Network Working Group M. Kucherawy
Request for Comments: 5451 Sendmail, Inc.
Category: Standards Track April 2009
Message Header Field for Indicating Message Authentication Status
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This memo defines a new header field 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), may use this message header field to relay that information
in a convenient way to users or to make sorting and filtering
decisions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 5
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
1.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 5
1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 5
1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 6
1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 7
2. Definition and Format of the Header Field . . . . . . . . . . 8
2.1. General Description . . . . . . . . . . . . . . . . . . . 8
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 8
2.3. Authentication Identifier Field . . . . . . . . . . . . . 10
2.4. Result Values . . . . . . . . . . . . . . . . . . . . . . 12
2.4.1. DKIM and DomainKeys Results . . . . . . . . . . . . . 12
2.4.2. SPF and Sender-ID Results . . . . . . . . . . . . . . 13
2.4.3. "iprev" Results . . . . . . . . . . . . . . . . . . . 14
2.4.4. SMTP AUTH Results . . . . . . . . . . . . . . . . . . 14
2.4.5. Extension Result Codes . . . . . . . . . . . . . . . . 15
2.5. Authentication Methods . . . . . . . . . . . . . . . . . . 15
2.5.1. Definition of Initial Methods . . . . . . . . . . . . 16
2.5.2. Extension Methods . . . . . . . . . . . . . . . . . . 16
3. The "iprev" Authentication Method . . . . . . . . . . . . . . 17
4. Adding the Header Field to A Message . . . . . . . . . . . . . 18
4.1. Header Field Position and Interpretation . . . . . . . . . 19
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 20
5. Removing the Header Field . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6.1. The Authentication-Results Header Field . . . . . . . . . 22
6.2. Email Authentication Method Name Registry . . . . . . . . 22
6.3. Email Authentication Result Name Registry . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 26
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 27
7.3. Header Field Position . . . . . . . . . . . . . . . . . . 28
7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 28
7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 28
7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 28
7.7. Attacks against Authentication Methods . . . . . . . . . . 28
7.8. Intentionally Malformed Header Fields . . . . . . . . . . 29
7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 29
7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 29
7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 29
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8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Normative References . . . . . . . . . . . . . . . . . . . 30
8.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 32
Appendix B. Authentication-Results Examples . . . . . . . . . . . 33
B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 33
B.2. Nearly Trivial Case; Service Provided, But No
Authentication Done . . . . . . . . . . . . . . . . . . . 34
B.3. Service Provided, Authentication Done . . . . . . . . . . 35
B.4. Service Provided, Several Authentications Done, Single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
B.5. Service Provided, Several Authentications Done,
Different MTAs . . . . . . . . . . . . . . . . . . . . . . 37
B.6. Service Provided, Multi-Tiered Authentication Done . . . . 39
Appendix C. Operational Considerations about Message
Authentication . . . . . . . . . . . . . . . . . . . 41
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
This memo defines a new header field for electronic mail messages
that presents the results of a message authentication effort in a
machine-readable format. The intent 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 integrity of its
content.
End users are not expected to be direct consumers of this header
field. This header field is intended for consumption by programs
that will then use or render such data in a human-usable form.
This memo defines both the format of this new header field and
discusses the implications of its presence or absence. However, it
does not discuss how the data contained in the header field should be
used (i.e. what filtering decisions are appropriate, or how an MUA
might render these results) as these are local policy and/or user
interface design questions that are not appropriate for this memo.
At the time of publication of this memo, [AUTH], [DKIM],
[DOMAINKEYS], [SENDERID], and [SPF] are published DNS domain-level
email authentication methods in common use. This proposal is not
intended to be restricted to domain-based authentication, but this
has proven to be a good starting point for implementations. As
various methods emerge, it is necessary to prepare for their
appearance and encourage convergence in the area of interfacing
verifiers to filters and MUAs.
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Although [SPF] defined a header field called Received-SPF and
[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.
1.1. Purpose
The header field defined in this memo is expected to serve several
purposes:
1. Convey the results of various message authentication checks being
applied by upstream filters and Mail Transfer Agents (MTAs) to
MUAs and downstream filters within the same "trust domain", as
such agents may wish to render those results to end users or use
that 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 should not be
construed as meaning that its data is valid, but rather that it is
asserting validity based on one or more authentication schemes
somewhere upstream. For an MUA or downstream filter to treat the
assertions as actually valid, there must be an assessment of the
trust relationship between such agents and the validating MTA.
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 creator of the header field to the
consumer must occur within a context of trust that the creator's
information is correct. How this trust is obtained is outside the
scope of this document. It is entirely a local matter.
