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draft-kucherawy-sender-auth-header-05.txt --> draft-kucherawy-sender-auth-header-06.txt

Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track May 19, July 26, 2007
Expires: November 20, 2007 January 27, 2008

Message Header Field for Indicating Sender Authentication Status
draft-kucherawy-sender-auth-header-05
draft-kucherawy-sender-auth-header-06

Status of this Memo

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This Internet-Draft will expire on November 20, 2007. January 27, 2008.

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Abstract

This memo defines a new message header field for use with electronic
mail messages to indicate the results of sender authentication
efforts. 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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definition and Format of the Header . . . . . . . . . . . . . 6
2.1. General Description . . . . . . . . . . . . . . . . . . . 6
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 6
2.3. Authentication Identifier Fields . . . . . . . . . . . . . 8
2.4. Result Values . . . . . . . . . . . . . . . . . . . . . . 9
2.5. Definition Of Initial Methods . . . . . . . . . . . . . . 9
2.6. Extension Fields . . . . . . . . . . . . . . . . . . . . . 10
3. Adding The Header To A Message . . . . . . . . . . . . . . . . 11
3.1. Header Position and Interpretation . . . . . . . . . . . . 11
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Removing The Header . . . . . . . . . . . . . . . . . . . 13
5. Conformance and Usage Requirements . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1. The Authentication-Results: header . . . . . . . . . . . . 15
6.2. Method Registry . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Non-conformant MTAs . . . . . . . . . . . . . . . . . . . 16
7.2. Header Position . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 20
Appendix C. Authentication-Results Examples . . . . . . . . . . . 21
C.1. Trivial case; header field not present . . . . . . . . . . 21
C.2. Nearly-trivial case; service provided, but no
authentication done . . . . . . . . . . . . . . . . . . . 22
C.3. Service provided, authentication done . . . . . . . . . . 22
C.4. Service provided, several authentications done, single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
C.5. Service provided, several authentications done,
different MTAs . . . . . . . . . . . . . . . . . . . . . . 24
Appendix D. Public Discussion . . . . . . . . . . . . . . . . . . 26

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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28

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1. Introduction

This memo defines a new message header field for electronic mail
messages which presents the results of a sender authentication effort
in a machine-readable format. The intent is to create a place to
collect such data when sender authentication mechanisms are in use so
that an
MUA a Mail User Agent (MUA) can provide a recommendation to the user
as to the trustworthiness of the message's origin and content.

This memo defines both the format of this new header, header field, and
discusses the implications of its presence or absence.

[REMOVE OR REWORD

[UPDATE PRIOR TO FINAL VERSION] At the time of publication of this
draft, [AUTH], [SENDERID], [SPF] [DKIM], [DOMAINKEYS], [SENDERID] and [DKIM] [SPF] are the
published e-mail authentication methods in common use. As various
methods emerge, it is necessary to prepare for their appearance and
encourage convergence in the area of interfacing these filters to
MUAs.

1.1. Purpose

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

1. Convey to MUAs from filters and MTAs Mail Transfer Agents (MTAs) the
results of various sender authentication checks being applied;

2. Provide a common location for the presentation of this data;

3. Create an extensible framework for specifying new authentication
methods as such emerge;

4. Convey the results of sender authentication tests to later
filtering agents within the same "trust domain", as such agents
might apply more or less stringent checks based on sender
authentication results.

1.2. Requirements

This memo establishes no new requirements on existing protocols or
servers, as there is currently no standard place for the described
data to be collected or presented.

1.3. Definitions

This document occasionally uses terms that appear in capital letters.
When the terms

The key words "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "MAY" appear capitalized, they "OPTIONAL" in this
document are being used to indicate
particular requirements of this specification. A discussion of the be interpreted as described in RFC2119.

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meanings of these terms appears in RFC2119.

Generally it

A "border MTA" is assumed that the work of applying sender
authentication schemes takes place at a border MTA, that is, an MTA which acts as a gateway between the general
Internet and the users within an organizational boundary. This specification

A "delivery MTA" (or Mail Delivery Agent or MDA) is written
with that assumption in mind. However, there an MTA which
actually enacts delivery of a message to a user's inbox or other
final delivery.

Generally it is assumed that the work of applying sender
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. See [I-D.DRAFT-CROCKER-EMAIL-ARCH] for further
discussion on e-mail system architecture.

