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Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 7601 August 2015
Obsoletes: 7001, 7410
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/rfc7601.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
Kucherawy Standards Track [Page 1]
RFC 7601 Authentication-Results Header Field August 2015
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 . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 6
1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . 7
1.5.3. Email Architecture . . . . . . . . . . . . . . . . . 7
1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 8
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 8
2. Definition and Format of the Header Field . . . . . . . . . . 9
2.1. General Description . . . . . . . . . . . . . . . . . . . 9
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10
2.3. Property Types (ptypes) and Properties . . . . . . . . . 12
2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . 13
2.5. Authentication Identifier Field . . . . . . . . . . . . . 14
2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . 15
2.7. Defined Methods and Result Values . . . . . . . . . . . . 15
2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16
2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . 22
4. Adding the Header Field to a Message . . . . . . . . . . . . 23
4.1. Header Field Position and Interpretation . . . . . . . . 25
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . 26
5. Removing Existing Header Fields . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
6.1. The Authentication-Results Header Field . . . . . . . . . 27
6.2. "Email Authentication Methods" Registry Description . . . 28
6.3. "Email Authentication Methods" Registry Update . . . . . 29
6.4. "Email Authentication Property Types" Registry . . . . . 30
6.5. "Email Authentication Result Names" Description . . . . . 31
6.6. "Email Authentication Result Names" Update . . . . . . . 32
6.7. 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 . . . . . . . 35
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 36
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . 36
Kucherawy Standards Track [Page 2]
RFC 7601 Authentication-Results Header Field August 2015
7.7. Attacks against Authentication Methods . . . . . . . . . 36
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 36
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . 36
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . 37
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 37
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.1. Normative References . . . . . . . . . . . . . . . . . . 37
8.2. Informative References . . . . . . . . . . . . . . . . . 38
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
B.5. Service Provided, Several Authentications Done, Different
MTAs . . . . . . . . . . . . . . . . . . . . . . . . . . 46
B.6. Service Provided, Multi-tiered Authentication Done . . . 48
B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 49
Appendix C. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 50
Appendix D. Changes since RFC 7001 . . . . . . . . . . . . . . . 51
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 53
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 53
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.
Kucherawy Standards Track [Page 3]
RFC 7601 Authentication-Results Header Field August 2015
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 Author Domain Signing Practices ([ADSP]) (Historic)
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])
o DomainKeys ([DOMAINKEYS]) (Historic)
o Sender ID ([SENDERID]) (Experimental)
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.
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.
Kucherawy Standards Track [Page 4]
RFC 7601 Authentication-Results Header Field August 2015
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.
Kucherawy Standards Track [Page 5]
RFC 7601 Authentication-Results Header Field August 2015
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].
Kucherawy Standards Track [Page 6]
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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 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.
Kucherawy Standards Track [Page 7]
RFC 7601 Authentication-Results Header Field August 2015
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.
1.5.4. 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
Kucherawy Standards Track [Page 8]
RFC 7601 Authentication-Results Header Field August 2015
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 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. 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.
Kucherawy Standards Track [Page 9]
RFC 7601 Authentication-Results Header Field August 2015
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
Kucherawy Standards Track [Page 10]
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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, 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 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 2.7.
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].
Kucherawy Standards Track [Page 11]
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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. 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, they indicate from which particular part of the message the
reported data were extracted.
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.
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.
Kucherawy Standards Track [Page 12]
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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-Result field:
... dkim=fail policy.dkim-rules=unsigned-subject ...
In this case, the property is "dkim-rules", indicating some local
check by that name took place and that check returned a result of
"unsigned-subject". These are arbitrary names selected by (and
presumably used within) the 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, but without a complete description or example of intended use.
Kucherawy Standards Track [Page 13]
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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 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.
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.
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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".
2.6. 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.
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2.7.1. 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.
Section 3.8 of [DOMAINKEYS] enumerates some possible results of a
DomainKeys evaluation. Those results are not used when generating
this header field; rather, the results returned are listed below.