Thus, this document defines a "trust boundary" as the delineation
between "external" and "internal" entities; "external" here includes
all hosts that do not deliberately provide some kind of messaging
service for the receiving ADMD's users, and "internal" includes those
hosts that do. By this definition, the hosts within a "trust
boundary" may lie entirely within a receiving ADMD's direct control,
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or they can include hosts managed by another ADMD (such as an ISP or
commercial filtering service) but that also provide services for the
former.
1.3. Processing Scope
This proposal is intended to address the needs of authenticating
messages or properties of messages during their actual transport. It
is not meant to address the security of messages that might be
encapsulated within other messages, such as a message/rfc822 [MIME]
part within a message.
1.4. Requirements
This memo establishes no new requirements on existing protocols or
servers.
In particular, this memo establishes no requirement on MTAs to reject
or filter arriving messages that do not pass authentication checks.
The data conveyed by the defined header field's contents are for the
information of MUAs and filters and should be used at their
discretion.
1.5. Definitions
This section defines various terms used throughout this document.
1.5.1. General
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
1.5.2. Security
[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.
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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 [SENDERID] 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
[SMTP] client to relay messages on its behalf but do not actually
validate any property of the message itself. By contrast, [DKIM] is
agnostic as to the routing of a message but uses cryptographic
signatures to authenticate agents claiming responsibility for the
message (which implies authorization) and ensure it was 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), or both.
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 an MTA that handles messages after a
border MTA and before a delivery MTA.
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The following diagram illustrates the flow of mail among these
defined components:
+-----+ +-----+ +------------+
| 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" may 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 from this specification.
See [EMAIL-ARCH] for further discussion on general email system
architecture, and Appendix C of this memo for discussion about the
common aspects of email authentication in current environments.
1.6. Trust Environment
This new header field permits one or more message validation
mechanisms to communicate its output to one or more separate
assessment mechanisms. These mechanisms operate within a unified
trust boundary that defines an Administrative Management Domain
(ADMD). An ADMD contains one or more entities that perform
validation and generate the header field, and one or more that
consume it for some type of assessment. The field contains no
integrity or validation mechanism of its own, so its presence must be
trusted implicitly. Hence, use of the header field depends upon
ensuring that mail entering the ADMD has instances of the header
field claiming to be valid within its boundaries removed, so that
occurrences of such header fields can be used safely by consumers.
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The "authserv-id" token defined in Section 2.2 can be used to label
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 within this proposal.
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 new header field being defined here is called "Authentication-
Results". It is a Structured Header Field as defined in [MAIL] and
thus all of the related definitions in that document apply.
This new header field SHOULD be 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 therefore should be
treated as 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 [MAIL] comments)
consists of an authentication identifier, an optional version, and
then a series of "method=result" statements indicating which
authentication method(s) were applied and their respective results,
and then, for each applied method, an optional "reason" string plus
optional "property=value" statements indicating which message
properties were evaluated to reach that conclusion.
The header field MAY appear more than once in a single message, or
more than one result MAY be represented in a single header field, or
a combination of these MAY be applied.
2.2. Formal Definition
Formally, the header field is specified as follows using [ABNF]:
authres-header = "Authentication-Results:" [CFWS] authserv-id
[ CFWS version ]
( [CFWS] ";" [CFWS] "none" / 1*resinfo ) [CFWS] CRLF
; the special case of "none" is used to indicate that no
; message authentication is performed
authserv-id = dot-atom
; see below for a description of this element
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version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use;
; this specification is version "1"; the absence of a
; version implies this version of the specification
resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*( CFWS propspec )
methodspec = [CFWS] method [CFWS] "=" [CFWS] result
; indicates which authentication method was evaluated
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 and would be
; useful to reveal to end users as authenticated
method = dot-atom [ [CFWS] "/" [CFWS] 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
result = dot-atom
; indicates the results of the attempt to authenticate
; the message; see below for details
ptype = "smtp" / "header" / "body" / "policy"
; indicates whether the property being evaluated was
; a parameter to an [SMTP] command, or was a value taken
; from a message header field, or was some property of
; the message body, or some other property evaluated by
; the receiving MTA
property = dot-atom
; if "ptype" is "smtp", this indicates which [SMTP]
; command provided the value that was evaluated by the
; authentication scheme being applied; if "ptype" is
; "header", this indicates from which header field the
; value being evaluated was extracted; if "ptype" is
; "body", this indicates the offset into the body at which
; content of interest was detected; if "ptype" is "policy"
; then this indicates the name of the policy that caused
; this header field to be added (see below)
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pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
[CFWS]
; the value extracted from the message property defined
; by the "ptype.property" construction; if the value
; identifies something intended to be an e-mail identity,
; then it MUST use the right hand portion of this ABNF
; definition
The "local-part" is as defined in Section 3.4.1, and "dot-atom" is
defined in Section 3.2.3, of [MAIL].
The "value" is as defined in Section 5.1 of [MIME].
The "domain-name" is as defined in Section 3.5 of [DKIM].
