DKIM
Network Working Group                                          E. Allman
Internet-Draft                                            Sendmail, Inc.
Intended status: Standards Track                              M. Delany                               J. Fenton
Expires: August 4, 26, 2008                                     Yahoo! Inc.
                                                               J. Fenton                             Cisco Systems, Inc.
                                                               M. Delany
                                                             Yahoo! Inc.
                                                               J. Levine
                                                    Taughannock Networks
                                                       February 1, 23, 2008

                  DKIM Sender Author Signing Practices
                         draft-ietf-dkim-ssp-02 (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
   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 regarding for signing their outgoing mail,
   and how other hosts can access, parse and interpret access 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

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Language and Terminology . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terms Imported from DKIM Signatures Specification  . . . .  5  4
     2.2.  Evaluator  . . .  Valid Signature  . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  SSP Checker  .  Author Address . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Valid Signature  Author Domain  . . . . . . . . . . . . . . . . . . . . . .  5
     2.5.  Alleged Author . . . . . . . . . . . . . . . . . . . . . .  5
     2.6.  Author Address . . . . . Signing Practices . . . . . . . . . . . . . . . . .  6  5
     2.7.  Author Domain Signature . . . . . . . . . . . . . . . . . . . . .  5
   3.  Operation Overview .  6
     2.8.  Author Signature . . . . . . . . . . . . . . . . . . . . .  6
     2.9.  Sender Signing Practices Record  .
     3.1.  ASP Usage  . . . . . . . . . . . .  6
   3.  Operational Description . . . . . . . . . . . .  6
     3.2.  ASP Results  . . . . . . .  6
     3.1.  Publication of SSP Records . . . . . . . . . . . . . . . .  6
     3.2.  Lookup of SSP Records  . .
   4.  Detailed Description . . . . . . . . . . . . . . . .  8
     3.3.  SSP Record Syntax . . . . .  7
     4.1.  DNS Representation . . . . . . . . . . . . . . .  9
   4.  IANA Considerations . . . . .  7
     4.2.  Publication of ASP Records . . . . . . . . . . . . . . . . 11  7
   5.  Security  IANA Considerations  . . . . . . . . . . . . . . . . . . . 11
     5.1.  DNS Attacks . . 10
     5.1.  ASP Specification Tag Registry . . . . . . . . . . . . . . 10
     5.2.  ASP Outbound Signing Practices Registry  . . . . . . . 11
     5.2.  DNS Wildcards . . 10
     5.3.  ASP Flags Registry . . . . . . . . . . . . . . . . . . . . 11
   6.  References  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     6.1.  ASP Threat Model . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . 11
     6.2.  DNS Attacks  . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Usage Examples (INFORMATIVE)
     6.3.  DNS Wildcards  . . . . . . . . . . . . 13
     A.1.  Single Location Domains . . . . . . . . . . 12
   7.  References . . . . . . . 13
     A.2.  Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 13
     A.3.  Bulk Mailing Domains with Discardable Mail
     7.1.  References - Normative . . . . . . . . 14
     A.4.  Third Party Senders . . . . . . . . . . 13
     7.2.  References - Informative . . . . . . . . . 14
   Appendix B.  Acknowledgements . . . . . . . . 13
   Appendix A.  Usage Examples  . . . . . . . . . . 14
   Appendix C.  Change Log . . . . . . . . . 13
     A.1.  Single Location Domains  . . . . . . . . . . . . 14
     C.1.  Changes since -ietf-dkim-ssp-01 . . . . . 14
     A.2.  Bulk Mailing Domains . . . . . . . . 15 . . . . . . . . . . . 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-00 -ietf-dkim-ssp-01  . . . . . . . . . . . . . 16
     C.3.  Changes since -ietf-dkim-ssp-00  . . . . . . . . . . . . . 17
     C.4.  Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 16
     C.4. 17
     C.5.  Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 16
     C.5. 18
     C.6.  Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 17 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19 20

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, signed, and the absence of a signature on a message is not an
   a priori indication of forgery.  In fact, during early phases of DKIM
   deployment it must be expected is very likely that most messages will remain unsigned.  Nevertheless,
   However, some domains may find it highly desirable might decide 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
   mail, for example, to do so, the benefits of DKIM are limited protect their brand name.  It is desirable for
   such domains to
   cases in which a valid signature exists and cannot be extended able to
   cases in which signatures are missing or are invalid.  Defining such
   a mechanism advertise that fact to other hosts.  This
   is the purpose topic of Sender Author 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 (ASP).