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2. Definition and Format of the Header

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". "Authentication-
Results". It qualifies in [MAIL] [6] terms as is a Structured Header Field, Field as defined in [MAIL] and
thus all of the related definitions in that document apply.

This new header SHOULD field MUST be added at the top of the message as it
transits MTAs which do authentication checks so that some idea of how far
away the checks were done can be inferred. It therefore also
qualifies in [MAIL] [6] terms should be
treated as a Trace Header Field, Field as defined in [MAIL] and thus all of
the related definitions in that document apply.

The decommented value of the header field consists of a hostname, an
authentication identifier, some whitespace, a "property=value"
statement indicating which property was selected to determine who
sent the message and what value was extracted from that property,
followed by zero or more authentication method names and a result
associated with each, returned by the code that implements the
method.

As it is currently a matter of some debate, the

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

2.2. Formal Definition

Formally, the header field is specified as follows:

header = "Authentication-Results:" hostname CFWS
headerspec *(CFWS [CFWS] authres-id
*([CFWS] ";" CFWS method CFWS "=" CFWS result)
CFWS

hostname methodspec propspec )

authres-id = domain dot-atom-text
; as see below for a description of this element;
; "dot-atom-text" is defined in section 3.4.1 3.2.4 of [MAIL]

methodspec = [CFWS] method [CFWS] "=" [CFWS] result [CFWS]
; indicates which authentication method was evaluated

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propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" value
; an indication of which property of the message
; was evaluated by the authentication scheme being
; applied to yield the reported result

method = token [ "-" "/" version ]
; a method indicates which method's result is
; is represented by "value", and is one of the methods
; explicitly defined as valid in this document
; or is an extension method as defined below

version = ( 1*( ALPHA / DIGIT ) 1*( 0*( "." 1*( ALPHA / DIGIT ) )
; indicates which version of the method was applied

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result = "pass" / "fail" "hardfail" / "softfail" / "neutral" /
"temperror" / "permerror"
; an indication of the results of the attempt to
; authenticate the sender

headerspec = ptype CWFS "." CWFS property CWFS "=" CFWS value
; an indication of which property of the message
; was evaluated by the authentication scheme being
; applied to yield the reported result

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, header field, or was some property of the
; the message body, or some other property evaluated by
; the receiving MTA

property = token
; if "ptype" is "smtp", this indicates which [SMTP]
; command provided the value which was evaluated by the
; authentication scheme being applied; if "ptype" is
; "header", this indicates from which header field the value
; 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 which caused
; this header field to be added (see below)

value = [CFWS] token [CFWS] / mailbox
; the value extracted from the message property defined
; by the "ptype.property" construction; if the value is
; intended ; to identify identifies a mailbox, then it is a "mailbox"
; as defined in section 3.4 of [MAIL]

The "token" is as defined in [MAIL];

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; "mailbox" allows CFWS

The "token" is as defined in Appendix A of [MIME] [7]. [MIME].

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

The list of commands eligible for use with the "smtp" ptype can be
found in [SMTP] [9] and subsequent amendments.

"CFWS" is as defined in section 3.2.3 of [MAIL] [6]. [MAIL].

The "ptype" and "property" values used by each authentication method
should be defined in the specification for that method (or its
amendments).

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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.

If the parsed "ptype.property" construction clearly identifies a
mailbox (in particular, smtp.mail, smtp.rcpt, header.from,
header.sender), then the "value" MUST be a "mailbox". Other
properties (e.g. smtp.helo) may be evaluated, but the property MUST
still be expressed as a "token" for simplified parsing.

2.3. Authentication Identifier Fields

Every Authentication-Results header field MUST have an authentication
identifier field ("authres-id" above) which is a single result
identifier. This is similar in syntax to a fully-qualified domain
name.

The authentication identifier field provides a unique identifier that
refers to the authenticating service within a given mail
administrative domain. The uniqueness of the identifier is
guaranteed by the mail administrative domain that generates it and
must pertain to exactly that one mail administrative domain. This
identifier is intended to be machine-readable and not necessarily
meaningful to users. MUAs may use this identifier to determine
whether or not the data contained in an Authentication-Results header
field can be trusted.

The mail administrative domain's unique domain name MUST be used as

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the last portion of the identifier.

