draft-ietf-dmarc-rfc7601bis-04.txt   draft-ietf-dmarc-rfc7601bis-05.txt 
DMARC Working Group M. Kucherawy Individual submission M. Kucherawy
Internet-Draft November 14, 2018 Internet-Draft January 20, 2019
Updates: 7601 (if approved) Obsoletes: 7601 (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: May 18, 2019 Expires: July 24, 2019
Message Header Field for Indicating Message Authentication Status Message Header Field for Indicating Message Authentication Status
draft-ietf-dmarc-rfc7601bis-04 draft-ietf-dmarc-rfc7601bis-05
Abstract Abstract
This document specifies a message header field called Authentication- This document specifies a message header field called Authentication-
Results for use with electronic mail messages to indicate the results Results for use with electronic mail messages to indicate the results
of message authentication efforts. Any receiver-side software, such of message authentication efforts. Any receiver-side software, such
as mail filters or Mail User Agents (MUAs), can use this header field as mail filters or Mail User Agents (MUAs), can use this header field
to relay that information in a convenient and meaningful way to users to relay that information in a convenient and meaningful way to users
or to make sorting and filtering decisions. or to make sorting and filtering decisions.
This document obsoletes [RFC7601].
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 18, 2019. This Internet-Draft will expire on July 24, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7 1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7
1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8 1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8
1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9
2. Definition and Format of the Header Field . . . . . . . . . . 9 2. Definition and Format of the Header Field . . . . . . . . . . 10
2.1. General Description . . . . . . . . . . . . . . . . . . . 9 2.1. General Description . . . . . . . . . . . . . . . . . . . 10
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10
2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13
2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14
2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 2.5. Authentication Identifier Field . . . . . . . . . . . . . 15
2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16
2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16
2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 17
2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 18 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 18
2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 20
2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 21
2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 22
2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 22
2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 23
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23
4. Adding the Header Field to a Message . . . . . . . . . . . . . 24 4. Adding the Header Field to a Message . . . . . . . . . . . . . 24
4.1. Header Field Position and Interpretation . . . . . . . . . 25 4.1. Header Field Position and Interpretation . . . . . . . . . 26
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 27
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
6.1. The Authentication-Results Header Field . . . . . . . . . 28 6.1. The Authentication-Results Header Field . . . . . . . . . 29
6.2. "Email Authentication Methods" Registry Description . . . 28 6.2. "Email Authentication Methods" Registry Description . . . 29
6.3. "Email Authentication Methods" Registry Update . . . . . . 28 6.3. "Email Authentication Methods" Registry Update . . . . . . 30
6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 29 6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 31
6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 29 6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 32
6.4. "Email Authentication Property Types" Registry . . . . . . 29 6.4. "Email Authentication Property Types" Registry
6.5. "Email Authentication Result Names" Description . . . . . 29 Description . . . . . . . . . . . . . . . . . . . . . . . 32
6.6. "Email Authentication Result Names" Update . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30 6.5. "Email Authentication Property Types" Registry Update . . 33
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 30 6.6. "Email Authentication Result Names" Registry
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 32 Description . . . . . . . . . . . . . . . . . . . . . . . 33
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 32 6.7. "Email Authentication Result Names" Registry Update . . . 33
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 32 6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 34
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 32 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 32 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 34
7.7. Attacks against Authentication Methods . . . . . . . . . . 33 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 36
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 33 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 36
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 33 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 36
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 33 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 37
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 34 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 37
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.7. Attacks against Authentication Methods . . . . . . . . . . 37
8.1. Normative References . . . . . . . . . . . . . . . . . . . 34 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 37
8.2. Informative References . . . . . . . . . . . . . . . . . . 36 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 37
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 38 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 38
Appendix B. Authentication-Results Examples . . . . . . . . . . . 38 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 38
B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 39 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
8.1. Normative References . . . . . . . . . . . . . . . . . . . 38
8.2. Informative References . . . . . . . . . . . . . . . . . . 39
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 43
Appendix B. Authentication-Results Examples . . . . . . . . . . . 43
B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 43
B.2. Nearly Trivial Case; Service Provided, but No B.2. Nearly Trivial Case; Service Provided, but No
Authentication Done . . . . . . . . . . . . . . . . . . . 39 Authentication Done . . . . . . . . . . . . . . . . . . . 44
B.3. Service Provided, Authentication Done . . . . . . . . . . 40 B.3. Service Provided, Authentication Done . . . . . . . . . . 45
B.4. Service Provided, Several Authentications Done, Single B.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
B.5. Service Provided, Several Authentications Done, B.5. Service Provided, Several Authentications Done,
Different MTAs . . . . . . . . . . . . . . . . . . . . . . 42 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 47
B.6. Service Provided, Multi-tiered Authentication Done . . . . 44 B.6. Service Provided, Multi-tiered Authentication Done . . . . 49
B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 45 B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 50
Appendix C. Operational Considerations about Message Appendix C. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 46 Authentication . . . . . . . . . . . . . . . . . . . 51
Appendix D. Changes Since RFC7601 . . . . . . . . . . . . . . . . 47 Appendix D. Changes since RFC7601 . . . . . . . . . . . . . . . . 52
Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 47 Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 53
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 53
1. Introduction 1. Introduction
This document describes a header field called Authentication-Results This document describes a header field called Authentication-Results
for electronic mail messages that presents the results of a message for electronic mail messages that presents the results of a message
authentication effort in a machine-readable format. The intent of authentication effort in a machine-readable format. The intent of
the header field is to create a place to collect such data when the header field is to create a place to collect such data when
message authentication mechanisms are in use so that a Mail User message authentication mechanisms are in use so that a Mail User
Agent (MUA) and downstream filters can make filtering decisions Agent (MUA) and downstream filters can make filtering decisions
and/or provide a recommendation to the user as to the validity of the and/or provide a recommendation to the user as to the validity of the
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2. Provide a common location within a message for this data; 2. Provide a common location within a message for this data;
3. Create an extensible framework for reporting new authentication 3. Create an extensible framework for reporting new authentication
methods as they emerge. methods as they emerge.
In particular, the mere presence of this header field does not mean In particular, the mere presence of this header field does not mean
its contents are valid. Rather, the header field is reporting its contents are valid. Rather, the header field is reporting
assertions made by one or more authentication schemes applied assertions made by one or more authentication schemes applied
somewhere upstream. For an MUA or downstream filter to treat the somewhere upstream. For an MUA or downstream filter to treat the
assertions as actually valid, there must be an assessment of the assertions as actually valid, there must be an assessment of the
trust relationship among such agents, the validating MTA, and the trust relationship among such agents, the validating MTA, the paths
mechanism for conveying the information. between them, and the mechanism for conveying the information.
1.2. Trust Boundary 1.2. Trust Boundary
This document makes several references to the "trust boundary" of an This document makes several references to the "trust boundary" of an
administrative management domain (ADMD). Given the diversity among administrative management domain (ADMD). Given the diversity among
existing mail environments, a precise definition of this term isn't existing mail environments, a precise definition of this term isn't
possible. possible.
Simply put, a transfer from the producer of the header field to the Simply put, a transfer from the producer of the header field to the
consumer must occur within a context that permits the consumer to consumer must occur within a context that permits the consumer to
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its data center. its data center.
It is possible for a message to be evaluated inside a trust boundary It is possible for a message to be evaluated inside a trust boundary
but then depart and re-enter the trust boundary. An example might be but then depart and re-enter the trust boundary. An example might be
a forwarded message such as a message/rfc822 attachment (see a forwarded message such as a message/rfc822 attachment (see
Multipurpose Internet Mail Extensions [MIME]) or one that is part of Multipurpose Internet Mail Extensions [MIME]) or one that is part of
a multipart/digest. The details reported by this field cannot be a multipart/digest. The details reported by this field cannot be
trusted in that case. Thus, this field found within one of those trusted in that case. Thus, this field found within one of those
media types is typically ignored. media types is typically ignored.
