wdiff rfc8601v1.txt rfc8601.txt

Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 8601 April May 2019
Obsoletes: 7601
Category: Standards Track
ISSN: 2070-1721

Message Header Field for Indicating Message Authentication Status

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 or to make sorting and
filtering decisions.

This document obsoletes RFC 7601.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8601.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . 5
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7
1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . 7
1.5.4. Email Architecture . . . . . . . . . . . . . . . . . 8
1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9
2. Definition and Format of the Header Field . . . . . . . . . . 10
2.1. General Description . . . . . . . . . . . . . . . . . . . 10
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10
2.3. Property Types (ptypes) and Properties . . . . . . . . . 13
2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . 14
2.5. Authentication Service Identifier Field . . . . . . . . . 15
2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . 16
2.7. Defined Methods and Result Values . . . . . . . . . . . . 16
2.7.1. DKIM . . . . . . . . . . . . . . . . . . . . . . . . 17
2.7.2. SPF . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19
2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20
2.7.5. Other Registered Codes . . . . . . . . . . . . . . . 21
2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21
2.7.7. Extension Result Codes . . . . . . . . . . . . . . . 22
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23 22
4. Adding the Header Field to a Message . . . . . . . . . . . . 24
4.1. Header Field Position and Interpretation . . . . . . . . 25
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . 26
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
6.1. The Authentication-Results Header Field . . . . . . . . . 28
6.2. "Email Authentication Methods" Registry Description . . . 28
6.3. "Email Authentication Methods" Registry Update . . . . . 30
6.3.1. "header.a" for DKIM . . . . . . . . . . . . . . . . . 30
6.3.2. "header.s" for DKIM . . . . . . . . . . . . . . . . . 31
6.4. "Email Authentication Property Types" Registry
Description . . . . . . . . . . . . . . . . . . . . . . . 31
6.5. "Email Authentication Property Types" Registry Update . . 32
6.6. "Email Authentication Result Names" Registry Description 32
6.7. "Email Authentication Result Names" Registry Update . . . 32
6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . 33
7. Security Considerations . . . . . . . . . . . . . . . . . . . 33
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . 33
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . 35
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 35
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . 36
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . 36
7.7. Attacks against Authentication Methods . . . . . . . . . 36
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 36
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . 37
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . 37
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 37
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
8.1. Normative References . . . . . . . . . . . . . . . . . . 38
8.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . 42
Appendix B. Authentication-Results Examples . . . . . . . . . . 42
B.1. Trivial Case: Header Field Not Present . . . . . . . . . 42
B.2. Nearly Trivial Case: Service Provided, but No
Authentication Done . . . . . . . . . . . . . . . . . . . 43
B.3. Service Provided, Authentication Done . . . . . . . . . . 44
B.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
B.5. Service Provided, Several Authentications Done, Different
MTAs . . . . . . . . . . . . . . . . . . . . . . . . . . 45
B.6. Service Provided, Multi-tiered Authentication Done . . . 47
B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 49
Appendix C. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 50
Appendix D. Changes since RFC 7601 . . . . . . . . . . . . . . . 51
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 52
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 52

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.

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 email authentication methods:

o SMTP Service Extension for Authentication [AUTH]

o DomainKeys Identified Mail Signatures [DKIM]

o Domain-based Message Authentication, Reporting, and Conformance
[DMARC]

o Sender Policy Framework [SPF]

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

o Require-Recipient-Valid-Since Header Field and SMTP Service
Extension [RRVS]

o S/MIME Signature Verification [SMIME-REG]

o Vouch By Reference [VBR]

The following Historic specifications were previously supported by
this framework but have since become obsolete:

o Author Domain Signing Practices [ADSP] (Historic)

o DomainKeys [DOMAINKEYS] (Historic)

Note that at the time of publication of this document the Sender ID
specification [SENDERID] (Experimental) is no longer supported by
this framework. Discussion regarding moving it to Historic status is
underway.

There exist registries for tokens used within this header field that
refer to the specifications listed above. Section 6 describes the
registries and their contents and specifies the process by which
entries are added or updated. It also updates the existing contents
to match the current states of these specifications.

The goal of this work 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 such 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 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, the paths
between them, 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 a remote service provider and reside physically within
its data center.

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

Note that an MUA could be configured to retrieve messages from a
receiver yet not be within the receiver's ADMD. In this case, for
the purposes of this work, that MUA is considered to be within the
receiver's ADMD if it is configured to identify and ascribe value to
authentication results recorded by that ADMD.

1.3. Processing Scope

The content of this header field is meant to convey to message
consumers that authentication work on the message was already done
within its trust boundary, and those results are being presented. It
is not intended to provide message parameters to consumers so that
they can perform authentication protocols on their own.

1.4. Requirements

This document establishes no new requirements on existing protocols,
insofar as a non-participating service will continue to interoperate
with the deployed messaging infrastructure.

In particular, this document establishes no requirement on MTAs to
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.

A participating ADMD does undertake some filtering and message
modification obligations as described in Section 5.

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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

1.5.2. Internationalized Email

In this document, there are references to messages formatted to
support Email Address Internationalization (EAI). Reference material
for this can be found in [RFC6530], [RFC6531], and [RFC6532].
Generally speaking, these documents allow UTF-8 in most places that
free-form text can be found and U-labels where domain names can be
used, and this document extends Authentication-Results accordingly.

1.5.3. Security

"Guidelines for Writing RFC Text on Security Considerations"
[SECURITY] discusses authentication and authorization and the
conflation of the two concepts. The use of those terms within the
context of recent message security work has given rise to slightly
different definitions, and this document reflects those current
usages, as follows:

o "Authorization" is the establishment of permission to use a
resource or represent an identity. In this context, authorization
indicates that a message from a particular ADMD arrived via a
route the ADMD has explicitly approved.

o "Authentication" is the assertion of validity of a piece of data
about a message (such as the sender's identity) or the message in
its entirety.

