DKIM Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: Informational August 10, 2010
Expires: February 11, 2011
DKIM And Mailing Lists
draft-ietf-dkim-mailinglists-02
Abstract
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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Copyright Notice
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Other Terms . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. DKIM-Specific References . . . . . . . . . . . . . . . . . 6
2.3. 'DKIM-Friendly' . . . . . . . . . . . . . . . . . . . . . 6
2.4. Feedback Loop References . . . . . . . . . . . . . . . . . 6
2.5. Message Streams . . . . . . . . . . . . . . . . . . . . . 6
3. Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . . 7
3.1. Roles and Realities . . . . . . . . . . . . . . . . . . . 7
3.2. Types Of Mailing Lists . . . . . . . . . . . . . . . . . . 8
3.3. Current MLM Effects On Signatures . . . . . . . . . . . . 9
4. Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 11
4.1. Author-Related Signing . . . . . . . . . . . . . . . . . . 11
4.2. Verification Outcomes at Receivers . . . . . . . . . . . . 12
4.3. Handling Choices at Receivers . . . . . . . . . . . . . . 12
4.4. Wrapping A Non-Participating MLM . . . . . . . . . . . . . 12
5. Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 13
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. DKIM Author Domain Signing Practices . . . . . . . . . . . 13
5.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 14
5.4. Author-Related Signing . . . . . . . . . . . . . . . . . . 14
5.5. Verification Outcomes at MLMs . . . . . . . . . . . . . . 15
5.6. Pros and Cons of Signature Removal . . . . . . . . . . . . 15
5.7. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 17
5.8. Verification Outcomes at Final Receiving Sites . . . . . . 18
5.9. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 18
5.10. Handling Choices at Receivers . . . . . . . . . . . . . . 19
6. DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8.1. Authentication Results When Relaying . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Appendix B. Example Scenarios . . . . . . . . . . . . . . . . . . 26
B.1. MLMs and ADSP . . . . . . . . . . . . . . . . . . . . . . 26
B.2. MLMs and FBLs . . . . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27
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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
identifiers?
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
signature?
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.
In general there are, in relation to DKIM, two categories of MLMs:
participating and non-participating. As both types have their own
issues regarding DKIM-signed messages that are either handled or
produced by them (or both), they are discussed in separate sections.
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
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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
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 are enumerated
and described in Section 3.3. This does not consider consider
changes the MTA might make independent of what changes the MLM
chooses to apply.
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 RFC5322.From
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
themselves.
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
(see [EMAIL-ARCH], Section 5.3). However, the fact that the
RFC5322.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
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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
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 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.
See Section 6 for additional discussion.
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
implementation. This is done to emphasize the functional
independence of MLM services and responsibilities from those of an
MTA.
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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
document.
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. '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 [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. Feedback Loop References
FBLs tend to use the ARF ([I-D.DRAFT-IETF-MARF-BASE]) or the IODEF
([IODEF]) format.
2.5. 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.
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
involved.
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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 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
DKIM-signed.
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 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 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
widest range of possible interactions 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 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: Altering the RFC5322.Subject field by adding a list-
specific prefix or suffix will invalidate the signer's signature
if that header field was covered by a hash of 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 (though see notes about the
"h=" tag in Section 3.5 of [DKIM]). Therefore this is seen as
less of a concern.
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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 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
will render any exisitng 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 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.
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, an MLM subscriber cannot expect signatures applied before
hte message was processed by the MLM to be valid. 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 verifiy. There
exist DKIM modules that incorrectly penalize messages with signatures
that do not validate, so this may have have detrimental effects
outside of the author's control. (Additional discussion of this is
below.) This problem could be compounded further 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 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 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. 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
MLMs.
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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 signing decision
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 behaviour.
Note that the [DKIM] specification explicitly directs verifiers 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. 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 5.6
below.)
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 provide DKIM signing services.
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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
ADSP-aware.
5.1. General
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]).
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
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 DKIM body hash, but
this is not workable for [MIME] messages and moreover has security
considerations (see Section 3.5 of [DKIM]). Its use is therefore
discouraged.
There is currently no header field proposed for relaying general list
policy 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 needed.
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
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
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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 first signing 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
verifier can either discard the message (i.e. accept the message at
the [SMTP] level but discard it without delivery) or conduct an SMTP
rejection 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, and disallow or present a warning
to 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.
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.
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 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
forwarded around either by MLMs or users, should come from a
different mail stream than a stream that serves a broader purpose.
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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, however, 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 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.
5.6. Pros and Cons of Signature Removal
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,
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.
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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
message;
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 to the contrary.
The MLM could re-evaluate exisiting 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
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
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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.
The use of MLM signatures will likely be used by recipient systems to
recognize list mail and gives 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 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, one signed and one not, which can confuse DKIM-
aware verifiers and receivers.
As is typical with DKIM signing, the MLM signature must be generated
only after all modifications the MLM wishes to apply have been
completed. Failing to do so generates a signature that can not be
expected to validate.
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
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.
Such MLMs are advised to ensure the signature's header hash will
cover:
o Any [AUTH-RESULTS] fields added by the MLM;
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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].
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.
Some concern has been expressed about an MLM applying its signature
to unsigned mail, which some verifiers or receivers might interpret
as conferring more authority to the message content. The working
group feels this is no different than present-day lists relaying
traffic and affixing RFC5322.Subject tags or similar, and thus it
doesn't 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.
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 all domains 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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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 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 or remove all unsigned externally-
applied Authentication-Results header fields, or 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, a receiver may 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 5.7.2
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
"discardable". In such cases where the submission fails that test,
the receiver 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
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 about issues with DKIM
infrastructure back to signers. 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 to do so.
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.
[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.
9.2. Informative References
[AUTH-RESULTS]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[DKIM-DEPLOYMENT]
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.
[DKIM-OVERVIEW]
Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
Identified Mail (DKIM) Service Overview", RFC 5585,
July 2009.
[DKIM-UPDATE]
Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
Signatures -- Update", RFC 5672, August 2009.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[ENHANCED]
Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[I-D.DRAFT-IETF-MARF-BASE]
Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", I-D DRAFT-
IETF-MARF-BASE, April 2010.
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[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
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[LIST-URLS]
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.
[MIME-TYPES]
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, John Levine, 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
Problem:
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
receiver
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
Problem:
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
manager
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
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
US
Phone: +1 415 946 3800
Email: msk@cloudmark.com
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