wdiff rfc7001.original rfc7001.txt

Individual submission
Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 7001 September 2013
Obsoletes: 5451, 6577
(if approved)
Intended status:
Category: Standards Track
Expires: January 12, 2014
ISSN: 2070-1721

Message Header Field for Indicating Message Authentication Status
draft-ietf-appsawg-rfc5451bis-10

Abstract

This document specifies a message header field called Authentication-
Results for use with electronic mail messages to indicate the results
of message authentication efforts. Any receiver-side software, such
as mail filters or Mail User Agents (MUAs), can use this header field
to relay that information in a convenient and meaningful way to
users, users
or to make sorting and filtering decisions.

Status of This Memo

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provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 12, 2014.
http://www.rfc-editor.org/info/rfc7001.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 ....................................................3
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 ....................................................4
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 5 .............................................5
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 ...........................................6
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 ...............................................6
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 ................................................6
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 ...........................................6
1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7 ............................................6
1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 7 ..................................7
1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 8 .........................................8
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 ..........................................8
2. Definition and Format of the Header Field . . . . . . . . . . 9 .......................9
2.1. General Description . . . . . . . . . . . . . . . . . . . 9 ........................................9
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 ..........................................9
2.3. The "policy" ptype ........................................12
2.4. Authentication Identifier Field . . . . . . . . . . . . . 12
2.4. ...........................13
2.5. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 13
2.5. ............................................14
2.6. Defined Methods and Result Values . . . . . . . . . . . . 14
2.5.1. .........................14
2.6.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 14
2.5.2. ................................14
2.6.2. SPF and Sender-ID . . . . . . . . . . . . . . . . . . 15
2.5.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 16
2.5.4. Sender ID ..................................15
2.6.3. "iprev" ............................................17
2.6.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 17
2.5.5. ..........................................17
2.6.5. Other Registered Codes . . . . . . . . . . . . . . . . 17
2.5.6. .............................18
2.6.6. Extension Methods . . . . . . . . . . . . . . . . . . 18
2.5.7. ..................................18
2.6.7. Extension Result Codes . . . . . . . . . . . . . . . . 18 .............................19
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 19 ..............................19
4. Adding the Header Field to a Message . . . . . . . . . . . . . 20 ...........................20
4.1. Header Field Position and Interpretation . . . . . . . . . 21 ..................22
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 22 ..................................23
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 23 ................................23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 ............................................24
6.1. The Authentication-Results Header Field . . . . . . . . . 24 ...................25
6.2. Email "Email Authentication Method Name Methods" Registry . . . . . . . . 24 ...................25
6.3. Email "Email Authentication Result Names Names" Registry . . . . . . . . 25 ..............26
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 25 Security Considerations ........................................26
7.1. Google Mail . . . . . . . . . . . . . . . . . . . . . . . 26 Forged Header Fields ......................................26
7.2. Yahoo! Mail . . . . . . . . . . . . . . . . . . . . . . . 26 Misleading Results ........................................28
7.3. Hotmail . . . . . . . . . . . . . . . . . . . . . . . . . 27 Header Field Position .....................................28
7.4. Courier MTA . . . . . . . . . . . . . . . . . . . . . . . 27 Reverse IP Query Denial-of-Service Attacks ................28
7.5. sid-milter . . . . . . . . . . . . . . . . . . . . . . . . 27 Mitigation of Backscatter .................................29
7.6. opendkim . . . . . . . . . . . . . . . . . . . . . . . . . 28 Internal MTA Lists ........................................29
7.7. opendmarc . . . . . . . . . . . . . . . . . . . . . . . . 28 Attacks against Authentication Methods ....................29
7.8. authres . . . . . . . . . . . . . . . . . . . . . . . . . 29 Intentionally Malformed Header Fields .....................29
7.9. Compromised Internal Hosts ................................29
7.10. Encapsulated Instances ...................................30
7.11. Reverse Mapping ..........................................30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 29 References .....................................................30
8.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 29
8.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 31
8.3. Normative References ......................................30
8.2. Informative References ....................................31
Appendix A. Acknowledgements .....................................33
Appendix B. Legacy MUAs ..........................................33
Appendix C. Authentication-Results Examples ......................33
C.1. Trivial Case; Header Field Position . . . . . . . . . . . . . . . . . . 31
8.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 32
8.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 32
8.6. Internal Not Present ....................34
C.2. Nearly Trivial Case; Service Provided, but No
Authentication Done .......................................34
C.3. Service Provided, Authentication Done .....................35
C.4. Service Provided, Several Authentications Done, Single
MTA Lists . . . . . . . . . . . . . . . . . . . . 32
8.7. Attacks against Authentication Methods . . . . . . . . . . 32
8.8. Intentionally Malformed Header Fields . . . . . . . . . . 32
8.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 33
8.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 33
8.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 33
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.1. Normative References . . . . . . . . . . . . . . . . . . . 33
9.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 36
Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 36
Appendix C. Authentication-Results Examples . . . . . . . . . . . 36
C.1. Trivial Case; Header Field Not Present . . . . . . . . . . 37
C.2. Nearly Trivial Case; Service Provided, But No
Authentication Done . . . . . . . . . . . . . . . . . . . 37
C.3. Service Provided, Authentication Done . . . . . . . . . . 38
C.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.5. Service Provided, Several Authentications Done,
Different MTAs . . . . . . . . . . . . . . . . . . . . . . 40
C.6. Service Provided, Multi-Tiered .......................................................36
C.5. Service Provided, Several Authentications Done,
Different MTAs ............................................37
C.6. Service Provided, Multi-Tiered Authentication Done . . . . 42 ........38
C.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 43 .....................................40
Appendix D. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 44 .......................................40
Appendix E. Changes since RFC5451 . . . . . . . . . . . . . . . . 45

1. Introduction

This document describes a header field called Authentication-Results
for electronic mail messages that presents the results of a message
authentication effort in a machine-readable format. The intent of
the header field is to create a place to collect such data when
message authentication mechanisms are in use so that a Mail User
Agent (MUA) and downstream filters can make filtering decisions
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
content.

This document revises the original definition found in [RFC5451]
based upon various authentication protocols in current use and
incorporates errata logged since the publication of the original
specification.

End users are not expected to be direct consumers of this header
field. This header field is intended for consumption by programs
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
the implications of its presence or absence. However, it does not
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
render those results) as these are local policy and/or user interface
design questions that are not appropriate for this document.

At the time of publication of this document, the following are
published, domain-level email authentication methods in common use:

o Author Domain Signing Practices ([ADSP])

o SMTP Service Extension for Authentication ([AUTH])

o DomainKeys Identified Mail Signatures ([DKIM])

o Sender Policy Framework ([SPF])

o Vouch-By-Reference ([VBR])

o reverse IP address name validation ("iprev", defined in Section 3)

In addition, the following are non-standard methods recognized by
this specification that are no longer common:

o DomainKeys ([DOMAINKEYS]) (Historic)
o Sender ID ([SENDERID]) (Experimental)

This specification is not intended to be restricted to domain-based
authentication schemes, but the existing schemes in that family have
proven to be a good starting point for implementations. The goal is
to give current and future authentication schemes a common framework
within which to deliver their results to downstream agents and
discourage the creation of unique header fields for each.

Although SPF defined a header field called "Received-SPF" and the
historic DomainKeys defined one called "DomainKey-Status" for this
purpose, those header fields are specific to the conveyance of their
respective results only and thus are insufficient to satisfy the
requirements enumerated below. In addition, many SPF implementations
have adopted the header field specified here at least as an option,
and DomainKeys has been obsoleted by DKIM.

1.1. Purpose

The header field defined in this document is expected to serve
several purposes:

1. Convey the results of various message authentication checks,
which are applied by upstream filters and Mail Transfer Agents
(MTAs) and then passed to MUAs and downstream filters within the
same "trust domain". Such agents might wish to render those
results to end users or to use those data to apply more or less
stringent content checks based on authentication results;

2. Provide a common location within a message for this data;

3. Create an extensible framework for reporting new authentication
methods as they emerge.

In particular, the mere presence of this header field does not mean
its contents are valid. Rather, the header field is reporting
assertions made by one or more authentication schemes (supposedly)
applied somewhere upstream. For an MUA or downstream filter to treat
the assertions as actually valid, there must be an assessment of the
trust relationship among such agents, the validating MTA, and the
mechanism for conveying the information.

1.2. Trust Boundary

This document makes several references to the "trust boundary" of an
administrative management domain (ADMD). Given the diversity among
existing mail environments, a precise definition of this term isn't
possible.

Simply put, a transfer from the producer of the header field to the
consumer must occur within a context that permits the consumer to
treat assertions by the producer as being reliable and accurate
(trustworthy). How this trust is obtained is outside the scope of
this document. It is entirely a local matter.

Thus, this document defines a "trust boundary" as the delineation
between "external" and "internal" entities. Services that are
internal -- within the trust boundary -- are provided by the ADMD's
infrastructure for its users. Those that are external are outside of
the authority of the ADMD. By this definition, hosts that are within
a trust boundary are subject to the ADMD's authority and policies,
independent of their physical placement or their physical operation.
For example, a host within a trust boundary might actually be
operated by E. Changes since RFC 5451 ...............................42

1. Introduction

This document describes a remote service provider and reside physically within
its data center.

It is possible header field called Authentication-Results
for a message to be evaluated inside a trust boundary,
but then depart and re-enter the trust boundary. An example might be
a forwarded message such as a message/rfc822 attachment (see
Multipurpose Internet Mail Extensions [MIME]), or one electronic mail messages that is part presents the results of a multipart/digest. The details reported by this field cannot be
trusted message
authentication effort in that case. Thus, this field found within one of those
media types is typically ignored.

1.3. Processing Scope a machine-readable format. The content intent of this
the header field is meant to convey create a place to collect such data when
message
consumers that authentication work on mechanisms are in use so that a Mail User
Agent (MUA) and downstream filters can make filtering decisions
and/or provide a recommendation to the message was already done
within user as to the validity of the
message's origin and possibly the safety and integrity of its trust boundary,
content.

This document revises the original definition found in [RFC5451]
based upon various authentication protocols in current use and those results
incorporates errata logged since the publication of the original
specification.

End users are being presented. It
is not intended to provide message parameters expected to be direct consumers so that
they can perform authentication protocols on their own.

1.4. Requirements of this header
field. This document establishes no new requirements on existing protocols header field is intended for consumption by programs
that will then use such data or servers.

In particular, this render it in a human-usable form.

This document establishes no requirement on MTAs to
reject specifies the format of this header field and discusses
the implications of its presence or filter arriving messages that do absence. However, it does not pass authentication
checks. The
discuss how the data conveyed by contained in the specified header field's contents field ought to be used,
such as what filtering decisions are appropriate or how an MUA might
render those results, as these are local policy and/or user interface
design questions that are not appropriate for this document.

At the information time of publication of MUAs and filters and are to be used at
their discretion.

1.5. Definitions

This section defines various terms used throughout this document.

1.5.1. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" document, the following are
published, domain-level email authentication methods in common use:

o Author Domain Signing Practices ([ADSP])

o SMTP Service Extension for Authentication ([AUTH])

o DomainKeys Identified Mail Signatures ([DKIM])

o Sender Policy Framework ([SPF])

o Vouch By Reference ([VBR])

o reverse IP address name validation ("iprev", defined in Section 3)

In addition, the following are non-standard methods recognized by
this
document specification that are no longer common:

o DomainKeys ([DOMAINKEYS]) (Historic)

o Sender ID ([SENDERID]) (Experimental)

This specification is not intended to be interpreted as described in [KEYWORDS].

1.5.2. Security

Guidelines for Writing RFC Text on Security Considerations
([SECURITY]) discusses restricted to domain-based
authentication and authorization and the
conflation of schemes, but the two concepts. The use of those terms existing schemes in that family have
proven to be a good starting point for implementations. The goal is
to give current and future authentication schemes a common framework
within which to deliver their results to downstream agents and
discourage the
context creation of recent message security work has given rise to slightly
different definitions, unique header fields for each.

Although SPF defined a header field called "Received-SPF" and the
historic DomainKeys defined one called "DomainKey-Status" for this document reflects
purpose, those current
usages, as follows:

o "Authorization" is header fields are specific to the establishment conveyance of permission their
respective results only and thus are insufficient to use a
resource or represent an identity. satisfy the
requirements enumerated below. In this context, authorization
indicates that a message from a particular ADMD arrived via a
route addition, many SPF implementations
have adopted the ADMD header field specified here at least as an option,
and DomainKeys has explicitly approved.

o "Authentication" been obsoleted by DKIM.

