One document matched: draft-ietf-dkim-ssp-02.txt
Differences from draft-ietf-dkim-ssp-01.txt
DKIM Working Group E. Allman
Internet-Draft Sendmail, Inc.
Intended status: Standards Track M. Delany
Expires: August 4, 2008 Yahoo! Inc.
J. Fenton
Cisco Systems, Inc.
February 1, 2008
DKIM Sender Signing Practices
draft-ietf-dkim-ssp-02
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and
contents of messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary DKIM protocol is described in
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[RFC4871].
This document describes the records that authors' domains can use to
advertise their practices regarding signing their outgoing mail, and
how other hosts can access, parse and interpret those records.
Requirements Language
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 [RFC2119].
(Unresolved Issues/To Be Done)
o Need to consider handling of multiple responses to a DNS query for
the SSP record.
o Security Considerations needs a detailed examination.
o IANA Considerations should be formalized (e.g., as in 4871).
o Check over the references.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4
2.1. Terms Imported from DKIM Signatures Specification . . . . 5
2.2. Evaluator . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. SSP Checker . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Author Address . . . . . . . . . . . . . . . . . . . . . . 6
2.7. Author Domain . . . . . . . . . . . . . . . . . . . . . . 6
2.8. Author Signature . . . . . . . . . . . . . . . . . . . . . 6
2.9. Sender Signing Practices Record . . . . . . . . . . . . . 6
3. Operational Description . . . . . . . . . . . . . . . . . . . 6
3.1. Publication of SSP Records . . . . . . . . . . . . . . . . 6
3.2. Lookup of SSP Records . . . . . . . . . . . . . . . . . . 8
3.3. SSP Record Syntax . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 13
A.1. Single Location Domains . . . . . . . . . . . . . . . . . 13
A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 13
A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 14
A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 14
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 14
C.1. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 15
C.2. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 16
C.3. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 16
C.4. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 16
C.5. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain.
However, the legacy of the Internet is such that not all messages
will be signed. Therefore, the absence of a signature is not an a
priori indication of forgery. In fact, during early phases of DKIM
deployment it must be expected that most messages will remain
unsigned. Nevertheless, some domains may find it highly desirable to
advertise that they sign all of their outgoing mail making the
absence of a valid signature a potential indication of forgery.
Without a mechanism to do so, the benefits of DKIM are limited to
cases in which a valid signature exists and cannot be extended to
cases in which signatures are missing or are invalid. Defining such
a mechanism is the purpose of Sender Signing Practices (SSP).
This specification focuses on information which is relevant in the
absence of an acceptable signature. Expressions of signing practice
which require outside auditing are out of scope for this
specification because they fall under the purview of reputation and
accreditation. Sender Signing Practices can be extended in the
future to include additional information that a receiver might use as
input to a processing decision.
More specifically, this specification defines the SSP Checker, a
module that retrieves the SSP information for a given domain, and the
format of the data returned. An module called the Evaluator combines
information from DKIM signatures, SSP Checker results, and any other
data sources it cares to use in order to make a decision regarding
how the message should be processed. The Evaluator is explicitly out
of scope of this document, and is described herein in order to make
the limits of this specification clear.
The detailed requirements for Sender Signing Practices are given in
[RFC5016], which the protocol described in this document attempts to
satisfy. This document refers extensively to [RFC4871], which should
be read as a prerequisite to this document.
2. Language and Terminology
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2.1. Terms Imported from DKIM Signatures Specification
Some terminology used herein is derived directly from [RFC4871].
Briefly,
o A "Signer" is the agent that signs a message. In many cases it
will correspond closely with the original author of the message or
an agent working on the author's behalf.
o "Selectors" describe the keys published by a signing domain.
Signing domains may have multiple Selectors. Selectors subdivide
the address space to allow a single sending domain to publish
multiple keys.
o A "Verifier" is the agent that verifies a message by checking
actual signature(s) in the message header against the message
itself using the public key published in the Selector referenced
by a given signature.
2.2. Evaluator
The "Evaluator" is the module that makes the ultimate decision on how
an incoming message should be processed at a given site. In some
cases it may be colocated with the Verifier. The Evaluator combines
information from the DKIM signature(s) (if any), the output of the
SSP Checker, and any other information it cares to consult in order
to make a processing decision about the message. The specification
of the Evaluator is out of scope of this document.
2.3. SSP Checker
The "SSP Checker" module performs the SSP queries on behalf of the
Evaluator. It is the primary module defined by this document. The
input to the SSP Checker is an address extracted from the From header
field of the message being evaluated; the output is either the Sender
Signing Practices associated with that domain, or an error code.
