Network Working Group                                          E. Allman
Internet-Draft                                            Sendmail, Inc.
Intended status: Standards Track                               J. Fenton
Expires: August 26, 2008 January 3, 2009                             Cisco Systems, Inc.
                                                               M. Delany
                                                             Yahoo! Inc.
                                                               J. Levine
                                                    Taughannock Networks
                                                       February 23,
                                                            July 2, 2008

              DKIM Author Domain Signing Practices (ASP)
                         draft-ietf-dkim-ssp-03 (ADSP)
                         draft-ietf-dkim-ssp-04

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Copyright Notice

   Copyright (C) The IETF Trust (2008). January 3, 2009.

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.  This document specifies an adjunct
   mechanism to aid in assessing messages by either Mail Transport Agents (MTAs) or Mail
   User Agents (MUAs).  The primary that do not contain a DKIM protocol is described
   signature for the domain used in
   [RFC4871].  This document describes the records author's address.  It defines a
   record that authors' domains can use to advertise their practices for signing whether they sign their outgoing mail, and
   how other hosts can access those records.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  3
   2.  Language and Terminology . . . . . . . . . . . . . . . . . . .  4  3
     2.1.  Terms Imported from DKIM Signatures Specification  . . . .  4  3
     2.2.  Valid Signature  . . . . . . . . . . . . . . . . . . . . .  5  4
     2.3.  Author Address . . . . . . . . . . . . . . . . . . . . . .  5  4
     2.4.  Author Domain  . . . . . . . . . . . . . . . . . . . . . .  5  4
     2.5.  Alleged Author . . . . . . . . . . . . . . . . . . . . . .  5  4
     2.6.  Author Domain Signing Practices  . . . . . . . . . . . . . . . . .  5  4
     2.7.  Author Signature . . . . . . . . . . . . . . . . . . . . .  5  4
   3.  Operation Overview . . . . . . . . . . . . . . . . . . . . . .  6  5
     3.1.  ASP  ADSP Applicability . . . . . . . . . . . . . . . . . . . .  5
     3.2.  ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  ASP  5
     3.3.  ADSP Results . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Detailed Description . . . . . . . . . . . . . . . . . . . . .  7  6
     4.1.  DNS Representation . . . . . . . . . . . . . . . . . . . .  7  6
     4.2.  Publication of ASP ADSP Records  . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . .  6
     4.3.  ADSP Lookup Procedure  . . . . . . . . . . . . . . . . . . 10
     5.1.  ASP Specification Tag Registry  7
   5.  IANA Considerations  . . . . . . . . . . . . . . 10
     5.2.  ASP Outbound Signing Practices Registry . . . . . . .  8
     5.1.  ADSP Specification Tag Registry  . . 10
     5.3.  ASP Flags Registry . . . . . . . . . . .  8
     5.2.  ADSP Outbound Signing Practices Registry . . . . . . . . . 11  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11  9
     6.1.  ASP  ADSP Threat Model  . . . . . . . . . . . . . . . . . . . . . 11  9
     6.2.  DNS Attacks  . . . . . . . . . . . . . . . . . . . . . . . 12 10
     6.3.  DNS Wildcards  . . . . . . . . . . . . . . . . . . . . . . 12 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 11
     7.1.  References - Normative . . . . . . . . . . . . . . . . . . 13 11
     7.2.  References - Informative . . . . . . . . . . . . . . . . . 13 11
   Appendix A.  Usage Examples  . . . . . . . . . . . . . . . . . . . 13 11
     A.1.  Single Location Domains  . . . . . . . . . . . . . . . . . 14 12
     A.2.  Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 14 12
     A.3.  Bulk Mailing Domains with Discardable Mail . . . . . . . . 14 13
     A.4.  Third Party Senders  . . . . . . . . . . . . . . . . . . . 15 13
     A.5.  Non-email Domains  . . . . . . . . . . . . . . . . . . . . 13
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15 13
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 15 13
     C.1.  Changes since -ietf-dkim-02 -ietf-dkim-03  . . . . . . . . . . . . . . . 15 14
     C.2.  Changes since -ietf-dkim-ssp-01 -ietf-dkim-02  . . . . . . . . . . . . . 16 . . 14
     C.3.  Changes since -ietf-dkim-ssp-00 -ietf-dkim-ssp-01  . . . . . . . . . . . . . 17 15
     C.4.  Changes since -ietf-dkim-ssp-00  . . . . . . . . . . . . . 16
     C.5.  Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17
     C.5. 16
     C.6.  Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18
     C.6. 17
     C.7.  Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 18 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 20 19

1.  Introduction

   DomainKeys Identified Mail (DKIM) defines a mechanism by which email
   messages can be cryptographically signed, permitting a signing domain
   to claim responsibility for the introduction of a message into the
   mail stream.  Message recipients can verify the signature by querying
   the signer's domain directly to retrieve the appropriate public key,
   and thereby confirm that the message was attested to by a party in
   possession of the private key for the signing domain.

