draft-ietf-dmarc-rfc7601bis-06.txt   rfc8601.txt 
Individual submission M. Kucherawy Internet Engineering Task Force (IETF) M. Kucherawy
Internet-Draft January 28, 2019 Request for Comments: 8601 May 2019
Obsoletes: 7601 (if approved) Obsoletes: 7601
Intended status: Standards Track Category: Standards Track
Expires: August 1, 2019 ISSN: 2070-1721
Message Header Field for Indicating Message Authentication Status Message Header Field for Indicating Message Authentication Status
draft-ietf-dmarc-rfc7601bis-06
Abstract Abstract
This document specifies a message header field called Authentication- This document specifies a message header field called
Results for use with electronic mail messages to indicate the results "Authentication-Results" for use with electronic mail messages to
of message authentication efforts. Any receiver-side software, such indicate the results of message authentication efforts. Any
as mail filters or Mail User Agents (MUAs), can use this header field receiver-side software, such as mail filters or Mail User Agents
to relay that information in a convenient and meaningful way to users (MUAs), can use this header field to relay that information in a
or to make sorting and filtering decisions. convenient and meaningful way to users or to make sorting and
filtering decisions.
This document obsoletes [RFC7601].
Status of this Memo This document obsoletes RFC 7601.
This Internet-Draft is submitted in full conformance with the Status of This Memo
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering This is an Internet Standards Track document.
Task Force (IETF). Note that other groups may also distribute
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 This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on August 1, 2019. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8601.
Copyright Notice Copyright Notice
Copyright (c) 2019 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction ....................................................4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Purpose ....................................................5
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Trust Boundary .............................................6
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 1.3. Processing Scope ...........................................7
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 1.4. Requirements ...............................................7
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 1.5. Definitions ................................................7
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 1.5.1. Key Words ...........................................7
1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7 1.5.2. Internationalized Email .............................7
1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.3. Security ............................................8
1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8 1.5.4. Email Architecture ..................................8
1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 1.5.5. Other Terms .........................................9
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 1.6. Trust Environment .........................................10
2. Definition and Format of the Header Field . . . . . . . . . . 10 2. Definition and Format of the Header Field ......................10
2.1. General Description . . . . . . . . . . . . . . . . . . . 10 2.1. General Description .......................................10
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 2.2. Formal Definition .........................................11
2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13 2.3. Property Types (ptypes) and Properties ....................13
2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14 2.4. The "policy" ptype ........................................15
2.5. Authentication Identifier Field . . . . . . . . . . . . . 15 2.5. Authentication Service Identifier Field ...................15
2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16 2.6. Version Tokens ............................................17
2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 2.7. Defined Methods and Result Values .........................17
2.7.1. DKIM . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.7.1. DKIM ...............................................17
2.7.2. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.7.2. SPF ................................................19
2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 2.7.3. "iprev" ............................................20
2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20 2.7.4. SMTP AUTH ..........................................21
2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 2.7.5. Other Registered Codes .............................22
2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 2.7.6. Extension Methods ..................................22
2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 2.7.7. Extension Result Codes .............................23
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23 3. The "iprev" Authentication Method ..............................23
4. Adding the Header Field to a Message . . . . . . . . . . . . . 24 4. Adding the Header Field to a Message ...........................25
4.1. Header Field Position and Interpretation . . . . . . . . . 25 4.1. Header Field Position and Interpretation ..................26
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 4.2. Local Policy Enforcement ..................................27
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
6.1. The Authentication-Results Header Field . . . . . . . . . 28
6.2. "Email Authentication Methods" Registry Description . . . 29
6.3. "Email Authentication Methods" Registry Update . . . . . . 30
6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 30
6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 31
6.4. "Email Authentication Property Types" Registry
Description . . . . . . . . . . . . . . . . . . . . . . . 31
6.5. "Email Authentication Property Types" Registry Update . . 32 5. Removing Existing Header Fields ................................28
6.6. "Email Authentication Result Names" Registry 6. IANA Considerations ............................................29
Description . . . . . . . . . . . . . . . . . . . . . . . 32 6.1. The Authentication-Results Header Field ...................29
6.7. "Email Authentication Result Names" Registry Update . . . 33 6.2. "Email Authentication Methods" Registry Description .......30
6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 33 6.3. "Email Authentication Methods" Registry Update ............31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 6.3.1. "header.a" for DKIM ................................32
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 34 6.3.2. "header.s" for DKIM ................................32
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 35 6.4. "Email Authentication Property Types" Registry
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 36 Description ...............................................32
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 36 6.5. "Email Authentication Property Types" Registry Update .....33
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36 6.6. "Email Authentication Result Names" Registry Description ..33
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 36 6.7. "Email Authentication Result Names" Registry Update .......34
7.7. Attacks against Authentication Methods . . . . . . . . . . 36 6.8. SMTP Enhanced Status Codes ................................34
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 37 7. Security Considerations ........................................35
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 37 7.1. Forged Header Fields ......................................35
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 37 7.2. Misleading Results ........................................36
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 38 7.3. Header Field Position .....................................37
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7.4. Reverse IP Query Denial-of-Service Attacks ................37
8.1. Normative References . . . . . . . . . . . . . . . . . . . 38 7.5. Mitigation of Backscatter .................................37
8.2. Informative References . . . . . . . . . . . . . . . . . . 39 7.6. Internal MTA Lists ........................................37
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 42 7.7. Attacks against Authentication Methods ....................38
Appendix B. Authentication-Results Examples . . . . . . . . . . . 42 7.8. Intentionally Malformed Header Fields .....................38
B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 43 7.9. Compromised Internal Hosts ................................38
B.2. Nearly Trivial Case; Service Provided, but No 7.10. Encapsulated Instances ...................................38
Authentication Done . . . . . . . . . . . . . . . . . . . 43 7.11. Reverse Mapping ..........................................39
B.3. Service Provided, Authentication Done . . . . . . . . . . 44 8. References .....................................................39
B.4. Service Provided, Several Authentications Done, Single 8.1. Normative References ......................................39
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 8.2. Informative References ....................................40
B.5. Service Provided, Several Authentications Done, Appendix A. Legacy MUAs ...........................................44
Different MTAs . . . . . . . . . . . . . . . . . . . . . . 46 Appendix B. Authentication-Results Examples .......................44
B.6. Service Provided, Multi-tiered Authentication Done . . . . 48 B.1. Trivial Case: Header Field Not Present .....................44
B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 49 B.2. Nearly Trivial Case: Service Provided, but No
Appendix C. Operational Considerations about Message Authentication Done ........................................45
Authentication . . . . . . . . . . . . . . . . . . . 50 B.3. Service Provided, Authentication Done ......................46
Appendix D. Changes since RFC7601 . . . . . . . . . . . . . . . . 51 B.4. Service Provided, Several Authentications Done, Single
Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 52 MTA ........................................................47
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 52 B.5. Service Provided, Several Authentications Done, Different
MTAs .......................................................48
B.6. Service Provided, Multi-tiered Authentication Done .........50
B.7. Comment-Heavy Example ......................................51
Appendix C. Operational Considerations about Message
Authentication ........................................52
Appendix D. Changes since RFC 7601 ................................53
Acknowledgments ...................................................54
Author's Address ..................................................54
1. Introduction 1. Introduction
This document describes a header field called Authentication-Results This document describes a header field called "Authentication-
for electronic mail messages that presents the results of a message Results" for electronic mail messages that presents the results of a
authentication effort in a machine-readable format. The intent of message authentication effort in a machine-readable format. The
the header field is to create a place to collect such data when intent of the header field is to create a place to collect such data
message authentication mechanisms are in use so that a Mail User when message authentication mechanisms are in use so that a Mail User
Agent (MUA) and downstream filters can make filtering decisions Agent (MUA) and downstream filters can make filtering decisions
and/or provide a recommendation to the user as to the validity of the and/or provide a recommendation to the user as to the validity of the
message's origin and possibly the safety and integrity of its message's origin and possibly the safety and integrity of its
content. content.
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.
At the time of publication of this document, the following are At the time of publication of this document, the following are
published email authentication methods: published email authentication methods:
o SMTP Service Extension for Authentication ([AUTH]) o SMTP Service Extension for Authentication [AUTH]
o DomainKeys Identified Mail Signatures ([DKIM]) o DomainKeys Identified Mail Signatures [DKIM]
o Domain-based Message Authentication, Reporting and Conformance o Domain-based Message Authentication, Reporting, and Conformance
([DMARC]) [DMARC]
o Sender Policy Framework ([SPF]) o Sender Policy Framework [SPF]
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 Vouch By Reference ([VBR]) o S/MIME Signature Verification [SMIME-REG]
The following historic specifications were previously supported by o Vouch By Reference [VBR]
this framework, but have since become obsolete: The following Historic specifications were previously supported by
this framework but have since become obsolete:
o Author Domain Signing Practices ([ADSP]) (Historic) o Author Domain Signing Practices [ADSP] (Historic)
o DomainKeys ([DOMAINKEYS]) (Historic) o DomainKeys [DOMAINKEYS] (Historic)
o Sender ID ([SENDERID]) (Historic) Note that at the time of publication of this document the Sender ID
specification [SENDERID] (Experimental) is no longer supported by
this framework. Discussion regarding moving it to Historic status is
underway.
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.
The goal of this work is to give current and future authentication The goal of this work is to give current and future authentication
schemes a common framework within which to deliver their results to schemes a common framework within which to deliver their results to
downstream agents and discourage the creation of unique header fields downstream agents and discourage the creation of unique header fields
skipping to change at page 5, line 40 skipping to change at page 5, line 45
1.1. Purpose 1.1. Purpose
The header field defined in this document is expected to serve The header field defined in this document is expected to serve
several purposes: several purposes:
1. Convey the results of various message authentication checks, 1. Convey the results of various message authentication checks,
which are applied by upstream filters and Mail Transfer Agents which are applied by upstream filters and Mail Transfer Agents
(MTAs) and then passed to MUAs and downstream filters within the (MTAs) and then passed to MUAs and downstream filters within the
same "trust domain". Such agents might wish to render those same "trust domain". Such agents might wish to render those
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 such data.
3. Create an extensible framework for reporting new authentication 3. Create an extensible framework for reporting new authentication
methods as they emerge. methods as they emerge.
In particular, the mere presence of this header field does not mean In particular, the mere presence of this header field does not mean
its contents are valid. Rather, the header field is reporting its contents are valid. Rather, the header field is reporting
assertions made by one or more authentication schemes applied assertions made by one or more authentication schemes applied
somewhere upstream. For an MUA or downstream filter to treat the somewhere upstream. For an MUA or downstream filter to treat the
assertions as actually valid, there must be an assessment of the assertions as actually valid, there must be an assessment of the
trust relationship among such agents, the validating MTA, the paths trust relationship among such agents, the validating MTA, the paths
between them, and the 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
treat assertions by the producer as being reliable and accurate treat assertions by the producer as being reliable and accurate
(trustworthy). How this trust is obtained is outside the scope of (trustworthy). How this trust is obtained is outside the scope of
this document. It is entirely a local matter. this document. It is entirely a local matter.