2.4. Valid Signature
A "Valid Signature" is any signature on a message which correctly
verifies using the procedure described in section 6.1 of [RFC4871].
2.5. Alleged Author
An "Alleged Author" is the Author Address of a message received by an
Evaluator; it is "alleged" because it has not yet been verified.
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2.6. Author Address
The "Author Address" is an email address in the From header field of
a message [RFC2822]. If the From header field contains multiple
addresses, the message has multiple Author Addresses, which may
potentially cause the Evaluator to perform multiple SSP Checks for a
given message.
2.7. Author Domain
The "Author Domain" is everything to the right of the "@" in the
Author Address (excluding the "@" itself).
2.8. Author Signature
An "Author Signature" is any Valid Signature where the identity of
the user or agent on behalf of which the message is signed (listed in
the "i=" tag or its default value from the "d=" tag) matches an
Author Address in the message.
2.9. Sender Signing Practices Record
A "Sender Signing Practices Record" consists of a machine-readable
record published by the domain of an Alleged Author which includes
information on whether that domain signs all of their email, and
related information. That record is defined in detail in section
Section 3.3.
3. Operational Description
The use of Sender Signing Practices consists of two parts:
Publication of SSP records by author domains wishing to do so
Lookup of SSP records by an SSP Checker under the direction of an
Evaluator.
3.1. Publication of SSP Records
3.1.1. DNS Representation
Sender Signing Practices Records are published using the DNS "TXT"
resource record type.
*[[DRAFT DISCUSSION, TO BE DELETED BEFORE PUBLICATION*: There has
been considerable discussion on the DKIM WG mailing list regarding
the relative advantages of TXT and a new resource record (RR)
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type. Many DNS server and resolver implementations are incapable
of quickly and easily supporting new resource record types. For
this reason, support of TXT records is required whether a new RR
type is defined or not. However, without a "flag day" on which
SSP TXT record support is to be withdrawn, such support is likely
to continue indefinitely. As a result, this specification defines
no new RR type for SSP.
Another alternative proposed by P. Hallam-Baker is the publication
of both a TXT record and, when implementations permit, a new RR,
referred to as XPTR, which gives the location from which SSP and
other policy information relating to a give domain can be
retrieved. This has the advantage of supporting a variety of
policies in a scalable manner, with better handling of wildcards
and centralized publication of policy records, with caching
advantages. However, the above implementation issues also apply
to XPTR, and an additional lookup is required to retrieve SSP via
the XPTR method. At the time of publication of this draft,
consensus on this proposal was unclear.*]]*
The RDATA for SSP resource records is textual in format, with
specific syntax and semantics relating to their role in describing
sender signing practices. SSP records follow the tag-list syntax
described in section 3.2 of [RFC4871], including the restriction on
duplicate tags, the use of white space, and case sensitivity.
Records not in overall compliance with that syntax MUST be ignored
(considered equivalent to a "NODATA" result), although they MAY cause
the logging of warning messages via an appropriate system logging
mechanism. All syntactically valid tags MUST be made available to
the Evaluator.
3.1.2. Location of SSP Records
SSP records for a domain are published at a location in the domain's
DNS hierarchy prefixed by "_ssp._domainkey"; e.g., the SSP record for
"example.com" would be a "TXT" record that is published at
"_ssp._domainkey.example.com".
Sender Signing Practices are intended to apply to all mail sent from
the domain of an Alleged Author. In order to ensure that SSP applies
to any hosts within that domain (e.g., www.example.com,
ftp.example.com, etc.) the SSP lookup algorithm looks up one level in
the domain tree. For example, mail signed by www.example.com may
optionally be covered by the SSP record for example.com. This
prevents administrators from having to include an SSP record for
every name within a given domain.
Normally, a domain expressing Sender Signing Practices will want to
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do so for both itself and all of its "descendents" (child domains at
all lower levels). Domains wishing to do so MUST publish SSP records
for the domain itself and any subdomains.
Wildcards within a domain publishing SSP records pose a particular
problem. This is discussed in more detail in Section 5.2.
3.2. Lookup of SSP Records
NON-NORMATIVE NOTE: While the operation of the Evaluator is outside
the scope of this specification, it is generally not worthwhile for
an Evaluator to request an SSP check when the results of that check
will not affect the disposition of the message. Since the
information provided by SSP is only relevant in the absence of valid
Author Signature(s), there is little to be gained by performing an
SSP check on domains corresponding to valid Author Signatures. SSP
checks may also be unnecessary when the Evaluator has some other
basis for deciding to process the message "normally", including, but
not limited to, the presence of a DKIM signature that the Evaluator
has some basis to trust sufficiently for this purpose.