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.
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 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
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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".
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 the message
will be treated as if it had not been signed.
Section 3.1 of [DOMAINKEYS] describes a process by which the sending
address of the message is determined. DomainKeys results are thus
reported along with the signing domain name, the sending address of
the message, and the name of the header field from which the latter
was extracted. This means that a DomainKeys result includes a ptype-
property combination of "header.d", plus one of "header.from" and
"header.sender". The sending address extracted from the header is
included with any [MAIL]-style comments removed; moreover, the local-
part of the address and the "@" character are removed if it has not
been authenticated in some way.
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2.7.2. SPF and Sender ID
SPF and Sender ID use the "spf" and "sender-id" method names,
respectively. The result values for SPF are defined in Section 2.6
of [SPF], and those definitions are included here by reference:
+-----------+--------------------------------+
| Code | Meaning |
+-----------+--------------------------------+
| none | [RFC7208], Section 2.6.1 |
+-----------+--------------------------------+
| pass | [RFC7208], Section 2.6.3 |
+-----------+--------------------------------+
| fail | [RFC7208], Section 2.6.4 |
+-----------+--------------------------------+
| softfail | [RFC7208], Section 2.6.5 |
+-----------+--------------------------------+
| policy | RFC 7601, Section 2.4 |
+-----------+--------------------------------+
| neutral | [RFC7208], Section 2.6.2 |
+-----------+--------------------------------+
| temperror | [RFC7208], Section 2.6.6 |
+-----------+--------------------------------+
| permerror | [RFC7208], Section 2.6.7 |
+-----------+--------------------------------+
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".)
The "sender-id" method is described in [SENDERID]. For this method,
the ptype used is "header" and the property will be the name of the
header field from which the Purported Responsible Address (see [PRA])
was extracted -- namely, one of "Resent-Sender", "Resent-From",
"Sender", or "From".
The results for Sender ID are listed and described in Section 4.2 of
[SENDERID], but for the purposes of this specification, the SPF
definitions enumerated above are used instead. Also, [SENDERID]
specifies result codes that use mixed case, but they are typically
used all lowercase in this context.
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For both methods, 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 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".
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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.
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
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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").
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 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)
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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.
2.7.7. 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.
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.
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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.
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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 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 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
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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 (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 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 "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.
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
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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 2.7).
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
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
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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 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 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 tables as
described below. These registry actions were originally defined by
[RFC5451] and updated by [RFC6577] and [RFC7001]. The created
registries are being further updated here to increase their
completeness.
6.1. The Authentication-Results Header Field
[RFC5451] added the 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:
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RFC 7601 Authentication-Results Header Field August 2015
Header field name: Authentication-Results
Applicable protocol: mail ([MAIL])
Status: Standard
Author/Change controller: IETF
Specification document(s): RFC 7601
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 is 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.
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Value: a brief description of the value to be supplied with that
method/ptype/property tuple.
Status: the status of this entry, which is either:
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 changes have been made to this registry per this
document:
1. The "Defined" field has been renamed "Definition", to be
consistent with the other registries in this group.
2. The entry for the "dkim" method, "header" ptype, and "b" property
now reference [RFC6008] as the defining document, and the
reference has be removed from the description.
3. All other "dkim", "domainkeys", "iprev", "sender-id", and "spf"
method entries have had their "Definition" fields changed to
refer to this document, as this document contains a complete
description of the registry and these corresponding values.
4. All "smime" entries have had their "Definition" fields changed to
[SMIME-REG].
5. The "value" field of the "smime" entry using property "smime-
part" has been changed to read: "The MIME body part reference
that contains the S/MIME signature. See Section 3.2.1 of RFC
7281 for full syntax."
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RFC 7601 Authentication-Results Header Field August 2015
6. The single entry for the "auth" method was intended to reflect
the identity indicated by the "AUTH" parameter to the SMTP "MAIL
FROM" command verb. However, there is also an "AUTH" command
verb. To clarify this ambiguity, the entry for the "auth" method
has had its "property" field changed to "mailfrom", and its
"Definition" field changed to this document.