   Hosts implementing this specification because they fall under the purview of reputation and
   accreditation.  Sender can inquire what Author 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 domain advertises.  This inquiry is described herein in order to make
   the limits of this specification clear. called an Author
   Signing Practices check.

   The detailed requirements for Sender Author Signing Practices are given in
   [RFC5016], which the protocol described in this document attempts to
   satisfy.
   [RFC5016].  This document refers extensively to [RFC4871], which should [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 read interpreted as a prerequisite to this document.
      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.  In many cases it
      will correspond closely with the original author message, as defined in
      section 2.1 of the message or
      an agent working on the author's behalf. [RFC4871].

   o  "Selectors" describe  A "Selector" specifies which of the keys published by a signing domain.
      Signing domains may have multiple Selectors.  Selectors subdivide
      the address space to allow a single sending
      domain is to publish
      multiple keys. be queried, as defined in section 3.1 of [RFC4871].

   o  A "Verifier" "Local-part" is the agent that verifies a message by checking
      actual signature(s) in the message header against the message
      itself using part of an address preceding the public key published @ sign, as
      defined in the Selector referenced
      by a given signature. [RFC2822] and used in [RFC4871].

2.2.  Evaluator

   The "Evaluator"  Valid Signature

   A "Valid Signature" 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 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.

2.6.

2.3.  Author Address

   The

   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, which may
   potentially cause the Evaluator to perform multiple SSP Checks for a
   given message.

2.7. Addresses.

2.4.  Author Domain

   The

   An "Author Domain" is everything to the right of the "@" in the an Author
   Address (excluding the "@" itself).

2.8.

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=" ""i="" tag or its default value from the "d=" ""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  When the domain identity of an Alleged Author which the user or
   agent includes
   information on whether that domain signs all of their email, a Local-part, the identities match if the Local-parts
   match 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 the domains wishing to do so

      Lookup of SSP records by an SSP Checker under match.  Otherwise, the direction of an
      Evaluator.

3.1.  Publication of SSP Records

3.1.1.  DNS Representation

   Sender Signing Practices Records are published using identities match if the DNS "TXT"
   resource record type.

      *[[DRAFT DISCUSSION, TO BE DELETED BEFORE PUBLICATION*:  There
   domains match.

   For example, if a message has
      been considerable discussion on a Valid Signature, with the DKIM WG mailing list regarding DKIM-
   Signature field containing "i=a@domain.example", then domain.example
   is asserting that it takes responsibility for the relative advantages of TXT and a new resource record (RR)
      type.  Many DNS server and resolver implementations are incapable
      of quickly message.  If the
   message's From: field contains the address "b@domain.example" and easily supporting new resource record types.  For
      this reason, support of TXT records is required whether an
   ASP query produces a new RR
      type is defined "dkim=all" or not.  However, without "dkim=discardable" result, that
   would mean that the message does not have a "flag day" on which
      SSP TXT record support is to be withdrawn, such support valid Author Signature.
   Even though the message is likely
      to continue indefinitely.  As a result, this specification defines
      no new RR type for SSP.

      Another alternative proposed signed by P. Hallam-Baker is the publication
      of both a TXT record and, when implementations permit, a new RR,
      referred same domain, its failure to
   satisfy ASP could be problematic.

3.  Operation Overview

   Domain owners can publish Author Signing Practices via a query
   mechanism such as XPTR, which gives the location from which SSP and
      other policy information relating to a give domain Domain Name System; specific details are given
   in Section 4.1.

   Hosts can be
      retrieved.  This has look up the advantage of supporting a variety Author Signing Practices of
      policies the domain(s)
   specified by the Author Address(es) as described in Section 4.2.2.
   If a scalable manner, with better handling of wildcards
      and centralized publication of policy records, with caching
      advantages.  However, message has multiple Author Addresses the above implementation issues also apply ASP lookups SHOULD be
   performed independently on each address.  This standard does not
   address the process a host might use to XPTR, 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 additional lookup Author Domain, ASP
      provides no benefit relative to that domain since the message is required
      already known to retrieve SSP via 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 XPTR method.  At receiver can use both the time Signature and the ASP
      result in its evaluation of publication 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 draft,
      consensus on 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 proposal was unclear.*]]* domain are signed.

   o  All messages from this domain are signed and discardable.

   o  The domain does not exist.

