Network Working Group E. Allman
Internet-Draft Sendmail, Inc.
Intended status: Standards Track J. Fenton
Expires: August 26, 2008 Cisco Systems, Inc.
M. Delany
Yahoo! Inc.
J. Levine
Taughannock Networks
February 23, 2008
DKIM Author Signing Practices (ASP)
draft-ietf-dkim-ssp-03
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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
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contents of messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary DKIM protocol is described in
[RFC4871]. This document describes the records that authors' domains
can use to advertise their practices for signing their outgoing mail,
and how other hosts can access those records.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4
2.1. Terms Imported from DKIM Signatures Specification . . . . 4
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Author Signing Practices . . . . . . . . . . . . . . . . . 5
2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. ASP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. ASP Results . . . . . . . . . . . . . . . . . . . . . . . 6
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7
4.2. Publication of ASP Records . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. ASP Specification Tag Registry . . . . . . . . . . . . . . 10
5.2. ASP Outbound Signing Practices Registry . . . . . . . . . 10
5.3. ASP Flags Registry . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. ASP Threat Model . . . . . . . . . . . . . . . . . . . . . 11
6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. References - Normative . . . . . . . . . . . . . . . . . . 13
7.2. References - Informative . . . . . . . . . . . . . . . . . 13
Appendix A. Usage Examples . . . . . . . . . . . . . . . . . . . 13
A.1. Single Location Domains . . . . . . . . . . . . . . . . . 14
A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 14
A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 14
A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 15
C.1. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 15
C.2. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 16
C.3. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 17
C.4. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17
C.5. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18
C.6. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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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, and the absence of a signature on a message is not an
a priori indication of forgery. In fact, during early phases of
deployment it is very likely that most messages will remain unsigned.
However, some domains might decide to sign all of their outgoing
mail, for example, to protect their brand name. It is desirable for
such domains to be able to advertise that fact to other hosts. This
is the topic of Author Signing Practices (ASP).
Hosts implementing this specification can inquire what Author Signing
Practices a domain advertises. This inquiry is called an Author
Signing Practices check.
The detailed requirements for Author Signing Practices are given in
[RFC5016]. This document refers extensively to [RFC4871] and assumes
the reader is familiar with it.
Requirements Notation: 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]
2. Language and Terminology
2.1. Terms Imported from DKIM Signatures Specification
Some terminology used herein is derived directly from [RFC4871]. In
several cases, references in that document to Sender have been
changed to Author here, to emphasize the relationship to the Author
address(es) in the From: header field described in [RFC2822].
Briefly,
o A "Signer" is the agent that signs a message, as defined in
section 2.1 of [RFC4871].
o A "Selector" specifies which of the keys published by a signing
domain is to be queried, as defined in section 3.1 of [RFC4871].
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o A "Local-part" is the part of an address preceding the @ sign, as
defined in [RFC2822] and used in [RFC4871].
2.2. Valid Signature
A "Valid Signature" is any signature on a message which correctly
verifies using the procedure described in section 6.1 of [RFC4871].
2.3. Author Address
An "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.
2.4. Author Domain
An "Author Domain" is everything to the right of the "@" in an Author
Address (excluding the "@" itself).
2.5. Alleged Author
An "Alleged Author" is an Author Address of a message; it is
"alleged" because it has not yet been verified.
2.6. Author Signing Practices
"Author Signing Practices" (or just "practices") consist of a
machine-readable record published by the domain of an Alleged Author
which includes statements about the domain's practices with respect
to mail it sends with its domain in the From: line.
2.7. 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. When the identity of the user or
agent includes a Local-part, the identities match if the Local-parts
match and the domains match. Otherwise, the identities match if the
domains match.
For example, if a message has a Valid Signature, with the DKIM-
Signature field containing "i=a@domain.example", then domain.example
is asserting that it takes responsibility for the message. If the
message's From: field contains the address "b@domain.example" and an
ASP query produces a "dkim=all" or "dkim=discardable" result, that
would mean that the message does not have a valid Author Signature.