3.2.1. SSP Checker Results
A Sender Signing Practices check produces one of four possible
results for use by the Evaluator:
1. The domain does not exist in DNS.
2. The domain does exist, but no SSP Record is present.
3. The SSP Record exists, and that value is also returned.
4. The DNS information could not be determined due to a transient
error such as "SERVFAIL".
3.2.2. SSP Lookup Algorithm
SSP Checkers doing an SSP lookup MUST produce a result that is
semantically equivalent to applying the following steps in the order
listed below. In practice, several of these steps can be performed
in parallel in order to improve performance. However,
implementations SHOULD avoid doing unnecessary DNS lookups. For the
purposes of this section a "valid SSP record" is one that is both
syntactically and semantically correct; in particular, it must match
the ABNF for a "tag-list" and must include a defined "dkim=" tag.
1. _Fetch Named SSP Record._ The SSP Checker MUST query DNS for a
TXT record corresponding to the Author Domain prefixed by
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""_ssp._domainkey."" (note the trailing dot). If the result of
this query is a "NOERROR" response with one or more answers which
are valid SSP records, return that record for interpretation by
the Evaluator; otherwise, continue to the next step.
2. _Verify Domain Exists._ The SSP Checker MUST perform a DNS query
for a record corresponding to the Author Domain (with no prefix).
The type of the query can be of any type, since this step is only
to determine if the domain itself exists in DNS. This query MAY
be done in parallel with the query made in step 2. If the result
of this query is an "NXDOMAIN" error, the SSP Checker MUST return
an appropriate error to the Evaluator and terminate the
algorithm.
NON-NORMATIVE DISCUSSION: Any resource record type could be
used for this query since the existence of a resource record
of any type will prevent an "NXDOMAIN" error. "MX" is a
reasonable choice for this purpose is because this record type
is thought to be the most common for likely domains, and will
therefore result in a result which can be more readily cached
than a negative result.
3. _Try Parent Domain._ The SSP Checker MUST query DNS for a TXT
record for the immediate parent domain, prefixed with
""_ssp._domainkey."" If the result of this query is anything
other than a "NOERROR" response with a valid SSP record, the
algorithm terminates returning a result indicating that no SSP
record was present. If the SSP "t" tag exists in the response
and any of the flags is "s" (indicating it should not apply to a
subdomain), the SSP Checker MUST also return a "No SSP Record"
result. Otherwise, return that record for interpretation by the
Evaluator.
If any of the queries involved in the Sender Signing Practices Check
result in a "SERVFAIL" error response, the SSP Checker MUST return
that information to the Evaluator; possible actions include queuing
the message or returning an SMTP error indicating a temporary
failure.
3.3. SSP Record Syntax
SSP Records MUST match the "tag-list" syntax defined in [RFC4871].
The specific tags used in SSP records are described below.
Unrecognized tags MUST be ignored.
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dkim= Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows:
unknown The domain may sign none, some, or all email.
all All mail from the domain is signed with an Author Signature.
discardable All mail from the domain is signed with an Author
Signature. Furthermore, if a message arrives without a valid
Author Signature due to modification in transit, submission via
a path without access to a signing key, or other reason, the
domain encourages the recipient(s) to discard it.
NON-NORMATIVE DISCUSSION: Sender signing practices of
"discardable" would be usually inappropriate for domains of end
users, because of the potential for mailing lists and similar
agents to modify messages in such a way as to render the
signature invalid. Domains sending mail that is expected to
pass with no significant modification to the recipient, such as
domains sending only transactional messages, are appropriate
places to consider the publication of a "discardable" practice.
See [RFC5016] section 5.3 and Appendix A for further
discussion.
ABNF:
ssp-dkim-tag = "dkim" *WSP "=" *WSP ("unknown" /
"all" / "discardable")
t= Flags, represented as a colon-separated list of names (plain-text;
OPTIONAL, default is that no flags are set). Flag values are:
s The signing practices apply only to the named domain, and not
to subdomains.
ABNF:
ssp-t-tag = %x75 *WSP "=" *WSP ssp-t-tag-flag
0*( *WSP ":" *WSP ssp-t-tag-flag )
ssp-t-tag-flag = "s" / hyphenated-word
; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ]
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Unrecognized flags MUST be included in the result that is provided
to the Evaluator.
