DKIM Working Group                                          M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Informational                          October 15, 2010
Expires: April 18, 2011

                         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).

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   This Internet-Draft will expire on April 18, 2011.

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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.  'DKIM-Friendly'  . . . . . . . . . . . . . . . . . . . . .  7
     2.4.  Message Streams  . . . . . . . . . . . . . . . . . . . . .  7
   3.  Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Roles and Realities  . . . . . . . . . . . . . . . . . . .  8
     3.2.  Types Of Mailing Lists . . . . . . . . . . . . . . . . . .  9
     3.3.  Current MLM Effects On Signatures  . . . . . . . . . . . . 10
   4.  Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Author-Related Signing . . . . . . . . . . . . . . . . . . 12
     4.2.  Verification Outcomes at Receivers . . . . . . . . . . . . 13
     4.3.  Handling Choices at Receivers  . . . . . . . . . . . . . . 13
     4.4.  Wrapping A Non-Participating MLM . . . . . . . . . . . . . 13
   5.  Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2.  DKIM Author Domain Signing Practices . . . . . . . . . . . 14
     5.3.  Subscriptions  . . . . . . . . . . . . . . . . . . . . . . 15
     5.4.  Author-Related Signing . . . . . . . . . . . . . . . . . . 15
     5.5.  Verification Outcomes at MLMs  . . . . . . . . . . . . . . 16
     5.6.  Signature Removal Issues . . . . . . . . . . . . . . . . . 16
     5.7.  MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 18
     5.8.  Verification Outcomes at Final Receiving Sites . . . . . . 19
     5.9.  Use With FBLs  . . . . . . . . . . . . . . . . . . . . . . 19
     5.10. Handling Choices at Receivers  . . . . . . . . . . . . . . 20
   6.  DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     8.1.  Authentication Results When Relaying . . . . . . . . . . . 24
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 25
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 27
   Appendix B.  Example Scenarios . . . . . . . . . . . . . . . . . . 28
     B.1.  MLMs and ADSP  . . . . . . . . . . . . . . . . . . . . . . 28
     B.2.  MLMs and FBLs  . . . . . . . . . . . . . . . . . . . . . . 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29

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1.  Introduction

   DomainKeys Identified Mail ([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 validity of the DKIM

   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, often 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  Under what circumstances is it advisable for an author, or its
      organization, to apply DKIM to 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 existing
      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
   views that are useful to consider.

   In general there are, in relation to DKIM, two categories of MLMs:
   participating and non-participating.  As each types has its own
   issues regarding DKIM-signed messages that are either handled or
   produced by them (or both), the types are discussed in separate

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 validated domain-level identifier 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

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   general Internet.

   A DKIM signature will fail to verify if a portion of the message
   covered by one of its hashes is altered.  An MLM commonly alters
   messages to provide information specific to the mailing list for
   which it is providing service.  Common modifications are enumerated
   and described in Section 3.3.  However, note that MLMs vary widely in
   behaviour as well as often allowing subscribers to select individual
   behaviours.  Further, this does not consider changes the MTA might
   make independent of what changes the MLM chooses to apply.

   The DKIM specification document deliberately refrains from the notion
   of tying the signing domain (the "d=" tag in a DKIM signature) to any
   identifier within a message; any ADMD that handles a message could
   sign it, regardless of its origin or author domain.  In particular,
   DKIM does not define any meaning to the occurrence of a match between
   the content of a "d=" tag and the value of, for example, a domain
   name in the RFC5322.From field, 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, nor less, meaning than
   a signature with any other "d=" value.

1.2.  MLMs In Infrastructure

   The previous section describes some of the things MLMs commonly do
   that produce broken signatures and thus reducing the perceived value
   of DKIM.

   Further, while 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.  Although it can be argued that
   they frustrate widespread DKIM adoption, it cannot be said that such
   behaviours are not standards compliant.  Thus, the best approach is
   to provide these best practices to all parties involved, imposing the
   minimum requirements possible to MLMs themselves.

   An MLM is an autonomous agent that takes delivery of a message 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 (see [EMAIL-ARCH],
   Section 5.3).  However, the fact that the RFC5322.From field of such
   a message is typically the same as that of 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

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   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 issue, and one
   this document intends to address, is some idea of how such a
   signature might be used by a recipient's evaluation module after the
   message has gone through a mailing list and may or may not have been
   invalidated, and if it has, where the invalidation may have happened.

