Individual Submission                                       M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Informational                              June 1, 2010
Expires: December 3, 2010

                         DKIM And Mailing Lists


   DomainKeys Identified Mail (DKIM) allows an administrative mail
   domain (ADMD) to assume some responsibility for a message.  As the
   industry has now gained some deployment experience, the goal for this
   document is to explore the use of DKIM for scenarios that include
   intermediaries, such as Mailing List Managers (MLMs).

Status of this Memo

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   This Internet-Draft will expire on December 3, 2010.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  MLMs In Infrastructure . . . . . . . . . . . . . . . . . .  4
     1.3.  Feedback Loops And Other Bi-Lateral Agreements . . . . . .  5
     1.4.  Document Scope and Goals . . . . . . . . . . . . . . . . .  5
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Other Terms  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  DKIM-Specific References . . . . . . . . . . . . . . . . .  7
     2.3.  Feedback Loop References . . . . . . . . . . . . . . . . .  7
     2.4.  Message Streams  . . . . . . . . . . . . . . . . . . . . .  7
   3.  Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Roles and Realities  . . . . . . . . . . . . . . . . . . .  8
     3.2.  Types Of Mailing Lists Lists . . . . . . . . . . . . . . .  9
     3.3.  Current MLM Effects On Signatures  . . . . . . . . . . . . 10
     3.4.  Alternatives of Participation and Conformance  . . . . . . 11
   4.  Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Author-Related Signing . . . . . . . . . . . . . . . . . . 13
     4.2.  Verification Outcomes at Receivers . . . . . . . . . . . . 13
     4.3.  Handling Choices at Receivers  . . . . . . . . . . . . . . 14
   5.  Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Subscriptions  . . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Author-Related Signing . . . . . . . . . . . . . . . . . . 15
     5.3.  Verification Outcomes at MLMs  . . . . . . . . . . . . . . 15
     5.4.  Pros and Cons of Signature Removal . . . . . . . . . . . . 16
     5.5.  MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 17
     5.6.  Verification Outcomes at Final Receiving Sites . . . . . . 18
     5.7.  Use With FBLs  . . . . . . . . . . . . . . . . . . . . . . 18
     5.8.  Handling Choices at Receivers  . . . . . . . . . . . . . . 19
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     7.1.  Authentication Results When Relaying . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 24
   Appendix B.  Example Scenarios . . . . . . . . . . . . . . . . . . 25
     B.1.  MLMs and ADSP  . . . . . . . . . . . . . . . . . . . . . . 25
     B.2.  MLMs and FBLs  . . . . . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26

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1.  Introduction

   [DKIM] allows an Administrative Mail Domain to take some
   responsibility for a [MAIL] message.  This can be an author's
   organization, an operational relay (Mail Transfer Agent, or MTA) or
   one of their agents.  Assertion of responsibility is made through a
   cryptographic signature.  Message transit from author to recipient is
   through relays that typically make no substantive change to the
   message content and thus preserve the DKIM signature.

   In contrast to relays, there are intermediaries, such as mailing list
   managers (MLMs), that actively take delivery of messages, re-format
   them, and re-post them, almost always invalidating DKIM signatures.
   The goal for this document is to explore the use of DKIM for
   scenarios that include intermediaries.  Questions that will be
   discussed include:

   o  When should an author, or its organization, use DKIM for mail sent
      to mailing lists?

   o  What are the tradeoffs regarding having an MLM verify and use DKIM

   o  What are the tradeoffs regarding having an MLM remove exisitng
      DKIM signatures prior to re-posting the message?

   o  What are the tradeoffs regarding having an MLM add its own DKIM

   These and others are open questions for which there may be no
   definitive answers.  However, based on experience since the
   publication of [DKIM] and its gradual deployment, there are some
   useful views worth considering.

   This document explores changes to common practice by the signers, the
   verifiers and the MLMs.