   However, the legacy of the Internet is such that not all messages
   will be signed, and the absence of a signature on a message is not an
   a priori indication of forgery.  In fact, during early phases of
   deployment it is very likely that most messages will remain unsigned.
   However, some domains might decide to sign all of their outgoing
   mail, for example, to protect their brand name. names.  It is desirable for
   such domains to be able to advertise that fact to other hosts.  This
   is the topic of Author Domain Signing Practices (ASP). (ADSP).

   Hosts implementing this specification can inquire what Author Signing
   Practices a domain advertises.  This inquiry is called an Author
   Signing Practices check.

   The detailed basic requirements for Author Signing Practices ADSP are given in [RFC5016].  This
   document refers extensively to [RFC4871] and assumes the reader is
   familiar with it.

   Requirements Notation:   The key words "MUST", "MUST NOT",
      "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
      "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
      interpreted as described in [RFC2119]

2.  Language and Terminology

2.1.  Terms Imported from DKIM Signatures Specification

   Some terminology used herein is derived directly from [RFC4871].  In
   several cases, references in that document to Sender have been
   changed to Author here, to emphasize the relationship to the Author
   address(es) in the From: header field described in [RFC2822].
   Briefly,

   o  A "Signer" is the agent that signs a message, as defined in
      section 2.1 of [RFC4871].

   o  A "Selector" specifies which of the keys published by a signing
      domain is to be queried, as defined in section 3.1 of [RFC4871].

   o  A "Local-part" is "Local-part" is the part of an address preceding the @ sign,
      character, as defined in [RFC2822] and used in [RFC4871].

2.2.  Valid Signature

   A "Valid Signature" is any signature on a message which correctly
   verifies using the procedure described in section 6.1 of [RFC4871].

2.3.  Author Address

   An "Author Address" is an email address in the From header field of a
   message [RFC2822].  If the From header field contains multiple
   addresses, the message has multiple Author Addresses.

2.4.  Author Domain

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

2.5.  Alleged Author

   An "Alleged Author" is an Author Address of a message; it is
   "alleged" because it has not yet been verified.

2.6.  Author Domain Signing Practices

   "Author Domain 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.  When the identity of the user or
   agent includes a Local-part, the identities match if the Local-parts
   match
   are the same string, and the domains match. are the same string.  Otherwise,
   the identities match if the domains match. are the same string.  Following
   [RFC2821], Local-part comparisons are case sensitive, domain
   comparisons are case insensitive.

   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
   ADSP 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 it fails to
   satisfy ASP could be problematic. ADSP.

3.  Operation Overview

   Domain owners can publish Author Signing Practices ADSP information via a query mechanism such
   as the Domain Name System; specific details are given in Section 4.1.

   Hosts can look up the Author Signing Practices ADSP information of the domain(s) specified by
   the Author Address(es) as described in Section 4.2.2. 4.3.  If a message has
   multiple Author Addresses the ASP ADSP lookups SHOULD be performed
   independently on each address.  This standard does not address the
   process a host might use to combine the lookup results.

3.1.  ASP  ADSP Applicability

   ADSP as defined in this document is bound to DNS.  For this reason,
   ADSP is applicable only to Author Domains with appropriate DNS
   records (see Note below).  The handling of other Author Domains is
   outside the scope of this document.  However, attackers may use such
   domain names in a deliberate attempt to sidestep an organization's
   ADSP policy statements.  It is up to the ADSP verifier implementation
   to return an appropriate error result for Author Domains outside the
   scope of ADSP.

   Note:   The results from DNS queries that are intended to validate a
      domain name unavoidably approximate the set of Author Domains that
      can appear in legitimate email.  For example, a DNS A record could
      belong to a device that does not even have an email
      implementation.  It is up to the verifier to decide what degree of
      approximation is acceptable.

