Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track May 19, 2007
Expires: November 20, 2007
Message Header for Indicating Sender Authentication Status
draft-kucherawy-sender-auth-header-05
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This memo defines a new message header for use with electronic mail
messages to indicate the results of sender authentication efforts.
Mail user agents (MUAs) may use this message header to relay that
information in a convenient way to users or to make sorting and
filtering decisions.
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1. Introduction
This memo defines a new message header 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 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, and discusses
the implications of its presence or absence.
[REMOVE OR REWORD PRIOR TO FINAL VERSION] At the time of publication
of this draft, [AUTH], [SENDERID], [SPF] and [DKIM] 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 defined in this memo is expected to serve several
purposes:
1. Convey to MUAs from filters and 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 "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to indicate
particular requirements of this specification. A discussion of the
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meanings of these terms appears in RFC2119.
Generally it 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 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.
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2. Definition and Format of the Header
The new header being defined here is called "Authentication-Results".
It qualifies in [MAIL] [6] terms as a Structured Header Field, and
thus all of the related definitions in that document apply.
This new header SHOULD 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 as a Trace Header Field, and thus all
of the related definitions in that document apply.
The decommented value of the header consists of a hostname, 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 header MAY appear
more than once in a single message, or more than one result MAY be
represented in a single header, or a combination of these MAY be
applied.
Formally, the header is specified as follows:
header = "Authentication-Results:" hostname CFWS
headerspec *(CFWS ";" CFWS method CFWS "=" CFWS result)
CFWS
hostname = domain
; as defined in section 3.4.1 of [MAIL]
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 = ( ALPHA / DIGIT ) 1*( "." ALPHA / DIGIT )
; indicates which version of the method was applied
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result = "pass" / "fail" / "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, 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 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 to be added (see below)
value = token / mailbox
; the value extracted from the message property defined
; by the "ptype.property" construction; if the value is
; intended ; to identify a mailbox, then it is a "mailbox"
; as defined in section 3.4 of [MAIL]
The "token" is as defined in Appendix A of [MIME] [7].
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].
The "ptype" and "property" 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" 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 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.
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: The message failed the authentication tests. (This may
require accessing an authentication policy of some kind published
by the sending domain.)
softfail: 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.
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.
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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3. 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
[SENDERID] [8]. Therefore, the authentication method identifiers
"auth", "dkim", "spf" and "senderid" 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 authentication methods that may become
available. As such, the order of the presented authentication
methods and results are not relevant since ultimately the importance
on any given method over another is the decision of the MUA that is
interpreting the value of the header.
The "method" MUST refer to an authentication method declared in this
memo, or in a subsequent one, or to an authentication method name
assigned by IANA.
An MTA compliant with this specification MUST add this header (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 once per test, or one header 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 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
which claims to have been added within its trust boundary. For
example, a border MTA at mx.example.com MUST delete any instance of
this header claiming to come from mx.example.com and SHOULD delete
any instance of this header claiming to come from any host in
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. However, care must be taken not to
remove headers added on messages which remain entirely within the
originator's trust boundary (i.e. local-to-local mail).
An MTA MAY add this header containing only the "hostname" portion to
explicitly indicate that no sender authentication schemes were
applied prior to delivery of this message.
4.1. Header Position and Interpretation
In order to ensure non-ambiguous results and avoid the impact of
false headers, an MUA SHOULD NOT interpret this header unless
specifically instructed to do so by the user. That is, this should
not be "on by default". Naturally then, users would not activate
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such a feature unless they are certain the header will be added by
the receiving MTA that accepts the mail which is ultimately read by
the MUA, and instances of the header 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:
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, such as:
Authentication-Results: mx.example.com; foo=pass (2 of 3 tests OK)
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5. 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 of this proposal should be aware that many MUAs are
unlikely to be retrofit to support the new header 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 as well as the header defined by this specification.
One suggestion is to provide a Priority: header with 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
authentication.
Certainly some modern MUAs can filter based on the content of this
header, but as there is keen interest in having MUAs make some kind
of graphical representation of this header's meaning, other interim
means of doing so may be necessary while this proposal is adopted.
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6. 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
corresponding to this specification after doing so and prior to
delivery.
MTAs that are relaying mail rather than delivering it MAY perform
sender authentication or even take actions based on the results
found, but MUST NOT add a "Authentication-Results" header if relaying
rather than rejecting or discarding at the gateway. Conversely, an
MTA doing local delivery MUST add this header prior to delivery the
message in order to be compliant.
