< draft-ietf-dkim-mailinglists-08.txt   draft-ietf-dkim-mailinglists-09.txt >
DKIM Working Group M. Kucherawy DKIM Working Group M. Kucherawy
Internet-Draft Cloudmark Internet-Draft Cloudmark
Intended status: BCP April 27, 2011 Intended status: BCP May 9, 2011
Expires: October 29, 2011 Expires: November 10, 2011
DKIM And Mailing Lists DKIM And Mailing Lists
draft-ietf-dkim-mailinglists-08 draft-ietf-dkim-mailinglists-09
Abstract Abstract
DomainKeys Identified Mail (DKIM) allows an administrative mail DomainKeys Identified Mail (DKIM) allows an administrative mail
domain (ADMD) to assume some responsibility for a message. Based on domain (ADMD) to assume some responsibility for a message. Based on
deployment experience with DKIM, this Best Current Practices document deployment experience with DKIM, this Best Current Practices document
provides guidance for the use of DKIM with scenarios that include provides guidance for the use of DKIM with scenarios that include
Mailing List Managers (MLMs). {DKIM 12} Mailing List Managers (MLMs).
Status of this Memo Status of this Memo
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This Internet-Draft will expire on October 29, 2011. This Internet-Draft will expire on November 10, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Notes to Editor and Reviewers . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. MLMs In Infrastructure . . . . . . . . . . . . . . . . . . 4
2.2. MLMs In Infrastructure . . . . . . . . . . . . . . . . . . 5 1.3. Feedback Loops And Other Bi-Lateral Agreements . . . . . . 5
2.3. Feedback Loops And Other Bi-Lateral Agreements . . . . . . 6 1.4. Document Scope and Goals . . . . . . . . . . . . . . . . . 5
2.4. Document Scope and Goals . . . . . . . . . . . . . . . . . 6 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Messaging Terms . . . . . . . . . . . . . . . . . . . . . 7
3.2. Messaging Terms . . . . . . . . . . . . . . . . . . . . . 8 2.3. DKIM-Specific References . . . . . . . . . . . . . . . . . 7
3.3. DKIM-Specific References . . . . . . . . . . . . . . . . . 8 2.4. 'DKIM-Friendly' . . . . . . . . . . . . . . . . . . . . . 7
3.4. 'DKIM-Friendly' . . . . . . . . . . . . . . . . . . . . . 8 2.5. Message Streams . . . . . . . . . . . . . . . . . . . . . 7
3.5. Message Streams . . . . . . . . . . . . . . . . . . . . . 8 3. Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . . 8
4. Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . . 9 3.1. Roles and Realities . . . . . . . . . . . . . . . . . . . 8
4.1. Roles and Realities . . . . . . . . . . . . . . . . . . . 9 3.2. Types Of Mailing Lists . . . . . . . . . . . . . . . . . . 9
4.2. Types Of Mailing Lists . . . . . . . . . . . . . . . . . . 10 3.3. Current MLM Effects On Signatures . . . . . . . . . . . . 10
4.3. Current MLM Effects On Signatures . . . . . . . . . . . . 11 4. Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 13
5. Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 14 4.1. Author-Related Signing . . . . . . . . . . . . . . . . . . 13
5.1. Author-Related Signing . . . . . . . . . . . . . . . . . . 14 4.2. Verification Outcomes at Receivers . . . . . . . . . . . . 14
5.2. Verification Outcomes at Receivers . . . . . . . . . . . . 15 4.3. Handling Choices at Receivers . . . . . . . . . . . . . . 14
5.3. Handling Choices at Receivers . . . . . . . . . . . . . . 15 4.4. Wrapping A Non-Participating MLM . . . . . . . . . . . . . 14
5.4. Wrapping A Non-Participating MLM . . . . . . . . . . . . . 15 5. Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 15
6. Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 16 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.2. DKIM Author Domain Signing Practices . . . . . . . . . . . 15
6.2. DKIM Author Domain Signing Practices . . . . . . . . . . . 16 5.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 16
6.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 17 5.4. Exceptions To ADSP Recommendations . . . . . . . . . . . . 16
6.4. Exceptions To ADSP Recommendations . . . . . . . . . . . . 17 5.5. Author-Related Signing . . . . . . . . . . . . . . . . . . 16
6.5. Author-Related Signing . . . . . . . . . . . . . . . . . . 17 5.6. Verification Outcomes at MLMs . . . . . . . . . . . . . . 17
6.6. Verification Outcomes at MLMs . . . . . . . . . . . . . . 18 5.7. Signature Removal Issues . . . . . . . . . . . . . . . . . 18
6.7. Signature Removal Issues . . . . . . . . . . . . . . . . . 19 5.8. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 19
6.8. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 20 5.9. Verification Outcomes at Final Receiving Sites . . . . . . 20
6.9. Verification Outcomes at Final Receiving Sites . . . . . . 21 5.10. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 21
6.10. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 22 5.11. Handling Choices at Receivers . . . . . . . . . . . . . . 21
6.11. Handling Choices at Receivers . . . . . . . . . . . . . . 23 6. DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 23
7. DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 24 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26 8.1. Security Considerations from DKIM and ADSP . . . . . . . . 25
9.1. Security Considerations from DKIM and ADSP . . . . . . . . 26 8.2. Authentication Results When Relaying . . . . . . . . . . . 25
9.2. Authentication Results When Relaying . . . . . . . . . . . 26 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . . 27 9.2. Informative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . . 27 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 28
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 29 Appendix B. Example Scenarios . . . . . . . . . . . . . . . . . . 29
Appendix B. Example Scenarios . . . . . . . . . . . . . . . . . . 30 B.1. MLMs and ADSP . . . . . . . . . . . . . . . . . . . . . . 29
B.1. MLMs and ADSP . . . . . . . . . . . . . . . . . . . . . . 30 B.2. MLMs and FBLs . . . . . . . . . . . . . . . . . . . . . . 29
B.2. MLMs and FBLs . . . . . . . . . . . . . . . . . . . . . . 30 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31
1. Notes to Editor and Reviewers
This version of the memo contains notations such as "{DKIM 2}".
These correspond to DKIM working group issue tracker items. They
should be deleted prior to publication.
2. Introduction 1. Introduction
DomainKeys Identified Mail ([DKIM]) allows an Administrative Mail DomainKeys Identified Mail ([DKIM]) allows an Administrative Mail
Domain to take some responsibility for a [MAIL] message. This can be Domain to take some responsibility for a [MAIL] message. This can be
an author's organization, an operational relay (Mail Transfer Agent, an author's organization, an operational relay (Mail Transfer Agent,
or MTA) or one of their agents. Assertion of responsibility is made or MTA) or one of their agents. Assertion of responsibility is made
through a cryptographic signature. Message transit from author to through a cryptographic signature. Message transit from author to
recipient is through relays that typically make no substantive change recipient is through relays that typically make no substantive change
to the message content and thus preserve the validity of the DKIM to the message content and thus preserve the validity of the DKIM
signature. signature.
skipping to change at page 4, line 45 skipping to change at page 3, line 45
These and others are open questions for which there may be no These and others are open questions for which there may be no
definitive answers. However, based on experience since the definitive answers. However, based on experience since the
publication of [DKIM] and its gradual deployment, there are some publication of [DKIM] and its gradual deployment, there are some
views that are useful to consider and some recommended procedures. views that are useful to consider and some recommended procedures.
In general there are, in relation to DKIM, two categories of MLMs: In general there are, in relation to DKIM, two categories of MLMs:
participating and non-participating. As each type has its own issues participating and non-participating. As each type has its own issues
regarding DKIM-signed messages that are either handled or produced by regarding DKIM-signed messages that are either handled or produced by
them (or both), the types are discussed in separate sections. them (or both), the types are discussed in separate sections.
2.1. Background 1.1. Background
DKIM signatures permit an agent of the email architecture (see DKIM signatures permit an agent of the email architecture (see
[EMAIL-ARCH]) to make a claim of responsibility for a message by [EMAIL-ARCH]) to make a claim of responsibility for a message by
affixing a validated domain-level identifier to the message as it affixing a validated domain-level identifier to the message as it
passes through a relay. {DKIM 12} Although not the only possibility, passes through a relay. Although not the only possibility, this is
this is most commonly done as a message passes through a boundary most commonly done as a message passes through a boundary Mail
Mail Transport Agent (MTA) as it departs an Administrative Mail Transport Agent (MTA) as it departs an Administrative Mail Domain
Domain (ADMD) across the open Internet. {DKIM 12} (ADMD) across the open Internet.
A DKIM signature will fail to verify if a portion of the message 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 covered by one of its hashes is altered. An MLM commonly alters
messages to provide information specific to the mailing list for messages to provide information specific to the mailing list for
which it is providing service. Common modifications are enumerated which it is providing service. Common modifications are enumerated
and described in Section 4.3. However, note that MLMs vary widely in and described in Section 3.3. However, note that MLMs vary widely in
behaviour as well as often allowing subscribers to select individual behaviour as well as often allowing subscribers to select individual
behaviours. Further, the MTA might make changes that are independent behaviours. Further, the MTA might make changes that are independent
of those applied by the MLM. {DKIM 12} of those applied by the MLM.
