draft-ietf-dkim-ssp-02.txt   draft-ietf-dkim-ssp-03.txt 
DKIM Working Group E. Allman Network Working Group E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Intended status: Standards Track M. Delany Intended status: Standards Track J. Fenton
Expires: August 4, 2008 Yahoo! Inc. Expires: August 26, 2008 Cisco Systems, Inc.
J. Fenton M. Delany
Cisco Systems, Inc. Yahoo! Inc.
February 1, 2008 J. Levine
Taughannock Networks
February 23, 2008
DKIM Sender Signing Practices DKIM Author Signing Practices (ASP)
draft-ietf-dkim-ssp-02 draft-ietf-dkim-ssp-03
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
Abstract Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and key server technology to permit verification of the source and
contents of messages by either Mail Transport Agents (MTAs) or Mail contents of messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary DKIM protocol is described in User Agents (MUAs). The primary DKIM protocol is described in
[RFC4871]. [RFC4871]. This document describes the records that authors' domains
can use to advertise their practices for signing their outgoing mail,
This document describes the records that authors' domains can use to and how other hosts can access those records.
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.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4 2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4
2.1. Terms Imported from DKIM Signatures Specification . . . . 5 2.1. Terms Imported from DKIM Signatures Specification . . . . 4
2.2. Evaluator . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.3. SSP Checker . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5 2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5 2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Author Address . . . . . . . . . . . . . . . . . . . . . . 6 2.6. Author Signing Practices . . . . . . . . . . . . . . . . . 5
2.7. Author Domain . . . . . . . . . . . . . . . . . . . . . . 6 2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5
2.8. Author Signature . . . . . . . . . . . . . . . . . . . . . 6 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6
2.9. Sender Signing Practices Record . . . . . . . . . . . . . 6 3.1. ASP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Operational Description . . . . . . . . . . . . . . . . . . . 6 3.2. ASP Results . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Publication of SSP Records . . . . . . . . . . . . . . . . 6 4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7
3.2. Lookup of SSP Records . . . . . . . . . . . . . . . . . . 8 4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7
3.3. SSP Record Syntax . . . . . . . . . . . . . . . . . . . . 9 4.2. Publication of ASP Records . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5.1. ASP Specification Tag Registry . . . . . . . . . . . . . . 10
5.1. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 11 5.2. ASP Outbound Signing Practices Registry . . . . . . . . . 10
5.2. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 11 5.3. ASP Flags Registry . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12 6.1. ASP Threat Model . . . . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . . 12 6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 13 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Single Location Domains . . . . . . . . . . . . . . . . . 13 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 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.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 14
A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 14 A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 14 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 15
C.1. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 15 C.1. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 15
C.2. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 16 C.2. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 16
C.3. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 16 C.3. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 17
C.4. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 16 C.4. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17
C.5. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 17 C.5. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 C.6. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 20
1. Introduction 1. Introduction
DomainKeys Identified Mail (DKIM) defines a mechanism by which email DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key, the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain. possession of the private key for the signing domain.
However, the legacy of the Internet is such that not all messages 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 will be signed, and the absence of a signature on a message is not an
priori indication of forgery. In fact, during early phases of DKIM a priori indication of forgery. In fact, during early phases of
deployment it must be expected that most messages will remain deployment it is very likely that most messages will remain unsigned.
unsigned. Nevertheless, some domains may find it highly desirable to However, some domains might decide to sign all of their outgoing
advertise that they sign all of their outgoing mail making the mail, for example, to protect their brand name. It is desirable for
absence of a valid signature a potential indication of forgery. such domains to be able to advertise that fact to other hosts. This
Without a mechanism to do so, the benefits of DKIM are limited to is the topic of Author Signing Practices (ASP).
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 Hosts implementing this specification can inquire what Author Signing
absence of an acceptable signature. Expressions of signing practice Practices a domain advertises. This inquiry is called an Author
which require outside auditing are out of scope for this Signing Practices check.