Even though the message is signed by the same domain, its failure to
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satisfy ASP could be problematic.
3. Operation Overview
Domain owners can publish Author Signing Practices via a query
mechanism such as the Domain Name System; specific details are given
in Section 4.1.
Hosts can look up the Author Signing Practices of the domain(s)
specified by the Author Address(es) as described in Section 4.2.2.
If a message has multiple Author Addresses the ASP lookups SHOULD be
performed independently on each address. This standard does not
address the process a host might use to combine the lookup results.
3.1. ASP Usage
Depending on the Author Domain(s) and the signatures in a message, a
recipient gets varying amounts of useful information from each ASP
lookup.
o If a message has no Valid Signature, the ASP result is directly
relevant to the message.
o If a message has a Valid Signature from an Author Domain, ASP
provides no benefit relative to that domain since the message is
already known to be compliant with any possible ASP for that
domain.
o If a message has a Valid Signature from a domain other than an
Author Domain, the receiver can use both the Signature and the ASP
result in its evaluation of the message.
3.2. ASP Results
An Author Signing Practices lookup for an Author Address produces one
of four possible results:
o Messages from this domain might or might not have an author
signature. This is the default if the domain exists in the DNS
but no record is found.
o All messages from this domain are signed.
o All messages from this domain are signed and discardable.
o The domain does not exist.
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4. Detailed Description
4.1. DNS Representation
Author Signing Practices records are published using the DNS TXT
resource record type.
NON-NORMATIVE DISCUSSION [to be removed 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) type. Read the archive for details.
The RDATA for ASP resource records is textual in format, with
specific syntax and semantics relating to their role in describing
Author Signing Practices. The "Tag=Value List" syntax described in
section 3.2 of [RFC4871] is used. Records not in compliance with
that syntax or the syntax of individual tags described in Section 4.3
MUST be ignored (considered equivalent to a NODATA result) for
purposes of ASP, although they MAY cause the logging of warning
messages via an appropriate system logging mechanism. If the RDATA
contains multiple character strings, the strings are logically
concatenated with no delimiters between the strings.
The ASP record for a domain is published at a location in the
domain's DNS hierarchy prefixed by _asp._domainkey.; e.g., the ASP
record for example.com would be a TXT record that is published at
"_asp._domainkey.example.com". A domain MUST NOT publish more than
one ASP record; the semantics of an ASP lookup that returns multiple
ASP records for a single domain are undefined. (Note that
example.com and mail.example.com are different domains.)
4.2. Publication of ASP Records
Author Signing Practices are intended to apply to all mail sent from
the domain of an Alleged Author. In order to ensure that ASP applies
to any hosts within that domain (e.g., www.example.com,
ftp.example.com.) the ASP lookup algorithm looks up one level in the
domain tree. For example, mail signed by www.example.com could be
covered by the ASP record for example.com. This avoids the need to
include an ASP record for every name within a given domain.
Normally, a domain expressing Author Signing Practices will want to
do so for both itself and all of its "descendants" (child domains at
all lower levels). Domains wishing to do so MUST publish ASP records
for the domain itself and any subdomains.
Wildcards within a domain publishing ASP records pose a particular
problem. This is discussed in more detail in Section 6.3.
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4.2.1. Record Syntax
ASP records use the "tag=value" syntax described in section 3.2 of
[RFC4871].
Tags used in ASP records are described below. Unrecognized tags MUST
be ignored. In the ABNF below, the WSP token is imported from
[RFC2822]. The ALPHA and DIGIT tokens are imported from [RFC5234].
dkim= Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows:
unknown The domain might sign 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.