Note that an MUA could be configured to retrieve messages from a
Receiver yet not be within the Receiver's ADMD. In this case, for
the purposes of this work, that MUA is considered to be within the
Receiver's ADMD if is configured to identify and ascribe value to
authentication results recorded by that ADMD.
1.3. Processing Scope 1.3. Processing Scope
The content of this header field is meant to convey to message The content of this header field is meant to convey to message
consumers that authentication work on the message was already done consumers that authentication work on the message was already done
within its trust boundary, and those results are being presented. It within its trust boundary, and those results are being presented. It
is not intended to provide message parameters to consumers so that is not intended to provide message parameters to consumers so that
they can perform authentication protocols on their own. they can perform authentication protocols on their own.
1.4. Requirements 1.4. Requirements
This document establishes no new requirements on existing protocols This document establishes no new requirements on existing protocols,
or servers. insofar as a non-participating service will continue to interoperate
with the deployed messaging infrastructure.
In particular, this document establishes no requirement on MTAs to In particular, this document establishes no requirement on MTAs to
reject or filter arriving messages that do not pass authentication reject or filter arriving messages that do not pass authentication
checks. The data conveyed by the specified header field's contents checks. The data conveyed by the specified header field's contents
are for the information of MUAs and filters and are to be used at are for the information of MUAs and filters and are to be used at
their discretion. their discretion.
A participating ADMD does undertake some filtering and message
modification obligations described in Section 5.
1.5. Definitions 1.5. Definitions
This section defines various terms used throughout this document. This section defines various terms used throughout this document.
1.5.1. Key Words 1.5.1. 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]. document are to be interpreted as described in BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
1.5.2. Internationalized Email 1.5.2. Internationalized Email
In this document, there are references to messages formatted to In this document, there are references to messages formatted to
support Email Address Internationalization (EAI). Reference material support Email Address Internationalization (EAI). Reference material
for this can be found in [RFC6530], [RFC6531], and [RFC6532]. for this can be found in [RFC6530], [RFC6531], and [RFC6532].
Generally speaking, these documents allow UTF-8 in most places that Generally speaking, these documents allow UTF-8 in most places that
free-form text can be found and U-labels where domain names can be free-form text can be found and U-labels where domain names can be
used, and this document extends Authentication-Results accordingly. used, and this document extends Authentication-Results accordingly.
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o "Authorization" is the establishment of permission to use a o "Authorization" is the establishment of permission to use a
resource or represent an identity. In this context, authorization resource or represent an identity. In this context, authorization
indicates that a message from a particular ADMD arrived via a indicates that a message from a particular ADMD arrived via a
route the ADMD has explicitly approved. route the ADMD has explicitly approved.
o "Authentication" is the assertion of validity of a piece of data o "Authentication" is the assertion of validity of a piece of data
about a message (such as the sender's identity) or the message in about a message (such as the sender's identity) or the message in
its entirety. its entirety.
As examples: SPF and Sender ID are authorization mechanisms in that As examples: SPF and Sender ID are authorization mechanisms in that
they express a result that shows whether or not the ADMD that they express a result that shows whether the ADMD that apparently
apparently sent the message has explicitly authorized the connecting sent the message has explicitly authorized the connecting Simple Mail
Simple Mail Transfer Protocol ([SMTP]) client to relay messages on Transfer Protocol ([SMTP]) client to relay messages on its behalf,
its behalf, but they do not actually validate any other property of but they do not actually validate any other property of the message
the message itself. By contrast, DKIM is agnostic as to the routing itself. By contrast, DKIM is agnostic as to the routing of a message
of a message but uses cryptographic signatures to authenticate but uses cryptographic signatures to authenticate agents, assign
agents, assign (some) responsibility for the message (which implies (some) responsibility for the message (which implies authorization),
authorization), and ensure that the listed portions of the message and ensure that the listed portions of the message were not modified
were not modified in transit. Since the signatures are not tied to in transit. Since the signatures are not tied to SMTP connections,
SMTP connections, they can be added by either the ADMD of origin, they can be added by either the ADMD of origin, intermediate ADMDs
intermediate ADMDs (such as a mailing list server), other handling (such as a mailing list server), other handling agents, or any
agents, or any combination. combination.
Rather than create a separate header field for each class of Rather than create a separate header field for each class of
solution, this specification groups them both into a single header solution, this specification groups them both into a single header
field. field.
1.5.4. Email Architecture 1.5.4. Email Architecture
o A "border MTA" is an MTA that acts as a gateway between the o A "border MTA" is an MTA that acts as a gateway between the
general Internet and the users within an organizational boundary. general Internet and the users within an organizational boundary.
(See also Section 1.2.) (See also Section 1.2.)
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supporting data can include a "reason" string and one or more supporting data can include a "reason" string and one or more
"property=value" statements indicating which message properties were "property=value" statements indicating which message properties were
evaluated to reach that conclusion. evaluated to reach that conclusion.
The header field can appear more than once in a single message, more The header field can appear more than once in a single message, more
than one result can be represented in a single header field, or a than one result can be represented in a single header field, or a
combination of these can be applied. combination of these can be applied.
2.2. Formal Definition 2.2. Formal Definition
Formally, the header field is specified as follows using Augmented Formally, the header field is specified as shown below using
Backus-Naur Form ([ABNF]): Augmented Backus-Naur Form ([ABNF]). Examples of valid header fields
with explanations of their semantics can be found in Appendix B.
authres-header-field = "Authentication-Results:" authres-payload authres-header-field = "Authentication-Results:" authres-payload
authres-payload = [CFWS] authserv-id authres-payload = [CFWS] authserv-id
[ CFWS authres-version ] [ CFWS authres-version ]
( no-result / 1*resinfo ) [CFWS] CRLF ( no-result / 1*resinfo ) [CFWS] CRLF
authserv-id = value authserv-id = value
; see below for a description of this element ; see below for a description of this element
authres-version = 1*DIGIT [CFWS] authres-version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use; ; indicates which version of this specification is in use;
; this specification is version "1", and the absence of a ; this specification is version "1", and the absence of a
; version implies this version of the specification ; version implies this version of the specification
no-result = [CFWS] ";" [CFWS] "none" no-result = [CFWS] ";" [CFWS] "none"
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; indicates more specifically than "ptype" what the ; indicates more specifically than "ptype" what the
; source of the evaluated property is; the exact meaning ; source of the evaluated property is; the exact meaning
; is specific to the method whose result is being reported ; is specific to the method whose result is being reported
; and is defined more clearly below ; and is defined more clearly below
special-smtp-verb = "mailfrom" / "rcptto" special-smtp-verb = "mailfrom" / "rcptto"
; special cases of [SMTP] commands that are made up ; special cases of [SMTP] commands that are made up
; of multiple words ; of multiple words
pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS] [CFWS]
; the value extracted from the message property defined ; the value extracted from the message property defined
; by the "ptype.property" construction ; by the "ptype.property" construction
"local-part" is defined in Section 3.4.1 of [MAIL], as modified by "local-part" is defined in Section 3.4.1 of [MAIL], as modified by
[RFC6531]. [RFC6531].
"CFWS" is defined in Section 3.2.2 of [MAIL]. "CFWS" is defined in Section 3.2.2 of [MAIL].
"domain-name" is as defined in Section 3.5 of [DKIM] where the "d="
tag is defined, with "sub-domain" as modified by [RFC6531].
