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Versions: 00 draft-ietf-dmarc-interoperability

DMARC Working Group                                       F. Martin, Ed.
Internet-Draft                                                  LinkedIn
Intended status: Informational                              E. Lear, Ed.
Expires: July 23, 2015                                Cisco Systems GmbH
                                                         T. Draegen, Ed.
                                                                Eudaemon
                                                        January 19, 2015


     Interoperability Issues Between DMARC and Indirect Email Flows
                    draft-dmarc-interoperability-00

Abstract

   DMARC introduces a mechanism for expressing domain-level policies and
   preferences for message validation, disposition, and reporting.  The
   DMARC mechanism can encounter interoperability issues when messages
   originate from third party sources, are modified in transit, or are
   forwarded enroute to their final destination.  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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

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

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Conventions  . . . . . . . . . . . . . . . . . .   3
   2.  Causes of Interoperability Issues . . . . . . . . . . . . . .   3
     2.1.  Identifier Alignment  . . . . . . . . . . . . . . . . . .   4
     2.2.  Message Forwarding  . . . . . . . . . . . . . . . . . . .   4
     2.3.  Message Modification  . . . . . . . . . . . . . . . . . .   5
   3.  Internet Mail Architecture, DMARC, and Indirect Email Flows .   5
     3.1.  Message Handling System . . . . . . . . . . . . . . . . .   5
       3.1.1.  Message Submission Agents . . . . . . . . . . . . . .   5
       3.1.2.  Message Transfer Agents . . . . . . . . . . . . . . .   6
         3.1.2.1.  Message Encoding  . . . . . . . . . . . . . . . .   6
         3.1.2.2.  Anti-spam . . . . . . . . . . . . . . . . . . . .   7
         3.1.2.3.  Email Address Internationalization  . . . . . . .   7
       3.1.3.  Message Delivery Agents . . . . . . . . . . . . . . .   7
     3.2.  Mediators . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Alias . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  ReSenders . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.3.  Mailing Lists . . . . . . . . . . . . . . . . . . . .   9
       3.2.4.  Gateways  . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.5.  Boundary Filters  . . . . . . . . . . . . . . . . . .  10
     3.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Possible Solutions to Interoperability Issues . . . . . . . .  10
     4.1.  Identifier Alignment  . . . . . . . . . . . . . . . . . .  11
     4.2.  Message Modification  . . . . . . . . . . . . . . . . . .  11
     4.3.  Message Forwarding  . . . . . . . . . . . . . . . . . . .  11
       4.3.1.  Original-Authentication-Results . . . . . . . . . . .  12
     4.4.  Message Handling Services . . . . . . . . . . . . . . . .  12
       4.4.1.  Message Transfer Agents . . . . . . . . . . . . . . .  12
         4.4.1.1.  Encoding  . . . . . . . . . . . . . . . . . . . .  12
         4.4.1.2.  Filters . . . . . . . . . . . . . . . . . . . . .  12
         4.4.1.3.  Email Address Internationalization  . . . . . . .  12
     4.5.  Mediators . . . . . . . . . . . . . . . . . . . . . . . .  12
       4.5.1.  Mailing Lists . . . . . . . . . . . . . . . . . . . .  12
     4.6.  Getting More Radical: Requiring New Communication Paths
           Between MUA and the Message Store . . . . . . . . . . . .  13
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14



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   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   DMARC [I-D.kucherawy-dmarc-base] introduces a mechanism for
   expressing domain-level policies and preferences for message
   validation, disposition, and reporting.  DMARC is used to combat
   exact-domain phishing, to gain visibility into email infrastructure,
   and to provide email egress controls.  Due to wide adoption, the
   impact of DMARC-based email rejection policies on both direct and
   indirect email flows can be significant.

   The DMARC mechanism can encounter several different types of
   interoperability issues due to message transformation or rerouting.
   This is known collectively as indirect email flows.  In some of these
   cases, message content is modified as a result.

   The next section describes interoperability issues between DMARC and
   indirect email flows.  These issues are first described in the
   context of configuration behavior that DMARC requires from underlying
   authentication technology, and then described as they appear in
   context of the Internet Mail Architecture [RFC5598].

   Lastly, possible methods for addressing interoperability issues are
   presented.  There are often multiple ways to address any given
   interoperability issue.  Whereas 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].

2.  Causes of Interoperability Issues

   What do we mean by "interoperability issues"?  We say that DMARC
   introduces interoperability issues or problems, when conformance to
   the DMARC specification leads an implementation to reject a message
   that is both compliant with the architecture as specified in
   [RFC5598] and would have been viewed as legitimate in the eyes of the
   intended recipient.  This the secondary effects of behaviors caused
   by that rejection.  Therefore, we can already conclude that DMARC
   poses no interoperability problems when messages properly validate
   through its specified processes.  The rest of this section,
   therefore, delves into how legitimate messages may get rejected.