The "dot-atom" used in a "result" above is further constrained by the
necessity of being enumerated in Section 2.4 or an amendment to it.
See Section 2.3 for a description of the "authserv-id" element.
The list of commands eligible for use with the "smtp" ptype can be
found in [SMTP] and subsequent amendments.
"CFWS" is as defined in Section 3.2.2 of [MAIL].
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.
The "ptype" and "property" values used by each authentication method
should be defined in the specification for that method (or its
amendments).
The "ptype" and "property" are case-insensitive.
A "ptype" value of "policy" indicates a policy decision about the
message not specific to a property of the message that could be
extracted. For example, if a method would normally report a
"ptype.property" of "header.From" and no From: header field was
present, the method can use "policy" to indicate that no conclusion
about the authenticity of the message could be reached.
2.3. Authentication Identifier Field
Every Authentication-Results header field has an authentication
identifier field ("authserv-id" above). This is similar in syntax to
a fully-qualified domain name.
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The authentication identifier field provides a unique identifier that
refers to the authenticating service within a given ADMD. The
uniqueness of the identifier MUST be guaranteed by the ADMD that
generates it and must pertain to exactly that one ADMD. This
identifier is intended to be machine-readable and not necessarily
meaningful to users. MUAs or downstream filters SHOULD use this
identifier to determine whether or not the data contained in an
Authentication-Results header field should be used.
For simplicity and scalability, the authentication identifier SHOULD
be a common token used throughout the ADMD, such as the DNS domain
name used by or within that ADMD.
For tracing and debugging purposes, the authentication identifier MAY
instead be the hostname of the MTA performing the authentication
check whose result is being reported. This is also useful for
another purpose, as described in Section 4. Moreover, some
implementations have considered appending a delimiter such as "/" and
following it with useful transport tracing data such as the [SMTP]
queue ID or a timestamp.
It should be noted, however, that using a local, relative identifier
like a single 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
must use the identifier to determine whether to use the contents of
the header field:
o Changes to the identifier impose a large, centralized
administrative burden.
o Ongoing administrative changes require constantly updating this
centralized table, making it difficult to ensure that an MUA or
downstream filter will have access to accurate information for
assessing the usability of the header field's content. In
particular, consumers of the header field will need to know not
only the current identifier(s) in use, but previous ones as well
to account for delivery latency or later re-assessment of the
header field's contents.
Examples of valid authentication identifiers are "example.com",
"mail.example.org", "ms1.newyork.example.com", and "example-auth".
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2.4. Result Values
Each individual authentication method returns one of a set of
specific result values. The subsections below define these results
for the authentication methods specifically supported by this memo,
and verifiers SHOULD use these values as described below. New
methods not specified in this document intended to be supported by
the header field defined in this memo MUST include a similar result
table either in its defining memo or in a supplementary one.
2.4.1. DKIM and DomainKeys Results
The result values used by [DKIM] and [DOMAINKEYS] are as follows:
none: The message was not signed.
pass: The message was signed, the signature or signatures were
acceptable to the verifier, and the signature(s) passed
verification tests.
fail: The message was signed and the signature or signatures were
acceptable to the verifier, but they failed the verification
test(s).
policy: The message was signed but the signature or signatures were
not acceptable to the verifier.
neutral: The message was signed but the signature or signatures
contained syntax errors or were not otherwise able to be
processed. This result SHOULD also be 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.
A signature is "acceptable to the verifier" if it passes local policy
checks (or there are no specific local policy checks). For example,
a verifier 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.
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[DKIM] advises that if a message fails verification, it should be
treated as an unsigned message. A report of "fail" here permits the
receiver of the report to decide how to handle the failure. A report
of "neutral" or "none" preempts that choice, ensuring the message
will be treated as if it had not been signed.
2.4.2. SPF and Sender-ID Results
The result values are used by [SPF] and [SENDERID] as follows:
none: No policy records were published at the sender's DNS domain.
neutral: The sender's ADMD has asserted that it cannot or does not
want to assert whether or not the sending IP address is authorized
to send mail using the sender's DNS domain.
pass: The client is authorized by the sender's ADMD to inject or
relay mail on behalf of the sender's DNS domain.
policy: The client is 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.
hardfail: This client is explicitly not authorized to inject or
relay mail using the sender's DNS domain.
softfail: The sender's ADMD believes the client was not authorized
to inject or relay mail using the sender's DNS domain, but is
unwilling to make a strong assertion to that effect.
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 policy record from DNS. 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 or
a syntax error in a retrieved DNS TXT record. A later attempt is
unlikely to produce a final result.
The distinction between and interpretation of "none" and "neutral"
under these methods is discussed further in [SPF].
The "policy" result would be returned if, for example, [SPF] returned
as "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).