"Keyword" is defined in Section 4.1.2 of [SMTP]. It is further "Keyword" is defined in Section 4.1.2 of [SMTP]. It is further
constrained by the necessity of being registered in the IANA registry constrained by the necesity of being registered in the IANA registry
relevant to the context in which it is used. See Section 2.7, relevant to the context in which it it is used. See Section 2.7, and
Section 2.3, and Section 6. Section 2.3, and Section 6.
"value" is as defined in Section 5.1 of [MIME], with "quoted-string" The "value" is as defined in Section 5.1 of [MIME], with "quoted-
updated as specified in RFC 6532. string" updated as specified in [RFC6532].
The "domain-name" is as defined in Section 3.5 of [DKIM].
The "Keyword" used in "result" above is further constrained by the
necessity of being enumerated in Section 2.7.
See Section 2.5 for a description of the authserv-id element. See Section 2.5 for a description of the authserv-id element.
If the value portion of a "pvalue" construction identifies something If the value portion of a "pvalue" construction identifies something
intended to be an email identity, then it MUST use the right hand intended to be an email identity, then it MUST use the right hand
portion of that ABNF definition. portion of that ABNF definition.
The list of commands eligible for use with the "smtp" ptype can be The list of commands eligible for use with the "smtp" ptype can be
found in Section 4.1 of [SMTP]. found in Section 4.1 of [SMTP].
skipping to change at page 13, line 11 skipping to change at page 13, line 28
Examples of complete messages using this header field can be found in Examples of complete messages using this header field can be found in
Appendix B. Appendix B.
2.3. Property Types (ptypes) and Properties 2.3. Property Types (ptypes) and Properties
The "ptype" in the ABNF above indicates the general type of property The "ptype" in the ABNF above indicates the general type of property
being described by the result being reported, upon which the reported being described by the result being reported, upon which the reported
result was based. Coupled with the "property", which is more result was based. Coupled with the "property", which is more
specific, they indicate from where the reported data were extracted. specific, they indicate from where the reported data were extracted.
This can include part of the message header or body, some part of the This can include a particular part of the message header or body,
SMTP session, a secondary output of an authentication method (apart some part of the SMTP session, a secondary output of an
from its pure result), or some other aspect of the message's authentication method (apart from its pure result), or some other
handling. aspect of the message's handling.
Combinations of ptypes and properties are registered and described in Combinations of ptypes and properties are registered and described in
the "Email Authentication Methods" registry, coupled with the the "Email Authentication Methods" registry, coupled with the
authentication methods with which they are used. This is further authentication methods with which they are used. This is further
described in Section 6. described in Section 6.
Legal values of "ptype" are as defined in the IANA "Email Legal values of "ptype" are as defined in the IANA "Email
Authentication Property Types" registry, created by [RFC7410]. The Authentication Property Types" registry, created by [RFC7410]. The
initial values and what they typically indicate are as follows, based initial values and what they typically indicate are as follows, based
on [RFC7001]: on [RFC7001]:
body: Information that was extracted from the body of the message. body: Information that was extracted from the body of the message.
This might be an arbitrary string of bytes, a hash of a string of This might be an arbitrary string of bytes, a hash of a string of
bytes, a Uniform Resource Identifier, or some other content of bytes, a Uniform Resource Identifier, or some other content of
interest. The "property" is an indication of where within the interest. The "property" is an indication of where within the
message body the extracted content was found, and can indicate an message body the extracted content was found, and can indicate an
offset, identify a MIME part, etc. offset, identify a MIME part, etc. (At the time of this revision,
no properties matching this ptype have been registered.
Accordingly, this ptype may be deprecated in the future.)
header: Indicates information that was extracted from the header of header: Indicates information that was extracted from the header of
the message. This might be the value of a header field or some the message. This might be the value of a header field or some
portion of a header field. The "property" gives a more precise portion of a header field. The "property" gives a more precise
indication of the place in the header from which the extraction indication of the place in the header from which the extraction
took place. took place.
policy: A local policy mechanism was applied that augments or policy: A local policy mechanism was applied that augments or
overrides the result returned by the authentication mechanism. overrides the result returned by the authentication mechanism.
(See Section 2.4.) (See Section 2.4.)
skipping to change at page 14, line 35 skipping to change at page 15, line 7
In this case, the property is "dkim-rules", indicating some local In this case, the property is "dkim-rules", indicating some local
check by that name took place and that check returned a result of check by that name took place and that check returned a result of
"unsigned-subject". These are arbitrary names selected by (and "unsigned-subject". These are arbitrary names selected by (and
presumably used within) the ADMD making use of them, so they are not presumably used within) the ADMD making use of them, so they are not
normally registered with IANA or otherwise specified apart from normally registered with IANA or otherwise specified apart from
setting syntax restrictions that allow for easy parsing within the setting syntax restrictions that allow for easy parsing within the
rest of the header field. rest of the header field.
This ptype existed in the original specification for this header This ptype existed in the original specification for this header
field, but without a complete description or example of intended use. field ([RFC5451]), but without a complete description or example of
As a result, it has not seen any practical use to date that matches intended use. As a result, it has not seen any practical use to date
its intended purpose. These added details are provided to guide that matches its intended purpose. These added details are provided
implementers toward proper use. to guide implementers toward proper use.
2.5. Authentication Identifier Field 2.5. Authentication Identifier Field
Every Authentication-Results header field has an authentication Every Authentication-Results header field has an authentication
service identifier field (authserv-id above). Specifically, this is service identifier field (authserv-id above). Specifically, this is
any string intended to identify the authentication service within the any string intended to identify the authentication service within the
ADMD that conducted authentication checks on the message. This ADMD that conducted authentication checks on the message. This
identifier is intended to be machine-readable and not necessarily identifier is intended to be machine-readable and not necessarily
meaningful to users. meaningful to users.
skipping to change at page 18, line 29 skipping to change at page 19, line 5
included with any [MAIL]-style comments removed; moreover, the local- included with any [MAIL]-style comments removed; moreover, the local-
part of the address and the "@" character are removed if it has not part of the address and the "@" character are removed if it has not
been authenticated in some way. been authenticated in some way.
2.7.2. SPF and Sender ID 2.7.2. SPF and Sender ID
SPF and Sender ID use the "spf" and "sender-id" method names, SPF and Sender ID use the "spf" and "sender-id" method names,
respectively. The result values for SPF are defined in Section 2.6 respectively. The result values for SPF are defined in Section 2.6
of [SPF], and those definitions are included here by reference: of [SPF], and those definitions are included here by reference:
+-----------+--------------------------------+ +-----------+------------------------------+
| Code | Meaning | | Code | Meaning |
+-----------+--------------------------------+ +-----------+------------------------------+
| none | [RFC7208], Section 2.6.1 | | none | [SPF], Section 2.6.1 |
+-----------+--------------------------------+ +-----------+------------------------------+
| pass | [RFC7208], Section 2.6.3 | | pass | [SPF], Section 2.6.3 |
+-----------+--------------------------------+ +-----------+------------------------------+
| fail | [RFC7208], Section 2.6.4 | | fail | [SPF], Section 2.6.4 |
+-----------+--------------------------------+ +-----------+------------------------------+
| softfail | [RFC7208], Section 2.6.5 | | softfail | [SPF], Section 2.6.5 |
+-----------+--------------------------------+ +-----------+------------------------------+
| policy | RFC 7601, Section 2.4 | | policy | [this document], Section 2.4 |
+-----------+--------------------------------+ +-----------+------------------------------+
| neutral | [RFC7208], Section 2.6.2 | | neutral | [SPF], Section 2.6.2 |
+-----------+--------------------------------+ +-----------+------------------------------+
| temperror | [RFC7208], Section 2.6.6 | | temperror | [SPF], Section 2.6.6 |
+-----------+--------------------------------+ +-----------+------------------------------+
| permerror | [RFC7208], Section 2.6.7 | | permerror | [SPF], Section 2.6.7 |
+-----------+--------------------------------+ +-----------+------------------------------+
These result codes are used in the context of this specification to These result codes are used in the context of this specification to
reflect the result returned by the component conducting SPF reflect the result returned by the component conducting SPF
evaluation. evaluation.