The DKIM signing specification deliberately rejects the notion of The DKIM signing specification deliberately rejects the notion of
tying the signing {DKIM 12} domain (the "d=" tag in a DKIM signature) tying the signing domain (the "d=" tag in a DKIM signature) to any
to any other identifier {DKIM 12} within a message; any ADMD that other identifier within a message; any ADMD that handles a message
handles a message could sign it, regardless of its origin or author could sign it, regardless of its origin or author domain. In
domain. In particular, DKIM does not define any meaning to the particular, DKIM does not define any meaning to the occurrence of a
occurrence of a match between the content of a "d=" tag and the value match between the content of a "d=" tag and the value of, for
of, for example, a domain name in the RFC5322.From field, nor is example, a domain name in the RFC5322.From field, nor is there any
there any obvious degraded value to a signature where they do not obvious degraded value to a signature where they do not match. Since
match. Since any DKIM signature is merely an assertion of "some" any DKIM signature is merely an assertion of "some" responsibility by
responsibility by an ADMD, a DKIM signature added by an MLM has no an ADMD, a DKIM signature added by an MLM has no more, nor less,
more, nor less, meaning than a signature with any other "d=" value. meaning than a signature with any other "d=" value.
2.2. MLMs In Infrastructure 1.2. MLMs In Infrastructure
An MLM is an autonomous agent that takes delivery of a message and 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 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], 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 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 a message (in the non-digest case) is typically the same as that of
the original message, and that recipients perceive the message as the original message, and that recipients perceive the message as
"from" the original author rather than the MLM, creates confusion "from" the original author rather than the MLM, creates confusion
about responsibility and autonomy for the re-posted message. This about responsibility and autonomy for the re-posted message. This
has important implications for use of DKIM. {DKIM 12} has important implications for use of DKIM.
Section 4.3 describes some of the things MLMs commonly do that Section 3.3 describes some of the things MLMs commonly do that
produce broken signatures, thus reducing the perceived value of DKIM. produce broken signatures, thus reducing the perceived value of DKIM.
Further, while there are published standards that are specific to MLM Further, while there are published standards that are specific to MLM
behaviour (e.g. [MAIL], [LIST-ID] and [LIST-URLS]), their adoption behaviour (e.g. [MAIL], [LIST-ID] and [LIST-URLS]), their adoption
has been spotty at best. Hence, efforts to specify the use of DKIM 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. in the context of MLMs needs to be incremental and value-based.
Some MLM behaviours are well-established and their effects on DKIM Some MLM behaviours are well-established and their effects on DKIM
signature validity can be argued as frustrating wider DKIM adoption. signature validity can be argued as frustrating wider DKIM adoption.
Still, those behaviors are not standards violations. Hence, the best Still, those behaviors are not standards violations. Hence, the best
approach for a BCP effort is to specify practices for all parties approach for a BCP effort is to specify practices for all parties
involved, defining the minimum changes possible to MLMs themselves. involved, defining the minimum changes possible to MLMs themselves.
{DKIM 12}
A DKIM signature on a message is an expression of some responsibility A DKIM signature on a message is an expression of some responsibility
for the message taken by the signing domain. An open issue that is for the message taken by the signing domain. An open issue that is
addressed by this document is the ways a signature might be used by a 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 recipient's evaluation module, after the message has gone through a
mailing list and might or might not have been rendered invalid. The mailing list and might or might not have been rendered invalid. The
document also considers how invalidation might have happened. {DKIM document also considers how invalidation might have happened.
12}
Note that where in this document there is discussion of an MLM Note that where in this document there is discussion of an MLM
conducting validation of DKIM signatures or ADSP policies, the actual conducting validation of DKIM signatures or ADSP policies, the actual
implementation could be one where the validation is done by the MTA 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 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 by a trusted channel not specified here. See [AUTH-RESULTS] for a
discussion of this. This document does not favour any particular discussion of this. This document does not favour any particular
arrangement of these agents over another, but merely talks about the arrangement of these agents over another, but merely talks about the
MLM itself doing the work as a matter of simplicity. MLM itself doing the work as a matter of simplicity.
2.3. Feedback Loops And Other Bi-Lateral Agreements 1.3. Feedback Loops And Other Bi-Lateral Agreements
A Feedback Loop (FBL) is a bi-lateral agreement between two parties A Feedback Loop (FBL) is a bi-lateral agreement between two parties
to exchange reports of abuse. Typically, a sender registers with a to exchange reports of abuse. Typically, a sender registers with a
receiving site to receive abuse reports from that site for mail receiving site to receive abuse reports from that site for mail
coming from the sender. coming from the sender.
An FBL reporting address (i.e., an address to which FBL reports are An FBL reporting address (i.e., an address to which FBL reports are
sent) is part of this bi-lateral registration. Some FBLs require sent) is part of this bi-lateral registration. Some FBLs require
DKIM use by the registrant. DKIM use by the registrant.
See Section 7 for additional discussion. See Section 6 for additional discussion.
FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) formats. FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) formats.
{DKIM 12}
2.4. Document Scope and Goals 1.4. Document Scope and Goals
This document provides discussion on the above issues, to improve the This document provides discussion on the above issues, to improve the
handling of possible interactions between DKIM and MLMs. In general, handling of possible interactions between DKIM and MLMs. In general,
the preference is to impose changes to behaviour at the signer and the preference is to impose changes to behaviour at the signer and
verifier rather than at the MLM. {DKIM 12} verifier rather than at the MLM.
Wherever possible, the document's discussion of MLMs is conceptually Wherever possible, the document's discussion of MLMs is conceptually
decoupled from MTAs despite the very tight integration that is decoupled from MTAs despite the very tight integration that is
sometimes observed in implementation. This is done to emphasize the sometimes observed in implementation. This is done to emphasize the
functional independence of MLM services and responsibilities from functional independence of MLM services and responsibilities from
those of an MTA. {DKIM 12} those of an MTA.
Parts of this document explore possible changes to common practice by Parts of this document explore possible changes to common practice by
signers, verifiers and MLMs. The suggested enhancements are largely signers, verifiers and MLMs. The suggested enhancements are largely
predictive {DKIM 12} in nature, taking into account the current email predictive in nature, taking into account the current email
infrastructure, the facilities DKIM can provide as it gains wider infrastructure, the facilities DKIM can provide as it gains wider
deployment, and working group consensus. There is no substantial deployment, and working group consensus. There is no substantial
implementation history upon which these suggestions are based, and implementation history upon which these suggestions are based, and
the efficacy, performance and security characteristics of them have the efficacy, performance and security characteristics of them have
not yet been fully explored. not yet been fully explored.
3. Definitions 2. Definitions
3.1. Key Words 2.1. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS]. {DKIM 15} document are to be interpreted as described in [KEYWORDS].
3.2. Messaging Terms 2.2. Messaging Terms
See [EMAIL-ARCH] for a general description of the current messaging See [EMAIL-ARCH] for a general description of the current messaging
architecture, and for definitions of various terms used in this architecture, and for definitions of various terms used in this
document. document.
3.3. DKIM-Specific References 2.3. DKIM-Specific References
Readers are encouraged to become familiar with [DKIM] and [ADSP], Readers are encouraged to become familiar with [DKIM] and [ADSP],
which are core specification documents, as well as [DKIM-OVERVIEW] which are core specification documents, as well as [DKIM-OVERVIEW]
and [DKIM-DEPLOYMENT], which are DKIM's primary tutorial documents. and [DKIM-DEPLOYMENT], which are DKIM's primary tutorial documents.
3.4. 'DKIM-Friendly' 2.4. 'DKIM-Friendly'
The term "DKIM-Friendly" is used to describe an email intermediary The term "DKIM-Friendly" is used to describe an email intermediary
that, when handling a message, makes no changes to that message which that, when handling a message, makes no changes to that message which
cause valid [DKIM] signatures present on the message on input to fail cause valid [DKIM] signatures present on the message on input to fail
to verify on output. to verify on output.
Various features of MTAs and MLMs seen as helpful to users often have Various features of MTAs and MLMs seen as helpful to users often have
side effects that do render DKIM signatures unverifiable. These side effects that do render DKIM signatures unverifiable. These
would not qualify for this label. would not qualify for this label.
3.5. Message Streams 2.5. Message Streams
A "message stream" identifies a group of messages originating from A "message stream" identifies a group of messages originating from
within an {DKIM 12} ADMD that are distinct in intent, origin and/or within an ADMD that are distinct in intent, origin and/or use, and
use, and partitions them somehow (i.e., via {DKIM 12} changing the partitions them somehow (i.e., via changing the value in the "d=" tag
value in the "d=" tag value in the context of DKIM) so as to keep value in the context of DKIM) so as to keep them associated to users
them associated to users yet distinct in terms of their evaluation yet distinct in terms of their evaluation and handling by verifiers
and handling by verifiers or receivers. or receivers.