3.2.  ADSP Usage

   Depending on the Author Domain(s) and the signatures in a message, a
   recipient gets varying amounts of useful information from each ASP ADSP
   lookup.

   o  If a message has no Valid Signature, the ASP ADSP result is directly
      relevant to the message.

   o  If a message has a Valid Signature from an Author Domain, ASP ADSP
      provides no benefit relative to that domain since the message is
      already known to be compliant with any possible ASP ADSP for that
      domain.

   o  If a message has a Valid Signature from a domain other than an
      Author Domain, the receiver can use both the Signature and the ASP
      ADSP result in its evaluation of the message.

3.2.  ASP

3.3.  ADSP Results

   An Author Signing Practices ADSP lookup for an Author Address produces one of four possible
   results:

   o  Messages from this domain might or might not have an author
      signature.  This is the default if the domain exists in the DNS
      but no record is found.

   o  All messages from this domain are signed.

   o  All messages from this domain are signed and discardable.

   o  The domain does is not exist. a valid mail domain.

4.  Detailed Description

4.1.  DNS Representation

   Author Signing Practices

   ADSP records are published using the DNS TXT resource record type.

   NON-NORMATIVE DISCUSSION [to be removed before publication]: There
   has been considerable discussion on the DKIM WG mailing list
   regarding the relative advantages of TXT and a new resource record
   (RR) type.  Read the archive for details.

   The RDATA for ASP ADSP resource records is textual in format, with
   specific syntax and semantics relating to their role in describing
   Author Signing Practices.
   ADSP.  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, ADSP,
   although they MAY cause the logging of warning messages via an
   appropriate system logging mechanism.  If the RDATA contains multiple
   character strings, the strings are logically concatenated with no
   delimiters between the strings.

   The ASP ADSP record for a domain is published at a location in the
   domain's DNS hierarchy prefixed by _asp._domainkey.; _adsp._domainkey.; e.g., the ASP ADSP
   record for example.com would be a TXT record that is published at
   "_asp._domainkey.example.com".
   "_adsp._domainkey.example.com".  A domain MUST NOT publish more than
   one ASP ADSP record; the semantics of an ASP ADSP lookup that returns
   multiple
   ASP ADSP records for a single domain are undefined.  (Note that
   example.com and mail.example.com are different domains.)

4.2.  Publication of ASP ADSP Records

   Author Signing Practices are

   ADSP is intended to apply to all mail sent from using the domain name
   string of an Alleged Author.  In order to ensure that ASP applies
   to any hosts

   Wildcards within that a domain (e.g., www.example.com,
   ftp.example.com.) the ASP lookup algorithm looks up one level publishing ADSP records pose a particular
   problem.  This is discussed in more detail in the
   domain tree.  For example, mail signed by www.example.com could be
   covered by the ASP record for example.com.  This avoids the need to
   include an ASP record for every name within a given domain.

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

   Wildcards within a domain publishing ASP records pose a particular
   problem.  This is discussed in more detail in Section 6.3.

4.2.1.  Record Syntax

   ASP records use Section 6.3.

4.2.1.  Record Syntax

   ADSP records use the "tag=value" syntax described in section 3.2 of
   [RFC4871].

   Tags used in ASP ADSP records are described below.  Unrecognized tags
   MUST be ignored.  In the ABNF below, the WSP FWS token is imported from
   [RFC2822].
   [RFC4871].  The ALPHA and DIGIT tokens are imported from [RFC5234].

   dkim=   Outbound signing practices for the domain (plain-text;
      REQUIRED).  Possible values are as follows:

      unknown   The domain might sign some or all email.

      all    All mail from the domain is signed with an Author
         Signature.

      discardable   All mail from the domain is signed with an Author
         Signature.  Furthermore, if a message arrives without a valid
         Author Signature due to modification in transit, submission via
         a path without access to a signing key, or other reason, the
         domain encourages the recipient(s) to discard it.

      ABNF:

         asp-dkim-tag
            adsp-dkim-tag = %x64.6b.69.6d *WSP *FWS "="
               *WSP *FWS
                            ("unknown" / "all" / "discardable")

   t= Flags, represented as a colon-separated list of names (plain-text;
      OPTIONAL, default is that no flags are set).  Flag values are:

      s  The signing practices apply only to the named domain, and not
         to subdomains.

