Comments inline.
A mass of small, localized suggestions. Most are merely wording improvements,
to tighten things up or clarify things a bit better.
A few substantive points, with suggested revision text, where possible.
There are more such points than I expected, later in the document, as things
seek to get more normative.
d/
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.
As the... Practices
->
Based on deployment experience with DKIM, this Best Current Practices document
provides guidance for the use of DKIM with scenarios that include Mailing List
Managers (MLMs).
1.1. Background
DKIM signatures permit an agent of the email architecture (see
[EMAIL-ARCH]) to make a claim of responsibility for a message by
affixing a validated domain-level identifier to the message as it
passes through a gateway. Although not the only possibility, this is
gateway??? I don't think so, given the email-arch reference.
perhaps:
as it is processed by an email actor
most commonly done as a message passes through a Mail Transport Agent
through a boundary mail Transport Agent
(MTA) as it departs an Administrative Mail Domain (ADMD) toward the
general Internet.
as it departs an Administrative Mail Domain (ADMD) across the open Internet.
{{ when departing, the act of departing puts it into the public Internet, not
'towards' it. }}
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.
Further, the MTA might make changes that are independent of those the MLM
applies.
The DKIM specification document deliberately refrains from the notion
specification document -> signing specification
refrains -> rejects
{{ "This document does not require the value of the SDID or AUID to match an
identifier in any other message header field." That's an overt dis-sociation,
not a passive ignoring. }}
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
any identifier -> any other identifier
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.
Some MLM behaviours are well-established and their effects on DKIM signature
validity can be argued as frustrating wider DKIM adoption. Still, those
behaviors are not standards violation. Hence the best approach for a BCP
effort
is to specify practices for all parties involved, defining the minimum changes
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
has important implications for use of DKIM.
{{ The above paragraph strikes me as the strongest and clearest description of
the underlying tension for this topic. I suggest moving it sooner in the
document and somehow highlighting it. Then again, it's still Section 1, so
maybe
its current location is fine. }}
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.
An open issue that is addressed by this document is the ways a signature might
be used by a recipient's evaluation module, after the message has gone through
a
mailing list and might or might have been rendered invalid. The document also
considers how invalidation might have happened.
{{ also note switch to non-normative vocabulary. }}
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
one of the final
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.
meaning... -> meaning that for them the FBL report is not actionable, might be
confusing and is thus undesirable.
See Section 6 for additional discussion.
FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) format.
format -> formats
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.
consensus shows a preference toward imposing -> the preference is to impose
{{ referring to consensus invites the reader to declare their disagreement; the
alternative language is more generic... }}
Wherever possible, MLMs will be conceptually decoupled from MTAs
MLMs will be -> the document's discussion of MLMs is
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
theoretical -> predictive
{{ perhaps slightly less inviting of being dismissed by the skeptical reader? }}
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.
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.
{{ stray thought: since the real concern is breakage, wouldn't it make sense
to
focus on DKIM-Hostile, rather than DKIM-Friendly? Friendly largely means being
passive. }}
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
This document...identify groups
->
A "message stream" identifies a group
ADMD that are distinct in intent, origin and/or use, and partition
and it partitions
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).
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
MLM-like agents -> intermediaries.
{{ what does 'MLM-like' mean? }}
impacts -> effects
DKIM-aware environment.
3.1. Roles and Realities
In DKIM parlance, there are several key roles in the transit of a
In DKIM parlance, -> Across DKIM activities,
{{ generic flag is raised: having this document repeat definitions from other
documents invites divergence. }}
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.
{{ And indeed, here's an example of divergence. In email-arch, the author does
not (necessarily) perform submission. Submission is performed by the
Originator, in email-arch... }}
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).
{{ "relays it towards its destinations" is more like the definition of a...
relay. }}
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
MTA -> boundary MTA
general -> public
Internet and the receiver's ADMD. Note that any agent that
handles a signed message could conduct verification; this document
could -> can
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.
{{ email-arch: "2.2.4. Receiver The Receiver performs final delivery" }}
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.
3.2. Types Of Mailing Lists
There are four common MLM implementation modes:
{{ mumble. given the focus of this document, i'm guessing this is one set of
terms that really /does/ warrant discussion directly in the document... }}
aliasing: An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one
that makes no changes to a message as it redistributes; any
to a message as -> to the message itself, as
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.
body at all and only [MAIL] trace header fields are added.
->
body or header at all, except for the addition of [MAIL] trace header fields.
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
message. -> message transit.
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
This is a special case of... act as an intermediary.
->
This is not a "mediator" in terms of [EMAIL-ARCH], since it originates the
message, but after creation, its message processing and posting behavior
otherwise do match the MLM paradigm.
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.
