draft-ietf-dkim-ssp-09.txt   draft-ietf-dkim-ssp-10.txt 
Network Working Group E. Allman Network Working Group E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Intended status: Standards Track J. Fenton Intended status: Standards Track J. Fenton
Expires: August 9, 2009 Cisco Systems, Inc. Expires: November 12, 2009 Cisco Systems, Inc.
M. Delany M. Delany
Yahoo! Inc. Yahoo! Inc.
J. Levine J. Levine
Taughannock Networks Taughannock Networks
February 5, 2009 May 11, 2009
DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP) DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
draft-ietf-dkim-ssp-09 draft-ietf-dkim-ssp-10
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 37 skipping to change at page 1, line 37
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
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This Internet-Draft will expire on August 9, 2009. This Internet-Draft will expire on November 12, 2009.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents in effect on the date of
(http://trustee.ietf.org/license-info) in effect on the date of publication of this document (http://trustee.ietf.org/license-info).
publication of this document. Please review these documents Please review these documents carefully, as they describe your rights
carefully, as they describe your rights and restrictions with respect and restrictions with respect to this document.
to this document.
Abstract Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email to permit verification of the authentication framework for email to permit verification of the
source and contents of messages. This document specifies an adjunct source and contents of messages. This document specifies an adjunct
mechanism to aid in assessing messages that do not contain a DKIM mechanism to aid in assessing messages that do not contain a DKIM
signature for the domain used in the author's address. It defines a signature for the domain used in the author's address. It defines a
record that can advertise whether a domain signs its outgoing mail, record that can advertise whether a domain signs its outgoing mail,
and how other hosts can access that record. and how other hosts can access that record.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4 2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4
2.1. Terms Imported from DKIM Signatures Specification . . . . 4 2.1. Terms Imported from DKIM Signatures Specification . . . . 4
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5 2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5 2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Author Domain Signing Practices . . . . . . . . . . . . . 5 2.6. Author Domain Signing Practices . . . . . . . . . . . . . 5
2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5 2.7. Author Domain Signature . . . . . . . . . . . . . . . . . 5
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 5
3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 6 3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 6
3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 7
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7 4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7 4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7
4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 8 4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 8
4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 9 4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 10 5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 10
5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 11 5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5.3. Authentication-Results Method Registry Update . . . . . . 11
6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 11 5.4. Authentication-Results Result Registry Update . . . . . . 11
6.2. DNS Considerations . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12 6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.2. DNS Considerations . . . . . . . . . . . . . . . . . . . . 14
7.1. References - Normative . . . . . . . . . . . . . . . . . . 13 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 15
7.2. References - Informative . . . . . . . . . . . . . . . . . 13 6.4. Inappropriate Application of Author Domain Signatures . . 15
Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 14 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
A.1. Domain and ADSP exist . . . . . . . . . . . . . . . . . . 14 7.1. References - Normative . . . . . . . . . . . . . . . . . . 15
A.2. Domain exists, ADSP does not exist . . . . . . . . . . . . 14 7.2. References - Informative . . . . . . . . . . . . . . . . . 16
A.3. Domain does not exist . . . . . . . . . . . . . . . . . . 15 Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 16
Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 15 A.1. Domain and ADSP exist . . . . . . . . . . . . . . . . . . 17
B.1. Single Location Domains . . . . . . . . . . . . . . . . . 15 A.2. Domain exists, ADSP does not exist . . . . . . . . . . . . 17
B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 15 A.3. Domain does not exist . . . . . . . . . . . . . . . . . . 17
B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 16 Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 17
B.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 16 B.1. Single Location Domains . . . . . . . . . . . . . . . . . 18
B.5. Domains with Independent Users and Liberal Use Policies . 17 B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 18
B.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 17 B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 19
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 17 B.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 19
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 17 B.5. Domains with Independent Users and Liberal Use Policies . 19
D.1. Changes since -ietf-dkim-08 . . . . . . . . . . . . . . . 17 B.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 19
D.2. Changes since -ietf-dkim-07 . . . . . . . . . . . . . . . 17 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 19
D.3. Changes since -ietf-dkim-06 . . . . . . . . . . . . . . . 18 Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 20
D.4. Changes since -ietf-dkim-05 . . . . . . . . . . . . . . . 18 D.1. Changes since -ietf-dkim-09 . . . . . . . . . . . . . . . 20
D.5. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 18 D.2. Changes since -ietf-dkim-08 . . . . . . . . . . . . . . . 20
D.6. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 18 D.3. Changes since -ietf-dkim-07 . . . . . . . . . . . . . . . 20
D.7. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 19 D.4. Changes since -ietf-dkim-06 . . . . . . . . . . . . . . . 20
D.8. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 20 D.5. Changes since -ietf-dkim-05 . . . . . . . . . . . . . . . 21
D.9. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 21 D.6. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 21
D.10. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 21 D.7. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 21
D.11. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 22 D.8. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 22
D.12. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 22 D.9. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 D.10. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 23
D.11. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 24
D.12. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 24
D.13. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
DomainKeys Identified Mail (DKIM) defines a mechanism by which email DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key, the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain. possession of the private key for the signing domain.