For SPF, the ptype used is "smtp", and the property is either For SPF, the ptype used is "smtp", and the property is either
"mailfrom" or "helo", since those values are the ones SPF can "mailfrom" or "helo", since those values are the ones SPF can
evaluate. (If the SMTP client issued the EHLO command instead of evaluate. (If the SMTP client issued the EHLO command instead of
HELO, the property used is "helo".) HELO, the property used is "helo".)
Note that in an EAI-formatted message, the "mailfrom" value can be Note that in an EAI-formatted message, the local-part of the
expressed in UTF-8. "mailfrom" can be expressed in UTF-8 and the domain part can be
expressed as a U-label.
The "sender-id" method is described in [SENDERID]. For this method, The "sender-id" method is described in [SENDERID]. For this method,
the ptype used is "header" and the property will be the name of the the ptype used is "header" and the property will be the name of the
header field from which the Purported Responsible Address (see [PRA]) header field from which the Purported Responsible Address (see [PRA])
was extracted -- namely, one of "Resent-Sender", "Resent-From", was extracted -- namely, one of "Resent-Sender", "Resent-From",
"Sender", or "From". "Sender", or "From".
The results for Sender ID are listed and described in Section 4.2 of The results for Sender ID are listed and described in Section 4.2 of
[SENDERID], but for the purposes of this specification, the SPF [SENDERID], but for the purposes of this specification, the SPF
definitions enumerated above are used instead. Also, [SENDERID] definitions enumerated above are used instead. Also, [SENDERID]
specifies result codes that use mixed case, but they are typically specifies result codes that use mixed case, but they are used all
used all lowercase in this context. lowercase in this context.
For both methods, an additional result of "policy" is defined, which For both methods, an additional result of "policy" is defined, which
means the client was authorized to inject or relay mail on behalf of means the client was authorized to inject or relay mail on behalf of
the sender's DNS domain according to the authentication method's the sender's DNS domain according to the authentication method's
algorithm, but local policy dictates that the result is unacceptable. algorithm, but local policy dictates that the result is unacceptable.
For example, "policy" might be used if SPF returns a "pass" result, For example, "policy" might be used if SPF returns a "pass" result,
but a local policy check matches the sending DNS domain to one found but a local policy check matches the sending DNS domain to one found
in an explicit list of unacceptable DNS domains (e.g., spammers). in an explicit list of unacceptable DNS domains (e.g., spammers).
If the retrieved sender policies used to evaluate SPF and Sender ID If the retrieved sender policies used to evaluate SPF and Sender ID
skipping to change at page 21, line 12 skipping to change at page 21, line 38
such as a permanent directory service lookup error. A later such as a permanent directory service lookup error. A later
attempt is not likely to produce a final result. attempt is not likely to produce a final result.
The result of AUTH is reported using a ptype of "smtp" and a property The result of AUTH is reported using a ptype of "smtp" and a property
of either: of either:
o "auth", in which case the value is the authorization identity o "auth", in which case the value is the authorization identity
generated by the exchange initiated by the AUTH command; or generated by the exchange initiated by the AUTH command; or
o "mailfrom", in which case the value is the mailbox identified by o "mailfrom", in which case the value is the mailbox identified by
the AUTH parameter used with the MAIL FROM command. Note that in the AUTH parameter used with the MAIL FROM command.
an EAI-formatted message, these values can be expressed in UTF-8.
Note that in an EAI-formatted message, the local-part can be
expressed in UTF-8, and the domain can be expressed as a U-label.
If both identities are available, both can be reported. For example, If both identities are available, both can be reported. For example,
consider this command issued by a client that has completed session consider this command issued by a client that has completed session
authentication with the AUTH command resulting in an authorized authentication with the AUTH command resulting in an authorized
identity of "client@c.example": identity of "client@c.example":
MAIL FROM:<alice@a.example> AUTH=<bob@b.example> MAIL FROM:<alice@a.example> AUTH=<bob@b.example>
This could result in a "resinfo" construction like so: This could result in a "resinfo" construction like so:
skipping to change at page 21, line 46 skipping to change at page 22, line 25
o Authorized Third-Party Signatures (in [ATPS], represented by o Authorized Third-Party Signatures (in [ATPS], represented by
"dkim-atps"); "dkim-atps");
o Author Domain Signing Practices (in [ADSP], represented by "dkim- o Author Domain Signing Practices (in [ADSP], represented by "dkim-
adsp"); adsp");
o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs");
o S/MIME (in [SMIME-REG], represented by "smime"). o S/MIME (in [SMIME-REG], represented by "smime").
Note that "vbr.mv" and "vbr.md", which are already registered, are Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which
permitted to be UTF-8 in an EAI-formatted message. are already registered, can be expressed as U-labels.
2.7.6. Extension Methods 2.7.6. Extension Methods
Additional authentication method identifiers (extension methods) may Additional authentication method identifiers (extension methods) may
be defined in the future by later revisions or extensions to this be defined in the future by later revisions or extensions to this
specification. These method identifiers are registered with the specification. These method identifiers are registered with the
Internet Assigned Numbers Authority (IANA) and, preferably, published Internet Assigned Numbers Authority (IANA) and, preferably, published
in an RFC. See Section 6 for further details. in an RFC. See Section 6 for further details.
Extension methods can be defined for the following reasons: Extension methods can be defined for the following reasons:
skipping to change at page 22, line 31 skipping to change at page 23, line 10
an MUA developer to understand or relay ancillary details of an MUA developer to understand or relay ancillary details of
authentication results. For example, if it might be of interest to authentication results. For example, if it might be of interest to
relay what data was used to perform an evaluation, such information relay what data was used to perform an evaluation, such information
could be relayed as a comment in the header field, such as: could be relayed as a comment in the header field, such as:
Authentication-Results: example.com; Authentication-Results: example.com;
foo=pass bar.baz=blob (2 of 3 tests OK) foo=pass bar.baz=blob (2 of 3 tests OK)
Experimental method identifiers MUST only be used within ADMDs that Experimental method identifiers MUST only be used within ADMDs that
have explicitly consented to use them. These method identifiers and have explicitly consented to use them. These method identifiers and
the parameters associated with them are not documented in RFCs. the parameters associated with them are not documented formally.
Therefore, they are subject to change at any time and not suitable Therefore, they are subject to change at any time and not suitable
for production use. Any MTA, MUA, or downstream filter intended for for production use. Any MTA, MUA, or downstream filter intended for
production use SHOULD ignore or delete any Authentication-Results production use SHOULD ignore or delete any Authentication-Results
header field that includes an experimental (unknown) method header field that includes an experimental (unknown) method
identifier. identifier.
2.7.7. Extension Result Codes 2.7.7. Extension Result Codes
Additional result codes (extension results) might be defined in the Additional result codes (extension results) might be defined in the
future by later revisions or extensions to this specification. Non- future by later revisions or extensions to this specification. Non-
skipping to change at page 26, line 10 skipping to change at page 26, line 36
Furthermore, MUAs and downstream filters SHOULD NOT interpret this Furthermore, MUAs and downstream filters SHOULD NOT interpret this
header field unless the authentication service identifier of the header field unless the authentication service identifier of the
header field is used within the ADMD as configured by the user or header field is used within the ADMD as configured by the user or
administrator. administrator.
