rfc7601.txt   draft-kucherawy-dmarc-rfc7601bis.txt 
Internet Engineering Task Force (IETF) M. Kucherawy Individual submission M. Kucherawy
Request for Comments: 7601 August 2015
Obsoletes: 7001, 7410 Obsoletes: 7601 (if approved)
Category: Standards Track Intended status: Standards Track
ISSN: 2070-1721 Expires: July 9, 2019
Message Header Field for Indicating Message Authentication Status Message Header Field for Indicating Message Authentication Status
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.
Status of This Memo This document obsoletes [RFC7601].
This is an Internet Standards Track document. Status of this Memo
This document is a product of the Internet Engineering Task Force This Internet-Draft is submitted in full conformance with the
(IETF). It represents the consensus of the IETF community. It has provisions of BCP 78 and BCP 79.
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, Internet-Drafts are working documents of the Internet Engineering
and how to provide feedback on it may be obtained at Task Force (IETF). Note that other groups may also distribute
http://www.rfc-editor.org/info/rfc7601. working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 9, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2015 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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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . 5 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . 6 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . 6 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 6 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . 7 1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7
1.5.3. Email Architecture . . . . . . . . . . . . . . . . . 7 1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 8 1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 8 1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9
2. Definition and Format of the Header Field . . . . . . . . . . 9 2. Definition and Format of the Header Field . . . . . . . . . . 9
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 . . . . . . . . . 12 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13
2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . 13 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14
2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 2.5. Authentication Identifier Field . . . . . . . . . . . . . 14
2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . 15 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16
2.7. Defined Methods and Result Values . . . . . . . . . . . . 15 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16
2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16
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" . . . . . . . . . . . . . . . . . . . . . . . 19
2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20
2.7.5. Other Registered Codes . . . . . . . . . . . . . . . 21 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21
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 . . . . . . . . . . . . . . 22 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23
4. Adding the Header Field to a Message . . . . . . . . . . . . 23 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 . . . . . . . . . . . . . . . 26 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
6.1. The Authentication-Results Header Field . . . . . . . . . 27 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 . . . . . 29 6.3. "Email Authentication Methods" Registry Update . . . . . . 30
6.4. "Email Authentication Property Types" Registry . . . . . 30 6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 31
6.5. "Email Authentication Result Names" Description . . . . . 31 6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 32
6.6. "Email Authentication Result Names" Update . . . . . . . 32 6.4. "Email Authentication Property Types" Registry
6.7. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . 33 Description . . . . . . . . . . . . . . . . . . . . . . . 32
7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 6.5. "Email Authentication Property Types" Registry Update . . 33
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . 33 6.6. "Email Authentication Result Names" Registry
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . 35 Description . . . . . . . . . . . . . . . . . . . . . . . 33
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 35 6.7. "Email Authentication Result Names" Registry Update . . . 33
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . 35 6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 34
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . 36 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 34
7.7. Attacks against Authentication Methods . . . . . . . . . 36 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 36
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 36 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 36
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . 36 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 36
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . 37 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 37
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 37 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 37
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 7.7. Attacks against Authentication Methods . . . . . . . . . . 37
8.1. Normative References . . . . . . . . . . . . . . . . . . 37 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 37
8.2. Informative References . . . . . . . . . . . . . . . . . 38 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 37
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . 42 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 38
Appendix B. Authentication-Results Examples . . . . . . . . . . 42 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 38
B.1. Trivial Case; Header Field Not Present . . . . . . . . . 42 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
8.1. Normative References . . . . . . . . . . . . . . . . . . . 38
8.2. Informative References . . . . . . . . . . . . . . . . . . 39
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 42
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 . . . . . . . . . . . . . . . . . . . 43 Authentication Done . . . . . . . . . . . . . . . . . . . 44
B.3. Service Provided, Authentication Done . . . . . . . . . . 44 B.3. Service Provided, Authentication Done . . . . . . . . . . 45
B.4. Service Provided, Several Authentications Done, Single B.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
B.5. Service Provided, Several Authentications Done, Different B.5. Service Provided, Several Authentications Done,
MTAs . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 47
B.6. Service Provided, Multi-tiered Authentication Done . . . 48 B.6. Service Provided, Multi-tiered Authentication Done . . . . 49
B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 49 B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 50
Appendix C. Operational Considerations about Message Appendix C. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 50 Authentication . . . . . . . . . . . . . . . . . . . 51
Appendix D. Changes since RFC 7001 . . . . . . . . . . . . . . . 51 Appendix D. Changes since RFC7601 . . . . . . . . . . . . . . . . 52
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 53 Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 53
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 53 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 and/ Agent (MUA) and downstream filters can make filtering decisions
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
message's origin and possibly the safety and integrity of its message's origin and possibly the safety and integrity of its
content. content.
This document revises the original definition found in [RFC5451]
based upon various authentication protocols in current use and
incorporates errata logged since the publication of the original
specification.
End users are not expected to be direct consumers of this header End users are not expected to be direct consumers of this header
field. This header field is intended for consumption by programs field. This header field is intended for consumption by programs
that will then use such data or render it in a human-usable form. that will then use such data or render it in a human-usable form.
This document specifies the format of this header field and discusses This document specifies the format of this header field and discusses
the implications of its presence or absence. However, it does not the implications of its presence or absence. However, it does not
discuss how the data contained in the header field ought to be used, discuss how the data contained in the header field ought to be used,
such as what filtering decisions are appropriate or how an MUA might such as what filtering decisions are appropriate or how an MUA might
render those results, as these are local policy and/or user interface render those results, as these are local policy and/or user interface
design questions that are not appropriate for this document. design questions that are not appropriate for this document.
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o reverse IP address name validation ("iprev", defined in Section 3) o reverse IP address name validation ("iprev", defined in Section 3)
o Require-Recipient-Valid-Since Header Field and SMTP Service o Require-Recipient-Valid-Since Header Field and SMTP Service
Extension ([RRVS]) Extension ([RRVS])
o S/MIME Signature Verification ([SMIME-REG]) o S/MIME Signature Verification ([SMIME-REG])
o Vouch By Reference ([VBR]) o Vouch By Reference ([VBR])
o DomainKeys ([DOMAINKEYS]) (Historic) o DomainKeys ([DOMAINKEYS]) (Historic)
o Sender ID ([SENDERID]) (Experimental) o Sender ID ([SENDERID]) (Experimental)
There exist registries for tokens used within this header field that There exist registries for tokens used within this header field that
refer to the specifications listed above. Section 6 describes the refer to the specifications listed above. Section 6 describes the
registries and their contents and specifies the process by which registries and their contents and specifies the process by which
entries are added or updated. It also updates the existing contents entries are added or updated. It also updates the existing contents
to match the current states of these specifications. to match the current states of these specifications.