      ABNF:

         asp-t-tag    = %x74 *WSP "=" *WSP { asp-t-tag-flag
               0*( *WSP ":" *WSP asp-t-tag-flag )
         asp-t-tag-flag = "s" / hyphenated-word
               ; for future extension
         hyphenated-word =  ALPHA [ *(ALPHA / DIGIT / "-")
               (ALPHA / DIGIT) ]

      Unrecognized flags MUST be ignored.

4.2.2.  Author Signing Practices

4.3.  ADSP Lookup Procedure

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

   1.  _Fetch Named ASP Record._ The host

   Verify Domain Scope:   An ADSP verifier implementation MUST query DNS for determine
      whether a TXT
       record corresponding to the given Author Domain prefixed by
       "_asp._domainkey." (note is within scope for ADSP.  Given the trailing dot).  If
      background in Section 3.1 the result verifier MUST decide which degree of
       this query
      over-approximation is a "NOERROR" response with acceptable.  The verifier MUST return an answer which is a
       valid ASP record, use
      appropriate error result for Author Domains that record; otherwise, continue to are outside the
       next step.

   2.  _Verify Domain Exists._
      scope of ADSP.

      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. to fetch the Named ADSP Record.  If the result of this query
      is that the Author domain does not exist in the DNS (often called
      an "NXDOMAIN" error, error), the algorithm MUST terminate with an appropriate error. error
      indicating that the domain is out of scope.

         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, domains used in e-mail, and will
         therefore result in produce a result which can be more readily cached
         than a negative result.

   3.  _Try Parent Domain._

      If the domain does exist, the verifier MAY make more extensive
      checks to verify the existence of the domain, such as the ones
      described in Section 5 of [RFC2821].  If those checks indicate
      that the Author domain does not exist for mail, e.g., the domain
      has no MX, A, or AAAA record, the verifier SHOULD terminate with
      an error indicating that the domain is out of scope.

   Fetch Named ADSP Record:   The host MUST query DNS for a TXT record for
      corresponding to the immediate parent domain, Author Domain prefixed with "_asp._domainkey." by "_adsp._domainkey."
      (note the trailing dot).

      If the result of this query is anything other than a "NOERROR" response with an answer
      which is a valid ASP ADSP record, the algorithm terminates with a
       result indicating use that no ASP record was present.  If the ASP "t"
       tag exists in the response record, 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.
      terminates.

      If any of the queries involved in the Author Signing Practices Check
   result a query results 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

   ADSP adds the following namespaces that have been registered with
   IANA. to the IANA registry.  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  ADSP Specification Tag Registry

   An ASP ADSP record provides for a list of specification tags.  IANA has
   established the ASP ADSP Specification Tag Registry for specification
   tags that can be used in ASP ADSP fields.

   The initial entries entry in the registry comprise: is:
                         +------+-----------------+
                         | TYPE | REFERENCE       |
                         +------+-----------------+
                         | dkim | (this document) |
         | t    | (this document) |
                         +------+-----------------+

   ASP
   ADSP Specification Tag Registry Initial Values

5.2.  ASP  ADSP 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 ADSP
   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
   ADSP 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 ADSP are mostly related to attempts on
   the part of malicious senders to represent themselves as authors for
   whom they are not authorized to send mail, often in an attempt to
   defraud either the recipient or an Alleged Author.

   Additional security considerations regarding Author Domain Signing
   Practices are found in the DKIM threat analysis [RFC4686].

6.1.  ASP  ADSP 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, author's name-
   recognition among recipients, by using its the author's 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

   ADSP 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

   ADSP checkers may perform up to three multiple 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 ADSP
   checkers can become participants in traffic multiplication attacks.

6.2.  DNS Attacks

   An attacker might attack the DNS infrastructure in an attempt to
   impersonate ASP records, in an attempt ADSP records to influence a receiver's decision on how it
   will handle mail.  However, such an attacker is more likely to attack
   at a higher level, e.g., redirecting A or MX record lookups in order
   to capture traffic that was legitimately intended for the target
   domain.  These DNS security issues are addressed by DNSSEC [RFC4033].

