I'll send this through to the IETF Secretariat just to make it available
for wider commentary later this week unless there are some big
showstoppers reported.
Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track July 2007
Expires: January 2, 2008
Message Header Field for Indicating Message Authentication Status
draft-kucherawy-sender-auth-header-06
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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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 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 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.
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 consists of 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.
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] ";" methodspec propspec )
authres-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
; 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 of [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] 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 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.)
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
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 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. See Section 6 for further information.
An MTA compliant with this specification MUST add this header field
(after performing one or more message 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.
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 it bears appears to be one within its own trust domain as
configured by the user.
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 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 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 of this header field added by
outside MTAs that appear to come from inside an MTA's trust boundary
must be removed. 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
desirable since there's no value in validating the signature on a
message with forged headers. However, signing agents MAY 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.
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.
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, 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], 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. Method Registry
As stated above, names of message authentication methods supported by
this specification must be registered with IANA, with the exception
of experimental names as described above.
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:
+------------+---------+--------+----------------------------------+
| Method | defined | ptype | property |
+------------+---------+--------+----------------------------------+
| auth | RFC2554 | smtp | auth |
+------------+---------+--------+----------------------------------+
| dkim | RFC4871 | header | value of signature "i" tag |
+------------+---------+--------+----------------------------------+
| domainkeys | RFC4870 | header | From or Sender |
+------------+---------+--------+----------------------------------+
| senderid | RFC4406 | header | name of header field used by PRA |
| | | smtp | from (envelope sender) |
+------------+---------+--------+----------------------------------+
| spf | RFC4408 | smtp | from |
+------------+---------+--------+----------------------------------+
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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
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 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 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 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 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.
[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.
[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, 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 graphical representation of this header field's meaning,
other interim means of doing so may be necessary while this proposal
is 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(_at_)example(_dot_)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
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver(_at_)sendmail(_dot_)com
Message-Id: <12345(_dot_)abc(_at_)example(_dot_)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(_at_)example(_dot_)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
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver(_at_)sendmail(_dot_)com
Message-Id: <12345(_dot_)abc(_at_)example(_dot_)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(_dot_)mail=sender(_at_)example(_dot_)com
From: sender(_at_)example(_dot_)net
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.128.1])
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(_at_)example(_dot_)com
Message-Id: <12345(_dot_)abc(_at_)example(_dot_)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 to
this specification and applied three different message authentication
checks:
Authentication-Results: mail-router.example.com;
auth=pass (cram-md5)
smtp(_dot_)mail=sender(_at_)example(_dot_)com;
spf=pass smtp(_dot_)mail=sender(_at_)example(_dot_)com
Authentication-Results: mail-router.example.com;
sender-id=pass header(_dot_)from=sender(_at_)example(_dot_)com
From: sender(_at_)example(_dot_)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(_at_)example(_dot_)net
Message-Id: <12345(_dot_)abc(_at_)example(_dot_)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 multiple message authentication
checks:
Authentication-Results: auth-checker.example.com;
sender-id=fail
header(_dot_)from=sender(_at_)example(_dot_)com;
dkim=pass (good signature)
header(_dot_)i=sender(_at_)example(_dot_)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(_dot_)mail=sender(_at_)example(_dot_)com;
spf=fail smtp(_dot_)mail=sender(_at_)example(_dot_)com
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.128.1])
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: a=rsa-sha1; s=gatsby; d=example.com;
c=simple; q=dns;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender(_at_)example(_dot_)com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: receiver(_at_)sendmail(_dot_)com
Message-Id: <12345(_dot_)abc(_at_)example(_dot_)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]
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.
The second MTA, identifying itself as "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.
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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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Appendix D. Public Discussion
[REMOVE BEFORE PUBLICATION]
Public discussion of this proposed specification is handled via the
mail-vet-discuss(_at_)mipassoc(_dot_)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(_at_)sendmail(_dot_)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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Kucherawy Expires January 2, 2008 [Page 28]
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