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 The detailed requirements for Author Signing Practices are given in
module that retrieves the SSP information for a given domain, and the [RFC5016]. This document refers extensively to [RFC4871] and assumes
format of the data returned. An module called the Evaluator combines the reader is familiar with it.
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 Requirements Notation: The key words "MUST", "MUST NOT", "REQUIRED",
[RFC5016], which the protocol described in this document attempts to "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
satisfy. This document refers extensively to [RFC4871], which should "MAY", and "OPTIONAL" in this document are to be interpreted as
be read as a prerequisite to this document. described in [RFC2119]
2. Language and Terminology 2. Language and Terminology
2.1. Terms Imported from DKIM Signatures Specification 2.1. Terms Imported from DKIM Signatures Specification
Some terminology used herein is derived directly from [RFC4871]. 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, Briefly,
o A "Signer" is the agent that signs a message. In many cases it o A "Signer" is the agent that signs a message, as defined in
will correspond closely with the original author of the message or section 2.1 of [RFC4871].
an agent working on the author's behalf.
o "Selectors" describe the keys published by a signing domain. o A "Selector" specifies which of the keys published by a signing
Signing domains may have multiple Selectors. Selectors subdivide domain is to be queried, as defined in section 3.1 of [RFC4871].
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 o A "Local-part" is the part of an address preceding the @ sign, as
actual signature(s) in the message header against the message defined in [RFC2822] and used in [RFC4871].
itself using the public key published in the Selector referenced
by a given signature.
2.2. Evaluator 2.2. Valid Signature
The "Evaluator" is the module that makes the ultimate decision on how A "Valid Signature" is any signature on a message which correctly
an incoming message should be processed at a given site. In some verifies using the procedure described in section 6.1 of [RFC4871].
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 2.3. Author Address
The "SSP Checker" module performs the SSP queries on behalf of the An "Author Address" is an email address in the From header field of a
Evaluator. It is the primary module defined by this document. The message [RFC2822]. If the From header field contains multiple
input to the SSP Checker is an address extracted from the From header addresses, the message has multiple Author Addresses.
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 2.4. Author Domain
A "Valid Signature" is any signature on a message which correctly An "Author Domain" is everything to the right of the "@" in an Author
verifies using the procedure described in section 6.1 of [RFC4871]. Address (excluding the "@" itself).
2.5. Alleged Author 2.5. Alleged Author
An "Alleged Author" is the Author Address of a message received by an An "Alleged Author" is an Author Address of a message; it is
Evaluator; it is "alleged" because it has not yet been verified. "alleged" because it has not yet been verified.
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 2.6. Author Signing Practices
The "Author Domain" is everything to the right of the "@" in the "Author Signing Practices" (or just "practices") consist of a
Author Address (excluding the "@" itself). 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.
2.8. Author Signature 2.7. Author Signature
An "Author Signature" is any Valid Signature where the identity of 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 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 the ""i="" tag or its default value from the ""d="" tag) matches an
Author Address in the message. Author Address in the message. When the identity of the user or
agent includes a Local-part, the identities match if the Local-parts
2.9. Sender Signing Practices Record match and the domains match. Otherwise, the identities match if the
domains match.
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 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
satisfy ASP could be problematic.
3.1.1. DNS Representation 3. Operation Overview
Sender Signing Practices Records are published using the DNS "TXT" Domain owners can publish Author Signing Practices via a query
resource record type. mechanism such as the Domain Name System; specific details are given
in Section 4.1.
*[[DRAFT DISCUSSION, TO BE DELETED BEFORE PUBLICATION*: There has Hosts can look up the Author Signing Practices of the domain(s)
been considerable discussion on the DKIM WG mailing list regarding specified by the Author Address(es) as described in Section 4.2.2.
the relative advantages of TXT and a new resource record (RR) If a message has multiple Author Addresses the ASP lookups SHOULD be
type. Many DNS server and resolver implementations are incapable performed independently on each address. This standard does not
of quickly and easily supporting new resource record types. For address the process a host might use to combine the lookup results.