Examples of valid authentication identifiers are mail.example.org,
engineering.example.edu and ms1.newyork.example.com.

2.4. Result Values

The six possible values of the "result" are:

pass: The message passed the authentication tests. (This may
require accessing an authentication policy of some kind published
by the sending domain.)

fail:

hardfail: The message failed the authentication tests. (This may
require accessing an authentication policy of some kind published
by the sending domain.)

softfail: The message failed the authentication tests, and the
authentication method has either an explicit (published by the
sending domain) or implicit policy, but the policy being used
doesn't require successful authentication of all messages from
that domain, and the message failed the
authentication tests. domain.

neutral: The authentication method completed without errors, but was
unable to reach either a positive or negative result about the
message.

temperror: A temporary (recoverable) error occurred attempting to
authenticate the sender; either the process couldn't be completed
locally, or (for methods requiring a policy to be accessed) there
was a temporary failure retrieving the sending domain's policy. A
later retry may produce a more final result.

permerror: A permanent (unrecoverable) error occurred attempting to
authenticate the sender; either the process couldn't be completed
locally, or (for methods requiring a policy to be accessed) there
was a permanent failure retrieving the sending domain's policy. A
later retry is unlikely to yield a final result.

New methods not specified in this document MUST indicate which of
these should be returned when exceptions such as syntax errors are

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detected.

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2.5. Definition Of Initial Methods

As they are currently existing specifications for sender
authentication, it is appropriate to define an authentication method
identifier for each of [AUTH] [1], [DKIM] [3], [SPF] [10] and [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] [8]. and

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[SPF]. Therefore, the authentication method identifiers "auth",
"dkim", "spf" and "domainkeys", "senderid" and "spf" respectively are hereby
defined for MTAs applying those specifications for e-mail sender
authentication.

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4. Adding The Header To A Message

This specification makes no attempt to evaluate the relative
strengths of various sender See Section 6 for details.

2.6. Extension Fields

Additional authentication methods method identifiers may be defined in the
future by later revisions or extensions to this specification.
Extension identifiers 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 identifiers may be defined for the following reasons:

1. To allow additional information from emergent authentication
systems to be communicated to MUAs. 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" which indicate
different levels of authentication and differentiate between
their relative strengths, e.g. "auth1-weak" and "auth1-strong".

Authentication method implementors are encouraged to provide adequate
information, via [MAIL] comments if necessary, to allow an MUA
developer to understand or relay ancilliary 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: mx.example.com;
foo=pass bar.baz=blob (2 of 3 tests OK)

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3. Adding The Header To A Message

This specification makes no attempt to evaluate the relative
strengths of various sender authentication methods that may become
available. As such, the order of the presented authentication
methods and results are not relevant since ultimately MUST NOT be used to determine the importance
on or
strength of any given method over another is the decision of another. Instead, the MUA that is
interpreting must
interpret the value result of the header.

The "method" MUST refer to an authentication each method declared in this
memo, or in a subsequent one, or based on its knowledge of what
that method evaluates.

The "method" MUST refer to an authentication method name
assigned by IANA. declared in the
IANA registry. See Section 6 for further information.

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

An MTA adding this header in either form MUST use its own hostname
only. It MUST be a fully-qualified domain name.

For security reasons, an a border MTA conforming to this specification
MUST
remove any discovered instance of this header for which the
"hostname" is its own, i.e. headers which claim to be from the MTA
but were added before the mail arrived at the MTA for processing. A
border MTA SHOULD also delete any discovered instance of this header field which claims
to have been added within its trust boundary. For example, a border
MTA at mx.example.com for example.com receiving a message from outside of its mail
domain MUST delete any instance of this header claiming to come from mx.example.com and SHOULD delete
any instance of this field bearing an
authentication identifier indicating the header claiming to come from any host in field was added
within example.com prior to adding its own headers. This applies in both
directions so that hosts outside the domain cannot claim results MUAs
inside the domain might trust. header fields. However,
care must be taken not to remove headers header fields added on messages which that
remain entirely within the originator's trust boundary (i.e. local-to-local (e.g. local-
to-local mail).

A "border MTA" in this discussion is considered to be the first MTA
which is listed as a mail exchanger (MX) for the recipient domain.