1.1. Purpose

The header field defined in this document is expected to serve
several purposes:

1. Convey the assertion of validity of a piece results of data
about a message (such as the sender's identity) or the various message in
its entirety.

As examples: SPF and Sender-ID authentication checks,
which are authorization mechanisms in that
they express a result that shows whether or not the ADMD that
apparently sent the message has explicitly authorized the connecting
Simple applied by upstream filters and Mail Transfer Protocol ([SMTP]) client Agents
(MTAs) and then passed to relay messages MUAs and downstream filters within the
same "trust domain". Such agents might wish to render those
results to end users or to use those data to apply more or less
stringent content checks based on
its behalf, but authentication results;
2. Provide a common location within a message for this data;

3. Create an extensible framework for reporting new authentication
methods as they do not actually validate any other property emerge.

In particular, the mere presence of this header field does not mean
its contents are valid. Rather, the message itself. By contrast, DKIM header field is agnostic as reporting
assertions made by one or more authentication schemes (supposedly)
applied somewhere upstream. For an MUA or downstream filter to treat
the routing assertions as actually valid, there must be an assessment of a message but uses cryptographic signatures to authenticate the
trust relationship among such agents, assign (some) responsibility for the message (which implies
authorization), validating MTA, and ensure that the listed portions of the message
were not modified in transit. Since
mechanism for conveying the signatures are not tied information.

1.2. Trust Boundary

This document makes several references to
SMTP connections, they can be added by either the ADMD "trust boundary" of origin,
intermediate ADMDs (such as a mailing list server), other handling
agents, or any combination.

Rather than create an
administrative management domain (ADMD). Given the diversity among
existing mail environments, a separate header field for each class precise definition of
solution, this proposal groups them both into a single header field.

1.5.3. Email Architecture

o A "border MTA" is an MTA that acts as term isn't
possible.

Simply put, a gateway between the
general Internet and the users within an organizational boundary.
(See also Section 1.2.)
o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
actually enacts delivery transfer from the producer of a message the header field to the
consumer must occur within a user's inbox or other
final delivery.

o An "intermediate MTA" is any MTA context that is not a delivery MTA and is
also not permits the first MTA consumer to handle
treat assertions by the message.

The following diagram illustrates producer as being reliable and accurate
(trustworthy). How this trust is obtained is outside the flow scope of mail among these
defined components. See Internet Mail Architecture [EMAIL-ARCH]
this document. It is entirely a local matter.

Thus, this document defines a "trust boundary" as the delineation
between "external" and "internal" entities. Services that are
internal -- within the trust boundary -- are provided by the ADMD's
infrastructure for
further discussion on general email system architecture, which
includes detailed descriptions its users. Those that are external are outside of these components, and Appendix D
the authority of the ADMD. By this document for discussion about definition, hosts that are within
a trust boundary are subject to the common aspects ADMD's authority and policies,
independent of email
authentication in current environments.

+-----+ +-----+ +------------+
| MUA |-->| MSA |-->| Border MTA |
+-----+ +-----+ +------------+
|
|
V
+----------+
| Internet |
+----------+
|
|
V
+-----+ +-----+ +------------------+ +------------+
| MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA |
+-----+ +-----+ +------------------+ +------------+

Generally, it their physical placement or their physical operation.
For example, a host within a trust boundary might actually be
operated by a remote service provider and reside physically within
its data center.

It is assumed that possible for a message to be evaluated inside a trust boundary
but then depart and re-enter the work of applying trust boundary. An example might be
a forwarded message
authentication schemes takes place at such as a border MTA message/rfc822 attachment (see
Multipurpose Internet Mail Extensions [MIME]) or a delivery MTA.
This specification is written with one that assumption in mind. However,
there are some sites at which the entire mail infrastructure consists is part of
a single host. In such cases, such terms as "border MTA" and
"delivery MTA" might well apply multipart/digest. The details reported by this field cannot be
trusted in that case. Thus, this field found within one of those
media types is typically ignored.

1.3. Processing Scope

The content of this header field is meant to convey to message
consumers that authentication work on the same machine or even the very
same agent. message was already done
within its trust boundary, and those results are being presented. It
is also possible that some not intended to provide message parameters to consumers so that
they can perform authentication
tests could take place protocols on an intermediate MTA. Although their own.

1.4. Requirements

This document establishes no new requirements on existing protocols
or servers.

In particular, this document establishes no requirement on MTAs to
reject or filter arriving messages that do not pass authentication
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
their discretion.

1.5. Definitions

This section defines various terms used throughout this document.

1.5.1. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document doesn't specifically describe such cases, they are not meant to be excluded.

1.5.4. Other Terms

In this document, the term "producer" refers to any component that
adds this header field to messages it is handling, interpreted as described in [KEYWORDS].

1.5.2. Security

"Guidelines for Writing RFC Text on Security Considerations"
([SECURITY]) discusses authentication and "consumer"
refers to any component that identifies, extracts, authorization and parses the
header field to use as part
conflation of a handling decision.

1.6. Trust Environment

This header field permits one or more message validation mechanisms
to communicate output to one or more separate assessment mechanisms.
These mechanisms operate within a unified trust boundary that defines
an Administrative Management Domain (ADMD). An ADMD contains one or
more entities that perform validation and generate the header field,
and one or more that consume it for some type of assessment. two concepts. The
field often contains no integrity or validation mechanism of its own,
so its presence must be trusted implicitly. Hence, valid use of those terms within the
header field requires removing any occurrences
context of it that are present
when the recent message enters the ADMD. This ensures that later
occurrences have been added within security work has given rise to slightly
different definitions, and this document reflects those current
usages, as follows:

o "Authorization" is the trust boundary establishment of the ADMD.

The "authserv-id" token defined in Section 2.2 can be used permission to
reference an entire ADMD or use a specific validation engine within an
ADMD. Although the labeling scheme is left as an operational choice,
some guidance for selecting
resource or represent an identity. In this context, authorization
indicates that a token message from a particular ADMD arrived via a
route the ADMD has explicitly approved.

o "Authentication" is provided in later sections the assertion of
this document.

2. Definition and Format validity of the Header Field

This section gives a general overview of the format piece of the header
field being defined, and then provides more formal specification.

2.1. General Description

The header field specified here is called "Authentication-Results".
It is data
about a Structured Header Field message (such as defined the sender's identity) or the message in Internet Message Format
([MAIL])
its entirety.

As examples: SPF and thus all of the related definitions Sender ID are authorization mechanisms in that document
apply.

This header field is added at
they express a result that shows whether or not the top of ADMD that
apparently sent the message as it transits
MTAs that has explicitly authorized the connecting
Simple Mail Transfer Protocol ([SMTP]) client to relay messages on
its behalf, but they do authentication checks, so some idea not actually validate any other property of how far away
the
checks were done can be inferred. It message itself. By contrast, DKIM is therefore considered to be a
Trace Field agnostic as defined in [MAIL], and thus all of the related
definitions in that document apply.

The value of to the header field (after removing comments) consists routing
of
an authentication identifier, an optional version, and then a series
of statements and supporting data. The statements are of the form
"method=result", and indicate which authentication method(s) were
applied and their respective results. For each such statement, message but uses cryptographic signatures to authenticate
agents, assign (some) responsibility for the
supporting data can include a "reason" string, message (which implies
authorization), and one or more
"property=value" statements indicating which ensure that the listed portions of the message properties
were
evaluated to reach that conclusion.

The header field can appear more than once not modified in a single message, or
more than one result transit. Since the signatures are not tied to
SMTP connections, they can be represented in added by either the ADMD of origin,
intermediate ADMDs (such as a single header field, mailing list server), other handling
agents, or any combination.

Rather than create a combination of these can be applied.

2.2. Formal Definition

Formally, the separate header field is specified as follows using Augmented
Backus-Naur Form ([ABNF]):

authres-header = "Authentication-Results:" [CFWS] authserv-id
[ CFWS authres-version ]
( no-result / 1*resinfo ) [CFWS] CRLF

authserv-id = value
; see below for a description of this element

authres-version = 1*DIGIT [CFWS]
; indicates which version each class of
solution, this specification is in use;
; this specification is version "1", and the absence of proposal groups them both into a
; version implies this version of the specification

no-result = [CFWS] ";" [CFWS] "none"
; the special case of "none" single header field.

1.5.3. Email Architecture

o A "border MTA" is used to indicate an MTA that no
; message authentication was performed

resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*( CFWS propspec )

methodspec = [CFWS] method [CFWS] "=" [CFWS] result
; indicates which authentication method was evaluated
; and what its output was

reasonspec = "reason" [CFWS] "=" [CFWS] value
; acts as a free-form comment on gateway between the reason
general Internet and the given result
; was returned

propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
; users within an indication of which properties organizational boundary.
(See also Section 1.2.)

o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that
actually enacts delivery of the a message
; were evaluated by the authentication scheme being
; applied to yield the reported result

method = Keyword [ [CFWS] "/" [CFWS] method-version ]
; a method indicates which method's result user's inbox or other
final delivery.

o An "intermediate MTA" is
; represented by "result", and any MTA that is one of the methods
; explicitly defined as valid in this document
; or not a delivery MTA and is an extension method as defined below

method-version = 1*DIGIT [CFWS]
; indicates which version of
also not the method specification is
; in use, corresponding first MTA to handle the matching entry in message.

The following diagram illustrates the IANA
; Email Authentication Methods registry; a value flow of mail among these
defined components. See Internet Mail Architecture [EMAIL-ARCH] for
further discussion on general email system architecture, which
includes detailed descriptions of these components, and Appendix D of "1"
; is assumed if
this version string document for discussion about the common aspects of email
authentication in current environments.

Generally, it is absent

result = Keyword
; indicates assumed that the results work of the attempt to authenticate
; the message; see below for details

ptype = "smtp" / "header" / "body" / "policy"
; indicates whether the property being evaluated was
; applying message
authentication schemes takes place at a parameter to an [SMTP] command, border MTA or was a value taken
; from a message header field, or was delivery MTA.
This specification is written with that assumption in mind. However,
there are some property sites at which the entire mail infrastructure consists
of
; a single host. In such cases, such terms as "border MTA" and
"delivery MTA" might well apply to the message body, same machine or some other property evaluated by
; even the receiving MTA

property = special-smtp-verb / Keyword
; if "ptype" very
same agent. It is "smtp", also possible that some message authentication
tests could take place on an intermediate MTA. Although this indicates which [SMTP]
; command provided
document doesn't specifically describe such cases, they are not meant
to be excluded.

1.5.4. Other Terms

In this document, the value term "producer" refers to any component that was evaluated by the
; authentication scheme being applied; if "ptype" is
; "header",
adds this indicates from which header field the
; value being evaluated was extracted; if "ptype" is
; "body", this indicates where in the message body
; a value being evaluated can be found (e.g., a specific
; offset into the message or a reference to a MIME part);
; if "ptype" messages it is "policy" then this indicates the name
; of the policy handling, and "consumer"
refers to any component that caused this identifies, extracts, and parses the
header field to be
; added (see below)

special-smtp-verb = "mailfrom" / "rcptto"
; special cases of [SMTP] commands that are made up
; use as part of multiple words

pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS]
; the value extracted from the a handling decision.

1.6. Trust Environment

This header field permits one or more message property defined
; by validation mechanisms
to communicate output to one or more separate assessment mechanisms.
These mechanisms operate within a unified trust boundary that defines
an Administrative Management Domain (ADMD). An ADMD contains one or
more entities that perform validation and generate the "ptype.property" construction

"local-part" is defined in Section 3.4.1, header field
and "CFWS" is defined in
Section 3.2.2, of [MAIL].

"Keyword" is defined in Section 4.1.2 of [SMTP].

The "value" is as defined in Section 5.1 of [MIME].

The "domain-name" is as defined in Section 3.5 one or more that consume it for some type of [DKIM]. assessment. The "Keyword" used in "result" above is further constrained by the
necessity
field often contains no integrity or validation mechanism of being enumerated in Section 2.5.

See Section 2.3 for a description its own,
so its presence must be trusted implicitly. Hence, valid use of the "authserv-id" element.

If the value portion
header field requires removing any occurrences of a "pvalue" construction identifies something
intended to be an e-mail identity, then it MUST use that are present
when the right hand
portion of message enters the ADMD. This ensures that ABNF definition.

The list later
occurrences have been added within the trust boundary of commands eligible for use with the "smtp" ptype can be
found ADMD.