However, the legacy of the Internet is such that not all messages However, the legacy of the Internet is such that not all messages
will be signed, and the absence of a signature on a message is not an will be signed, and the absence of a signature on a message is not an
a priori indication of forgery. In fact, during early phases of a priori indication of forgery. In fact, during early phases of
deployment it is very likely that most messages will remain unsigned. deployment it is very likely that most messages will remain unsigned.
However, some domains might decide to sign all of their outgoing However, some domains might decide to sign all of their outgoing
mail, for example, to protect their brand names. It might be mail, for example, to protect their brand names. It might be
desirable for such domains to be able to advertise that fact to other desirable for such domains to be able to advertise that fact to other
hosts. This is the topic of Author Domain Signing Practices (ADSP). hosts. This is the topic of Author Domain Signing Practices (ADSP).
Hosts implementing this specification can inquire what Author Signing Hosts implementing this specification can inquire what Author Domain
Practices a domain advertises. This inquiry is called an Author Signing Practices a domain advertises. This inquiry is called an
Signing Practices check. Author Domain Signing Practices check.
The basic requirements for ADSP are given in [RFC5016]. This The basic requirements for ADSP are given in [RFC5016]. This
document refers extensively to [RFC4871] and assumes the reader is document refers extensively to [RFC4871] and assumes the reader is
familiar with it. familiar with it.
Requirements Notation: The key words "MUST", "MUST NOT", Requirements Notation: The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119] interpreted as described in [RFC2119]
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An "Alleged Author" is an Author Address of a message; it is An "Alleged Author" is an Author Address of a message; it is
"alleged" because it has not yet been checked. "alleged" because it has not yet been checked.
2.6. Author Domain Signing Practices 2.6. Author Domain Signing Practices
"Author Domain Signing Practices" (or just "practices") consist of a "Author Domain Signing Practices" (or just "practices") consist of a
machine-readable record published by the domain of an Alleged Author machine-readable record published by the domain of an Alleged Author
which includes statements about the domain's practices with respect which includes statements about the domain's practices with respect
to mail it sends with its domain in the From: line. to mail it sends with its domain in the From: line.
2.7. Author Signature 2.7. Author Domain Signature
An "author signature" is a Valid Signature that has the same domain
name in the DKIM signing identity as the domain name in the Author
Address. If the DKIM signing identity has a Local-part, it is be
identical to the Local-part in the Author Address. Following
[RFC5321], Local-part comparisons are case sensitive, but domain
comparisons are case insensitive.
For example, if a message has a Valid Signature, with the DKIM- An "Author Domain Signature" is a Valid Signature in which the domain
Signature field containing "i=a@domain.example", then domain.example name of the DKIM signing entity, the d= tag in the DKIM-Signature
is asserting that it takes responsibility for the message. If the header field, is the same as the domain name in the Author Address.
message's From: field contains the address "b@domain.example", that Following [RFC5321], domain name comparisons are case insensitive.
would mean that the message does not have a valid Author Signature.
Even though the message is signed by the same domain, it will not
satisfy ADSP that specifies "dkim=all" or "dkim=discardable".
Note: ADSP is incompatible with valid DKIM usage in which a signer For example, if the From: line address is bob@domain.example, and the
uses "i=" with values that are not the same as addresses in mail message has a Valid Signature, with the DKIM-Signature header field
headers. In that case, a possible workaround could be to add a containing "d=domain.example", then the message has an Author Domain
second DKIM signature a "d=" value that matches the Author Signature.
Address, but no "i=".
3. Operation Overview 3. Operation Overview
Domain owners publish ADSP information via a query mechanism such as Domain owners publish ADSP information via a query mechanism such as
the Domain Name System; specific details are given in Section 4.1. the Domain Name System; specific details are given in Section 4.1.
