wdiff rfc7001v1.txt rfc7001.txt

Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 7001 August 2013
Obsoletes: 5451, 6577
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.

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 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7001.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 7
1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 8
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9
2. Definition and Format of the Header Field . . . . . . . . . . 9
2.1. General Description . . . . . . . . . . . . . . . . . . . 9
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10
2.3. Authentication Identifier Field . . . . . . . . . . . . . 12
2.4. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 13
2.5. Defined Methods and Result Values . . . . . . . . . . . . 14
2.5.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 14
2.5.2. SPF and Sender-ID Sender ID . . . . . . . . . . . . . . . . . . 15
2.5.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 16
2.5.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 17
2.5.5. Other Registered Codes . . . . . . . . . . . . . . . . 17
2.5.6. Extension Methods . . . . . . . . . . . . . . . . . . 18
2.5.7. Extension Result Codes . . . . . . . . . . . . . . . . 18
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 19
4. Adding the Header Field to a Message . . . . . . . . . . . . . 20
4.1. Header Field Position and Interpretation . . . . . . . . . 21
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 22
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
6.1. The Authentication-Results Header Field . . . . . . . . . 24
6.2. "Email Authentication Methods" Registry . . . . . . . . . 24
6.3. "Email Authentication Result Names" Registry . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 25
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 27
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 27
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 28
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 28
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 28
7.7. Attacks against Authentication Methods . . . . . . . . . . 28
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 28
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 29
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 29
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 29
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Normative References . . . . . . . . . . . . . . . . . . . 30
8.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 32
Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 32
Appendix C. Authentication-Results Examples . . . . . . . . . . . 32
C.1. Trivial Case; Header Field Not Present . . . . . . . . . . 33
C.2. Nearly Trivial Case; Service Provided, but No
Authentication Done . . . . . . . . . . . . . . . . . . . 33
C.3. Service Provided, Authentication Done . . . . . . . . . . 34
C.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
C.5. Service Provided, Several Authentications Done,
Different MTAs . . . . . . . . . . . . . . . . . . . . . . 36
C.6. Service Provided, Multi-Tiered Authentication Done . . . . 38
C.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 39
Appendix D. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 40
Appendix E. Changes since RFC 5451 . . . . . . . . . . . . . . . 41

1. Introduction

This document describes a header field called Authentication-Results
for electronic mail messages that presents the results of a message
authentication effort in a machine-readable format. The intent of
the header field is to create a place to collect such data when
message authentication mechanisms are in use so that a Mail User
Agent (MUA) and downstream filters can make filtering decisions
and/or provide a recommendation to the user as to the validity of the
message's origin and possibly the safety and integrity of its
content.

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

End users are not expected to be direct consumers of this header
field. This header field is intended for consumption by programs
that will then use such data or render it in a human-usable form.

This document specifies the format of this header field and discusses
the implications of its presence or absence. However, it does not
discuss how the data contained in the header field ought to be used,
such as what filtering decisions are appropriate or how an MUA might
render those results, as these are local policy and/or user interface
design questions that are not appropriate for this document.

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

o Author Domain Signing Practices ([ADSP])

o SMTP Service Extension for Authentication ([AUTH])

o DomainKeys Identified Mail Signatures ([DKIM])

o Sender Policy Framework ([SPF])

o Vouch By Reference ([VBR])

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

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

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

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

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

1.1. Purpose

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

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

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

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

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

1.2. Trust Boundary

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

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

Thus, this document defines a "trust boundary" as the delineation
between "external" and "internal" entities. Services that are
internal -- within the trust boundary -- are provided by the ADMD's
infrastructure for its users. Those that are external are outside of
the authority of the ADMD. By this definition, hosts that are within
a trust boundary are subject to the ADMD's authority and policies,
independent of their physical placement or their physical operation.
For example, a host within a trust boundary might actually be
operated by 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, this field found within one of those
media types is typically ignored.

1.3. Processing Scope

The content of this header field is meant to convey to message
consumers that authentication work on the 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
or servers.

In particular, this document establishes no requirement on MTAs to
reject or filter arriving messages that do not pass authentication
checks. The data conveyed by the specified header field's contents
are for the information of MUAs and filters and are to be used at
their discretion.

1.5. Definitions

This section defines various terms used throughout this document.

1.5.1. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].

1.5.2. 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 and Sender-ID Sender ID are authorization mechanisms in that
they express a result that shows whether or not the ADMD that
apparently sent the message has explicitly authorized the connecting
Simple Mail Transfer Protocol ([SMTP]) client to relay messages on
its behalf, but they do 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 either the ADMD of origin,
intermediate ADMDs (such as a mailing list server), other handling
agents, or any combination.

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

1.5.3. Email Architecture

o A "border MTA" is an MTA that acts as 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.

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 D 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.

1.5.4. 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 Administrative Management Domain (ADMD). An ADMD contains one or
more entities that perform validation and generate the header field
and one or more that consume it for some type of assessment. 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 are present
when the message enters the ADMD. This ensures that later
occurrences have been added within the trust boundary of the ADMD.

