Individual submission M. Kucherawy
Internet-Draft Sendmail, Inc.
Intended status: Standards Track September 23, 2008
Expires: March 27, 2009
Message Header Field for Indicating Message Authentication Status
draft-kucherawy-sender-auth-header-16
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Copyright (C) The IETF Trust (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 . . . . . . . . . . . . . . . . . . . . . . . 5
2. Definition and Format of the Header . . . . . . . . . . . . . 7
2.1. General Description . . . . . . . . . . . . . . . . . . . 7
2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 7
2.3. Authentication Identifier Fields . . . . . . . . . . . . . 10
2.4. Result Values . . . . . . . . . . . . . . . . . . . . . . 10
2.4.1. DKIM and DomainKeys Results . . . . . . . . . . . . . 10
2.4.2. DKIM ADSP Results . . . . . . . . . . . . . . . . . . 11
2.4.3. SPF and Sender-ID Results . . . . . . . . . . . . . . 12
2.4.4. iprev Results . . . . . . . . . . . . . . . . . . . . 13
2.4.5. SMTP AUTH Results . . . . . . . . . . . . . . . . . . 13
2.4.6. Extension Result Codes . . . . . . . . . . . . . . . . 14
2.5. Authentication Methods . . . . . . . . . . . . . . . . . . 15
2.5.1. Definition Of Initial Methods . . . . . . . . . . . . 15
2.5.2. Extension Methods . . . . . . . . . . . . . . . . . . 15
3. The 'iprev' Authentication Method . . . . . . . . . . . . . . 17
4. Adding The Header Field To A Message . . . . . . . . . . . . . 18
4.1. Header Position and Interpretation . . . . . . . . . . . . 18
4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 19
5. Removing The Header Field . . . . . . . . . . . . . . . . . . 20
6. Conformance and Usage Requirements . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7.1. The Authentication-Results: header . . . . . . . . . . . . 22
7.2. Email Authentication Method Name Registry . . . . . . . . 22
7.3. Email Authentication Result Name Registry . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8.1. Forged Headers . . . . . . . . . . . . . . . . . . . . . . 26
8.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 26
8.3. Other Protocols . . . . . . . . . . . . . . . . . . . . . 27
8.4. Header Position . . . . . . . . . . . . . . . . . . . . . 27
8.5. Reverse IP Query Denial-Of-Service Attacks . . . . . . . . 27
8.6. Mitigation of Backscatter . . . . . . . . . . . . . . . . 27
8.7. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 28
8.8. Attacks Against Authentication Methods . . . . . . . . . . 28
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8.9. Intentionally Malformed Header Fields . . . . . . . . . . 28
8.10. Compromised Internal Hosts . . . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1. Normative References . . . . . . . . . . . . . . . . . . . 29
9.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 31
Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 32
Appendix C. Authentication-Results Examples . . . . . . . . . . . 33
C.1. Trivial case; header field not present . . . . . . . . . . 33
C.2. Nearly-trivial case; service provided, but no
authentication done . . . . . . . . . . . . . . . . . . . 34
C.3. Service provided, authentication done . . . . . . . . . . 34
C.4. Service provided, several authentications done, single
MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
C.5. Service provided, several authentications done,
different MTAs . . . . . . . . . . . . . . . . . . . . . . 36
C.6. Service provided, multi-tiered authentication done . . . . 38
Appendix D. Public Discussion . . . . . . . . . . . . . . . . . . 40
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 42
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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 validity of the message's origin and possibly the
integrity of its 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.
Although [SPF] defined a header field called Received-SPF for this
purpose, that header field is specific to the conveyance of SPF and
similar results only and thus is insufficient to satisfy the
requirements enumerated below.
1.1. Purpose
The header field defined in this memo is expected to serve several
purposes:
1. Convey 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 reporting 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.
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In particular, this memo establishes no requirement on MTAs to reject
or filter arriving messages which do not pass authentication checks.
The data conveyed by the defined header field's contents are for the
information of MUAs and filters and should be used at their
discretion.
1.3. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
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 describe such cases, they are not meant to be excluded
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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 value of the header field (after removing [MAIL] comments)
consists of an authentication identifier, an optional version and
then a series of "method=result" statements indicating which
authentication method(s) were applied and their respective results,
and then, for each applied method, an optional "reason" string plus
optional "property=value" statements indicating which message
properties were evaluated to reach that conclusion.