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
steps -> phases
{{ methinks there are lots of 'steps' here, inside each phase... }}
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
can make typically -> typically make
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.
impacts of each -> effects of
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
included -> included in the hash
signature. [DKIM] lists RFC5322.Subject as one that should be
covered, so this is expected to be an issue for any list that
covered, -> covered and it contains important user-visible text,
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.
{{ "less"? seems like it means it is not a concern at all. }}
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.
have been signed -> have been included in the signature hash
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.
{{ I'd have thought that citing the l= capability here would be appropriate,
for
completeness. }}
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
in cases -> in some cases
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
a few scattered -> some
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.
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
{{ I don't know what to suggest, but this section presumes that we can
reasonably expect authors to know about DKIM issues and in particular to know
that some MLMs screw up signatures. But we already know that virtually no
author is aware of any of these issues, nevermind know what it means to be
'cautious'. Mumble. }}
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
{{ I'm going to quibble and say that the problematic modules are not really
part
of DKIM. If there's misbehaving software, let's not blame any part of DKIM...
So, perhaps: }}
There exists...control
->
Some mail filtering software incorrectly penalizes a message containing a DKIM
signature that fails verification. This may have detrimental effects outside
of
the author's control.
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.
could -> can / were -> are / applied -> apply / published -> publishes
{{ we live in an active world... }}
{{ by the way, does a term like "strict ADSP policy" have an obvious and
precise
meaning? I suspect not, so that more fully explaining what is meant might be
safer. }}
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
a sub-domain -> a signing and author sub-domain
{{ Since ADSP is being cited, the issue is more than what is earlier defined as
a message stream, since it requires consonance with the From: field. }}
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.)
{{ Hmmm. Seems like the normative language needs to be directed at the site
administrators and not the authors. So... }}
Therefore, site administrators wishing to employ ADSP with a "discardable"
setting, SHOULD separate the controlled mail stream warranting this handling
from other mail streams that are less controlled, such as personal mail that
transits MLMs.
4.2. Verification Outcomes at Receivers
There does not appear to be a reliable way to determine that a piece
does not appear to be a -> is no
{{ we know 'truth' here, so let's not be iffy about it... }}
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.
{{ what does it mean to "be prepared", here? Perhaps this is similar to above
and the directive that is needed is along the lines of: }}
Sites whose users subscribe to non-participating MLMs SHOULD ensure that such
user mail streams are not subject to strict DKIM-related handling policies.
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.
In order to exempt some mail from the expectation of signature verification, as
discussed in Section 4.1, receiving ADMDs would need to register
non-participating lists and confirm that mail transited them. However such an
approach requires excessive effort and even then is likely to be unreliable.
Hence it is not a scalable solution.
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.
{{ Even with an ADSP directive, it's outside of the scope of DKIM. So... }}
Any treatment of a verification failure as having special meaning is a
violation
of the basic DKIM signing specification. The only valid, standardized basis
for
going beyond that specification is with specific ADSP direction.
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
to adding -> for adding
MLM is to "wrap" it, or in essence place it between other DKIM-aware
{{ it?? what is the referrent? }}
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
{{ submissions? isn't this meant to refer to the receive-side of the MLM?
since
'submission' is an email term of art, this reference is confusing. }}
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.
{{ overall, I think I understand the intent of this sub-section but find the
text confusing and the guidance more limited than is likely to be helpful. I
think all that's needed is a bit more beef, rather than any change in intent. }}
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.
regarding DKIM-signed mail that transits an MLM that is DKIM-aware
5.1. General
As DKIM becomes more widely deployed, it is highly desirable that MLM
software adopt more DKIM-friendly processing.
{{ I suggested deleting the above. It doesn't add anything. }}
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]).
{{ Suggest breaking the above into two or more sentences. I nearly ran out of
breath reading it... }}
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
arrange to affix to outgoing messages expressions -> affix 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,
It is RECOMMENDED that 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.
{{ Mumble. The logic leading to this recommendation is reasonable, but there is
no way it will have any meaningful effect and I've no idea what else to
suggest. }}
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.
{{ Specs and BCPs do not make statements about wg consensus. Rather they
simply
document normative directives. All references to consensus should be changed to
simple normative declarations. }}
Where the above practice is not observed and "discardable" mail
{{ "above practice is not observed"? Sorry but I don't know what that's
referring to. }}
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
{{ Is this describing anything different than would/should take place for mail
that did NOT go througha list? The text seems to be describing a special case
but in fact it isn't. It's just an ADSP failure. }}
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
{{ this strikes me as one of the more useful sub-sections in the document... }}
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,
applied -> created
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
create a mail stream specifically used for generating signatures when
sending traffic to MLMs.
"signed message from an author".