   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 sender registers with a
   receiving site to receive abuse reports from that site for mail
   coming from the sender.

   An FBL reporting address (i.e., an address to which FBL reports are
   sent) 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 is undesirable.

   See Section 6 for additional discussion.

   FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) format.

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.  In general,
   consensus shows a preference toward 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
   implementation.  This is done to emphasize the functional

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   independence of MLM services and responsibilities from those of an

   Parts of this document explore possible changes to common practice by
   signers, verifiers and MLMs.  The suggested enhancements are largely
   theoretical in nature, taking into account the current email
   infrastructure, the facilities DKIM can provide as it gains wider
   deployment, and working group consensus.  There is no substantial
   implementation history upon which these suggestions are based, and
   the efficacy, performance and security characteristics of them have
   not yet been fully explored.

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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 core specification documents as well as [DKIM-OVERVIEW] and
   [DKIM-DEPLOYMENT] which are DKIM's primary tutorial documents.

2.3.  'DKIM-Friendly'

   The term "DKIM-Friendly" is used to describe an email intermediary
   that, when handling a message, makes no changes to that message which
   cause valid [DKIM] signatures present on the message on input to fail
   to verify on output.

   Various features of MTAs and MLMs seen as helpful to users often have
   side effects that do render DKIM signatures unverifiable.  These
   would not qualify for this label.

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 (i.e., via changing the value in the "d=" tag value in
   the context of DKIM) so as to keep them associated to users yet
   distinct in terms of their evaluation and handling by verifiers or

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

   verifier:  Any agent that conducts DKIM signature analysis.  One
      typically running at the MTA that sits between the general
      Internet and the receiver's ADMD.  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.  Filtering decisions could be made by this agent
      based on verificaiton results.

   receiver:  The agent that is the final transit relay for the message
      prior to being delivered to the recipient(s) of the message.
      Filtering decisions based on results made by the verifier could be
      applied by the receiver.  The verifier and the receiver could be
      the same agent.

   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

   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 an
      address that expands directly in the MTA, such as a list of local
      system administrators used for relaying operational or other
      internal-only messages.  See also Section 3.9.2 of [SMTP].

   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 that 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 resending 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

   In the remainder of this document we distinguish two relevant steps,
   corresponding to the following SMTP transactions:

   MLM Input:  Originating user is author; originating ADMD is
      originator and signer; MLM's ADMD is verifier; MLM's input
      function is receiver.

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   MLM Output:  MLM (sending its reconstructed copy of the originating
      user's message) is author; MLM's ADMD is originator and 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 class of 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.

   Some issues about these activities are discussed in Section 3.6.4 of
   [MAIL] and in Section 3.4.1 of [EMAIL-ARCH].

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 creating new content and
   thus there is never an existing signature.  However, the changes a
   resending MLM can make typically affect the RFC5322.Subject header
   field, addition of some list-specific header fields, and/or
   modification of the message body.  The impacts of each of these on
   DKIM verification are discussed below.

   Subject tags:  A popular feature of MLMs is the "tagging" of an
      RFC5322.Subject field by prefixing the field's contents with the
      name of the list, such as "[example]" for a list called "example".
      Altering the RFC5322.Subject field on new submissions by adding a
      list-specific prefix or suffix will invalidate the signer's
      signature if that header field was included when creating that
      signature.  [DKIM] lists RFC5322.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 (see notes about the "h=" tag
      in Section 3.5 of [DKIM]).  Therefore this is seen as less of a

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   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
      signed, and those signatures will thus be broken.

   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
      will render any existing signatures unverifiable.

   Major body changes:  There are some MLMs that make more substantial
      changes to message bodies when preparing them for re-distribution,
      such as adding, 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.

   MIME part removal:  Some MLMs that are MIME-aware will remove large
      MIME parts from submissions and replace them with URLs to reduce
      the size of the distributed form of the message and to prevent
      inadvertent automated malware delivery.  Except in cases where a
      body length limit is applied in generation of the DKIM signature,
      the signature will be broken.

   There reportedly still exist a few scattered mailing lists in
   operation that are actually run manually by a human list manager,
   whose workings in preparing a message for distribution could include
   the above or even some other changes.