The "authserv-id" 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 more formal specification.

2.1. General Description

The header field specified here is called "Authentication-Results". 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 identifier, an optional version, and then a series
of statements and supporting data. The statements are of the form
"method=result" and indicate which authentication method(s) were
applied and their respective results. For each such statement, 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 follows using Augmented
Backus-Naur Form ([ABNF]):

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

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

authres-version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use;
; this specification is version "1", and 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 )

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", and 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 = "smtp" / "header" / "body" / "policy"
; indicates whether the property being evaluated was
; a parameter to an [SMTP] command, 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

property = special-smtp-verb / Keyword
; if "ptype" is "smtp", this indicates which [SMTP]
; command provided the value that was evaluated by the
; authentication scheme being applied; if "ptype" is
; "header", this indicates from which header field the
; value being evaluated was extracted; if "ptype" is
; "body", this indicates where in the message body
; a value being evaluated can be found (e.g., a specific
; offset into the message or a reference to a MIME part);
; if "ptype" is "policy", then this indicates the name
; of the policy that caused this header field to be
; added (see below)

special-smtp-verb = "mailfrom" / "rcptto"
; special cases of [SMTP] commands 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], and "CFWS" is
defined in Section 3.2.2 of [MAIL].

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

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

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

The "Keyword" used in "result" above is further constrained by the
necessity of being enumerated in Section 2.5.

See Section 2.3 for a description of the "authserv-id" authserv-id element.

If the value portion of a "pvalue" construction identifies something
intended to be an e-mail identity, then it MUST use the 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 has a result to
report.

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. For example, if a method would normally report a
"ptype.property" of "header.From" and no From: From header field was
present, the method can use "policy" to indicate that no conclusion
about the authenticity of the message could be reached.

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

2.3. Authentication Identifier Field

Every Authentication-Results header field has an authentication
service identifier field ("authserv-id" (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.

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 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; these 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 re-assessment of the header
field's contents.

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

2.4. Version Tokens

The grammar above provides for 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.

2.5. 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 its their defining document documents or in a supplementary one. ones.

2.5.1. DKIM and DomainKeys

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

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: From header field of the
message, thus making third-party signatures unacceptable even if they
verify.

Both DKIM and DomainKeys use the same result set, 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] 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 the message
will be treated as if it had not been signed.

2.5.2. SPF and Sender-ID Sender ID

SPF and Sender ID use the "spf" and "sender-id" method names,
respectively. The result values for SPF are defined in Section 2.5
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.

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

Note that both of those documents specify result codes that use mixed
case, but they are typically used all lowercase in this context.

In both cases, 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 and Sender ID
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 these mechanisms SHOULD NOT include the local-
part.

2.5.3. "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.

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

2.5.4. SMTP AUTH

SMTP AUTH (defined in [AUTH]) is represented by the "auth" method,
and 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, yet
continued to send the message about which a result is being
reported.

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.

An agent making use of the data provided by this header field SHOULD
consider "fail" and "temperror" to be synonymous in terms of message
authentication, i.e., the client did not authenticate in either case.

2.5.5. Other Registered Codes

Result codes were also registered in other RFCs for Vouch By
Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party
Signatures (in [ATPS], represented by "dkim-atps"), and the DKIM-
related Author Domain Signing Practices (in [ADSP], represented by
"dkim-adsp").

2.5.6. 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 the
Internet Assigned Numbers Authority (IANA) and, preferably, published
in an RFC. See Section 6 for further details.

Extension methods can be defined for the following reasons:

1. To allow additional information from new authentication systems
to 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 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 in RFCs.
Therefore, they are subject to change at any time and not suitable
for production use. Any MTA, MUA, or downstream filter intended for
production use SHOULD ignore or delete any Authentication-Results
header field that includes an experimental (unknown) method
identifier.

2.5.7. Extension Result Codes

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

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

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 5.5 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
acknowledgement 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 2.5.6,
and each "result" MUST refer to a result code declared in the IANA
registry or an extension result code as defined in Section 2.5.7.
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 got
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
identifier portion and the "none" token (see Section 2.2) to 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 and Sender ID differ in that they can
yield a result based on more than one identifier; specifically, SPF
can evaluate the RFC5321.Helo RFC5321.HELO parameter or the RFC5321.MailFrom
parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter
or the Purported Responsible Address (PRA) identity. 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 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 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 instances of the header field
appearing 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 it bears
appears to be one used within its own 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 corresponding registry for such values as
defined in Section 6. Moreover, such agents MUST ignore a result
indicated for any "method" they do not specifically support.

An MUA SHOULD NOT reveal these results to end users, absent careful
human factors design considerations and testing, for the presentation
of trust-related materials. For example, an attacker could register
examp1e.com (note the digit "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.

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 2.5).

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

For security reasons, 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 MTA will have to be equipped
with a list of internal MTAs known to be compliant (and hence
trustworthy).

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

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 relaying
the message is not from a trusted internal MTA. This means the MTA
needs to be able to understand versions of this header field at last 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 two tables
as described below. These registry actions were originally defined
by [RFC5451] and are repeated here to provide a single, current
reference.