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 SSP ASP 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
   Author Signing Practices.  The "Tag=Value List" syntax described in
   section 3.2 of [RFC4871], including the restriction on
   duplicate tags, the use of white space, and case sensitivity. [RFC4871] is used.  Records not in overall compliance with
   that syntax MUST be ignored
   (considered equivalent to a "NODATA" result), although 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.  All syntactically valid tags MUST be made available to  If the Evaluator.

3.1.2.  Location of SSP Records

   SSP records RDATA
   contains multiple character strings, the strings are logically
   concatenated with no delimiters between the strings.

   The ASP record for a domain are is published at a location in the
   domain's DNS hierarchy prefixed by "_ssp._domainkey"; _asp._domainkey.; e.g., the SSP ASP
   record for
   "example.com" example.com would be a "TXT" TXT record that is published at
   "_ssp._domainkey.example.com".

   Sender
   "_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 SSP ASP applies
   to any hosts within that domain (e.g., www.example.com,
   ftp.example.com, etc.)
   ftp.example.com.) the SSP ASP lookup algorithm looks up one level in the
   domain tree.  For example, mail signed by www.example.com may
   optionally could be
   covered by the SSP ASP record for example.com.  This
   prevents administrators from having avoids the need to
   include an SSP ASP record for every name within a given domain.

   Normally, a domain expressing Sender Author Signing Practices will want to
   do so for both itself and all of its "descendents" "descendants" (child domains at
   all lower levels).  Domains wishing to do so MUST publish SSP ASP records
   for the domain itself and any subdomains.

   Wildcards within a domain publishing SSP ASP 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 6.3.

4.2.1.  Record Syntax

   ASP records use the operation "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 Evaluator is outside ABNF below, the scope of this specification, it WSP token is generally not worthwhile imported from
   [RFC2822].  The ALPHA and DIGIT tokens are imported from [RFC5234].

   dkim=  Outbound signing practices for
   an Evaluator to request an SSP check when the results of that check
   will not affect the disposition of the message.  Since domain (plain-text;
      REQUIRED).  Possible values are as follows:

      unknown  The domain might sign some or all email.

      all  All mail from the
   information provided by SSP domain is only relevant in the absence of valid signed with an Author Signature(s), there Signature.

      discardable  All mail from the domain is little to be gained by performing signed with an
   SSP check on domains corresponding to Author
         Signature.  Furthermore, if a message arrives without a valid
         Author Signatures.  SSP
   checks may also be unnecessary when the Evaluator has some other
   basis for deciding Signature due to process the message "normally", including, but
   not limited to, the presence of modification in transit, submission via
         a DKIM signature that the Evaluator
   has some basis path without access 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 a signing key, or other reason, the Evaluator:

   1.  The domain does not exist in DNS.

   2.  The
         domain does exist, but no SSP Record 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 present.

   3.  The SSP Record exists, and that value is also returned.

   4. no flags are set).  Flag values are:

      s  The DNS information could signing practices apply only to the named domain, and not be determined due
         to a transient
       error such as "SERVFAIL".

3.2.2.  SSP 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.

4.2.2.  Author Signing Practices Lookup Algorithm

   SSP Checkers Procedure

   Hosts doing an SSP 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 SSP ASP record" is one that is both syntactically and semantically
   correct; in particular, it must match matches the ABNF for a "tag-list" and must include
   includes a defined "dkim=" tag.

   1.  _Fetch Named SSP ASP Record._ The SSP Checker host MUST query DNS for a TXT
       record corresponding to the Author Domain prefixed by
       ""_ssp._domainkey.""
       "_asp._domainkey." (note the trailing dot).  If the result of
       this query is a "NOERROR" response with one or more answers an answer which
       are is a
       valid SSP records, return ASP record, use that record for interpretation by
       the Evaluator; record; otherwise, continue to the
       next step.

   2.  _Verify Domain Exists._ The SSP Checker 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 SSP Checker algorithm MUST return terminate
       with an appropriate error to the Evaluator and terminate the
       algorithm. 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"  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 host MUST query DNS for a TXT record for
       the immediate parent domain, prefixed with
       ""_ssp._domainkey."" "_asp._domainkey."  If
       the result of this query is anything other than a "NOERROR"
       response with a valid SSP ASP record, the algorithm terminates returning with a
       result indicating that no SSP ASP record was present.  If the SSP ASP "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.