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 3.1. ASP Usage
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 Depending on the Author Domain(s) and the signatures in a message, a
specific syntax and semantics relating to their role in describing recipient gets varying amounts of useful information from each ASP
sender signing practices. SSP records follow the tag-list syntax lookup.
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 o If a message has no Valid Signature, the ASP result is directly
relevant to the message.
SSP records for a domain are published at a location in the domain's o If a message has a Valid Signature from an Author Domain, ASP
DNS hierarchy prefixed by "_ssp._domainkey"; e.g., the SSP record for provides no benefit relative to that domain since the message is
"example.com" would be a "TXT" record that is published at already known to be compliant with any possible ASP for that
"_ssp._domainkey.example.com". domain.
Sender Signing Practices are intended to apply to all mail sent from o If a message has a Valid Signature from a domain other than an
the domain of an Alleged Author. In order to ensure that SSP applies Author Domain, the receiver can use both the Signature and the ASP
to any hosts within that domain (e.g., www.example.com, result in its evaluation of the message.
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 3.2. ASP Results
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 An Author Signing Practices lookup for an Author Address produces one
problem. This is discussed in more detail in Section 5.2. of four possible results:
3.2. Lookup of SSP Records 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.
NON-NORMATIVE NOTE: While the operation of the Evaluator is outside o All messages from this domain are signed.
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 o All messages from this domain are signed and discardable.
A Sender Signing Practices check produces one of four possible o The domain does not exist.
results for use by the Evaluator:
1. The domain does not exist in DNS. 4. Detailed Description
2. The domain does exist, but no SSP Record is present. 4.1. DNS Representation
3. The SSP Record exists, and that value is also returned. Author Signing Practices records are published using the DNS TXT
resource record type.
4. The DNS information could not be determined due to a transient NON-NORMATIVE DISCUSSION [to be removed before publication]: There
error such as "SERVFAIL". 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.
3.2.2. SSP Lookup Algorithm 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.
SSP Checkers doing an SSP lookup MUST produce a result that is The ASP record for a domain is published at a location in the
semantically equivalent to applying the following steps in the order domain's DNS hierarchy prefixed by _asp._domainkey.; e.g., the ASP
listed below. In practice, several of these steps can be performed record for example.com would be a TXT record that is published at
in parallel in order to improve performance. However, "_asp._domainkey.example.com". A domain MUST NOT publish more than
implementations SHOULD avoid doing unnecessary DNS lookups. For the one ASP record; the semantics of an ASP lookup that returns multiple
purposes of this section a "valid SSP record" is one that is both ASP records for a single domain are undefined. (Note that
syntactically and semantically correct; in particular, it must match example.com and mail.example.com are different domains.)
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 4.2. Publication of ASP Records
TXT record corresponding to the Author Domain prefixed by
""_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 Author Signing Practices are intended to apply to all mail sent from
for a record corresponding to the Author Domain (with no prefix). the domain of an Alleged Author. In order to ensure that ASP applies
The type of the query can be of any type, since this step is only to any hosts within that domain (e.g., www.example.com,
to determine if the domain itself exists in DNS. This query MAY ftp.example.com.) the ASP lookup algorithm looks up one level in the
be done in parallel with the query made in step 2. If the result domain tree. For example, mail signed by www.example.com could be
of this query is an "NXDOMAIN" error, the SSP Checker MUST return covered by the ASP record for example.com. This avoids the need to
an appropriate error to the Evaluator and terminate the include an ASP record for every name within a given domain.
algorithm.
NON-NORMATIVE DISCUSSION: Any resource record type could be Normally, a domain expressing Author Signing Practices will want to
used for this query since the existence of a resource record do so for both itself and all of its "descendants" (child domains at
of any type will prevent an "NXDOMAIN" error. "MX" is a all lower levels). Domains wishing to do so MUST publish ASP records
reasonable choice for this purpose is because this record type for the domain itself and any subdomains.
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 Wildcards within a domain publishing ASP records pose a particular
record for the immediate parent domain, prefixed with problem. This is discussed in more detail in Section 6.3.