1.1.  Background

   DKIM signatures permit an agent of the email architecture (see
   [EMAIL-ARCH]) to make a claim of responsibility for a message by
   affixing a domain-level digital signature to the message as it passes
   through a gateway.  Although not the only possibility, this is most
   commonly done as a message passes through a Mail Transport Agent
   (MTA) as it departs an Administrative Mail Domain (ADMD) toward the
   general Internet.

   DKIM signatures will fail to verify if a portion of the message

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   covered by one of its hashes is altered.  MLMs commonly alter
   messages to provide information specific to the mailing list for
   which it is providing service.  Common modifications include:

   o  Prefix the Subject: header field with a short string for easy
      sorting by receivers' Mail User Agents (MUAs) or other filtering

   o  Prepend or append list management information to the message's
      body, such as some text and/or a URL to which subscribers can go
      to make administrative changes to their subscriptions;

   o  Add header fields such as Reply-To:, Sender:, Resent-Sender:
      ([MAIL]), List-Id: ([LIST-ID]), List-Post: or List-Unsubscribe:
      ([LIST-URLS]).  In some cases, such header fields are replaced if
      the original message already contained them.

   The DKIM specification documents deliberately refrain from the notion
   of tying the signing domain (the "d=" tag in a DKIM signature) to any
   identifier within a message; any ADMD could sign any message
   regardless of its origin or author domain.  As such, there is no
   specification of any additional value if the content of the "d=" tag
   in the DKIM signature and the value of (for example) the From header
   field match, nor is there any obvious degraded value to a signature
   where they do not match.  Since any DKIM signature is merely an
   assertion of "some" responsibility by an ADMD, a DKIM signature added
   by an MLM has no more, or less, meaning as a signature with any other
   "d=" value.

1.2.  MLMs In Infrastructure

   The previous section describes some of the things MLMs commonly do
   that are not DKIM-friendly, producing broken signatures and thus
   reducing the perceived value of DKIM.

   Further, despite the advent of standards that are specific to MLM
   behaviour (e.g.  [MAIL], [LIST-ID] and [LIST-URLS]), their adoption
   has been spotty at best.  Hence, efforts to specify the use of DKIM
   in the context of MLMs needs to be incremental and value-based.

   MLM behaviors are well-established and standards compliant.  Thus,
   the best approach is to provide these best practices to all parties
   involved, imposing the minimum requirements possible to MLMs

   An MLM is an autonomous agent that takes delivery of a message
   delivered to it and can re-post it as a new message (or construct a
   digest of it along with other messages) to the members of the list

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   (see [EMAIL-ARCH], Section 5.3).  However, the fact that the From
   field of such a message is typically the same as for the original
   message and that recipients perceive the message as "from" the
   original author rather than the MLM creates confusion about
   responsibility and autonomy for the re-posted message.  This has
   important implications for use of DKIM.

   A DKIM signature on a message is an expression of some responsibility
   for the message taken by the signing domain.  An open question, one
   this document intends to address, is some idea of how such a
   signature might be applied by an recipient's evaluation module after
   the message has gone through a mailing list, and may or may not have
   been invalidated, and if so, where the invalidation may have

   Note that where in this document there is discussion of an MLM
   conducting validation of DKIM signatures or ADSP policies, the actual
   implementation could be one where the validation is done by the MTA
   or an agent attached to it, and the results of that work are relayed
   by a trusted channel not specified here.  See [AUTH-RESULTS] for a
   discussion of this.  This document does not favour any particular
   arrangement of these agents over another, but merely talks about the
   MLM itself doing the work as a matter of simplicity.

1.3.  Feedback Loops And Other Bi-Lateral Agreements

   A Feedback Loop (FBL) is a bi-lateral agreement between two parties
   to exchange reports of abuse.  Typically, a bulk mail sender
   registers with an email receiving site to receive abuse reports from
   that site for mail coming from the sender.

   An FBL reporting address is part of this bi-lateral registration.
   Some FBLs require DKIM use by the registrant.  Messages signed and
   sent by a registrant through an MLM can therefore result in having
   abuse reports sent to the original author when the actual problem
   pertains to the operation of the MLM.  However, the original author
   has no involvement in operation of the MLM, meaning the FBL report is
   not actionable and thus undesirable.