The authserv-id token defined in Section 4.1 of [SMTP].

The "propspec" may 2.2 can be omitted if, for example, the method was unable
to extract any properties used to do its evaluation yet has reference
an entire ADMD or a result to
report.

Where specific validation engine within an SMTP command name ADMD.
Although the labeling scheme is being reported left as an operational choice, some
guidance for selecting a "property", the
agent generating the header field represents that command by
converting it to lowercase token is provided in later sections of this
document.

2. Definition and dropping any spaces (e.g., "MAIL FROM"
becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).

A "ptype" value Format of "policy" indicates a policy decision about the
message not specific to Header Field

This section gives a property general overview of the message that could be
extracted. For example, if a method would normally report a
"ptype.property" format of "header.From" the header
field being defined and no From: then provides more formal specification.

2.1. General Description

The header field was
present, the method can use "policy" to indicate that no conclusion
about the authenticity specified here is called Authentication-Results. It
is a Structured Header Field as defined in Internet Message Format
([MAIL]), and thus all of the message could be reached.

Examples of complete messages using this header field can be found related definitions in
Appendix C.

2.3. Authentication Identifier Field

Every Authentication-Results that document
apply.

This header field has an authentication
service identifier field ("authserv-id" above). Specifically, this is any string intended to identify added at the authentication service within top of the ADMD message as it transits
MTAs that conducted do authentication checks on checks, so some idea of how far away the message. This
identifier
checks were done can be inferred. It is intended therefore considered to be machine-readable a
trace field as defined in [MAIL], and not necessarily
meaningful to users.

Since agents consuming this thus all of the related
definitions in that document apply.

The value of the header field will use this identifier to
determine whether its contents are (after removing comments) consists of interest (and
an authentication identifier, an optional version, and then a series
of statements and supporting data. The statements are safe to use),
the uniqueness of the identifier MUST be guaranteed by form
"method=result" and indicate which authentication method(s) were
applied and their respective results. For each such statement, the ADMD that
generates it
supporting data can include a "reason" string and MUST pertain to that ADMD. MUAs one or downstream
filters SHOULD use this identifier more
"property=value" statements indicating which message properties were
evaluated to determine whether or not the
data contained in an Authentication-Results reach that conclusion.

The header field ought to can appear more than once in a single message, more
than one result can be
used represented in a single header field, or ignored.

For simplicity a
combination of these can be applied.

2.2. Formal Definition

Formally, the header field is specified as follows using Augmented
Backus-Naur Form ([ABNF]):

authres-header = "Authentication-Results:" [CFWS] authserv-id
[ CFWS authres-version ]
( no-result / 1*resinfo ) [CFWS] CRLF

authserv-id = value
; see below for a description of this element
authres-version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use;
; this specification is version "1", and scalability, the authentication service identifier
SHOULD be absence of a common token used throughout
; version implies this version of the ADMD. Common practice
is to use specification

no-result = [CFWS] ";" [CFWS] "none"
; the DNS domain name special case of "none" is used by or within to indicate that ADMD, sometimes
called the "organizational domain", but this is not strictly
necessary.

For tracing no
; message authentication was performed

resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*( CFWS propspec )

methodspec = [CFWS] method [CFWS] "=" [CFWS] result
; indicates which authentication method was evaluated
; and debugging purposes, what its output was

reasonspec = "reason" [CFWS] "=" [CFWS] value
; a free-form comment on the authentication identifier can
instead be reason the specific hostname given result
; was returned

propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
; an indication of which properties of the MTA performing message
; were evaluated by the authentication check whose result is scheme being reported. Moreover, some
implementations define a sub-structure
; applied to yield the identifier; these are
outside reported result

method = Keyword [ [CFWS] "/" [CFWS] method-version ]
; a method indicates which method's result is
; represented by "result", and is one of the scope of methods
; explicitly defined as valid in this specification.

Note, however, that using a local, relative identifier like a flat
hostname, rather than a hierarchical and globally unique ADMD
identifier like a DNS domain name, makes configuration more difficult
for large sites. The hierarchical identifier permits aggregating
related, trusted systems together under a single, parent identifier, document
; or is an extension method as defined below

method-version = 1*DIGIT [CFWS]
; indicates which version of the method specification is
; in turn permits assessing use, corresponding to the trust relationship with matching entry in the IANA
; "Email Authentication Methods" registry; a single
reference. The alternative value of "1"
; is a flat namespace requiring
individually listing each trusted system. Since consumers will use
the identifier to determine whether to use assumed if this version string is absent

result = Keyword
; indicates the contents results of the header
field:

o Changes attempt to authenticate
; the identifier impose message; see below for details

ptype = "smtp" / "header" / "body" / "policy"
; indicates whether the property being evaluated was
; a large, centralized
administrative burden.

o Ongoing administrative changes require constantly updating this
centralized table, making it difficult parameter to ensure that an MUA or
downstream filter will have access to accurate information for
assessing the usability of the [SMTP] command, was a value taken
; from a message header field's content. In
particular, consumers field, was some property of
; the header field will need to know not
only the current identifier(s) in use, but previous ones as well
to account for delivery latency message body, or later re-assessment of was some other property evaluated by
; the
header field's contents.

Examples of valid authentication identifiers are "example.com",
"mail.example.org", "ms1.newyork.example.com", and "example-auth".

2.4. Version Tokens

The grammar above provides for receiving MTA
property = special-smtp-verb / Keyword
; if "ptype" is "smtp", this indicates which [SMTP]
; command provided the optional inclusion of versions on
both value that was evaluated by the
; authentication scheme being applied; if "ptype" is
; "header", this indicates from which header field itself (attached to the authserv-id token) and
on each of the methods
; value being reported. The method version refers to
the method itself, which evaluated was extracted; if "ptype" is specified
; "body", this indicates where in the documents describing
those methods, while message body
; a value being evaluated can be found (e.g., a specific
; offset into the authserv-id version refers message or a reference to a MIME part);
; if "ptype" is "policy", then this document
and thus indicates the syntax name
; of the policy that caused this header field.

The purpose of including these is field to avoid misinterpretation be
; added (see below)

special-smtp-verb = "mailfrom" / "rcptto"
; special cases of the
results. That is, if a parser finds a version after an authserv-id [SMTP] commands that it does not explicitly know, it can immediately discontinue
trying to parse since what follows might not be in an expected
format. For a method version, are made up
; of multiple words

pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS]
; the parser SHOULD ignore a method
result if value extracted from the version message property defined
; by the "ptype.property" construction

"local-part" is not supported defined in case Section 3.4.1 of [MAIL], and "CFWS" is
defined in Section 3.2.2 of [MAIL].

"Keyword" is defined in Section 4.1.2 of [SMTP].

The "value" is as defined in Section 5.1 of [MIME].

The "domain-name" is as defined in Section 3.5 of [DKIM].

The "Keyword" used in "result" above is further constrained by the semantics
necessity of the
result have a different meaning than what is expected. For example,
if a hypothetical DKIM version 2 yielded a "pass" result being enumerated in Section 2.6.

See Section 2.4 for
different reasons than version 1 does, a consumer description of this field might
not want the authserv-id element.

If the value portion of a "pvalue" construction identifies something
intended to be an e-mail identity, then it MUST use the altered semantics. Allowing versions right hand
portion of that ABNF definition.

The list of commands eligible for use with the "smtp" ptype can be
found in Section 4.1 of [SMTP].

The "propspec" may be omitted if, for example, the
syntax is method was unable
to extract any properties to do its evaluation yet has a way result to indicate this and let
report.

Where an SMTP command name is being reported as a "property", the consumer of
agent generating the header field decide.

2.5. Defined Methods represents that command by
converting it to lowercase and Result Values

Each individual authentication method returns one dropping any spaces (e.g., "MAIL FROM"
becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).

A "ptype" value of "policy" indicates a set of policy decision about the
message not specific result values. The subsections below provide references to a property of the documents defining the authentication methods specifically
supported by message that could be
extracted. See Section 2.3 for details.

Examples of complete messages using this document, and their corresponding result values.
Verifiers SHOULD use these values as described below. New methods
not specified header field can be found in this document, but intended
Appendix C.

2.3. The "policy" ptype

A special ptype value of "policy" is defined. This ptype is provided
to be supported by indicate that some local policy mechanism was applied that
augments or even replaces (i.e., overrides) the
header field defined here, MUST include a similar result table either
in its defining document or in a supplementary one.

2.5.1. DKIM and DomainKeys

DKIM is represented returned by
the "dkim" method authentication mechanism. The property and is defined in [DKIM].
DomainKeys is defined value in [DOMAINKEYS] this case
identify the local policy that was applied and is represented by the
"domainkeys" method.

A result it
returned.

For example, a DKIM signature is "acceptable not required to include the ADMD" if it passes local policy
checks (or there Subject
header field in the set of fields that are no specific local policy checks). For example,
an signed. An ADMD policy receiving
such a message might require decide that such a signature is unacceptable,
even if it passes, because the signature(s) on the message be
added using the DNS domain present in content of the From: Subject header field of
could be altered post-signing without invalidating the
message, thus making third-party signatures unacceptable even if they
verify.

Both DKIM and DomainKeys use signature.
Such an ADMD could replace the same DKIM "pass" result set, as follows:

none: The message was not signed.

pass: The message was signed, with a "policy"
result and then also include the signature or signatures were
acceptable to following in the ADMD, and corresponding
Authentication-Result field:

... dkim=fail policy.dkim-rules=unsigned-subject ...

In this case, the signature(s) passed verification
tests.

fail: The message was signed property is "dkim-rules", indicating some local
check by that name took place and that check returned a result of
"unsigned-subject". These are arbitrary names selected by (and
presumably used within) the signature or signatures were
acceptable to the ADMD, but ADMD making use of them, so they failed are not
normally registered with IANA or otherwise specified apart from
setting syntax restrictions that allow for easy parsing within the verification test(s).

policy: The message was signed but some aspect
rest of the signature or
signatures were not acceptable to header field.

This ptype existed in the ADMD.

neutral: The message was signed original specification for this header
field, but the signature or signatures
contained syntax errors without a complete description or were example of intended use.
As a result, it has not otherwise able seen any practical use to be
processed. This result is also used for other failures not
covered elsewhere in date that matches
its intended purpose. These added details are provided to guide
implementers toward proper use.

2.4. Authentication Identifier Field

Every Authentication-Results header field has an authentication
service identifier field (authserv-id above). Specifically, this list.

temperror: The message could not be verified due is
any string intended to some error identify the authentication service within the
ADMD that conducted authentication checks on the message. This
identifier is likely transient in nature, such as a temporary inability intended to
retrieve a public key. A later attempt may produce a final
result.

permerror: The message could be machine-readable and not necessarily
meaningful to users.

Since agents consuming this field will use this identifier to
determine whether its contents are of interest (and are safe to use),
the uniqueness of the identifier MUST be verified due guaranteed by the ADMD that
generates it and MUST pertain to some error that
is unrecoverable, such as a required ADMD. MUAs or downstream
filters SHOULD use this identifier to determine whether or not the
data contained in an Authentication-Results header field being absent. A
later attempt is unlikely to produce a final result.

[DKIM] advises that if a message fails verification, it is ought to be
treated as an unsigned message. A report of "fail" here permits
used or ignored.

For simplicity and scalability, the
receiver of authentication service identifier
SHOULD be a common token used throughout the report to decide how ADMD. Common practice
is to handle use the failure. A report
of "neutral" DNS domain name used by or "none" preempts within that choice, ensuring ADMD, sometimes
called the message
will be treated as if it had "organizational domain", but this is not been signed.

2.5.2. SPF and Sender-ID

SPF strictly
necessary.

For tracing and Sender ID use debugging purposes, the "spf" and "sender-id" method names,
respectively. The authentication identifier can
instead be the specific hostname of the MTA performing the
authentication check whose result values for SPF is being reported. Moreover, some
implementations define a substructure to the identifier; these are defined in Section 2.5
outside of [SPF], the scope of this specification.

Note, however, that using a local, relative identifier like a flat
hostname, rather than a hierarchical and those definitions are included here by reference:

These result codes are used globally unique ADMD
identifier like a DNS domain name, makes configuration more difficult
for large sites. The hierarchical identifier permits aggregating
related, trusted systems together under a single, parent identifier,
which in turn permits assessing the context of this specification trust relationship with a single
reference. The alternative is a flat namespace requiring
individually listing each trusted system. Since consumers will use
the identifier to
reflect determine whether to use the result returned by contents of the component conducting SPF
evaluation.