""_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 4.2.1. Record Syntax
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 ASP records use the "tag=value" syntax described in section 3.2 of
[RFC4871].
SSP Records MUST match the "tag-list" syntax defined in [RFC4871]. Tags used in ASP records are described below. Unrecognized tags MUST
The specific tags used in SSP records are described below. be ignored. In the ABNF below, the WSP token is imported from
Unrecognized tags MUST be ignored. [RFC2822]. The ALPHA and DIGIT tokens are imported from [RFC5234].
dkim= Outbound signing practices for the domain (plain-text; dkim= Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows: REQUIRED). Possible values are as follows:
unknown The domain may sign none, some, or all email. unknown The domain might sign some or all email.
all All mail from the domain is signed with an Author Signature. all All mail from the domain is signed with an Author Signature.
discardable All mail from the domain is signed with an Author discardable All mail from the domain is signed with an Author
Signature. Furthermore, if a message arrives without a valid Signature. Furthermore, if a message arrives without a valid
Author Signature due to modification in transit, submission via Author Signature due to modification in transit, submission via
a path without access to a signing key, or other reason, the a path without access to a signing key, or other reason, the
domain encourages the recipient(s) to discard it. 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: ABNF:
ssp-dkim-tag = "dkim" *WSP "=" *WSP ("unknown" / asp-dkim-tag = %x64.6b.69.6d *WSP "="
"all" / "discardable") *WSP ("unknown" / "all" / "discardable")
t= Flags, represented as a colon-separated list of names (plain-text; t= Flags, represented as a colon-separated list of names (plain-text;
OPTIONAL, default is that no flags are set). Flag values are: OPTIONAL, default is that no flags are set). Flag values are:
s The signing practices apply only to the named domain, and not s The signing practices apply only to the named domain, and not
to subdomains. to subdomains.
ABNF: ABNF:
ssp-t-tag = %x75 *WSP "=" *WSP ssp-t-tag-flag asp-t-tag = %x74 *WSP "=" *WSP { asp-t-tag-flag
0*( *WSP ":" *WSP ssp-t-tag-flag ) 0*( *WSP ":" *WSP asp-t-tag-flag )
ssp-t-tag-flag = "s" / hyphenated-word asp-t-tag-flag = "s" / hyphenated-word
; for future extension ; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ] (ALPHA / DIGIT) ]
Unrecognized flags MUST be included in the result that is provided
to the Evaluator.
4. IANA Considerations Unrecognized flags MUST be ignored.
IANA is requested to create a "DKIM selector name" registry and to 4.2.2. Author Signing Practices Lookup Procedure
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 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.
5. Security Considerations 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.
Security considerations in the Sender Signing Practices are mostly 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.
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
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 related to attempts on the part of malicious senders to represent
themselves as other authors, often in an attempt to defraud either themselves as authors for whom they are not authorized to send mail,
the recipient or an Alleged Author. often in an attempt to defraud either the recipient or an Alleged
Author.
Additional security considerations regarding Sender Signing Practices Additional security considerations regarding Author Signing Practices
may be found in the DKIM threat analysis [RFC4686]. are found in the DKIM threat analysis [RFC4686].
*<<<THIS SECTION IS NOT COMPLETE.>>>* 6.1. ASP Threat Model
5.1. DNS Attacks 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.
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 An attacker might attack the DNS infrastructure in an attempt to
impersonate SSP records. However, such an attacker is more likely to impersonate ASP records, in an attempt to influence a receiver's
attack at a higher level, e.g., redirecting "A" or "MX" record decision on how it will handle mail. However, such an attacker is
lookups in order to capture traffic that was legitimately intended more likely to attack at a higher level, e.g., redirecting A or MX
for the target domain. Domains concerned about this should use record lookups in order to capture traffic that was legitimately
DNSSEC [RFC4033]. intended for the target domain. These DNS security issues are
addressed by DNSSEC [RFC4033].