The header field MAY appear more than once in a single message, or
more than one result MAY be represented in a single header field, or
a combination of these MAY be applied.
2.2. Formal Definition
Formally, the header field is specified as follows using [ABNF]:
header = "Authentication-Results:" [CFWS] authserv-id
[ CFWS version ]
( [CFWS] ";" [CFWS] "none" / 1*resinfo ) [CFWS] CRLF
; the special case of "none" is used to indicate that no
; message authentication is performed
authserv-id = dot-atom-text
; see below for a description of this element;
; "dot-atom-text" is defined in section 3.2.4 of [MAIL]
version = 1*DIGIT [CFWS]
; indicates which version of this specification is in use;
; this specification is version "1"; the absence of a version
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; implies this version of the specification
resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ]
*( CFWS propspec )
methodspec = [CFWS] method [CFWS] "=" [CFWS] result
; indicates which authentication method was evaluated
reasonspec = "reason" [CFWS] "=" [CFWS] value
; a free-form comment on the reason the given result
; was returned
propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue
; an indication of which properties of the message
; were evaluated by the authentication scheme being
; applied to yield the reported result
method = token [ [CFWS] "/" [CFWS] version ]
; a method indicates which method's result is
; represented by "result", and is one of the methods
; explicitly defined as valid in this document
; or is an extension method as defined below
result = token
; indicates the results of the attempt to authenticate
; the message; see below for details
ptype = "smtp" / "header" / "body" / "policy"
; indicates whether the property being evaluated was
; a parameter to an [SMTP] command, or was a value taken
; from a message header field, or was some property of
; the message body, or some other property evaluated by
; the receiving MTA
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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)
pvalue = [CFWS] ( token / addr-spec ) [CFWS]
; the value extracted from the message property defined
; by the "ptype.property" construction; if the value
; identifies an address, then it is an "addr-spec"
; as defined in section 3.4 of [MAIL]
The "token" and "value" are as defined in section 5.1 of [MIME].
The "token" used in a "result" above is further constrained by the
necessity of being enumerated in Section 2.4 or an amendment to it.
See Section 2.3 for a description of the "authserv-id" element.
The list of commands eligible for use with the "smtp" ptype can be
found in [SMTP] and subsequent amendments.
"CFWS" is as defined in section 3.2.3 of [MAIL].
The "propspec" may be omitted if for example the method was unable to
extract any properties to do its evaluation yet has a result to
report.
The "ptype" and "property" values used by each authentication method
should be defined in the specification for that method (or its
amendments).
The "ptype" and "property" are case-insensitive.
A "ptype" value of "policy" indicates a policy decision about the
message not specific to a property of the message that could be
extracted. For example, if a method would normally report a
"ptype.property" of "header.From" and no From: header field was
present, the method can use "policy" to indicate that no conclusion
about the authenticity of the message could be reached.
If the parsed "ptype.property" construction clearly identifies a
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mailbox (in particular, smtp.mailfrom, smtp.rcpt, header.from,
header.sender), then the "pvalue" MUST be an "addr-spec". 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 has an authentication
identifier field ("authserv-id" above). 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.
For tracing and debugging purposes, the authentication identifier
SHOULD be the domain name of the MTA performing the authentication
check whose result is being reported.
Examples of valid authentication identifiers are mail.example.org,
engineering.example.net and ms1.newyork.example.com.
2.4. Result Values
Each individual authentication method returns one of a set of
specific result values. The subsections below define these results
for the authentication methods specifically supported by this memo.
New methods not specified in this document intended to be supported
by the header field defined in this memo MUST include a similar
result table either in its defining memo or in a supplementary one.
2.4.1. DKIM and DomainKeys Results
The result values used by [DKIM] and [DOMAINKEYS] are as follows:
none: The message was not signed.
pass: The message was signed, the signature(s) is (were) acceptable
to the verifier, and the signature(s) passed verification tests.