1.4.  Document Scope and Goals

   This document provides discussion on the above issues, to improve the
   handling of possible interactions between DKIM and MLMs.  An attempt
   has been made to prefer imposing changes to behaviour at the signer
   and verifier rather than at the MLM.

   Wherever possible, MLMs will be conceptually decoupled from MTAs
   despite the very tight integration that is sometimes observed in

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   implementation.  This is done to emphasize the functional
   independence of MLM services and responsibilities from those of an

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2.  Definitions

2.1.  Other Terms

   See [EMAIL-ARCH] for a general description of the current messaging
   architecture, and for definitions of various terms used in this

2.2.  DKIM-Specific References

   Readers are encouraged to become familiar with [DKIM] and [ADSP]
   which are standards-track protocol documents as well as
   [DKIM-OVERVIEW] and [DKIM-DEPLOYMENT] which are DKIM's primary
   tutorial documents.

2.3.  Feedback Loop References

   FBLs tend to use the ARF ([I-D.DRAFT-IETF-MARF-BASE]) or the IODEF
   ([IODEF]) format.

2.4.  Message Streams

   This document makes reference to the concept of "message streams".
   The idea is to identify groups of messages originating from within an
   ADMD that are distinct in intent, origin and/or use, and partition
   them somehow (most commonly via DNS subdomains, and/or the "d=" tag
   value in the context of DKIM) so as to keep them associated to users
   yet operationally distinct.

   A good example might be user mail, generated by a company's
   employees, versus operational or transactional mail that comes from
   automated sources, versus marketing or sales campaigns; each of these
   could have different security policies imposed against them, or there
   might be a desire to insulate one from the other (e.g., a marketing
   campaign that gets reported by many spam filters could cause the
   marketing stream's reputation to degrade without automatically
   punishing the transactional or user streams).

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3.  Mailing Lists and DKIM

   It is important to make some distinctions among different MLM-like
   agents, their typical implementations, and the impacts they have in a
   DKIM-aware environment.

3.1.  Roles and Realities

   In DKIM parlance, there are several key roles in the transit of a
   message.  Most of these are defined in [EMAIL-ARCH].

   author:  The agent that actually constructed the message being sent
      through the system, and performed the initial submission.  This
      can be a human using an MUA or a common system utility such as
      "cron", etc.

   originator:  The agent that accepts a message from the author,
      ensures it conforms to the relevant standards such as [MAIL], and
      then relays it toward its destination(s).  This is often referred
      to as the Mail Submission Agent (MSA).

   signer:  The agent that affixes one or more DKIM signature(s) to a
      message on its way toward its ultimate destination.  It is
      typically running at the MTA that sits between the author's ADMD
      and the general Internet.  The signer and the originator may also
      be the same agent.

   verifier:  The agent that conducts DKIM signature analysis.  It is
      typically running at the MTA that sits between the receiver's ADMD
      and the general Internet.  Note that any agent that handles a
      signed message could conduct verification; this document only
      considers that action and its outcomes either at an MLM or at the

   receiver:  The agent that is the final transit relay for the message
      prior to being delivered to the recipient(s) of the message.

   In the case of simple user-to-user mail, these roles are fairly
   straightforward.  However, when one is sending mail to a list, which
   then gets relayed to all of that list's subscribers, the roles are
   often less clear to the general user, as particular agents may hold
   multiple important but separable roles.  The above definitions are
   intended to enable more precise discussion of the mechanisms

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3.2.  Types Of Mailing Lists Lists

   There are four common MLM implementation modes:

   aliasing:  An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one
      that makes no changes to a message as it redistributes; any
      modifications are constrained to changes to the [SMTP] envelope
      recipient list (RCPT commands) only.  There are no changes to the
      message body at all and only [MAIL] trace header fields are added.
      The output of such an MLM is considered to be a continuation of
      the author's original message.  An example of such an MLM is a
      address that expands directly in the MTA, such as a list of local
      system administrators used for relaying operational or other
      internal-only messages.