A good example might be user mail generated by a company's employees, A good example might be user mail generated by a company's employees,
versus operational or transactional mail that comes from automated versus operational or transactional mail that comes from automated
sources, versus marketing or sales campaigns. Each of these could sources, versus marketing or sales campaigns. Each of these could
have different security policies imposed against them, or there might have different security policies imposed against them, or there might
be a desire to insulate one from the other (e.g., a marketing be a desire to insulate one from the other (e.g., a marketing
campaign that gets reported by many spam filters could cause the campaign that gets reported by many spam filters could cause the
marketing stream's reputation to degrade without automatically marketing stream's reputation to degrade without automatically
punishing the transactional or user streams). punishing the transactional or user streams).
4. Mailing Lists and DKIM 3. Mailing Lists and DKIM
It is important to make some distinctions among different styles of It is important to make some distinctions among different styles of
intermediaries, their typical implementations, and the effects they intermediaries, their typical implementations, and the effects they
have in a DKIM-aware environment. {DKIM 12} have in a DKIM-aware environment.
4.1. Roles and Realities 3.1. Roles and Realities
Across DKIM activities, there are several key roles {DKIM 12} in the Across DKIM activities, there are several key roles in the transit of
transit of a message. Most of these are defined in [EMAIL-ARCH], but a message. Most of these are defined in [EMAIL-ARCH], but are
are reviewed here for quick reference. {DKIM 12} reviewed here for quick reference.
author: The agent that provided the content of the message being author: The agent that provided the content of the message being
sent through the system. The author delivers that content to the sent through the system. The author delivers that content to the
originator in order to begin a message's journey to its intended originator in order to begin a message's journey to its intended
final recipients. The author can be a human using an MUA (Mail final recipients. The author can be a human using an MUA (Mail
User Agent) or a common system utility such as "cron", etc. {DKIM User Agent) or a common system utility such as "cron", etc.
12}
originator: The agent that accepts a message from the author, originator: The agent that accepts a message from the author,
ensures it conforms to the relevant standards such as [MAIL], and ensures it conforms to the relevant standards such as [MAIL], and
then sends {DKIM 12} it toward its destination(s). This is often then sends it toward its destination(s). This is often referred
referred to as the Mail Submission Agent (MSA). to as the Mail Submission Agent (MSA).
signer: Any agent that affixes one or more DKIM signature(s) to a signer: Any agent that affixes one or more DKIM signature(s) to a
message on its way toward its ultimate destination. There is message on its way toward its ultimate destination. There is
typically a signer running at the MTA that sits between the typically a signer running at the MTA that sits between the
author's ADMD and the general Internet. The originator and/or author's ADMD and the general Internet. The originator and/or
author might also be a signer. author might also be a signer.
verifier: Any agent that conducts DKIM signature analysis. One is verifier: Any agent that conducts DKIM signature analysis. One is
typically running at the MTA that sits between the public Internet typically running at the MTA that sits between the public Internet
{DKIM 12} and the receiver's ADMD. Note that any agent that and the receiver's ADMD. Note that any agent that handles a
handles a signed message can conduct verification; {DKIM 12} this signed message can conduct verification; this document only
document only considers that action and its outcomes either at an considers that action and its outcomes either at an MLM or at the
MLM or at the receiver. Filtering decisions could be made by this receiver. Filtering decisions could be made by this agent based
agent based on verification results. on verification results.
receiver: The agent that is the final transit relay for the message receiver: The agent that is the final transit relay for the message
and performs final delivery to {DKIM 12} the recipient(s) of the and performs final delivery to the recipient(s) of the message.
message. Filtering decisions based on results made by the Filtering decisions based on results made by the verifier could be
verifier could be applied by the receiver. The verifier and the applied by the receiver. The verifier and the receiver could be
receiver could be the same agent. the same agent.
In the case of simple user-to-user mail, these roles are fairly In the case of simple user-to-user mail, these roles are fairly
straightforward. However, when one is sending mail to a list, which straightforward. However, when one is sending mail to a list, which
then gets relayed to all of that list's subscribers, the roles are 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 often less clear to the general user as particular agents may hold
multiple important but separable roles. The above definitions are multiple important but separable roles. The above definitions are
intended to enable more precise discussion of the mechanisms intended to enable more precise discussion of the mechanisms
involved. involved.
4.2. Types Of Mailing Lists 3.2. Types Of Mailing Lists
There are four common MLM implementation modes: There are four common MLM implementation modes:
aliasing: An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one aliasing: An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one
that makes no changes to the message itself as it redistributes; that makes no changes to the message itself as it redistributes;
any modifications are constrained to changes to the [SMTP] {DKIM any modifications are constrained to changes to the [SMTP]
12} envelope recipient list (RCPT commands) only. There are no envelope recipient list (RCPT commands) only. There are no
changes to the message header or body at all, except for the changes to the message header or body at all, except for the
addition of [MAIL] trace header fields. {DKIM 12} The output of addition of [MAIL] trace header fields. The output of such an MLM
such an MLM is considered to be a continuation of the author's is considered to be a continuation of the author's original
original message transit. {DKIM 12} An example of such an MLM is message transit. An example of such an MLM is an address that
an address that expands directly in the {DKIM 12} MTA, such as a expands directly in the MTA, such as a list of local system
list of local system administrators used for relaying operational administrators used for relaying operational or other internal-
or other internal-only messages. See also Section 3.9.2 of only messages. See also Section 3.9.2 of [SMTP].
[SMTP].
resending: A resending MLM (see Sections 5.2 and 5.3 of resending: A resending MLM (see Sections 5.2 and 5.3 of
[EMAIL-ARCH]) is one that may make changes to a message. The [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 output of such an MLM is considered to be a new message; delivery
of the original has been completed prior to distribution of the of the original has been completed prior to distribution of the
re-posted message. Such messages are often re-formatted, such as re-posted message. Such messages are often re-formatted, such as
with list-specific header fields or other properties, to with list-specific header fields or other properties, to
facilitate discussion among list subscribers. facilitate discussion among list subscribers.
authoring: An authoring MLM is one that creates the content being authoring: An authoring MLM is one that creates the content being
sent as well as initiating its transport, rather than basing it on sent as well as initiating its transport, rather than basing it on
one or more messages received earlier. This is not a "mediator" one or more messages received earlier. This is not a "mediator"
in terms of [EMAIL-ARCH] since it originates the message, but in terms of [EMAIL-ARCH] since it originates the message, but
after creation, its message processing and posting behavior after creation, its message processing and posting behavior
otherwise do match the MLM paradigm. Typically {DKIM 12} replies otherwise do match the MLM paradigm. Typically replies are not
are not generated, or if they are, they go to a specific recipient generated, or if they are, they go to a specific recipient and not
and not back to the list's full set of recipients. Examples back to the list's full set of recipients. Examples include
include newsletters and bulk marketing mail. newsletters and bulk marketing mail.
digesting: A special case of the resending MLM is one that sends a digesting: A special case of the resending MLM is one that sends a
single message comprising an aggregation of recent MLM single message comprising an aggregation of recent MLM
submissions, which might be a message of [MIME] type "multipart/ submissions, which might be a message of [MIME] type "multipart/
digest" (see [MIME-TYPES]). This is obviously a new message but digest" (see [MIME-TYPES]). This is obviously a new message but
it may contain a sequence of original messages that may themselves it may contain a sequence of original messages that may themselves
have been DKIM-signed. have been DKIM-signed.
In the remainder of this document we distinguish two relevant steps, In the remainder of this document we distinguish two relevant steps,
corresponding to the following SMTP transactions: corresponding to the following SMTP transactions:
skipping to change at page 11, line 18 skipping to change at page 10, line 18
MLM Output: MLM (sending its reconstructed copy of the originating MLM Output: MLM (sending its reconstructed copy of the originating
user's message) is author; MLM's ADMD is originator and signer; user's message) is author; MLM's ADMD is originator and signer;
the ADMD of each subscriber of the list is a verifier; each the ADMD of each subscriber of the list is a verifier; each
subscriber is a receiver. subscriber is a receiver.
Much of this document focuses on the resending class of MLM as it has Much of this document focuses on the resending class of MLM as it has
the most direct conflict operationally with DKIM. the most direct conflict operationally with DKIM.
The dissection of the overall MLM operation into these two distinct The dissection of the overall MLM operation into these two distinct
phases allows the DKIM-specific {DKIM 12} issues with respect to MLMs phases allows the DKIM-specific issues with respect to MLMs to be
to be isolated and handled in a logical way. The main issue is that isolated and handled in a logical way. The main issue is that the
the repackaging and reposting of a message by an MLM is actually 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 construction of a completely new message, and as such the MLM is
introducing new content into the email ecosystem, consuming the introducing new content into the email ecosystem, consuming the
author's copy of the message and creating its own. When considered 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. 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 Some issues about these activities are discussed in Section 3.6.4 of
[MAIL] and in Section 3.4.1 of [EMAIL-ARCH]. [MAIL] and in Section 3.4.1 of [EMAIL-ARCH].