This specification is not intended to be restricted to domain-based The goal of this work is to give current and future authentication
authentication schemes, but the existing schemes in that family have schemes a common framework within which to deliver their results to
proven to be a good starting point for implementations. The goal is downstream agents and discourage the creation of unique header fields
to give current and future authentication schemes a common framework for each.
within which to deliver their results to downstream agents and
discourage the creation of unique header fields for each.
Although SPF defined a header field called "Received-SPF" and the Although SPF defined a header field called "Received-SPF" and the
historic DomainKeys defined one called "DomainKey-Status" for this historic DomainKeys defined one called "DomainKey-Status" for this
purpose, those header fields are specific to the conveyance of their purpose, those header fields are specific to the conveyance of their
respective results only and thus are insufficient to satisfy the respective results only and thus are insufficient to satisfy the
requirements enumerated below. In addition, many SPF implementations requirements enumerated below. In addition, many SPF implementations
have adopted the header field specified here at least as an option, have adopted the header field specified here at least as an option,
and DomainKeys has been obsoleted by DKIM. and DomainKeys has been obsoleted by DKIM.
1.1. Purpose 1.1. Purpose
skipping to change at page 5, line 32 skipping to change at page 5, line 44
results to end users or to use those data to apply more or less results to end users or to use those data to apply more or less
stringent content checks based on authentication results; stringent content checks based on authentication results;
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 (supposedly) assertions made by one or more authentication schemes applied
applied somewhere upstream. For an MUA or downstream filter to treat somewhere upstream. For an MUA or downstream filter to treat the
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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their discretion. their discretion.
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. Security 1.5.2. Internationalized Email
In this document, there are references to messages formatted to
support Email Address Internationalization (EAI). Reference material
for this can be found in [RFC6530], [RFC6531], and [RFC6532].
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
used, and this document extends Authentication-Results accordingly.
1.5.3. Security
"Guidelines for Writing RFC Text on Security Considerations" "Guidelines for Writing RFC Text on Security Considerations"
([SECURITY]) discusses authentication and authorization and the ([SECURITY]) discusses authentication and authorization and the
conflation of the two concepts. The use of those terms within the conflation of the two concepts. The use of those terms within the
context of recent message security work has given rise to slightly context of recent message security work has given rise to slightly
different definitions, and this document reflects those current different definitions, and this document reflects those current
usages, as follows: usages, as follows:
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 proposal groups them both into a single header field. solution, this specification groups them both into a single header
field.
1.5.3. 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.)
o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
actually enacts delivery of a message to a user's inbox or other actually enacts delivery of a message to a user's inbox or other
final delivery. final delivery.
o An "intermediate MTA" is any MTA that is not a delivery MTA and is o An "intermediate MTA" is any MTA that is not a delivery MTA and is
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authentication schemes takes place at a border MTA or a delivery MTA. authentication schemes takes place at a border MTA or a delivery MTA.
This specification is written with that assumption in mind. However, This specification is written with that assumption in mind. However,
there are some sites at which the entire mail infrastructure consists there are some sites at which the entire mail infrastructure consists
of a single host. In such cases, such terms as "border MTA" and of a single host. In such cases, such terms as "border MTA" and
"delivery MTA" might well apply to the same machine or even the very "delivery MTA" might well apply to the same machine or even the very
same agent. It is also possible that some message authentication same agent. It is also possible that some message authentication
tests could take place on an intermediate MTA. Although this tests could take place on an intermediate MTA. Although this
document doesn't specifically describe such cases, they are not meant document doesn't specifically describe such cases, they are not meant
to be excluded. to be excluded.
1.5.4. Other Terms 1.5.5. Other Terms
In this document, the term "producer" refers to any component that In this document, the term "producer" refers to any component that
adds this header field to messages it is handling, and "consumer" adds this header field to messages it is handling, and "consumer"
refers to any component that identifies, extracts, and parses the refers to any component that identifies, extracts, and parses the
header field to use as part of a handling decision. header field to use as part of a handling decision.
1.6. Trust Environment 1.6. Trust Environment
This header field permits one or more message validation mechanisms This header field permits one or more message validation mechanisms
to communicate output to one or more separate assessment mechanisms. to communicate output to one or more separate assessment mechanisms.
These mechanisms operate within a unified trust boundary that defines These mechanisms operate within a unified trust boundary that defines
an Administrative Management Domain (ADMD). An ADMD contains one or an Administrative Management Domain (ADMD). An ADMD contains one or
more entities that perform validation and generate the header field more entities that perform validation and generate the header field
and one or more that consume it for some type of assessment. The and one or more that consume it for some type of assessment. The
field often contains no integrity or validation mechanism of its own, field often contains no integrity or validation mechanism of its own,
so its presence must be trusted implicitly. Hence, valid use of the so its presence must be trusted implicitly. Hence, valid use of the
header field requires removing any occurrences of it that are present header field requires removing any occurrences of it that claim to be
when the message enters the ADMD. This ensures that later associated with the ADMD when the message enters the ADMD. This
occurrences have been added within the trust boundary of the ADMD. ensures that later occurrences have been added within the trust
boundary of the ADMD.
The authserv-id token defined in Section 2.2 can be used to reference The authserv-id token defined in Section 2.2 can be used to reference
an entire ADMD or a specific validation engine within an ADMD. an entire ADMD or a specific validation engine within an ADMD.
Although the labeling scheme is left as an operational choice, some Although the labeling scheme is left as an operational choice, some
guidance for selecting a token is provided in later sections of this guidance for selecting a token is provided in later sections of this
document. document.
2. Definition and Format of the Header Field 2. Definition and Format of the Header Field
This section gives a general overview of the format of the header This section gives a general overview of the format of the header
field being defined and then provides more formal specification. field being defined and then provides a formal specification.
2.1. General Description 2.1. General Description
The header field specified here is called Authentication-Results. It The header field specified here is called Authentication-Results. It
is a Structured Header Field as defined in Internet Message Format is a Structured Header Field as defined in Internet Message Format
([MAIL]), and thus all of the related definitions in that document ([MAIL]), and thus all of the related definitions in that document
apply. apply.