4. IANA Considerations
IANA is requested to create a "DKIM selector name" registry and to
reserve the selector name ""_ssp"" to avoid confusion between DKIM
key records and SSP records.
*<<< Needs to be updated to be more complete; see 4871 for examples
>>>*
5. Security Considerations
Security considerations in the Sender Signing Practices are mostly
related to attempts on the part of malicious senders to represent
themselves as other authors, often in an attempt to defraud either
the recipient or an Alleged Author.
Additional security considerations regarding Sender Signing Practices
may be found in the DKIM threat analysis [RFC4686].
*<<<THIS SECTION IS NOT COMPLETE.>>>*
5.1. DNS Attacks
An attacker might attack the DNS infrastructure in an attempt to
impersonate SSP records. However, such an attacker is more likely to
attack at a higher level, e.g., redirecting "A" or "MX" record
lookups in order to capture traffic that was legitimately intended
for the target domain. Domains concerned about this should use
DNSSEC [RFC4033].
Because SSP operates within the framework of the legacy e-mail
system, the default result in the absence of an SSP record is that
the domain does not sign all of its messages. It is therefore
important that the SSP Checker distinguish a DNS failure such as
SERVFAIL from other DNS errors so that appropriate actions can be
taken.
5.2. DNS Wildcards
Wildcards within a domain publishing SSP records, including but not
limited to wildcard "MX" records, pose a particular problem. While
referencing the immediate parent domain allows the discovery of an
SSP record corresponding to an unintended immediate-child subdomain,
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wildcard records apply at multiple levels. For example, if there is
a wildcard "MX" record for "example.com", the domain
"foo.bar.example.com" can receive mail through the named mail
exchanger. Conversely, the existence of the record makes it
impossible to tell whether "foo.bar.example.com" is a legitimate name
since a query for that name will not return an "NXDOMAIN" error. For
that reason, SSP coverage for subdomains of domains containing a
wildcard record is incomplete.
NON-NORMATIVE NOTE: Complete SSP coverage of domains containing
(or where any parent contains) wildcards generally cannot be
guaranteed.
6. References
6.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
6.2. Informative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016,
October 2007.
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Appendix A. Usage Examples (INFORMATIVE)
These examples are intended to illustrate typical uses of SSP. They
are not intended to be exhaustive, nor to apply to every domain or
mail system's individual situation.
A.1. Single Location Domains
A common mail system configuration handles all of a domain's users'
incoming and outgoing mail through a single MTA or cluster of MTAs.
In that case, the MTA(s) can be configured to sign outgoing mail with
an Author Signature.
In this situation it might be appropriate to publish an SSP record
for the domain containing "all", depending on whether the users also
send mail through other MTAs that do not apply an Author Signature.
Such MTAs could include MTAs at hotels or hotspot networks used by
travelling users, or web sites that provide "mail an article"
features.
Domain managers are advised to consider the ways that mail processing
can modify messages in ways that will invalidate an existing DKIM
signature, such as mailing lists, courtesy forwarders, and other
paths that could add or modify headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature,
recipient MTAs will consider the mail not to have an Author
Signature, even though the signature was present when the mail was
originally sent.
A.2. Bulk Mailing Domains
Another common configuration uses a domain solely for bulk or
broadcast mail, with no individual human users, again typically
sending all the mail through a single MTA or cluster of MTAs that can
apply an Author Signature. In this case, the domain's management can
be confident that all of its outgoing mail will be sent through the
signing MTA. Lacking individual users, the domain is unlikely to
participate in mailing lists, but could still send mail through other
paths that might invalidate signatures.
Domain owners often use specialist mailing providers to send their
bulk mail. In that case, the mailing provider needs access to a
suitable signing key in order to apply an Author Signature. One
possible route would be for the domain owner to generate the key and
give it to the mailing provider. Another would be for the domain to
delegate a subdomain to the mailing provider, for example,
bigbank.example might delegate email.bigbank.example to such a
provider. In that case, the provider can generate the keys and DKIM
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DNS records itself and use the subdomain in the Author Address in the
mail.
A.3. Bulk Mailing Domains with Discardable Mail
In some cases, a domain might sign all its outgoing mail with an
Author Signature, but prefers that recipient systems discard mail
without a valid Author Signature to avoid confusion from mail sent
from sources that do not apply an Author Signature. (This latter
kind of mail is sometimes loosely called "forgeries".) In that case,
it may be appropriate to publish an SSP record containing
"discardable". Note that a domain SHOULD NOT publish a "discardable"
record if it wishes to maximize the likelihood that mail from the
domain is delivered, since it could cause some fraction of the mail
the domain sends to be discarded.