   resending:  A resending MLM (see Sections 5.2 and 5.3 of
      [EMAIL-ARCH]) is one that may make changes to a message.  The
      output of such an MLM is considered to be a new message; delivery
      of the original has been completed prior to distribution of the
      re-posted message.  Such messages are often re-formatted, such as
      with list-specific header fields or other properties, to
      facilitate discussion among list subscribers.

   authoring:  An authoring MLM is one that creates the content being
      sent as well as initiating its transport, rather than basing it on
      one or more messages received earlier.  This is a special case of
      the MLM paradigm, one which generates its own content and does not
      act as an intermediary.  Typically replies are not generated, or
      if they are, they go to a specific recipient and not back to the
      list's full set of recipients.  Examples include newsletters and
      bulk marketing mail.

   digesting:  A special case of the re-posting MLM is one that sends a
      single message comprising an aggregation of recent MLM submissons,
      which might be a message of [MIME] type "multipart/digest" (see
      [MIME-TYPES]).  This is obviously a new message but it may contain
      a sequence of original messages that may themselves have been

   The remainder of this document operates on the presumption that a
   message going through a resending MLM actually comprises two message

   1.  Originating user to MLM: Originating user is author; originating
       ADMD is signer; MLM's ADMD is verifier; MLM's input function is

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   2.  MLM to receivers: MLM (sending its reconstructed copy of the
       originating user's message) is author; MLM's ADMD is signer; the
       ADMD of each subscriber of the list is a verifier; each
       subscriber is a receiver.

   Much of this document focuses on the resending MLM as it has the most
   direct conflict operationally with DKIM.

   The dissection of the overall MLM operation into these two distinct
   steps allows the DKIM-specific issues with respect to MLMs to be
   isolated and handled in a logical way.  The main issue is that the
   repackaging and reposting of a message by an MLM is actually the
   construction of a completely new message, and as such the MLM is
   introducing new content into the email ecosystem, consuming the
   author's copy of the message and creating its own.  When considered
   in this way, the dual role of the MLM and its ADMD becomes clear.

3.3.  Current MLM Effects On Signatures

   As described above, an aliasing MLM does not affect any existing
   signature, and an authoring MLM is always new content and thus there
   is never an existing signature.  However, the changes a resending MLM
   can make typically affect the Subject: header field, addition of some
   list-specific header fields, and/or the addition of some list-
   specific text to the top or bottom of the message body.  The impacts
   of each of these on DKIM verification are discussed below.

   Subject tags:  Altering the Subject: header field will invalidate the
      signer's signature if that header field was covered by a hash of
      that signature.  [DKIM] lists Subject as one that should be
      covered, so this is expected to be an issue for any list that
      makes such changes.

   List-specific header fields:  Some lists will add header fields
      specific to list administrative functions such as those defined in
      [LIST-ID] and [LIST-URLS], or the "Resent-" fields defined in
      [MAIL].  It is unlikely that a typical MUA would include such
      fields in an original message, and DKIM is resilient to the
      addition of header fields in general (though see notes about the
      "h=" tag in Section 3.5 of [DKIM]).  Therefore this is seen as
      less of a concern.

   Other header fields:  Some lists will add or replace header fields
      such as "Reply-To" or "Sender" in order to establish that the
      message is being sent in the context of the mailing list, so that
      the list is identified ("Sender") and any user replies go to the
      list ("Reply-To").  If these fields were included in the original
      message, it is possible that one or more of them may have been

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      signed, and this could cause a concern for MLMs that add or
      replace them.

   Minor body changes:  Some lists prepend or append a few lines to each
      message to remind subscribers of an administrative URL for
      subscription issues, or of list policy, etc.  Changes to the body
      will alter the body hash computed at the DKIM verifier, so these
      pose an immediate problem.

   Major body changes:  There are some MLMs that make more substantial
      changes to message bodies when preparing them for re-distribution,
      such as deleting, reordering, or reformatting [MIME] parts,
      "flatten" HTML messages into plain text, or insert headers or
      footers within HTML messages.  Most or all of these changes will
      invalidate a DKIM signature with little or no hope of compensation
      by either the signer or the verifier.