Similarly, header
field:

o Changes to the results identifier impose a large, centralized
administrative burden.

o Ongoing administrative changes require constantly updating this
centralized table, making it difficult to ensure that an MUA or
downstream filter will have access to accurate information for Sender-ID are listed and described in
Section 4.2 of [SENDERID], which in turn uses
assessing the SPF definitions.

Note that both usability of those documents specify result codes that use mixed
case, but they are typically used all-lowercase in this context. the header field's content. In both cases, an additional result
particular, consumers of "policy" is defined, which
means the client was authorized header field will need to inject or relay mail on behalf of know not
only the sender's DNS domain according current identifier(s) in use but previous ones as well to
account for delivery latency or later re-assessment of the header
field's contents.

Examples of valid authentication method's
algorithm, but local policy dictates that identifiers are "example.com",
"mail.example.org", "ms1.newyork.example.com", and "example-auth".

2.5. Version Tokens

The grammar above provides for the result is unacceptable.
For example, "policy" might be used if SPF returns a "pass" result,
but a local policy check matches optional inclusion of versions on
both the header field itself (attached to the authserv-id token) and
on each of the sending DNS domain methods being reported. The method version refers to one found
the method itself, which is specified in an explicit list of unacceptable DNS domains (e.g., spammers).

If the retrieved sender policies used documents describing
those methods, while the authserv-id version refers to evaluate SPF this document
and Sender ID
do not contain explicit provisions for authenticating thus the local-part
(see Section 3.4.1 syntax of [MAIL]) this header field.

The purpose of an address, the "pvalue" reported
along with results for including these mechanisms SHOULD NOT include the local-
part.

2.5.3. "iprev"

The result values are used by is to avoid misinterpretation of the "iprev" method, defined
results. That is, if a parser finds a version after an authserv-id
that it does not explicitly know, it can immediately discontinue
trying to parse since what follows might not be in
Section 3, are as follows:

pass: The DNS evaluation succeeded, i.e., an expected
format. For a method version, the "reverse" and
"forward" lookup results were returned and were in agreement.

fail: The DNS evaluation failed. In particular, parser SHOULD ignore a method
result if the "reverse" and
"forward" lookups each produced results but they were version is not supported in
agreement, or case the "forward" query completed but produced no
result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an
RCODE semantics of 0 (NOERROR) in the
result have a reply containing no answers, was
returned.

temperror: The DNS evaluation could different meaning than what is expected. For example,
if a hypothetical DKIM version 2 yielded a "pass" result for
different reasons than version 1 does, a consumer of this field might
not be completed due want to some
error that is likely transient use the altered semantics. Allowing versions in nature, such as a temporary DNS
error, e.g., the
syntax is a DNS RCODE way to indicate this and let the consumer of the header
field decide.

2.6. Defined Methods and Result Values

Each individual authentication method returns one of 2, commonly known as SERVFAIL, or
other error condition resulted. A later attempt may produce a
final result.

permerror: set of
specific result values. The DNS evaluation could subsections below provide references to
the documents defining the authentication methods specifically
supported by this document, and their corresponding result values.
Verifiers SHOULD use these values as described below. New methods
not specified in this document, but intended to be completed because no PTR
data are published for supported by the connecting IP address, e.g.,
header field defined here, MUST include a DNS
RCODE of 3, commonly known as NXDOMAIN, similar result table either
in their defining documents or an RCODE of 0 (NOERROR) in a reply containing no answers, was returned. This prevented
completion of supplementary ones.

2.6.1. DKIM and DomainKeys

DKIM is represented by the evaluation. "dkim" method and is defined in [DKIM].
DomainKeys is defined in [DOMAINKEYS] and is represented by the
"domainkeys" method.

A later attempt signature is unlikely "acceptable to
produce a final result.

There is the ADMD" if it passes local policy
checks (or there are no "none" for this method since any TCP connection
delivering email has specific local policy checks). For example,
an IP address associated with it, so some kind
of evaluation will always ADMD policy might require that the signature(s) on the message be possible.

For discussion of
added using the format of DNS replies, see Domain Names -
Implementation And Specification ([DNS]).

2.5.4. SMTP AUTH

SMTP AUTH (defined domain present in [AUTH]) is represented by the "auth" method, From header field of the
message, thus making third-party signatures unacceptable even if they
verify.

Both DKIM and its DomainKeys use the same result values are set, as follows:

none: SMTP authentication The message was not attempted. signed.

pass: The SMTP client authenticated to message was signed, the server reporting signature or signatures were
acceptable to the
result using ADMD, and the protocol described in [AUTH]. signature(s) passed verification
tests.

fail: The SMTP client attempted to authenticate message was signed and the signature or signatures were
acceptable to the server using ADMD, but they failed the protocol described in [AUTH] verification test(s).

policy: The message was signed, but some aspect of the signature or
signatures was not successful, yet
continued acceptable to send the ADMD.

neutral: The message about which a was signed, but the signature or signatures
contained syntax errors or were not otherwise able to be
processed. This result is being
reported. also used for other failures not
covered elsewhere in this list.

temperror: The SMTP client attempted to authenticate using the
protocol described in [AUTH] but was message could not able to complete the
attempt be verified due to some error which that
is likely transient in nature, such as a temporary directory service lookup error. inability to
retrieve a public key. A later attempt may produce a final
result.

permerror: The SMTP client attempted to authenticate using the
protocol described in [AUTH] but was message could not able to complete the
attempt be verified due to some error that
is likely not transient in nature, unrecoverable, such as a permanent directory service lookup error. required header field being absent. A
later attempt is not likely unlikely to produce a final result.

An agent making use final result.

[DKIM] advises that if a message fails verification, it is to be
treated as an unsigned message. A report of the data provided by this header field SHOULD
consider "fail" and "temperror" here permits the
receiver of the report to be synonymous in terms decide how to handle the failure. A report
of message
authentication, i.e., "neutral" or "none" preempts that choice, ensuring the client did message
will be treated as if it had not authenticate in either case.

2.5.5. Other Registered Codes

Result codes were also registered in other RFCs for Vouch By
Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party
Signatures (in [ATPS], represented by "dkim-atps"), been signed.

2.6.2. SPF and Sender ID

SPF and Sender ID use the DKIM-
related Author Domain Signing Practices (in [ADSP], represented by
"dkim-adsp").

2.5.6. Extension Methods

Additional authentication "spf" and "sender-id" method identifiers (extension methods) may
be names,
respectively. The result values for SPF are defined in the future by later revisions or extensions to this
specification. These method identifiers Section 2.5
of [SPF], and those definitions are registered with the
Internet Assigned Numbers Authority (IANA) and, preferably, published
in an RFC. See included here by reference:

Extension methods can be defined for the following reasons:

These result codes are used in the context of
such identifiers ought this specification to
reflect the name of result returned by the method being
defined, but ought not be needlessly long.

2. To allow component conducting SPF
evaluation.

Similarly, the creation results for Sender ID are listed and described in
Section 4.2 of "sub-identifiers" [SENDERID], which in turn uses the SPF definitions.

Note that indicate
different levels both of authentication and differentiate between
their relative strengths, e.g., "auth1-weak" and "auth1-strong".

Authentication method implementers those documents specify result codes that use mixed
case, but they are encouraged to provide adequate
information, via message header field comments if necessary, to allow typically used all lowercase in this context.

In both cases, an MUA developer additional result of "policy" is defined, which
means the client was authorized to understand inject or relay ancillary details mail on behalf of
the sender's DNS domain according to the authentication results. method's
algorithm, but local policy dictates that the result is unacceptable.
For example, if it "policy" might be of interest to
relay what data was used to perform an evaluation, such information
could be relayed as if SPF returns a comment in "pass" result,
but a local policy check matches the header field, such as:

Authentication-Results: example.com;
foo=pass bar.baz=blob (2 of 3 tests OK)

Experimental method identifiers MUST only be used within ADMDs that
have explicitly consented sending DNS domain to use them. These method identifiers and
the parameters associated with them are not documented one found
in RFCs.
Therefore, they are subject an explicit list of unacceptable DNS domains (e.g., spammers).

If the retrieved sender policies used to change at any time evaluate SPF and Sender ID
do not suitable contain explicit provisions for production use. Any MTA, MUA, or downstream filter intended authenticating the local-part
(see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported
along with results for
production use these mechanisms SHOULD ignore or delete any Authentication-Results
header field that includes an experimental (unknown) method
identifier.

2.5.7. Extension Result Codes

Additional result codes (extension results) might be defined in NOT include the
future local-
part.

2.6.3. "iprev"

The result values used by later revisions or extensions to this specification.
Result codes MUST be registered with the Internet Assigned Numbers
Authority (IANA) and preferably published "iprev" method, defined in an RFC. See Section 6
for further details.

Extension results MUST only be used within ADMDs that have explicitly
consented to use them. These 3,
are as follows:

pass: The DNS evaluation succeeded, i.e., the "reverse" and
"forward" lookup results were returned and were in agreement.

fail: The DNS evaluation failed. In particular, the parameters associated
with them are not formally documented. Therefore, they are subject
to change at any time "reverse" and
"forward" lookups each produced results, but they were not suitable for production use. Any MTA,
MUA in
agreement, or downstream filter intended for production use SHOULD ignore the "forward" query completed but produced no
result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or
delete any Authentication-Results header field that includes an
extension result.

3. The "iprev" Authentication Method

This section defines an additional authentication method called
"iprev".

"iprev" is an attempt to verify that a client appears to be valid
based on some
RCODE of 0 (NOERROR) in a reply containing no answers, was
returned.

temperror: The DNS queries, which is evaluation could not be completed due to say some
error that the IP address is
explicitly associated with likely transient in nature, such as a domain name. Upon receiving temporary DNS
error, e.g., a session
initiation DNS RCODE of some kind from 2, commonly known as SERVFAIL, or
other error condition resulted. A later attempt may produce a client,
final result.

permerror: The DNS evaluation could not be completed because no PTR
data are published for the connecting IP address of the client
peer is queried for matching names (i.e., address, e.g., a number-to-name
translation, also DNS
RCODE of 3, commonly known as a "reverse lookup" NXDOMAIN, or a "PTR" record
query). Once that result is acquired, a lookup an RCODE of each 0 (NOERROR)
in a reply containing no answers, was returned. This prevented
completion of the names
(i.e., a name-to-number translation, or an "A" or "AAAA" record
query) thus retrieved evaluation. A later attempt is done. The response unlikely to
produce a final result.

There is no "none" for this second check method since any TCP connection
delivering email has an IP address associated with it, so some kind
of evaluation will typically result always be possible.

For discussion of the format of DNS replies, see "Domain Names -
Implementation and Specification" ([DNS]).

2.6.4. SMTP AUTH

SMTP AUTH (defined in at least one mapping back to [AUTH]) is represented by the client's IP
address.

Expressed "auth" method,
and its result values are as an algorithm: If follows:

none: SMTP authentication was not attempted.

pass: The SMTP client authenticated to the server reporting the client peer's IP address is I,
result using the
list of names protocol described in [AUTH].

fail: The SMTP client attempted to authenticate to which I maps (after a "PTR" query) is the set N, and server using
the union of IP addresses protocol described in [AUTH] but was not successful, yet
continued to send the message about which each member of N maps (after
corresponding "A" and "AAAA" queries) is L, then this test is
successful if I a result is an element of L. being
reported.

temperror: The response SMTP client attempted to a PTR query could contain multiple names. To prevent
heavy DNS loads, agents performing these queries MUST be implemented
such that authenticate using the number of names evaluated by generation of
corresponding A or AAAA queries is limited so as
protocol described in [AUTH] but was not unduly taxing able to complete the DNS infrastructure, though it MAY be configurable by an
administrator. As an example, Section 5.5 of [SPF] chose a limit of
10 for its implementation of this algorithm.

DNS Extensions
attempt due to Support IP Version 6 ([DNS-IP6]) discusses the
query formats for the IPv6 case.

There is some contention regarding the wisdom and reliability of this
test. For example, error that is likely transient in some regions it can be difficult for this test
ever nature, such
as a temporary directory service lookup error. A later attempt
may produce a final result.

permerror: The SMTP client attempted to pass because authenticate using the practice of arranging
protocol described in [AUTH] but was not able to match complete the forward
and reverse DNS
attempt due to some error that is infrequently observed. Therefore, the precise
implementation details of how a verifier performs an "iprev" test are likely not specified here. The verifier MAY report transient in nature,
such as a successful or failed
"iprev" test at its discretion having done some kind of check of the
validity of the connection's identity using DNS. It permanent directory service lookup error. A later
attempt is incumbent
upon an not likely to produce a final result.