   In general, absent a general movement by MLM developers and operators
   toward more DKIM-friendly practices, an MLM subscriber cannot expect
   signatures applied before the message was processed by the MLM to be
   valid on delivery to a receiver.  Such an evolution is not expected
   in the short term due to general development and deployment inertia.
   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.

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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 send to the list when that mail would
   be signed.  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 verify.  There
   exist DKIM modules that incorrectly penalize messages with signatures
   that do not validate, so this may have detrimental effects outside of
   the author's control.  (Additional discussion of this is below.)
   This problem could be compounded if there were receivers that applied
   signing policies (e.g., [ADSP]) and the author published any kind of
   strict policy.

   For domains that do publish strict ADSP policies, the originating
   site can consider using a separate message stream, such as a sub-
   domain, for the "personal" mail -- a subdomain that is different from
   domain(s) used for other mail streams.  This allows each to develop
   an independent reputation, and more stringent policies (including
   ADSP) can be applied to the mail stream(s) that do not go through
   mailing lists or perhaps do not get signed at all.

   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.

   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 MLMs.  Therefore, 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 verifiers or recipients.  (This
   is discussed further in Section 5.6 below.)

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4.2.  Verification Outcomes at Receivers

   There does not appear to be a reliable way to determine that a piece
   of mail arrived via a non-participating MLM.  Sites whose users
   subscribe to non-participating MLMs should be prepared for legitimate
   mail to arrive with no valid signature, just as it always has in the
   absence of DKIM.

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 expectation of signed
   mail as discussed 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 behaviour.

   Note that the [DKIM] specification explicitly directs verifiers and
   receivers to treat a verification failure as though the message was
   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.

   Use of restrictive domain policies such as [ADSP] "discardable"
   presents an additional challenge.  In that case, when a message is
   unsigned or the signature can no longer be verified, discarding of
   the message is requested.  There is no exception in the policy for a
   message that may have been altered by an MLM, nor is there a reliable
   way to identify such mail.  Participants are thus advised to honor
   the policy and disallow the message.

4.4.  Wrapping A Non-Participating MLM

   One approach to adding DKIM support to an otherwise non-participating
   MLM is to "wrap" it, or in essence place it between other DKIM-aware
   components (such as MTAs) that provide some DKIM services.  For
   example, the ADMD operating a non-participating MLM could have a DKIM
   verifier act on submissions, enforcing some of the features and
   recommendations of Section 5 on behalf of the MLM, and the MTA or MSA
   receiving the MLM Output could also add a DKIM signature for the
   MLM's domain.

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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.  General

   As DKIM becomes more widely deployed, 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]).

   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 message.  Thus, the following sections also discuss
   and suggest 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 DKIM body hash, but
   this is not workable for [MIME] messages; moreover, it has security
   considerations (see Section 3.5 of [DKIM]).  Its use is therefore

   MLM operators often arrange to affix to outgoing messages expressions
   of list-specific policy and related information (e.g., rules for
   participation, small advertisements, etc.).  There is currently no
   header field proposed for relaying such general operational MLM
   details apart from what [LIST-URLS] already supports.  This sort of
   information is what is commonly included in appended footer text or
   prepended header text.  The working group recommends periodic,
   automatic mailings to the list to remind subscribers of list policy.
   These will be repetitive, of course, but by being generally the same
   each time they can be easily filtered if desired.

5.2.  DKIM Author Domain Signing Practices

   [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

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   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 verifiers or 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 discourage such subscribers when they first sign up to the
   list.  Further discussion of this appears in Section 5.3.

   Where the above practice is not observed and "discardable" mail
   arrives via a list to a verifier that applies ADSP checks, the
   message can either be discarded (i.e. accept the message at the
   [SMTP] level but discard it without delivery) or the message can be
   rejected by returning a 5xx error code.  In the latter case, some
   advice for how to conduct the rejection in a potentially meaningful
   way can be found in Section 5.10.

   See also Appendix B.5 of [ADSP] for further discussion.

5.3.  Subscriptions

   At subscription time, an ADSP-aware MLM could check for a published
   ADSP record for the new subscriber's domain.  If the policy specifies
   "discardable", the MLM might disallow the subscription or present a
   warning that the subscriber's submissions to the mailing list might
   not be deliverable to some recipients because subscriber's ADMD's
   published policy.