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If the retrieved sender policies used to evaluate [SPF] and
[SENDERID] do not contain explicit provisions for authenticating the
local-part (see Section 3.4.1 of [MAIL]) of an address, the "pvalue"
reported along with results for these mechanisms SHOULD NOT include
the local-part.
2.4.3. "iprev" Results
The result values are 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.
There is no "none" for this method since any TCP connection
delivering email has an IP address associated with it, so some kind
of evaluation will always be possible.
For discussion of the format of DNS replies, see [DNS].
2.4.4. SMTP AUTH Results
The result values are used by the [AUTH] method are as follows:
none: SMTP authentication was not attempted.
pass: The SMTP client had authenticated to the server reporting the
result using the protocol described in [AUTH].
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fail: The SMTP client had attempted to authenticate to the server
using the protocol described in [AUTH] but was not successful, yet
continued to send the message about which a result is being
reported.
temperror: The SMTP client attempted to authenticate using the
protocol described in [AUTH] but was not able to complete the
attempt due to some error which is likely transient in nature,
such as a temporary Lightweight Directory Access Protocol (LDAP)
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 LDAP lookup error. A later attempt is not
likely produce a final result.
Note that an agent making use of the data provided by this header
field SHOULD consider "fail" and "temperror" to be the synonymous in
terms of message authentication, i.e., the client did not
authenticate.
2.4.5. Extension Result Codes
Additional result codes (extension results) might be defined in the
future by later revisions or extensions to this specification.
Extension results beginning with "x-" will never be defined as
standard fields; such names are reserved for experimental use.
Result codes not beginning with "x-" MUST be registered with the
Internet Assigned Numbers Authority (IANA) and published in an RFC.
See Section 6 for further details.
Implementations reporting new result codes MUST use the "x-" prefix
until such time as the new method is registered by IANA.
Extension results MUST only be used within ADMDs that have explicitly
consented to use them. These results and the parameters associated
with them are not 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
extension result.
2.5. Authentication Methods
This section defines the supported authentication methods and
discusses the proper means for applying experimental and other
extension methods.
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2.5.1. Definition of Initial Methods
As they are currently existing specifications for message
authentication, it is appropriate to define an authentication method
identifier for each of [AUTH], [DKIM], [DOMAINKEYS], [SENDERID], and
[SPF]. Therefore, the authentication method identifiers "auth",
"dkim", "domainkeys", "sender-id", and "spf", respectively are hereby
defined for MTAs applying those specifications for email message
authentication.
Furthermore, method "iprev" is defined in Section 3.
See Section 6 for details.
2.5.2. Extension Methods
Additional authentication method identifiers (extension methods) may
be defined in the future by later revisions or extensions to this
specification. Extension methods beginning with "x-" will never be
defined as standard fields; such names are reserved for experimental
use. Method identifiers not beginning with "x-" MUST be registered
with the Internet Assigned Numbers Authority (IANA) and published in
an RFC. See Section 6 for further details.
Extension methods may 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 should reflect the name of the method being
defined, but should 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".
Implementations of new methods MUST use the "x-" prefix until such
time as the new method is registered by IANA.
Authentication method implementors are encouraged to provide adequate
information, via [MAIL] 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 method identifier.
3. The "iprev" Authentication Method
This section defines an additional authentication method called
"iprev".
In general, "iprev" is an attempt to verify that a client appears to
be valid based on some DNS queries. 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
should result in at least one mapping back to the client's IP
address.
More algorithmically: if the client peer's IP address is I, the list
of names to which I maps (after a "PTR" query) is the set N, and the
union of IP addresses to which each member of N maps (after
corresponding "A" and "AAAA" queries) is L, then this test is
successful if I is an element of L.
The response to a PTR query could contain multiple names. To prevent
heavy DNS loads, agents performing these queries MUST be implemented
such that the number of names evaluated by generation of
corresponding A or AAAA queries is finite, though it MAY be
configurable by an administrator. As an example, Section 5.5 of
[SPF] chose a limit of 10 for its implementation of this algorithm.
[DNS-IP6] discusses the query formats for the IPv6 case.
A successful test using this algorithm constitutes a result of "pass"
since the ADMD in which the client's PTR claims it belongs has
confirmed that claim by including corresponding data in its DNS
domain. A failure to match constitutes a "fail". There is no case
in which a "neutral" result can be returned. The remaining
"temperror" and "permerror" cases refer, respectively, to temporary
and permanent DNS query errors.
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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 actual
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 [DNSOP-REVERSE]. In particular, it recommends that
applications avoid using this test as a means of authentication or
security. Its presence in this memo is not an endorsement, but is
merely acknowledgement that the method remains common and provides
the means to relay the results of that test.
4. Adding the Header Field to A Message
This specification makes no attempt to evaluate the relative
strengths of various message authentication methods that may become
available. As such, the order of the presented authentication
methods and results MUST NOT be used either to imply or infer the
importance or strength of any given method over another. Instead,
the MUA or downstream filter consuming this header field must
interpret the result of each method based on its own knowledge of
what that method evaluates.