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2.1.  Identifier Alignment

   A fundamental aspect of message source validation is understanding
   how the originator of a message is validated.  Each of the underlying
   mechanisms that DMARC uses (DKIM [RFC6376] or SPF [RFC7208]) takes a
   different approach.  Therefore, the DMARC [I-D.kucherawy-dmarc-base]
   mechanism attempts to predictably specify the domain of the
   originator that will be used for its purposes (reporting/message
   disposition).  This step is referred to as Identifier Alignment.

   DKIM provides a cryptographic means for a domain to be associated
   with a particular message.  However, the signing domain is not
   required to be related to the domain contained in the 5322.From field
   [RFC5322].  The DMARC identifier alignment process posits just such a
   relationship.  For an identifier to align in DKIM, the signing domain
   must be part of the same organizational domain as the domain in the
   5322.From field, and the signature must be valid.

   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: if an implementation cannot process multiple DKIM signatures
   it may lead to perfectly valid messages being flagged as not
   authentic.

   SPF provides authenticated domain identifiers based on either the
   5321.MailFrom [RFC5321] domain or, if the 5321.MailFrom address is
   absent (as in the case of "bounces"), the domain found in the HELO/
   EHLO SMTP command.  More often than not operators have not put an SPF
   record on domains found in the HELO/EHLO SMTP command and when
   present, it could be difficult to change this domain to the domain in
   the 5322.From, especially when several mail streams share the same
   sending IP address.

2.2.  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.

   When a message has been transmitted through a forwarder, there will
   be no such relationship.  With SPF, the domain of the 5321.MailFrom
   or 5321.HELO/EHLO must match that of the 5322.From.  Because
   forwarders generally do not modify the 5322.From, this test will




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   fail.  Thus a DMARC implementation must rely on DKIM, if available,
   in these cases.

2.3.  Message Modification

   Modification of email content invalidates most DKIM signatures.
   While DKIM provides a length flag so that content can be appended
   (See Section 8.2 of RFC6376 [RFC6376] for additional discussion), in
   practice, particularly with MIME-encoded messages, a mailing list
   processor will do more than append (See Section 5.3 of [RFC5598] for
   details).

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 fulfill
   the purpose of 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.

   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.




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   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 5322.From header 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 domain identifiers
   related to the domain found in the 5322.From header.  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 5322.From header 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 modifying the domain present in a
   message's 5322.From header.  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 freemail services where the freemail service publishes 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.

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 change the message encoding.  For instance it may convert
   8bits mail sections to quoted-printable 7bits sections, or just
   change the character set from US-ASCII to ISO-8859-4.  Such
   transformations invalidate any present DKIM signature.





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3.1.2.2.  Anti-spam

   To keep its reputation, an MTA that transfers message, may block or
   otherwise remove harmful content from messages that are likely to be
   unwanted by the next MTA.  Any such modifications would invalidate a
   DKIM signature.

3.1.2.3.  Email Address Internationalization

   A DMARC interoperability issue arises in the context of Email Address
   Internationalization [RFC6530].  [RFC6854] allows group syntax in the
   5322.From header during the transition period to SMTPUTF8.  In the
   case where an EAI/SMTPUTF8-aware MTA relays a message to a non-aware
   MTA, the EAI/SMTPUTF8-aware MTA may transform the 5322.From header of
   the message to include group syntax that allows the non-aware MTA to
   process the email.

   This transformation modifies the content of the message and will
   invalide any DKIM signatures and possibly remove the ability for the
   DMARC mechanism to receive an authenticated domain identifier from a
   DKIM module.

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
   domain identifiers as inputs to the DMARC mechanism.

3.2.  Mediators

   Descriptions of Mediators are largely imported from [RFC5598].

   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.



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   DMARC interoperability issues are prevalent within the context of
   Mediators, as Mediators represent a class of transformations that
   mirror the concept of "indirect email flows".

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 alternate addresses.

   Aliases can be implemented by mailbox-level forwarding (e.g. through
   "dot-forwarding") or SIEVE-level forwarding (through the SIEVE
   'redirect' action).  When an Alias preserves message content, DKIM
   signatures may remain valid.  However, Aliases extend the delivery
   path beyond SPF's ability to grant authorization.

   Examples of Aliasing include:

   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.

   A fundamental challenge in dealing with workarounds involving Aliases
   is that in many instances, the aMSA has no administrative
   relationship to the ADMD of the alias.  Another challenge is that the
   underlying mechanisms upon which DMARC relies do not consider the
   local-part of the addr-spec.  While normally considered a perfectly
   reasonable scaling feature, the underlying assumption is that Authors
   will make use of aMSAs that are always authorized for a given domain.
   For vanity domains, this assumption turns out to not hold.

3.2.2.  ReSenders

   ReSenders "splice" a message's addressing information to connect the
   Author of the original message with the Recipient 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




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   authorization via alignement is removed as the new Recipient receives
   the message from the Mediator.

   Without an ability to produce authenticated domain identifiers
   relevant to the Author's 5322.From 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 calendering
   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.

   Mailing Lists share the same DMARC interoperability issues as
   ReSenders (Section 3.2.2), plus the following:

   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.

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).

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





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   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 companies
   domains by rewriting messages to use the domain of the acquiring
   company.

3.2.5.  Boundary Filters

   To enforce security boundaries, organizations can subject messages to
   analysis for conformance with its 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.