As examples: SPF is an authorization mechanism in that it expresses a
result that shows whether the ADMD that apparently sent the message
has explicitly authorized the connecting Simple Mail Transfer
Protocol (SMTP) client [SMTP] to relay messages on its behalf, but it
does not actually validate any other property of the message itself.
By contrast, DKIM is agnostic as to the routing of a message but uses
cryptographic signatures to authenticate agents, assign (some)
responsibility for the message (which implies authorization), and
ensure that the listed portions of the message were not modified in
transit. Since the signatures are not tied to SMTP connections, they
can be added by the ADMD of origin, intermediate ADMDs (such as a
mailing list server), other handling agents, or any combination of
these.

Rather than create a separate header field for each class of
solution, this specification groups them both into a single header
field.

1.5.4. Email Architecture

o A "border MTA" is an MTA that acts as 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 of a message to a user's inbox or other
final delivery.

o An "intermediate MTA" is any MTA that is not a delivery MTA and is
also not the first MTA to handle the message.

o A Message Submission Agent (MSA) is an agent that accepts a
message from an MUA, introducing it to the mail-handling stream.

The following diagram illustrates the flow of mail among these
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 C of
this document for discussion about the common aspects of email
authentication in current environments.

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

Generally, it is assumed that the work of applying message
authentication schemes takes place at a border MTA or a delivery MTA.
This specification is written with that assumption in mind. However,
there are some 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 same machine or even the very
same agent. It is also possible that some message authentication
tests could take place on an intermediate MTA. Although this
document doesn't specifically describe such cases, they are not meant
to be excluded.

1.5.5. Other Terms

In this document, the term "producer" refers to any component that
adds this header field to messages it is handling, and "consumer"
refers to any component that identifies, extracts, and parses the
header field to use as part 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 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. The field often contains no
integrity or validation mechanism of its own, so its presence must be
trusted implicitly. Hence, valid use of the header field requires
removing any occurrences of it that claim to be associated with the
ADMD when the message enters the ADMD. This ensures that later
occurrences have been added within the trust boundary of the ADMD.

The authserv-id token defined in Section 2.2 can be used to reference
an entire ADMD or a specific validation engine within an ADMD.
Although the labeling scheme is left as an operational choice, some
guidance for selecting a token is provided in later sections of this
document.

2. Definition and Format of the Header Field

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

2.1. General Description

The header field specified here is called "Authentication-Results".
It is a structured header field as defined in "Internet Message
Format" [MAIL], and thus all of the related definitions in that
document apply.

This header field is added at the top of the message as it transits
MTAs that do authentication checks, so some idea of how far away the
checks were done can be inferred. It is therefore considered to be a
trace field as defined in [MAIL], and thus all of the related
definitions in that document apply.

The value of the header field (after removing comments) consists of
an authentication service 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 or
methods were applied and their respective results. For each such
statement, the supporting data can include a "reason" string and one
or more "property=value" statements indicating which message
properties were evaluated to reach that conclusion.

The header field can appear more than once in a single message, more
than one result can be represented in a single header field, or a
combination of these can be applied.

2.2. Formal Definition

Formally, the header field is specified as shown below using
Augmented Backus-Naur Form [ABNF]. Examples of valid header fields
with explanations of their semantics can be found in Appendix B.

authres-header-field = "Authentication-Results:" authres-payload

authres-payload = [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 the absence of a
; version implies this version of the specification

no-result = [CFWS] ";" [CFWS] "none"
; the special case of "none" is used to indicate that no
; message authentication was performed

resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*(
[ CFWS propspec ) 1*propspec ]

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

reasonspec = "reason" [CFWS] "=" [CFWS] value
; a free-form comment on the reason the given result
; was returned

propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
; an indication of which properties of the 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 is
; represented by "result"; it is one of the methods
; explicitly defined as valid in this document
; or is an extension method as defined below

method-version = 1*DIGIT [CFWS]
; indicates which version of the method specification is
; in use, corresponding to the matching entry in the IANA
; "Email Authentication Methods" registry; a value of "1"
; is assumed if this version string is absent

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

ptype = Keyword
; indicates whether the property being evaluated was
; a parameter to an SMTP command [SMTP], was a value taken
; from a message header field, was some property of
; the message body, or was some other property evaluated by
; the receiving MTA; expected to be one of the "property
; types" explicitly defined as valid, or an extension
; ptype, as defined below

property = special-smtp-verb / Keyword
; indicates more specifically than "ptype" what the
; source of the evaluated property is; the exact meaning
; is specific to the method whose result is being reported
; and is defined more clearly below

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

pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS]
; the value extracted from the message property defined
; by the "ptype.property" construction

"local-part" is defined in Section 3.4.1 of [MAIL], as modified by
[RFC6531].

"CFWS" is defined in Section 3.2.2 of [MAIL].

"Keyword" is defined in Section 4.1.2 of [SMTP]. It is further
constrained by the necessity of being registered in the Internet
Assigned Numbers Authority (IANA) registry relevant to the context in
which it is used. See Sections 2.3, 2.7, and 6.

The "value" is as defined in Section 5.1 of [MIME], with
"quoted-string" updated as specified in [RFC6532].

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

The "Keyword" used in "result" above is further constrained by the
necessity of being enumerated in Section 2.7.

See Section 2.5 for a description of the authserv-id element.

If the value portion of a "pvalue" construction identifies something
intended to be an email identity, then it MUST use the 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 method was unable
to extract any properties to do its evaluation yet still has a result
to report. It may also be omitted if the agent generating this
result wishes not to reveal such properties to downstream agents.