   Of course, such a policy record could be applied after subscription,
   so this is not a universal solution.  An MLM implementation could do
   periodic checks of its subscribers and issue warnings where such a
   policy is detected, or simply check upon each submission.

5.4.  Author-Related Signing

   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,
   sometimes coupled with other messages from other sources.  In the
   future, as DKIM signature outputs (e.g. the AUID or SDID of
   [DKIM-UPDATE]) are used as inputs to reputation modules, there may be
   a desire to insulate one's reputation from influence by the unknown
   results of sending mail through an MLM.  In that case, authors may be
   well-advised to create a mail stream specifically used for generating
   signatures when sending traffic to MLMs.

   This suggestion can be made more general.  Mail that is of a
   transactional or generally end-to-end nature, and not likely to be

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   forwarded around either by MLMs or users, should come from a
   different mail stream than a stream that serves more varied uses.

5.5.  Verification Outcomes at MLMs

   MLMs typically attempt to authenticate messages posted through them.
   They usually do this through the trivial (and insecure) means of
   verifying the RFC5322.From field email address (or, less frequently,
   the RFC5321.MailFrom parameter) 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
   RFC5322.From address also have a DKIM signature with a corresponding
   "d=" domain.  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.

   As described, 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, not all signed mail will
   include an author signature (see [ADSP]).  MLM implementors are
   advised to accomodate such in their configurations.  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 present.

   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 submissions, 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.

5.6.  Signature Removal Issues

   A message that arrives signed with DKIM means some domain prior to
   MLM Input has made a claim of some responsibility for the message.
   An obvious benefit to leaving the input-side signatures intact, then,

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   is to preserve that chain of responsibility of the message so that
   the receivers of the final message have an opportunity to evaluate
   the message with that information available to them.

   However, 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.  (Note, however, that such filtering actions are plainly
   wrong; [DKIM] stipulates that an invalid signature is to be treated
   as no signature at all.)

   A possible solution would be to:

   1.  Attempt verification of all DKIM signatures present on the input

   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 (see Section 5.7).

   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
   the message; although [DKIM] stipulates that an invalid signature is
   the same as no signature, it is anticipated that there will be some
   implementations that ignore this advice.

   The MLM could re-evaluate existing signatures after making its
   message changes to determine whether or not any of them have been
   invalidated.  The cost of this is reduced by the fact that,
   presumably, the necessary public keys have already been downloaded
   and one or both of the message hashes could be reused.

   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.  However, should that field be the
   only one on the message when the verifier gets it, and the verifier
   explicitly trusts the signer (in this case, the MLM), the verifier is

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   in a position to believe that a valid author signature was present on
   the 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.

5.7.  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.

   MLM signatures will likely be used by recipient systems to recognize
   list mail, and they give the MLM's ADMD an opportunity to develop a
   good reputation for the list itself.

   A signing MLM is, as any other MLM, free to omit redistribution of a
   message if that message was not signed in accordance with its own
   local configuration or policy.  It could also redistribute but not
   sign such mail.  However, selective signing is discouraged;
   essentially that would create two message streams from the MLM, one
   signed and one not, which can confuse DKIM-aware verifiers and

   As is typical with DKIM signing, the MLM signature must be generated
   immediately prior to sending, only after all other processing the MLM
   wishes to apply has been completed.  Failing to do so generates a
   signature that can not be expected to validate.

   A signing MLM could 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 could be used by
   verifiers or receivers to identify the DKIM signature that was added
   by the MLM.  This is not required, however; it is believed the
   reputation of the signer will be a more critical data point rather
   than this suggested binding.  Furthermore, this is not a binding
   recognized by any current specification document.

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   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
   after being prepared for distribution (i.e. the "MLM Output" from
   Section 3.2), including any original signatures.

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

   One concern is that having an MLM apply its signature to unsigned
   mail might cause some verifiers or receivers to interpret the
   signature as conferring more authority or authenticity to the message
   content than is defined by [DKIM].  This is an issue beyond MLMs and
   primarily entails receive-side processing outside of the scope of
   [DKIM].  It is nevertheless worth noting here.  In the case of MLMs,
   the presence of an MLM signature is best treated as similar to MLM
   handling that affixes an RFC5322.Subject tag or similar information.
   It therefore does not introduce any new concerns.