Thus, this document defines a "trust boundary" as the delineation Thus, this document defines a "trust boundary" as the delineation
skipping to change at page 6, line 33 skipping to change at page 6, line 40
the authority of the ADMD. By this definition, hosts that are within the authority of the ADMD. By this definition, hosts that are within
a trust boundary are subject to the ADMD's authority and policies, a trust boundary are subject to the ADMD's authority and policies,
independent of their physical placement or their physical operation. independent of their physical placement or their physical operation.
For example, a host within a trust boundary might actually be For example, a host within a trust boundary might actually be
operated by a remote service provider and reside physically within operated by a remote service provider and reside physically within
its data center. its data center.
It is possible for a message to be evaluated inside a trust boundary It is possible for a message to be evaluated inside a trust boundary
but then depart and re-enter the trust boundary. An example might be but then depart and re-enter the trust boundary. An example might be
a forwarded message such as a message/rfc822 attachment (see a forwarded message such as a message/rfc822 attachment (see
Multipurpose Internet Mail Extensions [MIME]) or one that is part of "Multipurpose Internet Mail Extensions" [MIME]) or one that is part
a multipart/digest. The details reported by this field cannot be of a multipart/digest. The details reported by this field cannot be
trusted in that case. Thus, this field found within one of those trusted in that case. Thus, if found within one of those media
media types is typically ignored. types, this field is typically ignored.
Note that an MUA could be configured to retrieve messages from a Note that an MUA could be configured to retrieve messages from a
Receiver yet not be within the Receiver's ADMD. In this case, for receiver yet not be within the receiver's ADMD. In this case, for
the purposes of this work, that MUA is considered to be within the the purposes of this work, that MUA is considered to be within the
Receiver's ADMD if is configured to identify and ascribe value to receiver's ADMD if it is configured to identify and ascribe value to
authentication results recorded by that ADMD. authentication results recorded by that ADMD.
1.3. Processing Scope 1.3. Processing Scope
The content of this header field is meant to convey to message The content of this header field is meant to convey to message
consumers that authentication work on the message was already done consumers that authentication work on the message was already done
within its trust boundary, and those results are being presented. It within its trust boundary, and those results are being presented. It
is not intended to provide message parameters to consumers so that is not intended to provide message parameters to consumers so that
they can perform authentication protocols on their own. they can perform authentication protocols on their own.
skipping to change at page 7, line 18 skipping to change at page 7, line 26
insofar as a non-participating service will continue to interoperate insofar as a non-participating service will continue to interoperate
with the deployed messaging infrastructure. with the deployed messaging infrastructure.
In particular, this document establishes no requirement on MTAs to In particular, this document establishes no requirement on MTAs to
reject or filter arriving messages that do not pass authentication reject or filter arriving messages that do not pass authentication
checks. The data conveyed by the specified header field's contents checks. The data conveyed by the specified header field's contents
are for the information of MUAs and filters and are to be used at are for the information of MUAs and filters and are to be used at
their discretion. their discretion.
A participating ADMD does undertake some filtering and message A participating ADMD does undertake some filtering and message
modification obligations described in Section 5. modification obligations as described in Section 5.
1.5. Definitions 1.5. Definitions
This section defines various terms used throughout this document. This section defines various terms used throughout this document.
1.5.1. Key Words 1.5.1. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in BCP 14 [RFC2119] "OPTIONAL" in this document are to be interpreted as described in
[RFC8174] when, and only when, they appear in all capitals, as shown BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
here. capitals, as shown here.
1.5.2. Internationalized Email 1.5.2. Internationalized Email
In this document, there are references to messages formatted to In this document, there are references to messages formatted to
support Email Address Internationalization (EAI). Reference material support Email Address Internationalization (EAI). Reference material
for this can be found in [RFC6530], [RFC6531], and [RFC6532]. for this can be found in [RFC6530], [RFC6531], and [RFC6532].
Generally speaking, these documents allow UTF-8 in most places that Generally speaking, these documents allow UTF-8 in most places that
free-form text can be found and U-labels where domain names can be free-form text can be found and U-labels where domain names can be
used, and this document extends Authentication-Results accordingly. used, and this document extends Authentication-Results accordingly.
1.5.3. Security 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 is an authorization mechanism in that it expresses a As examples: SPF is an authorization mechanism in that it expresses a
result that shows whether the ADMD that apparently sent the message result that shows whether the ADMD that apparently sent the message
has explicitly authorized the connecting Simple Mail Transfer has explicitly authorized the connecting Simple Mail Transfer
Protocol ([SMTP]) client to relay messages on its behalf, but they do Protocol (SMTP) client [SMTP] to relay messages on its behalf, but it
not actually validate any other property of the message itself. By does not actually validate any other property of the message itself.
contrast, DKIM is agnostic as to the routing of a message but uses By contrast, DKIM is agnostic as to the routing of a message but uses
cryptographic signatures to authenticate agents, assign (some) cryptographic signatures to authenticate agents, assign (some)
responsibility for the message (which implies authorization), and responsibility for the message (which implies authorization), and
ensure that the listed portions of the message were not modified in ensure that the listed portions of the message were not modified in
transit. Since the signatures are not tied to SMTP connections, they transit. Since the signatures are not tied to SMTP connections, they
can be added by either the ADMD of origin, intermediate ADMDs (such can be added by the ADMD of origin, intermediate ADMDs (such as a
as a mailing list server), other handling agents, or any combination. mailing list server), other handling agents, or any combination of
these.
Rather than create a separate header field for each class of Rather than create a separate header field for each class of
solution, this specification groups them both into a single header solution, this specification groups them both into a single header
field. field.
1.5.4. Email Architecture 1.5.4. Email Architecture
o A "border MTA" is an MTA that acts as a gateway between the o A "border MTA" is an MTA that acts as a gateway between the
general Internet and the users within an organizational boundary. general Internet and the users within an organizational boundary.
(See also Section 1.2.) (See also Section 1.2.)
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
also not the first MTA to handle the message. also not the first MTA to handle the message.
o A Message Submission Agent (MSA) is an agent that accepts a
message from an MUA, introducing it to the mail-handling stream.
The following diagram illustrates the flow of mail among these The following diagram illustrates the flow of mail among these
defined components. See Internet Mail Architecture [EMAIL-ARCH] for defined components. See "Internet Mail Architecture" [EMAIL-ARCH]
further discussion on general email system architecture, which for further discussion on general email system architecture, which
includes detailed descriptions of these components, and Appendix C of includes detailed descriptions of these components, and Appendix C of
this document for discussion about the common aspects of email this document for discussion about the common aspects of email
authentication in current environments. authentication in current environments.
+-----+ +-----+ +------------+ +-----+ +-----+ +------------+
| MUA |-->| MSA |-->| Border MTA | | MUA |-->| MSA |-->| Border MTA |
+-----+ +-----+ +------------+ +-----+ +-----+ +------------+
| |
| |
V V
skipping to change at page 9, line 44 skipping to change at page 10, line 10
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 ADMD. An ADMD contains one or more entities that perform
more entities that perform validation and generate the header field validation and generate the header field and one or more that consume
and one or more that consume it for some type of assessment. The it for some type of assessment. The field often contains no
field often contains no integrity or validation mechanism of its own, integrity or validation mechanism of its own, so its presence must be
so its presence must be trusted implicitly. Hence, valid use of the trusted implicitly. Hence, valid use of the header field requires
header field requires removing any occurrences of it that claim to be removing any occurrences of it that claim to be associated with the
associated with the ADMD when the message enters the ADMD. This ADMD when the message enters the ADMD. This ensures that later
ensures that later occurrences have been added within the trust occurrences have been added within the trust boundary of the ADMD.
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 a 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".
is a Structured Header Field as defined in Internet Message Format It is a structured header field as defined in "Internet Message
([MAIL]), and thus all of the related definitions in that document Format" [MAIL], and thus all of the related definitions in that
apply. document 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
checks were done can be inferred. It is therefore considered to be a checks were done can be inferred. It is therefore considered to be a
trace field as defined in [MAIL], and thus all of the related trace field as defined in [MAIL], and thus all of the related
definitions in that document apply. definitions in that document apply.
The value of the header field (after removing comments) consists of The value of the header field (after removing comments) consists of
an authentication identifier, an optional version, and then a series an authentication service identifier, an optional version, and then a
of statements and supporting data. The statements are of the form series of statements and supporting data. The statements are of the
"method=result" and indicate which authentication method(s) were form "method=result" and indicate which authentication method or
applied and their respective results. For each such statement, the methods were applied and their respective results. For each such
supporting data can include a "reason" string and one or more statement, the supporting data can include a "reason" string and one
"property=value" statements indicating which message properties were or more "property=value" statements indicating which message
evaluated to reach that conclusion. properties were 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 shown below using Formally, the header field is specified as shown below using
Augmented Backus-Naur Form ([ABNF]). Examples of valid header fields Augmented Backus-Naur Form [ABNF]. Examples of valid header fields
with explanations of their semantics can be found in Appendix B. with explanations of their semantics can be found in Appendix B.