Hosts can look up the ADSP information of the domain(s) specified by Hosts can look up the ADSP information of the domain(s) specified by
the Author Address(es) as described in Section 4.3. If a message has the Author Address(es) as described in Section 4.3. If a message has
multiple Author Addresses the ADSP lookups SHOULD be performed multiple Author Addresses the ADSP lookups SHOULD be performed
independently on each address. This document does not address the independently on each address. This document does not address the
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3.2. ADSP Usage 3.2. ADSP Usage
Depending on the Author Domain(s) and the signatures in a message, a Depending on the Author Domain(s) and the signatures in a message, a
recipient gets varying amounts of useful information from each ADSP recipient gets varying amounts of useful information from each ADSP
lookup. lookup.
o If a message has no Valid Signature, the ADSP result is directly o If a message has no Valid Signature, the ADSP result is directly
relevant to the message. relevant to the message.
o If a message has an Author Signature, ADSP provides no benefit o If a message has an Author Domain Signature, ADSP provides no
relative to that domain since the message is already known to be benefit relative to that domain since the message is already known
compliant with any possible ADSP for that domain. to be compliant with any possible ADSP for that domain.
o If a message has a Valid Signature other than an Author Signature, o If a message has a Valid Signature other than an Author Domain
the receiver can use both the Signature and the ADSP result in its Signature, the receiver can use both the Signature and the ADSP
evaluation of the message. result in its evaluation of the message.
3.3. ADSP Results 3.3. ADSP Results
An ADSP lookup for an Author Address produces one of four possible An ADSP lookup for an Author Address produces one of four possible
results: results:
o Messages from this domain might or might not have an author o Messages from this domain might or might not have an author domain
signature. This is the default if the domain exists in the DNS signature. This is the default if the domain exists in the DNS
but no ADSP record is found. but no ADSP record is found.
o All messages from this domain are signed with an Author Signature. o All messages from this domain are signed with an Author Domain
Signature.
o All messages from this domain are signed with an Author Signature o All messages from this domain are signed with an Author Domain
and discardable, i.e., if a message arrives without a valid Author Signature and discardable, i.e., if a message arrives without a
Signature, the domain encourages the recipient(s) to discard it. valid Author Domain Signature, the domain encourages the
recipient(s) to discard it.
o This domain is out of scope, i.e., the domain does not exist in o This domain is out of scope, i.e., the domain does not exist in
the DNS. the DNS.
An ADSP lookup could terminate without producing any result if a DNS An ADSP lookup could terminate without producing any result if a DNS
lookup results in a temporary failure. lookup results in a temporary failure.
4. Detailed Description 4. Detailed Description
4.1. DNS Representation 4.1. DNS Representation
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[RFC4871] is used, modified to use WSP rather than FWS. Records not [RFC4871] is used, modified to use WSP rather than FWS. Records not
in compliance with that syntax or the syntax of individual tags in compliance with that syntax or the syntax of individual tags
described in Section 4.3 MUST be ignored (considered equivalent to a described in Section 4.3 MUST be ignored (considered equivalent to a
NODATA result) for purposes of ADSP, although they MAY cause the NODATA result) for purposes of ADSP, although they MAY cause the
logging of warning messages via an appropriate system logging logging of warning messages via an appropriate system logging
mechanism. If the RDATA contains multiple character strings, the mechanism. If the RDATA contains multiple character strings, the
strings are logically concatenated with no delimiters between the strings are logically concatenated with no delimiters between the
strings. strings.
Note: ADSP changes the "Tag=Value List" syntax from [RFC4871] to Note: ADSP changes the "Tag=Value List" syntax from [RFC4871] to
use WSP rather than FWS in its DNS records. Domains MUST NOT use WSP rather than FWS in its DNS records.
publish ADSP records with wildcard names. Wildcards within a
domain publishing ADSP records pose a particular problem, as Note: Domains MUST NOT publish ADSP records with wildcard names.
discussed in more detail in Section 6.3. Wildcards within a domain publishing ADSP records pose a
particular problem, as discussed in more detail in Section 6.3.
4.2. Publication of ADSP Records 4.2. Publication of ADSP Records
ADSP is intended to apply to all mail sent using the domain name ADSP is intended to apply to all mail sent using the domain name
string of an Alleged Author. string of an Alleged Author.
4.2.1. Record Syntax 4.2.1. Record Syntax
ADSP records use the "tag=value" syntax described in section 3.2 of ADSP records use the "tag=value" syntax described in section 3.2 of
[RFC4871], modified to use WSP rather than FWS. Every ADSP record [RFC4871], modified to use WSP rather than FWS. Every ADSP record
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Tags used in ADSP records are described below. Unrecognized tags Tags used in ADSP records are described below. Unrecognized tags
MUST be ignored. In the ABNF below, the WSP token, and the ALPHA and MUST be ignored. In the ABNF below, the WSP token, and the ALPHA and
DIGIT tokens are imported from [RFC5234]. DIGIT tokens are imported from [RFC5234].
dkim= Outbound signing practices for the domain (plain-text; dkim= Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows: REQUIRED). Possible values are as follows:
unknown The domain might sign some or all email. unknown The domain might sign some or all email.
all All mail from the domain is signed with an Author all All mail from the domain is signed with an Author Domain
Signature. Signature.
discardable All mail from the domain is signed with an Author discardable All mail from the domain is signed with an Author
Signature. Furthermore, if a message arrives without a valid Domain Signature. Furthermore, if a message arrives without a
Author Signature due to modification in transit, submission via valid Author Domain Signature due to modification in transit,
a path without access to a signing key, or any other reason, submission via a path without access to a signing key, or any
the domain encourages the recipient(s) to discard it. other reason, the domain encourages the recipient(s) to discard
it.