4.3. Current MLM Effects On Signatures 3.3. Current MLM Effects On Signatures
As described above, an aliasing MLM does not affect any existing As described above, an aliasing MLM does not affect any existing
signature, and an authoring MLM is always creating new content and signature, and an authoring MLM is always creating new content and
thus there is never an existing signature. However, the changes a thus there is never an existing signature. However, the changes a
resending MLM typically make affect {DKIM 12} the RFC5322.Subject resending MLM typically make affect the RFC5322.Subject header field,
header field, addition of some list-specific header fields, and/or addition of some list-specific header fields, and/or modification of
modification of the message body. The effects of each of these {DKIM the message body. The effects of each of these on DKIM verification
12} on DKIM verification are discussed below. are discussed below.
Subject tags: A popular feature of MLMs is the "tagging" of an Subject tags: A popular feature of MLMs is the "tagging" of an
RFC5322.Subject field by prefixing the field's contents with the RFC5322.Subject field by prefixing the field's contents with the
name of the list, such as "[example]" for a list called "example". name of the list, such as "[example]" for a list called "example".
Altering the RFC5322.Subject field on new submissions by adding a Altering the RFC5322.Subject field on new submissions by adding a
list-specific prefix or suffix will invalidate the signer's list-specific prefix or suffix will invalidate the signer's
signature if that header field was included in the hash when signature if that header field was included in the hash when
creating that signature. Section 5.5 of [DKIM] lists creating that signature. Section 5.5 of [DKIM] lists
RFC5322.Subject as one that should be covered as it contains RFC5322.Subject as one that should be covered as it contains
important user-visible text, so this is expected to be an issue important user-visible text, so this is expected to be an issue
for any list that makes such changes. {DKIM 12} for any list that makes such changes.
List-specific header fields: Some lists will add header fields List-specific header fields: Some lists will add header fields
specific to list administrative functions such as those defined in specific to list administrative functions such as those defined in
[LIST-ID] and [LIST-URLS], or the "Resent-" fields defined in [LIST-ID] and [LIST-URLS], or the "Resent-" fields defined in
[MAIL]. It is unlikely that a typical MUA would include such [MAIL]. It is unlikely that a typical MUA would include such
fields in an original message, and DKIM is resilient to the fields in an original message, and DKIM is resilient to the
addition of header fields in general (see notes about the "h=" tag addition of header fields in general (see notes about the "h=" tag
in Section 3.5 of [DKIM]). Therefore not seen as a concern. {DKIM in Section 3.5 of [DKIM]). Therefore not seen as a concern.
12}
Other header fields: Some lists will add or replace header fields Other header fields: Some lists will add or replace header fields
such as "Reply-To" or "Sender" in order to establish that the 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 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 the list is identified ("Sender") and any user replies go to the
list ("Reply-To"). If these fields were included in the original list ("Reply-To"). If these fields were included in the original
message, it is possible that one or more of them may have been message, it is possible that one or more of them may have been
included in the signature hash, and those {DKIM 12} signatures included in the signature hash, and those signatures will thus be
will thus be broken. broken.
Minor body changes: Some lists prepend or append a few lines to each Minor body changes: Some lists prepend or append a few lines to each
message to remind subscribers of an administrative URL for message to remind subscribers of an administrative URL for
subscription issues, or of list policy, etc. Changes to the body subscription issues, or of list policy, etc. Changes to the body
will alter the body hash computed at the DKIM verifier, so these will alter the body hash computed at the DKIM verifier, so these
will render any existing signatures that cover those portions of will render any existing signatures that cover those portions of
the message body unverifiable. [DKIM] includes the capability to the message body unverifiable. [DKIM] includes the capability to
limit the length of the body covered by its body hash so that limit the length of the body covered by its body hash so that
appended text will not interfere with signature validation, but appended text will not interfere with signature validation, but
this has security implications. {DKIM 12} this has security implications.
Major body changes: There are some MLMs that make more substantial Major body changes: There are some MLMs that make more substantial
changes to message bodies when preparing them for re-distribution, changes to message bodies when preparing them for re-distribution,
such as adding, deleting, reordering, or reformatting [MIME] such as adding, deleting, reordering, or reformatting [MIME]
parts, "flattening" HTML messages into plain text, or inserting parts, "flattening" HTML messages into plain text, or inserting
{DKIM 9} headers or footers within HTML messages. Most or all of headers or footers within HTML messages. Most or all of these
these changes will invalidate a DKIM signature. changes will invalidate a DKIM signature.
MIME part removal: Some MLMs that are MIME-aware will remove large MIME part removal: Some MLMs that are MIME-aware will remove large
MIME parts from submissions and replace them with URLs to reduce MIME parts from submissions and replace them with URLs to reduce
the size of the distributed form of the message and to prevent the size of the distributed form of the message and to prevent
inadvertent automated malware delivery. Except in some cases inadvertent automated malware delivery. Except in some cases
where {DKIM 12} a body length limit is applied in generation of where a body length limit is applied in generation of the DKIM
the DKIM signature, the signature will be broken. signature, the signature will be broken.
There reportedly still exist some {DKIM 12} mailing lists in There reportedly still exist some mailing lists in operation that are
operation that are actually run manually by a human list manager, actually run manually by a human list manager, whose workings in
whose workings in preparing a message for distribution could include preparing a message for distribution could include the above or even
the above or even some other changes. some other changes.
In general, absent a general movement by MLM developers and operators In general, absent a general movement by MLM developers and operators
toward more DKIM-friendly practices, an MLM subscriber cannot expect toward more DKIM-friendly practices, an MLM subscriber cannot expect
signatures applied before the message was processed by the MLM to be signatures applied before the message was processed by the MLM to be
valid on delivery to a receiver. Such an evolution is not expected valid on delivery to a receiver. Such an evolution is not expected
in the short term due to general development and deployment inertia. in the short term due to general development and deployment inertia.
Moreover, even if an MLM currently passes messages unmodified such Moreover, even if an MLM currently passes messages unmodified such
that author signatures validate, it is possible that a configuration that author signatures validate, it is possible that a configuration
change or software upgrade to that MLM will cause that no longer to change or software upgrade to that MLM will cause that no longer to
be true. be true.
5. Non-Participating MLMs 4. Non-Participating MLMs
This section contains a discussion of issues regarding sending DKIM- This section contains a discussion of issues regarding sending DKIM-
signed mail to or through an MLM that is not DKIM-aware. signed mail to or through an MLM that is not DKIM-aware.
Specifically, the header fields introduced by [DKIM] and Specifically, the header fields introduced by [DKIM] and
[AUTH-RESULTS] carry no special meaning to such an MLM. [AUTH-RESULTS] carry no special meaning to such an MLM.
5.1. Author-Related Signing 4.1. Author-Related Signing
In an idealized world, if an author knows that the MLM to which a In an idealized world, if an author knows that the MLM to which a
message {DKIM 12} is being sent is a non-participating resending MLM, message is being sent is a non-participating resending MLM, the
the author SHOULD be cautious when deciding whether or not to send a author SHOULD be cautious when deciding whether or not to send a
signed message to the list {DKIM 9}. The MLM could make a change signed message to the list. The MLM could make a change that would
that would invalidate the author's signature but not remove it prior invalidate the author's signature but not remove it prior to re-
to re-distribution. Hence, list recipients would receive a message distribution. Hence, list recipients would receive a message
purportedly from the author but bearing a DKIM signature that would purportedly from the author but bearing a DKIM signature that would
not verify. Some mail filtering software incorrectly penalizes a not verify. Some mail filtering software incorrectly penalizes a
message containing a DKIM signature that fails verification. This message containing a DKIM signature that fails verification. This
may have {DKIM 12} detrimental effects outside of the author's may have detrimental effects outside of the author's control.
control. (Additional discussion of this is below.) This problem can (Additional discussion of this is below.) This problem can be
be compounded if there are receivers that apply signing {DKIM 12} compounded if there are receivers that apply signing policies (e.g.,
policies (e.g., [ADSP]) and the author publishes any kind of strict [ADSP]) and the author publishes any kind of strict policy, i.e., a
policy, i.e., a policy that requests that receivers reject or policy that requests that receivers reject or otherwise deal severely
otherwise deal severely with non-compliant messages. {DKIM 12} with non-compliant messages.
For domains that do publish strict ADSP policies, the originating For domains that do publish strict ADSP policies, the originating
site SHOULD use a separate message stream (see Section 3.5), such as site SHOULD use a separate message stream (see Section 2.5), such as
a signing and author subdomain {DKIM 12}, for the "personal" mail -- a signing and author subdomain, for the "personal" mail -- a
a subdomain that is different from domain(s) used for other mail subdomain that is different from domain(s) used for other mail
streams. This allows each to develop an independent reputation, and streams. This allows each to develop an independent reputation, and
more stringent policies (including ADSP) can be applied to the mail 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 stream(s) that do not go through mailing lists or perhaps do not get
signed at all. signed at all.