Where an SMTP command name is being reported as a "property", the
agent generating the header field represents that command by
converting it to lowercase and dropping any spaces (e.g., "MAIL FROM"
becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).

A "ptype" value of "policy" indicates a policy decision about the
message not specific to a property of the message that could be
extracted. See Section 2.4 for details.

Examples of complete messages using this header field can be found in
Appendix B.

2.3. Property Types (ptypes) and Properties

The "ptype" in the ABNF above indicates the general type of property
being described by the result being reported, upon which the reported
result was based. Coupled with the "property", which is more
specific, it indicates from where the reported "pvalue" was
extracted. This can include a particular part of the message header
or body, some part of the SMTP session, a secondary output of an
authentication method (apart from its pure result), or some other
aspect of the message's handling.

Combinations of ptypes and properties are registered and described in
the "Email Authentication Methods" registry, coupled with the
authentication methods with which they are used. This is further
described in Section 6.

Legal values of "ptype" are as defined in the IANA "Email
Authentication Property Types" registry, created by [RFC7410]. The
initial values and what they typically indicate are as follows, based
on [RFC7001]:

body: Information that was extracted from the body of the message.
This might be an arbitrary string of bytes, a hash of a string of
bytes, a Uniform Resource Identifier, or some other content of
interest. The "property" is an indication of where within the
message body the extracted content was found and can indicate an
offset, identify a MIME part, etc. (At the time of this revision,
no properties matching this ptype have been registered.
Accordingly, this ptype may be deprecated in the future.)

header: Indicates information that was extracted from the header of
the message. This might be the value of a header field or some
portion of a header field. The "property" gives a more precise
indication of the place in the header from which the extraction
took place.

policy: A local policy mechanism was applied that augments or
overrides the result returned by the authentication mechanism.
(See Section 2.4.)

smtp: Indicates information that was extracted from an SMTP command
that was used to relay the message. The "property" indicates
which SMTP command included the extracted content as a parameter.

Results reported using unknown ptypes MUST NOT be used in making
handling decisions. They can be safely ignored by consumers.

Entries in the "Email Authentication Methods" registry can define
properties that deviate from these definitions when appropriate.
Such deviations need to be clear in the registry and/or in the
defining document. See Section 2.7.1 for an example.

2.4. The "policy" ptype

A special ptype value of "policy" is also defined. This ptype is
provided to indicate that some local policy mechanism was applied
that augments or even replaces (i.e., overrides) the result returned
by the authentication mechanism. The property and value in this case
identify the local policy that was applied and the result it
returned.

For example, a DKIM signature is not required to include the Subject
header field in the set of fields that are signed. An ADMD receiving
such a message might decide that such a signature is unacceptable,
even if it passes, because the content of the Subject header field
could be altered post-signing without invalidating the signature.
Such an ADMD could replace the DKIM "pass" result with a "policy"
result and then also include the following in the corresponding
Authentication-Results field:

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

In this case, the property is "dkim-rules", indicating that 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 ADMD making use of them, so they are not
normally registered with IANA or otherwise specified apart from
setting syntax restrictions that allow for easy parsing within the
rest of the header field.

This ptype existed in the original specification for this header
field [RFC5451], but without a complete description or example of
intended use. As a result, it has not seen any practical use to date
that matches its intended purpose. These added details are provided
to guide implementers toward proper use.

2.5. Authentication Service Identifier Field

Every Authentication-Results header field has an authentication
service identifier field (authserv-id above). Specifically, this is
any string intended to identify the authentication service within the
ADMD that conducted authentication checks on the message. This
identifier is intended to be machine-readable and not necessarily
meaningful to users.

Note that in an EAI-formatted message, this identifier may be
expressed in UTF-8.

Since agents consuming this field will use this identifier to
determine whether its contents are of interest (and are safe to use),
the uniqueness of the identifier MUST be guaranteed by the ADMD that
generates it and MUST pertain to that ADMD. MUAs or downstream
filters SHOULD use this identifier to determine whether or not the
data contained in an Authentication-Results header field ought to be
used or ignored.

For simplicity and scalability, the authentication service identifier
SHOULD be a common token used throughout the ADMD. Common practice
is to use the DNS domain name used by or within that ADMD, sometimes
called the "organizational domain", but this is not strictly
necessary.

For tracing and debugging purposes, the authentication service
identifier can instead be the specific hostname of the MTA performing
the authentication check whose result is being reported. Moreover,
some implementations define a substructure to the identifier; such
structures are outside of the scope of 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,
which in turn permits assessing the 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 determine whether to use the contents of the header
field:

o Changes to the 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
assessing the usability of the header field's content. In
particular, consumers 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 or later reassessment of the header
field's content.

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

2.6. Version Tokens

The grammar above provides for the optional inclusion of versions on
both the header field itself (attached to the authserv-id token) and
on each of the methods being reported. The method version refers to
the method itself, which is specified in the documents describing
those methods, while the authserv-id version refers to this document
and thus the syntax of this header field.

The purpose of including these is to avoid misinterpretation of the
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 an expected
format. For a method version, the parser SHOULD ignore a method
result if the version is not supported in case the semantics of the
result have a 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 want to use the altered semantics. Allowing versions in the
syntax is a way to indicate this and let the consumer of the header
field decide.

2.7. Defined Methods and Result Values

Each individual authentication method returns one of a set of
specific result values. The 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 supported by the
header field defined here, MUST include a similar result table either
in their defining documents or in supplementary ones.

2.7.1. DKIM

DKIM is represented by the "dkim" method and is defined in [DKIM].