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fail: The message was signed and the signature(s) is (were)
acceptable to the verifier, but it (they) failed the verification
test(s).
policy: The message was signed but the signature(s) is (were) not
acceptable to the verifier.
neutral: The message was signed but the signature(s) contained
syntax errors or were not otherwise able to be processed. This
result SHOULD also be used for other failures not covered
elsewhere in this list.
temperror: The message could not be verified due to some error which
is likely transient in nature, such as a temporary inability to
retrieve a public key. A later attempt may produce a final
result.
permerror: The message could not be verified due to some error which
is unrecoverable, such as a required header field being absent. A
later attempt is unlikley to produce a final result.
A signature is "acceptable to the verifier" if it passes local policy
checks (or there are no specific local policy checks). For example,
a verifier might require that the signature(s) on the message be
added by the domain identified in the From: header field of the
message, thus making third-party signatures unacceptable.
2.4.2. DKIM ADSP Results
The result values are used by [I-D.DRAFT-IETF-DKIM-SSP] as follows:
none: No DKIM author domain signing practises (ADSP) record was
published.
pass: A DKIM ADSP record was published which indicated the mail
should be signed with an author signature, and this message had
such a signature that validated.
unknown: No valid author signature was found on the message and
either the published ADSP was "unknown", or no policy was
published.
signed: A valid author signature was found on the message and the
published ADSP was "unknown".
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fail: No valid author signature was found on the message and the
published ASDP record indicated an "all" policy.
discard: No valid author signature was found on the message and the
published ADSP record indicated a "discardable" policy.
nxdomain: Evaluating the ADSP for the author's domain indicated that
the author's domain does not exist.
temperror: A DKIM policy could not be retrieved due to some error
which is likely transient in nature, such as a temporary DNS
error. A later attempt may produce a final result.
permerror: A DKIM policy could not be retrieved due to some error
which is likely not transient in nature, such as a permanent DNS
error. A later attempt is unlikely to produce a final result.
2.4.3. SPF and Sender-ID Results
The result values are used by [SPF] and [SENDERID] as follows:
none: No policy records were published by the sender's domain.
neutral: The sender's domain has asserted that it cannot or does not
want to assert whether or not the sending IP address is authorized
to send mail on behalf of the sender's domain.
pass: The client is authorized to inject or relay mail on behalf of
the sender's domain.
policy: The client is authorized to inject or relay mail on behalf
of the sender's domain according to the authentication method's
algorithm, but local policy dictates that the result is
unacceptable.
hardfail: This client is explicitly not authorized to inject or
relay mail on behalf of the sender's domain.
softfail: The sender's domain believes the client was not authorized
to inject or relay mail on its behalf but is unwilling to make a
strong assertion to that effect.
temperror: The message could not be verified due to some error which
is likely transient in nature, such as a temporary inability to
retrieve a policy record from DNS. A later attempt may produce a
final result.
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permerror: The message could not be verified due to some error which
is unrecoverable, such as a required header field being absent. A
later attempt is unlikley to produce a final result.
The distinction between and interpretation of "none" and "neutral"
under these methods is discussed further in [SPF].
The "policy" result would be returned if, for example, [SPF] returned
as "pass" result, but a local policy check matches the sending domain
to one found in an explicit list of unacceptable domains (e.g.
spammers).
2.4.4. iprev Results
The result values are used by the "iprev" method, defined in
Section 3, are as follows:
pass: The reverse DNS evaluation succeeded, i.e. the "reverse" and
"forward" lookup results were in agreement.
hardfail: The reverse DNS evaluation failed. In particular, the
"reverse" and "forward" lookups each produced results but they
were not in agreement.
softfail: The reverse DNS evaluation failed. In particular, one or
both of the "reverse" and forward lookups returned no data (i.e. a
DNS reply code of NODATA).
temperror: The reverse DNS evaluation could not be completed due to
some error which is likely transient in nature, such as a
temporary DNS error. A later attempt may produce a final result.
permerror: The reverse DNS evaluation could not be completed due to
some error which is unrecoverable (e.g. a DNS reply code of NODATA
or NXDOMAIN). A later attempt is unlikely to produce a final
result.
There is no "none" for this method since any TCP connection
delivering e-mail has an IP address associated with it, so some kind
of evaluation will always be possible.
