperability issues as ReSenders
   (Section 3.2.2).




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   Gateways may share also the same DMARC interoperability issues as
   MTAs (Section 3.1.2).

   Gateway-level forwarding can introduce DMARC interoperability issues
   if the Gateway is configured to rewrite the message to map between
   recipient domains.  For example, an acquisition may lead the
   acquiring company to decide to decommission the acquired company's
   domains by rewriting messages to use the domain of the acquiring
   company.  Since the RFC5322.To header field is usually DKIM-signed,
   this kind of rewriting will also cause DKIM signatures to fail.

3.2.5.  Boundary Filters

   To enforce security boundaries, organizations can subject messages to
   analysis for conformance with their safety policies.  A filter might
   alter the content to render it safe, such as by removing content
   deemed unacceptable.

   Boundary Filters share the same DMARC interoperability issues as
   ReSenders.

   Issues may arise if SPF and DKIM is evaluated after the filter
   modifications.

   Examples of Boundary Filters include:

   o  Anti-spam: To keep its reputation, an MTA that transfers a message
      may remove harmful content from messages that are likely to be
      unwanted by the next MTA and/or add text in the body to indicate
      the message has been scanned.  Any such modifications would
      invalidate a DKIM signature.

   o  Any service that reformulates the RFC5322.body for any other
      reason, for instance adding an organizational disclaimer.

   o  Secondary MX services.  In this case, however, it is inappropriate
      for a primary MX server to perform an SPF check against its own
      secondaries.  Rather, the secondary MX should perform this
      function.

3.3.  Combinations

   The causes of indirect email flows can be combined.  For example, a
   university student may subscribe to a mailing list (using his
   university email address) while this university email address is
   configured to forward all emails to a freemail provider where a more
   permanent email address for this student exists.




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   Within an organization the message may pass through various MTAs
   (Section 3.1.2), each of which performs a different function
   (authentication, filtering, distribution, etc.)

4.  Possible Mitigations of Interoperability Issues

   Solutions to interoperability issues between DMARC and indirect email
   flows vary widely in their scope and implications.  They range from
   improvements to underlying processors, such as proper handling of
   multiple DKIM signatures, to more radical approaches to the messaging
   architecture.  This section describes possible ways to address
   interoperability issues.

   Mail systems are diverse and widely deployed and are expected to
   continue to work with old systems.  For instance, Qmail is still used
   and the base code has not been updated since 1998.  Ezmlm, a once
   popular mailing list manager, is still deployed and has not been
   updated since 1997, although a new version, Ezmlm-idx exists.  In
   this constrained environment, some solutions may be time-consuming
   and/or disruptive to implement.

   DMARC provides for receivers to make decisions about identity
   alignment acceptability based on information outside DMARC and
   communicate those decisions as "overrides" to the sender.  This
   facility can be used to ease some interoperability issues, although
   care is needed to ensure that this does not create loopholes that
   abusers can use arbitrarily.

4.1.  Mitigations in Current Use

   At many places where DMARC is already deployed, mitigations are in
   use.  These mitigations vary in their effectiveness and side effects,
   but have the advantage that they are currently available.

4.1.1.  Mitigations for Senders

4.1.1.1.  Identifier Alignment

   o  MTAs handling multiple domains may choose to change
      RFC5321.MailFrom to align with RFC5322.From to improve SPF
      usability for DMARC.

   o  MTAs handling multiple domains may also choose to align
      RFC5321.HELO/EHLO to RFC5322.From, particularly when sending
      bounce messages.  Adjusting dynamically the RFC5321.HELO based on
      the RFC5322.From may not be possible for some MTA software.





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   o  MTAs may choose to DKIM sign bounces with an aligned domain to
      allow DKIM-based DMARC pass.

   o  MTAs handling multiple domains may require DMARC-using senders to
      provide DKIM keys and use DKIM to avoid SPF alignment issues.
      Handling DKIM keys with a third party has its security challenges.

   o  Senders who are sending on behalf of users in other Administrative
      Domains may choose to use an RFC5322.From under the sender's
      control.  The new From can be either a forwarding address in a
      domain controlled by the Sender, or a placeholder address, with
      the original user's address in a RFC5322.Reply-to header field.
      However this may affect what the recipient expects in its MUA.

   o  Senders can use different domains with different DMARC policies
      for email sent directly and email known to use indirect mail flow.
      However for known brands, all active domains are likely to be
      targeted equally by abusers.