MUAs and downstream filters MUST ignore any result reported using a MUAs and downstream filters MUST ignore any result reported using a
"result" not specified in the IANA "Result Code" registry or a "result" not specified in the IANA "Result Code" registry or a
"ptype" not listed in the "Email Authentication Property Types" "ptype" not listed in the "Email Authentication Property Types"
registry for such values as defined in Section 6. Moreover, such registry for such values as defined in Section 6. Moreover, such
agents MUST ignore a result indicated for any "method" they do not agents MUST ignore a result indicated for any "method" they do not
specifically support. specifically support. The exception to this is experimental methods
as discussed in Section 2.7.6.
An MUA SHOULD NOT reveal these results to end users, absent careful An MUA SHOULD NOT reveal these results to end users, absent careful
human factors design considerations and testing, for the presentation human factors design considerations and testing, for the presentation
of trust-related materials. For example, an attacker could register of trust-related materials. For example, an attacker could register
examp1e.com (note the digit "1" (one)) and send signed mail to examp1e.com (note the digit "1" (one)) and send signed mail to
intended victims; a verifier would detect that the signature was intended victims; a verifier would detect that the signature was
valid and report a "pass" even though it's clear the DNS domain name valid and report a "pass" even though it's clear the DNS domain name
was intended to mislead. See Section 7.2 for further discussion. was intended to mislead. See Section 7.2 for further discussion.
As stated in Section 2.1, this header field MUST be treated as though As stated in Section 2.1, this header field MUST be treated as though
skipping to change at page 26, line 35 skipping to change at page 27, line 14
Note that there are a few message handlers that are only capable of Note that there are a few message handlers that are only capable of
appending new header fields to a message. Strictly speaking, these appending new header fields to a message. Strictly speaking, these
handlers are not compliant with this specification. They can still handlers are not compliant with this specification. They can still
add the header field to carry authentication details, but any signal add the header field to carry authentication details, but any signal
about where in the handling chain the work was done may be lost. about where in the handling chain the work was done may be lost.
Consumers SHOULD be designed such that this can be tolerated, Consumers SHOULD be designed such that this can be tolerated,
especially from a producer known to have this limitation. especially from a producer known to have this limitation.
MUAs SHOULD ignore instances of this header field discovered within MUAs SHOULD ignore instances of this header field discovered within
message/rfc822 MIME attachments. message/rfc822 MIME attachments. They are likely to contain the
results of authentication checks done in the past, possibly long ago,
and have no contemporary value. Due caution to this needs to be
taken when choosing to consume them.
Further discussion of these topics can be found in Section 7 below. Further discussion of these topics can be found in Section 7 below.
4.2. Local Policy Enforcement 4.2. Local Policy Enforcement
Some sites have a local policy that considers any particular Some sites have a local policy that considers any particular
authentication policy's non-recoverable failure results (typically authentication policy's non-recoverable failure results (typically
"fail" or similar) as justification for rejecting the message. In "fail" or similar) as justification for rejecting the message. In
such cases, the border MTA SHOULD issue an SMTP rejection response to such cases, the border MTA SHOULD issue an SMTP rejection response to
the message, rather than adding this header field and allowing the the message, rather than adding this header field and allowing the
skipping to change at page 27, line 21 skipping to change at page 28, line 4
To mitigate the impact of forged header fields, any MTA conforming to To mitigate the impact of forged header fields, any MTA conforming to
this specification MUST delete any discovered instance of this header this specification MUST delete any discovered instance of this header
field that claims, by virtue of its authentication service field that claims, by virtue of its authentication service
identifier, to have been added within its trust boundary but that did identifier, to have been added within its trust boundary but that did
not come directly from another trusted MTA. For example, an MTA for not come directly from another trusted MTA. For example, an MTA for
example.com receiving a message MUST delete or otherwise obscure any example.com receiving a message MUST delete or otherwise obscure any
instance of this header field bearing an authentication service instance of this header field bearing an authentication service
identifier indicating that the header field was added within identifier indicating that the header field was added within
example.com prior to adding its own header fields. This could mean example.com prior to adding its own header fields. This could mean
each MTA will have to be equipped with a list of internal MTAs known each internal MTA will need to be configured with a list of other
to be compliant (and hence trustworthy). known, trusted MTAs that are thus expected to be using that same
identifier.
For messages that are EAI-formatted messages, this test is done after For messages that are EAI-formatted messages, this test is done after
converting A-labels into U-labels. converting A-labels into U-labels.
For simplicity and maximum security, a border MTA could remove all For simplicity and maximum security, a border MTA could remove all
instances of this header field on mail crossing into its trust instances of this header field on mail crossing into its trust
boundary. However, this may conflict with the desire to access boundary. However, this may conflict with the desire to access
authentication results performed by trusted external service authentication results performed by trusted external service
providers. It may also invalidate signed messages whose signatures providers. It may also invalidate signed messages whose signatures
cover external instances of this header field. A more robust border cover external instances of this header field. A more robust border
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could invalidate one or more signatures on the message if they could invalidate one or more signatures on the message if they
covered the header field to be removed. This behavior can be covered the header field to be removed. This behavior can be
desirable since there's little value in validating the signature on a desirable since there's little value in validating the signature on a
message with forged header fields. However, signing agents MAY message with forged header fields. However, signing agents MAY
therefore elect to omit these header fields from signing to avoid therefore elect to omit these header fields from signing to avoid
this situation. this situation.
An MTA SHOULD remove any instance of this header field bearing a An MTA SHOULD remove any instance of this header field bearing a
version (express or implied) that it does not support. However, an version (express or implied) that it does not support. However, an
MTA MUST remove such a header field if the [SMTP] connection relaying MTA MUST remove such a header field if the [SMTP] connection relaying
the message is not from a trusted internal MTA. This means the MTA the message is not from a trusted internal MTA. (As discussed above,
needs to be able to understand versions of this header field at least this too can result in invalidation of signatures.) This means the
as late as the ones understood by the MUAs or other consumers within MTA needs to be able to understand versions of this header field at
its ADMD. least as late as the ones understood by the MUAs or other consumers
within its ADMD.
6. IANA Considerations 6. IANA Considerations
IANA has registered the defined header field and created registries IANA has registered the defined header field and created registries
as described below. These registry actions were originally defined as described below. These registry actions were originally defined
by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created
registries were further updated in [RFC7601] to make them more registries were further updated in [RFC7601] to make them more
complete. complete.
Each is listed below, though generally they are not changed by this Each registry has two related sections below. The first describes
document. the registry and its update procedures, which are unchanged from
[RFC7601]. The second enumerates changes to entries that are
relevant to this document.
6.1. The Authentication-Results Header Field 6.1. The Authentication-Results Header Field
The Authentication-Results header field was added to the IANA The Authentication-Results header field was added to the IANA
"Permanent Message Header Field Names" registry, per the procedure "Permanent Message Header Field Names" registry, per the procedure
found in [IANA-HEADERS]. That entry will be updated to reference found in [IANA-HEADERS]. That entry will be updated to reference
this document. The following is the registration template: this document. The following is the registration template:
Header field name: Authentication-Results Header field name: Authentication-Results
Applicable protocol: mail ([MAIL]) Applicable protocol: mail ([MAIL])
Status: Standard Status: Standard
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): [this document] Specification document(s): [this document]
Related information: none Related information: none
6.2. "Email Authentication Methods" Registry Description 6.2. "Email Authentication Methods" Registry Description
No changes are made to the description of this registry. Names of message authentication methods supported by this
specification have been registered with IANA, with the exception of
experimental names as described in Section 2.7.6. Along with each
method is recorded the properties that accompany the method's result.
The "Email Authentication Parameters" group, and within it the "Email
Authentication Methods" registry, were created by [RFC5451] for this
purpose. [RFC6577] added a "status" field for each entry. [RFC7001]
amended the rules governing that registry and also added a "version"
field to the registry.
The reference for that registry will be updated to reference this
document.