Each "method" MUST refer to an authentication method declared in the
IANA registry, or an extension method as defined in Section 2.5.2,
and each "result" MUST refer to a result code declared in the IANA
registry, or an extension result code as defined in Section 2.4.5.
See Section 6 for further information about the registered methods
and result codes.
An MTA compliant with this specification MUST add 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 MUST add 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 to indicate explicitly that no message
authentication schemes were applied prior to delivery of this
message.
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An MTA adding this header field must take steps to identify it as
legitimate to the MUAs or downstream filters that will ultimately
consume its content. One required process to do so is described in
Section 5. Further measures may be required in some environments.
Some possible solutions are enumerated in Section 7.1. This memo
does not mandate any specific solution to this issue as each
environment has its own facilities and limitations.
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 allows easy detection of
header fields that can be trusted.
End users making direct use of this header field may inadvertently
trust information that has not been properly vetted. If, for
example, a basic [SPF] result were to be relayed that claims an
authenticated addr-spec, the local-part of that addr-spec has
actually not been authenticated. Thus, an MTA adding this header
field SHOULD NOT include any data that has not been authenticated by
the method(s) being applied. Moreover, MUAs SHOULD NOT render to
users such information if it is presented by a method known not to
authenticate it.
4.1. Header Field Position and Interpretation
In order to ensure non-ambiguous results and avoid the impact of
false header fields, MUAs and downstream filters SHOULD NOT interpret
this header field unless specifically instructed to do so by the user
or administrator. That is, this interpretation should not be "on by
default". Naturally then, users or administrators should not
activate such a feature unless they are certain the header field will
be added by the border MTA that accepts the mail that is ultimately
read by the MUA, and instances of the header field appearing to be
from within the ADMD but actually added by foreign MTAs will be
removed before delivery.
Furthermore, MUAs and downstream filters SHOULD NOT interpret this
header field unless the authentication 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 result code registry, or a "ptype" not
listed in the corresponding registry for such values as defined in
Section 6. Moreover, such agents MUST ignore a result indicated for
any "method" they do not specifically support.
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An MUA SHOULD NOT reveal these results to end users unless the
results are accompanied by, at a minimum, some associated reputation
data about the authenticated origin identifiers within the message.
For example, an attacker could register examp1e.com (note the digit
"one") and send signed mail to intended victims; a verifier would
detect that the signature was valid and report a "pass" even though
it's clear the DNS domain name was intended to mislead. See
Section 7.2 for further discussion.
As stated in Section 2.1, this header field SHOULD 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.
MUAs SHOULD ignore instances of this header field discovered within
message/rfc822 [MIME] attachments.
Further discussion of this can be found in Section 7 below.
4.2. Local Policy Enforcement
If a site's local policy is to consider a non-recoverable failure
result (e.g., "fail" for DKIM, "hardfail" for SPF) for any particular
authentication method as justification to reject the message
completely, the border MTA SHOULD issue an [SMTP] rejection response
to the message rather than adding this header field with the failure
result and allowing it 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
[DSN] to a forged originator.
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 2.4).
Such rejections at the SMTP protocol level are not possible if local
policy is enforced at the MUA and not the MTA. Unfortunately, this
may be a common scenario.
5. Removing the Header Field
For security reasons, any MTA conforming to this specification MUST
delete any discovered instance of this header field that claims to
have been added within its trust boundary and that did not come from
another trusted MTA. For example, an MTA (border or otherwise) for
example.com receiving a message MUST delete any instance of this
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header field bearing an authentication identifier indicating the
header field was added within example.com prior to adding its own
header fields. This may 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 MAY remove all
instances of this header field on mail crossing into its trust
boundary. However, this may conflict with the desire to access
authentication results performed by trusted external service
providers. It may also invalidate signed messages whose signatures
cover external instances of this header field. A more robust border
MTA could allow a specific list of authenticating MTAs whose
information should be let in, removing 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 might 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 may invalidate
one or more signatures on the message if they covered the header
field to be removed at the time of signing. This behavior can be
desirable since there's little value in validating the signature on a
message with forged headers. 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 if the [SMTP] connection relaying the
message is not from a trusted internal MTA.
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6. IANA Considerations
IANA has registered a new header field and created two new tables as
described below.
6.1. The Authentication-Results Header Field
Per [IANA-HEADERS], the "Authentication-Results" header field has
been added to the IANA Permanent Message Header Field Registry. The
following is the registration template:
Header field name: Authentication-Results
Applicable protocol: mail ([MAIL])
Status: Standard
Author/Change controller: IETF
Specification document(s): RFC 5451
Related information:
Requesting review of any proposed changes and additions to
this field is recommended.