4.1.1.2.  Message Modification

   o  Senders can maximize survivability of DKIM signatures by limiting
      the header fields they sign, using relaxed canonicalization and by
      using length to allow appended signatures.

   o  Senders can also maximize survivability by starting with RFC-
      compliant headers and common body formats.

   o  In order to minimize the chances of transport conversions, Senders
      can convert the message to a suitable MIME content-transfer
      encoding such as quoted-printable or base64 before signing
      ([RFC6376] Section 5.3).

4.1.2.  Mitigations for Receivers

4.1.2.1.  Identifier Alignment

   o  Receivers should update DKIM handling libraries to ensure that
      they process all valid DKIM signatures and check them for
      alignment.

4.1.2.2.  Policy Override

   o  Receivers can amalgamate data from their user base to identify
      forwarders and use such list for a DMARC local policy override.
      This process may be easier for large receivers, where there is
      data and resources to create such lists, than for small receivers.




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4.1.2.3.  Email Address Internationalization

   o  During the transition from email systems that do not allow EAI
      (SMTPUTF8) to email systems that allow it, [RFC6854] allows the
      MUA or MSA to use the group syntax for the RFC5322.From header
      field to avoid a known MTA to reject the message (if RFC5322 is
      implemented strictly).  While this will alleviate the EAI problem,
      it will allow a simple DMARC workaround since the message may no
      longer have a single usable domain in the RFC5322.From header
      field, making DMARC inapplicable.  DMARC implementations that pay
      attention to the validity of the domains in the RFC5322.From may
      then have to choose between EAI transitioning compliance
      (accepting the email without a domain in RFC5322.From) and
      security (rejecting RFC-compliant email for lack of a domain in
      the RFC5322.From).  The practical solution is to upgrade MTAs to
      support EAI (SMTPUTF8), avoiding the transition issue.

4.1.3.  Mitigations for ReSenders

4.1.3.1.  Changes to the RFC5322.From

   Many ReSender issues can be avoided by using an RFC5322.From under
   the ReSender's control, instead of the initial RFC5322.From.  This
   will correct identifier alignment issues and allow arbitrary message
   modification, for instance.  When ReSenders change the RFC5322.From,
   it is desirable to preserve the information about the original
   initiator of the message.  The Original-From [RFC5703] (or X-
   Original-From) header field is used for this purpose in various
   contexts (X- header fields name are discouraged by [RFC6648]).

   However, handling of Original-From (or X-Original-From) is not
   defined anywhere.  It is not currently used consistently or displayed
   to the user, but in any situation where it is used, it is a new
   unauthenticated identifier available for exploitation.

4.1.3.2.  Avoiding Message Modification

   o  Forwarders can choose to add email header fields instead of
      modifying existing headers or bodies, for instance to indicate a
      message may be spam.

   o  Forwarders can minimize the circumstances in which they choose to
      fix messages, preferring to preserve non-compliant headers to
      creating DKIM failures.

   o  Forwarders can choose to reject messages with suspect or harmful
      content instead of modifying them.




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4.1.3.3.  Mailing Lists

   [RFC6377] provides some guidance on using DKIM with Mailing lists.
   Here are some remediation techniques on using DMARC with Mailing
   lists:

   o  One mitigation policy, which is now present on several Mailing
      List software, is to configure the Mailing List Manager (MLM) to
      alter the RFC5322.From header field to use the domain of the MLM.
      Since most list subscribers prefer to know the identity of the
      author of the original message, typically this information may be
      provided in the display name part of the RFC5322.From header
      field.  This display name needs to be carefully crafted as to not
      collide with the original display name of the author, nor contain
      something that looks like an email address or domain name.  These
      modifications may to some extent defeat the purpose of DMARC
      itself.  It may make it difficult to ensure that users of all
      email clients can easily reply to the author, the list, or all
      using the email client features provided for that purpose.  Use of
      RFC5322.Reply-To header field can alleviate this problem depending
      on whether the mailing list is configured to reply-to-list, reply-
      to-author or reply-to-fixed-address, however it is important to
      note that this header field can take multiple email addresses.
      When altering the RFC5322.From there are two possibilities, to
      change it to put the mailing list email address, or to change it
      to add a suffix like ".invalid" to the domain of the email address
      present there.  The later modification may create issues because
      it is an invalid domain name, and some MTAs may take particular
      attention to the validity of email addresses in RFC5322.From and
      the reputation of the domains present there.

   o  Another mitigation policy is to configure the MLM to "wrap" the
      message in a MIME message/rfc822 part and to send as the Mailing
      List email address.  Many email clients (as of August 2014) have
      difficulty reading such messages.

   o  Another mitigation policy, is to configure the MLM to not modify
      the message so that the DKIM signature remains valid.  Some
      Mailing Lists are mainly setup this way and require little
      modifications to ensure the DKIM signature is preserved.  However
      moving to this policy a list that do extensive modification to the
      email, may be too much of a change for the members of such list.

   o  Another mitigation policy, is to reject posts from domains with a
      DMARC policy other than p=none.  However members of such Mailing
      Lists may complain of unfair exclusion.





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   o  To alleviate unsubscribes to the Mailing List due to the messages
      bouncing because of DMARC, the MLM needs to not act on bounces due
      to Message Authentication issues.  [RFC3463] specifies Enhanced
      Mail System Status Codes which help differentiate between various
      bounces.  Correctly interpreting Extended SMTP error messages is
      useful in this case in particular codes defined in [RFC7372] and
      in DMARC.

   All these techniques may provide some specific challenges in MUAs and
   different operational usages for end users (like rewriting filters to
   sort emails in folders).  There will be some time before all
   implications are understood and alleviated.

4.2.  Proposed and In-Progress Mitigations

   The following mitigations are based on Internet Drafts which have not
   yet received broad consensus.  They are described here to offer
   exploratory path for solutions.  These solutions should not be used
   in a production environment.

   o  Third party authorization, [RFC6541], [I-D.otis-tpa-label] and
      [I-D.kucherawy-dkim-delegate] provide ways to extend identifier
      alignment under the control of the domain owner.

   o  DKIM with constrained transformations,
      [I-D.kucherawy-dkim-list-canon] is proposed to allow message
      modification.

   o  DKIM with recorded transformations, [I-D.kucherawy-dkim-transform]
      is proposed to indicate what limited transformations were done to
      the message so that a receiver could reverse them and confirm the
      validity of the orignal DKIM signature.

   o  [I-D.kucherawy-original-authres] is intended as a way to pass
      along Original Authentication Results to "downstream" receivers.
      It is not widely implemented and relies on a trust relationship
      between the forwarder and the other receivers.

   o  [I-D.levine-dkim-conditional] could be used to have the sender add
      a limited DKIM signature, that signs only a very limited set of
      header fields and not the body of the message.  This DKIM
      signature would come with the condition that a subsequent known
      domain fully DKIM sign the message.  For instance a Mailing List
      could transform the message, add its DKIM signature and there
      would be a valid DKIM signature aligned with the RFC5322.From that
      would satisfy DMARC while limiting the possibilities of replay
      attack.




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4.2.1.  Getting More Radical: Requiring New Communication Paths Between
        MUAs

   In practice a number of operators are using strict alignment mode in
   DMARC in order to avoid receiving new and innovative forms of
   unwanted and unauthentic email through systems purporting to be
   mailing list handlers.  The receiving ADMD has no knowledge of which
   lists the user has subscribed to and which they have not.  One avenue
   of exploration would be for the user to authorize mailing lists as
   proxies for authentication, at which point the receiving ADMD would
   be vesting some trust in the mailing list service.  The creators of
   DKIM foresaw precisely this possibility at the time by not tightly
   binding any semantics to the RFC5322.From header field.  Some
   experimental work has taken place in this area, as mentioned above.
   Additional work might examine a new communication path to the user to
   authorize some form of transitive trust.