Any other values are treated as "unknown". Any other values are treated as "unknown".
ABNF: ABNF:
adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP
("unknown" / "all" / "discardable") ("unknown" / "all" / "discardable")
4.3. ADSP Lookup Procedure 4.3. ADSP Lookup Procedure
Hosts doing an ADSP lookup MUST produce a result that is semantically Hosts doing an ADSP lookup MUST produce a result that is semantically
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The initial entries in the registry comprise: The initial entries in the registry comprise:
+-------------+-----------------+ +-------------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+-------------+-----------------+ +-------------+-----------------+
| unknown | (this document) | | unknown | (this document) |
| all | (this document) | | all | (this document) |
| discardable | (this document) | | discardable | (this document) |
+-------------+-----------------+ +-------------+-----------------+
ADSP Outbound Signing Practices Registry Initial Values ADSP Outbound Signing Practices Registry Initial Values
5.3. Authentication-Results Method Registry Update
IANA is requested to add the following to the Email Authentication
Method Name Registry:
Method: dkim-adsp
Defined In: this memo
ptype: header
property: from
value: Contents of the [RFC5322] From: header field, with comments
removed
5.4. Authentication-Results Result Registry Update
IANA is requested to add or update the following in the Email
Authentication Result Name Registry:
Code: none
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: No DKIM author domain signing practises (ADSP) record was
published.
Code: pass
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: This message had an Author Domain Domain Signature that was
validated. (An ADSP check is not strictly required to be
performed for this result, since a valid Author Domain Signature
satisfies all possible ADSP policies.)
Code: unknown
Existing/New Code: new
Defined In: this memo
Auth Method: dkim-adsp
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "unknown".
Code: fail
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "all".
Code: discard
Existing/New Code: new
Defined In: this memo
Auth Method: dkim-adsp
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "discardable".
Code: nxdomain
Existing/New Code: new
Defined In: this memo
Auth Method: dkim-adsp
Meaning: Evaluating the ADSP for the Author's DNS domain indicated
that the Author's DNS domain does not exist.
Code: temperror
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: An ADSP record could not be retrieved due to some error
that is likely transient in nature, such as a temporary DNS error.
A later attempt may produce a final result.
Code: permerror
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: An ADSP record could not be retrieved due to some error
that is likely not transient in nature, such as a permanent DNS
error. A later attempt is unlikely to produce a final result.
6. Security Considerations 6. Security Considerations
Security considerations in the ADSP are mostly related to attempts on Security considerations in the ADSP are mostly related to attempts on
the part of malicious senders to represent themselves as authors for the part of malicious senders to represent themselves as authors for
whom they are not authorized to send mail, often in an attempt to whom they are not authorized to send mail, often in an attempt to
defraud either the recipient or an Alleged Author. defraud either the recipient or an Alleged Author.
Additional security considerations regarding Author Domain Signing Additional security considerations regarding Author Domain Signing
Practices are found in the DKIM threat analysis [RFC4686]. Practices are found in the DKIM threat analysis [RFC4686].
skipping to change at page 12, line 52 skipping to change at page 15, line 14
6.3. DNS Wildcards 6.3. DNS Wildcards
DNS wildcards (described in [RFC4592]) that exist in the DNS DNS wildcards (described in [RFC4592]) that exist in the DNS
hierarchy at or above the domain being checked interfere with the hierarchy at or above the domain being checked interfere with the
ability to verify the scope of the ADSP check described in ability to verify the scope of the ADSP check described in
Section 4.3. For example, a wildcard record for *.domain.example Section 4.3. For example, a wildcard record for *.domain.example
makes all subdomains such as foo.domain.example exist in the DNS. makes all subdomains such as foo.domain.example exist in the DNS.
Domains that intend to make active use of ADSP by publishing a Domains that intend to make active use of ADSP by publishing a
practice other than Unknown are advised to avoid the use of wildcards practice other than Unknown are advised to avoid the use of wildcards
elsewhere in their hierarchy. in their hierarchy.
