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draft-kucherawy-sender-auth-header-07.txt --> draft-kucherawy-sender-auth-header-08.txt

Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track September 20, October 12, 2007
Expires: March 23, April 14, 2008

Message Header Field for Indicating Message Authentication Status
draft-kucherawy-sender-auth-header-07
draft-kucherawy-sender-auth-header-08

Status of this Memo

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This Internet-Draft will expire on March 23, April 14, 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 message 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 Field To A Message . . . . . . . . . . . . . . . . 11
3.1. Header Position and Interpretation . . . . . . . . . . . . 11
4. Discussion Removing The Header Field . . . . . . . . . . . . . . . . . . 13
5. Conformance and Usage Requirements . . . . . . . . 13
4.1. Removing The Header . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . 13
5. Conformance and Usage Requirements . . . . . . . . 15
6.1. The Authentication-Results: header . . . . . . 14
6. IANA Considerations . . . . . . 15
6.2. Email Authentication Method Name Registry . . . . . . . . 15
7. Security Considerations . . . . . . . 15
6.1. The Authentication-Results: header . . . . . . . . . . . . 15
6.2. Method Registry 17
7.1. Forged Headers . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . 17
7.2. Misleading Results . . . . . . . . . . . . . . . . . . . 16
7.1. Non-conformant MTAs . 17
7.3. Other Protocols . . . . . . . . . . . . . . . . . . . 16
7.2. . . 18
7.4. Header Position . . . . . . . . . . . . . . . . . . . . . 16 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17 19
8.2. Informative References . . . . . . . . . . . . . . . . . . 17 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19 21
Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 20 22
Appendix C. Authentication-Results Examples . . . . . . . . . . . 21 23
C.1. Trivial case; header field not present . . . . . . . . . . 21 23
C.2. Nearly-trivial case; service provided, but no
authentication done . . . . . . . . . . . . . . . . . . . 22 24
C.3. Service provided, authentication done . . . . . . . . . . 22 24
C.4. Service provided, several authentications done, single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 25
C.5. Service provided, several authentications done,
different MTAs . . . . . . . . . . . . . . . . . . . . . . 24 26

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C.6. Service provided, multi-tiered authentication done . . . . 28
Appendix D. Public Discussion . . . . . . . . . . . . . . . . . . 26

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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27 31
Intellectual Property and Copyright Statements . . . . . . . . . . 28 32

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1. Introduction

This memo defines a new message header field for electronic mail
messages which 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) 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 field, and
discusses the implications of its presence or absence.

[UPDATE PRIOR TO FINAL VERSION] At the time of publication of this
draft, [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] and [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 Mail Transfer Agents (MTAs) the
results of various message 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 message authentication tests to later
filtering agents within the same "trust domain", as such agents
might apply more or less stringent checks based on message
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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

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document are to be interpreted as described in RFC2119.

A "border MTA" is an MTA which acts as a gateway between the general
Internet and the users within an organizational boundary.

A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA which
actually enacts delivery of a message to a user's inbox or other
final delivery.

An "intermediate MTA" is an MTA which handles messages after a border
MTAs and before a delivery MTA.

+-----+ +-----+ +------------+
| 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 message authentication could
take place on an intermediate MTA. Although this document doesn't
specifically include such cases, they are not meant to be excluded
from this specification.

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". 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 MUST be added at the top of the message as it
transits MTAs which 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 Header Field as defined in [MAIL] and thus all of
the related definitions in that document apply.

The decommented value of the header field (after removing [MAIL] comments)
consists of an authentication identifier, some whitespace, identifier 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, a "property=value" statement indicating which property was selected to determine who
sent the message and what value
property 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. evaluated.

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:

header = "Authentication-Results:" [CFWS] authres-id
*([CFWS] ";" authserv-id
[CFWS] *(";" methodspec propspec )

authres-id [CFWS])

authserv-id = dot-atom-text
; see below for a description of this element;
; "dot-atom-text" is defined in section 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

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

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

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 = 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 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
; identifies a mailbox, then it is a "mailbox"
; as defined in section 3.4 of [MAIL];

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; "mailbox" allows CFWS

The "token" is as defined in Appendix A section 5.1 of [MIME].

See Section 2.3 for

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See Section 2.3 for a description of the "authres-id" "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 3.2.3 of [MAIL].

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.

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 has an authentication
identifier field ("authres-id" ("authserv-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

For tracing and debugging purposes, the authentication identifier
SHOULD be used as

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check whose result is being reported.