Because SSP operates within the framework of the legacy e-mail Because ASP operates within the framework of the legacy e-mail
system, the default result in the absence of an SSP record is that 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 the domain does not sign all of its messages. It is therefore
important that the SSP Checker distinguish a DNS failure such as important that the ASP clients distinguish a DNS failure such as
SERVFAIL from other DNS errors so that appropriate actions can be "SERVFAIL" from other DNS errors so that appropriate actions can be
taken. taken.
5.2. DNS Wildcards 6.3. DNS Wildcards
Wildcards within a domain publishing SSP records, including but not Wildcards within a domain publishing ASP records, including but not
limited to wildcard "MX" records, pose a particular problem. While limited to wildcard MX records, pose a particular problem. While
referencing the immediate parent domain allows the discovery of an referencing the immediate parent domain allows the discovery of an
SSP record corresponding to an unintended immediate-child subdomain, ASP record corresponding to an unintended immediate-child subdomain,
wildcard records apply at multiple levels. For example, if there is wildcard records apply at multiple levels. For example, if there is
a wildcard "MX" record for "example.com", the domain a wildcard MX record for "example.com", the domain
"foo.bar.example.com" can receive mail through the named mail "foo.bar.example.com" can receive mail through the named mail
exchanger. Conversely, the existence of the record makes it exchanger. Conversely, the existence of the record makes it
impossible to tell whether "foo.bar.example.com" is a legitimate name 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 since a query for that name will not return an "NXDOMAIN" error. For
that reason, SSP coverage for subdomains of domains containing a that reason, ASP coverage for subdomains of domains containing a
wildcard record is incomplete. wildcard record is incomplete.
NON-NORMATIVE NOTE: Complete SSP coverage of domains containing NON-NORMATIVE NOTE: Complete ASP coverage of domains containing (or
(or where any parent contains) wildcards generally cannot be where any parent contains) wildcards generally cannot be provided by
guaranteed. standard DNS servers.
6. References
6.1. Normative References 7. References
[RFC1035] Mockapetris, P., "Domain names - implementation and 7.1. References - Normative
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 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, [RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001. 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. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006. 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 [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016, (DKIM) Signing Practices Protocol", RFC 5016,
October 2007. October 2007.
Appendix A. Usage Examples (INFORMATIVE) Appendix A. Usage Examples
These examples are intended to illustrate typical uses of SSP. They These examples are intended to illustrate typical uses of ASP. They
are not intended to be exhaustive, nor to apply to every domain or are not intended to be exhaustive, nor to apply to every domain's or
mail system's individual situation. 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 Domain managers are advised to consider the ways that mail processing
can modify messages in ways that will invalidate an existing DKIM can modify messages in ways that will invalidate an existing DKIM
signature, such as mailing lists, courtesy forwarders, and other signature, such as mailing lists, courtesy forwarders, and other
paths that could add or modify headers, or modify the message body. paths that could add or modify headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature, In that case, if the modifications invalidate the DKIM signature,
recipient MTAs will consider the mail not to have an Author recipient hosts will consider the mail not to have an Author
Signature, even though the signature was present when the mail was Signature, even though the signature was present when the mail was
originally sent. 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 A.2. Bulk Mailing Domains
Another common configuration uses a domain solely for bulk or Another common configuration uses a domain solely for bulk or
broadcast mail, with no individual human users, again typically broadcast mail, with no individual human users, again typically
sending all the mail through a single MTA or cluster of MTAs that can 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 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 be confident that all of its outgoing mail will be sent through the
signing MTA. Lacking individual users, the domain is unlikely to signing MTA. Lacking individual users, the domain is unlikely to
participate in mailing lists, but could still send mail through other participate in mailing lists, but could still send mail through other
paths that might invalidate signatures. paths that might invalidate signatures.
Domain owners often use specialist mailing providers to send their Domain owners often use specialist mailing providers to send their
bulk mail. In that case, the mailing provider needs access to a bulk mail. In that case, the mailing provider needs access to a
suitable signing key in order to apply an Author Signature. One suitable signing key in order to apply an Author Signature. One
possible route would be for the domain owner to generate the key and 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 give it to the mailing provider. Another would be for the domain to
delegate a subdomain to the mailing provider, for example, delegate a subdomain to the mailing provider, for example,
bigbank.example might delegate email.bigbank.example to such a bigbank.example might delegate email.bigbank.example to such a
provider. In that case, the provider can generate the keys and DKIM 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 DNS records itself and use the subdomain in the Author address in the
mail. mail.