An agent making use of the reported "iprev" result to understand
what exactly that particular verifier is attempting to report.

Extensive discussion of reverse DNS mapping data provided by this header field SHOULD
consider "fail" and its implications can "temperror" to be found synonymous in Considerations terms of message
authentication, i.e., the client did not authenticate in either case.

2.6.5. Other Registered Codes

Result codes were also registered in other RFCs for Vouch By
Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party
Signatures (in [ATPS], represented by "dkim-atps"), and the use of DNS Reverse Mapping
([DNSOP-REVERSE]). In particular, it recommends that applications
avoid using this test as a means of DKIM-
related Author Domain Signing Practices (in [ADSP], represented by
"dkim-adsp").

2.6.6. Extension Methods

Additional authentication or security. Its
presence method identifiers (extension methods) may
be defined in this document is not an endorsement, but is merely
acknowledgement that the future by later revisions or extensions to this
specification. These method remains common and provides identifiers are registered with the means
Internet Assigned Numbers Authority (IANA) and, preferably, published
in an RFC. See Section 6 for further details.

Extension methods can be defined for the following reasons:

1. To allow additional information from new authentication systems
to relay be communicated to MUAs or downstream filters. The names of
such identifiers ought to reflect the results name of that test.

4. Adding the Header Field to a Message

This specification makes no attempt to evaluate method being
defined but ought not be needlessly long.

2. To allow the relative
strengths creation of various message authentication methods "sub-identifiers" that may become
available. The methods listed are an order-independent set; their
sequence does not indicate relative strength or importance
different levels of one authentication and differentiate between
their relative strengths, e.g., "auth1-weak" and "auth1-strong".

Authentication method over another. Instead, the MUA or downstream filter consuming
this implementers are encouraged to provide adequate
information, via message header field is comments if necessary, to interpret the result allow
an MUA developer to understand or relay ancillary details of each method based on
its own knowledge
authentication results. For example, if it might be of interest to
relay what that method evaluates.

Each "method" MUST refer data was used to perform an authentication method declared evaluation, such information
could be relayed as a comment in the
IANA registry, or an extension header field, such as:

Authentication-Results: example.com;
foo=pass bar.baz=blob (2 of 3 tests OK)

Experimental method as described in Section 2.5.6,
and each "result" identifiers MUST refer only be used within ADMDs that
have explicitly consented to a use them. These method identifiers and
the parameters associated with them are not documented in RFCs.
Therefore, they are subject to change at any time and not suitable
for production use. Any MTA, MUA, or downstream filter intended for
production use SHOULD ignore or delete any Authentication-Results
header field that includes an experimental (unknown) method
identifier.

2.6.7. Extension Result Codes

Additional result code declared codes (extension results) might be defined in the IANA
registry,
future by later revisions or an extension result code as defined extensions to this specification.
Result codes MUST be registered with the Internet Assigned Numbers
Authority (IANA) and preferably published in Section 2.5.7. an RFC. See Section 6
for further information about the registered methods details.

Extension results MUST only be used within ADMDs that have explicitly
consented to use them. These results and result codes.

An MTA compliant the parameters associated
with this specification adds this them are not formally documented. Therefore, they are subject
to change at any time and not suitable for production use. Any MTA,
MUA, or downstream filter intended for production use SHOULD ignore
or delete any Authentication-Results header field
(after performing one or more message that includes an
extension result.

3. The "iprev" Authentication Method

This section defines an additional authentication tests) method called
"iprev".

"iprev" is an attempt to
indicate verify that a client appears to be valid
based on some DNS queries, which MTA or ADMD performed is to say that the test, which test got
applied, and what IP address is
explicitly associated with a domain name. Upon receiving a session
initiation of some kind from a client, the result was. If an MTA applies more than one
such test, it adds this header field either once per test, IP address of the client
peer is queried for matching names (i.e., a number-to-name
translation, also known as a "reverse lookup" or once
indicating all a "PTR" record
query). Once that result is acquired, a lookup of each of the results. An MTA MUST NOT add names
(i.e., a result to name-to-number translation, or an
existing header field.

An MTA MAY add this header field containing only the authentication
identifier portion and the "none" token (see Section 2.2) to indicate
explicitly that no message authentication schemes were applied prior "A" or "AAAA" record
query) thus retrieved is done. The response to delivery of this message.

An MTA adding this header field has to take steps second check
will typically result in at least one mapping back to identify it the client's IP
address.

Expressed as
legitimate to an algorithm: If the MUAs or downstream filters that will ultimately
consume its content. One process client peer's IP address is I, the
list of names to do so which I maps (after a "PTR" query) is described in Section 5.
Further measures may be necessary in some environments. Some
possible solutions are enumerated in Section 8.1. This document does
not mandate any specific solution the set N, and
the union of IP addresses to this issue as which each environment
has its own facilities member of N maps (after
corresponding "A" and limitations.

Most known message authentication methods focus on a particular
identifier "AAAA" queries) is L, then this test is
successful if I is an element of L.

The response to evaluate. SPF and Sender ID differ in that they can
yield a result based on more than one identifier; specifically, SPF
can evaluate the RFC5321.Helo parameter or the RFC5321.MailFrom
parameter, and Sender ID can evaluate PTR query could contain multiple names. To prevent
heavy DNS loads, agents performing these queries MUST be implemented
such that the RFC5321.MailFrom parameter number of names evaluated by generation of
corresponding A or AAAA queries is limited so as not to be unduly
taxing to the PRA identity. When generating DNS infrastructure, though it MAY be configurable by an
administrator. As an example, Section 5.5 of [SPF] chose a limit of
10 for its implementation of this field algorithm.

"DNS Extensions to report those
results, only Support IP Version 6" ([DNS-IP6]) discusses the parameter that yielded
query formats for the result IPv6 case.

There is included.

For MTAs that add some contention regarding the wisdom and reliability of this header field, adding header fields
test. For example, in order
(at some regions, it can be difficult for this
test ever to pass because the top), per Section 3.6 practice of [MAIL], arranging to match the
forward and reverse DNS is particularly important.
Moreover, this header field SHOULD be inserted above any other trace
header fields such MTAs might prepend. This placement allows easy
detection infrequently observed. Therefore, the
precise implementation details of header fields that can be trusted.

End users how a verifier performs an "iprev"
test are not specified here. The verifier MAY report a successful or
failed "iprev" test at its discretion having done some kind of check
of the validity of the connection's identity using DNS. It is
incumbent upon an agent making direct use of this header field might inadvertently
trust information that has not been properly vetted. If, for
example, a basic SPF the reported "iprev" result were to
understand what exactly that particular verifier is attempting to
report.

Extensive discussion of reverse DNS mapping and its implications can
be relayed that claims an
authenticated addr-spec, found in "Considerations for the local-part use of DNS Reverse Mapping"
([DNSOP-REVERSE]). In particular, it recommends that addr-spec has
actually applications
avoid using this test as a means of authentication or security. Its
presence in this document is not been authenticated. Thus, an MTA adding this header
field SHOULD NOT include any data endorsement but is merely
acknowledgement that has not been authenticated by the method(s) being applied. Moreover, MUAs SHOULD NOT render to
users such information if it is presented by a method known not remains common and provides the means
to
authenticate it.

4.1. relay the results of that test.

4. Adding the Header Field Position and Interpretation

In order to ensure non-ambiguous results and avoid a Message

This specification makes no attempt to evaluate the impact relative
strengths of
false header fields, MUAs and various message authentication methods that may become
available. The methods listed are an order-independent set; their
sequence does not indicate relative strength or importance of one
method over another. Instead, the MUA or downstream filters SHOULD NOT interpret filter consuming
this header field unless specifically configured is to do so by interpret the user
or administrator. That is, this interpretation should not be "on by
default". Naturally then, users result of each method based on
its own knowledge of what that method evaluates.

Each "method" MUST refer to an authentication method declared in the
IANA registry or administrators ought not activate
such an extension method as described in Section 2.6.6,
and each "result" MUST refer to a feature unless they are certain result code declared in the IANA
registry or an extension result code as defined in Section 2.6.7.
See Section 6 for further information about the registered methods
and result codes.

An MTA compliant with this specification adds this header field will be
validly added by an agent within the
(after performing one or more message authentication tests) to
indicate which MTA or ADMD that accepts the mail that
is ultimately read by performed the MUA, test, which test got
applied, and instances of what the result was. If an MTA applies more than one
such test, it adds this header field
appearing to originate within either once per test or once
indicating all of the ADMD but are actually added by
foreign MTAs will be removed before delivery.

Furthermore, MUAs and downstream filters SHOULD results. An MTA MUST NOT interpret add a result to an
existing header field.

An MTA MAY add this header field unless containing only the authentication service
identifier it bears
appears portion and the "none" token (see Section 2.2) to be one used within its own ADMD indicate
explicitly that no message authentication schemes were applied prior
to delivery of this message.

An MTA adding this header field has to take steps to identify it as configured by
legitimate to the user
or administrator. MUAs and or downstream filters MUST ignore that will ultimately
consume its content. One process to do so is described in Section 5.
Further measures may be necessary in some environments. Some
possible solutions are enumerated in Section 7.1. This document does
not mandate any result reported using specific solution to this issue as each environment
has its own facilities and limitations.

Most known message authentication methods focus on a
"result" not specified particular
identifier to evaluate. SPF and Sender ID differ in the that they can
yield a result code registry, based on more than one identifier; specifically, SPF
can evaluate the RFC5321.HELO parameter or a "ptype" not
listed in the corresponding registry for such values as defined in
Section 6. Moreover, such agents MUST ignore a result indicated for
any "method" they do not specifically support.

An MUA SHOULD NOT reveal these results to end users, absent careful
human factors design considerations RFC5321.MailFrom
parameter, and testing, for Sender ID can evaluate the presentation
of trust related materials. For example, an attacker could register
examp1e.com (note RFC5321.MailFrom parameter
or the digit "one") and send signed mail Purported Responsible Address (PRA) identity. When generating
this field to intended
victims; a verifier would detect that the signature was valid and report a "pass" even though it's clear those results, only the DNS domain name was
intended to mislead. See Section 8.2 for further discussion.

As stated parameter that yielded
the result is included.

For MTAs that add this header field, adding header fields in order
(at the top), per Section 2.1, 3.6 of [MAIL], is particularly important.
Moreover, this header field MUST SHOULD be treated as though
it were a inserted above any other trace
header field as defined in Section 3.6.7 fields such MTAs might prepend. This placement allows easy
detection of [MAIL],
and hence MUST NOT be reordered and MUST be prepended to the message,
so header fields that there is generally some indication upon delivery of where in
the chain can be trusted.

End users making direct use of handling MTAs the message authentication was done.

Note this header field might inadvertently
trust information that there are has not been properly vetted. If, for
example, a few message handlers basic SPF result were to be relayed that are only capable claims an
authenticated addr-spec, the local-part of
appending new header fields to a message. Strictly speaking, these
handlers are that addr-spec has
actually not compliant with been authenticated. Thus, an MTA adding this specification. They can still
add the header
field to carry authentication details, but SHOULD NOT include any signal
about where in the handling chain data that has not been authenticated by
the work was done may be lost.
Consumers method(s) being applied. Moreover, MUAs SHOULD be designed NOT render to
users such that this can be tolerated,
especially from information if it is presented by a producer method known not to have this limitation.
authenticate it.

4.1. Header Field Position and Interpretation

In order to ensure non-ambiguous results and avoid the impact of
false header fields, MUAs and downstream filters SHOULD ignore instances of NOT interpret
this header field discovered within
message/rfc822 MIME attachments.

Further discussion of these topics can unless specifically configured to do so by the user
or administrator. That is, this interpretation should not be found in Section 8 below.

4.2. Local Policy Enforcement

Some sites have a local policy that considers any particular
authentication policy's non-recoverable failure results (typically
"fail" "on by
default". Naturally then, users or similar) as justification for rejecting the message. In administrators ought not activate
such cases, a feature unless they are certain the border MTA SHOULD issue header field will be
validly added by an SMTP rejection response agent within the ADMD that accepts the mail that
is ultimately read by the MUA, and instances of the header field
appearing to originate within the message, rather than adding ADMD but are actually added by
foreign MTAs will be removed before delivery.

Furthermore, MUAs and downstream filters SHOULD NOT interpret this
header field and allowing unless the
message authentication service identifier it bears
appears to proceed toward delivery. This is more desirable than
allowing be one used within its own ADMD as configured by the message to reach an internal host's MTA user
or spam filter,
thus possibly generating administrator.