   There reportedly still exist a few scattered mailing lists in
   operation that are actually run manually by a human list manager.

   In general, an MLM subscriber cannot be expected to be able to
   reconstruct the original message as it appeared at time of signing
   and thus whether or not an author signature is actually valid after
   MLM rewriting.  Moreover, even if an MLM currently passes messages
   unmodified such that author signatures validate, it is possible that
   a configuration change or software upgrade to that MLM will cause
   that no longer to be true.

3.4.  Alternatives of Participation and Conformance

   As DKIM becomes more entrenched, it is highly desirable that MLM
   software adopt more DKIM-friendly processing.

   Changes that merely add new header fields, such as those specified by
   [LIST-ID], [LIST-URLS] and [MAIL] are generally the most friendly to
   a DKIM-participating email infrastructure in that their addition by
   an MLM will not affect any existing DKIM signatures unless those
   fields were already present and covered by a signature's hash or a
   signature was created specifically to disallow their addition (see
   the note about "h=" in Section 3.5 of [DKIM]).  The shortest path to
   success for DKIM would be to mandate that all MLM software be re-
   designed or re-configured with that goal in mind.

   However, the practice of applying headers and footers to message
   bodies is common and not expected to fade regardless of what
   documents this or any standards body might produce.  This sort of
   change will invalidate the signature on a message where the body hash
   covers the entire entire message.  Thus, the following sections also

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   investigate and recommend other processing alternatives.

   A possible mitigation to this incompatibility is use of the "l=" tag
   to bound the portion of the body covered by the body hash, but this
   not workable for [MIME] messages and moreover has security
   considerations (see Section 3.5 of [DKIM]).  Its use is therefore

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4.  Non-Participating MLMs

   This section contains a discussion of issues regarding sending DKIM-
   signed mail to or through an MLM that is not DKIM-aware.
   Specifically, the header fields introduced by [DKIM] and
   [AUTH-RESULTS] carry no special meaning to such an MLM.

4.1.  Author-Related Signing

   If an author knows that the MLM to which a message is being sent is a
   non-participating resending MLM, the author is advised to be cautious
   when deciding whether or not to sign the message.  The MLM could make
   a change that would invalidate the author's signature but not remove
   it prior to re-distribution.  Hence, list recipients would receive a
   message purportedly from the author but bearing a DKIM signature that
   would not verifiy.  This problem would be compounded further if there
   were receivers that applied signing policies ([ADSP]) and the author
   published any kind of strict policy.

   If this is cause for concern, the originating site can consider using
   a separate message stream, such as a sub-domain, for the "personal"
   mail that is different from domain(s) used for other mail streams, so
   that they develop independent reputations, and more stringent
   policies (including ADSP) can be applied to the mail stream(s) that
   do not go through mailing lists.

   However, all of this presupposes a level of infrastructure
   understanding that is not expected to be common.  Thus, it will be
   incumbent upon site administrators to consider how support of users
   wishing to participate in mailing lists might be accomplished as DKIM
   achieves wider adoption.  A common suggestion is to establish
   subdomains in the DNS that are used for separating different streams
   of mail from within an ADMD, such as user-created "direct" mail from
   transactional or automated mail; some of these may be signed and some
   not, some with published ADSP records, some not.  In general, the
   more strict practices and policies are likely to be successful only
   for the mail streams subject to the most end-to-end control by the
   originating organization.  That typically excludes mail going through

4.2.  Verification Outcomes at Receivers

   Verifiers that receive mail bearing DKIM signatures that fail to
   verify might benefit from attempting to detect that such mail passed
   through a non-participating MLM and then decide not to apply [ADSP]
   in order to avoid aggressive filtering of mail that should otherwise
   have been delivered.

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   Unfortunately, there may not be a reliable way of making such
   determinations, as there is no uniform MLM behaviour, and any tagging
   mechanism meant to relay such information could easily be abused.

   Note that the underlying problem is the operational choice to use
   ADSP in a message stream that does not maintain the signature.