   Examples of Boundary Filters include:

   o  Antispam services that remove or modify content.

   o  Any service that reformulates the 5322.body for any other reason.

   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.

   Within an organisation the message may passes through various MTAs
   (Section 3.1.2) that performs each one a different function,
   authentication, filtering, distribution,...

4.  Possible Solutions to Interoperability Issues

   Solutions to interoperability issues between DMARC and indirect email
   flows vary from improving of underlying processors, such as proper
   handling multiple DKIM signatures, to more radical approaches to the
   messaging architecture.  This section decribes possible ways to
   address interoperability issues.




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   Through external knowledge it may be possible to determinine that the
   DMARC mechanism cannot be applied to a particular message because
   modification and/or forwarding is to be expected through the normal
   course of operations for a given sender.  This is known as an
   "override".

4.1.  Identifier Alignment

   In the case of forwarders, identity alignment poses a substantial
   concern.  A receiving ADMD may track subscriptions to mailing lists,
   so that different processing may be applied to those messages.

   Various proposals have been submitted to provide either some form of
   transitive trust between an email sender, a realy and an email
   receiver, or to allow a modified version of DKIM to survive light
   transformation to the message:

   o  DKIM conditional signatures, [I-D.levine-dkim-conditional]

   o  Third party authorization, [RFC6541], [I-D.otis-tpa-label] and
      [I-D.kucherawy-dkim-delegate]

   o  DKIM with light transformations, [I-D.kucherawy-dkim-list-canon]

4.2.  Message Modification

   Message modification invalidates DKIM signatures and complicates a
   receiver's ability to generate authenticated domain identifiers from
   a message.  It is therefore important to review the MTAs
   (Section 3.1.2) configuration to ensure no modification of the
   message is done when simply forwarding the message.  Furthermore
   Filters should not add to or modify the body of the message, but
   either rejecting the message or adding email headers to indicate the
   result of the filter.

4.3.  Message Forwarding

   Forwarding messages without modification is referred to as
   "transparent forwarding", and is a way to preserve the validity of
   DKIM signatures.

   The X-Original-From header is used in various contexts.

   Note that Original-From is merely adding complexity to the 'who was
   the author of this message' assessment, very possibly creating yet-
   another security hole.





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4.3.1.  Original-Authentication-Results

   Mentioned in early drafts [I-D.kucherawy-original-authres] as a way
   to pass along Original Authentication Results to "downstream"
   receivers.

4.4.  Message Handling Services

4.4.1.  Message Transfer Agents

4.4.1.1.  Encoding

   There is very little reason to modify the encoding of the message,
   compatibility issues between international character sets are few
   nowadays.  No modification of the message should be done when simply
   forwarding the message.

4.4.1.2.  Filters

   Filters should not add to or modify the body of the message, but
   either should reject the message or add new email headers (not under
   DKIM) to indicate the result of the filter.

4.4.1.3.  Email Address Internationalization

   The postmaster will choose the solution best for its users but really
   to avoid DMARC not finding a single useable domain in the 5322.From,
   the real solution is to upgrade your MTAs, if you want to do DMARC,
   to support EAI (SMTPUTF8) so that the group syntax in the 5322.From
   is not needed for interoperability issues during transition, and such
   syntax be considered at least as suspicious when present.

4.5.  Mediators

4.5.1.  Mailing Lists

   o  One common mitigation policy is to configure the Mailing List
      Manager (MLM) to alter the RFC5322.From 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
      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
      modification may to some extent defeats the purpose of DMARC
      itself.  It may make it difficult to ensure that users of all
      email clients can easily reply to author, list, or all using the
      email client features provided for that purpose.  Use of "Reply-



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      To" can alleviate this problem depending if the mailing list is
      configured to reply-to-list, reply-to-author or reply-to-fixed-
      address.

   o  Another common mitigation policy is to configure the MLM to "wrap"
      the message in a MIME message/rfc822 part.  This completely
      bypasses the DMARC policy in clients that allow reading the part
      as a message.  Many email clients (as of August 2014) have
      difficulty reading such messages.

   o  Finally a less common mitigation policy, is to configure the MLM
      to not modify the message so that the DKIM signature remains
      valid.

   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.  Correctly interpreting Extended
      SMTP error messages is useful in this case.

   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.

4.6.  Getting More Radical: Requiring New Communication Paths Between
      MUA and the Message Store

   In practice a number of operators are using strict alignement mode in
   DMARC in order to avoid receiving new and innovative forms of
   unwanted and unauthentic mail 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 5322.From.  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 third party
   signatures.

5.  IANA Considerations

   No IANA considerations.







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6.  Security Considerations

   No Security considerations.

7.  Acknowledgments

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

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

8.  References

8.1.  Normative References

   [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.

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

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






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

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-dmarc-base]
              Kucherawy, M. and E. Zwicky, "Domain-based Message
              Authentication, Reporting and Conformance (DMARC)", draft-
              kucherawy-dmarc-base-11 (work in progress), January 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.

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)
   Eudaemon
   NC
   USA

   Email: tim@eudaemon.net



































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