ABNF:
asp-dkim-tag = %x64.6b.69.6d *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:
asp-t-tag = %x74 *WSP "=" *WSP { asp-t-tag-flag
0*( *WSP ":" *WSP asp-t-tag-flag )
asp-t-tag-flag = "s" / hyphenated-word
; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ]
Unrecognized flags MUST be ignored.
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4.2.2. Author Signing Practices Lookup Procedure
Hosts doing an ASP 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 ASP record" is one that is both syntactically and semantically
correct; in particular, it matches the ABNF for a "tag-list" and
includes a defined "dkim=" tag.
1. _Fetch Named ASP Record._ The host MUST query DNS for a TXT
record corresponding to the Author Domain prefixed by
"_asp._domainkey." (note the trailing dot). If the result of
this query is a "NOERROR" response with an answer which is a
valid ASP record, use that record; otherwise, continue to the
next step.
2. _Verify Domain Exists._ The host 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 algorithm MUST terminate
with an appropriate error.
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 host MUST query DNS for a TXT record for
the immediate parent domain, prefixed with "_asp._domainkey." If
the result of this query is anything other than a "NOERROR"
response with a valid ASP record, the algorithm terminates with a
result indicating that no ASP record was present. If the ASP "t"
tag exists in the response and any of the flags is "s"
(indicating it does not apply to a subdomain), the algorithm also
terminates without finding an ASP record. Otherwise, use that
record.
If any of the queries involved in the Author Signing Practices Check
result in a "SERVFAIL" error response, the algorithm terminates
without returning a result; possible actions include queuing the
message or returning an SMTP error indicating a temporary failure.
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5. IANA Considerations
ASP introduces some new namespaces that have been registered with
IANA. In all cases, new values are assigned only for values that
have been documented in a published RFC that has IETF Consensus
[RFC2434].
INFORMATIVE NOTE [ to be removed before publication ]: RFC 4871
defines a selector as a sub-domain, importing the term from RFC 2822.
A sub-domain starts with a letter or digit, hence names such as _asp
that start with an underscore cannot collide with valid selectors.
5.1. ASP Specification Tag Registry
An ASP record provides for a list of specification tags. IANA has
established the ASP Specification Tag Registry for specification tags
that can be used in ASP fields.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| dkim | (this document) |
| t | (this document) |
+------+-----------------+
ASP Specification Tag Registry Initial Values
5.2. ASP Outbound Signing Practices Registry
The "dkim=" tag spec, defined in Section 4.2.1, provides for a value
specifying Outbound Signing Practices. IANA has established the ASP
Outbound Signing Practices Registry for Outbound Signing Practices.
The initial entries in the registry comprise:
+-------------+-----------------+
| TYPE | REFERENCE |
+-------------+-----------------+
| unknown | (this document) |
| all | (this document) |
| discardable | (this document) |
+-------------+-----------------+
ASP Outbound Signing Practices Registry Initial Values
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5.3. ASP Flags Registry
The "t=" tag spec, defined in Section 4.2.1, provides for a value
specifying Flags. IANA has established the ASP Flags Registry for
ASP Flags.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| s | (this document) |
+------+-----------------+
ASP Flags Registry Initial Values
6. Security Considerations
Security considerations in the Author Signing Practices are mostly
related to attempts on the part of malicious senders to represent
themselves as authors for whom they are not authorized to send mail,
often in an attempt to defraud either the recipient or an Alleged
Author.
Additional security considerations regarding Author Signing Practices
are found in the DKIM threat analysis [RFC4686].
6.1. ASP Threat Model
Email recipients often have a core set of content authors that they
already trust. Common examples include financial institutions with
which they have an existing relationship and Internet web transaction
sites with which they conduct business.
Email abuse often seeks to exploit the name-recognition that
recipients will have, for a legitimate email author, by using its
domain name in the From: header field. Especially since many popular
MUAs do not display the author's email address, there is no empirical
evidence of the extent that this particular unauthorized use of a
domain name contributes to recipient deception or that eliminating it
will have significant effect.