This header field is added at the top of the message as it transits This header field is added at the top of the message as it transits
MTAs that do authentication checks, so some idea of how far away the MTAs that do authentication checks, so some idea of how far away the
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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 = "Authentication-Results:" [CFWS] authserv-id authres-header-field = "Authentication-Results:" authres-payload
authres-payload = "Authentication-Results:" [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
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; the message body, or was some other property evaluated by ; the message body, or was some other property evaluated by
; the receiving MTA; expected to be one of the "property ; the receiving MTA; expected to be one of the "property
; types" explicitly defined as valid, or an extension ; types" explicitly defined as valid, or an extension
; ptype, as defined below ; ptype, as defined below
property = special-smtp-verb / Keyword property = special-smtp-verb / Keyword
; 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], and "CFWS" is "local-part" is defined in Section 3.4.1 of [MAIL], as modified by
defined in Section 3.2.2 of [MAIL]. [RFC6531].
"Keyword" is defined in Section 4.1.2 of [SMTP]. "CFWS" is defined in Section 3.2.2 of [MAIL].
The "value" is as defined in Section 5.1 of [MIME]. "Keyword" is defined in Section 4.1.2 of [SMTP]. It is further
constrained by the necesity of being registered in the IANA registry
relevant to the context in which it it is used. See Section 2.7, and
Section 2.3, and Section 6.
The "value" is as defined in Section 5.1 of [MIME], with "quoted-
string" updated as specified in [RFC6532].
The "domain-name" is as defined in Section 3.5 of [DKIM]. The "domain-name" is as defined in Section 3.5 of [DKIM].
The "Keyword" used in "result" above is further constrained by the The "Keyword" used in "result" above is further constrained by the
necessity of being enumerated in Section 2.7. 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].
The "propspec" may be omitted if, for example, the method was unable The "propspec" may be omitted if, for example, the method was unable
to extract any properties to do its evaluation yet has a result to to extract any properties to do its evaluation yet still has a result
report. to report. It may also be omitted if the agent generating this
result wishes not to reveal such properties to downstream agents.
Where an SMTP command name is being reported as a "property", the Where an SMTP command name is being reported as a "property", the
agent generating the header field represents that command by agent generating the header field represents that command by
converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" converting it to lowercase and dropping any spaces (e.g., "MAIL FROM"
becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).
A "ptype" value of "policy" indicates a policy decision about the A "ptype" value of "policy" indicates a policy decision about the
message not specific to a property of the message that could be message not specific to a property of the message that could be
extracted. See Section 2.4 for details. extracted. See Section 2.4 for details.
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 which particular part of the message the specific, they indicate from where the reported data were extracted.
reported data were extracted. This can include a particular part of the message header or body,
some part of the SMTP session, a secondary output of an
authentication method (apart from its pure result), or some other
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]:
skipping to change at page 13, line 39 skipping to change at page 14, line 29
For example, a DKIM signature is not required to include the Subject For example, a DKIM signature is not required to include the Subject
header field in the set of fields that are signed. An ADMD receiving header field in the set of fields that are signed. An ADMD receiving
such a message might decide that such a signature is unacceptable, such a message might decide that such a signature is unacceptable,
even if it passes, because the content of the Subject header field even if it passes, because the content of the Subject header field
could be altered post-signing without invalidating the signature. could be altered post-signing without invalidating the signature.
Such an ADMD could replace the DKIM "pass" result with a "policy" Such an ADMD could replace the DKIM "pass" result with a "policy"
result and then also include the following in the corresponding result and then also include the following in the corresponding
Authentication-Result field: Authentication-Result field:
... dkim=fail policy.dkim-rules=unsigned-subject ... ... dkim=policy policy.dkim-rules=unsigned-subject ...
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
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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.
Note that in an EAI-formatted message, this identifier may be
expressed in UTF-8.
Since agents consuming this field will use this identifier to Since agents consuming this field will use this identifier to
determine whether its contents are of interest (and are safe to use), determine whether its contents are of interest (and are safe to use),
the uniqueness of the identifier MUST be guaranteed by the ADMD that the uniqueness of the identifier MUST be guaranteed by the ADMD that
generates it and MUST pertain to that ADMD. MUAs or downstream generates it and MUST pertain to that ADMD. MUAs or downstream
filters SHOULD use this identifier to determine whether or not the filters SHOULD use this identifier to determine whether or not the
data contained in an Authentication-Results header field ought to be data contained in an Authentication-Results header field ought to be
used or ignored. used or ignored.
For simplicity and scalability, the authentication service identifier For simplicity and scalability, the authentication service identifier
SHOULD be a common token used throughout the ADMD. Common practice SHOULD be a common token used throughout the ADMD. Common practice
is to use the DNS domain name used by or within that ADMD, sometimes is to use the DNS domain name used by or within that ADMD, sometimes
called the "organizational domain", but this is not strictly called the "organizational domain", but this is not strictly
necessary. necessary.
For tracing and debugging purposes, the authentication identifier can For tracing and debugging purposes, the authentication identifier can
instead be the specific hostname of the MTA performing the instead be the specific hostname of the MTA performing the
authentication check whose result is being reported. Moreover, some authentication check whose result is being reported. Moreover, some
implementations define a substructure to the identifier; these are implementations define a substructure to the identifier; such
outside of the scope of this specification. structures are outside of the scope of this specification.
Note, however, that using a local, relative identifier like a flat Note, however, that using a local, relative identifier like a flat
hostname, rather than a hierarchical and globally unique ADMD hostname, rather than a hierarchical and globally unique ADMD
identifier like a DNS domain name, makes configuration more difficult identifier like a DNS domain name, makes configuration more difficult
for large sites. The hierarchical identifier permits aggregating for large sites. The hierarchical identifier permits aggregating
related, trusted systems together under a single, parent identifier, related, trusted systems together under a single, parent identifier,
which in turn permits assessing the trust relationship with a single which in turn permits assessing the trust relationship with a single
reference. The alternative is a flat namespace requiring reference. The alternative is a flat namespace requiring
individually listing each trusted system. Since consumers will use individually listing each trusted system. Since consumers will use
the identifier to determine whether to use the contents of the header the identifier to determine whether to use the contents of the header
skipping to change at page 17, line 8 skipping to change at page 17, line 44
is unrecoverable, such as a required header field being absent. A is unrecoverable, such as a required header field being absent. A
later attempt is unlikely to produce a final result. later attempt is unlikely to produce a final result.