Examples of valid authentication identifiers are mail.example.org,

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engineering.example.net 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.)

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.

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 message; 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 message; 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
detected.

2.5. 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

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

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authentication. See Section 6 for details.

2.6. 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. 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 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 to determine the importance or
strength of any given method over another. Instead, the MUA must
interpret the result of each method based on its knowledge of what
that method evaluates.

The "method" MUST refer to an authentication method declared in the
IANA registry. registry, or an extension method as defined in Section 2.6. See
Section 6 for further information. information about the registered methods.

An MTA compliant with this specification MUST add this header field
(after performing one or more message authentication tests) to
indicate at which host which 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.

For security reasons, a border MTA conforming to this specification
MUST delete any discovered instance of this header field which claims
to have been added within its trust boundary. For example, a border
MTA for example.com receiving a message from outside of its mail
domain MUST delete any instance of this header field bearing an
authentication identifier indicating the header field was added
within example.com prior to adding its own header fields. However,
care must be taken not to remove header fields added on messages that
remain entirely within the originator's trust boundary (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 authentication
identifier portion to indicate explicitly that no message
authentication schemes were applied prior to delivery of this
message.

3.1. Header Position and Interpretation

In order to ensure non-ambiguous results and avoid the impact of
false 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 such a feature unless they are certain the header
field will be added by the receiving MTA that accepts the mail 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 field unless the
hostname
authentication identifier it bears appears to be one within its own
trust domain as configured by the user.

An MUA should MUST ignore any result reported using a "result" or "ptype"
not reveal these results to enumerated by this specification or a later amendment to it.

An MUA should not reveal these results to end users unless the
results are accompanied by, at a minimum, some associated reputation

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data about the message that was authenticated.

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 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.

Further discussion of this can be found in the Security
Considerations section Section 7 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.

4.1. Removing The Header

As specified in Section 3, instances Field

For security reasons, a border MTA conforming to this specification
MUST delete any discovered instance of this header field added by
outside MTAs that appear which claims
to come have been added within its trust boundary. For example, a border
MTA for example.com receiving a message from inside outside of its mail
domain MUST delete any instance of this header field bearing an MTA's trust boundary
authentication identifier indicating the header field was added
within example.com prior to adding its own header fields. However,
care must be removed. taken not to remove header fields added on messages that
remain entirely within the originator's trust boundary (e.g. local-
to-local mail).

Furthermore, a border MTA MAY elect simply to remove all instances of
this header field on mail crossing its trust into its trust boundary.

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 C.

Similarly, in the case of messages signed using [DKIM] or other
message signing methods that sign headers, this may invalidate one or
more signature on the message if they included the header field to be
removed at the time of signing. This behaviour is can be desirable
since there's no 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.

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5. Conformance and Usage Requirements

An MTA or gateway conforms to this specification if it applies one or
more message authentication mechanisms and inserts a header field
corresponding to this specification after doing so and prior to
delivery.
delivery (per Section 3) and removes apparently improper headers (per
Section 4).

MTAs that are relaying mail rather than delivering it, i.e. are not
part of an intended recipient's trust boundary, MAY perform message
authentication or even take actions based on the results found, but
SHOULD NOT add an "Authentication-Results" header field if relaying
(rather than rejecting or discarding at the gateway). Conversely, an
MTA doing local delivery and some form of message authentication MUST
add this header field prior to delivery the message in order to be
compliant. An exception to the former case is described in
Section 4.

A minimal implementation that does at least one message
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 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

This specification introduces some new namespaces that will be
registered with IANA. In all cases,

IANA is requested to register a new entries are assigned only
for values that have been documented in header field and to create a published RFC that has IETF
Consensus, per [IANA-CONSIDERATIONS]. new
table as described below.

6.1. The Authentication-Results: header

Per [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.

6.2. Email Authentication Method Name Registry

As stated above, names

Names of message authentication methods supported by this
specification must be registered with IANA, with the exception of
experimental names as described above. in Section 2.6.

New entries are assigned only for values that have been documented in
a published RFC that has IETF Consensus, 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, 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 field:

7.1. Non-conformant MTAs Forged Headers

An MUA 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 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 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 should not have processing of the
"Authentication-Results" header field enabled by default; instead it
SHOULD
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" header fields are trustworthy; however, this
list SHOULD should initially be empty.

Proposed alternate solutions to this problem are nascent. Possibly
the simplest is a digital signature on the header 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 message authentication services and saw the
message in question, but this isn't especially palatable. In either
case, a method needs to exist 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.