2.4.5. SMTP AUTH Results
The result values are used by the [AUTH] method are as follows:
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none: SMTP authentication was not attempted.
pass: The SMTP client had authenicated to the server reporting the
result using the protocol described in [AUTH].
fail: The SMTP client had attempted to authenticate to the server
using the protocol described in [AUTH] but was not successful, yet
continued to send the message about which a result is being
reported.
temperror: The SMTP client attempted to authenticate using the
protocol described in [AUTH] but was not able to complete the
attempt due to some error which is likely transient in nature,
such as a temporary LDAP lookup error. A later attempt may
produce a final result.
permerror: The SMTP client attempted to authenticate using the
protocol described in [AUTH] but was not able to complete the
attempt due to some error which is likely not transient in nature,
such as a permanent LDAP lookup error. A later attempt is not
likely produce a final result.
Note that an agent making use of the data provided by this header
field SHOULD consider "fail" and "temperror" to be the synonymous in
terms of message authentication, i.e. the client did not
authenticate.
2.4.6. Extension Result Codes
Additional result codes (extension results) may be defined in the
future by later revisions or extensions to this specification.
Extension results beginning with "x-" will never be defined as
standard fields; such names are reserved for experimental use.
Result codes not beginning with "x-" MUST be registered with the
Internet Assigned Numbers Authority (IANA) and published in an RFC.
See Section 7 for further details.
Implementations reporting new result codes MUST use the "x-" prefix
until such time as the new method is registered by IANA.
Extension results MUST only be used within trust domains that have
explicitly consented to use them. These results and the parameters
associated with them are not documented in RFCs. Therefore, they are
subject to change at any time and not suitable for production use.
Any MTA or MUA intended for production use SHOULD ignore or delete
any Authentication-Results header field that includes an extension
result.
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2.5. Authentication Methods
This section defines the supported authentication methods and
discusses the proper means for applying experimental and other
extension methods.
2.5.1. Definition Of Initial Methods
As they are currently existing specifications for message
authentication, it is appropriate to define an authentication method
identifier for each of [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] and
[SPF]. Therefore, the authentication method identifiers "auth",
"dkim", "domainkeys", "senderid" and "spf" respectively are hereby
defined for MTAs applying those specifications for e-mail message
authentication.
Furthermore, method "iprev" is defined in Section 3.
Finally, as its publication is imminent, this document also defines
"dkim-adsp" per [I-D.DRAFT-IETF-DKIM-SSP].
See Section 7 for details.
2.5.2. Extension Methods
Additional authentication method identifiers (extension methods) may
be defined in the future by later revisions or extensions to this
specification. Extension methods beginning with "x-" will never be
defined as standard fields; such names are reserved for experimental
use. Method identifiers not beginning with "x-" MUST be registered
with the Internet Assigned Numbers Authority (IANA) and published in
an RFC. See Section 7 for further details.
Extension methods may be defined for the following reasons:
1. To allow additional information from new authentication systems
to be communicated to MUAs. 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".
Implementations of new methods MUST use the "x-" prefix until such
time as the new method is registered by IANA.
Authentication method implementors are encouraged to provide adequate
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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)
Experimental method identifiers MUST only be used within trust
domains that have explicitly consented to use them. These method
identifiers and the parameters associated with them are not
documented in RFCs. Therefore, they are subject to change at any
time and not suitable for production use. Any MTA or MUA intended
for production use SHOULD ignore or delete any Authentication-Results
header field that includes an experimental method identifier.
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3. The 'iprev' Authentication Method
This section defines an additional authentication method called
"iprev".
In general, "iprev" is an attempt to verify that a client appears to
be valid based on some DNS queries. Upon receiving a session
initiation of some kind from a client, the IP address of the client
peer is queried for matching names (i.e. a number-to-name
translation, also known as a "reverse lookup" or a "PTR" record
query). Once that result is acquired, a lookup of each of the names
(i.e. a name-to-number translation, or an "A" record query) thus
retrieved is done. The response to this second check should result
in at least one mapping back to the client's IP address.
More algorithmically: If the client peer's IP address is A, the list
of names to which A maps (after a "PTR" query) is the set N, and the
union of IP addresses to which each member of N maps (after an "A"
query) is L, then this test is successful if A is an element of L.
Section 5.5 of [SPF] contains more detail about this process as well
as some discussion of possible denial-of-service attacks. [DNS-IP6]
discusses the format of this query for the IPv6 case.
A successful test using this algorithm constitutes a result of "pass"
since the domain in which the client's PTR claims it belongs has
confirmed that claim. A failure to match constitutes a "hardfail".