However, all of this presupposes a level of infrastructure However, all of this presupposes a level of infrastructure
understanding that is not expected to be common. Thus, it will be understanding that is not expected to be common. Thus, it will be
incumbent upon site administrators to consider how support of users incumbent upon site administrators to consider how support of users
wishing to participate in mailing lists might be accomplished as DKIM wishing to participate in mailing lists might be accomplished as DKIM
achieves wider adoption. achieves wider adoption.
In general, the more strict practices and policies are likely to be 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 successful only for the mail streams subject to the most end-to-end
control by the originating organization. That typically excludes control by the originating organization. That typically excludes
mail going through MLMs. Therefore, site administrators wishing to mail going through MLMs. Therefore, site administrators wishing to
employ ADSP with a "discardable" setting SHOULD separate the employ ADSP with a "discardable" setting SHOULD separate the
controlled mail stream warranting this handling from other mail controlled mail stream warranting this handling from other mail
streams that are less controlled, such as personal mail that transits streams that are less controlled, such as personal mail that transits
MLMs. (See also in Section 6.7 below.) {DKIM 12} MLMs. (See also in Section 5.7 below.)
5.2. Verification Outcomes at Receivers 4.2. Verification Outcomes at Receivers
There is no reliable way to {DKIM 12} determine that a piece of mail There is no reliable way to determine that a piece of mail arrived
arrived via a non-participating MLM. Sites whose users subscribe to via a non-participating MLM. Sites whose users subscribe to non-
non-participating MLMs SHOULD ensure that such user mail streams are participating MLMs SHOULD ensure that such user mail streams are not
not subject to strict DKIM-related handling policies. {DKIM 12} subject to strict DKIM-related handling policies.
5.3. Handling Choices at Receivers 4.3. Handling Choices at Receivers
In order to exempt some mail from the expectation of signature In order to exempt some mail from the expectation of signature
verification, as discussed in Section 5.1, receiving ADMDs would need verification, as discussed in Section 4.1, receiving ADMDs would need
to register non-participating lists and confirm that mail transited to register non-participating lists and confirm that mail transited
them. However, such an approach requires excessive effort and even them. However, such an approach requires excessive effort and even
then is likely to be unreliable. Hence, it is not a scalable then is likely to be unreliable. Hence, it is not a scalable
solution. {DKIM 12} solution.
Any treatment of a verification failure as having special meaning is Any treatment of a verification failure as having special meaning is
a violation of the basic DKIM signing specification. The only valid, a violation of the basic DKIM signing specification. The only valid,
standardized basis for going beyond that specification is with standardized basis for going beyond that specification is with
specific ADSP direction. {DKIM 12} specific ADSP direction.
Use of restrictive domain policies such as [ADSP] "discardable" Use of restrictive domain policies such as [ADSP] "discardable"
presents an additional challenge. In that case, when a message is presents an additional challenge. In that case, when a message is
unsigned or the signature can no longer be verified, discarding of unsigned or the signature can no longer be verified, discarding of
the message is requested. There is no exception in the policy for a 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 message that may have been altered by an MLM, nor is there a reliable
way to identify such mail. Therefore, participants SHOULD honour the way to identify such mail. Therefore, participants SHOULD honour the
policy and disallow the message. policy and disallow the message.
5.4. Wrapping A Non-Participating MLM 4.4. Wrapping A Non-Participating MLM
One approach for adding DKIM support to an otherwise non- One approach for adding DKIM support to an otherwise non-
participating MLM is to "wrap" the MLM, or in essence place it participating MLM is to "wrap" the MLM, or in essence place it
between other DKIM-aware components (such as MTAs) that provide some between other DKIM-aware components (such as MTAs) that provide some
DKIM services. For example, the ADMD operating a non-participating DKIM services. For example, the ADMD operating a non-participating
MLM could have its DKIM verifier act on messages from list MLM could have its DKIM verifier act on messages from list
subscribers, enforcing some of the features and recommendations of subscribers, enforcing some of the features and recommendations of
Section 6 on behalf of the MLM, and the MTA or MSA receiving the MLM 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. {DKIM Output could also add a DKIM signature for the MLM's domain.
12}
6. Participating MLMs 5. Participating MLMs
This section contains a discussion of issues regarding DKIM-signed This section contains a discussion of issues regarding DKIM-signed
mail that transits an MLM which is DKIM-aware. mail that transits an MLM which is DKIM-aware.
6.1. General 5.1. General
Changes that merely add new header fields, such as those specified by Changes that merely add new header fields, such as those specified by
[LIST-ID], [LIST-URLS] and [MAIL], are generally the most friendly to [LIST-ID], [LIST-URLS] and [MAIL], are generally the most friendly to
a DKIM-participating email infrastructure. Their addition by an MLM a DKIM-participating email infrastructure. Their addition by an MLM
{DKIM 12} will not affect any existing DKIM signatures unless those will not affect any existing DKIM signatures unless those fields were
fields were already present and covered by a signature's hash, or a already present and covered by a signature's hash, or a signature was
signature {DKIM 12} was created specifically to disallow their created specifically to disallow their addition (see the note about
addition (see the note about "h=" in Section 3.5 of [DKIM]). "h=" in Section 3.5 of [DKIM]).
However, the practice of applying headers and footers to message However, the practice of applying headers and footers to message
bodies is common and not expected to fade regardless of what bodies is common and not expected to fade regardless of what
documents this or any standards body might produce. This sort of documents this or any standards body might produce. This sort of
change will invalidate the signature on a message where the body hash change will invalidate the signature on a message where the body hash
covers the entire message. Thus, the following sections also discuss covers the entire message. Thus, the following sections also discuss
and suggest other processing alternatives. and suggest other processing alternatives.
A possible mitigation to this incompatibility is use of the "l=" tag 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 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 this is not workable for [MIME] messages; moreover, it has security
considerations (see Section 3.5 of [DKIM]). Its use is therefore considerations (see Section 3.5 of [DKIM]). Its use is therefore
discouraged. discouraged.
Expressions of list-specific policy (e.g., rules for participation, Expressions of list-specific policy (e.g., rules for participation,
small advertisements, etc.) are often added to outgoing messages by small advertisements, etc.) are often added to outgoing messages by
MLM operators. There is currently no header field proposed for MLM operators. There is currently no header field proposed for
relaying such general operational MLM details apart from what relaying such general operational MLM details apart from what
[LIST-URLS] already supports. This sort of information is commonly [LIST-URLS] already supports. This sort of information is commonly
included footer text appended to the body of the message, or header included footer text appended to the body of the message, or header
text prepended above the original body {DKIM 9}. It is RECOMMENDED text prepended above the original body. It is RECOMMENDED that
that periodic, automatic mailings to the list to remind subscribers periodic, automatic mailings to the list to remind subscribers of
of list policy, and it is otherwise RECOMMENDED that the use of list policy, and it is otherwise RECOMMENDED that the use of standard
standard header fields to express list operation parameters be header fields to express list operation parameters be applied rather
applied rather than body changes. {DKIM 12} These periodic mailings than body changes. These periodic mailings will be repetitive, of
will be repetitive, of course, but by being generally the same each course, but by being generally the same each time they can be easily
time they can be easily filtered if desired. filtered if desired.
6.2. DKIM Author Domain Signing Practices 5.2. DKIM Author Domain Signing Practices
ADSP {DKIM 9} presents a particular challenge. An author domain ADSP presents a particular challenge. An author domain posting a
posting a policy of "discardable" imposes a very tight restriction on policy of "discardable" imposes a very tight restriction on the use
the use of mailing lists, essentially constraining that domain's of mailing lists, essentially constraining that domain's users to
users to lists operated by aliasing MLMs only; any MLM that alters a lists operated by aliasing MLMs only; any MLM that alters a message
message from such a domain or removes its signature subjects the from such a domain or removes its signature subjects the message to
message to severe action by verifiers or receivers. A resending severe action by verifiers or receivers. A resending MLM SHOULD
{DKIM 12} MLM SHOULD reject outright any mail from an author whose reject outright any mail from an author whose domain posts such a
domain posts such a policy, as those messages likely to be discarded policy, as those messages likely to be discarded or rejected by any
or rejected by any ADSP-aware recipients. See also the discussion in ADSP-aware recipients. See also the discussion in Section 5.3.
Section 6.3. {DKIM 9}
Where such rejection of "discardable" mail is not enforced, and such Where such rejection of "discardable" mail is not enforced, and such
mail arrives to a {DKIM 12} verifier that applies ADSP checks which mail arrives to a verifier that applies ADSP checks which fail, the
fail, the message SHOULD either be discarded (i.e. accept the message message SHOULD either be discarded (i.e. accept the message at the
at the [SMTP] level but discard it without delivery) or rejected by [SMTP] level but discard it without delivery) or rejected by
returning a 5xx error code. In the latter case, some advice for how 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 to conduct the rejection in a potentially meaningful way can be found
in Section 6.11. in Section 5.11.