7.2. Header Position

Despite Misleading Results

Until some form of service for querying the requirements reputation of [MAIL], header fields can sometimes be
reordered enroute by intermediate MTAs. The goal a sending
agent is widely deployed, the existence of requiring this header field addition only at the top of
indicating a "pass" does not render the message trustworthy. It is
possible for an acknowledgement
that some MTAs do reorder header fields, but most do not. Thus, in
the 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). In
particular, this issue is not resolved by forged header removal
discussed above.

Hence, MUAs must take some care with use of this header even after

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possibly malicious headers are scrubbed. There are several potential
heuristics that can be used to determine whether an authentication
results header is valid. For instance, an MUA could determine after
several replies that the user has replied to valid messages.
Alternatively, after many messages that contain the same results from
diverse paths, the MUA may decide to record that the sender is valid.
Similarly, when a different results header field appears, an MUA may
decide the older data is no longer trustworthy. In all of these
cases prudence dictates that the user be queried in these situations
until implementors have had some deployment experience.

7.3. Other Protocols

Mitigation of the forged header 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.

It is the intention of the author to follow this proposal with drafts
proposing the above two extensions.

7.4. Header 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.

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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

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

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

[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.

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

[IMAP] Bcdef, A., "Internet Message Access Protocol v4",
RFC 1234, April 2006.

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

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

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[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: Eric Allman, Mark Delany,
Jim Fenton, Tony Hansen, Paul Hoffman, Eliot Lear, John Levine, Miles
Libbey, Charles Lindsey, 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

Some modern MUAs can already filter based on the content of this
header field, but as field. However, there is keen interest in having MUAs make
some kind of graphical representation of this header field's meaning, meaning
to end users. Until this capability is added, other interim means of doing so
conveying authentication results may be necessary while this proposal
is
and its successors are 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 [192.0.2.1])
by server.sendmail.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@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 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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C.2. Nearly-trivial case; service provided, but no authentication done

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

Authentication-Results: mail-router.example.com
From: sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1])
by server.sendmail.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
From: sender@example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@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 field)
conforms to this specification. The absence of any method and result
tokens indicates that no message authentication was done.

C.3. Service provided, authentication done

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

Authentication-Results: mail-router.example.com;
spf=pass smtp.mail=sender@example.com
From: sender@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

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The "Authentication-Results" header field is present, indicating that
the border MTA (which is identified in the value of the header field)
conforms to this specification. Furthermore, the message was
authenticated by that MTA via the method specified in [SPF]. The MUA
could extract and relay this extra information if desired.

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

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

Authentication-Results: mail-router.example.com;
auth=pass (cram-md5) smtp.mail=sender@example.com;
spf=pass smtp.mail=sender@example.com
Authentication-Results: mail-router.example.com;
sender-id=pass header.from=sender@example.com
Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1])
by dialup-1-2-3-4.example.net (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 (which is identified in the value of the header field)
conforms to this specification. 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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C.5. Service provided, several authentications done, different MTAs

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

Authentication-Results: auth-checker.example.com;
sender-id=hardfail header.from=sender@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: mail-router.example.com;
auth=pass (cram-md5) smtp.mail=sender@example.com;
spf=pass
spf=hardfail smtp.mail=sender@example.com
Authentication-Results: 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 [192.0.2.1])
by 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;
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.net receiver@sendmail.com
Message-Id: <12345.abc@example.com>
Subject: here's a sample

Hello! Goodbye!

Example 4: 5: Headers reporting results from one MTA multiple MTAs

The "Authentication-Results" header field is present, indicating the
delivering MTA (which is identified in the value of the header field)
conforms
conformance to this specification. Furthermore, the sender
authenticated herself/himself to the MTA via a method specified It is present twice because two
different MTAs in
[AUTH], the chain of delivery did authentication tests.
The first, "mail-router.example.com" reports that [AUTH] and both [SPF] and [SENDERID] checks
were done both used, and passed.
The [AUTH] passed but [SPF] failed. In the [AUTH]
case, additional data is provided in the comment field, which the MUA could extract and relay this extra information
can choose to render if desired.

Two "Authentication-Results" header fields are not required since the
same host

The second MTA, identifying itself as "auth-checker.example.com",
reports that it did all a [SENDERID] test (which failed) and a [DKIM]
test (which passed). Again, additional data about one of the checking. The authenticating agent could
have consolidated all tests

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is provided as a comment, which the MUA may choose to render.