A signature is "acceptable to the ADMD" if it passes local policy
checks (or there are no specific local policy checks). For example,
an ADMD policy might require that the signature(s) on the message be
added using the DNS domain present in the From header field of the
message, thus making third-party signatures unacceptable even if they
verify.

The DKIM result set is as follows:

none: The message was not signed.

pass: The message was signed, the signature or signatures were
acceptable to the ADMD, and the signature(s) passed verification
tests.

fail: The message was signed and the signature or signatures were
acceptable to the ADMD, but they failed the verification test(s).

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

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

temperror: The message could not be verified due to some error that
is likely transient in nature, such as a temporary inability to
retrieve a public key. A later attempt may produce a final
result.

permerror: The message could not be verified due to some error that
is unrecoverable, such as a required header field being absent. A
later attempt is unlikely to produce a final result.

DKIM results are reported using a ptype of "header". The property,
however, represents one of the tags found in the DKIM-Signature
header field rather than a distinct header field. For example, the
ptype-property combination "header.d" refers to the content of the
"d" (signing domain) tag from within the signature header field, and
not a distinct header field called "d".

Note that in an EAI-formatted message, the values of the "d" and "i"
properties can be expressed in UTF-8.

In addition to previous registrations, this document registers the
DKIM tags "a" (cryptographic algorithm used to sign the message) and
"s" (selector) as reportable properties. These can be used to aid
receivers during post-verification processing. In particular,
[RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is
important to be able to distinguish such signatures from those using
preferred algorithms.

The ability to report different DKIM results for a message with
multiple signatures is described in [RFC6008].

[DKIM] advises that if a message fails verification, it is to be
treated as an unsigned message. A report of "fail" here permits the
receiver of the report to decide how to handle the failure. A report
of "neutral" or "none" preempts that choice, ensuring that the
message will be treated as if it had not been signed.

2.7.2. SPF

SPF uses the "spf" method name. The result values for SPF are
defined in Section 2.6 of [SPF], and those definitions are included
here by reference:

These result codes are used in the context of this specification to
reflect the result returned by the component conducting SPF
evaluation.

For SPF, the ptype used is "smtp", and the property is either
"mailfrom" or "helo", since those values are the ones SPF can
evaluate. (If the SMTP client issued the EHLO command instead of
HELO, the property used is "helo".)

Note that in an EAI-formatted message, the local-part of the
"mailfrom" can be expressed in UTF-8 and the domain part can be
expressed as a U-label.

For this method, an additional result of "policy" is defined, which
means the client was authorized to inject or relay mail on behalf of
the sender's DNS domain according to the authentication method's
algorithm, but local policy dictates that the result is unacceptable.
For example, "policy" might be used if SPF returns a "pass" result,
but a local policy check matches the sending DNS domain to one found
in an explicit list of unacceptable DNS domains (e.g., spammers).

If the retrieved sender policies used to evaluate SPF do not contain
explicit provisions for authenticating the local-part (see
Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported along
with results for this mechanism SHOULD NOT include the local-part or
the following "@" character.

2.7.3. "iprev"

The result values used by the "iprev" method, defined in Section 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 "reverse" and
"forward" lookups each produced results, but they were not in
agreement, or the "forward" query completed but produced no
result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an
RCODE of 0 (NOERROR) in a reply containing no answers, was
returned.

temperror: The DNS evaluation could not be completed due to some
error that is likely transient in nature, such as a temporary DNS
error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or
other error condition resulted. A later attempt may produce a
final result.

permerror: The DNS evaluation could not be completed because no PTR
data are published for the connecting IP address, e.g., a DNS
RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR)
in a reply containing no answers, was returned. This prevented
completion of the evaluation. A later attempt is unlikely to
produce a final result.

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

The result is reported using a ptype of "policy" (as this is not part
of any established protocol) and a property of "iprev".

For discussion of the format of DNS replies, see "Domain names -
implementation and specification" [DNS].

2.7.4. SMTP AUTH

SMTP AUTH (defined in [AUTH]) is represented by the "auth" method.
Its result values are as follows:

none: SMTP authentication was not attempted.

pass: The SMTP client authenticated to the server reporting the
result using the protocol described in [AUTH].

fail: The SMTP client attempted to authenticate to the server using
the protocol described in [AUTH] but was not successful (such as
providing a valid identity but an incorrect password).

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

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

The result of AUTH is reported using a ptype of "smtp" and a property
of either:

o "auth", in which case the value is the authorization identity
generated by the exchange initiated by the AUTH command; or

o "mailfrom", in which case the value is the mailbox identified by
the AUTH parameter used with the MAIL FROM command.

Note that in an EAI-formatted message, the local-part can be
expressed in UTF-8 and the domain can be expressed as a U-label.

If both identities are available, both can be reported. For example,
consider this command issued by a client that has completed session
authentication with the AUTH command resulting in an authorized
identity of "client@c.example":

MAIL FROM:<alice@a.example> AUTH=<bob@b.example>

This could result in a "resinfo" construction like so:

; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example

Note that in all cases other than "pass", the message was sent by an
unauthenticated client. All non-"pass" cases SHOULD thus be treated
as equivalent with respect to this method.

2.7.5. Other Registered Codes

Result codes were also registered in other RFCs as follows:

o Vouch By Reference (in [AR-VBR], represented by "vbr").

o Authorized Third-Party Signatures (in [ATPS], represented by
"dkim-atps").

o Author Domain Signing Practices (in [ADSP], represented by
"dkim-adsp").

o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs").

o S/MIME (in [SMIME-REG], represented by "smime").

Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which
are already registered, can be expressed as U-labels.

2.7.6. Extension Methods

Additional authentication method identifiers (extension methods) may
be defined in the future by later revisions or extensions to this
specification. These method identifiers are registered with 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 be communicated to MUAs or downstream filters. The names of
such identifiers ought to reflect the name of the method being
defined but ought not be needlessly long.