See also Appendix B.5 of [ADSP] for further discussion. See also Appendix B.5 of [ADSP] for further discussion.
6.3. Subscriptions 5.3. Subscriptions
At subscription time, an ADSP-aware MLM SHOULD check for a published At subscription time, an ADSP-aware MLM SHOULD check for a published
ADSP record for the new subscriber's domain. If the policy specifies ADSP record for the new subscriber's domain. If the policy specifies
"discardable", the MLM SHOULD disallow the subscription or present a "discardable", the MLM SHOULD disallow the subscription or present a
warning that the subscriber's submissions to the mailing list might warning that the subscriber's submissions to the mailing list might
not be deliverable to some recipients because of the subscriber's not be deliverable to some recipients because of the subscriber's
ADMD's published policy. ADMD's published policy.
Of course, such a policy record could be created {DKIM 12} after Of course, such a policy record could be created after subscription,
subscription, so this is not a universal solution. An MLM so this is not a universal solution. An MLM implementation MAY do
implementation MAY do periodic checks of its subscribers and issue periodic checks of its subscribers and issue warnings where such a
warnings where such a policy is detected, or simply check upon each policy is detected, or simply check upon each submission.
submission.
6.4. Exceptions To ADSP Recommendations 5.4. Exceptions To ADSP Recommendations
Where an ADMD has established some out-of-band trust agreement with Where an ADMD has established some out-of-band trust agreement with
another ADMD such that an Authentication-Results field applied by one another ADMD such that an Authentication-Results field applied by one
is trusted by the other, the above recommendations for MLM operation is trusted by the other, the above recommendations for MLM operation
with respect to ADSP do not apply because it is then possible to with respect to ADSP do not apply because it is then possible to
establish whether or not a valid author signature can be inferred establish whether or not a valid author signature can be inferred
even if one is not present on receipt. even if one is not present on receipt.
6.5. Author-Related Signing 5.5. Author-Related Signing
An important consideration is that authors rarely have any direct An important consideration is that authors rarely have any direct
influence over the management of an MLM. Specifically, the behavior influence over the management of an MLM. Specifically, the behavior
of an intermediary (e.g., an MLM that is not careful about filtering of an intermediary (e.g., an MLM that is not careful about filtering
out junk mail or being diligent about unsubscription requests) can out junk mail or being diligent about unsubscription requests) can
trigger recipient complaints that reflect back on those agents that trigger recipient complaints that reflect back on those agents that
appear to be responsible for the message, in this case an author via appear to be responsible for the message, in this case an author via
the address found in the RFC5322.From field. In the future, as DKIM the address found in the RFC5322.From field. In the future, as DKIM
signature outputs (i.e., the signing domain) are used as inputs to signature outputs (i.e., the signing domain) are used as inputs to
reputation modules, there may be a desire to insulate one's reputation modules, there may be a desire to insulate one's
reputation from influence by the unknown results of sending mail reputation from influence by the unknown results of sending mail
through an MLM. In that case, authors SHOULD create a mail stream through an MLM. In that case, authors SHOULD create a mail stream
specifically used for generating signatures when sending traffic to specifically used for generating signatures when sending traffic to
MLMs. {DKIM 12} MLMs.
This suggestion can be made more general. Mail that is of a This suggestion can be made more general. Mail that is of a
transactional or generally end-to-end nature, and not likely to be transactional or generally end-to-end nature, and not likely to be
forwarded around either by MLMs or users, SHOULD be signed with a forwarded around either by MLMs or users, SHOULD be signed with a
different mail stream identifier from a stream that serves more different mail stream identifier from a stream that serves more
varied uses. {DKIM 12} varied uses.
6.6. Verification Outcomes at MLMs 5.6. Verification Outcomes at MLMs
MLMs typically attempt to authenticate messages posted through them. MLMs typically attempt to authenticate messages posted through them.
They usually do this through the trivial (and insecure) means of They usually do this through the trivial (and insecure) means of
verifying the RFC5322.From field email address (or, less frequently, verifying the RFC5322.From field email address (or, less frequently,
the RFC5321.MailFrom parameter) against a list subscription registry. the RFC5321.MailFrom parameter) against a list subscription registry.
{DKIM 12} DKIM enables a stronger form of authentication: {DKIM 9} DKIM enables a stronger form of authentication: The MLM can require
The MLM can require that messages using a given RFC5322.From address that messages using a given RFC5322.From address also have a DKIM
also have a DKIM signature with a corresponding "d=" domain. This signature with a corresponding "d=" domain. This feature would be
feature would be somewhat similar to using ADSP, except that the somewhat similar to using ADSP, except that the requirement for it
requirement for it would be imposed by the MLM and not the author's would be imposed by the MLM and not the author's organization.
organization.
(Note, however, that this goes beyond DKIM's documented semantics. (Note, however, that this goes beyond DKIM's documented semantics.
It is presented as a possible workable enhancement.) {DKIM 12} It is presented as a possible workable enhancement.)
As described, the MLM might conduct DKIM verification of a signed As described, the MLM might conduct DKIM verification of a signed
message to attempt to confirm the identity of the author. Although message to attempt to confirm the identity of the author. Although
it is a common and intuitive conclusion, few signed messages will it is a common and intuitive conclusion, few signed messages will
include an author {DKIM 12} signature (see [ADSP]). MLM implementers include an author signature (see [ADSP]). MLM implementers adding
adding such support would have accommodate this. For example, an MLM such support would have accommodate this. For example, an MLM might
might be designed to accommodate a list of possible signing domains be designed to accommodate a list of possible signing domains (the
(the "d=" portion of a DKIM signature) for a given author, and "d=" portion of a DKIM signature) for a given author, and determine
determine at verification time if any of those are present. This at verification time if any of those are present. This enables a
enables a more reliable method of authentication at the expense of more reliable method of authentication at the expense of having to
having to store a mapping of authorized signing domains for store a mapping of authorized signing domains for subscribers and
subscribers and trusting that it will be kept current. {DKIM 12} trusting that it will be kept current.
A message that cannot be thus authenticated MAY be held for A message that cannot be thus authenticated MAY be held for
moderation or rejected outright. moderation or rejected outright.
This logic could apply to any list operation, not just list This logic could apply to any list operation, not just list
submission. In particular, this improved authentication MAY apply to submission. In particular, this improved authentication MAY apply to
subscription, unsubscription, and/or changes to subscriber options subscription, unsubscription, and/or changes to subscriber options
that are sent via email rather than through an authenticated, that are sent via email rather than through an authenticated,
interactive channel such as the web. interactive channel such as the web.
In the case of verification of signatures on submissions, MLMs SHOULD In the case of verification of signatures on submissions, MLMs SHOULD
add an [AUTH-RESULTS] header field to indicate the signature(s) add an [AUTH-RESULTS] header field to indicate the signature(s)
observed on the submission as it arrived at the MLM and what the observed on the submission as it arrived at the MLM and what the
outcome of the evaluation was. Downstream agents might or might not outcome of the evaluation was. Downstream agents might or might not
trust the content of that header {DKIM 12} field depending on their trust the content of that header field depending on their own a
own a priori knowledge of the operation of the ADMD generating (and, priori knowledge of the operation of the ADMD generating (and,
preferably, signing) that header field. See [AUTH-RESULTS] for preferably, signing) that header field. See [AUTH-RESULTS] for
further discussion. further discussion.
6.7. Signature Removal Issues 5.7. Signature Removal Issues
A message that arrives signed with DKIM means some domain prior to A message that arrives signed with DKIM means some domain prior to
MLM Input has made a claim of some responsibility for the message. MLM Input has made a claim of some responsibility for the message.
An obvious benefit to leaving the input-side signatures intact, then, An obvious benefit to leaving the input-side signatures intact, then,
is to preserve that original assertion of responsibility for the is to preserve that original assertion of responsibility for the
message so that the receivers of the final message have an message so that the receivers of the final message have an
opportunity to evaluate the message with that information available opportunity to evaluate the message with that information available
to them. {DKIM 12} to them.
However, if the MLM is configured to make changes to the message However, if the MLM is configured to make changes to the message
prior to re-posting that would invalidate the original signature(s), prior to re-posting that would invalidate the original signature(s),
further action is RECOMMENDED to prevent invalidated signatures from further action is RECOMMENDED to prevent invalidated signatures from
arriving at final recipients, possibly triggering unwarranted filter arriving at final recipients, possibly triggering unwarranted filter
actions. (Note, however, that such filtering actions are plainly actions. (Note, however, that such filtering actions are plainly
wrong; [DKIM] stipulates that an invalid signature is to be treated wrong; [DKIM] stipulates that an invalid signature is to be treated
as no signature at all.) as no signature at all.)