Since different hosts did the two sets of authentication checks, the results into one
header field.

This example illustrates a scenario fields cannot be consolidated in which this example.

This example illustrates more typical transmission of mail into
"example.com" from a remote user on a dialup connection (example.net) sends mail "example.net". The
user appears to be legitimate as he/she had a valid password allowing
authentication at the border MTA
(example.com) using SMTP authentication to prove identity. [AUTH]. The
dialup provider [SPF] and
[SENDERID] tests failed since "example.com" has been explicitly authorized not granted
"example.net" authority to relay mail as
"example.com" resulting in passes by on its behalf. However, the SPF
[DKIM] test passed because the sending user had a private key
matching one of "example.com"'s published public keys and SenderID checks. used it to
sign the message.

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C.5.

C.6. Service provided, several authentications done, different MTAs multi-tiered authentication done

A message that was relayed inbound by two different MTAs that conform
to this specification and applied multiple message had authentication
checks: done at various stages, one of
which was outside the receiving domain:

Authentication-Results: auth-checker.example.com;
sender-id=fail header.from=sender@example.com; chicago.example.com;
dkim=pass (good signature) header.i=sender@example.com header.i=@mail-router.example.net;
dkim=hardfail (bad signature) header.i=@newyork.example.com
Received: from mail-router.example.com
(mail-router.example.com [192.0.2.1]) mail-router.example.net
(mail-router.example.net [192.0.2.250])
by auth-checker.example.com chicago.example.com (8.11.6/8.11.6)
for <recipient@chicago.example.com>
with ESMTP id i7PK0sH7021929;
Fri, Feb 15 2002 17:19:22 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=furble;
d=mail-router.example.net; t=1188964198; c=relaxed/simple;
h=From:Date:To:Message-Id:Subject:Authentication-Results;
bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=;
b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
Authentication-Results: mail-router.example.com;
auth=pass (cram-md5) smtp.mail=sender@example.com;
spf=fail smtp.mail=sender@example.com mail-router.example.net;
dkim=pass (good signature) header.i=@newyork.example.com
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.2.200]) smtp.newyork.example.com
(smtp.newyork.example.com [192.0.2.220])
by mail-router.example.com 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=example.com; d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@example.com sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver@sendmail.com meetings@example.net
Message-Id: <12345.abc@example.com> <12345.abc@newyork.example.com>
Subject: here's a sample

Hello! Goodbye!

Example 5: 6: 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]
were both used, and [AUTH] passed but [SPF] failed. different
domains

In the [AUTH]
case, additional data is provided in the comment field, which the MUA
can choose to render if desired. this example we see multi-tiered authentication with an extended
trust boundary.

The second MTA, identifying itself as "auth-checker.example.com",
reports that it did a [SENDERID] test (which failed) and message was sent from someone at example.com's New York office
(newyork.example.com) to a [DKIM]
test, (which passed). Again, additional data about one of mailing list managed at an intermediary.
The message was signed at the tests origin using [DKIM].

The message was sent to a mailing list service provider called

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example.net which is provided as used by example.com. There meetings@example.net
is expanded to a comment, long list of recipients, one of which is at the MUA may choose to render.

Since different hosts did
Chicago office. In this example, we will assume that the two sets of authentication checks, trust
boundary for chicago.example.com includes 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". mailing list server at
example.net.

The
user appears mailing list server there first authenticated the message and
affixed an Authentication-Results: header field indicating such. It
then altered the message by affixing some footer text to be legitimate the body
including some administrivia such as he/she had unsubscription instructions.
Finally, the mailing list server affixes a valid password allowing
authentication at second [DKIM] signature
and begins distribution of the message.

The border MTA using [AUTH]. The [SPF] and
[SENDERID] tests failed since "example.com" has for chicago.example.com explicitly trusts results from
mail-router.example.net so that header is not granted
"example.net" authority to relay mail on its behalf. However, removed. It performs
evaluation of both signatures and determines that the
[DKIM] test passed first (most
recent) is a "pass" but, because of the sending user had aforementioned modifications,
the second is a private key
matching one of "example.com"'s published public keys and used "hardfail". However, the first signature included
the Authentication-Results: header added at mail-router.example.net
which validated the second signature. Thus, indirectly, it to
sign can be
determined that the message. authentication claimed by both signatures are
indeed valid.

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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.
6475 Christie Ave., Suite 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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