There is no case in which "softfail" or "neutral" can be returned.
The remaining "temperror" and "permerror" cases refer respectively to
temporary and permanent DNS query errors.
There is some contention regarding the wisdom and reliability of this
test. For example, in some regions it can be difficult for this test
ever to pass because the practise of arranging to match the forward
and reverse DNS is infrequently observed. Therefore, the actual
implementation details of how a verifier performs an "iprev" test are
not specified here. The verifier MAY report a successful or failed
"iprev" test at its discretion having done some kind of check of the
validity of the connection's identity using DNS. It is incumbent
upon an agent making use of the reported "iprev" result to understand
what exactly that particular verifier is attempting to report.
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4. Adding The Header Field To A Message
This specification makes no attempt to evaluate the relative
strengths of various message authentication methods that may become
available. As such, the order of the presented authentication
methods and results MUST NOT be used 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.
Each "method" MUST refer to an authentication method declared in the
IANA registry, or an extension method as defined in Section 2.5.2,
and each "result" MUST refer to a result code declared in the IANA
registry, or an extension result code as defined in Section 2.4.6.
See Section 7 for further information about the registered methods
and result codes.
An MTA compliant with this specification MUST add this header field
(after performing one or more message authentication tests) to
indicate 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.
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.
4.1. 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 or administrator.
That is, this interpretation should not be "on by default".
Naturally then, users or administrators should not activate such a
feature unless they are certain the header field will be added by the
border MTA that accepts the mail that is ultimately read by the MUA,
and instances of the header field appearing to be from within the
trust domain but actually added by foreign MTAs will be removed
before delivery.
Furthermore, an MUA SHOULD NOT interpret this header field unless the
authentication identifier it bears appears to be one within its own
trust domain as configured by the user or administrator.
An MUA MUST ignore any result reported using a "result" not specified
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in the result code registry, or a "ptype" not listed in the
corresponding registry for such values as defined in Section 7.
Moreover, an MUA MUST ignore a result indicated for any "method" it
does not support.
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. For example, an
attacker could register examp1e.com (note the digit "one") and send
signed mail to intended victims; a verifier would detect that the
signature was valid and report a "pass" even though it's clear the
domain name was intended to mislead. See Section 8.2 for further
discussion.
As stated in Section 2.1, this header field SHOULD be treated as
though it were a trace header field as defined in section 3.6 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 Section 8 below.
4.2. Local Policy Enforcement
If a site's local policy is to consider a non-recoverable failure
result (e.g. "fail" for DKIM, "hardfail" for SPF or "discard" for
DKIM-ADSP) for any particular authentication method as justification
to reject the message completely, the border MTA SHOULD issue an
[SMTP] rejection response to the message rather than adding this
header with the failure result and allowing it to proceed toward
delivery. This is more desirable than allowing the message to reach
an internal host's MTA or spam filter, thus possibly generating a
local rejection such as a [DSN] to a forged originator.
The same MAY also be done for local policy decisions overriding the
results of the authentication methods (e.g. the "policy" result codes
described in Section 4.2.
Such rejections at the SMTP protocol level are not possible if local
policy is enforced at the MUA and not the MTA. Unfortunately, this
may be a common scenario.
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5. Removing The Header Field
For security reasons, any MTA conforming to this specification MUST
delete any discovered instance of this header field which claims to
have been added within its trust boundary and did not come from
another trusted MTA. For example, an MTA (border or otherwise) for
example.com receiving a message MUST delete any instance of this
header field bearing an authentication identifier indicating the
header field was added within example.com prior to adding its own
header fields. This may mean each MTA will have to be equipped with
a list of internal MTAs known to be compliant (and hence
trustworthy).
Furthermore, a border MTA MAY elect simply to remove all instances of
this header field on mail crossing 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 can be desirable
since there's little value in validating the signature on a message
with forged headers. However, signing agents MAY therefore elect to
omit these header fields from signing to avoid this situation.
An MTA SHOULD remove any instance of this header bearing a version
(express or implied) that it does not support. However, an MTA MUST
remove such a header if the [SMTP] connection relaying the message is
from not a trusted internal MTA.
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6. 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 (per Section 4) and removes apparently improper headers (per
Section 5).
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 delivering the message in order to be
compliant. An exception to the former case is described in
Section 5.