authres-header-field = "Authentication-Results:" authres-payload authres-header-field = "Authentication-Results:" authres-payload
authres-payload = [CFWS] authserv-id authres-payload = [CFWS] authserv-id
[ CFWS authres-version ] [ CFWS authres-version ]
( no-result / 1*resinfo ) [CFWS] CRLF ( no-result / 1*resinfo ) [CFWS] CRLF
authserv-id = value authserv-id = value
; see below for a description of this element ; see below for a description of this element
skipping to change at page 11, line 19 skipping to change at page 11, line 34
authres-version = 1*DIGIT [CFWS] authres-version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use; ; indicates which version of this specification is in use;
; this specification is version "1", and the absence of a ; this specification is version "1", and the absence of a
; version implies this version of the specification ; version implies this version of the specification
no-result = [CFWS] ";" [CFWS] "none" no-result = [CFWS] ";" [CFWS] "none"
; the special case of "none" is used to indicate that no ; the special case of "none" is used to indicate that no
; message authentication was performed ; message authentication was performed
resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*( CFWS propspec ) [ CFWS 1*propspec ]
methodspec = [CFWS] method [CFWS] "=" [CFWS] result methodspec = [CFWS] method [CFWS] "=" [CFWS] result
; indicates which authentication method was evaluated ; indicates which authentication method was evaluated
; and what its output was ; and what its output was
reasonspec = "reason" [CFWS] "=" [CFWS] value reasonspec = "reason" [CFWS] "=" [CFWS] value
; a free-form comment on the reason the given result ; a free-form comment on the reason the given result
; was returned ; was returned
propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
; an indication of which properties of the message ; an indication of which properties of the message
; were evaluated by the authentication scheme being ; were evaluated by the authentication scheme being
; applied to yield the reported result ; applied to yield the reported result
method = Keyword [ [CFWS] "/" [CFWS] method-version ] method = Keyword [ [CFWS] "/" [CFWS] method-version ]
; a method indicates which method's result is ; a method indicates which method's result is
; represented by "result", and is one of the methods ; represented by "result"; it is one of the methods
; explicitly defined as valid in this document ; explicitly defined as valid in this document
; or is an extension method as defined below ; or is an extension method as defined below
method-version = 1*DIGIT [CFWS] method-version = 1*DIGIT [CFWS]
; indicates which version of the method specification is ; indicates which version of the method specification is
; in use, corresponding to the matching entry in the IANA ; in use, corresponding to the matching entry in the IANA
; "Email Authentication Methods" registry; a value of "1" ; "Email Authentication Methods" registry; a value of "1"
; is assumed if this version string is absent ; is assumed if this version string is absent
result = Keyword result = Keyword
; indicates the results of the attempt to authenticate ; indicates the results of the attempt to authenticate
; the message; see below for details ; the message; see below for details
ptype = Keyword ptype = Keyword
; indicates whether the property being evaluated was ; indicates whether the property being evaluated was
; a parameter to an [SMTP] command, was a value taken ; a parameter to an SMTP command [SMTP], was a value taken
; from a message header field, was some property of ; from a message header field, was some property of
; 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 [SMTP] that are made up
; of multiple words ; of multiple words
pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS] [CFWS]
; the value extracted from the message property defined ; the value extracted from the message property defined
; by the "ptype.property" construction ; by the "ptype.property" construction
"local-part" is defined in Section 3.4.1 of [MAIL], as modified by "local-part" is defined in Section 3.4.1 of [MAIL], as modified by
[RFC6531]. [RFC6531].
"CFWS" is defined in Section 3.2.2 of [MAIL]. "CFWS" is defined in Section 3.2.2 of [MAIL].
"Keyword" is defined in Section 4.1.2 of [SMTP]. It is further "Keyword" is defined in Section 4.1.2 of [SMTP]. It is further
constrained by the necesity of being registered in the IANA registry constrained by the necessity of being registered in the Internet
relevant to the context in which it it is used. See Section 2.7, and Assigned Numbers Authority (IANA) registry relevant to the context in
Section 2.3, and Section 6. which it is used. See Sections 2.3, 2.7, and 6.
The "value" is as defined in Section 5.1 of [MIME], with "quoted- The "value" is as defined in Section 5.1 of [MIME], with
string" updated as specified in [RFC6532]. "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
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 still has a result to extract any properties to do its evaluation yet still has a result
to report. It may also be omitted if the agent generating this to report. It may also be omitted if the agent generating this
result wishes not to reveal such properties to downstream agents. result wishes not to reveal such properties to downstream agents.
skipping to change at page 13, line 27 skipping to change at page 13, line 46
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 where the reported data were extracted. specific, it indicates from where the reported "pvalue" was
This can include a particular part of the message header or body, extracted. This can include a particular part of the message header
some part of the SMTP session, a secondary output of an or body, some part of the SMTP session, a secondary output of an
authentication method (apart from its pure result), or some other authentication method (apart from its pure result), or some other
aspect of the message's handling. aspect of the message's handling.
Combinations of ptypes and properties are registered and described in Combinations of ptypes and properties are registered and described in
the "Email Authentication Methods" registry, coupled with the the "Email Authentication Methods" registry, coupled with the
authentication methods with which they are used. This is further authentication methods with which they are used. This is further
described in Section 6. described in Section 6.
Legal values of "ptype" are as defined in the IANA "Email Legal values of "ptype" are as defined in the IANA "Email
Authentication Property Types" registry, created by [RFC7410]. The Authentication Property Types" registry, created by [RFC7410]. The
initial values and what they typically indicate are as follows, based initial values and what they typically indicate are as follows, based
on [RFC7001]: on [RFC7001]:
body: Information that was extracted from the body of the message. body: Information that was extracted from the body of the message.
This might be an arbitrary string of bytes, a hash of a string of This might be an arbitrary string of bytes, a hash of a string of
bytes, a Uniform Resource Identifier, or some other content of bytes, a Uniform Resource Identifier, or some other content of
interest. The "property" is an indication of where within the interest. The "property" is an indication of where within the
message body the extracted content was found, and can indicate an message body the extracted content was found and can indicate an
offset, identify a MIME part, etc. (At the time of this revision, offset, identify a MIME part, etc. (At the time of this revision,
no properties matching this ptype have been registered. no properties matching this ptype have been registered.
Accordingly, this ptype may be deprecated in the future.) Accordingly, this ptype may be deprecated in the future.)
header: Indicates information that was extracted from the header of header: Indicates information that was extracted from the header of
the message. This might be the value of a header field or some the message. This might be the value of a header field or some
portion of a header field. The "property" gives a more precise portion of a header field. The "property" gives a more precise
indication of the place in the header from which the extraction indication of the place in the header from which the extraction
took place. took place.
skipping to change at page 14, line 43 skipping to change at page 15, line 21
identify the local policy that was applied and the result it identify the local policy that was applied and the result it
returned. returned.
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-Results field:
... dkim=policy 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 that some
check by that name took place and that check returned a result of local check by that name took place and that check returned a result
"unsigned-subject". These are arbitrary names selected by (and of "unsigned-subject". These are arbitrary names selected by (and
presumably used within) the ADMD making use of them, so they are not presumably used within) the ADMD making use of them, so they are not
normally registered with IANA or otherwise specified apart from normally registered with IANA or otherwise specified apart from
setting syntax restrictions that allow for easy parsing within the setting syntax restrictions that allow for easy parsing within the
rest of the header field. rest of the header field.
This ptype existed in the original specification for this header This ptype existed in the original specification for this header
field ([RFC5451]), but without a complete description or example of field [RFC5451], but without a complete description or example of
intended use. As a result, it has not seen any practical use to date intended use. As a result, it has not seen any practical use to date
that matches its intended purpose. These added details are provided that matches its intended purpose. These added details are provided
to guide implementers toward proper use. to guide implementers toward proper use.
2.5. Authentication Identifier Field 2.5. Authentication Service 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 Note that in an EAI-formatted message, this identifier may be
expressed in UTF-8. expressed in UTF-8.
skipping to change at page 15, line 38 skipping to change at page 16, line 19
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 service
instead be the specific hostname of the MTA performing the identifier can instead be the specific hostname of the MTA performing
authentication check whose result is being reported. Moreover, some the authentication check whose result is being reported. Moreover,
implementations define a substructure to the identifier; such some implementations define a substructure to the identifier; such
structures are 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
skipping to change at page 16, line 16 skipping to change at page 16, line 45
o Changes to the identifier impose a large, centralized o Changes to the identifier impose a large, centralized
administrative burden. administrative burden.
o Ongoing administrative changes require constantly updating this o Ongoing administrative changes require constantly updating this
centralized table, making it difficult to ensure that an MUA or centralized table, making it difficult to ensure that an MUA or
downstream filter will have access to accurate information for downstream filter will have access to accurate information for
assessing the usability of the header field's content. In assessing the usability of the header field's content. In
particular, consumers of the header field will need to know not particular, consumers of the header field will need to know not
only the current identifier(s) in use but previous ones as well to only the current identifier(s) in use but previous ones as well to
account for delivery latency or later re-assessment of the header account for delivery latency or later reassessment of the header
field's contents. field's content.
Examples of valid authentication identifiers are "example.com", Examples of valid authentication service identifiers are
"mail.example.org", "ms1.newyork.example.com", and "example-auth". "example.com", "mail.example.org", "ms1.newyork.example.com", and
"example-auth".
2.6. Version Tokens 2.6. Version Tokens
The grammar above provides for the optional inclusion of versions on The grammar above provides for the optional inclusion of versions on
both the header field itself (attached to the authserv-id token) and both the header field itself (attached to the authserv-id token) and
on each of the methods being reported. The method version refers to on each of the methods being reported. The method version refers to
the method itself, which is specified in the documents describing the method itself, which is specified in the documents describing
those methods, while the authserv-id version refers to this document those methods, while the authserv-id version refers to this document
and thus the syntax of this header field. and thus the syntax of this header field.
The purpose of including these is to avoid misinterpretation of the The purpose of including these is to avoid misinterpretation of the
results. That is, if a parser finds a version after an authserv-id results. That is, if a parser finds a version after an authserv-id
that it does not explicitly know, it can immediately discontinue that it does not explicitly know, it can immediately discontinue
trying to parse since what follows might not be in an expected trying to parse, since what follows might not be in an expected
format. For a method version, the parser SHOULD ignore a method format. For a method version, the parser SHOULD ignore a method
result if the version is not supported in case the semantics of the result if the version is not supported in case the semantics of the
result have a different meaning than what is expected. For example, result have a different meaning than what is expected. For example,
if a hypothetical DKIM version 2 yielded a "pass" result for if a hypothetical DKIM version 2 yielded a "pass" result for
different reasons than version 1 does, a consumer of this field might different reasons than version 1 does, a consumer of this field might
not want to use the altered semantics. Allowing versions in the not want to use the altered semantics. Allowing versions in the
syntax is a way to indicate this and let the consumer of the header syntax is a way to indicate this and let the consumer of the header
field decide. field decide.
2.7. Defined Methods and Result Values 2.7. Defined Methods and Result Values
skipping to change at page 18, line 10 skipping to change at page 18, line 45
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" Note that in an EAI-formatted message, the values of the "d" and "i"
properties can be expressed in UTF-8. properties can be expressed in UTF-8.
In addition to previous registrations, this document registers the In addition to previous registrations, this document registers the
DKIM tags "a" (cryptographic algorithm used to sign the message) and DKIM tags "a" (cryptographic algorithm used to sign the message) and
"s" (selector) as reportable properties. This can be used to aid "s" (selector) as reportable properties. These can be used to aid
receivers during post-verification processing. In particular, receivers during post-verification processing. In particular,
[RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is [RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is
important to be able to distinguish such signatures from those using important to be able to distinguish such signatures from those using
preferred algorithms. 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 that the
will be treated as if it had not been signed. message will be treated as if it had not been signed.