However, closing this exploit could facilitate some types of
optimized processing by receive-side message filtering engines, since
it could permit them to maintain higher-confidence assertions about
From: header field uses of a domain, when the occurrence is
authorized.
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Unauthorized uses of domain names occur elsewhere in messages, as do
unauthorized uses of organizations' names. These attacks are outside
the scope of this specification.
ASP does not provide any benefit--nor, indeed, have any effect at
all--unless an external system acts upon the verdict, either by
treating the message differently during the delivery process or by
showing some indicator to the end recipient. Such a system is out of
scope for this specification.
ASP Checkers perform up to three DNS lookups per Alleged Author
Domain. Since these lookups are driven by domain names in email
message headers of possibly fraudulent email, legitimate ASP Checkers
can become participants in traffic multiplication attacks.
6.2. DNS Attacks
An attacker might attack the DNS infrastructure in an attempt to
impersonate ASP records, in an attempt to influence a receiver's
decision on how it will handle mail. 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. These DNS security issues are
addressed by DNSSEC [RFC4033].
Because ASP operates within the framework of the legacy e-mail
system, the default result in the absence of an ASP record is that
the domain does not sign all of its messages. It is therefore
important that the ASP clients distinguish a DNS failure such as
"SERVFAIL" from other DNS errors so that appropriate actions can be
taken.
6.3. DNS Wildcards
Wildcards within a domain publishing ASP records, including but not
limited to wildcard MX records, pose a particular problem. While
referencing the immediate parent domain allows the discovery of an
ASP record corresponding to an unintended immediate-child subdomain,
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, ASP coverage for subdomains of domains containing a
wildcard record is incomplete.
NON-NORMATIVE NOTE: Complete ASP coverage of domains containing (or
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where any parent contains) wildcards generally cannot be provided by
standard DNS servers.
7. References
7.1. References - Normative
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[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.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
7.2. References - Informative
[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016,
October 2007.
Appendix A. Usage Examples
These examples are intended to illustrate typical uses of ASP. They
are not intended to be exhaustive, nor to apply to every domain's or
mail system's individual situation.
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
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paths that could add or modify headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature,
recipient hosts will consider the mail not to have an Author
Signature, even though the signature was present when the mail was
originally sent.
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 group 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 ASP record
for the domain containing "all", depending on whether the users also
send mail through other paths that do not apply an Author Signature.
Such paths could include MTAs at hotels or hotspot networks used by
travelling users, or web sites that provide "mail an article"
features.
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 group 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
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 prefer 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
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kind of mail is sometimes loosely called "forgeries".) In that case,
it might be appropriate to publish an ASP 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" ASP 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.
Appendix B. Acknowledgements
This document greatly benefited from comments by Steve Atkins, Jon
Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael
Thomas, and Wietse Venema.
Appendix C. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
C.1. Changes since -ietf-dkim-02
o Merge in more text from ASP draft.
o Phrase actions as host's rather than checker.
o Explanatory description of i= matching.
o Lookup procedure consistently refers to one ASP record per lookup.
o Update security section w/ language from W. Venema
o Simplify imports of terms from other RFCs, add Local-part, 4234 ->
5234.
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o Add usage example appendix.
o Add IANA considerations.
o Update authors list
C.2. 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).
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o Removed check to avoid querying top-level domains.
o Changed ABNF use of whitespace from [FWS] to *WSP (partially
addresses issue 1543).
C.3. 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.4. 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 to be published, and effect of
wildcard records within the domain, on SSP.
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C.5. Changes since -allman-ssp-01
o Changed term "Sender Signing Policy" to "Sender Signing
Practices".
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.6. 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
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
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Jim Fenton
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
Phone: +1 408 526 5914
Email: fenton@cisco.com
Mark Delany
Yahoo! Inc.
701 First Avenue
Sunnyvale, CA 94089
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://www.taugh.com
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