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

6.3.  DNS Wildcards

   Wildcards within

   If a domain publishing ASP records, including but not
   limited to contains wildcards, then any name that matches the
   wildcard MX records, pose according to [RFC4592] is potentially a particular problem.  While
   referencing the immediate parent valid mail domain allows the discovery of an
   ASP record corresponding
   eligible for ADSP.  It is possible to an unintended immediate-child subdomain, add a wildcard records apply at multiple levels.  For example, if there is TXT record
   alongside a wildcard MX record that will provide suitable ADSP records for "example.com", the
   any 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 chosen by an attacker, since a query for that name if the wildcard synthesizes
   chosen-name.example.com IN MX, it will not return then also synthesize
   _adsp._domainkey.chosen-name.example.com IN TXT.  However multiple
   wildcard TXT records produce an "NXDOMAIN" error.  For
   that reason, ASP coverage undefined ADSP result, which means
   you cannot also publish both ADSP records and records for any other
   TXT-using protocol (such as SPF) for subdomains of domains containing a wildcard record is incomplete.

   NON-NORMATIVE NOTE: Complete ASP coverage of domains containing (or
   where any parent contains) wildcards generally cannot be provided by
   standard DNS servers. domain.

7.  References

7.1.  References - Normative

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, September 2006.

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2.  References - Informative

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC5016]  Thomas, M., "Requirements for a DomainKeys Identified Mail
              (DKIM) Signing Practices Protocol", RFC 5016,
              October 2007.

Appendix A.  Usage Examples

   These examples are intended to illustrate typical uses of ASP. ADSP.  They
   are not intended to be exhaustive, nor to apply to every domain's or
   mail system's individual situation.

   Domain managers are advised to consider the ways that mail processing
   can modify messages in ways that will invalidate an existing DKIM
   signature, such as mailing lists, courtesy forwarders, and other
   paths that could add or modify headers, or modify the message body.
   In that case, if the modifications invalidate the DKIM signature,
   recipient hosts will consider the mail not to have an Author
   Signature, even though the signature was present when the mail was
   originally sent.

A.1.  Single Location Domains

   A common mail system configuration handles all of a domain's users'
   incoming and outgoing mail through a single MTA or group of MTAs.  In
   that case, the MTA(s) can be configured to sign outgoing mail with an
   Author Signature.

   In this situation it might be appropriate to publish an ASP ADSP record
   for the domain containing "all", depending on whether the users also
   send mail through other paths that do not apply an Author Signature.
   Such paths could include MTAs at hotels or hotspot networks used by
   travelling users, or web sites that provide "mail an article"
   features.

A.2.  Bulk Mailing Domains

   Another common configuration uses a domain solely for bulk or
   broadcast mail, with no individual human users, again typically
   sending all the mail through a single MTA or group of MTAs that can
   apply an Author Signature.  In this case, the domain's management can
   be confident that all of its outgoing mail will be sent through the
   signing MTA.  Lacking individual users, the domain is unlikely to
   participate in mailing lists, but could still send mail through other
   paths that might invalidate signatures.

   Domain owners often use specialist mailing providers to send their
   bulk mail.  In that case, the mailing provider needs access to a
   suitable signing key in order to apply an Author Signature.  One
   possible route would be for the domain owner to generate the key and
   give it to the mailing provider.  Another would be for the domain to
   delegate a subdomain to the mailing provider, for example,
   bigbank.example might delegate email.bigbank.example to such a
   provider.  In that case, the provider can generate the keys and DKIM
   DNS records itself and use the subdomain in the Author address in the
   mail.

   Regardless of the DNS and key management strategy chosen, whoever
   maintains the DKIM records for the domain could also install an ADSP
   record containing "all".

A.3.  Bulk Mailing Domains with Discardable Mail

   In some cases, a domain might sign all of its outgoing mail with an
   Author Signature, but prefer that recipient systems discard mail
   without a valid Author Signature to avoid confusion from mail sent
   from sources that do not apply an Author Signature.  (This latter
   kind of mail is sometimes loosely called "forgeries".)  In that case,
   it might be appropriate to publish an ASP ADSP record containing
   "discardable".  Note that a domain SHOULD NOT publish a "discardable"
   record if it wishes to maximize the likelihood that mail from the
   domain is delivered, since it could cause some fraction of the mail
   the domain sends to be discarded.

   As a special case, if a domain sends no mail at all, it can safely
   publish a "discardable" ASP record, since any mail with an author
   address in the domain is a forgery.

A.4.  Third Party Senders

   Another common use case is for a third party to enter into an
   agreement whereby that third party will send bulk or other mail on
   behalf of a designated author or 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.