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.
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7. IANA Considerations
IANA is requested to register a new header field and to create a new
table as described below.
7.1. 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 ([MAIL])
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. Email Authentication Method Name Registry
Names of message authentication methods supported by this
specification must be registered with IANA, with the exception of
experimental names as described in Section 2.5.2.
New entries are assigned only for values that have been documented in
a published RFC that has IETF Review, per [IANA-CONSIDERATIONS].
Each method must register a name, the specification that defines it,
one or more "ptype" values appropriate for use with that method, 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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+------------+----------+--------+----------------+--------------------+
| Method | Defined | ptype | property | value |
+------------+----------+--------+----------------+--------------------+
| auth | RFC4954 | smtp | auth | AUTH parameter of |
| | | | | the SMTP MAIL |
| | | | | command |
+------------+----------+--------+----------------+--------------------+
| dkim | RFC4871 | header | d | value of |
| | | | | signature "d" tag |
| | | +----------------+--------------------+
| | | | i | value of |
| | | | | signature "i" tag |
+------------+----------+--------+----------------+--------------------+
| dkim-adsp | [TBD] | header | from | value of From |
| | | | | header field |
| | | | | w/comments removed |
+------------+----------+--------+----------------+--------------------+
| domainkeys | RFC4870 | header | from | value of From |
| | | | | header field |
| | | | | w/comments removed |
| | | +----------------+--------------------+
| | | | sender | value of Sender |
| | | | | header field |
| | | | | w/comments removed |
+------------+----------+--------+----------------+--------------------+
| iprev | this | policy | iprev | client IP address |
| | document | | | |
+------------+----------+--------+----------------+--------------------+
| senderid | RFC4406 | header | name of header | value of header |
| | | | field used by | field used by PRA |
| | | | PRA | w/comments removed |
+------------+----------+--------+----------------+--------------------+
| spf | RFC4408 | smtp | mailfrom | envelope sender |
| | +--------+----------------+--------------------+
| | | smtp | helo | HELO/EHLO value |
+------------+----------+--------+----------------+--------------------+
7.3. Email Authentication Result Name Registry
Names of message authentication result codes supported by this
specification must be registered with IANA, with the exception of
experimental codes as described in Section 2.4.6.
New entries are assigned only for result codes that have been
documented in a published RFC that has IETF Review, per
[IANA-CONSIDERATIONS]. Each code must register a name, the document
which establishes the registration, the authentication method(s)
which uses it, and either a definition of the semantics of its use or
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a reference to the place where those semantics are defined.
The initial set of entries in this registry is as follows:
+-----------+----------+----------------+------------------------------+
| Code | Defined | Auth Method(s) | Meaning |
+-----------+----------+----------------+------------------------------+
| none | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | dkim-adsp | section 2.4.2 |
| | +----------------+------------------------------+
| | | spf | section 2.4.3 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | auth | section 2.4.5 |
+-----------+----------+----------------+------------------------------+
| pass | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | dkim-adsp | section 2.4.2 |
| | +----------------+------------------------------+
| | | spf | section 2.4.3 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.4 |
| | +----------------+------------------------------+
| | | auth | section 2.4.5 |
+-----------+----------+----------------+------------------------------+
| fail | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | dkim-adsp | section 2.4.2 |
| | +----------------+------------------------------+
| | | auth | section 2.4.5 |
+-----------+----------+----------------+------------------------------+
| policy | this | dkim | section 2.4.1 |
| | document | domainkeys | |
+-----------+----------+----------------+------------------------------+
| neutral | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | spf | section 2.4.3 |
| | | sender-id | |
+-----------+----------+----------------+------------------------------+
| temperror | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
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| | | dkim-adsp | section 2.4.2 |
| | +----------------+------------------------------+
| | | spf | section 2.4.3 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.4 |
| | +----------------+------------------------------+
| | | auth | section 2.4.5 |
+-----------+----------+----------------+------------------------------+
| permerror | this | dkim | section 2.4.1 |
| | document | domainkeys | |
| | +----------------+------------------------------+
| | | dkim-adsp | section 2.4.2 |
| | +----------------+------------------------------+
| | | spf | section 2.4.3 |
| | | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.4 |
| | +----------------+------------------------------+
| | | auth | section 2.4.5 |