6.1. The Authentication-Results Header Field

[RFC5451] added the "Authentication-Results" Authentication-Results header field 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 7001
Related information:
Requesting review of any proposed changes and additions to
this field is recommended.

6.2. "Email Authentication Methods" Registry

Names of message authentication methods supported by this
specification are to be registered with IANA, with the exception of
experimental names as described in Section 2.5.6. A registry was
created by [RFC5451] for this purpose. This document changes the
rules governing that registry.

New entries are assigned only for values that have received Expert
Review, per [IANA-CONSIDERATIONS]. The designated expert shall be
appointed by the IESG. The designated expert has discretion to
request that a publication be referenced if 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".

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

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

IANA has also added a "version" field to all existing registry
entries. All current methods are recorded as version "1".

6.3. "Email Authentication Result Names" Registry

Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.5.7. A registry was
created by [RFC5451] for this purpose. This document changes the
rules governing that registry.

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

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

The definitions for the SPF and Sender ID authentication methods are
updated using the references found in Section 2.5.2.

7. Security Considerations

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

7.1. Forged Header Fields

An MUA or filter that accesses a mailbox whose messages are handled
by a non-conformant MTA, and understands Authentication-Results
header fields, could potentially make false conclusions based on
forged header fields. A malicious user or agent could forge a header
field using the DNS domain of a receiving ADMD as the authserv-id
token in the value of the header field and, 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"
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" 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: Received header
fields) to determine whether any of them conform to Section 5 of
this memo. This, too, has potentially high barriers to entry.

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

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

Support for some of these is being considered for future work.

In any case, a mechanism needs to exist for an MUA or filter to
verify that the host that appears to have added the header field (a)
actually did so 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 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 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
acknowledgement that some MTAs do reorder header fields, but most do
not. Thus, in the general case, there will be some indication of
which MTAs (if any) handled the message after the addition of the
header field defined here.

7.4. Reverse IP Query Denial-of-Service Attacks

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

7.5. Mitigation of Backscatter

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

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

7.8. Intentionally Malformed Header Fields

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

7.9. Compromised Internal Hosts

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

7.10. Encapsulated Instances

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

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

7.11. Reverse Mapping

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

8. References

8.1. Normative References

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

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

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

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

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

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

8.2. Informative References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Appendix A. Acknowledgements

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

Appendix B. Legacy MUAs

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

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

Appendix C. Authentication-Results Examples

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

C.1. Trivial Case; Header Field Not Present

The trivial case:

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

Hello! Goodbye!

Example 1: Trivial Case

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

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

C.3. Service Provided, Authentication Done

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

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

Hello! Goodbye!

Example 3: Header Reporting Results

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

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

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

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

Hello! Goodbye!

Example 4: Headers Reporting Results from One MTA

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

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

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

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

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

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

Hello! Goodbye!

Example 5: Headers Reporting Results from Multiple MTAs

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

The second MTA, "auth-checker.example.com", auth-checker.example.com, reports that it did a
Sender ID test (which failed) and a DKIM test (which passed). Again,
additional data about one of the tests is provided as a comment,
which the MUA may choose to render. Also noteworthy here is the fact
that there is a DKIM signature added by example.com that assured the
integrity of the lower Authentication-Results field.

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

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

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

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

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

Example 6: Headers Reporting Results 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.

C.7. Comment-Heavy Example

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

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

Example 7: A Very Comment-Heavy but Perfectly Legal Example

Appendix D. Operational Considerations about Message Authentication

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

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

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

3. MUAs rely upon the upstream MTAs within their trust boundaries to
make correct (as much as is possible) evaluations about the
message's envelope, header, and content. Thus, MUAs don't need
to know how to do the work that upstream MTAs do; they only need
the results of that work.

4. Evaluations about the quality of a message, from simple token
matching (e.g., a list of preferred DNS domains) to cryptanalysis
(e.g., public/private key work), are at least a little bit
expensive and thus need to be minimized. To that end, performing
those tests at the border MTA is far preferred to doing that work
at each MUA that handles a message. If an ADMD's environment
adheres to common messaging protocols, a reputation query or an
authentication check performed by a border MTA would return the
same result as the same query performed by an MUA. By contrast,
in an environment where the MUA does the work, a message arriving
for multiple recipients would thus cause authentication or
reputation evaluation to be done more than once for the same
message (i.e., at each MUA), 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 and IP address
reputations, etc., can change over time or even become invalid,
and users can take a long time to read a message once delivered.
The value of this work thus degrades, perhaps quickly, once the
delivery process has completed. This seriously diminishes the
value of this work when done other than at MTAs.

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

Appendix E. Changes since RFC 5451

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

o Requested Internet Standard status.

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

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

o Added references to ADSP, ATPS, and VBR.

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

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

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

o Erratum #2818 addressed.

o Erratum #3195 addressed.

o Performed some minor wordsmithing, wordsmithing and removed odd prose.

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

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

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

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

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

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

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

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

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

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

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

Author's Address

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

EMail: superuser@gmail.com