A minimal implementation which does at least one sender
authentication check will add the header defined by this memo prior
to invoking local delivery procedures.
This specification places no restrictions on the processing of the
header's contents by user agents or distribution lists. It is
presented to those packages solely for their own information.
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7. IANA Considerations
7.1. The Authentication-Results: header
Per [IANA-HEADERS] [5], 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. Method Registry
Following the policies outlined in [IANA-CONSIDERATIONS] [4], names
of sender authentication methods supported by this specification must
be registered with IANA under the IETF Consensus method, with the
exception of experimental names as described above.
Each method must register a name, the RFC which defines it, which
"ptype" is appropriate for use with that method, and which "property"
should be reported by that method.
The initial set of entries in this registry is as follows:
+------------+------+--------+----------------------------+
| Method | RFC | ptype | property |
+------------+------+--------+----------------------------+
| auth | 2554 | smtp | auth |
+------------+------+--------+----------------------------+
| domainkeys | TBD | header | From or Sender |
+------------+------+--------+----------------------------+
| dkim | 4871 | header | i |
+------------+------+--------+----------------------------+
| senderid | 4406 | header | name of header used by PRA |
+------------+------+--------+----------------------------+
| spf | 4408 | smtp | from |
+------------+------+--------+----------------------------+
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8. Security Considerations
The following security considerations apply when applying or
processing the "Authentication-Results" header:
8.1. 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
make false conclusions based on forged headers. A malicious user or
agent could forge a header 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, and the
MUA could trust it.
It is for this reason an MUA SHOULD NOT have processing of the
"Authentication-Results" header enabled by default; instead it must
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 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 which can be
verified by a posted public key. Another would be a means to
interrogate the MTA that added the header to see if it is actually
providing any sender authentication services and saw the message in
question. In either case, a method needs to exist to verify that the
host which appears to have added the header (a) actually did so, and
(b) is legitimately adding that header for this delivery.
8.2. Header Position
Despite the requirements of [MAIL] [6], headers can sometimes be
reordered enroute by intermediate MTAs. The goal of requiring header
addition only at the top of a message is an acknowledgement that some
MTAs do reorder headers, 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 defined here.
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9. References
[1] 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] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and
M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",
RFC 4817, May 2007.
[4] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
[5] 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] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.
[9] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[10] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for
Authorizing Use of Domains in E-Mail, Version 1", RFC 4408,
April 2006.
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Appendix A. 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 of Cisco, and
Eric Allman of Sendmail, Inc.
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Appendix B. Public Discussion
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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Appendix C. Authentication-Results Examples
This section presents some examples of the use of this header to
indicate authentication results.
C.1. Trivial case; header 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 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 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 is present, indicating that the
delivering MTA (which is named in the value of the header) 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 conforms to this
standard and applied some sender authentication:
Authentication-Results: mail-router.example.com
smtp.auth=sender@example.com; auth=pass (cram-md5)
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 is present, indicating that the
delivering MTA (which is named in the value of the header) 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 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 conforms to
this standard and applied two different sender authentication checks:
Authentication-Results: mail-router.example.com
smtp.mail=sender@example.com;
auth=pass (cram-md5); spf=pass
Authentication-Results: mail-router.example.com
header.from=sender@example.com; sender-id=pass
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 is present, indicating the
delivering MTA (which is named in the value of the header) conforms
to this specification. Furthermore, the sender authenticated
herself/himself to the MTA via a method specified in [AUTH], and both
SPF and Sender-ID checks were done and passed. The MUA could extract
and relay this extra information if desired.
Two "Authentication-Results" headers are required because the methods
applied did not all base their results on the same property of the
message.
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C.5. Service provided, several authentications done, different MTAs
A message that was relayed inbound by two different MTAs which
conform to this standard and applied multiple sender authentication
checks:
Authentication-Results: auth-checker.example.com
header.from=sender@example.com; sender-id=pass;
domainkeys=pass (good signature)
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; auth=pass (cram-md5);
spf=fail
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: a=rsa-sha1; s=gatsby; d=sendmail.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 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, "auth-
checker.example.com", reports that it did a Sender-ID test and a
DomainKeys [2] 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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Author's Address
Murray S. Kucherawy
Sendmail, Inc.
6425 Christie Ave., Suite 400
Emeryville, CA 94608
US
Phone: +1 510 594 5400
Email: msk+ietf@sendmail.com
Kucherawy Expires November 20, 2007 [Page 23]
Internet-Draft Authentication-Results Header May 2007
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Kucherawy Expires November 20, 2007 [Page 24]