2.7.2. SPF 2.7.2. SPF
SPF uses the "spf" method name. The result values for SPF are SPF uses the "spf" method name. The result values for SPF are
defined in Section 2.6 of [SPF], and those definitions are included defined in Section 2.6 of [SPF], and those definitions are included
here by reference: here by reference:
+-----------+------------------------------+ +-----------+------------------------------+
| Code | Meaning | | Code | Meaning |
+-----------+------------------------------+ +-----------+------------------------------+
| none | [SPF], Section 2.6.1 | | none | [SPF], Section 2.6.1 |
+-----------+------------------------------+ +-----------+------------------------------+
| pass | [SPF], Section 2.6.3 | | pass | [SPF], Section 2.6.3 |
+-----------+------------------------------+ +-----------+------------------------------+
| fail | [SPF], Section 2.6.4 | | fail | [SPF], Section 2.6.4 |
+-----------+------------------------------+ +-----------+------------------------------+
| softfail | [SPF], Section 2.6.5 | | softfail | [SPF], Section 2.6.5 |
+-----------+------------------------------+ +-----------+------------------------------+
| policy | [this document], Section 2.4 | | policy | RFC 8601, Section 2.4 |
+-----------+------------------------------+ +-----------+------------------------------+
| neutral | [SPF], Section 2.6.2 | | neutral | [SPF], Section 2.6.2 |
+-----------+------------------------------+ +-----------+------------------------------+
| temperror | [SPF], Section 2.6.6 | | temperror | [SPF], Section 2.6.6 |
+-----------+------------------------------+ +-----------+------------------------------+
| permerror | [SPF], 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".)
skipping to change at page 19, line 24 skipping to change at page 20, line 10
For this method, an additional result of "policy" is defined, which For this method, 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 do not contain If the retrieved sender policies used to evaluate SPF do not contain
explicit provisions for authenticating the local-part (see Section explicit provisions for authenticating the local-part (see
3.4.1 of [MAIL]) of an address, the "pvalue" reported along with Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported along
results for this mechanism SHOULD NOT include the local-part or the with results for this mechanism SHOULD NOT include the local-part or
following "@" character. the following "@" character.
2.7.3. "iprev" 2.7.3. "iprev"
The result values used by the "iprev" method, defined in Section 3, The result values used by the "iprev" method, defined in Section 3,
are as follows: are as follows:
pass: The DNS evaluation succeeded, i.e., the "reverse" and pass: The DNS evaluation succeeded, i.e., the "reverse" and
"forward" lookup results were returned and were in agreement. "forward" lookup results were returned and were in agreement.
fail: The DNS evaluation failed. In particular, the "reverse" and fail: The DNS evaluation failed. In particular, the "reverse" and
skipping to change at page 20, line 12 skipping to change at page 20, line 43
other error condition resulted. A later attempt may produce a other error condition resulted. A later attempt may produce a
final result. final result.
permerror: The DNS evaluation could not be completed because no PTR permerror: The DNS evaluation could not be completed because no PTR
data are published for the connecting IP address, e.g., a DNS data are published for the connecting IP address, e.g., a DNS
RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR)
in a reply containing no answers, was returned. This prevented in a reply containing no answers, was returned. This prevented
completion of the evaluation. A later attempt is unlikely to completion of the evaluation. A later attempt is unlikely to
produce a final result. produce a final result.
There is no "none" for this method since any TCP connection There is no "none" for this method, since any TCP connection
delivering email has an IP address associated with it, so some kind delivering email has an IP address associated with it, so some kind
of evaluation will always be possible. of evaluation will always be possible.
The result is reported using a ptype of "policy" (as this is not part The result is reported using a ptype of "policy" (as this is not part
of any established protocol) and a property of "iprev". of any established protocol) and a property of "iprev".
For discussion of the format of DNS replies, see "Domain Names - For discussion of the format of DNS replies, see "Domain names -
Implementation and Specification" ([DNS]). implementation and specification" [DNS].
2.7.4. SMTP AUTH 2.7.4. SMTP AUTH
SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. SMTP AUTH (defined in [AUTH]) is represented by the "auth" method.
Its result values are as follows: Its result values are as follows:
none: SMTP authentication was not attempted. none: SMTP authentication was not attempted.
pass: The SMTP client authenticated to the server reporting the pass: The SMTP client authenticated to the server reporting the
result using the protocol described in [AUTH]. result using the protocol described in [AUTH].
skipping to change at page 21, line 9 skipping to change at page 21, line 41
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, the local-part can be Note that in an EAI-formatted message, the local-part can be
expressed in UTF-8, and the domain can be expressed as a U-label. expressed in UTF-8 and the domain can be expressed as a U-label.
If both identities are available, both can be reported. For example, If both identities are available, both can be reported. For example,
consider this command issued by a client that has completed session consider this command issued by a client that has completed session
authentication with the AUTH command resulting in an authorized authentication with the AUTH command resulting in an authorized
identity of "client@c.example": identity of "client@c.example":
MAIL FROM:<alice@a.example> AUTH=<bob@b.example> MAIL FROM:<alice@a.example> AUTH=<bob@b.example>
This could result in a "resinfo" construction like so: This could result in a "resinfo" construction like so:
; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example
Note that in all cases other than "pass", the message was sent by an Note that in all cases other than "pass", the message was sent by an
unauthenticated client. All non-"pass" cases SHOULD thus be treated unauthenticated client. All non-"pass" cases SHOULD thus be treated
as equivalent with respect to this method. as equivalent with respect to this method.
2.7.5. Other Registered Codes 2.7.5. Other Registered Codes
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
adsp"); "dkim-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 Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which
are already registered, can be expressed as U-labels. are already registered, can be expressed as U-labels.
2.7.6. Extension Methods 2.7.6. Extension Methods
Additional authentication method identifiers (extension methods) may Additional authentication method identifiers (extension methods) may
be defined in the future by later revisions or extensions to this be defined in the future by later revisions or extensions to this
specification. These method identifiers are registered with the specification. These method identifiers are registered with IANA
Internet Assigned Numbers Authority (IANA) and, preferably, published and, preferably, published in an RFC. See Section 6 for further
in an RFC. See Section 6 for further details. details.
Extension methods can be defined for the following reasons: Extension methods can be defined for the following reasons:
1. To allow additional information from new authentication systems 1. To allow additional information from new authentication systems
to be communicated to MUAs or downstream filters. The names of to be communicated to MUAs or downstream filters. The names of
such identifiers ought to reflect the name of the method being such identifiers ought to reflect the name of the method being
defined but ought not be needlessly long. defined but ought not be needlessly long.
2. To allow the creation of "sub-identifiers" that indicate 2. To allow the creation of "sub-identifiers" that indicate
different levels of authentication and differentiate between different levels of authentication and differentiate between
their relative strengths, e.g., "auth1-weak" and "auth1-strong". their relative strengths, e.g., "auth1-weak" and "auth1-strong".
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 were 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 formally. the parameters associated with them are not documented formally.
Therefore, they are subject to change at any time and not suitable Therefore, they are subject to change at any time and not suitable
for production use. Any MTA, MUA, or downstream filter intended for for production use. Any MTA, MUA, or downstream filter intended for
production use SHOULD ignore or delete any Authentication-Results production use SHOULD ignore or delete any Authentication-Results
header field that includes an experimental (unknown) method header field that includes an experimental (unknown) method
identifier. identifier.
2.7.7. Extension Result Codes 2.7.7. Extension Result Codes
Additional result codes (extension results) might be defined in the Additional result codes (extension results) might be defined in the
future by later revisions or extensions to this specification. Non- future by later revisions or extensions to this specification.
experimental result codes MUST be registered with the Internet Non-experimental result codes MUST be registered with IANA (and,
Assigned Numbers Authority (IANA) and preferably published in an RFC. preferably, published in an RFC). See Section 6 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 23, line 34 skipping to change at page 24, line 20
the union of IP addresses to which each member of N maps (after the union of IP addresses to which each member of N maps (after
corresponding "A" and "AAAA" queries) is L, then this test is corresponding "A" and "AAAA" queries) is L, then this test is
successful if I is an element of L. successful if I is an element of L.
Often an MTA receiving a connection that fails this test will simply Often an MTA receiving a connection that fails this test will simply
reject the connection using the enhanced status code defined in reject the connection using the enhanced status code defined in
[AUTH-ESC]. If an operator instead wishes to make this information [AUTH-ESC]. If an operator instead wishes to make this information
available to downstream agents as a factor in handling decisions, it available to downstream agents as a factor in handling decisions, it
records a result in accordance with Section 2.7.3. records a result in accordance with Section 2.7.3.
The response to a PTR query could contain multiple names. To prevent The response to a "PTR" query could contain multiple names. To
heavy DNS loads, agents performing these queries MUST be implemented prevent heavy DNS loads, agents performing these queries MUST be
such that the number of names evaluated by generation of implemented such that the number of names evaluated by generation of
corresponding A or AAAA queries is limited so as not to be unduly corresponding "A" or "AAAA" queries is limited so as not to be unduly
taxing to the DNS infrastructure, though it MAY be configurable by an taxing to the DNS infrastructure, though it MAY be configurable by an
administrator. As an example, Section 4.6.4 of [SPF] chose a limit administrator. As an example, Section 4.6.4 of [SPF] chose a limit
of 10 for its implementation of this algorithm. of 10 for its implementation of this algorithm.
"DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the "DNS Extensions to Support IP Version 6" [DNS-IP6] discusses the
query formats for the IPv6 case. query formats for the IPv6 case.
There is some contention regarding the wisdom and reliability of this There is some contention regarding the wisdom and reliability of this
test. For example, in some regions, it can be difficult for this test. For example, in some regions, it can be difficult for this
test ever to pass because the practice of arranging to match the test ever to pass because the practice of arranging to match the
forward and reverse DNS is infrequently observed. Therefore, the forward and reverse DNS is infrequently observed. Therefore, the
precise implementation details of how a verifier performs an "iprev" precise implementation details of how a verifier performs an "iprev"
test are not specified here. The verifier MAY report a successful or test are not specified here. The verifier MAY report a successful or
failed "iprev" test at its discretion having done some kind of check failed "iprev" test at its discretion having done some kind of check
of the validity of the connection's identity using DNS. It is of the validity of the connection's identity using DNS. It is
incumbent upon an agent making use of the reported "iprev" result to incumbent upon an agent making use of the reported "iprev" result to
understand what exactly that particular verifier is attempting to understand what exactly that particular verifier is attempting to
report. report.
Extensive discussion of reverse DNS mapping and its implications can Extensive discussion of reverse DNS mapping and its implications can
be found in "Considerations for the use of DNS Reverse Mapping" be found in "Considerations for the use of DNS Reverse Mapping"
([DNSOP-REVERSE]). In particular, it recommends that applications [DNSOP-REVERSE]. In particular, it recommends that applications
avoid using this test as a means of authentication or security. Its avoid using this test as a means of authentication or security. Its
presence in this document is not an endorsement but is merely presence in this document is not an endorsement but is merely
acknowledgment that the method remains common and provides the means acknowledgment that the method remains common and provides the means
to relay the results of that test. to relay the results of that test.