New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to
request that a publication be referenced if a clear, concise
definition of the authentication method cannot be provided such that
interoperability is assured. Registrations should otherwise be
permitted. The designated expert can also handle requests to mark
any current registration as "deprecated".
No two entries can have the same combination of method, ptype, and
property.
An entry in this registry contains the following:
Method: the name of the method.
Definition: a reference to the document that created this entry, if
any (see below).
ptype: a "ptype" value appropriate for use with that method.
property: a "property" value matching that "ptype" also appropriate
for use with that method.
Value: a brief description of the value to be supplied with that
method/ptype/property tuple.
Status: the status of this entry, which is either:
active: The entry is in current use.
deprecated: The entry is no longer in current use.
Version: a version number associated with the method (preferably
starting at "1").
The "Definition" field will typically refer to a permanent document,
or at least some descriptive text, where additional information about
the entry being added can be found. This might in turn reference the
document where the method is defined so that all of the semantics
around creating or interpreting an Authentication-Results header
field using this method, ptype, and property can be understood.
6.3. "Email Authentication Methods" Registry Update 6.3. "Email Authentication Methods" Registry Update
The following two entries are added. The following entries in this registry are to be updated to replace
[RFC7601] with this document:
+------------+--------+----------------------------------+
| Method | ptype | Property |
+------------+--------+----------------------------------+
| auth | smtp | auth |
+------------+--------+----------------------------------+
| auth | smtp | mailfrom |
+------------+--------+----------------------------------+
| dkim | header | d |
+------------+--------+----------------------------------+
| dkim | header | i |
+------------+--------+----------------------------------+
| domainkeys | header | d |
+------------+--------+----------------------------------+
| domainkeys | header | from |
+------------+--------+----------------------------------+
| domainkeys | header | sender |
+------------+--------+----------------------------------+
| iprev | policy | iprev |
+------------+--------+----------------------------------+
| sender-id | header | name of header field used by PRA |
+------------+--------+----------------------------------+
| spf | smtp | mailfrom |
+------------+--------+----------------------------------+
| spf | smtp | helo |
+------------+--------+----------------------------------+
In addition, two new entries are added to this registry, as follows:
6.3.1. 'header.a' for DKIM 6.3.1. 'header.a' for DKIM
Method: dkim Method: dkim
Definition: [this document] Definition: [this document]
ptype: header ptype: header
property: a property: a
Description: value of signature "a" tag Description: value of signature "a" tag
Status: active Status: active
Version: 1 Version: 1
6.3.2. 'header.s' for DKIM 6.3.2. 'header.s' for DKIM
"header.s" for DKIM:
Method: dkim Method: dkim
Definition: [this document] Definition: [this document]
ptype: header ptype: header
property: s property: s
Description: value of signature "s" tag Description: value of signature "s" tag
Status: active Status: active
Version: 1 Version: 1
6.4. "Email Authentication Property Types" Registry 6.4. "Email Authentication Property Types" Registry Description
[RFC7410] created the "Email Authentication Property Types" registry. [RFC7410] created the "Email Authentication Property Types" registry.
No changes are made to the description of this registry. However, it Entries in this registry are subject to the Expert Review rules as
should be noted that Section 2.3 contains slightly different language described in [IANA-CONSIDERATIONS]. Each entry in the registry
than prior versions of this document, allowing a broader space from requires the following values:
which to extract meaningful identifiers and report them through this
mechanism.
6.5. "Email Authentication Result Names" Description ptype: The name of the ptype being registered, which must fit within
the ABNF described in Section 2.2.
No changes are made to the description of this registry. Definition: An optional reference to a defining specification.
6.6. "Email Authentication Result Names" Update Description: A brief description of what sort of information this
"ptype" is meant to cover.
No changes are made to entries in this registry. For new entries, the Designated Expert needs to assure that the
description provided for the new entry adequately describes the
intended use. An example would be helpful to include in the entry's
defining document, if any, although entries in the "Email
Authentication Methods" registry or the "Email Authentication Result
Names" registry might also serve as examples of intended use.
As this is a complete restatement of the definition and rules for
this registry, IANA will update this registry to show Section 2.3 of
this document as the current definitions for the "body", "header",
"policy", and "smtp" entries of that registry. References to other
documents will be removed.
6.5. "Email Authentication Property Types" Registry Update
All current entries in this registry are to be updated to replace
[RFC7601] with this document.
6.6. "Email Authentication Result Names" Registry Description
Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.7.7.
New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to
request that a publication be referenced if a clear, concise
definition of the authentication result cannot be provided such that
interoperability is assured. Registrations should otherwise be
permitted. The designated expert can also handle requests to mark
any current registration as "deprecated".
No two entries can have the same combination of method and code.
An entry in this registry contains the following:
Auth Method: an authentication method for which results are being
returned using the header field defined in this document.
Code: a result code that can be returned for this authentication
method.
Specification: either free form text explaining the meaning of this
method-code combination, or a reference to such a definition.
Status: the status of this entry, which is either:
active: The entry is in current use.
deprecated: The entry is no longer in current use.
6.7. "Email Authentication Result Names" Registry Update
The following entries in this registry are to be updated to reflect
new Specifications as follows:
o All "auth" method result codes ("fail", "none", "pass",
"permerror", "temperror") are now specified in Section 2.7.4 of
this document.
o All "dkim" method result names ("fail", "neutral", "none", "pass",
"permerror", "policy", "temperror") are now specified in
Section 2.7.1 of this document.
o All "iprev" method result names ("fail", "pass", "permerror",
"temperror") are now specified in Section 2.7.3 of this document.
o The "sender-id" and "spf" method result names "fail", "neutral",
"none", "pass", "permerror", "policy", "softfail", and "temperror"
are now specified in Section 2.7.2 of this document. The
registrations for result name "hardfail" are not updated.
6.8. SMTP Enhanced Status Codes
The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of
the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry" is to be updated to refer only to Section 3.3 of [AUTH-ESC]
as that is where that registration was done.
7. Security Considerations 7. Security Considerations
The following security considerations apply when adding or processing The following security considerations apply when adding or processing
the Authentication-Results header field: the Authentication-Results header field:
7.1. Forged Header Fields 7.1. Forged Header Fields
An MUA or filter that accesses a mailbox whose messages are handled An MTA not applying the filtering discussed in Section 5 exposes MUAs
by a non-conformant MTA, and understands Authentication-Results to false conclusions based on forged header fields. A malicious user
header fields, could potentially make false conclusions based on or agent could forge a header field using the DNS domain of a
forged header fields. A malicious user or agent could forge a header receiving ADMD as the authserv-id token in the value of the header
field using the DNS domain of a receiving ADMD as the authserv-id field and, with the rest of the value, claim that the message was
token in the value of the header field and, with the rest of the properly authenticated. The non-conformant MTA would fail to strip
value, claim that the message was properly authenticated. The non- the forged header field, and the MUA could inappropriately trust it.
conformant MTA would fail to strip the forged header field, and the
MUA could inappropriately trust it.
For this reason, it is best not to have processing of the For this reason, it is best not to have processing of the
Authentication-Results header field enabled by default; instead, it Authentication-Results header field enabled by default; instead, it
should be ignored, at least for the purposes of enacting filtering should be ignored, at least for the purposes of enacting filtering
decisions, unless specifically enabled by the user or administrator decisions, unless specifically enabled by the user or administrator
after verifying that the border MTA is compliant. It is acceptable after verifying that the border MTA is compliant. It is acceptable
to have an MUA aware of this specification but have an explicit list to have an MUA aware of this specification but have an explicit list
of hostnames whose Authentication-Results header fields are of hostnames whose Authentication-Results header fields are
trustworthy; however, this list should initially be empty. trustworthy; however, this list should initially be empty.
skipping to change at page 33, line 18 skipping to change at page 37, line 32
If an attack becomes known against an authentication method, clearly If an attack becomes known against an authentication method, clearly
then the agent verifying that method can be fooled into thinking an then the agent verifying that method can be fooled into thinking an
inauthentic message is authentic, and thus the value of this header inauthentic message is authentic, and thus the value of this header
field can be misleading. It follows that any attack against the field can be misleading. It follows that any attack against the
authentication methods supported by this document is also a security authentication methods supported by this document is also a security
consideration here. consideration here.