DKIM results are reported using a ptype of "header". The property, DKIM results are reported using a ptype of "header". The property,
however, represents one of the tags found in the DKIM-Signature however, represents one of the tags found in the DKIM-Signature
header field rather than a distinct header field. For example, the header field rather than a distinct header field. For example, the
ptype-property combination "header.d" refers to the content of the ptype-property combination "header.d" refers to the content of the
"d" (signing domain) tag from within the signature header field, and "d" (signing domain) tag from within the signature header field, and
not a distinct header field called "d". not a distinct header field called "d".
Note that in an EAI-formatted message, the values of the "d" and "i"
properties can be expressed in UTF-8.
In addition to previous registrations, this document registers the
DKIM tag "a" (cryptographic algorithm used to sign the message) as a
reportable property. This can be used to aid receivers during post-
verification processing. In particular, [RFC8301] obsoleted use of
the "rsa-sha1" algorithm in DKIM, so it is important to be able to
distinguish such signatures from those using preferred algorithms.
The ability to report different DKIM results for a message with The ability to report different DKIM results for a message with
multiple signatures is described in [RFC6008]. multiple signatures is described in [RFC6008].
[DKIM] advises that if a message fails verification, it is to be [DKIM] advises that if a message fails verification, it is to be
treated as an unsigned message. A report of "fail" here permits the treated as an unsigned message. A report of "fail" here permits the
receiver of the report to decide how to handle the failure. A report receiver of the report to decide how to handle the failure. A report
of "neutral" or "none" preempts that choice, ensuring the message of "neutral" or "none" preempts that choice, ensuring the message
will be treated as if it had not been signed. will be treated as if it had not been signed.
Section 3.1 of [DOMAINKEYS] describes a process by which the sending Section 3.1 of [DOMAINKEYS] describes a process by which the sending
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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
expressed in UTF-8.
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
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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. the AUTH parameter used with the MAIL FROM command.
Note that in an EAI-formatted message, these values can be expressed
in UTF-8.
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:
; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example
skipping to change at page 21, line 19 skipping to change at page 22, line 4
Result codes were also registered in other RFCs as follows: Result codes were also registered in other RFCs as follows:
o Vouch By Reference (in [AR-VBR], represented by "vbr"); o Vouch By Reference (in [AR-VBR], represented by "vbr");
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 in an EAI-formatted message, "vbr.mv" and "vbr.md", which
are already registered, can be expressed in UTF-8.
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:
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Authentication method implementers are encouraged to provide adequate Authentication method implementers are encouraged to provide adequate
information, via message header field comments if necessary, to allow information, via message header field comments if necessary, to allow
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. future by later revisions or extensions to this specification. Non-
Result codes MUST be registered with the Internet Assigned Numbers experimental result codes MUST be registered with the Internet
Authority (IANA) and preferably published in an RFC. See Section 6 Assigned Numbers Authority (IANA) and preferably published in an RFC.
for further details. See Section 6 for further details.
Experimental results MUST only be used within ADMDs that have Experimental results MUST only be used within ADMDs that have
explicitly consented to use them. These results and the parameters explicitly consented to use them. These results and the parameters
associated with them are not formally documented. Therefore, they associated with them are not formally documented. Therefore, they
are subject to change at any time and not suitable for production are subject to change at any time and not suitable for production
use. Any MTA, MUA, or downstream filter intended for production use use. Any MTA, MUA, or downstream filter intended for production use
SHOULD ignore or delete any Authentication-Results header field that SHOULD ignore or delete any Authentication-Results header field that
includes an extension result. includes an extension result.
3. The "iprev" Authentication Method 3. The "iprev" Authentication Method
skipping to change at page 24, line 14 skipping to change at page 25, line 5
Each "method" MUST refer to an authentication method declared in the Each "method" MUST refer to an authentication method declared in the
IANA registry or an extension method as described in Section 2.7.6, IANA registry or an extension method as described in Section 2.7.6,
and each "result" MUST refer to a result code declared in the IANA and each "result" MUST refer to a result code declared in the IANA
registry or an extension result code as defined in Section 2.7.7. registry or an extension result code as defined in Section 2.7.7.
See Section 6 for further information about the registered methods See Section 6 for further information about the registered methods
and result codes. and result codes.
An MTA compliant with this specification adds this header field An MTA compliant with this specification adds this header field
(after performing one or more message authentication tests) to (after performing one or more message authentication tests) to
indicate which MTA or ADMD performed the test, which test got indicate which MTA or ADMD performed the test, which test was
applied, and what the result was. If an MTA applies more than one applied, and what the result was. If an MTA applies more than one
such test, it adds this header field either once per test or once such test, it adds this header field either once per test or once
indicating all of the results. An MTA MUST NOT add a result to an indicating all of the results. An MTA MUST NOT add a result to an
existing header field. existing header field.
An MTA MAY add this header field containing only the authentication An MTA MAY add this header field containing only the authentication
identifier portion and the "none" token (see Section 2.2) to indicate identifier portion and the "none" token (see Section 2.2) to indicate
explicitly that no message authentication schemes were applied prior explicitly that no message authentication schemes were applied prior
to delivery of this message. to delivery of this message.
skipping to change at page 25, line 23 skipping to change at page 26, line 19
this header field unless specifically configured to do so by the user this header field unless specifically configured to do so by the user
or administrator. That is, this interpretation should not be "on by or administrator. That is, this interpretation should not be "on by
default". Naturally then, users or administrators ought not activate default". Naturally then, users or administrators ought not activate
such a feature unless (1) they are certain the header field will be such a feature unless (1) they are certain the header field will be
validly added by an agent within the ADMD that accepts the mail that validly added by an agent within the ADMD that accepts the mail that
is ultimately read by the MUA, and (2) instances of the header field is ultimately read by the MUA, and (2) instances of the header field
that appear to originate within the ADMD but are actually added by that appear to originate within the ADMD but are actually added by
foreign MTAs will be removed before delivery. foreign MTAs will be removed before delivery.
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 it bears header field unless the authentication service identifier of the
appears to be one used within its own ADMD as configured by the user header field is used within the ADMD as configured by the user or
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 9 skipping to change at page 27, line 6
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
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The same MAY also be done for local policy decisions overriding the The same MAY also be done for local policy decisions overriding the
results of the authentication methods (e.g., the "policy" result results of the authentication methods (e.g., the "policy" result
codes described in Section 2.7). codes described in Section 2.7).
Such rejections at the SMTP protocol level are not possible if local Such rejections at the SMTP protocol level are not possible if local
policy is enforced at the MUA and not the MTA. policy is enforced at the MUA and not the MTA.