An MTA MAY add this header field containing only the "hostname" authentication
identifier portion to
explicitly indicate explicitly that no sender
authentication schemes were applied prior to delivery of this
message.

4.1.

3.1. Header Position and Interpretation

In order to ensure non-ambiguous results and avoid the impact of
false headers, header fields, an MUA SHOULD NOT interpret this header field
unless specifically instructed to do so by the user. That is, this
interpretation should not be "on by default". Naturally then, users
would not activate

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field will be added by the receiving MTA that accepts the mail which that

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is ultimately read by the MUA, and instances of the header field
appearing to be from within the trust domain but actually added by
foreign MTAs will be removed before delivery.

Furthermore, an MUA SHOULD NOT interpret this header unless the
hostname it bears appears to be one within its own trust domain as
configured by the user.

This header field SHOULD be treated as though it were a trace header
field as defined in section 3.6 of [MAIL] [6], and hence SHOULD not
be reordered and SHOULD be prepended to the message, so that there is
generally some indication upon delivery of where in the chain of
handling MTAs the sender authentcation was done.

Further discussion of this can be found in the Security
Considerations section below.

4.2. Extension Fields

Additional authentication method identifiers may be defined in the
future by later revisions or extensions to this specification.
Extension identifiers 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.

Extension identifiers may be defined for the following reasons:

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

Authentication method implementors are encouraged to provide adequate
information, via [MAIL] comments if necessary, to allow an MUA
developer to understand or relay ancilliary details of authentication
results. For example, if SHOULD NOT interpret this header field unless the
hostname it might bears appears to be of interest one within its own trust domain as
configured by the user.

An MUA should not reveal these results to relay what naive end users unless the
results are accompanied by, at a minimum, some associated reputation
data about the sender that was used to perform an evaluation, such information could authenticated.

As stated in Section 2.1, this header field SHOULD be relayed treated as
though it were a comment trace header field as defined in section 3.6 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 header, such as:

Authentication-Results: mx.example.com; foo=pass (2 chain of 3 tests OK) handling MTAs the sender authentcation was
done.

Further discussion of this can be found in the Security
Considerations section below.

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4. Discussion

This section discusses various implementation issues not specifically
related to security. Security issues are discussed in a later
section.

5.1. Legacy MUAs

Implementors

4.1. Removing The Header

As specified in Section 3, instances of this proposal should be aware header field added by
outside MTAs that many MUAs are
unlikely appear to come from inside an MTA's trust boundary
must be retrofit to support the new header and its semantics. removed. In the interests of convenience and quicker adaptation, a delivery
MTA might want to consider adding things that are processed by
existing MUAs as well as the header defined by this specification.
One suggestion is to provide a Priority: header with a value that
reflects the strength case of the authentication that was accomplished,
e.g. "low" for weak messages signed using [DKIM] or no authentication, "normal"
other message signing methods that sign headers, this may invalidate
one or "high" for good
authentication.

Certainly some modern MUAs can filter based more signature on the content message if they included the header
field to be removed at the time of this
header, but as there signing. This behaviour is keen interest
desirable since there's no value in having MUAs make some kind
of graphical representation of this header's meaning, other interim
means of doing so may be necessary while validating the signature on a
message with forged headers. However, signing agents MAY elect to
omit these header fields from signing to avoid this proposal is adopted. situation.

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5. Conformance and Usage Requirements

An MTA or gateway conforms to this specification if it applies one or
more sender authentication mechanisms and inserts a header field
corresponding to this specification after doing so and prior to
delivery.

MTAs that are relaying mail rather than delivering it it, i.e. are not
part of an intended recipient's trust boundary, MAY perform sender
authentication or even take actions based on the results found, but MUST
SHOULD NOT add a an "Authentication-Results" header field if relaying
rather
(rather than rejecting or discarding at the gateway. gateway). Conversely, an
MTA doing local delivery and some form of sender authentication MUST
add this header field prior to delivery the message in order to be
compliant.

A minimal implementation which that does at least one sender authentication
check will add the header field defined by this memo prior to
invoking local delivery procedures.

This specification places no restrictions on the processing of the
header's
header field's contents by user agents or distribution lists. It is
presented to those packages solely for their own information.

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6. IANA Considerations

7.1.