2. To allow the creation of "sub-identifiers" that indicate
different levels of authentication and differentiate between
their relative strengths, e.g., "auth1-weak" and "auth1-strong".

Authentication method implementers are encouraged to provide adequate
information, via message header field comments if necessary, to allow
an MUA developer to understand or relay ancillary details of
authentication results. For example, if it might be of interest to
relay what data was were used to perform an evaluation, such information
could be relayed as a comment in 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 to use them. These method identifiers and
the parameters associated with them are not documented formally.
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.

2.7.7. Extension Result Codes

Additional result codes (extension results) might be defined in the
future by later revisions or extensions to this specification.
Non-experimental result codes MUST be registered with IANA (and,
preferably, published in an RFC). See Section 6 for further details.

Experimental results MUST only be used within ADMDs that have
explicitly consented to use them. These results and the parameters
associated with 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 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 DNS queries, which is to say that the IP address is
explicitly associated with a domain name. Upon receiving a session
initiation of some kind from a client, the 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 a "PTR" record
query). Once that result is acquired, a lookup of each of the names
(i.e., a name-to-number translation, or an "A" or "AAAA" record
query) thus retrieved is done. The response to this second check
will typically result in at least one mapping back to the client's IP
address.

Expressed as an algorithm: If the client peer's IP address is I, the
list of names to which I maps (after a "PTR" query) is the set N, and
the union of IP addresses to which each member of N maps (after
corresponding "A" and "AAAA" queries) is L, then this test is
successful if I is an element of L.

Often an MTA receiving a connection that fails this test will simply
reject the connection using the enhanced status code defined in
[AUTH-ESC]. If an operator instead wishes to make this information
available to downstream agents as a factor in handling decisions, it
records a result in accordance with Section 2.7.3.

The response to a "PTR" query could contain multiple names. To
prevent heavy DNS loads, agents performing these queries MUST be
implemented such that the number of names evaluated by generation of
corresponding "A" or "AAAA" queries is limited so as not to be unduly
taxing to the DNS infrastructure, though it MAY be configurable by an
administrator. As an example, Section 4.6.4 of [SPF] chose a limit
of 10 for its implementation of this algorithm.

"DNS Extensions 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, in some regions, it can be difficult for this
test ever to pass because the practice of arranging to match the
forward and reverse DNS is infrequently observed. Therefore, the
precise implementation details of 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 use of the reported "iprev" result to
understand what exactly that particular verifier is attempting to
report.

Extensive discussion of reverse DNS mapping and its implications can
be found in "Considerations for the use of DNS Reverse Mapping"
[DNSOP-REVERSE]. In particular, it recommends that applications
avoid using this test as a means of authentication or security. Its
presence in this document is not an endorsement but is merely
acknowledgment that the method remains common and provides the means
to relay the results of that test.

4. Adding the Header Field to a Message

This specification makes no attempt to evaluate the relative
strengths of 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 filter consuming
this header field is to interpret the 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 an extension method as described in Section 2.7.6,
and each "result" MUST refer to a result code declared in the IANA
registry or an extension result code as defined in Section 2.7.7.
See Section 6 for further information about the registered methods
and result codes.

An MTA compliant with this specification adds this header field
(after performing one or more message authentication tests) to
indicate which MTA or ADMD performed the test, which test was
applied, and what the result was. If an MTA applies more than one
such test, it adds this header field either once per test or once
indicating all of the results. An MTA MUST NOT add a result to an
existing header field.

An MTA MAY add this header field containing only the authentication
service identifier portion and the "none" token (see Section 2.2) to
explicitly
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
legitimate to the MUAs or downstream filters 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 specific solution to this issue, as each environment
has its own facilities and limitations.

Most known message authentication methods focus on a particular
identifier to evaluate. SPF differs in that it can yield a result
based on more than one identifier; specifically, SPF can evaluate the
RFC5321.HELO parameter or the RFC5321.MailFrom parameter. When
generating this field to report those results, only the parameter
that yielded the result is included.

For MTAs that add this header field, adding header fields in order
(at the top), per Section 3.6 of [MAIL], 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 of header fields that can be trusted.

End users making direct use of this header field might inadvertently
trust information that has not been properly vetted. If, for
example, a basic SPF result were to be relayed that claims an
authenticated addr-spec, the local-part of that addr-spec has
actually not been authenticated. Thus, an MTA adding this header
field SHOULD NOT include any data that has have 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 to
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 NOT interpret
this header field unless specifically configured to do so by the user
or administrator. That is, this interpretation should not be "on by
default". Naturally then, users or administrators ought not activate
such a feature unless (1) they are certain the header field will be
validly added by an agent within the ADMD that accepts the mail that
is ultimately read by the MUA, and (2) instances of the header field
that appear to originate within the ADMD but are actually added by
foreign MTAs will be removed before delivery.

Furthermore, MUAs and downstream filters SHOULD NOT interpret this
header field unless the authentication service identifier of the
header field is used within the ADMD as configured by the user or
administrator.

MUAs and downstream filters MUST ignore any result reported using a
"result" not specified in the IANA "Result Code" registry or a
"ptype" not listed in the "Email Authentication Property Types"
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. The exception to this is experimental methods
as discussed in Section 2.7.6.

An MUA SHOULD NOT reveal these results to end users, absent careful
"human factors" design considerations and testing, for the
presentation of trust-related materials. For example, an attacker
could register examp1e.com (note the digit "1" (one)) and send signed
mail to intended victims; a verifier would detect that the signature
was valid and report a "pass" even though it's clear the DNS domain
name was intended to mislead. See Section 7.2 for further
discussion.