4.3.  Handling Choices at Receivers

   A receiver's ADMD would have to have some way to register such non-
   participating lists to exempt them from the filtering described in
   Section 4.1.  This is, however, probably not a scalable solution as
   it imposes a burden on the receiver that is predicated on sender

   Note that the [DKIM] specification explicitly directs verifiers to
   treat a verification failure as though the message were not signed in
   the first place.  In the absence of specific ADSP direction, any
   treatment of a verification failure as having special meaning is
   either outside the scope of DKIM or is in violation of it.

   [ADSP] presents an additional challenge.  Per that specification,
   when a message is unsigned or the signature can no longer be
   verified, the verifier must discard the message.  There is no
   exception in the policy for a message that may have been altered by
   an MLM.  Verifiers are thus advised to honor the policy and disallow
   the message.  Furthermore, authors whose ADSP is published as
   "discardable" are advised not to send mail to MLMs as it is likely to
   be rejected by ADSP-aware recipients.  (This is discussed further in
   Section 5.4 below.)

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5.  Participating MLMs

   This section contains a discussion of issues regarding sending DKIM-
   signed mail to or through an MLM that is DKIM-aware, and may also be

5.1.  Subscriptions

   At subscription time, an ADSP-aware MLM could check for a published
   ADSP record for the new subscriber, and present a warning for one
   whose ADMD's published policy is "discardable" indicating that
   submissions from that ADMD may not be deliverable because of
   modifications that are likely to be made to the message.

5.2.  Author-Related Signing

   MLMs typically attempt to authenticate messages posted through them.
   They usually do this through the trivial (and insecure) means of
   verifying the From field email address against a list registry.  DKIM
   enables a stronger form of authentication, although this is not yet
   formally documented: It can require that messages using a given From
   address also have a DKIM signature with a corresponding "d=" domain.
   (Note, however, that it is entirely reasonable for an MLM to permit
   registration of some other "d=" domain as valid evidence of such
   authentication.)  This feature would be somewhat similar to using
   ADSP, except that the requirement for it would be imposed by the MLM
   and not the author's organization.

   An important consideration is that authors rarely have any direct
   influence over the management of an MLM.  As such, a signed message
   from an author will in essence go to a set of unexpected places.
   Authors may be well-advised to create a mail stream specifically used
   for generating signatures when sending traffic to MLMs.  This becomes
   important as domain-based reputation systems begin to appear as
   components of mail filtering modules.

   This suggestion can be made more general.  Mail that is of a
   transactional or generally end-to-end nature, and not likely to be
   forwarded around either by MLMs or users, should come from a
   different mail stream than a stream that serves a broader purpose.

5.3.  Verification Outcomes at MLMs

   As described above, the MLM might conduct DKIM verification of a
   signed message to attempt to confirm the identity of the author.
   Although it is a common and intuitive conclusion, however, not all
   signed mail will include an author signature (see [ADSP]).  MLM
   implementors are advised to accomodate such in their configurations.

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   For example, an MLM might be designed to accomodate a list of
   possible signing domains (the "d=" portion of a DKIM signature) for a
   given author, and determine at verification time if any of those are

   A message that cannot be thus authenticated could be held for
   moderation or rejected outright.

   This logic could apply to any list operation, not just list
   submission.  In particular, this improved authentication could apply
   to subscription, unsubscription, and/or changes to subscriber options
   that are sent via email rather than through an authenticated,
   interactive channel such as the web.

   In the case of verification of signatures on subscriptions, MLMs are
   advised to add an [AUTH-RESULTS] header field to indicate the
   signature(s) observed on the submission as it arrived at the MLM and
   what the outcome of the evaluation was.  Downstream agents may or may
   not trust the content of that header field depending on their own a
   priori knowledge of the operation of the ADMD generating (and,
   preferably, signing) that header field.  See [AUTH-RESULTS] for
   further discussion.