5.8.  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 since specifications such as [LIST-URLS]
   and [LIST-ID] are not universally deployed and can be trivially

   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 5.6, especially where an ADMD
   has misused ADSP.

5.9.  Use With FBLs

   An FBL operator may wish to act on a complaint from a user about a
   posting to a list.  Some FBLs could choose to generate feedback
   reports based on DKIM verifications in the subject message.  Such
   operators are advised to send a report to each domain with a valid
   signature that has an FBL agreement established, as DKIM signatures
   are claims of some responsibility for that message.  Because authors

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   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).

   Use of FBLs in this way should be made explicit to list subscribers.
   For example, if it is the policy of the MLM's ADMD to handle an FBL
   item by unsubscribing the user that was the apparent sender of the
   offending message, advising subscribers of this in advance would help
   to avoid surprises later.

5.10.  Handling Choices at Receivers

   A recipient that explicitly trusts signatures from a particular 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

   Receivers are advised to ignore or remove all unsigned externally-
   applied Authentication-Results header fields, and those not signed by
   an ADMD that can be trusted by the receiver.  See Section 5 and
   Section 7 of [AUTH-RESULTS] for further discussion.

   Upon DKIM and ADSP evaluation during an SMTP session (a common
   implementation), an agent might decide to reject a message during an
   SMTP session.  If this is done, use of an [SMTP] failure code not
   normally used for "user unknown" (550) is suggested; 554 seems an
   appropriate candidate.  If the rejecting SMTP server supports
   [ENHANCED] status codes, 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 and some appropriate wording in the text part of
   the reply, in contrast to a code of 5.1.1 indicating the user does
   not exist.  Those MLMs that automatically attempt to remove users
   with prolonged delivery problems (such as account deletion) will thus
   be able to tell the difference between policy rejection and other
   delivery failures, and act accordingly.  SMTP servers doing so are
   also advised to use appropriate wording in the text portion of the
   reply, perhaps explicitly using the string "ADSP" to facilitate
   searching of relevant data in logs.

   The preceding paragraph does not apply to an [ADSP] policy of

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   "discardable".  In such cases where the submission fails that test,
   the receiver or verifier is strongly advised to discard the message
   but return an SMTP success code, i.e. accept the message but drop it
   without delivery.  An SMTP rejection of such mail instead of the
   requested discard action causes more harm than good.

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6.  DKIM Reporting

   The MARF working group is developing mechanisms for reporting
   forensic details about DKIM verification failures.  At the time of
   this writing, this is still a work in progress.

   MLMs are encouraged to apply these or other DKIM failure reporting
   mechanisms as a method for providing feedback to signers about issues
   with DKIM infrastructure.  This is especially important for MLMs that
   implement DKIM verification as a mechanism for authentication of list
   configuration commands and submissions from subscribers.

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7.  IANA Considerations

   This document includes no IANA actions.

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8.  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.

8.1.  Authentication Results When Relaying

   Section 5 advocates addition of an [AUTH-RESULTS] header field to
   indicate authentication status of a message received as MLM Input.
   Per Section 7.2 of [AUTH-RESULTS], receivers generally should not
   trust such data without a good reason to do so, such as an a priori
   agreement with the MLM's ADMD.

   Such agreements are strongly advised to include a requirement that
   those header fields be covered by a [DKIM] signature added by the
   MLM's ADMD.

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9.  References

9.1.  Normative References

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

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

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

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

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

9.2.  Informative References

              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., "RFC 4871 DomainKeys Identified Mail (DKIM)
              Signatures -- Update", RFC 5672, August 2009.

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

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

   [LIST-ID]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field

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              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.

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

   [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: Serge Aumont, Daniel Black,
   Dave Crocker, JD Falk, Tony Hansen, Eliot Lear, Charles Lindsey, John
   Levine, Jeff Macdonald, S. Moonesamy, Rolf E. Sonneveld, and
   Alessandro Vesely.

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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 advertises 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 the MLMs are certain they make no
   changes that invalidate DKIM signatures.

B.2.  MLMs and FBLs


   o  subscriber sends signed 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 might also strip
   existing signatures; FBLs should report to list operators instead of
   subscribers where such can be distinguished, otherwise to all parties
   with valid signatures.

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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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