6.2. Email Authentication Method Name Registry
Names of message authentication methods supported by this
specification must be registered with IANA, with the exception of
experimental names as described in Section 2.5.2.
New entries are assigned only for values that have been documented in
a published RFC that has had IETF Review, per [IANA-CONSIDERATIONS].
Each method must register a name, the specification that defines it,
one or more "ptype" values appropriate for use with that method,
which "property" value(s) should be reported by that method, and a
description of the "value" to be used with each.
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The initial set of entries in this registry is as follows:
+------------+----------+--------+----------------+--------------------+
| Method | Defined | ptype | property | value |
+------------+----------+--------+----------------+--------------------+
| auth | RFC4954 | smtp | auth | AUTH parameter of |
| | | | | the SMTP MAIL |
| | | | | command |
+------------+----------+--------+----------------+--------------------+
| dkim | RFC4871 | header | d | value of |
| | | | | signature "d" tag |
| | | +----------------+--------------------+
| | | | i | value of |
| | | | | signature "i" tag |
+------------+----------+--------+----------------+--------------------+
| domainkeys | RFC4870 | header | d | value of |
| | | | | signature "d" tag |
| | | +----------------+--------------------+
| | | | from | value of From |
| | | | | header field after |
| | | | | removing comments |
| | | | | and local-part if |
| | | | | not authenticated |
| | | +----------------+--------------------+
| | | | sender | value of Sender |
| | | | | header field after |
| | | | | removing comments |
| | | | | and local-part if |
| | | | | not authenticated |
+------------+----------+--------+----------------+--------------------+
| iprev | this | policy | iprev | client IP address |
| | document | | | |
+------------+----------+--------+----------------+--------------------+
| sender-id | RFC4406 | header | name of header | value of header |
| | | | field used by | field used by PRA |
| | | | the Purported | after removing |
| | | | Responsible | comments and parts |
| | | | Address (PRA) | not authenticated |
+------------+----------+--------+----------------+--------------------+
| spf | RFC4408 | smtp | mailfrom | envelope sender |
| | | | | after removing |
| | | | | parts not |
| | | | | authenticated |
| | +--------+----------------+--------------------+
| | | smtp | helo | HELO/EHLO value |
+------------+----------+--------+----------------+--------------------+
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6.3. Email Authentication Result Name Registry
Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.4.5.
New entries are assigned only for result codes that have been
documented in a published RFC that has had IETF Review, per
[IANA-CONSIDERATIONS]. Each code must register a name, the document
that establishes the registration, the authentication method(s) that
uses it, and either a definition of the semantics of its use or a
reference to the place where those semantics are defined.
The initial set of entries in this registry is as follows:
+-----------+----------+----------------+------------------------------+
| Code | Defined | Auth Method(s) | Meaning |
+-----------+----------+----------------+------------------------------+
| none | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | auth | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
| pass | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.3 |
| | +----------------+------------------------------+
| | | auth | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
| fail | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.3 |
| | +----------------+------------------------------+
| | | auth | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
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| policy | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
+-----------+----------+----------------+------------------------------+
| neutral | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
+-----------+----------+----------------+------------------------------+
| temperror | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.3 |
| | +----------------+------------------------------+
| | | auth | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
| permerror | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.2 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.3 |
| | +----------------+------------------------------+
| | | auth | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
| hardfail | this | spf | section 2.4.2 |
| | document | sender-id | |
+-----------+----------+----------------+------------------------------+
| softfail | this | spf | section 2.4.2 |
| | document | sender-id | |
+-----------+----------+----------------+------------------------------+
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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 mail is 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.
It is for this reason an MUA should not 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 alternate solutions to this problem are nascent:
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 memo 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.
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
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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 host names 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 planned 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 memo.
Mitigation of the forged header field attack can also be accomplished
by moving the authentication results data into meta-data associated
with the message. In particular, an [SMTP] extension could be
established which is used to communicate authentication results from
the border MTA to intermediate and delivery MTAs; the latter of these
could arrange to store the authentication results as meta-data
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.
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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 enroute by intermediate MTAs. The goal of requiring header
field addition only at the top of a message is an acknowledgement
that some MTAs do reorder header fields, but most do not. Thus, in
the general case, there will be some indication of which MTAs (if
any) handled the message after the addition of the header field
defined here.
7.4. Reverse IP Query Denial-of-Service Attacks
Section 5.5 of [SPF] describes a DNS-based denial-of-service attack
for verifiers that attempt DNS-based identity verification of
arriving client connections. A verifier wishing to do this check and
report this information SHOULD 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 memo SHOULD be
aware of the algorithm used by the verifier reporting the result and
thus be aware of its limitations.
7.5. Mitigation of Backscatter
Failing to follow the instructions of Section 4.2 can result in a
denial-of-service attack caused by the generation of [DSN] messages
(or equivalent) to addresses that did not send the messages being
rejected.