5.4.  Pros and Cons of Signature Removal

   If the MLM is configured to make changes to the message prior to re-
   posting that would invalidate the original signature(s), further
   action is recommended to prevent invalidated signatures from arriving
   at final recipients, possibly triggering unwarranted filter actions.
   A possible solution would be to:

   1.  Attempt verification of all DKIM signatures present on the

   2.  Apply local policy to authenticate the identity of the author;

   3.  Add an [AUTH-RESULTS] header field to the message to indicate the
       results of the above;

   4.  Remove all previously-evaluated DKIM signatures;

   5.  Affix a new signature that covers the Authentication-Results
       header field just added.

   Removing the original signature(s) seems particularly appropriate
   when the MLM knows it is likely to invalidate any or all of them due
   to the nature of the reformatting it will do.  This avoids false
   negatives at the list's subscribers in their roles as receivers of

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   the message.

   However, per the discussion in [AUTH-RESULTS], there is no a priori
   reason for the final receivers to put any faith in the veracity of
   that header field when added by the MLM.  Thus, the final recipients
   of the message have no way to verify on their own the authenticity of
   the author's identity on that message.

   Since an aliasing MLM makes no substantive changes to a message, it
   need not consider the issue of signature removal as the original
   signatures should arrive at least to the next MTA unmodified.  It is
   possible that future domain-based reputations would prefer a more
   rich data set on receipt of a message, and in that case signature
   removal would be undesirable.

   An authoring MLM is closed to outside submitters, thus much of this
   discussion does not apply in that case.

   [ADSP] presents a particular challenge.  An author domain posting a
   policy of "discardable" imposes a very tight restriction on the use
   of mailing lists, essentially constraining that domain's users to
   lists operated by aliasing MLMs only; any MLM that alters a message
   from such a domain or removes its signature subjects the message to
   severe action by receivers.  It is the consensus of the working group
   that a resending MLM is advised to reject outright any mail from an
   author whose domain posts such a policy as it is likely to be
   rejected by any ADSP-aware recipients, and might also be well advised
   to present a warning to such subscribers when first signing up to the

5.5.  MLM Signatures

   DKIM-aware resending MLMs and authoring MLMs are encouraged to affix
   their own signatures when distributing messages.  The MLM is
   responsible for the alterations it makes to the original messages it
   is re-sending, and should express this via a signature.  This is also
   helpful for getting feedback from any FBLs that might be set up so
   that undesired list mail can generate appropriate action.

   A signing MLM is, as any other MLM, free to omit redistribution of a
   message from an author if that message was not signed in accordance
   with its own local configuration or policy.  However, selective
   signing is discouraged; essentially that would create two message
   streams from the MLM, which can confuse verifiers and receivers.

   A signing MLM is advised to add a List-Post: header field (see
   [LIST-URLS]) using a DNS domain matching what will be used in the
   "d=" tag of the DKIM signature it will add to the new message.  This

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   could be used by verifiers or receivers to identify the DKIM
   signature that was added by the MLM.

   Such MLMs are advised to ensure the signature's header hash will

   o  Any [AUTH-RESULTS] fields added by the MLM;

   o  Any [LIST-ID] or [LIST-URLS] fields added by the MLM;

   o  Any [MAIL] fields, especially Sender and Reply-To, added or
      replaced by the MLM.

   A DKIM-aware resending MLM is encouraged to sign the entire message
   as it arrived, especially including the original signatures.

   DKIM-aware authoring MLMs are advised to sign the mail they send
   according to the regular signing guidelines given in [DKIM].

   Operators of non-DKIM-aware MLMs could arrange to submit MLM mail
   through an MSA that is DKIM-aware so that its mail will be signed.

5.6.  Verification Outcomes at Final Receiving Sites

   In general, verifiers and receivers can treat a signed message from
   an MLM like any other signed message; indeed, it would be difficult
   to discern any difference.

   However, because the author domain will commonly be different from
   the MLM's signing domain, there may be a conflict with [ADSP] as
   discussed in Section 4.3 and Section 5.4.