5. Removing Existing Header Fields 5. Removing Existing Header Fields
For security reasons, any MTA conforming to this specification MUST To mitigate the impact of forged header fields, any MTA conforming to
delete any discovered instance of this header field that claims, by this specification MUST delete any discovered instance of this header
virtue of its authentication service identifier, to have been added field that claims, by virtue of its authentication service
within its trust boundary but that did not come directly from another identifier, to have been added within its trust boundary but that did
trusted MTA. For example, an MTA for example.com receiving a message not come directly from another trusted MTA. For example, an MTA for
MUST delete or otherwise obscure any instance of this header field example.com receiving a message MUST delete or otherwise obscure any
bearing an authentication service identifier indicating that the instance of this header field bearing an authentication service
header field was added within example.com prior to adding its own identifier indicating that the header field was added within
header fields. This could mean each MTA will have to be equipped example.com prior to adding its own header fields. This could mean
with a list of internal MTAs known to be compliant (and hence each internal MTA will need to be configured with a list of other
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
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
MTA could allow a specific list of authenticating MTAs whose MTA could allow a specific list of authenticating MTAs whose
information is to be admitted, removing the header field originating information is to be admitted, removing the header field originating
from all others. from all others.
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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 tables as IANA has registered the defined header field and created registries
described below. These registry actions were originally defined by as described below. These registry actions were originally defined
[RFC5451] and updated by [RFC6577] and [RFC7001]. The created by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created
registries are being further updated here to increase their registries were further updated in [RFC7601] to make them more
completeness. complete.
Each registry has two related sections below. The first describes
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
[RFC5451] added the Authentication-Results header field 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 has been 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): RFC 7601 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
Names of message authentication methods supported by this Names of message authentication methods supported by this
specification have been registered with IANA, with the exception of specification have been registered with IANA, with the exception of
experimental names as described in Section 2.7.6. Along with each experimental names as described in Section 2.7.6. Along with each
method is recorded the properties that accompany the method's result. method is recorded the properties that accompany the method's result.
The "Email Authentication Parameters" group, and within it the "Email The "Email Authentication Parameters" group, and within it the "Email
Authentication Methods" registry, were created by [RFC5451] for this Authentication Methods" registry, were created by [RFC5451] for this
purpose. [RFC6577] added a "status" field for each entry. [RFC7001] purpose. [RFC6577] added a "status" field for each entry. [RFC7001]
amended the rules governing that registry and also added a "version" amended the rules governing that registry and also added a "version"
field to the registry. field to the registry.
The reference for that registry has been updated to reference this The reference for that registry will be updated to reference this
document. document.
New entries are assigned only for values that have received Expert New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to appointed by the IESG. The designated expert has discretion to
request that a publication be referenced if a clear, concise request that a publication be referenced if a clear, concise
definition of the authentication method cannot be provided such that definition of the authentication method cannot be provided such that
interoperability is assured. Registrations should otherwise be interoperability is assured. Registrations should otherwise be
permitted. The designated expert can also handle requests to mark permitted. The designated expert can also handle requests to mark
any current registration as "deprecated". any current registration as "deprecated".
skipping to change at page 29, line 26 skipping to change at page 30, line 36
The "Definition" field will typically refer to a permanent document, The "Definition" field will typically refer to a permanent document,
or at least some descriptive text, where additional information about or at least some descriptive text, where additional information about
the entry being added can be found. This might in turn reference the 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 document where the method is defined so that all of the semantics
around creating or interpreting an Authentication-Results header around creating or interpreting an Authentication-Results header
field using this method, ptype, and property can be understood. 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 changes have been made to this registry per this The following entries in this registry are to be updated to replace
document: [RFC7601] with this document:
1. The "Defined" field has been renamed "Definition", to be +------------+--------+----------------------------------+
consistent with the other registries in this group. | 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 |
+------------+--------+----------------------------------+
2. The entry for the "dkim" method, "header" ptype, and "b" property In addition, two new entries are added to this registry, as follows:
now reference [RFC6008] as the defining document, and the
reference has be removed from the description.
3. All other "dkim", "domainkeys", "iprev", "sender-id", and "spf" 6.3.1. 'header.a' for DKIM
method entries have had their "Definition" fields changed to
refer to this document, as this document contains a complete
description of the registry and these corresponding values.
4. All "smime" entries have had their "Definition" fields changed to Method: dkim
[SMIME-REG].
5. The "value" field of the "smime" entry using property "smime- Definition: [this document]
part" has been changed to read: "The MIME body part reference
that contains the S/MIME signature. See Section 3.2.1 of RFC
7281 for full syntax."
6. The single entry for the "auth" method was intended to reflect
the identity indicated by the "AUTH" parameter to the SMTP "MAIL
FROM" command verb. However, there is also an "AUTH" command
verb. To clarify this ambiguity, the entry for the "auth" method
has had its "property" field changed to "mailfrom", and its
"Definition" field changed to this document.
7. The following entry has been added: ptype: header
Method: auth property: a
Definition: this document (RFC 7601) Description: value of signature "a" tag
ptype: smtp Status: active
property: auth Version: 1
Value: identity confirmed by the AUTH command 6.3.2. 'header.s' for DKIM
Status: active Method: dkim
Version: 1 Definition: [this document]
8. The values of the "domainkeys" entries for ptype "header" have ptype: header
been updated as follows:
from: contents of the [MAIL] From: header field, after removing property: s
comments, and removing the local-part and following "@" if not
authenticated
sender: contents of the [MAIL] Sender: header field, after Description: value of signature "s" tag
removing comments, and removing the local-part and following
"@" if not authenticated
9. For all entries for "dkim-adsp" and "domainkeys", their Status Status: active
values have been changed to "deprecated", reflecting the fact
that the corresponding specifications now have Historic status.
Their "Definition" fields have also been modified to include a
reference to this document.
6.4. "Email Authentication Property Types" Registry Version: 1
6.4. "Email Authentication Property Types" Registry Description
[RFC7410] created the "Email Authentication Property Types" registry. [RFC7410] created the "Email Authentication Property Types" registry.
Entries in this registry are subject to the Expert Review rules as Entries in this registry are subject to the Expert Review rules as
described in [IANA-CONSIDERATIONS]. Each entry in the registry described in [IANA-CONSIDERATIONS]. Each entry in the registry
requires the following values: requires the following values:
ptype: The name of the ptype being registered, which must fit within ptype: The name of the ptype being registered, which must fit within
the ABNF described in Section 2.2. the ABNF described in Section 2.2.
skipping to change at page 31, line 21 skipping to change at page 32, line 45
"ptype" is meant to cover. "ptype" is meant to cover.