5.  IANA Considerations

   This document contains no actions for IANA.  [RFC Editor: Please
   delete this section prior to publication.]

6.  Security Considerations

   This document is an analysis of DMARC's impact on indirect email
   flows.  It describes the possibility of accidental denial-of-service
   that can be created by rejections of messages by DMARC-aware Mail
   Receivers.  However, it introduces no new security issues to Internet
   messaging.

7.  Acknowledgments

   Miles Fidelman, John Levine, David Crocker, Stephen J.  Turnbull,
   Rolf E.  Sonneveld, Tim Draegen and Franck Martin contributed to the
   IETF DMARC Working Group's wiki page listing all known
   interoperability issues with DMARC and indirect email flows.

   Tim Draegen created the first draft of this document from these
   contributions and by hamfistedly mapping contributions into the
   language of [RFC5598].

8.  References

8.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.



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   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
              3463, January 2003.

   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July
              2009.

   [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
              Tests, Iteration, Extraction, Replacement, and Enclosure",
              RFC 5703, October 2009.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
              September 2011.

   [RFC6377]  Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, September 2011.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

   [RFC6541]  Kucherawy, M., "DomainKeys Identified Mail (DKIM)
              Authorized Third-Party Signatures", RFC 6541, February
              2012.

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

   [RFC6854]  Leiba, B., "Update to Internet Message Format to Allow
              Group Syntax in the "From:" and "Sender:" Header Fields",
              RFC 6854, March 2013.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              April 2014.

   [RFC7372]  Kucherawy, M., "Email Authentication Status Codes", RFC
              7372, September 2014.




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8.2.  Informative References

   [I-D.kucherawy-dkim-delegate]
              Kucherawy, M. and D. Crocker, "Delegating DKIM Signing
              Authority", draft-kucherawy-dkim-delegate-01 (work in
              progress), June 2014.

   [I-D.kucherawy-dkim-list-canon]
              Kucherawy, M., "A List-safe Canonicalization for
              DomainKeys Identified Mail (DKIM)", draft-kucherawy-dkim-
              list-canon-00 (work in progress), June 2014.

   [I-D.kucherawy-dkim-transform]
              Kucherawy, M., "Recognized Transformations of Messages
              Bearing DomainKeys Identified Mail (DKIM) Signatures",
              draft-kucherawy-dkim-transform-00 (work in progress),
              April 2015.

   [I-D.kucherawy-original-authres]
              Chew, M. and M. Kucherawy, "Original-Authentication-
              Results Header Field", draft-kucherawy-original-authres-00
              (work in progress), February 2012.

   [I-D.levine-dkim-conditional]
              Levine, J., "DKIM Conditional Signatures", draft-levine-
              dkim-conditional-00 (work in progress), June 2014.

   [I-D.otis-tpa-label]
              Otis, D. and D. Black, "Third-Party Authorization Label",
              draft-otis-tpa-label-00 (work in progress), May 2014.

   [RFC7489]  Kucherawy, M. and E. Zwicky, "Domain-based Message
              Authentication, Reporting, and Conformance (DMARC)", RFC
              7489, March 2015.

Authors' Addresses

   Franck Martin (editor)
   LinkedIn
   Mountain View, CA
   USA

   Email: fmartin@linkedin.com








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   Eliot Lear (editor)
   Cisco Systems GmbH
   Richtistrasse 7
   Wallisellen, ZH  CH-8304
   Switzerland

   Phone: +41 44 878 9200
   Email: lear@cisco.com


   Tim Draegen (editor)
   Eudaemonic Development LLC
   PO Box 19443
   Asheville, NC  28815
   USA

   Email: tim@eudev.net


   Elizabeth Zwicky (editor)
   Yahoo
   Sunnyvale, CA
   USA

   Email: zwicky@yahoo-inc.com


























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