A possible solution would be to: A possible solution would be to:
skipping to change at page 19, line 51 skipping to change at page 18, line 48
3. Remove all existing [AUTH-RESULTS] fields (optional); 3. Remove all existing [AUTH-RESULTS] fields (optional);
4. Add an [AUTH-RESULTS] header field to the message to indicate the 4. Add an [AUTH-RESULTS] header field to the message to indicate the
results of the above; results of the above;
5. Remove all previously-evaluated DKIM signatures; 5. Remove all previously-evaluated DKIM signatures;
6. Affix a new signature that includes in in its hashes the entire 6. Affix a new signature that includes in in its hashes the entire
message on the output side, including the Authentication-Results message on the output side, including the Authentication-Results
header field just added (see Section 6.8). header field just added (see Section 5.8).
Removing the original signature(s) seems particularly appropriate Removing the original signature(s) seems particularly appropriate
when the MLM knows it is likely to invalidate any or all of them due 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 to the nature of the reformatting it will do. This avoids false
negatives at the list's subscribers in their roles as receivers of negatives at the list's subscribers in their roles as receivers of
the message; although [DKIM] stipulates that an invalid signature is the message; although [DKIM] stipulates that an invalid signature is
the same as no signature, it is anticipated that there will be some the same as no signature, it is anticipated that there will be some
implementations that ignore this advice. implementations that ignore this advice.
The MLM could re-evaluate existing signatures after making its The MLM could re-evaluate existing signatures after making its
message changes to determine whether or not any of them have been message changes to determine whether or not any of them have been
invalidated. The cost of this is reduced by the fact that, invalidated. The cost of this is reduced by the fact that,
presumably, the necessary public keys have already been downloaded presumably, the necessary public keys have already been downloaded
and one or both of the message hashes could be reused. and one or both of the message hashes could be reused.
Per the discussion in [AUTH-RESULTS], a receiver's choice to put any Per the discussion in [AUTH-RESULTS], a receiver's choice to put any
faith in the veracity of that header field requires an a priori faith in the veracity of that header field requires an a priori
assessment of the agent that created it. Absent that assessment, a assessment of the agent that created it. Absent that assessment, a
receiver cannot interpret the field as valid. Thus, the final receiver cannot interpret the field as valid. Thus, the final
recipients of the {DKIM 12} message have no way to verify on their recipients of the message have no way to verify on their own the
own the authenticity of the author's identity on that message. authenticity of the author's identity on that message. However, if
However, if that field is the only one on the message when the that field is the only one on the message when the verifier gets it,
verifier gets it, and the verifier explicitly trusts the signer that and the verifier explicitly trusts the signer that included the
included the Authentication-Results field in its header hash (in this Authentication-Results field in its header hash (in this case, the
case, the MLM), the verifier is in a position to believe that a valid MLM), the verifier is in a position to believe that a valid author
author signature was present on the message. {DKIM 12} signature was present on the message.
This can be generalized as follows: A receiver SHOULD consider only This can be generalized as follows: A receiver SHOULD consider only
[AUTH-RESULTS] fields bearing an authserv-id that appears in a list [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 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 hash of a [DKIM] signature added by a domain in the same trusted
list. list.
Since an aliasing MLM makes no substantive changes to a message, it Since an aliasing MLM makes no substantive changes to a message, it
need not consider the issue of signature removal as the original need not consider the issue of signature removal as the original
signatures should arrive at least to the next MTA unmodified. It is signatures should arrive at least to the next MTA unmodified. It is
possible that future domain-based reputations would prefer a more possible that future domain-based reputations would prefer a more
rich data set on receipt of a message, and in that case signature rich data set on receipt of a message, and in that case signature
removal would be undesirable. removal would be undesirable.
An authoring MLM is closed to outside submitters, thus much of this An authoring MLM is closed to outside submitters, thus much of this
discussion does not apply in that case. discussion does not apply in that case.
6.8. MLM Signatures 5.8. MLM Signatures
DKIM-aware resending MLMs and authoring MLMs SHOULD affix their own DKIM-aware resending MLMs and authoring MLMs SHOULD affix their own
signatures when distributing messages. The MLM is responsible for signatures when distributing messages. The MLM is responsible for
the alterations it makes to the original messages it is re-sending, the alterations it makes to the original messages it is re-sending,
and should express this via a signature. This is also helpful for 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 getting feedback from any FBLs that might be set up so that undesired
list mail can generate appropriate action. list mail can generate appropriate action.
MLM signatures will likely be used by recipient systems to recognize 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 list mail, and they give the MLM's ADMD an opportunity to develop a
good reputation for the list itself. good reputation for the list itself.
A signing MLM is, as any other MLM, free to omit redistribution of a 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 message if that message was not signed in accordance with its own
local configuration or policy. It could also redistribute but not local configuration or policy. It could also redistribute but not
sign such mail. However, selective signing is NOT RECOMMENDED; sign such mail. However, selective signing is NOT RECOMMENDED;
essentially that would create two message streams from the MLM, one essentially that would create two message streams from the MLM, one
signed and one not, which can confuse DKIM-aware verifiers and signed and one not, which can confuse DKIM-aware verifiers and
receivers. receivers.
A signing MLM could add a List-Post: {DKIM 12} header field (see A signing MLM could add a List-Post: header field (see [LIST-URLS])
[LIST-URLS]) using that DNS domain matching the one used in the "d=" using that DNS domain matching the one used in the "d=" tag of the
tag of the DKIM signature that is added by the MLM. This can be used DKIM signature that is added by the MLM. This can be used by
by {DKIM 12} verifiers or receivers to identify the DKIM signature verifiers or receivers to identify the DKIM signature that was added
that was added by the MLM. This is not required, however; it is by the MLM. This is not required, however; it is believed the
believed the reputation of the signer will be a more critical data reputation of the signer will be a more critical data point rather
point rather than this suggested binding. Furthermore, this is not a than this suggested binding. Furthermore, this is not a binding
binding recognized by any current specification document. recognized by any current specification document.
A DKIM-aware resending MLM SHOULD sign the entire message after the A DKIM-aware resending MLM SHOULD sign the entire message after the
message is prepared for distribution (i.e. the "MLM Output" from message is prepared for distribution (i.e. the "MLM Output" from
Section 4.2). Any other configuration might generate signatures that Section 3.2). Any other configuration might generate signatures that
will not validate. {DKIM 12} will not validate.
DKIM-aware authoring MLMs MUST sign the mail they send according to DKIM-aware authoring MLMs MUST sign the mail they send according to
the regular signing guidelines given in [DKIM]. the regular signing guidelines given in [DKIM].
One concern is that having an MLM apply its signature to unsigned One concern is that having an MLM apply its signature to unsigned
mail might cause some verifiers or receivers to interpret the mail might cause some verifiers or receivers to interpret the
signature as conferring more authority or authenticity to the message signature as conferring more authority or authenticity to the message
content than is defined by [DKIM]. This is an issue beyond MLMs and content than is defined by [DKIM]. This is an issue beyond MLMs and
primarily entails receive-side processing outside of the scope of primarily entails receive-side processing outside of the scope of
[DKIM]. It is nevertheless worth noting here. {DKIM 12} [DKIM]. It is nevertheless worth noting here.
6.9. Verification Outcomes at Final Receiving Sites 5.9. Verification Outcomes at Final Receiving Sites
In general, verifiers and receivers SHOULD treat a signed message In general, verifiers and receivers SHOULD treat a signed message
from an MLM like any other signed message; indeed, it would be from an MLM like any other signed message; indeed, it would be
difficult to discern any difference since specifications such as difficult to discern any difference since specifications such as
[LIST-URLS] and [LIST-ID] are not universally deployed and can be [LIST-URLS] and [LIST-ID] are not universally deployed and can be
trivially spoofed. trivially spoofed.
However, because the author domain will commonly be different from However, because the author domain will commonly be different from
the MLM's signing domain, there may be a conflict with [ADSP] as the MLM's signing domain, there may be a conflict with [ADSP] as
discussed in Section 5.3 and Section 6.7, especially where an ADMD discussed in Section 4.3 and Section 5.7, especially where an ADMD
has misused ADSP. has misused ADSP.
6.10. Use With FBLs 5.10. Use With FBLs
An FBL operator might wish to act on a complaint from a user about a An FBL operator might wish to act on a complaint from a user about a
message sent to a list. Some {DKIM 12} FBLs could choose to generate message sent to a list. Some FBLs could choose to generate feedback
feedback reports based on DKIM verifications in the subject message. reports based on DKIM verifications in the subject message. Such
Such operators SHOULD send a report to each domain with a valid operators SHOULD send a report to each domain with a valid signature
signature that has an FBL agreement established, as DKIM signatures that has an FBL agreement established, as DKIM signatures are claims
are claims of some responsibility for that message. Because authors of some responsibility for that message. Because authors generally
generally have limited control over the operation of a list, this have limited control over the operation of a list, this point makes
point makes MLM signing all the more important. MLM signing all the more important.
MLM operators SHOULD register with FBLs from major service providers. MLM operators SHOULD register with FBLs from major service providers.
In the context of DKIM, there SHOULD be an exchange of information In the context of DKIM, there SHOULD be an exchange of information
with the FBL provider including what signing domain the MLM will use, with the FBL provider including what signing domain the MLM will use,
if any. {DKIM 12} if any.