+-----------+----------+----------------+------------------------------+
| nxdomain | this | dkim-adsp | section 2.4.2 |
| | document | | |
+-----------+----------+----------------+------------------------------+
| signed | this | dkim-adsp | section 2.4.2 |
| | document | | |
+-----------+----------+----------------+------------------------------+
| unknown | this | dkim-adsp | section 2.4.2 |
| | document | | |
+-----------+----------+----------------+------------------------------+
| discard | this | dkim-adsp | section 2.4.2 |
| | document | | |
+-----------+----------+----------------+------------------------------+
| hardfail | this | spf | section 2.4.3 |
| | document | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
| softfail | this | spf | section 2.4.3 |
| | document | sender-id | |
| | +----------------+------------------------------+
| | | iprev | section 2.4.4 |
+-----------+----------+----------------+------------------------------+
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8. Security Considerations
The following security considerations apply when applying or
processing the "Authentication-Results" header field:
8.1. 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 have processing of the
"Authentication-Results" header field enabled by default; instead it
should be ignored, at least for the purposes of enacting filtering
decisions, unless specifically enabled by the user or administrator
after verifying that the border MTA is compliant. It is acceptable
to have an MUA aware of this specification, but have an explicit list
of hostnames whose "Authentication-Results" header fields are
trustworthy; however, this list should initially be empty.
Proposed alternate solutions to this problem are nascent. 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 mechanism 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.
8.2. Misleading Results
Until some form of service for querying the reputation of a sending
agent is widely deployed, the existence of this header field
indicating a "pass" does not render the message trustworthy. It is
possible for an arriving piece of spam or other undesirable mail to
pass checks by several of the methods enumerated above (e.g. a piece
of spam signed using [DKIM] by the originator of the spam, which
might be a spammer or a compromised system). In particular, this
issue is not resolved by forged header 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.
8.3. 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.
8.4. 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.
8.5. Reverse IP Query Denial-Of-Service Attacks
Section 5.5 of [SPF] describes a DNS-based denial-of-service attack
for verifiers that attempt to DNS-based identity verification of
arriving client connections. A verifier wishing to do this check and
report this information SHOULD take care not to go to unbounded
lengths to resolve "A" and "PTR" queries. MUAs or other filters
making use of an "iprev" result specified by this memo SHOULD be
aware of the algorithm used by the verifier reporting the result and
thus be aware of its limitations.
8.6. Mitigation of Backscatter
Failing to follow the instructions of Section 4.2 can result in a
denial-of-service attack caused by the generation of [DSN] messages
(or equivalent) to addresses which did not send the messages being
rejected.
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8.7. Internal MTA Lists
Section 5 describes a procedure for scrubbing headers which may
contain forged authentication results about a message. A compliant
installation will have to include at each MTA a list of other MTAs
known to be compliant and trustworthy. Failing to keep this list
current as internal infrastructure changes may expose a domain to
attack.
8.8. Attacks Against Authentication Methods
If an attack becomes known against an authentication method, clearly
then the agent verifying that method can be fooled into thinking an
inauthentic message is authentic, and thus the value of this header
field can be misleading. It follows that any attack against the
authentication methods supported by this document (and later
amendments to it) is also a security consideration here.
8.9. Intentionally Malformed Header Fields
It is possible for an attacker to add an Authentication-Results:
header field which is extraordinarily large or otherwise malformed in
an attempt to discover or exploit weaknesses in header field parsing
code. Implementors must thoroughly verify all such header fields
received from MTAs and be robust against intentionally as well as
unintentionally malformed header fields.
8.10. Compromised Internal Hosts
An internal MUA or MTA which has been compromised could generate mail
with a forged From: header and a forged Authentication-Results:
header which endorses it. Although it is clearly a larger concern to
have compromised internal machines than it is to prove the value of
this header, this risk can be mitigated by arranging that internal
MTAs will remove this header field if it claims to have been added by
a trusted border MTA (as described above) yet the [SMTP] connection
is not coming from an internal machine known to be running an
authorized MTA. However, in such a configuration, legitimate MTAs
will have to add this header field when legitimate internal-only
messages are generated. This is also covered in Section 5.
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9. References
9.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[]
Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", RFC 3864,
September 2004.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[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.