{{all messages are from the author. what distinctive condition is this trying
to describe? messages signed with the author domain? I don't understand the
point of the "As such" sentence. The rest of the paragraph is also confusing
to
me. I'm not sure what to suggest to make it clearer. }}
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.
come from a different mail stream
->
signed with a different mail stream identifier
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
list registry -> list subscription registry
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.
Note that this also uses a DKIM signature beyond its formal semantics, since a
signature does not validate the RFC5322.From address.
As described, the MLM MAY conduct DKIM verification of a signed
message to attempt to confirm the identity of the author. Although
{{ Hmmm. This really does go entirely beyond DKIM semantics. Is the text
meaning to imply that any signature validates any From: field? Generally, the
document ought to distinguish between what can/should be done today within
existing specifications, versus what would be enhancements beyond them. }}
it is a common and intuitive conclusion, not all signed mail will
include an author signature (see [ADSP]). MLM implementers SHOULD
not all... will include
->
few signed messages will include
{{ state the affirmative and the likely long-term reality. }}
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.
{{ As soon as a document like this puts forward a hypothetical, exploratory
design choice, it should document likely tradeoffs. For example, here the
assumption is that the author will know what the list should be and that the
list is stable. Neither is likely to be true. }}
A message that cannot be thus authenticated MAY be held for
moderation or rejected outright.
{{ The follow-on problem with this line of specification is that providing
normative language lends more claim of certitude and benefit than is justified.
I strongly suggest removing normative language from this type of discussion.
The discussion is an exploration and should cast things in those terms. Hence,
MAY -> might.}}
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
may or may -> might or might
trust the content of that header field depending on their own a
priori knowledge of the operation of the ADMD generating (and,
preferably, signing) that header field. See [AUTH-RESULTS] for
further discussion.
5.6. Signature Removal Issues
A message that arrives signed with DKIM means some domain prior to
MLM Input has made a claim of some responsibility for the message.
An obvious benefit to leaving the input-side signatures intact, then,
is to preserve that chain of responsibility of the message so that
{{ hmmm. it's not really a chain. this is not a model of transitive trust but
of "packaged" trust that is preserved. So, perhaps: }}
to preserve that chain of responsibility
->
to preserve that original assertion of responsibility
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.
{{ in fact, the actual work is only in comparing the hashes, not in performing
any additional public key computations. }}
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
Per the...MLM
->
Per the discussion in [AUTH-RESULTS], a receiver's putting any faith in the
veracity of that header field requires a priori assessment of the agent that
created it. Absent that assessment, a receiver cannot interpret the field as
valid.
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
{{ huh? "verify their own authenticity"? what exactly does this mean and how
does the DKIM signing spec support it? }}
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
signer -> signer that is covering the Auth-Results field
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.
{{ I thought FBL behavior required prior registration. }}
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
{{ seems like this "policy" applies to any receiving MLM and not just ones that
do signing. That is, it's about incoming messages and not outgoing ones. }}
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])
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
using a DNS domain matching what will be used in the "d=" tag of the
DKIM signature it will add to the new message
->
using that DNS domain matching the one used in the "d=" tag of the
DKIM signature that is added by the MLM
This could -> This can
DKIM signature it will add to the new message
verifiers or receivers to identify the DKIM signature that was added
by the MLM. This is not required, however; it is believed the
{{ this is describing a policy that is otherwise undocumented and undeclared.
If there is an explicit relationship between a d= name and a list-post name, it
should be declared explicitly, such as with a special header-field defined to
assert the relationship. }}
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.
{{ This implies that there is semantics or security 'protection' in the choice
of header-fields that are hashed. None of this is valid in terms of the
current
DKIM Signing specification. In order to make it valid, there needs to be an
additional specification defining it and specifying how a recipient can know
that the added semantics apply. }}
A DKIM-aware resending MLM SHOULD sign the entire message after being
after being -> after the message is
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
{{ "cannot be expected" is odd language to use about verification. either is
will verify or it won't. how can this be statistical, other than later, in
transit vagaries? }}
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.
{{ after re-reading this paragraph a few times, I find I don't know what it is
adding and also the next-to-last sentence confuses me. }}
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
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
{{ "a complaint from a user about a posting to a list" I think I don't know
what
this means. "a posting"? }}
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.
{{ this implies a rather important point that should be made explicitly and
perhaps strongly: MLMs should register with FBLs. }}
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
{{ The passive tense of this sentence winds up confusing me. "is suggested"?
by
whom and where? Given that the second part negates it, I now don't really know
what is done, recommended or what. }}
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.
{{ I tend to discourage directly discussing work that is underway. The text is
quickly overtaken by event. Perhaps:
As mechanisms are available for reporting forensive details about DKIM
verification failures, MLMs will benefit from their use.
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.
7. IANA Considerations
This document includes no IANA actions.
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.
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
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