As stated in Section 2.1, this header field MUST be treated as though
it were a trace header field as defined in Section 3.6.7 of [MAIL]
and hence MUST NOT be reordered and MUST be prepended to the message,
so that there is generally some indication upon delivery of where in
the chain of handling MTAs the message authentication was done.

Note that there are a few message handlers that are only capable of
appending new header fields to a message. Strictly speaking, these
handlers are not compliant with this specification. They can still
add the header field to carry authentication details, but any signal
about where in the handling chain the work was done may be lost.
Consumers SHOULD be designed such that this can be tolerated,
especially from a producer known to have this limitation.

MUAs SHOULD ignore instances of this header field discovered within
message/rfc822 MIME attachments. They are likely to contain the
results of authentication checks done in the past, possibly long ago,
and have no contemporary value. Due caution therefore needs to be
taken when choosing to consume them.

Further discussion of these topics can be found in Section 7 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" or similar) as justification for rejecting the message. In
such cases, the border MTA SHOULD issue an SMTP rejection response to
the message, rather than adding this header field and allowing the
message to proceed toward delivery. This is more desirable than
allowing the message 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 done for local policy decisions overriding the
results of the authentication methods (e.g., the "policy" result
codes described in Section 2.7).

Such rejections at the SMTP protocol level are not possible if local
policy is enforced at the MUA and not the MTA.

5. Removing Existing Header Fields

To mitigate the impact of forged header fields, any MTA conforming to
this specification MUST delete any discovered instance of this header
field that claims, by virtue of its authentication service
identifier, to have been added within its trust boundary but that did
not come directly from another trusted MTA. For example, an MTA for
example.com receiving a message MUST delete or otherwise obscure any
instance of this header field bearing an authentication service
identifier indicating that the header field was added within
example.com prior to adding its own header fields. This could mean
each internal MTA will need to be configured with a list of other
known, trusted MTAs that are thus expected to be using that same
identifier.

In the case of EAI-formatted messages, this test is done after
converting A-labels into U-labels.

For simplicity and maximum security, a border MTA could remove all
instances of this header field on mail crossing into its trust
boundary. However, this may conflict with the desire to access
authentication results performed by trusted external service
providers. It may also invalidate signed messages whose signatures
cover external instances of this header field. A more robust border
MTA could allow a specific list of authenticating MTAs whose
information is to be admitted, removing the header field originating
from all others.

As stated in Section 1.2, a formal definition of "trust boundary" is
deliberately not made here. It is entirely possible that a border
MTA for example.com will explicitly trust authentication results
asserted by upstream host example.net even though they exist in
completely disjoint administrative boundaries. In that case, the
border MTA MAY elect not 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 this is provided in
Appendix B.

Similarly, in the case of messages signed using [DKIM] or other
message-signing methods that sign header fields, this removal action
could invalidate one or more signatures on the message if they
covered the header field to be removed. This behavior can be
desirable, since there's little value in validating the 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 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 [SMTP]
relaying the message is not from a trusted internal MTA. (As
discussed above, this too can result in invalidation of signatures.)
This means the MTA needs to be able to understand versions of this
header field at least as late as the ones understood by the MUAs or
other consumers within its ADMD.

6. IANA Considerations

IANA has registered the defined header field and created registries
as described below. These registry actions were originally defined
by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created
registries were further updated in [RFC7601] to make them more
complete.

Each registry has two related sections below. The first describes
the registry and its update procedures, which are unchanged from
[RFC7601]. The second enumerates changes to entries that are
relevant to this document.

6.1. The Authentication-Results Header Field

The Authentication-Results header field was added to the IANA
"Permanent Message Header Field Names" registry, per the procedure
found in [IANA-HEADERS]. That entry has been updated 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): RFC 8601
Related information: none

6.2. "Email Authentication Methods" Registry Description

Names of message authentication methods supported by this
specification have been registered with IANA, with the exception of
experimental names as described in Section 2.7.6. Along with each
method are recorded the properties that accompany the method's
result.

The "Email Authentication Parameters" group, and within it the "Email
Authentication Methods" registry, were created by [RFC5451] for this
purpose. [RFC6577] added a "Status" field for each entry. [RFC7001]
amended the rules governing that registry and also added a "Version"
field to the registry.

The reference for that registry has been updated to reference this
document.

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

No two entries can have the same combination of method, ptype, and
property.

An entry in this registry contains the following:

Method: the name of the method.

Definition: a reference to the document that created this entry, if
any (see below).

ptype: a "ptype" value appropriate for use with that method.

Property: a "property" value matching that "ptype" also appropriate
for use with that method.

Value: a brief description of the value to be supplied with that
method/ptype/property tuple.

Status: the status of this entry, which is one of the following:

active: The entry is in current use.

deprecated: The entry is no longer in current use.

Version: a version number associated with the method (preferably
starting at "1").

The "Definition" field will typically refer to a permanent document,
or at least some descriptive text, where additional information about
the entry being added can be found. This might in turn reference the
document where the method is defined so that all of the semantics
around creating or interpreting an Authentication-Results header
field using this method, ptype, and property can be understood.

6.3. "Email Authentication Methods" Registry Update

The following entries in this registry have been updated to replace
[RFC7601] with this document:

Notably, the DomainKeys and Sender ID entries are not updated to
refer to this revised specification, as they are considered obsolete.
Accordingly, IANA has changed the "Status" field of the "sender-id"
entry in this table to "deprecated".

Finally, two new entries have been added to this registry, as
follows:

6.3.1. "header.a" for DKIM

Method: dkim

Definition: RFC 8601

ptype: header

Property: a

Value: value of signature "a" tag

Status: active
Version: 1

6.3.2. "header.s" for DKIM

Method: dkim

Definition: RFC 8601

ptype: header

Property: s

Value: value of signature "s" tag

Status: active

Version: 1

6.4. "Email Authentication Property Types" Registry Description

[RFC7410] created the "Email Authentication Property Types" registry.