7. The following entry has been added:
Method: auth
Definition: this document (RFC 7601)
ptype: smtp
property: auth
Value: identity confirmed by the AUTH command
Status: active
Version: 1
8. The values of the "domainkeys" entries for ptype "header" have
been updated as follows:
from: contents of the [MAIL] From: header field, after removing
comments, and removing the local-part and following "@" if not
authenticated
sender: contents of the [MAIL] Sender: header field, after
removing comments, and removing the local-part and following
"@" if not authenticated
9. For all entries for "dkim-adsp" and "domainkeys", their Status
values have been changed to "deprecated", reflecting the fact
that the corresponding specifications now have Historic status.
Their "Definition" fields have also been modified to include a
reference to this document.
6.4. "Email Authentication Property Types" Registry
[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:
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RFC 7601 Authentication-Results Header Field August 2015
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 assure 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. References to
[RFC7001] and [RFC7410] have been removed.
6.5. "Email Authentication Result Names" 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. A registry was
created by [RFC5451] for this purpose. [RFC6577] added the "status"
column and [RFC7001] updated the rules governing that registry.
New entries are assigned only for values that have received Expert
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.
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RFC 7601 Authentication-Results Header Field August 2015
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 either:
active: The entry is in current use.
deprecated: The entry is no longer in current use.
6.6. "Email Authentication Result Names" Update
This document includes a complete description of the registry,
obsoleting [RFC7001]. Accordingly, the following changes have been
made to this registry per this document:
o The "Defined" field has been removed.
o The "Meaning" field has been renamed "Specification", as described
above.
o The "Auth Method" field now appears before the "Code" field.
o For easier searching, the table has been arranged such that it is
sorted first by Auth Method, then by Code within each Auth Method
grouping.
o All entries for the "dkim", "domainkeys", "spf", "sender-id",
"auth", and "iprev" methods have had their "Specification" fields
replaced as follows:
dkim: Section 2.7.1 of this document (RFC 7601)
domainkeys: Section 2.7.1 of this document (RFC 7601)
spf: for "hardfail", Section 2.4.2 of [RFC5451]; for all others,
Section 2.7.2 of this document (RFC 7601)
sender-id: for "hardfail", Section 2.4.2 of [RFC5451]; for all
others, Section 2.7.2 of this document (RFC 7601)
auth: Section 2.7.4 of this document (RFC 7601)
iprev: Section 2.7.3 of this document (RFC 7601)
o All entries for "dkim-adsp" that were missing an explicit
reference to a defining document now reference [ADSP] in their
"Specification" fields.
Kucherawy Standards Track [Page 32]
RFC 7601 Authentication-Results Header Field August 2015
o All entries for "dmarc" have had their "Specification" fields
changed to reference Section 11.2 of [DMARC].
o All entries for "dkim-adsp" and "domainkeys" have had their Status
values changed to "deprecated", reflecting the fact that the
corresponding specifications now have Historic status. Their
"Specification" fields have also been modified to include a
reference to this document.
6.7. SMTP Enhanced Status Codes
The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of
the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry" has been updated to refer to this document instead of
[RFC7001].
7. Security Considerations
The following security considerations apply when adding or processing
the 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 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:
Kucherawy Standards Track [Page 33]
RFC 7601 Authentication-Results Header Field August 2015
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, the header field proposed here 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.
Kucherawy Standards Track [Page 34]
RFC 7601 Authentication-Results Header Field August 2015
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
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] 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.
Kucherawy Standards Track [Page 35]
RFC 7601 Authentication-Results Header Field August 2015
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.