For new entries, the Designated Expert needs to assure that the For new entries, the Designated Expert needs to assure that the
description provided for the new entry adequately describes the description provided for the new entry adequately describes the
intended use. An example would be helpful to include in the entry's intended use. An example would be helpful to include in the entry's
defining document, if any, although entries in the "Email defining document, if any, although entries in the "Email
Authentication Methods" registry or the "Email Authentication Result Authentication Methods" registry or the "Email Authentication Result
Names" registry might also serve as examples of intended use. Names" registry might also serve as examples of intended use.
As this is a complete restatement of the definition and rules for As this is a complete restatement of the definition and rules for
this registry, IANA has updated this registry to show Section 2.3 of this registry, IANA will update this registry to show Section 2.3 of
this document as the current definitions for the "body", "header", this document as the current definitions for the "body", "header",
"policy", and "smtp" entries of that registry. References to "policy", and "smtp" entries of that registry. References to other
[RFC7001] and [RFC7410] have been removed. documents will be removed.
6.5. "Email Authentication Result Names" Description 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 Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.7.7. A registry was experimental codes as described in Section 2.7.7.
created by [RFC5451] for this purpose. [RFC6577] added the "status"
column and [RFC7001] updated the rules governing that registry.
New entries are assigned only for values that have received Expert New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to appointed by the IESG. The designated expert has discretion to
request that a publication be referenced if a clear, concise request that a publication be referenced if a clear, concise
definition of the authentication result cannot be provided such that definition of the authentication result cannot be provided such that
interoperability is assured. Registrations should otherwise be interoperability is assured. Registrations should otherwise be
permitted. The designated expert can also handle requests to mark permitted. The designated expert can also handle requests to mark
any current registration as "deprecated". any current registration as "deprecated".
skipping to change at page 32, line 14 skipping to change at page 33, line 44
Specification: either free form text explaining the meaning of this Specification: either free form text explaining the meaning of this
method-code combination, or a reference to such a definition. method-code combination, or a reference to such a definition.
Status: the status of this entry, which is either: Status: the status of this entry, which is either:
active: The entry is in current use. active: The entry is in current use.
deprecated: The entry is no longer in current use. deprecated: The entry is no longer in current use.
6.6. "Email Authentication Result Names" Update 6.7. "Email Authentication Result Names" Registry Update
This document includes a complete description of the registry,
obsoleting [RFC7001]. Accordingly, the following changes have been
made to this registry per this document:
o The "Defined" field has been removed.
o The "Meaning" field has been renamed "Specification", as described
above.
o The "Auth Method" field now appears before the "Code" field.
o For easier searching, the table has been arranged such that it is
sorted first by Auth Method, then by Code within each Auth Method
grouping.
o All entries for the "dkim", "domainkeys", "spf", "sender-id",
"auth", and "iprev" methods have had their "Specification" fields
replaced as follows:
dkim: Section 2.7.1 of this document (RFC 7601)
domainkeys: Section 2.7.1 of this document (RFC 7601)
spf: for "hardfail", Section 2.4.2 of [RFC5451]; for all others,
Section 2.7.2 of this document (RFC 7601)
sender-id: for "hardfail", Section 2.4.2 of [RFC5451]; for all
others, Section 2.7.2 of this document (RFC 7601)
auth: Section 2.7.4 of this document (RFC 7601) The following entries in this registry are to be updated to reflect
new Specifications as follows:
iprev: Section 2.7.3 of this document (RFC 7601) o All "auth" method result codes ("fail", "none", "pass",
"permerror", "temperror") are now specified in Section 2.7.4 of
this document.
o All entries for "dkim-adsp" that were missing an explicit o All "dkim" method result names ("fail", "neutral", "none", "pass",
reference to a defining document now reference [ADSP] in their "permerror", "policy", "temperror") are now specified in
"Specification" fields. Section 2.7.1 of this document.
o All entries for "dmarc" have had their "Specification" fields o All "iprev" method result names ("fail", "pass", "permerror",
changed to reference Section 11.2 of [DMARC]. "temperror") are now specified in Section 2.7.3 of this document.
o All entries for "dkim-adsp" and "domainkeys" have had their Status o The "sender-id" and "spf" method result names "fail", "neutral",
values changed to "deprecated", reflecting the fact that the "none", "pass", "permerror", "policy", "softfail", and "temperror"
corresponding specifications now have Historic status. Their are now specified in Section 2.7.2 of this document. The
"Specification" fields have also been modified to include a registrations for result name "hardfail" are not updated.
reference to this document.
6.7. SMTP Enhanced Status Codes 6.8. SMTP Enhanced Status Codes
The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of
the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry" has been updated to refer to this document instead of Registry" is to be updated to refer only to Section 3.3 of [AUTH-ESC]
[RFC7001]. 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.
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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
It is possible for an attacker to add an Authentication-Results As with any other header field found in the message, it is possible
header field that is extraordinarily large or otherwise malformed in for an attacker to add an Authentication-Results header field that is
an attempt to discover or exploit weaknesses in header field parsing extraordinarily large or otherwise malformed in an attempt to
code. Implementers must thoroughly verify all such header fields discover or exploit weaknesses in header field parsing code.
received from MTAs and be robust against intentionally as well as Implementers must thoroughly verify all such header fields received
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
header field that endorses it. Although it is clearly a larger header field that endorses it. Although it is clearly a larger
concern to have compromised internal machines than it is to prove the concern to have compromised internal machines than it is to prove the
value of this header field, this risk can be mitigated by arranging value of this header field, this risk can be mitigated by arranging
that internal MTAs will remove this header field if it claims to have that internal MTAs will remove this header field if it claims to have
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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 early guidance to MUA developers is
appropriate here. appropriate here.
Since MTAs are unlikely to strip Authentication-Results header fields Since MTAs are unlikely to strip Authentication-Results header fields
after mailbox delivery, MUAs are advised in Section 4.1 to ignore after mailbox delivery, MUAs are warned in Section 4.1 to ignore such
such instances within MIME attachments. Moreover, when extracting a instances within MIME attachments. Moreover, when extracting a
message digest to separate mail store messages or other media, such message digest to separate mail store messages or other media, such
header fields should be removed so that they will never be header fields should be removed so that they will never be
interpreted improperly by MUAs that might later consume them. interpreted improperly by MUAs that might later consume them.