4. Adding the Header Field to a Message 4. Adding the Header Field to a Message
This specification makes no attempt to evaluate the relative This specification makes no attempt to evaluate the relative
strengths of various message authentication methods that may become strengths of various message authentication methods that may become
available. The methods listed are an order-independent set; their available. The methods listed are an order-independent set; their
skipping to change at page 24, line 42 skipping to change at page 25, line 31
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 was 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 service identifier portion and the "none" token (see Section 2.2) to
explicitly that no message authentication schemes were applied prior indicate explicitly that no message authentication schemes were
to delivery of this message. applied prior to delivery of this message.
An MTA adding this header field has to take steps to identify it as An MTA adding this header field has to take steps to identify it as
legitimate to the MUAs or downstream filters that will ultimately legitimate to the MUAs or downstream filters that will ultimately
consume its content. One process to do so is described in Section 5. consume its content. One process to do so is described in Section 5.
Further measures may be necessary in some environments. Some Further measures may be necessary in some environments. Some
possible solutions are enumerated in Section 7.1. This document does possible solutions are enumerated in Section 7.1. This document does
not mandate any specific solution to this issue as each environment not mandate any specific solution to this issue, as each environment
has its own facilities and limitations. has its own facilities and limitations.
Most known message authentication methods focus on a particular Most known message authentication methods focus on a particular
identifier to evaluate. SPF differs in that it can yield a result identifier to evaluate. SPF differs in that it can yield a result
based on more than one identifier; specifically, SPF can evaluate the based on more than one identifier; specifically, SPF can evaluate the
RFC5321.HELO parameter or the RFC5321.MailFrom parameter. When RFC5321.HELO parameter or the RFC5321.MailFrom parameter. When
generating this field to report those results, only the parameter generating this field to report those results, only the parameter
that yielded the result is included. that yielded the result is included.
For MTAs that add this header field, adding header fields in order For MTAs that add this header field, adding header fields in order
(at the top), per Section 3.6 of [MAIL], is particularly important. (at the top), per Section 3.6 of [MAIL], is particularly important.
Moreover, this header field SHOULD be inserted above any other trace Moreover, this header field SHOULD be inserted above any other trace
header fields such MTAs might prepend. This placement allows easy header fields such MTAs might prepend. This placement allows easy
detection of header fields that can be trusted. detection of header fields that can be trusted.
End users making direct use of this header field might inadvertently End users making direct use of this header field might inadvertently
trust information that has not been properly vetted. If, for trust information that has not been properly vetted. If, for
example, a basic SPF result were to be relayed that claims an example, a basic SPF result were to be relayed that claims an
authenticated addr-spec, the local-part of that addr-spec has authenticated addr-spec, the local-part of that addr-spec has
actually not been authenticated. Thus, an MTA adding this header actually not been authenticated. Thus, an MTA adding this header
field SHOULD NOT include any data that has not been authenticated by field SHOULD NOT include any data that have not been authenticated by
the method(s) being applied. Moreover, MUAs SHOULD NOT render to the method(s) being applied. Moreover, MUAs SHOULD NOT render to
users such information if it is presented by a method known not to users such information if it is presented by a method known not to
authenticate it. authenticate it.
4.1. Header Field Position and Interpretation 4.1. Header Field Position and Interpretation
In order to ensure non-ambiguous results and avoid the impact of In order to ensure non-ambiguous results and avoid the impact of
false header fields, MUAs and downstream filters SHOULD NOT interpret false header fields, MUAs and downstream filters SHOULD NOT interpret
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
skipping to change at page 26, line 9 skipping to change at page 26, line 48
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. The exception to this is experimental methods specifically support. The exception to this is experimental methods
as discussed in Section 2.7.6. 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
of trust-related materials. For example, an attacker could register presentation of trust-related materials. For example, an attacker
examp1e.com (note the digit "1" (one)) and send signed mail to could register examp1e.com (note the digit "1" (one)) and send signed
intended victims; a verifier would detect that the signature was mail to intended victims; a verifier would detect that the signature
valid and report a "pass" even though it's clear the DNS domain name was valid and report a "pass" even though it's clear the DNS domain
was intended to mislead. See Section 7.2 for further discussion. name 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
it were a trace header field as defined in Section 3.6.7 of [MAIL] it were a trace header field as defined in Section 3.6.7 of [MAIL]
and hence MUST NOT be reordered and MUST be prepended to the message, and hence MUST NOT be reordered and MUST be prepended to the message,
so that there is generally some indication upon delivery of where in so that there is generally some indication upon delivery of where in
the chain of handling MTAs the message authentication was done. the chain of handling MTAs the message authentication was done.
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. They are likely to contain the message/rfc822 MIME attachments. They are likely to contain the
results of authentication checks done in the past, possibly long ago, results of authentication checks done in the past, possibly long ago,
and have no contemporary value. Due caution to this needs to be and have no contemporary value. Due caution therefore needs to be
taken when choosing to consume them. 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
skipping to change at page 27, line 25 skipping to change at page 28, line 20
identifier, to have been added within its trust boundary but that did identifier, to have been added within its trust boundary but that did
not come directly from another trusted MTA. For example, an MTA for not come directly from another trusted MTA. For example, an MTA for
example.com receiving a message MUST delete or otherwise obscure any example.com receiving a message MUST delete or otherwise obscure any
instance of this header field bearing an authentication service instance of this header field bearing an authentication service
identifier indicating that the header field was added within identifier indicating that the header field was added within
example.com prior to adding its own header fields. This could mean example.com prior to adding its own header fields. This could mean
each internal MTA will need to be configured with a list of other each internal MTA will need to be configured with a list of other
known, trusted MTAs that are thus expected to be using that same known, trusted MTAs that are thus expected to be using that same
identifier. identifier.
For messages that are EAI-formatted messages, this test is done after In the case of EAI-formatted messages, this test is done after
converting A-labels into U-labels. converting A-labels into U-labels.
For simplicity and maximum security, a border MTA could remove all For simplicity and maximum security, a border MTA could remove all
instances of this header field on mail crossing into its trust instances of this header field on mail crossing into its trust
boundary. However, this may conflict with the desire to access boundary. However, this may conflict with the desire to access
authentication results performed by trusted external service authentication results performed by trusted external service
providers. It may also invalidate signed messages whose signatures providers. It may also invalidate signed messages whose signatures
cover external instances of this header field. A more robust border cover external instances of this header field. A more robust border
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
skipping to change at page 28, line 6 skipping to change at page 28, line 48
border MTA MAY elect not to delete those results; moreover, the border MTA MAY elect not to delete those results; moreover, the
upstream host doing some authentication work could apply a signing upstream host doing some authentication work could apply a signing
technology such as [DKIM] on its own results to assure downstream technology such as [DKIM] on its own results to assure downstream
hosts of their authenticity. An example of this is provided in hosts of their authenticity. An example of this is provided in
Appendix B. Appendix B.
Similarly, in the case of messages signed using [DKIM] or other Similarly, in the case of messages signed using [DKIM] or other
message-signing methods that sign header fields, this removal action message-signing methods that sign header fields, this removal action
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
message with forged header fields. However, signing agents MAY a 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 [SMTP]
the message is not from a trusted internal MTA. (As discussed above, relaying the message is not from a trusted internal MTA. (As
this too can result in invalidation of signatures.) This means the discussed above, this too can result in invalidation of signatures.)
MTA needs to be able to understand versions of this header field at This means the MTA needs to be able to understand versions of this
least as late as the ones understood by the MUAs or other consumers header field at least as late as the ones understood by the MUAs or
within its ADMD. other consumers within its ADMD.
6. IANA Considerations 6. IANA Considerations
IANA has registered the defined header field and created registries IANA has registered the defined header field and created registries
as described below. These registry actions were originally defined as described below. These registry actions were originally defined
by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created
registries were further updated in [RFC7601] to make them more registries were further updated in [RFC7601] to make them more
complete. complete.
Each registry has two related sections below. The first describes Each registry has two related sections below. The first describes
the registry and its update procedures, which are unchanged from the registry and its update procedures, which are unchanged from
[RFC7601]. The second enumerates changes to entries that are [RFC7601]. The second enumerates changes to entries that are
relevant to this document. relevant to this document.
6.1. The Authentication-Results Header Field 6.1. The Authentication-Results Header Field
The Authentication-Results header field was added to the IANA The Authentication-Results header field was added to the IANA
"Permanent Message Header Field Names" registry, per the procedure "Permanent Message Header Field Names" registry, per the procedure
found in [IANA-HEADERS]. That entry will be updated to reference found in [IANA-HEADERS]. That entry has been updated to reference
this document. The following is the registration template: this document. The following is the registration template:
Header field name: Authentication-Results Header field name: Authentication-Results
Applicable protocol: mail ([MAIL]) Applicable protocol: mail [MAIL]
Status: Standard Status: standard
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): [this document] Specification document(s): RFC 8601
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 are 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 will be updated to reference this The reference for that registry has been 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".
No two entries can have the same combination of method, ptype, and No two entries can have the same combination of method, ptype, and
property. property.
An entry in this registry contains the following: An entry in this registry contains the following:
Method: the name of the method. Method: the name of the method.
Definition: a reference to the document that created this entry, if Definition: a reference to the document that created this entry, if
any (see below). any (see below).
ptype: a "ptype" value appropriate for use with that method. ptype: a "ptype" value appropriate for use with that method.
property: a "property" value matching that "ptype" also appropriate Property: a "property" value matching that "ptype" also appropriate
for use with that method. for use with that method.
Value: a brief description of the value to be supplied with that Value: a brief description of the value to be supplied with that
method/ptype/property tuple. method/ptype/property tuple.
Status: the status of this entry, which is either: Status: the status of this entry, which is one of the following:
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.
Version: a version number associated with the method (preferably Version: a version number associated with the method (preferably
starting at "1"). starting at "1").
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 entries in this registry are to be updated to replace The following entries in this registry have been updated to replace
[RFC7601] with this document: [RFC7601] with this document:
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| Method | ptype | Property | | Method | ptype | Property |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| auth | smtp | auth | | auth | smtp | auth |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| auth | smtp | mailfrom | | auth | smtp | mailfrom |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| dkim | header | d | | dkim | header | d |
skipping to change at page 30, line 44 skipping to change at page 31, line 46
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| iprev | policy | iprev | | iprev | policy | iprev |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| spf | smtp | mailfrom | | spf | smtp | mailfrom |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
| spf | smtp | helo | | spf | smtp | helo |
+------------+--------+----------------------------------+ +------------+--------+----------------------------------+
Notably, the DomainKeys and Sender ID entries are not updated to Notably, the DomainKeys and Sender ID entries are not updated to
refer to this revised specification, as they are considered obsolete. refer to this revised specification, as they are considered obsolete.