Where the FBL wishes to be more specific, it MAY act solely on a DKIM 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 signature where the signing domain matches the DNS domain found in a
List-Post: header field (or similar). List-Post: header field (or similar).
Use of FBLs in this way SHOULD be made explicit to list subscribers. 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 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 item by unsubscribing the user that was the apparent sender of the
offending message, advising subscribers of this in advance would help offending message, advising subscribers of this in advance would help
to avoid surprises later. to avoid surprises later.
A DKIM-signed message sent to an MLM, and then distributed to all of 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 a list's recipients, could result in a complaint from one of the
final recipients for some reason. This could be an actual complaint final recipients for some reason. This could be an actual complaint
from some subscriber that finds the message abusive or otherwise from some subscriber that finds the message abusive or otherwise
undesirable, or it {DKIM 12} could be an automated complaint such as undesirable, or it could be an automated complaint such as receiver
receiver detection of an invalidated DKIM signature or some other detection of an invalidated DKIM signature or some other condition.
condition. It could also be a complaint that results from It could also be a complaint that results from antagonistic
antagonistic behaviour, such as is common when a subscriber to a list behaviour, such as is common when a subscriber to a list is having
is having trouble unsubscribing, and then begins issuing complaints trouble unsubscribing, and then begins issuing complaints about all
about all submissions to the list. This would result in a complaint submissions to the list. This would result in a complaint being
being generated in the context of an FBL report back to the message generated in the context of an FBL report back to the message author.
author. However, the original author has no involvement in operation However, the original author has no involvement in operation of the
of the MLM itself, meaning the FBL report is not actionable, and is MLM itself, meaning the FBL report is not actionable, and is thus
thus undesirable. {DKIM 9, DKIM 12} undesirable.
6.11. Handling Choices at Receivers 5.11. Handling Choices at Receivers
A recipient that explicitly trusts signatures from a particular MLM A recipient that explicitly trusts signatures from a particular MLM
MAY wish to extend that trust to an [AUTH-RESULTS] header field MAY wish to extend that trust to an [AUTH-RESULTS] header field
signed by that MLM. The recipient MAY then do additional processing signed by that MLM. The recipient MAY then do additional processing
of the message, using the results recorded in the Authentication- of the message, using the results recorded in the Authentication-
Results header field instead of the original author's DKIM signature. Results header field instead of the original author's DKIM signature.
This includes possibly processing the message as per ADSP This includes possibly processing the message as per ADSP
requirements. requirements.
Receivers SHOULD ignore or remove all unsigned externally-applied Receivers SHOULD ignore or remove all unsigned externally-applied
Authentication-Results header fields, and those not signed by an ADMD 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 that can be trusted by the receiver. See Section 5 and Section 7 of
[AUTH-RESULTS] for further discussion. [AUTH-RESULTS] for further discussion.
Upon DKIM and ADSP evaluation during an SMTP session (a common Upon DKIM and ADSP evaluation during an SMTP session (a common
implementation), an agent MAY decide to reject a message during an implementation), an agent MAY decide to reject a message during an
SMTP session. If this is done, use of an [SMTP] failure code not SMTP session. If this is done, use of an [SMTP] failure code not
normally used for "user unknown" (550) is preferred; therefore, 554 normally used for "user unknown" (550) is preferred; therefore, 554
SHOULD be used. {DKIM 12} If the rejecting SMTP server supports SHOULD be used. If the rejecting SMTP server supports [ENHANCED]
[ENHANCED] status codes, it SHOULD make a distinction between status codes, it SHOULD make a distinction between messages rejected
messages rejected deliberately due to policy decisions rather than deliberately due to policy decisions rather than those rejected
those rejected because of other delivery issues {DKIM 9}. In because of other delivery issues. In particular, a policy rejection
particular, a policy rejection SHOULD be relayed using a 5.7.1 SHOULD be relayed using a 5.7.1 enhanced status code and some
enhanced status code and some appropriate wording in the text part of appropriate wording in the text part of the reply, in contrast to a
the reply, in contrast to a code of 5.1.1 indicating the user does code of 5.1.1 indicating the user does not exist. Those MLMs that
not exist. Those MLMs that automatically attempt to remove users automatically attempt to remove users with prolonged delivery
with prolonged delivery problems (such as account deletion) SHOULD problems (such as account deletion) SHOULD thus detect the difference
thus detect the difference between policy rejection and other between policy rejection and other delivery failures, and act
delivery failures, and act accordingly. SMTP servers doing so SHOULD accordingly. SMTP servers doing so SHOULD also use appropriate
also use appropriate wording in the text portion of the reply, wording in the text portion of the reply, perhaps explicitly using
perhaps explicitly using the string "ADSP" to facilitate searching of the string "ADSP" to facilitate searching of relevant data in logs.
relevant data in logs.
The preceding paragraph does not apply to an [ADSP] policy of The preceding paragraph does not apply to an [ADSP] policy of
"discardable". In such cases where the submission fails that test, "discardable". In such cases where the submission fails that test,
the receiver or verifier SHOULD discard the message but return an the receiver or verifier SHOULD discard the message but return an
SMTP success code, i.e. accept the message but drop it without SMTP success code, i.e. accept the message but drop it without
delivery. An SMTP rejection of such mail instead of the requested delivery. An SMTP rejection of such mail instead of the requested
discard action causes more harm than good. discard action causes more harm than good.
7. DKIM Reporting 6. DKIM Reporting
As mechanisms become available for reporting forensic details about As mechanisms become available for reporting forensic details about
DKIM verification failures, MLMs will benefit from their use. {DKIM DKIM verification failures, MLMs will benefit from their use.
12}
MLMs SHOULD apply DKIM failure reporting mechanisms as a method for MLMs SHOULD apply DKIM failure reporting mechanisms as a method for
providing feedback to signers about issues with DKIM infrastructure. providing feedback to signers about issues with DKIM infrastructure.
This is especially important for MLMs that implement DKIM This is especially important for MLMs that implement DKIM
verification as a mechanism for authentication of list configuration verification as a mechanism for authentication of list configuration
commands and submissions from subscribers. commands and submissions from subscribers.
8. IANA Considerations 7. IANA Considerations
This document includes no IANA actions. This document includes no IANA actions.
9. Security Considerations 8. Security Considerations
This document provides suggested or best current practices for use This document provides suggested or best current practices for use
with DKIM, and as such does not introduce any new technologies for with DKIM, and as such does not introduce any new technologies for
consideration. However, the following security issues should be consideration. However, the following security issues should be
considered when implementing the above practices. considered when implementing the above practices.
9.1. Security Considerations from DKIM and ADSP 8.1. Security Considerations from DKIM and ADSP
Readers should be familiar with the material in the Security Readers should be familiar with the material in the Security
Considerations in [DKIM], [ADSP] and [AUTH-RESULTS] as appropriate. Considerations in [DKIM], [ADSP] and [AUTH-RESULTS] as appropriate.
{DKIM 9}
9.2. Authentication Results When Relaying 8.2. Authentication Results When Relaying
Section 6 advocates addition of an [AUTH-RESULTS] header field to Section 5 advocates addition of an [AUTH-RESULTS] header field to
indicate authentication status of a message received as MLM Input. indicate authentication status of a message received as MLM Input.
Per Section 7.2 of [AUTH-RESULTS], receivers generally should not 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 trust such data without a good reason to do so, such as an a priori
agreement with the MLM's ADMD. agreement with the MLM's ADMD.
Such agreements are strongly advised to include a requirement that Such agreements are strongly advised to include a requirement that
those header fields be covered by a [DKIM] signature added by the those header fields be covered by a [DKIM] signature added by the
MLM's ADMD. MLM's ADMD.
10. References 9. References
10.1. Normative References 9.1. Normative References
[ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM [ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
Sender Signing Practises", RFC 5617, August 2009. Sender Signing Practises", RFC 5617, August 2009.
[AUTH-RESULTS] [AUTH-RESULTS]
Kucherawy, M., "Message Header Field for Indicating Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009. Message Authentication Status", RFC 5451, April 2009.
[DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", Identified Mail (DKIM) Signatures",
skipping to change at page 27, line 31 skipping to change at page 26, line 31
Crocker, D., "Internet Mail Architecture", RFC 5598, Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009. July 2009.
[KEYWORDS] [KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322, [MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008. October 2008.
10.2. Informative References 9.2. Informative References
[DKIM-DEPLOYMENT] [DKIM-DEPLOYMENT]
Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
"DomainKeys Identified Mail (DKIM) Development, Deployment "DomainKeys Identified Mail (DKIM) Development, Deployment
and Operations", I-D DRAFT-IETF-DKIM-DEPLOYMENT, and Operations", I-D DRAFT-IETF-DKIM-DEPLOYMENT,
January 2010. January 2010.
[DKIM-OVERVIEW] [DKIM-OVERVIEW]
Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
Identified Mail (DKIM) Service Overview", RFC 5585, Identified Mail (DKIM) Service Overview", RFC 5585,
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