7.8. Intentionally Malformed Header Fields 7.8. Intentionally Malformed Header Fields
As with any other header field found in a message, it is possible for As with any other header field found in the message, it is possible
an attacker to add an Authentication-Results header field that is for an attacker to add an Authentication-Results header field that is
extraordinarily large or otherwise malformed in an attempt to extraordinarily large or otherwise malformed in an attempt to
discover or exploit weaknesses in header field parsing code. discover or exploit weaknesses in header field parsing code.
Implementers must thoroughly verify all such header fields received Implementers must thoroughly verify all such header fields received
from MTAs and be robust against intentionally as well as from MTAs and be robust against intentionally as well as
unintentionally malformed header fields. unintentionally malformed header fields.
7.9. Compromised Internal Hosts 7.9. Compromised Internal Hosts
An internal MUA or MTA that has been compromised could generate mail An internal MUA or MTA that has been compromised could generate mail
with a forged From header field and a forged Authentication-Results with a forged From header field and a forged Authentication-Results
skipping to change at page 33, line 47 skipping to change at page 38, line 12
legitimate MTAs will have to add this header field when legitimate legitimate MTAs will have to add this header field when legitimate
internal-only messages are generated. This is also covered in internal-only messages are generated. This is also covered in
Section 5. Section 5.
7.10. Encapsulated Instances 7.10. Encapsulated Instances
MIME messages can contain attachments of type "message/rfc822", which MIME messages can contain attachments of type "message/rfc822", which
contain other messages. Such an encapsulated message can also contain other messages. Such an encapsulated message can also
contain an Authentication-Results header field. Although the contain an Authentication-Results header field. Although the
processing of these is outside of the intended scope of this document processing of these is outside of the intended scope of this document
(see Section 1.3), some early guidance to MUA developers is (see Section 1.3), some simple guidance to MUA developers is
appropriate here. appropriate here.
Since MTAs are unlikely to strip Authentication-Results header fields Since MTAs are generally unlikely to strip Authentication-Results
after mailbox delivery, MUAs are advised in Section 4.1 to ignore header fields during mailbox delivery, normative language exists in
such instances within MIME attachments. Moreover, when extracting a Section 4.1 cautioning MUAs to ignore such instances within MIME
message digest to separate mail store messages or other media, such attachments, as might be included when a message is forwarded.
header fields should be removed so that they will never be Moreover, when extracting a message digest to separate mail store
interpreted improperly by MUAs that might later consume them. messages or other media, such header fields should be removed so that
they will never be interpreted improperly by MUAs that might later
consume them.
There can be cases where these header fields included as part of
encapsulated messages might actually be of value, such as when they
are taken from messages within the same ADMD where they will be
consumed. Caution must be taken that the consumer fully understands
the semantics of what the header field is indicating and the
message's handling history before ascribing any value, positive or
negative, to such data.
7.11. Reverse Mapping 7.11. Reverse Mapping
Although Section 3 of this memo includes explicit support for the Although Section 3 of this memo includes explicit support for the
"iprev" method, its value as an authentication mechanism is limited. "iprev" method, its value as an authentication mechanism is limited.
Implementers of both this specification and agents that use the data Implementers of both this specification and agents that use the data
it relays are encouraged to become familiar with the issues raised by it relays are encouraged to become familiar with the issues raised by
[DNSOP-REVERSE] when deciding whether or not to include support for [DNSOP-REVERSE] when deciding whether or not to include support for
"iprev". "iprev".
8. References 8. References
8.1. Normative References 8.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008, RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<http://www.rfc-editor.org/info/rfc6376>.
[IANA-HEADERS] [IANA-HEADERS]
Klyne, G., Nottingham, M., and J. Mogul, "Registration Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>. <http://www.rfc-editor.org/info/rfc3864>.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>. <http://www.rfc-editor.org/info/rfc5322>.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<http://www.rfc-editor.org/info/rfc2045>. <http://www.rfc-editor.org/info/rfc2045>.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Message Authentication Status", RFC 5451, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC5451, April 2009, RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc5451>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6008] Kucherawy, M., "Authentication-Results Registration for
Differentiating among Cryptographic Results", RFC 6008,
DOI 10.17487/RFC6008, September 2010,
<https://www.rfc-editor.org/info/rfc6008>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>. February 2012, <https://www.rfc-editor.org/info/rfc6530>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>. <https://www.rfc-editor.org/info/rfc6531>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, Email Headers", RFC 6532, DOI 10.17487/RFC6532,
February 2012, <https://www.rfc-editor.org/info/rfc6532>. February 2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC6577] Kucherawy, M., "Authentication-Results Registration Update
for Sender Policy Framework (SPF) Results", RFC 6577,
DOI 10.17487/RFC6577, March 2012,
<https://www.rfc-editor.org/info/rfc6577>.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, DOI 10.17487/
RFC7001, September 2013,
<https://www.rfc-editor.org/info/rfc7001>.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating [RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601, DOI 10.17487/ Message Authentication Status", RFC 7601, DOI 10.17487/
RFC7601, August 2015, RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>. <https://www.rfc-editor.org/info/rfc7601>.
[RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
Update to DomainKeys Identified Mail (DKIM)", RFC 8301, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
DOI 10.17487/RFC8301, January 2018, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
<https://www.rfc-editor.org/info/rfc8301>.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>. <http://www.rfc-editor.org/info/rfc5321>.
8.2. Informative References 8.2. Informative References
[ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing "DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617,
skipping to change at page 36, line 32 skipping to change at page 40, line 27
[AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, DOI 10.17487/ Extension for Authentication", RFC 4954, DOI 10.17487/
RFC4954, July 2007, RFC4954, July 2007,
<http://www.rfc-editor.org/info/rfc4954>. <http://www.rfc-editor.org/info/rfc4954>.
[AUTH-ESC] [AUTH-ESC]
Kucherawy, M., "Email Authentication Status Codes", Kucherawy, M., "Email Authentication Status Codes",
RFC 7372, DOI 10.17487/RFC7372, September 2014, RFC 7372, DOI 10.17487/RFC7372, September 2014,
<http://www.rfc-editor.org/info/rfc7372>. <http://www.rfc-editor.org/info/rfc7372>.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<http://www.rfc-editor.org/info/rfc6376>.
[DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<http://www.rfc-editor.org/info/rfc7489>. <http://www.rfc-editor.org/info/rfc7489>.
[DNS] Mockapetris, P., "Domain names - implementation and [DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>. November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
skipping to change at page 37, line 18 skipping to change at page 41, line 19
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003, DOI 10.17487/RFC3464, January 2003,
<http://www.rfc-editor.org/info/rfc3464>. <http://www.rfc-editor.org/info/rfc3464>.
[EMAIL-ARCH] [EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598, Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009, DOI 10.17487/RFC5598, July 2009,
<http://www.rfc-editor.org/info/rfc5598>. <http://www.rfc-editor.org/info/rfc5598>.
[IANA-CONSIDERATIONS]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>. <http://www.rfc-editor.org/info/rfc3501>.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<http://www.rfc-editor.org/info/rfc1939>. <http://www.rfc-editor.org/info/rfc1939>.