This specification introduces some new namespaces that will be
registered with IANA. In all cases, new entries are assigned only
for values that have been documented in a published RFC that has IETF
Consensus, per [IANA-CONSIDERATIONS].

6.1. The Authentication-Results: header

Per [IANA-HEADERS] [5], [IANA-HEADERS], the "Authentication-Results:" header field is
added to the IANA Permanent Message Header Field Registry. The
following is the registration template:

Header field name: Authentication-Results
Applicable protocol: mail ([RFC2822])
Status: Standard
Author/Change controller: IETF
Specification document(s): [TBD]
Related information:
Requesting review of any proposed changes and additions to
this field is recommended.

7.2.

6.2. Method Registry

Following the policies outlined in [IANA-CONSIDERATIONS] [4],

As stated above, names of sender authentication methods supported by
this specification must be registered with IANA under the IETF Consensus method, IANA, with the exception
of experimental names as described above.

Each method must register a name, the RFC which specification that defines it, which
one or more "ptype" is values appropriate for use with that method, and
which "property" value(s) should be reported by that method.

The initial set of entries in this registry is as follows:

+------------+------+--------+----------------------------+

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7. Security Considerations

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

8.1. header field:

7.1. Non-conformant MTAs

An MUA that is aware of this specification which accesses a mailbox whose mail is handled by a non-conformant MTA is in a position to non-
conformant MTA, and understands Authentication-Results header fields,
could potentially make false conclusions based on forged headers. header
fields. A malicious user or agent could forge a header field using
the destination MX for a receiving domain as the hostname token in
the value of the header, and with the rest of the value claim that
the sender was properly authenticated. The non-conformant MTA would
fail to strip the forged header, 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 must
SHOULD be ignored, at least for the purposes of enacting filtering
decisions, unless specifically enabled by the user after verifying
that the MTA is compliant. It is acceptable to have an MUA aware of
this standard, but have an explicit list of hostnames whose
"Authentication-Results" headers header fields are trustworthy, however trustworthy; however, this
list SHOULD initially be empty.

Proposed alternate solutions to this problem are nascent. Possibly
the simplest is a digital signature on the header which field that can be
verified by a posted public key. Another would be a means to
interrogate the MTA that added the header field to see if it is
actually providing any sender authentication services and saw the
message in
question. question, but this isn't especially palatable. In either
case, a method needs to exist to verify that the host which that appears to
have added the header field (a) actually did so, and (b) is
legitimately adding that header field for this delivery.

8.2.

7.2. Header Position

Despite the requirements of [MAIL] [6], headers [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 headers, 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.

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8. References

8.1. Normative References

[MAIL] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.

[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

[1]

[AUTH] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.

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

[3]

[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4817, May 2007.

[4]

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

[I-D.DRAFT-CROCKER-EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture",
I-D draft-crocker-email-arch, May 2007.

[IANA-CONSIDERATIONS]
Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
October 1998.

[5]

[IANA-HEADERS]
Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", RFC 2434,
September 2004.

[6] Resnick, P., "Internet Message Format", RFC 2822, April 2001.

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

[8]

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

[9]

[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.

[10]

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

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RFC 4408, April 2006.

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Appendix A. Acknowledgements

The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Tony Hansen of AT&T, Mark
Delany and Miles Libbey of Yahoo! Inc., Jim Fenton proposal: Eric Allman, Mark Delany,
Jim Fenton, Tony Hansen, Paul Hoffman, John Levine, Miles Libbey, S.
Moonesamy, Michael Thomas.

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Appendix B. Legacy MUAs

Implementors of this proposal should be aware that many MUAs are
unlikely to be retrofit to support the new header field and its
semantics. In the interests of convenience and quicker adaptation, 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.

Certainly some modern MUAs can filter based on the content of this
header field, but as there is keen interest in having MUAs make some
kind of Cisco, and
Eric Allman graphical representation of Sendmail, Inc.

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Appendix B. Public Discussion

Public discussion this header field's meaning,
other interim means of doing so may be necessary while this proposed specification is handled via the
mail-vet-discuss@mipassoc.org mailing list. The list proposal
is open.
Access to subscription forms and to list archives can be found at
http://mipassoc.org/mailman/listinfo/mail-vet-discuss. adopted.