Kucherawy Standards Track [Page 36]
RFC 7601 Authentication-Results Header Field August 2015
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., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://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,
<http://www.rfc-editor.org/info/rfc3864>.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
Kucherawy Standards Track [Page 37]
RFC 7601 Authentication-Results Header Field August 2015
[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,
<http://www.rfc-editor.org/info/rfc2045>.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://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, <http://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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc4954>.
[AUTH-ESC]
Kucherawy, M., "Email Authentication Status Codes",
RFC 7372, DOI 10.17487/RFC7372, September 2014,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7489>.
Kucherawy Standards Track [Page 38]
RFC 7601 Authentication-Results Header Field August 2015
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc3464>.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<http://www.rfc-editor.org/info/rfc5598>.
[IANA-CONSIDERATIONS]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://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,
<http://www.rfc-editor.org/info/rfc1939>.
[PRA] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006,
<http://www.rfc-editor.org/info/rfc4407>.
Kucherawy Standards Track [Page 39]
RFC 7601 Authentication-Results Header Field August 2015
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451,
DOI 10.17487/RFC5451, April 2009,
<http://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,
<http://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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7410>.
[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,
<http://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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7281>.
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[SPF] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<http://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,
<http://www.rfc-editor.org/info/rfc5518>.
Kucherawy Standards Track [Page 41]
RFC 7601 Authentication-Results Header Field August 2015
Appendix A. 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 (i.e., while this
proposal and its successors are being 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: Trivial Case
Kucherawy Standards Track [Page 42]
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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 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.
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.
Kucherawy Standards Track [Page 43]
RFC 7601 Authentication-Results Header Field August 2015
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.
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RFC 7601 Authentication-Results Header Field August 2015
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;
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 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 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 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.com,
producing "pass" results from the SPF and Sender ID checks.
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RFC 7601 Authentication-Results Header Field August 2015
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;
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 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.
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RFC 7601 Authentication-Results Header Field August 2015
The second MTA, 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 from a user on a dial-up connection 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 has not granted 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 published public keys and used it to sign the message.
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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.
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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
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Appendix C. 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), 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.
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RFC 7601 Authentication-Results Header Field August 2015
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 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 7001
o Applied RFC 7410.
o Updated all references to RFC 4408 with RFC 7208.
o Added section explaining "property" values. (Addressed Erratum
#4201.)
o Did some minor text reorganization.
o Gave registry history -- enough that this is now the authoritative
registry definition.
o Added text explaining each of the method-ptype-property tuples
registered by this document.
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RFC 7601 Authentication-Results Header Field August 2015
o Changed the meaning of the "Defined" column of the methods
registry to be the place where each entry was created and
described; it is expected that this will then refer to the
method's defining document. Provided IANA with corresponding
update instructions.
o Cleaned up registry structure and content, and replaced all
references to RFC 7001 with pointers to this document.
o Added references: [DMARC], [PRA], [RFC6008], [RFC6577], [RRVS],
[SMIME-REG].
o Added description of values that can be extracted from SMTP AUTH
sessions and an example.
o Provided much more complete descriptions of reporting DomainKeys
results.
o Added more detail about Sender ID.
o Marked all ADSP and DomainKeys entries as deprecated since their
defining documents are as well.
o Reworked some text around ignoring unknown ptypes.
o Completely described the ptypes registry.
o Mentioned that EHLO is mapped to HELO for SPF.
o RFC 7208 uses all-lowercase result strings now, so adjusted prose
accordingly.
o Updated list of supported methods, and mentioned the registries
immediately below.
o Mentioned that when a local-part is removed, the "@" goes with it.
o Referred to RFC 7328 in the "iprev" definition.
o Corrected the "smime-part" prose.
o Updated examples that use SMTP AUTH to claim "with ESMTPA" in the
Received fields.
o Made minor editorial adjustments.
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Acknowledgments
The author wishes to acknowledge the following individuals for their
review and constructive criticism of this document: Stephane
Bortzmeyer, Scott Kitterman, John Levine, Tom Petch, and Pete
Resnick.
Author's Address
Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
United States
Email: superuser@gmail.com
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