A.5.  Non-email Domains

   If a domain sends no mail at all, it can safely publish a
   "discardable" ADSP record, since any mail with an author address in
   the domain is a forgery.

Appendix B.  Acknowledgements

   This document greatly benefited from comments by Steve Atkins, Jon
   Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael
   Thomas, and Wietse Venema.

Appendix C.  Change Log

   *NOTE TO RFC EDITOR: This section may be removed upon publication of
   this document as an RFC.*

C.1.  Changes since -ietf-dkim-03

   o  Name change for title and filename, to be ADSP

   o  String changes throughout, to author Domain signing practices and
      to aDsp.

   o  Added some keywords.

   o  Clarified comparison of local part and domain in Author Address.

   o  Streamlined the Abstract.

   o  Revised text of last bullet in Results list.

   o  Removed definitions not used in the document.

   o  Removed all specification details pertaining to sub-domains.

   o  Moved Lookup Procedure up one document level.

   o  Revised domain validity specification.  Part in ADSP Usage in
      Operations section, and part as it as first step in Lookup.

   o  Fixed xml for figures, including labeling ABNF with new xml2rfc
      construct.

   o  Revised wildcard text.

   o  Removed 't' tag.

   o  Removed ADSP Flags Registry section.

   o  Changed ABNF use of whitespace from WSP back to FWS, for
      consistency with dkim-base.

C.2.  Changes since -ietf-dkim-02

   o  Merge in more text from ASP ADSP draft.

   o  Phrase actions as host's rather than checker.

   o  Explanatory description of i= matching.

   o  Lookup procedure consistently refers to one ASP ADSP 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.

C.3.  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.3.

C.4.  Changes since -ietf-dkim-ssp-00

   o  Clarified Operation Overview and eliminated use of Legitimate as
      the counterpart of Suspicious since the words have different
      meanings.

   o  Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
      records in DNS vs. a new RR type.

   o  Clarified publication rules for multilevel names.

   o  Better description of overall record syntax, in particular that
      records with unknown tags are considered syntactically correct.

   o  Clarified Sender Signing Practices Check Procedure, primarily by
      use of new term Author Domain.

   o  Eliminated section "Third-Party Signatures and Mailing Lists" that
      is better included in the DKIM overview document.

   o  Added "handling" tag to express alleged sending domain's
      preference about handling of Suspicious messages.

   o  Clarified handling of SERVFAIL error in SSP check.

   o  Replaced "entity" with "domain", since with the removal of user-
      granularity SSP, the only entities having sender signing policies
      are domains.

C.4.

C.5.  Changes since -allman-ssp-02

   o  Removed user-granularity SSP and u= tag.

   o  Replaced DKIMP resource record with a TXT record.

   o  Changed name of the primary tag from "p" to "dkim".

   o  Replaced lookup algorithm with one which traverses upward at most
      one level.

   o  Added description of records to be published, and effect of
      wildcard records within the domain, on SSP.

C.5.

C.6.  Changes since -allman-ssp-01

   o  Changed term "Sender Signing Policy" to "Sender Signing
      Practices".

   o  Changed query methodology to use a separate DNS resource record
      type, DKIMP.

   o  Changed tag values from SPF-like symbols to words.

   o  User level policies now default to that of the domain if not
      specified.

   o  Removed the "Compliance" section since we're still not clear on
      what goes here.

   o  Changed the "parent domain" policy to only search up one level
      (assumes that subdomains will publish SSP records if appropriate).

   o  Added detailed description of SSP check procedure.

C.6.

C.7.  Changes since -allman-ssp-00

   From a "diff" perspective, the changes are extensive.  Semantically,
   the changes are:

   o  Added section on "Third-Party Signatures and Mailing Lists"

   o  Added "Compliance" (transferred from -base document).  I'm not
      clear on what needs to be done here.

   o  Extensive restructuring.

Authors' Addresses

   Eric Allman
   Sendmail, Inc.
   6475 Christie Ave, Suite 350
   Emeryville, CA  94608

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

   Jim Fenton
   Cisco Systems, Inc.
   MS SJ-9/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1 408 526 5914
   Email: fenton@cisco.com

   Mark Delany
   Yahoo! Inc.
   701 First Avenue
   Sunnyvale, CA  94089

   Phone: +1 408 349 6831
   Email: markd+dkim@yahoo-inc.com

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com
   URI:   http://www.taugh.com

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