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Appendix C. Authentication-Results Examples

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

C.1. Trivial case; header field not present

The trivial case:

From: sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [1.2.3.4])
by server.sendmail.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@sendmail.com
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 sender 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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C.2. Nearly-trivial case; service provided, but no authentication done

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

Authentication-Results: mail-router.example.com
From: sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [1.2.3.4])
by server.sendmail.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@sendmail.com
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, indicating that
the delivering MTA (which is named in the value of the header) header field)
conforms to this specification. The absence of any method and result
tokens indicates that no sender authentication was done.

C.3. Service provided, authentication done

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

Authentication-Results: mail-router.example.com
smtp.auth=sender@example.com; mail-router.example.com;
auth=pass (cram-md5) smtp.auth=sender@example.com
From: sender@example.com
Received: from dialup-1-2-3-4.example-isp.com
(dialup-1-2-3-4.example-isp.com [1.2.3.4])
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@sendmail.com
Message-Id: <12345.abc@example.com>
Subject: here's a sample

Hello! Goodbye!

Example 3: Header reporting results

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The "Authentication-Results" header field is present, indicating that
the delivering MTA (which is named identified in the value of the header) header
field) conforms to this specification. Furthermore, the sender
authenticated herself/himself to the MTA via a method specified in
[AUTH]. The actual method is identified in a header field comment
after the method's result is indicated. The MUA could extract and
relay this extra information if desired.

C.4. Service provided, several authentications done, single MTA

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

Authentication-Results: mail-router.example.com
smtp.mail=sender@example.com; mail-router.example.com;
auth=pass (cram-md5); (cram-md5) smtp.mail=sender@example.com;
spf=pass smtp.mail=sender@example.com
Authentication-Results: mail-router.example.com
header.from=sender@example.com; mail-router.example.com;
sender-id=pass header.from=sender@example.com
From: sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [1.2.3.4])
by dialup-1-2-3-4.example-isp.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@sendmail.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 (which is named identified in the value of the header) header field)
conforms to this specification. Furthermore, the sender
authenticated herself/himself to the MTA via a method specified in
[AUTH], and both
SPF [SPF] and Sender-ID [SENDERID] checks were done and passed.
The MUA could extract and relay this extra information if desired.

Two "Authentication-Results" headers header fields are not required because since the methods
applied
same host did not all base their results on the same property of the
message. checking. The authenticating agent could
have consolidated all the results into one header field.

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C.5. Service provided, several authentications done, different MTAs

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

Authentication-Results: auth-checker.example.com auth-checker.example.com;
sender-id=pass header.from=sender@example.com; sender-id=pass;
domainkeys=pass
dkim=pass (good signature) header.i=sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [10.11.12.13])
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: mail-router.example.com
smtp.mail=sender@example.com; mail-router.example.com;
auth=pass (cram-md5); (cram-md5) smtp.mail=sender@example.com;
spf=fail smtp.mail=sender@example.com
Received: from dialup-1-2-3-4.example-isp.com
(dialup-1-2-3-4.example-isp.com [1.2.3.4])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DomainKey-Signature:
DKIM-Signature: a=rsa-sha1; s=gatsby; d=sendmail.com; d=example.com;
c=simple; q=dns;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@sendmail.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. It 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 [SPF]
were both used used, and [AUTH] passed but SPF [SPF] failed. In the [AUTH]
case, additional data is provided in the comment field, which the MUA
can choose to render if desired.

The second MTA, "auth-
checker.example.com", identifying itself as "auth-checker.example.com",
reports that it did a Sender-ID [SENDERID] test and a
DomainKeys [2] [DKIM] test, both of
which passed. Again, additional data about one of the tests is
provided as a comment, which the MUA may choose to render.

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

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Appendix D. Public Discussion

[REMOVE BEFORE PUBLICATION]

Public discussion of this proposed specification is handled via the
mail-vet-discuss@mipassoc.org mailing list. The list is open.
Access to subscription forms and to list archives can be found at
http://mipassoc.org/mailman/listinfo/mail-vet-discuss.

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Author's Address

Murray S. Kucherawy
Sendmail, Inc.
6425
6475 Christie Ave., Suite 400 350
Emeryville, CA 94608
US

Phone: +1 510 594 5400
Email: msk+ietf@sendmail.com

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Full Copyright Statement

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contained in BCP 78, and except as set forth therein, the authors
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