A.3. Bulk Mailing Domains with Discardable Mail A.3. Bulk Mailing Domains with Discardable Mail
In some cases, a domain might sign all its outgoing mail with an In some cases, a domain might sign all its outgoing mail with an
Author Signature, but prefers that recipient systems discard mail Author Signature, but prefer that recipient systems discard mail
without a valid Author Signature to avoid confusion from mail sent without a valid Author Signature to avoid confusion from mail sent
from sources that do not apply an Author Signature. (This latter from sources that do not apply an Author Signature. (This latter
kind of mail is sometimes loosely called "forgeries".) In that case, kind of mail is sometimes loosely called "forgeries".) In that case,
it may be appropriate to publish an SSP record containing it might be appropriate to publish an ASP record containing
"discardable". Note that a domain SHOULD NOT publish a "discardable" "discardable". Note that a domain SHOULD NOT publish a "discardable"
record if it wishes to maximize the likelihood that mail from the record if it wishes to maximize the likelihood that mail from the
domain is delivered, since it could cause some fraction of the mail domain is delivered, since it could cause some fraction of the mail
the domain sends to be discarded. the domain sends to be discarded.
As a special case, if a domain sends no mail at all, it can safely 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 publish a "discardable" ASP record, since any mail with an author
address in the domain is a forgery. address in the domain is a forgery.
A.4. Third Party Senders A.4. Third Party Senders
Another common use case is for a third party to enter into an 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 agreement whereby that third party will send bulk or other mail on
behalf of a designated author domain, using that domain in the behalf of a designated author domain, using that domain in the
RFC2822 From: or other headers. Due to the many and varied RFC2822 From: or other headers. Due to the many and varied
complexities of such agreements, third party signing is not addressed complexities of such agreements, third party signing is not addressed
in this specification. The authors anticipate that as mail systems in this specification.
gain experience with DKIM, it will become possible to codify best
practices of this and other usages of DKIM.
Appendix B. Acknowledgements Appendix B. Acknowledgements
The authors wish to thank many members of the ietf-dkim mailing list This document greatly benefited from comments by Steve Atkins, Jon
for valuable suggestions and constructive criticism of earlier Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael
versions of this draft. Thomas, and Wietse Venema.
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 Appendix C. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of *NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.* this document as an RFC.*
C.1. Changes since -ietf-dkim-ssp-01 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.
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 Reworded introduction for clarity.
o Various definition clarifications. o Various definition clarifications.
o Changed names of practices to unknown, all, and discardable. o Changed names of practices to unknown, all, and discardable.
o Removed normative language mandating use of SSP in particular o Removed normative language mandating use of SSP in particular
situations (issue 1538). situations (issue 1538).
skipping to change at page 16, line 5 skipping to change at page 17, line 10
o Introduced the concepts of "SSP Checker" and "Evaluator". o Introduced the concepts of "SSP Checker" and "Evaluator".
o Multiple author case now handled my separate invocations of SSP o Multiple author case now handled my separate invocations of SSP
checker by Evaluator (issue 1525). checker by Evaluator (issue 1525).
o Removed check to avoid querying top-level domains. o Removed check to avoid querying top-level domains.
o Changed ABNF use of whitespace from [FWS] to *WSP (partially o Changed ABNF use of whitespace from [FWS] to *WSP (partially
addresses issue 1543). addresses issue 1543).