IANA is accordingly asked to change the Status of the "sender-id" Accordingly, IANA has changed the "Status" field of the "sender-id"
entry in this table to "deprecated". entry in this table to "deprecated".
Finally, two new entries are added to this registry, as follows: Finally, two new entries have been added to this registry, as
follows:
6.3.1. "header.a" for DKIM
6.3.1. 'header.a' for DKIM
Method: dkim Method: dkim
Definition: [this document] Definition: RFC 8601
ptype: header ptype: header
property: a Property: a
Description: value of signature "a" tag Value: value of signature "a" tag
Status: active Status: active
Version: 1 Version: 1
6.3.2. 'header.s' for DKIM 6.3.2. "header.s" for DKIM
Method: dkim Method: dkim
Definition: [this document] Definition: RFC 8601
ptype: header ptype: header
property: s Property: s
Description: value of signature "s" tag Value: value of signature "s" tag
Status: active Status: active
Version: 1 Version: 1
6.4. "Email Authentication Property Types" Registry Description 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.
Definition: An optional reference to a defining specification. Definition: an optional reference to a defining specification.
Description: A brief description of what sort of information this Description: a brief description of what sort of information this
"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 ensure 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 will update this registry to show Section 2.3 of this registry, IANA has updated 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 other "policy", and "smtp" entries of that registry.
documents will be removed.
6.5. "Email Authentication Property Types" Registry Update 6.5. "Email Authentication Property Types" Registry Update
All current entries in this registry are to be updated to replace All current entries in this registry have been updated to replace
[RFC7601] with this document. [RFC7601] with this document.
6.6. "Email Authentication Result Names" Registry Description 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. experimental codes as described in Section 2.7.7.
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".
No two entries can have the same combination of method and code. No two entries can have the same combination of method and code.
An entry in this registry contains the following: An entry in this registry contains the following:
Auth Method: an authentication method for which results are being Auth Method: an authentication method for which results are being
returned using the header field defined in this document. returned using the header field defined in this document.
Code: a result code that can be returned for this authentication Code: a result code that can be returned for this authentication
method. method.
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 one of the following:
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.
6.7. "Email Authentication Result Names" Registry Update 6.7. "Email Authentication Result Names" Registry Update
The following entries in this registry are to be updated to reflect For the following entries in this registry, the new "Specification"
this new Specification as follows: field has been set as follows:
o All "auth" method result codes ("fail", "none", "pass", o All "auth" method result codes ("fail", "none", "pass",
"permerror", "temperror") are now specified in Section 2.7.4 of "permerror", and "temperror") are now specified in Section 2.7.4
this document. of this document.
o All "dkim" method result names ("fail", "neutral", "none", "pass", o All "dkim" method result names ("fail", "neutral", "none", "pass",
"permerror", "policy", "temperror") are now specified in "permerror", "policy", and "temperror") are now specified in
Section 2.7.1 of this document. Section 2.7.1 of this document.
o All "iprev" method result names ("fail", "pass", "permerror", o All "iprev" method result names ("fail", "pass", "permerror", and
"temperror") are now specified in Section 2.7.3 of this document. "temperror") are now specified in Section 2.7.3 of this document.
o The "spf" method result names "fail", "neutral", "none", "pass", o The "spf" method result names "fail", "neutral", "none", "pass",
"permerror", "policy", "softfail", and "temperror" are now "permerror", "policy", "softfail", and "temperror" are now
specified in Section 2.7.2 of this document. The registration for specified in Section 2.7.2 of this document. The registration for
result name "hardfail" is not updated. result name "hardfail" is not updated.
The following entries in this registry are to be updated with a new The following entries in this registry have been updated with a new
Status of "deprecated", with no change to the Specification as they "Status" field set to "deprecated", and with no change to the
reference historic protocols: "Specification" field as they reference historic protocols:
o All "domainkeys" method result names ("fail", "neutral", "none", o All "domainkeys" method result names ("fail", "neutral", "none",
"pass", "permerror", "policy", and "temperror"). "pass", "permerror", "policy", and "temperror").
o All "sender-id" method result names ("fail", "neutral", "none", o All "sender-id" method result names ("fail", "neutral", "none",
"pass", "permerror", "policy", "softfail", and "temperror"). "pass", "permerror", "policy", "softfail", and "temperror").
6.8. 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" subregistry of
the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry" is to be updated to refer only to Section 3.3 of [AUTH-ESC] Registry" has been updated to refer only to Section 3.3 of
as that is where that registration was done. [AUTH-ESC], as that is where that registration was done.
7. Security Considerations 7. Security Considerations
The following security considerations apply when adding or processing The following security considerations apply when adding or processing
the Authentication-Results header field: the Authentication-Results header field:
7.1. Forged Header Fields 7.1. Forged Header Fields
An MTA not applying the filtering discussed in Section 5 exposes MUAs An MTA not applying the filtering discussed in Section 5 exposes MUAs
to false conclusions based on forged header fields. A malicious user to false conclusions based on forged header fields. A malicious user
skipping to change at page 35, line 13 skipping to change at page 36, line 18
this memo. This, too, has potentially high barriers to entry. this memo. This, too, has potentially high barriers to entry.
4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to
allow an MUA or filtering agent to acquire the authserv-id in use allow an MUA or filtering agent to acquire the authserv-id in use
within an ADMD, thus allowing it to identify which within an ADMD, thus allowing it to identify which
Authentication-Results header fields it can trust. Authentication-Results header fields it can trust.
5. On the presumption that internal MTAs are fully compliant with 5. On the presumption that internal MTAs are fully compliant with
Section 3.6 of [MAIL] and the compliant internal MTAs are using Section 3.6 of [MAIL] and the compliant internal MTAs are using
their own hostnames or the ADMD's DNS domain name as the their own hostnames or the ADMD's DNS domain name as the
authserv-id token, the header field proposed here should always authserv-id token, this header field should always appear above a
appear above a Received header added by a trusted MTA. This can Received header added by a trusted MTA. This can be used as a
be used as a test for header field validity. test for header field validity.
Support for some of these is being considered for future work. Support for some of these is being considered for future work.
In any case, a mechanism needs to exist for an MUA or filter to In any case, a mechanism needs to exist for an MUA or filter to
verify that the host that appears to have added the header field (a) verify that the host that appears to have added the header field
actually did so and (b) is legitimately adding that header field for (a) actually did so and (b) is legitimately adding that header field
this delivery. Given the variety of messaging environments deployed for this delivery. Given the variety of messaging environments
today, consensus appears to be that specifying a particular mechanism deployed today, consensus appears to be that specifying a particular
for doing so is not appropriate for this document. mechanism for doing so is not appropriate for this document.
Mitigation of the forged header field attack can also be accomplished Mitigation of the forged header field attack can also be accomplished
by moving the authentication results data into metadata associated by moving the authentication results data into metadata associated
with the message. In particular, an [SMTP] extension could be with the message. In particular, an SMTP extension [SMTP] could be
established to communicate authentication results from the border MTA established to communicate authentication results from the border MTA
to intermediate and delivery MTAs; the latter of these could arrange to intermediate and delivery MTAs; the latter of these could arrange
to store the authentication results as metadata retrieved and to store the authentication results as metadata retrieved and
rendered along with the message by an [IMAP] client aware of a rendered along with the message by an IMAP client [IMAP] aware of a
similar extension in that protocol. The delivery MTA would be told similar extension in that protocol. The delivery MTA would be told
to trust data via this extension only from MTAs it trusts, and border to trust data via this extension only from MTAs it trusts, and border
MTAs would not accept data via this extension from any source. There MTAs would not accept data via this extension from any source. There
is no vector in such an arrangement for forgery of authentication is no vector in such an arrangement for forgery of authentication
data by an outside agent. data by an outside agent.
7.2. Misleading Results 7.2. Misleading Results
Until some form of service for querying the reputation of a sending Until some form of service for querying the reputation of a sending
agent is widely deployed, the existence of this header field agent is widely deployed, the existence of this header field
indicating a "pass" does not render the message trustworthy. It is indicating a "pass" does not render the message trustworthy. It is
possible for an arriving piece of spam or other undesirable mail to possible for an arriving piece of spam or other undesirable mail to
pass checks by several of the methods enumerated above (e.g., a piece pass checks by several of the methods enumerated above (e.g., a piece
of spam signed using [DKIM] by the originator of the spam, which of spam signed using [DKIM] by the originator of the spam, which
might be a spammer or a compromised system). In particular, this might be a spammer or a compromised system). In particular, this
issue is not resolved by forged header field removal discussed above. issue is not resolved by forged header field removal (discussed
above).
Hence, MUAs and downstream filters must take some care with use of Hence, MUAs and downstream filters must take some care with use of
this header even after possibly malicious headers are scrubbed. this header even after possibly malicious headers are scrubbed.
7.3. Header Field Position 7.3. Header Field Position
Despite the requirements of [MAIL], header fields can sometimes be Despite the requirements of [MAIL], header fields can sometimes be
reordered en route by intermediate MTAs. The goal of requiring reordered en route by intermediate MTAs. The goal of requiring
header field addition only at the top of a message is an header field addition only at the top of a message is an
acknowledgment that some MTAs do reorder header fields, but most do acknowledgment that some MTAs do reorder header fields, but most do
not. Thus, in the general case, there will be some indication of not. Thus, in the general case, there will be some indication of
which MTAs (if any) handled the message after the addition of the which MTAs (if any) handled the message after the addition of the
header field defined here. header field defined here.
7.4. Reverse IP Query Denial-of-Service Attacks 7.4. Reverse IP Query Denial-of-Service Attacks
Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack Section 4.6.4 of [SPF] observes that limits are necessary on
for verifiers that attempt DNS-based identity verification of recursive evaluations of SPF records in order to avoid abuse of or
arriving client connections. A verifier wishing to do this check and attacks on the DNS when verifying arriving client connections. A
report this information needs to take care not to go to unbounded verifier wishing to do this check and report this information needs
lengths to resolve "A" and "PTR" queries. MUAs or other filters to take care not to go to unbounded lengths to resolve "A" and "PTR"
making use of an "iprev" result specified by this document need to be queries. MUAs or other filters making use of an "iprev" result
aware of the algorithm used by the verifier reporting the result and, specified by this document need to be aware of the algorithm used by
especially, its limitations. the verifier reporting the result and, especially, its limitations.