5.7.  Use With FBLs

   An FBL operator wishing act on a complaint by making use of DKIM
   verifications is advised to send a report to any domain with a valid
   signature that has an FBL agreement established, as DKIM signatures
   are claims of some responsibility for that message.  Because authors
   generally have limited control over the operation of a list, this
   point makes MLM signing all the more important.

   Where the FBL wishes to be more specific, it could act solely on a
   DKIM signature where the signing domain matches the DNS domain found
   in a List-Post: header field (or similar).

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5.8.  Handling Choices at Receivers

   A recipient that trusts signatures from an MLM may wish to extend
   that trust to an [AUTH-RESULTS] header field signed by that MLM.  The
   recipient may then do additional processing of the message, using the
   results recorded in the Authentication-Results header field instead
   of the original author's DKIM signature.  This includes possibly
   processing the message as per ADSP requirements.

   Receivers are advised to ignore all unsigned Authentication-Results
   header fields.

   Upon DKIM and ADSP evaluation, a receiver may decide to reject a
   message during an SMTP session.  If this is done, use of [ENHANCED]
   is advised to make a distinction between messages rejected
   deliberately due to policy decisions rather than those rejected
   because of other deliverability issues.  In particular, a policy
   rejection is advised to be relayed using a 5.7.1 enhanced status
   code, in contrast to a code of 5.1.1 indicating the user does not
   exist.  Those MLMs that attempt to automatically remove users with
   prolonged delivery problems (such as account deletion) will thus be
   able to tell the difference between policy rejection and delivery
   failures, and act accordingly.  Where the receiver's MTA does not
   support enhanced status codes, [SMTP] reply codes could also be
   carefully selected (554 and 550, respectively, for example).

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6.  IANA Considerations

   This document includes no IANA actions.

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7.  Security Considerations

   This document provides suggested or best current practices for use
   with DKIM, and as such does not introduce any new technologies for
   consideration.  However, the following security issues should be
   considered when implementing the above practices.

7.1.  Authentication Results When Relaying

   some stuff about the fact that the MLM's auth-results can't be
   trusted by default

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8.  References

8.1.  Normative References

   [ADSP]     Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
              Sender Signing Practises", RFC 5617, August 2009.

   [DKIM]     Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

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

8.2.  Informative References

              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

              Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
              "DomainKeys Identified Mail (DKIM) Development, Deployment
              and Operations", I-D DRAFT-IETF-DKIM-DEPLOYMENT,
              January 2010.

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

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

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

              Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", I-D DRAFT-
              IETF-MARF-BASE, April 2010.

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

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   [LIST-ID]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field
              and Namespace for the Identification of Mailing Lists",
              RFC 2919, March 2001.

              Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
              for Core Mail List Commands and their Transport through
              Message Header Fields", RFC 2369, July 1998.

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

              Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

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

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Appendix A.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this document: Dave Crocker, JD Falk, Tony
   Hansen, Eliot Lear, John Levine and S. Moonesamy.

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Appendix B.  Example Scenarios

   This section describes a few MLM-related DKIM scenarios that were
   part of the impetus for this work, and the recommended resolutions
   for each.

B.1.  MLMs and ADSP


   o  author ADMD advertise an ADSP policy of "dkim=discardable"

   o  author sends DKIM-signed mail to a non-participating MLM, which
      invalidates the signature

   o  receiver MTA checks DKIM and ADSP at SMTP time, and is configured
      to reject ADSP failures, so rejects this message

   o  process repeats a few times, after which the MLM unsubscribes the

   Solution: MLMs should refuse mail from domains advertising ADSP
   policies of "discardable" unless they are certain they make no
   changes that invalidate DKIM signatures.

B.2.  MLMs and FBLs


   o  subscriber sends sign mail to a non-participating MLM that does
      not invalidate the signature

   o  a recipient reports the message as spam

   o  FBL at recipient ADMD sends report to contributor rather than list

   Solution: MLMs should sign mail they send and should probably strip
   signatures; FBLs should report to list operators instead of to
   subscribers where such can be distinguished.

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Author's Address

   Murray S. Kucherawy
   128 King St., 2nd Floor
   San Francisco, CA  94107

   Phone: +1 415 946 3800

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