MUAs and downstream filters MUST ignore any result reported using a local rejection
"result" not specified in the IANA "Result Code" registry or a
"ptype" not listed in the corresponding registry for such values as
defined in Section 6. Moreover, such agents MUST ignore a [DSN] result
indicated for any "method" they do not specifically support.

An MUA SHOULD NOT reveal these results to a
forged originator. Such generated rejections are colloquially known
as "backscatter".

The same MAY also be done end users, absent careful
human factors design considerations and testing, for local policy decisions overriding the
results presentation
of trust-related materials. For example, an attacker could register
examp1e.com (note the authentication methods (e.g., the "policy" result
codes described in Section 2.5).

Such rejections at the SMTP protocol level are not possible if local
policy is enforced at digit "one") and send signed mail to intended
victims; a verifier would detect that the MUA signature was valid and not
report a "pass" even though it's clear the MTA.

5. Removing Existing Header Fields

For security reasons, any MTA conforming DNS domain name was
intended to mislead. See Section 7.2 for further discussion.

As stated in Section 2.1, this specification MUST
delete any discovered instance of this header field MUST be treated as though
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,
so that claims, by
virtue there is generally some indication upon delivery of its where in
the chain of handling MTAs the message authentication service identifier, to have been added
within its trust boundary but was done.

Note that did not come directly from another
trusted MTA. For example, an MTA for example.com receiving there are a few message
MUST delete or otherwise obscure any instance handlers that are only capable of this header field
bearing an authentication service identifier indicating the header
field was added within example.com prior to adding its own
appending new header
fields. This could mean each MTA will have fields to be equipped with a
list of internal MTAs known to be message. Strictly speaking, these
handlers are not compliant (and hence trustworthy).

For simplicity and maximum security, a border MTA could remove all
instances of with this specification. They can still
add the header field on mail crossing into its trust
boundary. However, this may conflict with the desire to access carry authentication results performed by trusted external service
providers. It details, but any signal
about where in the handling chain the work was done may also invalidate signed messages whose signatures
cover external instances of be lost.
Consumers SHOULD be designed such that this header field. A more robust border
MTA could allow can be tolerated,
especially from a specific list of authenticating MTAs whose
information is producer known to be admitted, removing the have this limitation.

MUAs SHOULD ignore instances of this header field originating
from all others.

As stated discovered within
message/rfc822 MIME attachments.

Further discussion of these topics can be found in Section 1.2, 7 below.

4.2. Local Policy Enforcement

Some sites have a formal definition of "trust boundary" is
deliberately not made here. It is entirely possible local policy that a border
MTA for example.com will explicitly trust considers any particular
authentication policy's non-recoverable failure results
asserted by upstream host example.net even though they exist in
completely disjoint administrative boundaries. (typically
"fail" or similar) as justification for rejecting the message. In that case,
such cases, the border MTA MAY elect not SHOULD issue an SMTP rejection response to delete those results; moreover,
the
upstream host doing some authentication work could apply a signing
technology such as [DKIM] on its own results to assure downstream
hosts of their authenticity. An example of message, rather than adding this is provided in
Appendix C.

Similarly, in header field and allowing the case of messages signed using [DKIM] or other
message signing methods that sign header fields, this removal action
could invalidate one or to proceed toward delivery. This is more signatures on desirable than
allowing the message if they
covered the header field to reach an internal host's MTA or spam filter,
thus possibly generating a local rejection such as a Delivery Status
Notification (DSN) [DSN] to a forged originator. Such generated
rejections are colloquially known as "backscatter".

The same MAY also be removed. This behavior can be
desirable since there's little value done for local policy decisions overriding the
results of the authentication methods (e.g., the "policy" result
codes described in validating Section 2.6).

Such rejections at the signature on a
message with forged header fields. However, signing agents MAY
therefore elect to omit these header fields from signing SMTP protocol level are not possible if local
policy is enforced at the MUA and not the MTA.

5. Removing Existing Header Fields

For security reasons, any MTA conforming to avoid this situation.

An MTA SHOULD remove specification MUST
delete any discovered instance of this header field bearing a
version (express or implied) that it does not support. However, an
MTA MUST remove such a header field if the [SMTP] connection relaying
the message is claims, by
virtue of its authentication service identifier, to have been added
within its trust boundary but that did not come directly from a another
trusted internal MTA. This means the For example, an MTA
needs to be able to understand versions for example.com receiving a message
MUST delete or otherwise obscure any instance of this header field at last
as late as the ones understood by the MUAs or other consumers within
its ADMD.

6. IANA Considerations

IANA has registered
bearing an authentication service identifier indicating that the defined
header field and created two tables
as described below. These registry actions were originally defined
by [RFC5451] and are repeated here to provide a single, current
reference.

6.1. The Authentication-Results Header Field

[RFC5451] was added the "Authentication-Results" header field to the IANA
Permanent Message Header Field Registry, per the procedure found in
[IANA-HEADERS]. That entry is to be updated within example.com prior to reference this
document. The following is the registration template:

Header field name: Authentication-Results
Applicable protocol: mail ([MAIL])
Status: Standard
Author/Change controller: IETF
Specification document(s): [this memo]
Related information:
Requesting review adding its own
header fields. This could mean each MTA will have to be equipped
with a list of any proposed changes and additions internal MTAs known to be compliant (and hence
trustworthy).

For simplicity and maximum security, a border MTA could remove all
instances of this header field is recommended.

6.2. Email Authentication Method Name Registry

Names of message on mail crossing into its trust
boundary. However, this may conflict with the desire to access
authentication methods supported results performed by trusted external service
providers. It may also invalidate signed messages whose signatures
cover external instances of this
specification are header field. A more robust border
MTA could allow a specific list of authenticating MTAs whose
information is to be registered with IANA, with admitted, removing the exception of
experimental names as described header field originating
from all others.

As stated in Section 2.5.6. A registry was
created by [RFC5451] for this purpose. This document changes the
rules governing that registry.

New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The Designated Expert shall be
appointed by the IESG. The Designated Expert has discretion to
request that a publication be referenced if 1.2, a clear, concise formal definition of the "trust boundary" is
deliberately not made here. It is entirely possible that a border
MTA for example.com will explicitly trust authentication method cannot be provided such results
asserted by upstream host example.net even though they exist in
completely disjoint administrative boundaries. In that
interoperability is assured. Registrations should otherwise be
permitted. The Designated Expert can also handle requests case, the
border MTA MAY elect not to mark
any current registration as "deprecated".

Each method must register a name, delete those results; moreover, the specification that defines it,
upstream host doing some authentication work could apply a version number associated with signing
technology such as [DKIM] on its own results to assure downstream
hosts of their authenticity. An example of this is provided in
Appendix C.

Similarly, in the method being registered
(preferably starting at "1"), and zero case of messages signed using [DKIM] or more "ptype" values
appropriate for use with that method, which "property" value(s)
should be reported by other
message-signing methods that method, and a description of sign header fields, this removal action
could invalidate one or more signatures on the "value" message if they
covered the header field to be used with each.

All existing registry entries that reference [RFC5451] are to removed. This behavior can be
updated to reference this document. [RFC Editor note: Section
numbers may have to change as well
desirable since they appear there's little value in validating the registry,
but numbering may change between now and publication. We can deal signature on a
message with forged header fields. However, signing agents MAY
therefore elect to omit these header fields from signing to avoid
this situation.

An MTA SHOULD remove any instance of this during header field bearing a
version (express or implied) that it does not support. However, an
MTA MUST remove such a header field if the IANA phase and/or AUTH48.].

IANA [SMTP] connection relaying
the message is also requested to add not from a "version" field to all existing
registry entries. All current methods are trusted internal MTA. This means the MTA
needs to be recorded as version
"1".

6.3. Email Authentication Result Names Registry

Names able to understand versions of message authentication result codes supported this header field at least
as late as the ones understood by this
specification must be the MUAs or other consumers within
its ADMD.

6. IANA Considerations

IANA has registered with IANA, with the exception of
experimental codes defined header field and created two tables
as described in Section 2.5.7. A below. These registry was
created actions were originally defined
by [RFC5451] for this purpose. This document changes the
rules governing that registry.

New entries and are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The Designated Expert shall be
appointed by the IESG. The Designated Expert has discretion repeated here to
request that a publication be referenced if provide a clear, concise
definition of the authentication result cannot be provided such that
interoperability is assured. Registrations should otherwise be
permitted. The Designated Expert can also handle requests to mark
any single, current registration as "deprecated".

All existing registry entries that reference
reference.

6.1. The Authentication-Results Header Field

[RFC5451] are to be
updated added the Authentication-Results header field to reference this document.

The definitions for the SPF and Sender ID authentication methods are
updated using IANA
"Permanent Message Header Field Names" registry, per the references procedure
found in Section 2.5.2.

7. Implementation Status

[RFC Editor: Please delete this section prior [IANA-HEADERS]. That entry has been updated to publication.]

This section records the status of known implementations of the
protocol defined by this specification at the time of posting of reference
this
Internet-Draft, and is based on a proposal described in
draft-sheffer-running-code. document. The description of implementations in
this section following is intended to assist the registration template:

Header field name: Authentication-Results
Applicable protocol: mail ([MAIL])
Status: Standard
Author/Change controller: IETF in its decision processes
in progressing drafts to RFCs. Please note that the listing
Specification document(s): RFC 7001
Related information:
Requesting review of any
individual implementation here does not imply endorsement proposed changes and additions to
this field is recommended.

6.2. "Email Authentication Methods" Registry

Names of message authentication methods supported by the
IETF. Furthermore, no effort has been spent this
specification are to verify be registered with IANA, with the
information presented here that exception of
experimental names as described in Section 2.6.6. A registry was supplied
created by IETF contributors. [RFC5451] for this purpose. This is not intended as, and must not be construed to be, a catalog
of available implementations or their features. Readers are advised
to note document changes the
rules governing that other implementations may exist.

According to draft-sheffer-running-code, "this will allow reviewers
and working groups to assign due consideration to documents registry.

New entries are assigned only for values that have
the benefit of running code, received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by considering the running code as
evidence of valuable experimentation and feedback that IESG. The designated expert has made the
implemented protocols more mature. It is up to the individual
working groups to use this information as they see fit".

7.1. Google Mail

Responsible Organization: Google

Implementation: Gmail; http://mail.google.com

Brief Description: Gmail is a popular, free, web-based mailbox
service provider

Maturity level: widely used

Coverage: all syntax required to report SPF and DKIM results

Licensing: proprietary

Implementation Experience: n/a

Contact Information: http://mail.google.com

7.2. Yahoo! Mail

Responsible Organization: Yahoo!, Inc.

Implementation: Yahoo! Mail; http://mail.yahoo.com

Brief Description: Yahoo! Mail is a popular, free, web-based
mailbox service provider

Maturity level: widely used

Coverage: all syntax required to report SPF and DKIM results

Licensing: proprietary
Implementation Experience: n/a

Contact Information: http://mail.yahoo.com

7.3. Hotmail

Responsible Organization: Microsoft Corp.

Implementation: Hotmail; http://www.hotmail.com

Brief Description: Hotmail is a popular, free, web-based mailbox
service provider

Maturity level: widely used

Coverage: all syntax required to report SPF and DKIM results

Licensing: proprietary

Implementation Experience: n/a

Contact Information: http://www.hotmail.com

7.4. Courier MTA

Responsible Organization: Double Precision, Inc.

Implementation: Courier MTA; http://www.courier-mta.org

Brief Description: Courier MTA is an open source mail server

Maturity level: production

Coverage: all syntax required to report SPF and DKIM results

Licensing: GPL

Implementation Experience: n/a

Contact Information: http://www.courier-mta.org

7.5. sid-milter

Responsible Organization: Sendmail, Inc.

Implementation: sid-milter;
http://sourceforge.net/projects/sid-milter

Brief Description: sid-milter is an open source MTA plugin that
implements both Sender ID and SPF

Maturity level: production (no longer maintained)

Coverage: all syntax required discretion to report SPF and Sender ID results

Licensing: Sendmail Open Source License

Implementation Experience: n/a

Contact Information: http://www.sendmail.com

7.6. opendkim

Responsible Organization: The Trusted Domain Project

Implementation: opendkim; http://www.opendkim.org

Brief Description: opendkim includes a library
request that implements DKIM
and an MTA plugin a publication be referenced if a clear, concise
definition of the authentication method cannot be provided such that uses this library to provide DKIM and
related services

Maturity level: widely used (Facebook, AOL, etc.)