If a domain contains wildcards, then any name that matches the If a domain contains wildcards, then any name that matches the
wildcard can appear to be a valid mail domain eligible for ADSP. But wildcard can appear to be a valid mail domain eligible for ADSP. But
the "_adsp._domainkey." prefix on ADSP records does not allow the "_adsp._domainkey." prefix on ADSP records does not allow
publication of wildcard records that cover ADSP records without also publication of wildcard records that cover ADSP records without also
covering non-ADSP records, nor of wildcard records that cover non- covering non-ADSP records, nor publication of wildcard records that
ADSP records without also covering ADSP records. Hence a domain MUST cover non-ADSP records without also covering ADSP records. A domain
NOT publish wildcard ADSP records. that uses ADSP practices other than unknown SHOULD NOT publish
wildcard records.
6.4. Inappropriate Application of Author Domain Signatures
In one model of DKIM usage, a domain signs messages that are in
transit through their system. Since any signature whose domain
matches the Author Domain is by definition an Author Domain
Signature, it would be unwise to sign mail whose Author Domain is the
signer's domain if the mail is not known to meet the domain's
standards for an Author Domain Signature.
One such use case is where a domain might apply such a signature is
following application of an Authentication-Results header field as
described in Section 7.1 of [RFC5451]. This problem can be easily
avoided either by not applying a signature that might be confused
with an Author Domain Signature or by applying a signature from some
other domain, such as a subdomain of the Author Domain.
7. References 7. References
7.1. References - Normative 7.1. References - Normative
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] 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.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008. October 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
7.2. References - Informative 7.2. References - Informative
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006. Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016, (DKIM) Signing Practices Protocol", RFC 5016,
October 2007. October 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
skipping to change at page 14, line 47 skipping to change at page 17, line 32
A mail message contains this From: header line: A mail message contains this From: header line:
From: alice@bbb.example (Old-fashioned Alice) From: alice@bbb.example (Old-fashioned Alice)
The ADSP Lookup first identifies the Author Address alice@bbb.example The ADSP Lookup first identifies the Author Address alice@bbb.example
and the Author Domain bbb.example. It does an MX DNS query for and the Author Domain bbb.example. It does an MX DNS query for
bbb.example, and gets back record (3). Since that query didn't bbb.example, and gets back record (3). Since that query didn't
return an error, it then proceeds to a TXT DNS query for return an error, it then proceeds to a TXT DNS query for
_adsp._domainkey.bbb.example, which returns NXDOMAIN. Since the _adsp._domainkey.bbb.example, which returns NXDOMAIN. Since the
domain exists but there is no ADSP record, ADSP returns the default domain exists but there is no ADSP record, ADSP returns the default
unknown result: messages may or may not have an author signature. unknown result: messages may or may not have an author domain
signature.
A.3. Domain does not exist A.3. Domain does not exist
A mail message contains this From: header line: A mail message contains this From: header line:
From: frank@ccc.example (Unreliable Frank) From: frank@ccc.example (Unreliable Frank)
The ADSP Lookup first identifies the Author Address frank@ccc.example The ADSP Lookup first identifies the Author Address frank@ccc.example
and the Author Domain ccc.example. It does an MX DNS query for and the Author Domain ccc.example. It does an MX DNS query for
ccc.example, and gets back an NXDOMAIN result since there are no ccc.example, and gets back an NXDOMAIN result since there are no
skipping to change at page 15, line 29 skipping to change at page 18, line 11
These examples are intended to illustrate typical uses of ADSP. They These examples are intended to illustrate typical uses of ADSP. They
are not intended to be exhaustive, nor to apply to every domain's or are not intended to be exhaustive, nor to apply to every domain's or
mail system's individual situation. mail system's individual situation.
Domain managers are advised to consider the ways that mail processing Domain managers are advised to consider the ways that mail processing
can modify messages in ways that will invalidate an existing DKIM can modify messages in ways that will invalidate an existing DKIM
signature, such as mailing lists, courtesy forwarders, and other signature, such as mailing lists, courtesy forwarders, and other
paths that could add or modify headers, or modify the message body. paths that could add or modify headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature, In that case, if the modifications invalidate the DKIM signature,
recipient hosts will consider the mail not to have an Author recipient hosts will consider the mail not to have an Author Domain
Signature, even though the signature was present when the mail was Signature, even though the signature was present when the mail was
originally sent. originally sent.
B.1. Single Location Domains B.1. Single Location Domains
A common mail system configuration handles all of a domain's users' A common mail system configuration handles all of a domain's users'
incoming and outgoing mail through a single MTA or group of MTAs. In incoming and outgoing mail through a single MTA or group of MTAs. In
that case, the MTA(s) can be configured to sign outgoing mail with an that case, the MTA(s) can be configured to sign outgoing mail with an
Author Signature. Author Domain Signature.