As a special case, if a domain sends no mail at all, it can safely
publish a "discardable" SSP record, since any mail with an author
address in the domain is a forgery.
A.4. Third Party Senders
Another common use case is for a third party to enter into an
agreement whereby that third party will send bulk or other mail on
behalf of a designated author domain, using that domain in the
RFC2822 From: or other headers. Due to the many and varied
complexities of such agreements, third party signing is not addressed
in this specification. The authors anticipate that as mail systems
gain experience with DKIM, it will become possible to codify best
practices of this and other usages of DKIM.
Appendix B. Acknowledgements
The authors wish to thank many members of the ietf-dkim mailing list
for valuable suggestions and constructive criticism of earlier
versions of this draft.
This draft incorporates content from a parallel "DKIM Author Signing
Policies" document edited by John Levine. The authors appreciate
this contribution.
Appendix C. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
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C.1. Changes since -ietf-dkim-ssp-01
o Reworded introduction for clarity.
o Various definition clarifications.
o Changed names of practices to unknown, all, and discardable.
o Removed normative language mandating use of SSP in particular
situations (issue 1538).
o Clarified possible confusion over handling of syntax errors.
o Removed normative language from Introduction (issue 1538).
o Changed "Originator" to "Author" throughout (issue 1529).
o Removed all references to Third-Party Signatures (issues 1512,
1521).
o Removed all mention of "Suspicious" (issues 1528, 1530).
o Removed "t=y" (testing) flag (issue 1540).
o Removed "handling" tag (issue 1513).
o Broke up the "Sender Signing Practices Check Procedure" into two
algorithms: fetching the SSP record and interpretation thereof
(issues 1531, 1535; partially addresses issue 1520).
Interpretation is now the responsibility of the Evaluator.
o Document restructuring for better flow and remove redundancies
(some may address issue 1523, but I'm not sure I understand that
issue completely; also issues 1532, 1537).
o Removed all mention of how this interacts with users, even though
it makes parts of the document harder to understand (issue 1526).
o Introduced the concepts of "SSP Checker" and "Evaluator".
o Multiple author case now handled my separate invocations of SSP
checker by Evaluator (issue 1525).
o Removed check to avoid querying top-level domains.
o Changed ABNF use of whitespace from [FWS] to *WSP (partially
addresses issue 1543).
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C.2. Changes since -ietf-dkim-ssp-00
o Clarified Operation Overview and eliminated use of Legitimate as
the counterpart of Suspicious since the words have different
meanings.
o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
records in DNS vs. a new RR type.
o Clarified publication rules for multilevel names.
o Better description of overall record syntax, in particular that
records with unknown tags are considered syntactically correct.
o Clarified Sender Signing Practices Check Procedure, primarily by
use of new term Author Domain.
o Eliminated section "Third-Party Signatures and Mailing Lists" that
is better included in the DKIM overview document.
o Added "handling" tag to express alleged sending domain's
preference about handling of Suspicious messages.
o Clarified handling of SERVFAIL error in SSP check.
o Replaced "entity" with "domain", since with the removal of user-
granularity SSP, the only entities having sender signing policies
are domains.
C.3. Changes since -allman-ssp-02
o Removed user-granularity SSP and u= tag.
o Replaced DKIMP resource record with a TXT record.
o Changed name of the primary tag from "p" to "dkim".
o Replaced lookup algorithm with one which traverses upward at most
one level.
o Added description of records which must be published, and effect
of wildcard records within the domain, on SSP.
C.4. Changes since -allman-ssp-01
o Changed term "Sender Signing Policy" to "Sender Signing
Practices".
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o Changed query methodology to use a separate DNS resource record
type, DKIMP.
o Changed tag values from SPF-like symbols to words.
o User level policies now default to that of the domain if not
specified.
o Removed the "Compliance" section since we're still not clear on
what goes here.
o Changed the "parent domain" policy to only search up one level
(assumes that subdomains will publish SSP records if appropriate).
o Added detailed description of SSP check procedure.
C.5. Changes since -allman-ssp-00
From a "diff" perspective, the changes are extensive. Semantically,
the changes are:
o Added section on "Third-Party Signatures and Mailing Lists"
o Added "Compliance" (transferred from -base document). I'm not
clear on what needs to be done here.
o Extensive restructuring.
Authors' Addresses
Eric Allman
Sendmail, Inc.
6475 Christie Ave, Suite 350
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
URI:
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Mark Delany
Yahoo! Inc.
701 First Avenue
Sunnyvale, CA 94089
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Jim Fenton
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
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