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 proposal and agents that use the data it Implementers of both this specification and agents that use the data
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, Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
DOI 10.17487/RFC5234, January 2008, RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532,
February 2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601, DOI 10.17487/
RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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, August Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617,
2009, <http://www.rfc-editor.org/info/rfc5617>. August 2009, <http://www.rfc-editor.org/info/rfc5617>.
[AR-VBR] Kucherawy, M., "Authentication-Results Registration for [AR-VBR] Kucherawy, M., "Authentication-Results Registration for
Vouch by Reference Results", RFC 6212, Vouch by Reference Results", RFC 6212, DOI 10.17487/
DOI 10.17487/RFC6212, April 2011, RFC6212, April 2011,
<http://www.rfc-editor.org/info/rfc6212>. <http://www.rfc-editor.org/info/rfc6212>.
[ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM)
Authorized Third-Party Signatures", RFC 6541, Authorized Third-Party Signatures", RFC 6541,
DOI 10.17487/RFC6541, February 2012, DOI 10.17487/RFC6541, February 2012,
<http://www.rfc-editor.org/info/rfc6541>. <http://www.rfc-editor.org/info/rfc6541>.
[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, Extension for Authentication", RFC 4954, DOI 10.17487/
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., [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76, "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011, RFC 6376, DOI 10.17487/RFC6376, September 2011,
skipping to change at page 39, line 36 skipping to change at page 41, line 11
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] [IANA-CONSIDERATIONS]
Narten, T. and H. Alvestrand, "Guidelines for Writing an Cotton, M., Leiba, B., and T. Narten, "Guidelines for
IANA Considerations Section in RFCs", BCP 26, RFC 5226, Writing an IANA Considerations Section in RFCs", BCP 26,
DOI 10.17487/RFC5226, May 2008, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc5226>. <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 [RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, Message Authentication Status", RFC 5451, DOI 10.17487/
DOI 10.17487/RFC5451, April 2009, RFC5451, April 2009,
<http://www.rfc-editor.org/info/rfc5451>. <https://www.rfc-editor.org/info/rfc5451>.
[RFC6008] Kucherawy, M., "Authentication-Results Registration for [RFC6008] Kucherawy, M., "Authentication-Results Registration for
Differentiating among Cryptographic Results", RFC 6008, Differentiating among Cryptographic Results", RFC 6008,
DOI 10.17487/RFC6008, September 2010, DOI 10.17487/RFC6008, September 2010,
<http://www.rfc-editor.org/info/rfc6008>. <https://www.rfc-editor.org/info/rfc6008>.
[RFC6577] Kucherawy, M., "Authentication-Results Registration Update [RFC6577] Kucherawy, M., "Authentication-Results Registration Update
for Sender Policy Framework (SPF) Results", RFC 6577, for Sender Policy Framework (SPF) Results", RFC 6577,
DOI 10.17487/RFC6577, March 2012, DOI 10.17487/RFC6577, March 2012,
<http://www.rfc-editor.org/info/rfc6577>. <https://www.rfc-editor.org/info/rfc6577>.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating [RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, Message Authentication Status", RFC 7001, DOI 10.17487/
DOI 10.17487/RFC7001, September 2013, RFC7001, September 2013,
<http://www.rfc-editor.org/info/rfc7001>. <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] Rescorla, E. and B. Korver, "Guidelines for Writing RFC [SECURITY]
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>.
[SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", [SENDERID]
Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, DOI 10.17487/RFC4406, April 2006, RFC 4406, DOI 10.17487/RFC4406, April 2006,
<http://www.rfc-editor.org/info/rfc4406>. <http://www.rfc-editor.org/info/rfc4406>.
[SMIME-REG] [SMIME-REG]
Melnikov, A., "Authentication-Results Registration for Melnikov, A., "Authentication-Results Registration for
S/MIME Signature Verification", RFC 7281, S/MIME Signature Verification", RFC 7281, DOI 10.17487/
DOI 10.17487/RFC7281, June 2014, RFC7281, June 2014,
<http://www.rfc-editor.org/info/rfc7281>. <http://www.rfc-editor.org/info/rfc7281>.
[SPF] Kitterman, S., "Sender Policy Framework (SPF) for [SPF] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208, Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014, DOI 10.17487/RFC7208, April 2014,
<http://www.rfc-editor.org/info/rfc7208>. <http://www.rfc-editor.org/info/rfc7208>.
[VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009, Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
<http://www.rfc-editor.org/info/rfc5518>. <http://www.rfc-editor.org/info/rfc5518>.
skipping to change at page 42, line 22 skipping to change at page 43, line 10
field. One suggestion is to include a Priority header field, on field. One suggestion is to include a Priority header field, on
messages that don't already have such a header field, containing a messages that don't already have such a header field, containing a
value that reflects the strength of the authentication that was value that reflects the strength of the authentication that was
accomplished, e.g., "low" for weak or no authentication, "normal" or accomplished, e.g., "low" for weak or no authentication, "normal" or
"high" for good or strong authentication. "high" for good or strong authentication.
Some modern MUAs can already filter based on the content of this Some modern MUAs can already filter based on the content of this
header field. However, there is keen interest in having MUAs make header field. However, there is keen interest in having MUAs make
some kind of graphical representation of this header field's meaning some kind of graphical representation of this header field's meaning
to end users. Until this capability is added (i.e., while this to end users. Until this capability is added (i.e., while this
proposal and its successors are being adopted), other interim means specification and its successors continue to be adopted), other
of conveying authentication results may be necessary. interim means of conveying authentication results may be necessary.
Appendix B. Authentication-Results Examples Appendix B. Authentication-Results Examples
This section presents some examples of the use of this header field This section presents some examples of the use of this header field
to indicate authentication results. to indicate authentication results.
B.1. Trivial Case; Header Field Not Present B.1. Trivial Case; Header Field Not Present
The trivial case: The trivial case:
skipping to change at page 45, line 43 skipping to change at page 46, line 43
receiving DNS domain name is used as the authserv-id. Furthermore, receiving DNS domain name is used as the authserv-id. Furthermore,
the sender authenticated herself/himself to the MTA via a method the sender authenticated herself/himself to the MTA via a method
specified in [AUTH], and both SPF and Sender ID checks were done and specified in [AUTH], and both SPF and Sender ID checks were done and
passed. The MUA could extract and relay this extra information if passed. The MUA could extract and relay this extra information if
desired. desired.
Two Authentication-Results header fields are not required since the Two Authentication-Results header fields are not required since the
same host did all of the checking. The authenticating agent could same host did all of the checking. The authenticating agent could
have consolidated all the results into one header field. have consolidated all the results into one header field.