In this situation it might be appropriate to publish an ADSP record In this situation it might be appropriate to publish an ADSP record
for the domain containing "all", depending on whether the users also for the domain containing "all", depending on whether the users also
send mail through other paths that do not apply an Author Signature. send mail through other paths that do not apply an Author Domain
Such paths could include MTAs at hotels or hotspot networks used by Signature. Such paths could include MTAs at hotels or hotspot
travelling users, web sites that provide "mail an article" features, networks used by travelling users, web sites that provide "mail an
user messages sent through mailing lists, or third party mail clients article" features, user messages sent through mailing lists, or third
that support multiple user identities. party mail clients that support multiple user identities.
B.2. Bulk Mailing Domains B.2. Bulk Mailing Domains
Another common configuration uses a domain solely for bulk or Another common configuration uses a domain solely for bulk or
broadcast mail, with no individual human users, again typically broadcast mail, with no individual human users, again typically
sending all the mail through a single MTA or group of MTAs that can sending all the mail through a single MTA or group of MTAs that can
apply an Author Signature. In this case, the domain's management can apply an Author Domain Signature. In this case, the domain's
be confident that all of its outgoing mail will be sent through the management can be confident that all of its outgoing mail will be
signing MTA. Lacking individual users, the domain is unlikely to sent through the signing MTA. Lacking individual users, the domain
participate in mailing lists, but could still send mail through other is unlikely to participate in mailing lists, but could still send
paths that might invalidate signatures. mail through other paths that might invalidate signatures.
Domain owners often use specialist mailing providers to send their Domain owners often use specialist mailing providers to send their
bulk mail. In that case, the mailing provider needs access to a bulk mail. In that case, the mailing provider needs access to a
suitable signing key in order to apply an Author Signature. One suitable signing key in order to apply an Author Domain Signature.
possible route would be for the domain owner to generate the key and One possible route would be for the domain owner to generate the key
give it to the mailing provider. Another would be for the domain to and give it to the mailing provider. Another would be for the domain
delegate a subdomain to the mailing provider, for example, to delegate a subdomain to the mailing provider, for example,
bigbank.example might delegate email.bigbank.example to such a bigbank.example might delegate email.bigbank.example to such a
provider. In that case, the provider can generate the keys and DKIM provider. In that case, the provider can generate the keys and DKIM
DNS records itself and use the subdomain in the Author address in the DNS records itself and use the subdomain in the Author address in the
mail. mail.
Regardless of the DNS and key management strategy chosen, whoever Regardless of the DNS and key management strategy chosen, whoever
maintains the DKIM records for the domain could also install an ADSP maintains the DKIM records for the domain could also install an ADSP
record containing "all". record containing "all".
B.3. Bulk Mailing Domains with Discardable Mail B.3. Bulk Mailing Domains with Discardable Mail
In some cases, a domain might sign all of its outgoing mail with an In some cases, a domain might sign all of its outgoing mail with an
Author Signature, but prefer that recipient systems discard mail Author Domain Signature, but prefer that recipient systems discard
without a valid Author Signature to avoid confusion from mail sent mail without a valid Author Domain Signature to avoid confusion from
from sources that do not apply an Author Signature. (In the case of mail sent from sources that do not apply an Author Domain Signature.
domains with tightly controlled outgoing mail, this latter kind of (In the case of domains with tightly controlled outgoing mail, this
mail is sometimes loosely called "forgeries".) In that case, it latter kind of mail is sometimes loosely called "forgeries".) In
might be appropriate to publish an ADSP record containing that case, it might be appropriate to publish an ADSP record
"discardable". Note that a domain SHOULD NOT publish a "discardable" containing "discardable". Note that a domain SHOULD NOT publish a
record if it wishes to maximize the likelihood that mail from the "discardable" record if it wishes to maximize the likelihood that
domain is delivered, since it could cause some fraction of the mail mail from the domain is delivered, since it could cause some fraction
the domain sends to be discarded. of the mail the domain sends to be discarded.
B.4. Third Party Senders B.4. Third Party Senders
Another common use case is for a third party to enter into an Another common use case is for a third party to enter into an
agreement whereby that third party will send bulk or other mail on agreement whereby that third party will send bulk or other mail on
behalf of a designated author or author domain, using that domain in behalf of a designated author or author domain, using that domain in
the RFC5322 From: or other headers. Due to the many and varied the RFC5322 From: or other headers. Due to the many and varied
complexities of such agreements, third party signing is not addressed complexities of such agreements, third party signing is not addressed
in this specification. in this specification.
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This document greatly benefited from comments by Steve Atkins, Jon This document greatly benefited from comments by Steve Atkins, Jon
Callas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, Ellen Callas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, Ellen
Siegel, Michael Thomas, and Wietse Venema. Siegel, Michael Thomas, and Wietse Venema.
Appendix D. Change Log Appendix D. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of *NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.* this document as an RFC.*
D.1. Changes since -ietf-dkim-08 D.1. Changes since -ietf-dkim-09
o Change author signature to use d=.
o Change all "author signature" to "author domain signature".
o Add authentication results for IANA per Murray K.
o Minor editorial clarifications.