Entries in this registry are subject to the Expert Review rules as
described in [IANA-CONSIDERATIONS]. Each entry in the registry
requires the following values:

ptype: the name of the ptype being registered, which must fit within
the ABNF described in Section 2.2.

Definition: an optional reference to a defining specification.

Description: a brief description of what sort of information this
"ptype" is meant to cover.

For new entries, the designated expert needs to ensure that the
description provided for the new entry adequately describes the
intended use. An example would be helpful to include in the entry's
defining document (if any), although entries in the "Email
Authentication Methods" registry or the "Email Authentication Result
Names" registry might also serve as examples of intended use.

As this is a complete restatement of the definition and rules for
this registry, IANA has updated this registry to show Section 2.3 of
this document as the current definitions for the "body", "header",
"policy", and "smtp" entries of that registry.

6.5. "Email Authentication Property Types" Registry Update

All current entries in this registry have been updated to replace
[RFC7601] with this document.

6.6. "Email Authentication Result Names" Registry Description

Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.7.7.

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

No two entries can have the same combination of method and code.

An entry in this registry contains the following:

Auth Method: an authentication method for which results are being
returned using the header field defined in this document.

Code: a result code that can be returned for this authentication
method.

Specification: either free-form text explaining the meaning of this
method-code combination or a reference to such a definition.

Status: the status of this entry, which is one of the following:

active: The entry is in current use.

deprecated: The entry is no longer in current use.

6.7. "Email Authentication Result Names" Registry Update

For the following entries in this registry, the new "Specification"
field has been set as follows:

o All "auth" method result codes ("fail", "none", "pass",
"permerror", and "temperror") are now specified in Section 2.7.4
of this document.

o All "dkim" method result names ("fail", "neutral", "none", "pass",
"permerror", "policy", and "temperror") are now specified in
Section 2.7.1 of this document.

o All "iprev" method result names ("fail", "pass", "permerror", and
"temperror") are now specified in Section 2.7.3 of this document.

o The "spf" method result names "fail", "neutral", "none", "pass",
"permerror", "policy", "softfail", and "temperror" are now
specified in Section 2.7.2 of this document. The registration for
result name "hardfail" is not updated.

The following entries in this registry have been updated with a new
"Status" field set to "deprecated", and with no change to the
"Specification" field as they reference historic protocols:

o All "domainkeys" method result names ("fail", "neutral", "none",
"pass", "permerror", "policy", and "temperror").

o All "sender-id" method result names ("fail", "neutral", "none",
"pass", "permerror", "policy", "softfail", and "temperror").

6.8. SMTP Enhanced Status Codes

The entry for X.7.25 in the "Enumerated Status Codes" subregistry of
the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry" has been updated to refer only to Section 3.3 of
[AUTH-ESC], as that is where that registration was done.

7. Security Considerations

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

7.1. Forged Header Fields

An MTA not applying the filtering discussed in Section 5 exposes MUAs
to 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, with the rest of the 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 header field enabled by default; 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 but have an explicit list
of hostnames whose Authentication-Results header fields are
trustworthy; however, this list should initially be empty.

Proposed alternative solutions to this problem were made some time
ago and are listed below. To date, they have not been developed due
to lack of 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 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 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] and the compliant internal MTAs are using
their own hostnames or the ADMD's DNS domain name as the
authserv-id token, this header field should always appear above a
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 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 metadata associated
with the message. In particular, an SMTP extension [SMTP] could be
established 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 metadata retrieved and
rendered along with the message by an IMAP client [IMAP] 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.

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.

7.3. Header Field Position

Despite the requirements of [MAIL], header fields can sometimes be
reordered en route by intermediate MTAs. The goal of requiring
header field addition only at the top of a message is an
acknowledgment 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.

7.4. Reverse IP Query Denial-of-Service Attacks

Section 4.6.4 of [SPF] observes that limits are necessary on
recursive evaluations of SPF records in order to avoid abuse of or
attacks on the DNS when verifying 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.

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
[DSN] (or equivalent) to addresses that did not send the messages
being rejected.

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.

7.7. Attacks against Authentication Methods

If an attack against an authentication method becomes known, 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.

7.8. Intentionally Malformed Header Fields

As with any other header field found in the message, it is possible
for an attacker to add an Authentication-Results header field that is
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.

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 [SMTP] 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.

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 simple guidance to MUA developers is
appropriate here.

Since MTAs are generally unlikely to strip Authentication-Results
header fields during mailbox delivery, normative language exists in
Section 4.1 cautioning MUAs to ignore such instances within MIME
attachments, as might be included when a message is forwarded.
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.

There can be cases where these header fields included as part of
encapsulated messages might actually be of value, such as when they
are taken from messages within the same ADMD where they will be
consumed. Caution must be taken to ensure that the consumer fully
understands the semantics of what the header field is indicating and
the message's handling history before ascribing any value, positive
or negative, to such data.

7.11. Reverse Mapping

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 specification 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

8.1. Normative References

[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[IANA-HEADERS]
Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.

[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.

[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>.

[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>.

[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <https://www.rfc-editor.org/info/rfc6532>.

[RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601,
DOI 10.17487/RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.

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, DOI 10.17487/RFC5617, August
2009, <https://www.rfc-editor.org/info/rfc5617>.

[AR-VBR] Kucherawy, M., "Authentication-Results Registration for
Vouch by Reference Results", RFC 6212,
DOI 10.17487/RFC6212, April 2011,
<https://www.rfc-editor.org/info/rfc6212>.

[ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM)
Authorized Third-Party Signatures", RFC 6541,
DOI 10.17487/RFC6541, February 2012,
<https://www.rfc-editor.org/info/rfc6541>.

[AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/info/rfc4954>.

[AUTH-ESC]
Kucherawy, M., "Email Authentication Status Codes",
RFC 7372, DOI 10.17487/RFC7372, September 2014,
<https://www.rfc-editor.org/info/rfc7372>.

[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.

[DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.

[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.

[DNSOP-REVERSE]
Senie, D. and A. Sullivan, "Considerations for the use of
DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop-
reverse-mapping-considerations-06, March 2008.

[DOMAINKEYS]
Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
DOI 10.17487/RFC4870, May 2007,
<https://www.rfc-editor.org/info/rfc4870>.

[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003,
<https://www.rfc-editor.org/info/rfc3464>.

[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.

[IANA-CONSIDERATIONS]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.

[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>.

[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451,
DOI 10.17487/RFC5451, April 2009,
<https://www.rfc-editor.org/info/rfc5451>.

[RFC6008] Kucherawy, M., "Authentication-Results Registration for
Differentiating among Cryptographic Results", RFC 6008,
DOI 10.17487/RFC6008, September 2010,
<https://www.rfc-editor.org/info/rfc6008>.

[RFC6577] Kucherawy, M., "Authentication-Results Registration Update
for Sender Policy Framework (SPF) Results", RFC 6577,
DOI 10.17487/RFC6577, March 2012,
<https://www.rfc-editor.org/info/rfc6577>.

[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001,
DOI 10.17487/RFC7001, September 2013,
<https://www.rfc-editor.org/info/rfc7001>.

[RFC7410] Kucherawy, M., "A Property Types Registry for the
Authentication-Results Header Field", RFC 7410,
DOI 10.17487/RFC7410, December 2014,
<https://www.rfc-editor.org/info/rfc7410>.

[RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage
Update to DomainKeys Identified Mail (DKIM)", RFC 8301,
DOI 10.17487/RFC8301, January 2018,
<https://www.rfc-editor.org/info/rfc8301>.

[RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
Since Header Field and SMTP Service Extension", RFC 7293,
DOI 10.17487/RFC7293, July 2014,
<https://www.rfc-editor.org/info/rfc7293>.

[SECURITY]
Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.

[SENDERID]
Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, DOI 10.17487/RFC4406, April 2006,
<https://www.rfc-editor.org/info/rfc4406>.

[SMIME-REG]
Melnikov, A., "Authentication-Results Registration for
S/MIME Signature Verification", RFC 7281,
DOI 10.17487/RFC7281, June 2014,
<https://www.rfc-editor.org/info/rfc7281>.

[SPF] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.

[VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
<https://www.rfc-editor.org/info/rfc5518>.

Appendix A. Legacy MUAs

Implementers of this specification should be aware that many MUAs are
unlikely to be retrofitted to support the Authentication-Results
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 (i.e., while this
specification and its successors continue to be adopted), other
interim means of conveying authentication results may be necessary.

Appendix B. Authentication-Results Examples

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

B.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: Header Field Not Present

The 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 (1) the message authentication services
were not available at the time of delivery, (2) no service is
provided, or (3) the MTA is not in compliance with this
specification.

B.2. Nearly Trivial Case: Service Provided, 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 but No Authentication Done

The 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 or 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.

B.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

The 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.

B.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; iprev=pass
policy.iprev=192.0.2.200
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 ESMTPA 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 from One MTA

The 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 themselves to the MTA via a method specified
in [AUTH], and both SPF and "iprev" checks were done and passed. The
MUA could extract and relay this extra information if desired.

Two 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
dial-up connection (example.net) sends mail to a border MTA
(example.com) using SMTP authentication to prove identity. The
dial-up provider has been explicitly authorized to relay mail as
example.net, producing a "pass" result from the SPF check.

B.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;
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 ESMTPA 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 from Multiple MTAs

The 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 MTA, mail-router.example.com,
reports that SMTP AUTH and SPF were both 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, reports that it did a DKIM
test (which passed). Again, additional data about one of the tests
is
are provided as a comment, which the MUA may choose to render. Also
noteworthy here is the fact that there is a DKIM signature added by
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 a message into
example.com from a user on a dial-up connection example.net. The
user appears to be legitimate, as they had a valid password allowing
authentication at the border MTA using SMTP AUTH. The SPF test
failed, since example.com has not granted example.net's dial-up
network authority to relay mail on its behalf. The DKIM test passed
because the sending user had a private key matching one of
example.com's published public keys and mail-router.example.com used
it to sign the message.

B.6. Service Provided, Multi-tiered Authentication Done
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 from Multiple MTAs in
Different ADMDs

In this 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, 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 metadata about the result are in
use here. In one case, the "reason=" clause is 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.

B.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 but Perfectly Legal Example

Appendix C. Operational Considerations about Message Authentication

Implementation of the Authentication-Results header field 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
header field 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 inside of an ADMD 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 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 cryptographic
verification (e.g., public/private key work), do have a cost 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), 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 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, 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 elsewhere 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 D. Changes since RFC 7601

o Added IANA registration for DKIM "a" and "s" properties.

o Included EAI guidance.

o Adjusted some ABNF tokens and names for easier inclusion by other
documents.

o Made minor editorial adjustments.

o Deprecated entries from RFCs that are now Historic.

o Erratum 4671 resolved.

o Erratum 5435 resolved.

Acknowledgments

The author wishes to acknowledge the following individuals for their
review and constructive criticism of this document: Kurt Andersen,
Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro
Vesely.

Author's Address

Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
United States of America

Email: superuser@gmail.com