7.6. Internal MTA Lists
Section 5 describes a procedure for scrubbing headers 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 (and later
amendments to it) is also a security consideration here.
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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. Implementors must thoroughly verify all such header fields
received from MTAs and be robust against intentionally as well as
unintentionally malformed header fields.
7.9. Compromised Internal Hosts
An internal MUA or MTA that has been compromised could generate mail
with a forged From header field and a forged Authentication-Results
header field that endorses it. Although it is clearly a larger
concern to have compromised internal machines than it is to prove the
value of this header field, this risk can be mitigated by arranging
that internal MTAs will remove this header field if it claims to have
been added by a trusted border MTA (as described above), yet the
[SMTP] connection is not coming from an internal machine known to be
running an authorized MTA. However, in such a configuration,
legitimate MTAs will have to add this header field when legitimate
internal-only messages are generated. This is also covered in
Section 5.
7.10. Encapsulated Instances
[MIME] messages may contain attachments of type "message/rfc822",
which contain other [MAIL] messages. Such an encapsulated message
may 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.
Implementors 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".
Kucherawy Standards Track [Page 29]
RFC 5451 Authentication-Results Header Field April 2009
8. References
8.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68,
RFC 5234, January 2008.
[IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul,
"Registration Procedures for Message Header
Fields", BCP 90, RFC 3864, September 2004.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[MAIL] Resnick, P., Ed., "Internet Message Format",
RFC 5322, October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose
Internet Mail Extensions (MIME) Part One:
Format of Internet Message Bodies", RFC 2045,
November 1996.
8.2. Informative References
[AUTH] Siemborski, R. and A. Melnikov, "SMTP Service
Extension for Authentication", RFC 4954,
July 2007.
[DKIM] Allman, E., Callas, J., Delany, M., Libbey,
M., Fenton, J., and M. Thomas, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 4871,
May 2007.
[DNS] Mockapetris, P., "Domain names -
implementation and specification", STD 13,
RFC 1035, November 1987.
[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M.
Souissi, "DNS Extensions to Support IP Version
6", RFC 3596, October 2003.
[DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for
the use of DNS Reverse Mapping", Work
in Progress, March 2008.
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[DOMAINKEYS] Delany, M., "Domain-Based Email Authentication
Using Public Keys Advertised in the DNS
(DomainKeys)", RFC 4870, May 2007.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible
Message Format for Delivery Status
Notifications", RFC 3464, January 2003.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture",
Work in Progress, October 2008.
[IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 5226, May 2008.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL
- VERSION 4rev1", RFC 3501, March 2003.
[POP3] Myers, J. and M. Rose, "Post Office Protocol -
Version 3", STD 53, RFC 1939, May 1996.
[SECURITY] Rescorla, E. and B. Korver, "Guidelines for
Writing RFC Text on Security Considerations",
BCP 72, RFC 3552, July 2003.
[SENDERID] Lyon, J. and M. Wong, "Sender ID:
Authenticating E-Mail", RFC 4406, April 2006.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.
[SPF] Wong, M. and W. Schlitt, "Sender Policy
Framework (SPF) for Authorizing Use of Domains
in E-Mail, Version 1", RFC 4408, April 2006.
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Appendix A. Legacy MUAs
Implementors of this proposal should be aware that many MUAs are
unlikely to be retrofitted to support the new header field and its
semantics. In the interests of convenience and quicker adoption, a
delivery MTA might want to consider adding things that are processed
by existing MUAs in addition to the Authentication-Results header
field. One suggestion is to include a Priority header field, on
messages that don't already have such a header field, containing a
value that reflects the strength of the authentication that was
accomplished, e.g., "low" for weak or no authentication, "normal" or
"high" for good or strong authentication.
Some modern MUAs can already filter based on the content of this
header field. However, there is keen interest in having MUAs make
some kind of graphical representation of this header field's meaning
to end users. Until this capability is added, other interim means of
conveying authentication results may be necessary while this proposal
and its successors are adopted.
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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
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.
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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; 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 and result tokens) indicates that no message
authentication was done.
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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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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.com;
spf=pass smtp.mailfrom=example.com
Authentication-Results: example.com;
sender-id=pass header.from=example.com
Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6)
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@example.net
From: sender@example.com
Message-Id: <12345.abc@example.com>
Subject: here's a sample
Hello! Goodbye!
Example 4: Headers reporting results from one MTA
The "Authentication-Results" header field is present, indicating 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 [SENDERID] 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
dialup connection (example.net) sends mail to a border MTA
(example.com) using SMTP authentication to prove identity. The
dialup provider has been explicitly authorized to relay mail as
"example.com" resulting in passes by the SPF and SenderID checks.