D.2. Changes since -ietf-dkim-08
o Simplify and clarify interpretation of d= and i=. o Simplify and clarify interpretation of d= and i=.
o Add note pointing out that i= usage conflicts with normal usage, o Add note pointing out that i= usage conflicts with normal usage,
and suggest workaround. and suggest workaround.
o Add note pointing out that you can mechanically add ADSP records o Add note pointing out that you can mechanically add ADSP records
for all the subdomains you use. for all the subdomains you use.
o in Section 3.2 fix text to say that only an Author Signature o in Section 3.2 fix text to say that only an Author Signature
counts. counts.
D.2. Changes since -ietf-dkim-07 D.3. Changes since -ietf-dkim-07
Clarify that ADSP records use WSP rather than FWS in 4.1 and 4.2.1. Clarify that ADSP records use WSP rather than FWS in 4.1 and 4.2.1.
D.3. Changes since -ietf-dkim-06 D.4. Changes since -ietf-dkim-06
Minor editorial changes suggested by AD: Minor editorial changes suggested by AD:
o expand DKIM in title o expand DKIM in title
o clarify that there's no subdomain matching in Section 3.1 o clarify that there's no subdomain matching in Section 3.1
o ADSP lookup can terminate without a result if the DNS lookup fails o ADSP lookup can terminate without a result if the DNS lookup fails
o random dkim= values are treated as unknown o random dkim= values are treated as unknown
skipping to change at page 18, line 27 skipping to change at page 21, line 14
o in 4.2 note WSP not FWS o in 4.2 note WSP not FWS
o in 4.3 note that NODATA is not NXDOMAIN o in 4.3 note that NODATA is not NXDOMAIN
o add new Appendix A with lookup examples o add new Appendix A with lookup examples
Also address Tony's nits in Also address Tony's nits in
http://mipassoc.org/pipermail/ietf-dkim/2008q3/010720.html. Make the http://mipassoc.org/pipermail/ietf-dkim/2008q3/010720.html. Make the
examples consistently use the .example domain. examples consistently use the .example domain.
D.4. Changes since -ietf-dkim-05 D.5. Changes since -ietf-dkim-05
Minor editorial nits: define NOERROR, SERVFAIL, NXDOMAIN as rfc1035 Minor editorial nits: define NOERROR, SERVFAIL, NXDOMAIN as rfc1035
rcodes, change some punctuation, IANA section change IETF Consensus rcodes, change some punctuation, IANA section change IETF Consensus
to the new IETF Review. to the new IETF Review.
D.5. Changes since -ietf-dkim-04 D.6. Changes since -ietf-dkim-04
o Require dkim at the front of each record. o Require dkim at the front of each record.
o Disparage wildcard records. o Disparage wildcard records.
o Changed ABNF use of whitespace from FWS back to WSP, dkim-base is o Changed ABNF use of whitespace from FWS back to WSP, dkim-base is
wrong. wrong.
o RFC 2434 -> 5226, make ref to 4686 informational since it's not o RFC 2434 -> 5226, make ref to 4686 informational since it's not
standards track. standards track.
o Improve examples with material from Ellen. o Improve examples with material from Ellen.
D.6. Changes since -ietf-dkim-03 D.7. Changes since -ietf-dkim-03
o Name change for title and filename, to be ADSP o Name change for title and filename, to be ADSP
o String changes throughout, to author Domain signing practices and o String changes throughout, to author Domain signing practices and
to aDsp. to aDsp.
o Added some keywords. o Added some keywords.
o Clarified comparison of local part and domain in Author Address. o Clarified comparison of local part and domain in Author Address.
skipping to change at page 19, line 34 skipping to change at page 22, line 24
o Revised wildcard text. o Revised wildcard text.
o Removed 't' tag. o Removed 't' tag.
o Removed ADSP Flags Registry section. o Removed ADSP Flags Registry section.
o Changed ABNF use of whitespace from WSP back to FWS, for o Changed ABNF use of whitespace from WSP back to FWS, for
consistency with dkim-base. consistency with dkim-base.