This example illustrates a scenario in which a remote user on a dial- This example illustrates a scenario in which a remote user on a
up connection (example.net) sends mail to a border MTA (example.com) dial-up connection (example.net) sends mail to a border MTA
using SMTP authentication to prove identity. The dial-up provider (example.com) using SMTP authentication to prove identity. The
has been explicitly authorized to relay mail as example.com, dial-up provider has been explicitly authorized to relay mail as
producing "pass" results from the SPF and Sender ID checks. example.net, producing "pass" results from the SPF and Sender ID
checks.
B.5. Service Provided, Several Authentications Done, Different MTAs B.5. Service Provided, Several Authentications Done, Different MTAs
A message that was relayed inbound by two different MTAs that conform A message that was relayed inbound by two different MTAs that conform
to this specification and applied multiple message authentication to this specification and applied multiple message authentication
checks: checks:
Authentication-Results: example.com; Authentication-Results: example.com;
sender-id=fail header.from=example.com; sender-id=fail header.from=example.com;
dkim=pass (good signature) header.d=example.com dkim=pass (good signature) header.d=example.com
skipping to change at page 47, line 18 skipping to change at page 48, line 16
which the MUA may choose to render. Also noteworthy here is the fact which the MUA may choose to render. Also noteworthy here is the fact
that there is a DKIM signature added by example.com that assured the that there is a DKIM signature added by example.com that assured the
integrity of the lower Authentication-Results field. integrity of the lower Authentication-Results field.
Since different hosts did the two sets of authentication checks, the Since different hosts did the two sets of authentication checks, the
header fields cannot be consolidated in this example. header fields cannot be consolidated in this example.
This example illustrates more typical transmission of mail into This example illustrates more typical transmission of mail into
example.com from a user on a dial-up connection example.net. The example.com from a user on a dial-up connection example.net. The
user appears to be legitimate as he/she had a valid password allowing user appears to be legitimate as he/she had a valid password allowing
authentication at the border MTA using SMTP AUTH. The SPF and Sender authentication at the border MTA using SMTP AUTH. The SPF test
ID tests failed since example.com has not granted example.net failed since example.com has not granted example.net's dial-up
network authority to relay mail on its behalf. The Sender ID test
failed since example.com has not granted mail-router.example.com
authority to relay mail on its behalf. However, the DKIM test passed authority to relay mail on its behalf. However, the DKIM test passed
because the sending user had a private key matching one of because the sending user had a private key matching one of
example.com's published public keys and used it to sign the message. example.com's published public keys and mail-router.example.com used
it to sign the message.
B.6. Service Provided, Multi-tiered Authentication Done B.6. Service Provided, Multi-tiered Authentication Done
A message that had authentication done at various stages, one of A message that had authentication done at various stages, one of
which was outside the receiving ADMD: which was outside the receiving ADMD:
Authentication-Results: example.com; Authentication-Results: example.com;
dkim=pass reason="good signature" dkim=pass reason="good signature"
header.i=@mail-router.example.net; header.i=@mail-router.example.net;
dkim=fail reason="bad signature" dkim=fail reason="bad signature"
skipping to change at page 48, line 45 skipping to change at page 49, line 45
t=1188964191; c=simple/simple; t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject; h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com> Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample Subject: here's a sample
Example 6: Headers Reporting Results from Multiple MTAs in Example 6: Headers Reporting Results from Multiple MTAs in Different
Different ADMDs ADMDs
In this example, we see multi-tiered authentication with an extended In this example, we see multi-tiered authentication with an extended
trust boundary. trust boundary.
The message was sent from someone at example.com's New York office The message was sent from someone at example.com's New York office
(newyork.example.com) to a mailing list managed at an intermediary. (newyork.example.com) to a mailing list managed at an intermediary.
The message was signed at the origin using DKIM. The message was signed at the origin using DKIM.
The message was sent to a mailing list service provider called The message was sent to a mailing list service provider called
example.net, which is used by example.com. There, example.net, which is used by example.com. There,
meetings@example.net is expanded to a long list of recipients, one of meetings@example.net is expanded to a long list of recipients, one of
whom is at the Chicago office. In this example, we will assume that whom is at the Chicago office. In this example, we will assume that
the trust boundary for chicago.example.com includes the mailing list the trust boundary for chicago.example.com includes the mailing list
server at example.net. server at example.net.
The mailing list server there first authenticated the message and The mailing list server there first authenticated the message and
skipping to change at page 50, 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 51, line 37 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 RFC 7001 Appendix D. Changes since RFC7601
o Applied RFC 7410.
o Updated all references to RFC 4408 with RFC 7208.
o Added section explaining "property" values. (Addressed Erratum
#4201.)
o Did some minor text reorganization.
o Gave registry history -- enough that this is now the authoritative
registry definition.
o Added text explaining each of the method-ptype-property tuples
registered by this document.
o Changed the meaning of the "Defined" column of the methods
registry to be the place where each entry was created and
described; it is expected that this will then refer to the
method's defining document. Provided IANA with corresponding
update instructions.
o Cleaned up registry structure and content, and replaced all
references to RFC 7001 with pointers to this document.
o Added references: [DMARC], [PRA], [RFC6008], [RFC6577], [RRVS],
[SMIME-REG].
o Added description of values that can be extracted from SMTP AUTH
sessions and an example.
o Provided much more complete descriptions of reporting DomainKeys
results.
o Added more detail about Sender ID.
o Marked all ADSP and DomainKeys entries as deprecated since their
defining documents are as well.
o Reworked some text around ignoring unknown ptypes.
o Completely described the ptypes registry.
o Mentioned that EHLO is mapped to HELO for SPF.
o RFC 7208 uses all-lowercase result strings now, so adjusted prose
accordingly.
o Updated list of supported methods, and mentioned the registries
immediately below.
o Mentioned that when a local-part is removed, the "@" goes with it.
o Referred to RFC 7328 in the "iprev" definition. o Added IANA registration for DKIM "a" and "s" properties.
o Corrected the "smime-part" prose. o Include EAI guidance.
o Updated examples that use SMTP AUTH to claim "with ESMTPA" in the o Adjust some ABNF tokes and names for easier inclusion by other
Received fields. documents.
o Made minor editorial adjustments. o Made minor editorial adjustments.
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: Stephane review and constructive criticism of this document: Seth Blank, Tim
Bortzmeyer, Scott Kitterman, John Levine, Tom Petch, and Pete Draegen, Scott Kitterman, John Levine, and Alessandro Vesely.
Resnick.
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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