DKIM Working Group M. Kucherawy
Internet-Draft Cloudmark
Intended status: BCP March 28, 2011
Expires: September 29, 2011
DKIM And Mailing Lists
draft-ietf-dkim-mailinglists-06
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) and describe the
Best Current Practices.
Status of this Memo
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This Internet-Draft will expire on September 29, 2011.
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described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . 15
5.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 15
5.4. Author-Related Signing . . . . . . . . . . . . . . . . . . 15
5.5. Verification Outcomes at MLMs . . . . . . . . . . . . . . 16
5.6. Signature Removal Issues . . . . . . . . . . . . . . . . . 17
5.7. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 18
5.8. Verification Outcomes at Final Receiving Sites . . . . . . 19
5.9. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 20
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
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, often invalidating DKIM signatures. The goal
for this document is to explore the use of DKIM for scenarios that
include intermediaries, and recommend Best Current Practices based on
acquired experience. 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
identifiers?
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
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
views that are useful to consider and some recommended procedures.
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
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 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
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(MTA) as it departs an Administrative Mail Domain (ADMD) toward the
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
Section 3.3 describes some of the things MLMs commonly do that
produce broken signatures, thus reducing the perceived value of DKIM.
Further, while there are published 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.
Other MLM behaviours 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 (in the non-digest case) 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
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has important implications for use of DKIM.
A DKIM signature on a message is an expression of some responsibility
for the message taken by the signing domain. An open 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 and how 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.
A DKIM-signed message sent to an MLM, and then distributed to all of
a list's recipients, could result in a complaint from one of the
recipients for some reason. This could be an actual complaint from
some subscriber that finds the message abusive or otherwise
undesirable, or it could be an automated complaint such as receiver
detection of an invalidated DKIM signature or some other condition.
It could also be a complaint that results from antagonistic
behaviour, such as is common when a subscriber to a list is having
trouble unsubscribing, and then begins issuing complaints about all
submissions to the list. This would result in a complaint being
generated in the context of an FBL report back to the message author.
However, the original author has no involvement in operation of the
MLM itself, meaning the FBL report is not actionable and thus
undesirable.
See Section 6 for additional discussion.
FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) format.
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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
independence of MLM services and responsibilities from those of an
MTA.
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
document.
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
receivers.
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 is
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 verification 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
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 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
submissions, 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
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
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 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 that cover those portions of
the message body 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, "flattening" 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 SHOULD 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 SHOULD use a separate message stream (see Section 2.4), 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" SHOULD NOT 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. Therefore, participants SHOULD honour 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
ADSP-aware.
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
discouraged.
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,
and otherwise RECOMMENDS use of standard header fields to express
list operation parameters rather than body changes. These periodic
mailings will be repetitive, of course, but by being generally the
same each time they can be easily filtered if desired.
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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 verifiers or receivers. It is the consensus of the
working group that a resending MLM SHOULD 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 SHOULD also 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 which fail,
the message SHOULD either be discarded (i.e. accept the message at
the [SMTP] level but discard it without delivery) or 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 SHOULD check for a published
ADSP record for the new subscriber's domain. If the policy specifies
"discardable", the MLM SHOULD disallow the subscription or present a
warning that the subscriber's submissions to the mailing list might
not be deliverable to some recipients because of the 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 MAY 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 SHOULD
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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 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 MAY 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 implementers SHOULD
accommodate such in their configurations. For example, an MLM might
be designed to accommodate 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 MAY be held for
moderation or rejected outright.
This logic could apply to any list operation, not just list
submission. In particular, this improved authentication MAY 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 SHOULD
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.
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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,
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
message;
2. Apply local policy to authenticate the identity of the author;
3. Remove all existing [AUTH-RESULTS] fields (optional);
4. Add an [AUTH-RESULTS] header field to the message to indicate the
results of the above;
5. Remove all previously-evaluated DKIM signatures;
6. Affix a new signature that covers the entire message on the
output side, including 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.
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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, if that field is 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.
This can be generalized as follows: A receiver SHOULD consider only
[AUTH-RESULTS] fields bearing an authserv-id that appears in a list
of sites the receiver trusts and which is also included in the header
hash of a [DKIM] signature added by a domain in the same trusted
list.
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 SHOULD 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 NOT RECOMMENDED;
essentially that would create two message streams from the MLM, one
signed and one not, which can confuse DKIM-aware verifiers and
receivers.
A signing MLM SHOULD add a List-Post: header field (see [LIST-URLS])
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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.
Such MLMs SHOULD ensure the signature's header hash will cover at
least:
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 SHOULD sign the entire message after being
prepared for distribution (i.e. the "MLM Output" from Section 3.2).
Any other configuration might generate signatures that cannot be
expected to validate. As with any other DKIM signing operation, the
choice of what portions of the header and body of the output message
should include those parts of the header and body for which the MLM
wishes to assert responsibility.
DKIM-aware authoring MLMs MUST 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 SHOULD 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 spoofed.
However, because the author domain will commonly be different from
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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 might 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 SHOULD 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 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 MAY 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
requirements.
Receivers SHOULD 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 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 and thus SHOULD be used. If the rejecting SMTP
server supports [ENHANCED] status codes, it SHOULD make a distinction
between messages rejected deliberately due to policy decisions rather
than those rejected because of other deliverability issues. In
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particular, a policy rejection SHOULD 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) SHOULD
thus detect the difference between policy rejection and other
delivery failures, and act accordingly. SMTP servers doing so SHOULD
also 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 or verifier SHOULD 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 SHOULD 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.
[AUTH-RESULTS]
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.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
9.2. Informative References
[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.
[ENHANCED]
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.
[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.
[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.
[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, 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
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 the MLMs 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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