[PRA] Lyon, J., "Purported Responsible Address in E-Mail [PRA] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006, Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006,
<http://www.rfc-editor.org/info/rfc4407>. <http://www.rfc-editor.org/info/rfc4407>.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, DOI 10.17487/
RFC5451, April 2009,
<https://www.rfc-editor.org/info/rfc5451>.
[RFC6008] Kucherawy, M., "Authentication-Results Registration for
Differentiating among Cryptographic Results", RFC 6008,
DOI 10.17487/RFC6008, September 2010,
<https://www.rfc-editor.org/info/rfc6008>.
[RFC6577] Kucherawy, M., "Authentication-Results Registration Update
for Sender Policy Framework (SPF) Results", RFC 6577,
DOI 10.17487/RFC6577, March 2012,
<https://www.rfc-editor.org/info/rfc6577>.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, DOI 10.17487/
RFC7001, September 2013,
<https://www.rfc-editor.org/info/rfc7001>.
[RFC7410] Kucherawy, M., "A Property Types Registry for the [RFC7410] Kucherawy, M., "A Property Types Registry for the
Authentication-Results Header Field", RFC 7410, Authentication-Results Header Field", RFC 7410,
DOI 10.17487/RFC7410, December 2014, DOI 10.17487/RFC7410, December 2014,
<http://www.rfc-editor.org/info/rfc7410>. <http://www.rfc-editor.org/info/rfc7410>.
[RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage
Update to DomainKeys Identified Mail (DKIM)", RFC 8301,
DOI 10.17487/RFC8301, January 2018,
<https://www.rfc-editor.org/info/rfc8301>.
[RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid- [RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
Since Header Field and SMTP Service Extension", RFC 7293, Since Header Field and SMTP Service Extension", RFC 7293,
DOI 10.17487/RFC7293, July 2014, DOI 10.17487/RFC7293, July 2014,
<http://www.rfc-editor.org/info/rfc7293>. <http://www.rfc-editor.org/info/rfc7293>.
[SECURITY] [SECURITY]
Rescorla, E. and B. Korver, "Guidelines for Writing RFC Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003, DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>. <http://www.rfc-editor.org/info/rfc3552>.
skipping to change at page 46, line 25 skipping to change at page 51, line 25
1. Service operators prefer to resolve the handling of problem 1. Service operators prefer to resolve the handling of problem
messages as close to the border of the ADMD as possible. This messages as close to the border of the ADMD as possible. This
enables, for example, rejection of messages at the SMTP level enables, for example, rejection of messages at the SMTP level
rather than generating a DSN internally. Thus, doing any of the rather than generating a DSN internally. Thus, doing any of the
authentication or reputation work exclusively at the MUA or authentication or reputation work exclusively at the MUA or
intermediate MTA renders this desire unattainable. intermediate MTA renders this desire unattainable.
2. Border MTAs are more likely to have direct access to external 2. Border MTAs are more likely to have direct access to external
sources of authentication or reputation information since modern sources of authentication or reputation information since modern
MUAs are more likely to be heavily firewalled. Thus, some MUAs MUAs inside of an ADMD are more likely to be heavily firewalled.
might not even be able to complete the task of performing Thus, some MUAs might not even be able to complete the task of
authentication or reputation evaluations without complex proxy performing authentication or reputation evaluations without
configurations or similar burdens. complex proxy configurations or similar burdens.
3. MUAs rely upon the upstream MTAs within their trust boundaries to 3. MUAs rely upon the upstream MTAs within their trust boundaries to
make correct (as much as is possible) evaluations about the make correct (as much as is possible) evaluations about the
message's envelope, header, and content. Thus, MUAs don't need message's envelope, header, and content. Thus, MUAs don't need
to know how to do the work that upstream MTAs do; they only need to know how to do the work that upstream MTAs do; they only need
the results of that work. the results of that work.
4. Evaluations about the quality of a message, from simple token 4. Evaluations about the quality of a message, from simple token
matching (e.g., a list of preferred DNS domains) to cryptanalysis matching (e.g., a list of preferred DNS domains) to cryptographic
(e.g., public/private key work), do have a cost and thus need to verification (e.g., public/private key work), do have a cost and
be minimized. To that end, performing those tests at the border thus need to be minimized. To that end, performing those tests
MTA is far preferred to doing that work at each MUA that handles at the border MTA is far preferred to doing that work at each MUA
a message. If an ADMD's environment adheres to common messaging that handles a message. If an ADMD's environment adheres to
protocols, a reputation query or an authentication check common messaging protocols, a reputation query or an
performed by a border MTA would return the same result as the authentication check performed by a border MTA would return the
same query performed by an MUA. By contrast, in an environment same result as the same query performed by an MUA. By contrast,
where the MUA does the work, a message arriving for multiple in an environment where the MUA does the work, a message arriving
recipients would thus cause authentication or reputation for multiple recipients would thus cause authentication or
evaluation to be done more than once for the same message (i.e., reputation evaluation to be done more than once for the same
at each MUA), causing needless amplification of resource use and message (i.e., at each MUA), causing needless amplification of
creating a possible denial-of-service attack vector. resource use and creating a possible denial-of-service attack
vector.
5. Minimizing change is good. As new authentication and reputation 5. Minimizing change is good. As new authentication and reputation
methods emerge, the list of methods supported by this header methods emerge, the list of methods supported by this header
field would presumably be extended. If MUAs simply consume the field would presumably be extended. If MUAs simply consume the
contents of this header field rather than actually attempt to do contents of this header field rather than actually attempt to do
authentication and/or reputation work, then MUAs only need to authentication and/or reputation work, then MUAs only need to
learn to parse this header field once; emergence of new methods learn to parse this header field once; emergence of new methods
requires only a configuration change at the MUAs and software requires only a configuration change at the MUAs and software
changes at the MTAs (which are presumably fewer in number). When changes at the MTAs (which are presumably fewer in number). When
choosing to implement these functions in MTAs vs. MUAs, the choosing to implement these functions in MTAs vs. MUAs, the
skipping to change at page 47, line 34 skipping to change at page 52, line 37
delivery process has completed. This seriously diminishes the delivery process has completed. This seriously diminishes the
value of this work when done elsewhere than at MTAs. value of this work when done elsewhere than at MTAs.
Many operational choices are possible within an ADMD, including the Many operational choices are possible within an ADMD, including the
venue for performing authentication and/or reputation assessment. venue for performing authentication and/or reputation assessment.
The current specification does not dictate any of those choices. The current specification does not dictate any of those choices.
Rather, it facilitates those cases in which information produced by Rather, it facilitates those cases in which information produced by
one stage of analysis needs to be transported with the message to the one stage of analysis needs to be transported with the message to the
next stage. next stage.
Appendix D. Changes Since RFC7601 Appendix D. Changes since RFC7601
o Added IANA registration for DKIM "a" and "s" properties. o Added IANA registration for DKIM "a" and "s" properties.
o Include EAI guidance. o Include EAI guidance.
o Adjust some ABNF tokens and names for easier inclusion by other o Adjust some ABNF tokes and names for easier inclusion by other
documents. documents.
o Made minor editorial adjustments.
Appendix E. Acknowledgments Appendix E. Acknowledgments
The author wishes to acknowledge the following individuals for their The author wishes to acknowledge the following individuals for their
review and constructive criticism of this document: Seth Blank, Tim review and constructive criticism of this document: Seth Blank, Tim
Draegen, John Levine, Scott Kitterman, and Alessandro Vesely. Draegen, Scott Kitterman, John Levine, and Alessandro Vesely.
Author's Address Author's Address
Murray S. Kucherawy Murray S. Kucherawy
270 Upland Drive 270 Upland Drive
San Francisco, CA 94127 San Francisco, CA 94127
United States United States
Email: superuser@gmail.com Email: superuser@gmail.com
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