Coverage: all syntax required to report DKIM, VBR, and some
extension results

Licensing: BSD two-clause license

Implementation Experience: n/a

Contact Information: http://www.trusteddomain.org

7.7. opendmarc

Responsible Organization:
interoperability is assured. Registrations should otherwise be
permitted. The Trusted Domain Project

Implementation: opendmarc; http://www.trusteddomain.org/opendmarc

Brief Description: opendmarc includes designated expert can also handle requests to mark
any current registration as "deprecated".

Each method must register a name, the specification that defines it,
a library version number associated with the method being registered
(preferably starting at "1"), zero or more "ptype" values appropriate
for use with that method, which "property" value(s) should be
reported by that implements
DMARC method, and an MTA plugin a description of the "value" to be used
with each.

All existing registry entries that uses reference [RFC5451] have been
updated to reference this library document, except where entries have already
been deprecated.

IANA has also added a "version" field to provide DMARC
and related services
Maturity level: production

Coverage: all syntax required existing registry
entries. All current methods are recorded as version "1".

6.3. "Email Authentication Result Names" Registry

Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.6.7. A registry was
created by [RFC5451] for this purpose. This document changes the
rules governing that registry.

New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to report DMARC results

Licensing: BSD two-clause license

Implementation Experience: n/a

Contact Information: http://www.trusteddomain.org

7.8. authres

Responsible Organization: Julian Mehnle, Scott Kitterman

Implementation: authres; https://pypi.python.org/pypi/authres/

Brief Description: authres is
request that a python module for parsing and
generating publication be referenced if a clear, concise
definition of the Authentication-Results header field. It authentication result cannot be provided such that
interoperability is used by
"pyspf", a python module assured. Registrations should otherwise be
permitted. The designated expert can also handle requests to mark
any current registration as "deprecated".

All existing registry entries that implements reference [RFC5451] have been
updated to reference this document.

The definitions for the SPF validation
service.

Maturity level: production

Coverage: supports all aspects of and Sender ID authentication methods are
updated using the protocol, including one new
method that is pending publication

Licensing: Apache 2.0 License

Implementation Experience: n/a

Contact Information:
https://launchpad.net/authentication-results-python

8. references found in Section 2.6.2.

7. Security Considerations

The following security considerations apply when adding or processing
the "Authentication-Results" Authentication-Results header field:

8.1.

7.1. Forged Header Fields

An MUA or filter that accesses a mailbox whose messages are handled
by a non-conformant MTA, and understands Authentication-Results
header fields, could potentially make false conclusions based on
forged header fields. A malicious user or agent could forge a header
field using the DNS domain of a receiving ADMD as the authserv-id
token in the value of the header field, and field and, with the rest of the
value
value, claim that the message was properly authenticated. The non-
conformant MTA would fail to strip the forged header field, and the
MUA could inappropriately trust it.

For this reason, it is best not to have processing of the
"Authentication-Results"
Authentication-Results header field enabled by default; instead instead, it
should be ignored, at least for the purposes of enacting filtering
decisions, unless specifically enabled by the user or administrator
after verifying that the border MTA is compliant. It is acceptable
to have an MUA aware of this specification, specification but have an explicit list
of hostnames whose "Authentication-Results" Authentication-Results header fields are
trustworthy; however, this list should initially be empty.

Proposed alternative solutions to this problem were made some time
ago,
ago and are listed below. To date, they have not been developed due
to lack of demand, demand but are documented here should the information be
useful at some point in the future:

1. Possibly the simplest is a digital signature protecting the
header field, such as using [DKIM], that can be verified by an
MUA by using a posted public key. Although one of the main
purposes of this document is to relieve the burden of doing
message authentication work at the MUA, this only requires that
the MUA learn a single authentication scheme even if a number of
them are in use at the border MTA. Note that [DKIM] requires
that the From header field be signed, although in this
application, the signing agent (a trusted MTA) likely cannot
authenticate that value, so the fact that it is signed should be
ignored. Where the authserv-id is the ADMD's domain name, the
authserv-id matching this valid internal signature's "d=" DKIM
value is sufficient.

2. Another would be a means to interrogate the MTA that added the
header field to see if it is actually providing any message
authentication services and saw the message in question, but this
isn't especially palatable given the work required to craft and
implement such a scheme.

3. Yet another might be a method to interrogate the internal MTAs
that apparently handled the message (based on Received: Received header
fields) to determine whether any of them conform to Section 5 of
this memo. This, too, has potentially high barriers to entry.

4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to
allow an MUA or filtering agent to acquire the "authserv-id" authserv-id in use
within an ADMD, thus allowing it to identify which
Authentication-Results header fields it can trust.

5. On the presumption that internal MTAs are fully compliant with
Section 3.6 of [MAIL], [MAIL] and the compliant internal MTAs are using
their own host names hostnames or the ADMD's DNS domain name as the
"authserv-id"
authserv-id token, the header field proposed here should always
appear above a Received: Received header added by a trusted MTA. This can
be used as a test for header field validity.

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
verify that the host that appears to have added the header field (a)
actually did so, so and (b) is legitimately adding that header field for
this delivery. Given the variety of messaging environments deployed
today, consensus appears to be that specifying a particular mechanism
for doing so is not appropriate for this document.

Mitigation of the forged header field attack can also be accomplished
by moving the authentication results data into meta-data metadata associated
with the message. In particular, an [SMTP] extension could be
established that is used to communicate authentication results from the border MTA
to intermediate and delivery MTAs; the latter of these could arrange
to store the authentication results as meta-data metadata retrieved and
rendered along with the message by an [IMAP] client aware of a
similar extension in that protocol. The delivery MTA would be told
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
is no vector in such an arrangement for forgery of authentication
data by an outside agent.

8.2.

7.2. Misleading Results

Until some form of service for querying the reputation of a sending
agent is widely deployed, the existence of this header field
indicating a "pass" does not render the message trustworthy. It is
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
of spam signed using [DKIM] by the originator of the spam, which
might be a spammer or a compromised system). In particular, this
issue is not resolved by forged header field removal discussed above.

Hence, MUAs and downstream filters must take some care with use of
this header even after possibly malicious headers are scrubbed.

8.3.

7.3. Header Field Position

Despite the requirements of [MAIL], header fields can sometimes be
reordered enroute en route by intermediate MTAs. The goal of requiring
header field addition only at the top of a message is an
acknowledgement that some MTAs do reorder header fields, but most do
not. Thus, in the general case, there will be some indication of
which MTAs (if any) handled the message after the addition of the
header field defined here.

8.4.

7.4. Reverse IP Query Denial-of-Service Attacks

Section 5.5 of [SPF] describes a DNS-based denial-of-service attack
for verifiers that attempt DNS-based identity verification of
arriving client connections. A verifier wishing to do this check and
report this information needs to take care not to go to unbounded
lengths to resolve "A" and "PTR" queries. MUAs or other filters
making use of an "iprev" result specified by this document need to be
aware of the algorithm used by the verifier reporting the result and,
especially, its limitations.

8.5.

7.5. Mitigation of Backscatter

Failing to follow the instructions of Section 4.2 can result in a
denial-of-service attack caused by the generation of [DSN] messages
(or equivalent) to addresses that did not send the messages being
rejected.

8.6.

7.6. Internal MTA Lists

Section 5 describes a procedure for scrubbing header fields that may
contain forged authentication results about a message. A compliant
installation will have to include, at each MTA, a list of other MTAs
known to be compliant and trustworthy. Failing to keep this list
current as internal infrastructure changes may expose an ADMD to
attack.

8.7.

7.7. Attacks against Authentication Methods

If an attack becomes known against an authentication method, clearly
then the agent verifying that method can be fooled into thinking an
inauthentic message is authentic, and thus the value of this header
field can be misleading. It follows that any attack against the
authentication methods supported by this document is also a security
consideration here.

8.8.

7.8. Intentionally Malformed Header Fields

It is possible for an attacker to add an Authentication-Results
header field that is extraordinarily large or otherwise malformed in
an attempt to discover or exploit weaknesses in header field parsing
code. Implementers must thoroughly verify all such header fields
received from MTAs and be robust against intentionally as well as
unintentionally malformed header fields.

8.9.

7.9. Compromised Internal Hosts

An internal MUA or MTA that has been compromised could generate mail
with a forged From header field and a forged Authentication-Results
header field that endorses it. Although it is clearly a larger
concern to have compromised internal machines than it is to prove the
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
been added by a trusted border MTA (as described above), yet the
[SMTP] connection is not coming from an internal machine known to be
running an authorized MTA. However, in such a configuration,
legitimate MTAs will have to add this header field when legitimate
internal-only messages are generated. This is also covered in
Section 5.

8.10.

7.10. Encapsulated Instances

MIME messages can contain attachments of type "message/rfc822", which
contain other messages. Such an encapsulated message can also
contain an Authentication-Results header field. Although the
processing of these is outside of the intended scope of this document
(see Section 1.3), some early guidance to MUA developers is
appropriate here.

Since MTAs are unlikely to strip Authentication-Results header fields
after mailbox delivery, MUAs are advised in Section 4.1 to ignore
such instances within MIME attachments. Moreover, when extracting a
message digest to separate mail store messages or other media, such
header fields should be removed so that they will never be
interpreted improperly by MUAs that might later consume them.

8.11.

7.11. Reverse Mapping

Although Section 3 of this memo includes explicit support for the
"iprev" method, its value as an authentication mechanism is limited.
Implementers of both this proposal and agents that use the data it
relays are encouraged to become familiar with the issues raised by
[DNSOP-REVERSE] when deciding whether or not to include support for
"iprev".

8. References

9.1.

8.1. Normative References

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[IANA-HEADERS]
Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.

[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.

9.2.

8.2. Informative References

[ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009.

[AR-VBR] Kucherawy, M., "Authentication-Results Registration for
Vouch by Reference Results", RFC 6212, April 2011.

[ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM)
Authorized Third-Party Signatures", RFC 6541,
February 2012.

[AUTH] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.

[DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
September 2011.

[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.

[DNSOP-REVERSE]
Senie, D. and A. Sullivan, "Considerations for the use of
DNS Reverse Mapping", Work in Progress, March 2008.

[DOMAINKEYS]
Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.

[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.

[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598, October 2008.
July 2009.

[IANA-CONSIDERATIONS]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.

[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.

[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.

[SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.

[SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.

[SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.

[VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, April 2009.

Appendix A. Acknowledgements

The author wishes to acknowledge the following individuals for their
review and constructive criticism of this update: document: Dave Cridland,
Dave Crocker, Bjoern Hoehrmann, Scott Kitterman, John Levine, Alexey
Melnikov, S. Moonesamy, and Alessandro Vesely.

Appendix B. Legacy MUAs

Implementers of this protocol should be aware that many MUAs are
unlikely to be retrofitted to support the new header field and its
semantics. In the interests of convenience and quicker adoption, a
delivery MTA might want to consider adding things that are processed
by existing MUAs in addition to the Authentication-Results header
field. One suggestion is to include a Priority header field, on
messages that don't already have such a header field, containing a
value that reflects the strength of the authentication that was
accomplished, e.g., "low" for weak or no authentication, "normal" or
"high" for good or strong authentication.

Some modern MUAs can already filter based on the content of this
header field. However, there is keen interest in having MUAs make
some kind of graphical representation of this header field's meaning
to end users. Until this capability is added, other interim means of
conveying authentication results may be necessary while this proposal
and its successors are adopted.

Appendix C. Authentication-Results Examples

This section presents some examples of the use of this header field
to indicate authentication results.

C.1. Trivial Case; Header Field Not Present

The trivial case:

Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1])
by server.example.org (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Message-Id: <12345.abc@example.com>
Subject: here's a sample

Hello! Goodbye!

Example 1: Trivial case Case

The "Authentication-Results" Authentication-Results header field is completely absent. The
MUA may make no conclusion about the validity of the message. This
could be the case because the message authentication services were
not available at the time of delivery, or no service is provided, or
the MTA is not in compliance with this specification.

C.2. Nearly Trivial Case; Service Provided, But but No Authentication Done

A message that was delivered by an MTA that conforms to this
specification but provides no actual message authentication service:

Authentication-Results: example.org 1; none
Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1])
by server.example.org (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.org
Message-Id: <12345.abc@example.com>
Subject: here's a sample

Hello! Goodbye!

Example 2: Header present Present but no authentication done No Authentication Done
The "Authentication-Results" Authentication-Results header field is present, showing that the
delivering MTA conforms to this specification. It used its DNS
domain name as the authserv-id. The presence of "none" (and the
absence of any method and result tokens) indicates that no message
authentication was done. The version number of the specification to
which the field's content conforms is explicitly provided.