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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=hardfail header.from=example.com;
dkim=pass (good signature) header.i=sender@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
Authentication-Results: example.com;
auth=pass (cram-md5) smtp.auth=sender@example.com;
spf=hardfail smtp.mailfrom=example.com
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com;
i=sender@example.com; t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
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, "mail-router.example.com" reports
that [AUTH] and [SPF] were both used, and [AUTH] passed but [SPF]
failed. In the [AUTH] case, additional data is provided in the
comment field, which the MUA can choose to render if desired.
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The second MTA, "auth-checker.example.com", reports that it did a
[SENDERID] 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.
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 dialup connection "example.net". The
user appears to be legitimate as he/she had a valid password allowing
authentication at the border MTA using [AUTH]. The [SPF] and
[SENDERID] 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 (good signature) header.i=@mail-router.example.net;
dkim=fail (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
with ESMTP id i7PK0sH7021929;
Fri, Feb 15 2002 17:19:22 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=furble;
d=mail-router.example.net; t=1188964198; c=relaxed/simple;
h=From:Date:To:Message-Id:Subject:Authentication-Results;
bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=;
b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
Authentication-Results: example.net;
dkim=pass (good signature) header.i=@newyork.example.com
Received: from smtp.newyork.example.com
(smtp.newyork.example.com [192.0.2.220])
by mail-router.example.net (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample
Example 6: Headers reporting results from multiple MTAs in different
ADMDs
In this example we see multi-tiered authentication with an extended
trust boundary.
The message was sent from someone at example.com's New York office
(newyork.example.com) to a mailing list managed at an intermediary.
The message was signed at the origin using [DKIM].
Kucherawy Standards Track [Page 39]
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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
that 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.
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Appendix C. Operational Considerations about Message Authentication
This proposal 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 the work of this proposal and its deployment to
date is 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, rejections of messages at the SMTP level
rather than generating a DSN internally. Thus, doing any of the
authentication or reputation work exclusively at the MUA or
intermediate MTA renders this desire unattainable.
2. Border MTAs are more likely to have direct access to external
sources of authentication or reputation information since modern
MUAs are more likely to be heavily firewalled. Thus, some MUAs
might not even be able to complete the task of performing
authentication or reputation evaluations without complex proxy
configurations or similar burdens.
3. MUAs rely upon the upstream MTAs within their trust boundaries to
make correct (as much as that is possible) evaluations about the
message's envelope, header and content. Thus, MUAs don't need to
know how to do the work that upstream MTAs do; they only need the
results of that work.
4. Evaluations about the quality of a message, from simple token
matching (e.g., a list of preferred DNS domains) to cryptanalysis
(e.g., public/private key work), are at least a little bit
expensive and thus should 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 5451 Authentication-Results Header Field April 2009
5. Minimizing change is good. As new authentication and reputation
methods emerge, the list of methods supported by this header
field would presumably be extended. If MUAs simply consume the
contents of this header field rather than actually attempting to
do authentication and/or reputation work, then MUAs only need to
learn to parse this header field once; emergence of new methods
requires only a configuration change at the MUAs and software
changes at the MTAs (which are presumably fewer in number). When
choosing to implement these functions in MTAs vs MUAs, the issues
of individual flexibility, infrastructure inertia and scale of
effort must be considered. It is typically easier to change a
single MUA than an MTA because the modification affects fewer
users and can be pursued with less care. However, changing many
MUAs is more effort than changing a smaller number of MTAs.
6. For decisions affecting message delivery and display, assessment
based on authentication and reputation is best performed close to
the time of message transit, as a message makes its journey
toward a user's inbox, not afterwards. DKIM keys and IP address
reputations, etc., can change over time or even become invalid,
and users can take a long time to read a message once delivered.
The value of this work thus degrades, perhaps quickly, once the
delivery process has completed. This seriously diminishes the
value of this work when done other than at MTAs.
Many operational choices are possible within an ADMD, including the
venue for performing authentication and/or reputation assessment.
The current specification does not dictate any of those choices.
Rather, it facilitates those cases in which information produced by
one stage of analysis needs to be transported with the message to the
next stage.
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Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Eric Allman, Mark Delany,
Victor Duchovni, Frank Ellermann, Jim Fenton, Philip Guenther, Tony
Hansen, Paul Hoffman, Scott Kitterman, Eliot Lear, John Levine, Miles
Libbey, Charles Lindsey, Alexey Melnikov, Douglas Otis, Juan Altmayer
Pizzorno, Michael Thomas, and Kazu Yamamoto.
Special thanks to Dave Crocker and S. Moonesamy for their logistical
support, and feedback on and contributions to the numerous proposed
edits throughout the lifetime of this work.
Author's Address
Murray S. Kucherawy
Sendmail, Inc.
6475 Christie Ave., Suite 350
Emeryville, CA 94608
US
Phone: +1 510 594 5400
EMail: msk+ietf@sendmail.com
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RFC, FYI, BCP