D.7. Changes since -ietf-dkim-02 D.8. Changes since -ietf-dkim-02
o Merge in more text from ADSP draft. o Merge in more text from ADSP draft.
o Phrase actions as host's rather than checker. o Phrase actions as host's rather than checker.
o Explanatory description of i= matching. o Explanatory description of i= matching.
o Lookup procedure consistently refers to one ADSP record per o Lookup procedure consistently refers to one ADSP record per
lookup. lookup.
skipping to change at page 20, line 9 skipping to change at page 22, line 46
o Simplify imports of terms from other RFCs, add Local-part, 4234 -> o Simplify imports of terms from other RFCs, add Local-part, 4234 ->
5234. 5234.
o Add usage example appendix. o Add usage example appendix.
o Add IANA considerations. o Add IANA considerations.
o Update authors list o Update authors list
D.8. Changes since -ietf-dkim-ssp-01 D.9. Changes since -ietf-dkim-ssp-01
o Reworded introduction for clarity. o Reworded introduction for clarity.
o Various definition clarifications. o Various definition clarifications.
o Changed names of practices to unknown, all, and discardable. o Changed names of practices to unknown, all, and discardable.
o Removed normative language mandating use of SSP in particular o Removed normative language mandating use of SSP in particular
situations (issue 1538). situations (issue 1538).
skipping to change at page 21, line 10 skipping to change at page 23, line 47
o Introduced the concepts of "SSP Checker" and "Evaluator". o Introduced the concepts of "SSP Checker" and "Evaluator".
o Multiple author case now handled my separate invocations of SSP o Multiple author case now handled my separate invocations of SSP
checker by Evaluator (issue 1525). checker by Evaluator (issue 1525).
o Removed check to avoid querying top-level domains. o Removed check to avoid querying top-level domains.
o Changed ABNF use of whitespace from [FWS] to *WSP (partially o Changed ABNF use of whitespace from [FWS] to *WSP (partially
addresses issue 1543). addresses issue 1543).
D.9. Changes since -ietf-dkim-ssp-00 D.10. Changes since -ietf-dkim-ssp-00
o Clarified Operation Overview and eliminated use of Legitimate as o Clarified Operation Overview and eliminated use of Legitimate as
the counterpart of Suspicious since the words have different the counterpart of Suspicious since the words have different
meanings. meanings.
o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
records in DNS vs. a new RR type. records in DNS vs. a new RR type.
o Clarified publication rules for multilevel names. o Clarified publication rules for multilevel names.
skipping to change at page 21, line 39 skipping to change at page 24, line 28
o Added "handling" tag to express alleged sending domain's o Added "handling" tag to express alleged sending domain's
preference about handling of Suspicious messages. preference about handling of Suspicious messages.
o Clarified handling of SERVFAIL error in SSP check. o Clarified handling of SERVFAIL error in SSP check.
o Replaced "entity" with "domain", since with the removal of user- o Replaced "entity" with "domain", since with the removal of user-
granularity SSP, the only entities having sender signing policies granularity SSP, the only entities having sender signing policies
are domains. are domains.
D.10. Changes since -allman-ssp-02 D.11. Changes since -allman-ssp-02
o Removed user-granularity SSP and u= tag. o Removed user-granularity SSP and u= tag.
o Replaced DKIMP resource record with a TXT record. o Replaced DKIMP resource record with a TXT record.
o Changed name of the primary tag from "p" to "dkim". o Changed name of the primary tag from "p" to "dkim".
o Replaced lookup algorithm with one which traverses upward at most o Replaced lookup algorithm with one which traverses upward at most
one level. one level.
o Added description of records to be published, and effect of o Added description of records to be published, and effect of
wildcard records within the domain, on SSP. wildcard records within the domain, on SSP.
D.11. Changes since -allman-ssp-01 D.12. Changes since -allman-ssp-01
o Changed term "Sender Signing Policy" to "Sender Signing o Changed term "Sender Signing Policy" to "Sender Signing
Practices". Practices".
o Changed query methodology to use a separate DNS resource record o Changed query methodology to use a separate DNS resource record
type, DKIMP. type, DKIMP.
o Changed tag values from SPF-like symbols to words. o Changed tag values from SPF-like symbols to words.
o User level policies now default to that of the domain if not o User level policies now default to that of the domain if not
specified. specified.
o Removed the "Compliance" section since we're still not clear on o Removed the "Compliance" section since we're still not clear on
what goes here. what goes here.
o Changed the "parent domain" policy to only search up one level o Changed the "parent domain" policy to only search up one level
(assumes that subdomains will publish SSP records if appropriate). (assumes that subdomains will publish SSP records if appropriate).
o Added detailed description of SSP check procedure. o Added detailed description of SSP check procedure.
D.12. Changes since -allman-ssp-00 D.13. Changes since -allman-ssp-00
From a "diff" perspective, the changes are extensive. Semantically, From a "diff" perspective, the changes are extensive. Semantically,
the changes are: the changes are:
o Added section on "Third-Party Signatures and Mailing Lists" o Added section on "Third-Party Signatures and Mailing Lists"
o Added "Compliance" (transferred from -base document). I'm not o Added "Compliance" (transferred from -base document). I'm not
clear on what needs to be done here. clear on what needs to be done here.
o Extensive restructuring. o Extensive restructuring.
 End of changes. 43 change blocks. 
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