C.3. Service Provided, Authentication Done

A message that was delivered by an MTA that conforms to this
specification and applied some message authentication:

Authentication-Results: example.com;
spf=pass smtp.mailfrom=example.net
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.net
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.com
Message-Id: <12345.abc@example.net>
Subject: here's a sample

Hello! Goodbye!

Example 3: Header reporting results Reporting Results

The "Authentication-Results" Authentication-Results header field is present, indicating that
the border MTA conforms to this specification. The authserv-id is
once again the DNS domain name. Furthermore, the message was
authenticated by that MTA via the method specified in [SPF]. Note
that since that method cannot authenticate the local-part, it has
been omitted from the result's value. The MUA could extract and
relay this extra information if desired.

C.4. Service Provided, Several Authentications Done, Single MTA

A message that was relayed inbound via a single MTA that conforms to
this specification and applied three different message authentication
checks:

Authentication-Results: example.com;
auth=pass (cram-md5) smtp.auth=sender@example.net;
spf=pass smtp.mailfrom=example.net
Authentication-Results: example.com;
sender-id=pass header.from=example.net
Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6)
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.com
From: sender@example.net
Message-Id: <12345.abc@example.net>
Subject: here's a sample

Hello! Goodbye!

Example 4: Headers reporting results Reporting Results from one One MTA

The "Authentication-Results" Authentication-Results header field is present, indicating that
the delivering MTA conforms to this specification. Once again, the
receiving DNS domain name is used as the authserv-id. Furthermore,
the sender authenticated herself/himself to the MTA via a method
specified in [AUTH], and both SPF and Sender ID checks were done and
passed. The MUA could extract and relay this extra information if
desired.

Two "Authentication-Results" Authentication-Results header fields are not required since the
same host did all of the checking. The authenticating agent could
have consolidated all the results into one header field.

This example illustrates a scenario in which a remote user on a
dialup connection (example.net) sends mail to a border MTA
(example.com) using SMTP authentication to prove identity. The
dialup provider has been explicitly authorized to relay mail as
"example.com"
example.com resulting in passes by the SPF and SenderID Sender ID checks.

C.5. Service Provided, Several Authentications Done, Different MTAs

A message that was relayed inbound by two different MTAs that conform
to this specification and applied multiple message authentication
checks:

Authentication-Results: example.com;
sender-id=fail header.from=example.com;
dkim=pass (good signature) header.d=example.com
Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1])
by auth-checker.example.com (8.11.6/8.11.6)
with ESMTP id i7PK0sH7021929;
Fri, Feb 15 2002 17:19:22 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com;
t=1188964191; c=simple/simple; h=From:Date:To:Subject:
Message-Id:Authentication-Results;
bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
Authentication-Results: example.com;
auth=pass (cram-md5) smtp.auth=sender@example.com;
spf=fail smtp.mailfrom=example.com
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.com
Message-Id: <12345.abc@example.com>
Subject: here's a sample

Hello! Goodbye!

Example 5: Headers reporting results Reporting Results from multiple Multiple MTAs

The "Authentication-Results" Authentication-Results header field is present, indicating
conformance to this specification. Once again, the authserv-id used
is the recipient's DNS domain name. The header field is present
twice because two different MTAs in the chain of delivery did
authentication tests. The first, "mail-router.example.com" first MTA, mail-router.example.com,
reports that SMTP AUTH and SPF were both used, used and that the former
passed while the latter failed. In the SMTP AUTH case, additional
information is provided in the comment field, which the MUA can
choose to render if desired.

The second MTA, "auth-checker.example.com", auth-checker.example.com, reports that it did a
Sender ID test (which failed) and a DKIM test (which passed). Again,
additional data about one of the tests is 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 example.com that assured the
integrity of the lower Authentication-Results field.

Since different hosts did the two sets of authentication checks, the
header fields cannot be consolidated in this example.

This example illustrates more typical transmission of mail into
"example.com"
example.com from a user on a dialup connection "example.net". example.net. The user
appears to be legitimate as he/she had a valid password allowing
authentication at the border MTA using SMTP AUTH. The SPF and Sender
ID tests failed since "example.com" example.com has not granted "example.net" example.net
authority to relay mail on its behalf. However, the DKIM test passed
because the sending user had a private key matching one of
"example.com"'s
example.com's published public keys and used it to sign the message.

C.6. Service Provided, Multi-Tiered Authentication Done

A message that had authentication done at various stages, one of
which was outside the receiving ADMD:

Authentication-Results: example.com;
dkim=pass reason="good signature"
header.i=@mail-router.example.net;
dkim=fail reason="bad signature"
header.i=@newyork.example.com
Received: from mail-router.example.net
(mail-router.example.net [192.0.2.250])
by chicago.example.com (8.11.6/8.11.6)
for <recipient@chicago.example.com>
with ESMTP id i7PK0sH7021929;
Fri, Feb 15 2002 17:19:22 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=furble;
d=mail-router.example.net; t=1188964198; c=relaxed/simple;
h=From:Date:To:Message-Id:Subject:Authentication-Results;
bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=;
b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
Authentication-Results: example.net;
dkim=pass (good signature) header.i=@newyork.example.com
Received: from smtp.newyork.example.com
(smtp.newyork.example.com [192.0.2.220])
by mail-router.example.net (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby;
d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample

Example 6: Headers reporting results Reporting Results from multiple Multiple MTAs in different Different
ADMDs

In this example example, we see multi-tiered authentication with an extended
trust boundary.

The message was sent from someone at example.com's New York office
(newyork.example.com) to a mailing list managed at an intermediary.
The message was signed at the origin using DKIM.

The message was sent to a mailing list service provider called
example.net, which is used by example.com. There,
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
the trust boundary for chicago.example.com includes the mailing list
server at example.net.

The mailing list server there first authenticated the message and
affixed an Authentication-Results header field indicating such using
its DNS domain name for the authserv-id. It then altered the message
by affixing some footer text to the body, including some
administrivia such as unsubscription instructions. Finally, the
mailing list server affixes a second DKIM signature and begins
distribution of the message.

The border MTA for chicago.example.com explicitly trusts results from
mail-router.example.net
mail-router.example.net, so that header field is not removed. It
performs evaluation of both signatures and determines that the first
(most recent) is a "pass" but, because of the aforementioned
modifications, the second is a "fail". However, the first signature
included the Authentication-Results header added at mail-
router.example.net that validated the second signature. Thus,
indirectly, it can be determined that the authentications claimed by
both signatures are indeed valid.

Note that two styles of presenting meta-data metadata about the result are in
use here. In one case, the "reason=" clause is present present, which is
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
comment. The latter can be harder for parsers to extract given the
varied supported syntaxes of mail header fields.

C.7. Comment-Heavy Example

The formal syntax permits comments within the content in a number of
places. For the sake of illustration, this example is also legal:

Authentication-Results: foo.example.net (foobar) 1 (baz);
dkim (Because I like it) / 1 (One yay) = (wait for it) fail
policy (A dot can go here) . (like that) expired
(this surprised me) = (as I wasn't expecting it) 1362471462

Example 7: A very comment-heavy Very Comment-Heavy but perfectly legal example Perfectly Legal Example

Appendix D. Operational Considerations about Message Authentication

This protocol is predicated on the idea that authentication (and
presumably in the future, reputation) work is typically done by
border MTAs rather than MUAs or intermediate MTAs; the latter merely
make use of the results determined by the former. Certainly this is
not mandatory for participation in electronic mail or message
authentication, but this protocol and its deployment to date are
based on that model. The assumption satisfies several common ADMD
requirements:

1. Service operators prefer to resolve the handling of problem
messages as close to the border of the ADMD as possible. This
enables, for example, rejection of messages at the SMTP level
rather than generating a DSN internally. Thus, doing any of the
authentication or reputation work exclusively at the MUA or
intermediate MTA renders this desire unattainable.

2. Border MTAs are more likely to have direct access to external
sources of authentication or reputation information since modern
MUAs are more likely to be heavily firewalled. Thus, some MUAs
might not even be able to complete the task of performing
authentication or reputation evaluations without complex proxy
configurations or similar burdens.

3. MUAs rely upon the upstream MTAs within their trust boundaries to
make correct (as much as that is possible) evaluations about the
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
the results of that work.

4. Evaluations about the quality of a message, from simple token
matching (e.g., a list of preferred DNS domains) to cryptanalysis
(e.g., public/private key work), are at least a little bit
expensive and thus need to be minimized. To that end, performing
those tests at the border MTA is far preferred to doing that work
at each MUA that handles a message. If an ADMD's environment
adheres to common messaging protocols, a reputation query or an
authentication check performed by a border MTA would return the
same result as the same query performed by an MUA. By contrast,
in an environment where the MUA does the work, a message arriving
for multiple recipients would thus cause authentication or
reputation evaluation to be done more than once for the same
message (i.e., at each MUA) MUA), causing needless amplification of
resource use and creating a possible denial-of-service attack
vector.

5. Minimizing change is good. As new authentication and reputation
methods emerge, the list of methods supported by this header
field would presumably be extended. If MUAs simply consume the
contents of this header field rather than actually attempting attempt to do
authentication and/or reputation work, then MUAs only need to
learn to parse this header field once; emergence of new methods
requires only a configuration change at the MUAs and software
changes at the MTAs (which are presumably fewer in number). When
choosing to implement these functions in MTAs vs. MUAs, the
issues of individual flexibility, infrastructure inertia, and
scale of effort must be considered. It is typically easier to
change a single MUA than an MTA because the modification affects
fewer users and can be pursued with less care. However, changing
many MUAs is more effort than changing a smaller number of MTAs.

6. For decisions affecting message delivery and display, assessment
based on authentication and reputation is best performed close to
the time of message transit, as a message makes its journey
toward a user's inbox, not afterwards. DKIM keys and IP address
reputations, etc., can change over time or even become invalid,
and users can take a long time to read a message once delivered.
The value of this work thus degrades, perhaps quickly, once the
delivery process has completed. This seriously diminishes the
value of this work when done other than at MTAs.

Many operational choices are possible within an ADMD, including the
venue for performing authentication and/or reputation assessment.
The current specification does not dictate any of those choices.
Rather, it facilitates those cases in which information produced by
one stage of analysis needs to be transported with the message to the
next stage.

Appendix E. Changes since RFC5451

[Note to IESG: This can be dropped prior to publication unless it's
desirable to carry the changes visibly in this way.] RFC 5451

o Errata Erratum #2617 was addressed in RFC6577 RFC 6577 and was incorporated here here.

o Request Requested Internet Standard status status.

o Change Changed IANA rules to Designated Expert from IETF Review "IETF Review" to "designated expert".

o Update Updated existing IANA registries from the old RFC to this one one.

o Add Added references to ADSP, ATPS, VBR and VBR.

o Remove Removed all the "X-" stuff, per BCP178 BCP 178.

o Adjust Adjusted language to indicate that this header field was already
defined,
defined and that we're just refreshing and revising revising.

o In a few places, RFC2119 RFC 2119 language had been used in lowercase
terms; fixed here here.

o Errata Erratum #2818 addressed addressed.

o Errata Erratum #3195 addressed addressed.

o Some Performed some minor wordsmithing and removal of removed odd prose prose.

o ABNF: change changed "dot-atom" to "Keyword" since "dot-atom" allows "=",
which leads to ambiguous productions productions.

o ABNF: the authserv-id can be a "value", not a "dot-atom" "dot-atom".

o ABNF: separate separated the spec version from the method version; they're
syntactically the same but semantically different; add different. Added a
section discussing them them.

o Call Called out the SMTP verb exceptions ("mailfrom" and "rcptto"); the
previous RFC didn't do this, leading to interoperability problems problems.

o Rather than deleting suspect header fields, they could also be
renamed to something harmless; there is at least one
implementation of this this.

o Update Updated IANA method "Email Authentication Methods" registry to include
version numbers numbers.

o Rather than repeating what RFC4408[bis] RFC 4408 says the SPF results are, just refer
referred to those documents documents.

o Constrain To avoid confusing consumers, constrained inclusion of unnecessary properties to avoid confusing
consumers
properties.

o Review Reviewed usage of "should" vs. SHOULD "SHOULD".

o Update Updated prose around authserv-id (Section 2.3)

o Merge Sections 2.5 and 2.6 (defined methods and result codes) 2.4).

Author's Address

Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
US

EMail: superuser@gmail.com