   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 in the potential for mailing lists response and similar
         agents any of the flags is "s"
       (indicating it does not apply to modify messages 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 such a way as to render "SERVFAIL" error response, the
         signature invalid.  Domains sending mail 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 is expected to
         pass have been registered with no significant modification to the recipient, such as
         domains sending only transactional messages,
   IANA.  In all cases, new values are appropriate
         places assigned only for values that
   have been documented in a published RFC that has IETF Consensus
   [RFC2434].

   INFORMATIVE NOTE [ to consider the be removed before publication of ]: RFC 4871
   defines a "discardable" practice.
         See [RFC5016] section 5.3 and Appendix selector as a sub-domain, importing the term from RFC 2822.
   A for further
         discussion.

      ABNF:

   ssp-dkim-tag   = "dkim" *WSP "=" *WSP ("unknown" /
                     "all" / "discardable")

   t= Flags, represented 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 colon-separated list of names (plain-text;
      OPTIONAL, default is specification tags.  IANA has
   established the ASP Specification Tag Registry for specification tags
   that no flags are set).  Flag values are:

      s can be used in ASP fields.

   The signing practices apply only to initial entries in 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
                           ; 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 future extension
   hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
                     (ALPHA / DIGIT) ]
      Unrecognized flags MUST be included a value
   specifying Outbound Signing Practices.  IANA has established the ASP
   Outbound Signing Practices Registry for Outbound Signing Practices.

   The initial entries 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 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 selector name ""_ssp"" to avoid confusion between DKIM
   key records and SSP records.

   *<<< Needs to be updated to be more complete; see 4871 ASP Flags Registry for examples
   >>>*

5.
   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 Sender Author Signing Practices are mostly
   related to attempts on the part of malicious senders to represent
   themselves as other authors, authors for whom they are not authorized to send mail,
   often in an attempt to defraud either
   the recipient or an 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.

   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.

   Additional security considerations regarding Sender Signing Practices
   may be found Author
   Domain.  Since these lookups are driven by domain names in the DKIM threat analysis [RFC4686].

      *<<<THIS SECTION IS NOT COMPLETE.>>>*

5.1. 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 SSP records. 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" A or "MX" MX
   record lookups in order to capture traffic that was legitimately
   intended for the target domain.  Domains concerned about this should use  These DNS security issues are
   addressed by DNSSEC [RFC4033].

   Because SSP ASP operates within the framework of the legacy e-mail
   system, the default result in the absence of an SSP ASP record is that
   the domain does not sign all of its messages.  It is therefore
   important that the SSP Checker ASP clients distinguish a DNS failure such as
   SERVFAIL
   "SERVFAIL" from other DNS errors so that appropriate actions can be
   taken.

5.2.

6.3.  DNS Wildcards

   Wildcards within a domain publishing SSP ASP records, including but not
   limited to wildcard "MX" MX records, pose a particular problem.  While
   referencing the immediate parent domain allows the discovery of an
   SSP
   ASP record corresponding to an unintended immediate-child subdomain,
   wildcard records apply at multiple levels.  For example, if there is
   a wildcard "MX" 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 ASP coverage for subdomains of domains containing a
   wildcard record is incomplete.

   NON-NORMATIVE NOTE: Complete SSP ASP coverage of domains containing (or
   where any parent contains) wildcards generally cannot be
      guaranteed.

6. provided by
   standard DNS servers.

7.  References

6.1.  Normative

7.1.  References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987. 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.

   [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.

   [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 (INFORMATIVE)

   These examples are intended to illustrate typical uses of SSP. ASP.  They
   are not intended to be exhaustive, nor to apply 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
   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 every domain or have an Author
   Signature, even though the signature was present when the mail system's individual situation. 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 cluster 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 SSP ASP record
   for the domain containing "all", depending on whether the users also
   send mail through other MTAs paths that do not apply an Author Signature.
   Such MTAs paths 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 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 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 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
   kind of mail is sometimes loosely called "forgeries".)  In that case,
   it may might be appropriate to publish an SSP 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" SSP 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.  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 greatly benefited from comments by John Levine.  The authors appreciate
   this contribution. 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.

   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).

   o  Removed check to avoid querying top-level domains.

   o  Changed ABNF use of whitespace from [FWS] to *WSP (partially
      addresses issue 1543).

C.2.

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.3.

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 which must to be published, and effect of
      wildcard records within the domain, on SSP.

C.4.

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.5.

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
   USA

   Phone: +1 510 594 5501
   Email: eric+dkim@sendmail.org
   URI:

   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

   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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