7.5. Mitigation of Backscatter 7.5. Mitigation of Backscatter
Failing to follow the instructions of Section 4.2 can result in a Failing to follow the instructions of Section 4.2 can result in a
denial-of-service attack caused by the generation of [DSN] messages denial-of-service attack caused by the generation of DSN messages
(or equivalent) to addresses that did not send the messages being [DSN] (or equivalent) to addresses that did not send the messages
rejected. being rejected.
7.6. Internal MTA Lists 7.6. Internal MTA Lists
Section 5 describes a procedure for scrubbing header fields that may Section 5 describes a procedure for scrubbing header fields that may
contain forged authentication results about a message. A compliant contain forged authentication results about a message. A compliant
installation will have to include, at each MTA, a list of other MTAs installation will have to include, at each MTA, a list of other MTAs
known to be compliant and trustworthy. Failing to keep this list known to be compliant and trustworthy. Failing to keep this list
current as internal infrastructure changes may expose an ADMD to current as internal infrastructure changes may expose an ADMD to
attack. attack.
7.7. Attacks against Authentication Methods 7.7. Attacks against Authentication Methods
If an attack becomes known against an authentication method, clearly If an attack against an authentication method becomes known, clearly
then the agent verifying that method can be fooled into thinking an then the agent verifying that method can be fooled into thinking an
inauthentic message is authentic, and thus the value of this header inauthentic message is authentic, and thus the value of this header
field can be misleading. It follows that any attack against the field can be misleading. It follows that any attack against the
authentication methods supported by this document is also a security authentication methods supported by this document is also a security
consideration here. consideration here.
7.8. Intentionally Malformed Header Fields 7.8. Intentionally Malformed Header Fields
As with any other header field found in the message, it is possible As with any other header field found in the message, it is possible
for an attacker to add an Authentication-Results header field that is for an attacker to add an Authentication-Results header field that is
skipping to change at page 37, line 23 skipping to change at page 38, line 32
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
been added by a trusted border MTA (as described above), yet the been added by a trusted border MTA (as described above), yet the SMTP
[SMTP] connection is not coming from an internal machine known to be connection [SMTP] is not coming from an internal machine known to be
running an authorized MTA. However, in such a configuration, running an authorized MTA. However, in such a configuration,
legitimate MTAs will have to add this header field when legitimate legitimate MTAs will have to add this header field when legitimate
internal-only messages are generated. This is also covered in internal-only messages are generated. This is also covered in
Section 5. Section 5.
7.10. Encapsulated Instances 7.10. Encapsulated Instances
MIME messages can contain attachments of type "message/rfc822", which MIME messages can contain attachments of type "message/rfc822", which
contain other messages. Such an encapsulated message can also contain other messages. Such an encapsulated message can also
contain an Authentication-Results header field. Although the contain an Authentication-Results header field. Although the
skipping to change at page 37, line 51 skipping to change at page 39, line 11
Section 4.1 cautioning MUAs to ignore such instances within MIME Section 4.1 cautioning MUAs to ignore such instances within MIME
attachments, as might be included when a message is forwarded. attachments, as might be included when a message is forwarded.
Moreover, when extracting a message digest to separate mail store Moreover, when extracting a message digest to separate mail store
messages or other media, such header fields should be removed so that messages or other media, such header fields should be removed so that
they will never be interpreted improperly by MUAs that might later they will never be interpreted improperly by MUAs that might later
consume them. consume them.
There can be cases where these header fields included as part of There can be cases where these header fields included as part of
encapsulated messages might actually be of value, such as when they encapsulated messages might actually be of value, such as when they
are taken from messages within the same ADMD where they will be are taken from messages within the same ADMD where they will be
consumed. Caution must be taken that the consumer fully understands consumed. Caution must be taken to ensure that the consumer fully
the semantics of what the header field is indicating and the understands the semantics of what the header field is indicating and
message's handling history before ascribing any value, positive or the message's handling history before ascribing any value, positive
negative, to such data. or negative, to such data.
7.11. Reverse Mapping 7.11. Reverse Mapping
Although Section 3 of this memo includes explicit support for the Although Section 3 of this memo includes explicit support for the
"iprev" method, its value as an authentication mechanism is limited. "iprev" method, its value as an authentication mechanism is limited.
Implementers of both this specification and agents that use the data Implementers of both this specification and agents that use the data
it relays are encouraged to become familiar with the issues raised by it relays are encouraged to become familiar with the issues raised by
[DNSOP-REVERSE] when deciding whether or not to include support for [DNSOP-REVERSE] when deciding whether or not to include support for
"iprev". "iprev".
8. References 8. References
8.1. Normative References 8.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234,
RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <https://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>. <https://www.rfc-editor.org/info/rfc3864>.
[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>. <https://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>. <https://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>. February 2012, <https://www.rfc-editor.org/info/rfc6530>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>. <https://www.rfc-editor.org/info/rfc6531>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, Email Headers", RFC 6532, DOI 10.17487/RFC6532,
February 2012, <https://www.rfc-editor.org/info/rfc6532>. February 2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating [RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601, DOI 10.17487/ Message Authentication Status", RFC 7601,
RFC7601, August 2015, DOI 10.17487/RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>. <https://www.rfc-editor.org/info/rfc7601>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, RFC 2119 Key Words", BCP 14, RFC 8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. 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>. <https://www.rfc-editor.org/info/rfc5321>.
8.2. Informative References 8.2. Informative References
[ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing "DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617,
August 2009, <http://www.rfc-editor.org/info/rfc5617>. August 2009, <https://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, DOI 10.17487/ Vouch by Reference Results", RFC 6212,
RFC6212, April 2011, DOI 10.17487/RFC6212, April 2011,
<http://www.rfc-editor.org/info/rfc6212>. <https://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>. <https://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, DOI 10.17487/ Extension for Authentication", RFC 4954,
RFC4954, July 2007, DOI 10.17487/RFC4954, July 2007,
<http://www.rfc-editor.org/info/rfc4954>. <https://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>. <https://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,
<http://www.rfc-editor.org/info/rfc6376>. <https://www.rfc-editor.org/info/rfc6376>.
[DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<http://www.rfc-editor.org/info/rfc7489>. <https://www.rfc-editor.org/info/rfc7489>.
[DNS] Mockapetris, P., "Domain names - implementation and [DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>. November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596, "DNS Extensions to Support IP Version 6", STD 88,
DOI 10.17487/RFC3596, October 2003, RFC 3596, DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>. <https://www.rfc-editor.org/info/rfc3596>.
[DNSOP-REVERSE] [DNSOP-REVERSE]
Senie, D. and A. Sullivan, "Considerations for the use of Senie, D. and A. Sullivan, "Considerations for the use
DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop- of DNS Reverse Mapping", Work in Progress,
reverse-mapping-considerations-06, March 2008. draft-ietf-dnsop-reverse-mapping-considerations-06,
March 2008.
[DOMAINKEYS] [DOMAINKEYS]
Delany, M., "Domain-Based Email Authentication Using Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
DOI 10.17487/RFC4870, May 2007, DOI 10.17487/RFC4870, May 2007,
<http://www.rfc-editor.org/info/rfc4870>. <https://www.rfc-editor.org/info/rfc4870>.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003, DOI 10.17487/RFC3464, January 2003,
<http://www.rfc-editor.org/info/rfc3464>. <https://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>. <https://www.rfc-editor.org/info/rfc5598>.
[IANA-CONSIDERATIONS] [IANA-CONSIDERATIONS]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>. <https://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>. <https://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>. <https://www.rfc-editor.org/info/rfc1939>.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating [RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, DOI 10.17487/ Message Authentication Status", RFC 5451,
RFC5451, April 2009, DOI 10.17487/RFC5451, April 2009,
<https://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,
<https://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,
<https://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, DOI 10.17487/ Message Authentication Status", RFC 7001,
RFC7001, September 2013, DOI 10.17487/RFC7001, September 2013,
<https://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>. <https://www.rfc-editor.org/info/rfc7410>.
[RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage [RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage
Update to DomainKeys Identified Mail (DKIM)", RFC 8301, Update to DomainKeys Identified Mail (DKIM)", RFC 8301,
DOI 10.17487/RFC8301, January 2018, DOI 10.17487/RFC8301, January 2018,
<https://www.rfc-editor.org/info/rfc8301>. <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>. <https://www.rfc-editor.org/info/rfc7293>.
[SECURITY] [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003, DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>. <https://www.rfc-editor.org/info/rfc3552>.
[SENDERID] [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
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>. <https://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, DOI 10.17487/ S/MIME Signature Verification", RFC 7281,
RFC7281, June 2014, DOI 10.17487/RFC7281, June 2014,
<http://www.rfc-editor.org/info/rfc7281>. <https://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>. <https://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>. <https://www.rfc-editor.org/info/rfc5518>.
Appendix A. Legacy MUAs Appendix A. Legacy MUAs
Implementers of this protocol should be aware that many MUAs are Implementers of this specification should be aware that many MUAs are
unlikely to be retrofitted to support the new header field and its unlikely to be retrofitted to support the Authentication-Results
semantics. In the interests of convenience and quicker adoption, a header field and its semantics. In the interests of convenience and
delivery MTA might want to consider adding things that are processed quicker adoption, a delivery MTA might want to consider adding things
by existing MUAs in addition to the Authentication-Results header that are processed by existing MUAs in addition to the
field. One suggestion is to include a Priority header field, on Authentication-Results header field. One suggestion is to include a
messages that don't already have such a header field, containing a Priority header field, on messages that don't already have such a
value that reflects the strength of the authentication that was header field, containing a value that reflects the strength of the
accomplished, e.g., "low" for weak or no authentication, "normal" or authentication that was accomplished, e.g., "low" for weak or no
"high" for good or strong authentication. authentication, "normal" or "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
specification and its successors continue to be adopted), other specification and its successors continue to be adopted), other
interim means 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:
Received: from mail-router.example.com Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1]) (mail-router.example.com [192.0.2.1])
by server.example.org (8.11.6/8.11.6) by server.example.org (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489; with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800 Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org To: receiver@example.org
Message-Id: <12345.abc@example.com> Message-Id: <12345.abc@example.com>
Subject: here's a sample Subject: here's a sample
Hello! Goodbye! Hello! Goodbye!
Example 1: Trivial Case Example 1: Header Field Not Present
The Authentication-Results header field is completely absent. The The Authentication-Results header field is completely absent. The
MUA may make no conclusion about the validity of the message. This MUA may make no conclusion about the validity of the message. This
could be the case because the message authentication services were could be the case because (1) the message authentication services
not available at the time of delivery, or no service is provided, or were not available at the time of delivery, (2) no service is
the MTA is not in compliance with this specification. provided, or (3) the MTA is not in compliance with this
specification.