9.2. Informative References
[AUTH] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[DOMAINKEYS]
Delany, M., "Domain-based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
[I-D.DRAFT-CROCKER-EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture",
I-D draft-crocker-email-arch, May 2007.
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[I-D.DRAFT-IETF-DKIM-SSP]
Allman, E., Delany, M., and J. Fenton, "DKIM Author
Signing Practices", I-D draft-ietf-dkim-ssp, January 2008.
[IANA-CONSIDERATIONS]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
October 1998.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[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",
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, Dave Crocker,
Mark Delany, Frank Ellermann, Jim Fenton, Philip Guenther, Tony
Hansen, Paul Hoffman, Eliot Lear, John Levine, Miles Libbey, Charles
Lindsey, S. Moonesamy, Juan Altmayer Pizzorno, Michael Thomas.
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Appendix B. Legacy MUAs
Implementors of this proposal should be aware that many MUAs are
unlikely to be retrofitted to support the new header field and its
semantics. In the interests of convenience and quicker 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.
Some modern MUAs can already filter based on the content of this
header field. However, there is keen interest in having MUAs make
some kind of graphical representation of this header field's meaning
to end users. Until this capability is added, other interim means of
conveying authentication results may be necessary while this proposal
and its successors are adopted.
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Appendix 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:
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
specification but provides no actual message authentication service:
Authentication-Results: mail-router.example.com; none
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 presence of "none" (and 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
specification and applied some message authentication:
Authentication-Results: mail-router.example.com;
spf=pass smtp.mailfrom=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 to
this specification and applied three different message authentication
checks:
Authentication-Results: mail-router.example.com;
auth=pass (cram-md5) smtp.auth=sender@example.com;
spf=pass smtp.mailfrom=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 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.auth=sender@example.com;
spf=hardfail smtp.mailfrom=sender@example.com
Received: from dialup-1-2-3-4.example.net
(dialup-1-2-3-4.example.net [192.0.2.200])
by mail-router.example.com (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com;
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@sendmail.com
Message-Id: <12345.abc@example.com>
Subject: here's a sample
Hello! Goodbye!
Example 5: Headers reporting results from multiple MTAs
The "Authentication-Results" header field is present, indicating
conformance to this specification. It is present twice because two
different MTAs in the chain of delivery did authentication tests.
The first, "mail-router.example.com" reports that [AUTH] and [SPF]
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
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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
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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C.6. Service provided, multi-tiered authentication done
A message that had authentication done at various stages, one of
which was outside the receiving domain:
Authentication-Results: chicago.example.com;
dkim=pass (good signature) header.i=@mail-router.example.net;
dkim=fail (bad signature) header.i=@newyork.example.com
Received: from mail-router.example.net
(mail-router.example.net [192.0.2.250])
by chicago.example.com (8.11.6/8.11.6)
for <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.net;
dkim=pass (good signature) header.i=@newyork.example.com
Received: from smtp.newyork.example.com
(smtp.newyork.example.com [192.0.2.220])
by mail-router.example.net (8.11.6/8.11.6)
with ESMTP id g1G0r1kA003489;
Fri, Feb 15 2002 17:19:07 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample
Example 6: Headers reporting results from multiple MTAs in different
domains
In this example we see multi-tiered authentication with an extended
trust boundary.
The message was sent from someone at example.com's New York office
(newyork.example.com) to a mailing list managed at an intermediary.
The message was signed at the origin using [DKIM].
The message was sent to a mailing list service provider called
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example.net which is used by example.com. There meetings@example.net
is expanded to a long list of recipients, one of which is at the
Chicago office. In this example, we will assume that the trust
boundary for chicago.example.com includes the mailing list server at
example.net.
The mailing list server there first authenticated the message and
affixed an Authentication-Results: header field indicating such. It
then altered the message by affixing some footer text to the body
including some administrivia such as unsubscription instructions.
Finally, the mailing list server affixes a second [DKIM] signature
and begins distribution of the message.
The border MTA for chicago.example.com explicitly trusts results from
mail-router.example.net so that header is not removed. It performs
evaluation of both signatures and determines that the first (most
recent) is a "pass" but, because of the aforementioned modifications,
the second is a "hardfail". However, the first signature included
the Authentication-Results: header added at mail-router.example.net
which validated the second signature. Thus, indirectly, it can be
determined that the 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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This document and the information contained herein are provided on an
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