C.2. Changes since -ietf-dkim-ssp-00 C.3. Changes since -ietf-dkim-ssp-00
o Clarified Operation Overview and eliminated use of Legitimate as o Clarified Operation Overview and eliminated use of Legitimate as
the counterpart of Suspicious since the words have different the counterpart of Suspicious since the words have different
meanings. meanings.
o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
records in DNS vs. a new RR type. records in DNS vs. a new RR type.
o Clarified publication rules for multilevel names. o Clarified publication rules for multilevel names.
skipping to change at page 16, line 34 skipping to change at page 17, line 39
o Added "handling" tag to express alleged sending domain's o Added "handling" tag to express alleged sending domain's
preference about handling of Suspicious messages. preference about handling of Suspicious messages.
o Clarified handling of SERVFAIL error in SSP check. o Clarified handling of SERVFAIL error in SSP check.
o Replaced "entity" with "domain", since with the removal of user- o Replaced "entity" with "domain", since with the removal of user-
granularity SSP, the only entities having sender signing policies granularity SSP, the only entities having sender signing policies
are domains. are domains.
C.3. Changes since -allman-ssp-02 C.4. Changes since -allman-ssp-02
o Removed user-granularity SSP and u= tag. o Removed user-granularity SSP and u= tag.
o Replaced DKIMP resource record with a TXT record. o Replaced DKIMP resource record with a TXT record.
o Changed name of the primary tag from "p" to "dkim". o Changed name of the primary tag from "p" to "dkim".
o Replaced lookup algorithm with one which traverses upward at most o Replaced lookup algorithm with one which traverses upward at most
one level. one level.
o Added description of records which must be published, and effect o Added description of records to be published, and effect of
of wildcard records within the domain, on SSP. wildcard records within the domain, on SSP.
C.4. Changes since -allman-ssp-01 C.5. Changes since -allman-ssp-01
o Changed term "Sender Signing Policy" to "Sender Signing o Changed term "Sender Signing Policy" to "Sender Signing
Practices". Practices".
o Changed query methodology to use a separate DNS resource record o Changed query methodology to use a separate DNS resource record
type, DKIMP. type, DKIMP.
o Changed tag values from SPF-like symbols to words. o Changed tag values from SPF-like symbols to words.
o User level policies now default to that of the domain if not o User level policies now default to that of the domain if not
specified. specified.
o Removed the "Compliance" section since we're still not clear on o Removed the "Compliance" section since we're still not clear on
what goes here. what goes here.
o Changed the "parent domain" policy to only search up one level o Changed the "parent domain" policy to only search up one level
(assumes that subdomains will publish SSP records if appropriate). (assumes that subdomains will publish SSP records if appropriate).
o Added detailed description of SSP check procedure. o Added detailed description of SSP check procedure.
C.5. Changes since -allman-ssp-00 C.6. Changes since -allman-ssp-00
From a "diff" perspective, the changes are extensive. Semantically, From a "diff" perspective, the changes are extensive. Semantically,
the changes are: the changes are:
o Added section on "Third-Party Signatures and Mailing Lists" o Added section on "Third-Party Signatures and Mailing Lists"
o Added "Compliance" (transferred from -base document). I'm not o Added "Compliance" (transferred from -base document). I'm not
clear on what needs to be done here. clear on what needs to be done here.
o Extensive restructuring. o Extensive restructuring.
Authors' Addresses Authors' Addresses
Eric Allman Eric Allman
Sendmail, Inc. Sendmail, Inc.
6475 Christie Ave, Suite 350 6475 Christie Ave, Suite 350
Emeryville, CA 94608 Emeryville, CA 94608
USA
Phone: +1 510 594 5501 Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org Email: eric+dkim@sendmail.org
URI: 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 Mark Delany
Yahoo! Inc. Yahoo! Inc.
701 First Avenue 701 First Avenue
Sunnyvale, CA 94089 Sunnyvale, CA 94089
USA
Phone: +1 408 349 6831 Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com Email: markd+dkim@yahoo-inc.com
URI:
Jim Fenton John Levine
Cisco Systems, Inc. Taughannock Networks
MS SJ-9/2 PO Box 727
170 W. Tasman Drive Trumansburg, NY 14886
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914 Phone: +1 831 480 2300
Email: fenton@cisco.com Email: standards@taugh.com
URI: URI: http://www.taugh.com
Full Copyright Statement Full Copyright Statement
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