B.2. Nearly Trivial Case; Service Provided, but No Authentication Done B.2. Nearly Trivial Case: Service Provided, but No Authentication Done
A message that was delivered by an MTA that conforms to this A message that was delivered by an MTA that conforms to this
specification but provides no actual message authentication service: specification but provides no actual message authentication service:
Authentication-Results: example.org 1; none Authentication-Results: example.org 1; none
Received: from mail-router.example.com Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1]) (mail-router.example.com [192.0.2.1])
by server.example.org (8.11.6/8.11.6) by server.example.org (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489; with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800 Fri, Feb 15 2002 17:19:07 -0800
skipping to change at page 45, line 34 skipping to change at page 47, line 34
Message-Id: <12345.abc@example.net> Message-Id: <12345.abc@example.net>
Subject: here's a sample Subject: here's a sample
Hello! Goodbye! Hello! Goodbye!
Example 4: Headers Reporting Results from One MTA Example 4: Headers Reporting Results from One MTA
The Authentication-Results header field is present, indicating that The Authentication-Results header field is present, indicating that
the delivering MTA conforms to this specification. Once again, the the delivering MTA conforms to this specification. Once again, the
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 themselves to the MTA via a method specified
specified in [AUTH], and both SPF and "iprev" checks were done and in [AUTH], and both SPF and "iprev" checks were done and passed. The
passed. The MUA could extract and relay this extra information if 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 This example illustrates a scenario in which a remote user on a
dial-up connection (example.net) sends mail to a border MTA dial-up connection (example.net) sends mail to a border MTA
(example.com) using SMTP authentication to prove identity. The (example.com) using SMTP authentication to prove identity. The
dial-up provider has been explicitly authorized to relay mail as dial-up provider has been explicitly authorized to relay mail as
example.net, producing a "pass" result from the SPF check. example.net, producing a "pass" result from the SPF check.
B.5. Service Provided, Several Authentications Done, Different MTAs B.5. Service Provided, Several Authentications Done, Different MTAs
skipping to change at page 47, line 4 skipping to change at page 49, line 7
is the recipient's DNS domain name. The header field is present is the recipient's DNS domain name. The header field is present
twice because two different MTAs in the chain of delivery did twice because two different MTAs in the chain of delivery did
authentication tests. The first MTA, mail-router.example.com, authentication tests. The first MTA, mail-router.example.com,
reports that SMTP AUTH and SPF were both used and that the former reports that SMTP AUTH and SPF were both used and that the former
passed while the latter failed. In the SMTP AUTH case, additional passed while the latter failed. In the SMTP AUTH case, additional
information is provided in the comment field, which the MUA can information is provided in the comment field, which the MUA can
choose to render if desired. choose to render if desired.
The second MTA, auth-checker.example.com, reports that it did a DKIM The second MTA, auth-checker.example.com, reports that it did a DKIM
test (which passed). Again, additional data about one of the tests test (which passed). Again, additional data about one of the tests
is provided as a comment, which the MUA may choose to render. Also are provided as a comment, which the MUA may choose to render. Also
noteworthy here is the fact that there is a DKIM signature added by noteworthy here is the fact that there is a DKIM signature added by
example.com that assured the integrity of the lower Authentication- example.com that assured the integrity of the lower Authentication-
Results field. 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 a message into This example illustrates more typical transmission of a message 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 they had a valid password allowing
authentication at the border MTA using SMTP AUTH. The SPF test authentication at the border MTA using SMTP AUTH. The SPF test
failed since example.com has not granted example.net's dial-up failed, since example.com has not granted example.net's dial-up
network authority to relay mail on its behalf. The DKIM test passed network authority to relay mail on its behalf. 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 mail-router.example.com used example.com's published public keys and mail-router.example.com used
it to sign the message. 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:
skipping to change at page 48, line 45 skipping to change at page 50, 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 Different Example 6: Headers Reporting Results from Multiple MTAs in
ADMDs Different 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
affixed an Authentication-Results header field indicating such using affixed an Authentication-Results header field indicating such using
its DNS domain name for the authserv-id. It then altered the message its DNS domain name for the authserv-id. It then altered the message
by affixing some footer text to the body, including some by affixing some footer text to the body, including some
administrivia such as unsubscription instructions. Finally, the administrivia such as unsubscription instructions. Finally, the
mailing list server affixes a second DKIM signature and begins mailing list server affixes a second DKIM signature and begins
distribution of the message. distribution of the message.
The border MTA for chicago.example.com explicitly trusts results from The border MTA for chicago.example.com explicitly trusts results from
mail-router.example.net, so that header field is not removed. It mail-router.example.net, so that header field is not removed. It
performs evaluation of both signatures and determines that the first performs evaluation of both signatures and determines that the first
(most recent) is a "pass" but, because of the aforementioned (most recent) is a "pass" but, because of the aforementioned
modifications, the second is a "fail". However, the first signature modifications, the second is a "fail". However, the first signature
included the Authentication-Results header added at mail- included the Authentication-Results header added at
router.example.net that validated the second signature. Thus, mail-router.example.net that validated the second signature. Thus,
indirectly, it can be determined that the authentications claimed by indirectly, it can be determined that the authentications claimed by
both signatures are indeed valid. both signatures are indeed valid.
Note that two styles of presenting metadata about the result are in Note that two styles of presenting metadata about the result are in
use here. In one case, the "reason=" clause is present, which is use here. In one case, the "reason=" clause is present, which is
intended for easy extraction by parsers; in the other case, the CFWS intended for easy extraction by parsers; in the other case, the CFWS
production of the ABNF is used to include such data as a header field production of the ABNF is used to include such data as a header field
comment. The latter can be harder for parsers to extract given the comment. The latter can be harder for parsers to extract given the
varied supported syntaxes of mail header fields. varied supported syntaxes of mail header fields.
skipping to change at page 50, line 7 skipping to change at page 52, line 7
Authentication-Results: foo.example.net (foobar) 1 (baz); Authentication-Results: foo.example.net (foobar) 1 (baz);
dkim (Because I like it) / 1 (One yay) = (wait for it) fail dkim (Because I like it) / 1 (One yay) = (wait for it) fail
policy (A dot can go here) . (like that) expired policy (A dot can go here) . (like that) expired
(this surprised me) = (as I wasn't expecting it) 1362471462 (this surprised me) = (as I wasn't expecting it) 1362471462
Example 7: A Very Comment-Heavy but Perfectly Legal Example Example 7: A Very Comment-Heavy but Perfectly Legal Example
Appendix C. Operational Considerations about Message Authentication Appendix C. Operational Considerations about Message Authentication
This protocol is predicated on the idea that authentication (and Implementation of the Authentication-Results header field is
presumably in the future, reputation) work is typically done by predicated on the idea that authentication (and presumably in the
border MTAs rather than MUAs or intermediate MTAs; the latter merely future, reputation) work is typically done by border MTAs rather than
make use of the results determined by the former. Certainly this is MUAs or intermediate MTAs; the latter merely make use of the results
not mandatory for participation in electronic mail or message determined by the former. Certainly this is not mandatory for
authentication, but this protocol and its deployment to date are participation in electronic mail or message authentication, but this
based on that model. The assumption satisfies several common ADMD header field and its deployment to date are based on that model. The
requirements: assumption satisfies several common ADMD requirements:
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 inside of an ADMD are more likely to be heavily firewalled. MUAs inside of an ADMD are more likely to be heavily firewalled.
Thus, some MUAs might not even be able to complete the task of Thus, some MUAs might not even be able to complete the task of
performing authentication or reputation evaluations without performing authentication or reputation evaluations without
complex proxy 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.
skipping to change at page 51, line 23 skipping to change at page 53, line 23
choosing to implement these functions in MTAs vs. MUAs, the choosing to implement these functions in MTAs vs. MUAs, the
issues of individual flexibility, infrastructure inertia, and issues of individual flexibility, infrastructure inertia, and
scale of effort must be considered. It is typically easier to scale of effort must be considered. It is typically easier to
change a single MUA than an MTA because the modification affects change a single MUA than an MTA because the modification affects
fewer users and can be pursued with less care. However, changing fewer users and can be pursued with less care. However, changing
many MUAs is more effort than changing a smaller number of MTAs. many MUAs is more effort than changing a smaller number of MTAs.
6. For decisions affecting message delivery and display, assessment 6. For decisions affecting message delivery and display, assessment
based on authentication and reputation is best performed close to based on authentication and reputation is best performed close to
the time of message transit, as a message makes its journey the time of message transit, as a message makes its journey
toward a user's inbox, not afterwards. DKIM keys and IP address toward a user's inbox, not afterwards. DKIM keys, IP address
reputations, etc., can change over time or even become invalid, reputations, etc., can change over time or even become invalid,
and users can take a long time to read a message once delivered. and users can take a long time to read a message once delivered.
The value of this work thus degrades, perhaps quickly, once the The value of this work thus degrades, perhaps quickly, once the
delivery process has completed. This seriously diminishes the delivery process has completed. This seriously diminishes the
value of this work when done elsewhere than at MTAs. value of this work when done elsewhere than at MTAs.
Many operational choices are possible within an ADMD, including the Many operational choices are possible within an ADMD, including the
venue for performing authentication and/or reputation assessment. venue for performing authentication and/or reputation assessment.
The current specification does not dictate any of those choices. The current specification does not dictate any of those choices.
Rather, it facilitates those cases in which information produced by Rather, it facilitates those cases in which information produced by
one stage of analysis needs to be transported with the message to the one stage of analysis needs to be transported with the message to the
next stage. next stage.
Appendix D. Changes since RFC7601 Appendix D. Changes since RFC 7601
o Added IANA registration for DKIM "a" and "s" properties. o Added IANA registration for DKIM "a" and "s" properties.
o Include EAI guidance. o Included EAI guidance.
o Adjust some ABNF tokes and names for easier inclusion by other o Adjusted some ABNF tokens and names for easier inclusion by other
documents. documents.
o Made minor editorial adjustments. o Made minor editorial adjustments.
o Deprecate entries from RFCs that are now Historic. o Deprecated entries from RFCs that are now Historic.
Appendix E. Acknowledgments o Erratum 4671 resolved.
o Erratum 5435 resolved.
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: Kurt Andersen, review and constructive criticism of this document: Kurt Andersen,
Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro
Vesely. Vesely.
Author's Address Author's Address
Murray S. Kucherawy Murray S. Kucherawy
270 Upland Drive 270 Upland Drive
San Francisco, CA 94127 San Francisco, CA 94127
United States United States of America
Email: superuser@gmail.com Email: superuser@gmail.com
 End of changes. 188 change blocks. 
451 lines changed or deleted 454 lines changed or added

This html diff was produced by rfcdiff 1.47. The latest version is available from http://tools.ietf.org/tools/rfcdiff/