DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
draft-ietf-dkim-ssp-10

Network Working Group                                          E. Allman
Internet-Draft                                            Sendmail, Inc.
Intended status: Standards Track                               J. Fenton
Expires: November 12, 2009                           Cisco Systems, Inc.
                                                               M. Delany
                                                             Yahoo! Inc.
                                                               J. Levine
                                                    Taughannock Networks
                                                            May 11, 2009

DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
                         draft-ietf-dkim-ssp-10

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 12, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   DomainKeys Identified Mail (DKIM) defines a domain-level
   authentication framework for email to permit verification of the
   source and contents of messages.  This document specifies an adjunct
   mechanism to aid in assessing messages that do not contain a DKIM
   signature for the domain used in the author's address.  It defines a
   record that can advertise whether a domain signs its outgoing mail,
   and how other hosts can access that record.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Language and Terminology . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terms Imported from DKIM Signatures Specification  . . . .  4
     2.2.  Valid Signature  . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Author Address . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Author Domain  . . . . . . . . . . . . . . . . . . . . . .  5
     2.5.  Alleged Author . . . . . . . . . . . . . . . . . . . . . .  5
     2.6.  Author Domain Signing Practices  . . . . . . . . . . . . .  5
     2.7.  Author Domain Signature  . . . . . . . . . . . . . . . . .  5
   3.  Operation Overview . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  ADSP Applicability . . . . . . . . . . . . . . . . . . . .  6
     3.2.  ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  ADSP Results . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Detailed Description . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  DNS Representation . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Publication of ADSP Records  . . . . . . . . . . . . . . .  8
     4.3.  ADSP Lookup Procedure  . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  ADSP Specification Tag Registry  . . . . . . . . . . . . . 10
     5.2.  ADSP Outbound Signing Practices Registry . . . . . . . . . 10
     5.3.  Authentication-Results Method Registry Update  . . . . . . 11
     5.4.  Authentication-Results Result Registry Update  . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     6.1.  ADSP Threat Model  . . . . . . . . . . . . . . . . . . . . 13
     6.2.  DNS Considerations . . . . . . . . . . . . . . . . . . . . 14
     6.3.  DNS Wildcards  . . . . . . . . . . . . . . . . . . . . . . 15
     6.4.  Inappropriate Application of Author Domain Signatures  . . 15
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  References - Normative . . . . . . . . . . . . . . . . . . 15
     7.2.  References - Informative . . . . . . . . . . . . . . . . . 16
   Appendix A.  Lookup Examples . . . . . . . . . . . . . . . . . . . 16
     A.1.  Domain and ADSP exist  . . . . . . . . . . . . . . . . . . 17
     A.2.  Domain exists, ADSP does not exist . . . . . . . . . . . . 17
     A.3.  Domain does not exist  . . . . . . . . . . . . . . . . . . 17
   Appendix B.  Usage Examples  . . . . . . . . . . . . . . . . . . . 17

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     B.1.  Single Location Domains  . . . . . . . . . . . . . . . . . 18
     B.2.  Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 18
     B.3.  Bulk Mailing Domains with Discardable Mail . . . . . . . . 19
     B.4.  Third Party Senders  . . . . . . . . . . . . . . . . . . . 19
     B.5.  Domains with Independent Users and Liberal Use Policies  . 19
     B.6.  Non-email Domains  . . . . . . . . . . . . . . . . . . . . 19
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 19
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 20
     D.1.  Changes since -ietf-dkim-09  . . . . . . . . . . . . . . . 20
     D.2.  Changes since -ietf-dkim-08  . . . . . . . . . . . . . . . 20
     D.3.  Changes since -ietf-dkim-07  . . . . . . . . . . . . . . . 20
     D.4.  Changes since -ietf-dkim-06  . . . . . . . . . . . . . . . 20
     D.5.  Changes since -ietf-dkim-05  . . . . . . . . . . . . . . . 21
     D.6.  Changes since -ietf-dkim-04  . . . . . . . . . . . . . . . 21
     D.7.  Changes since -ietf-dkim-03  . . . . . . . . . . . . . . . 21
     D.8.  Changes since -ietf-dkim-02  . . . . . . . . . . . . . . . 22
     D.9.  Changes since -ietf-dkim-ssp-01  . . . . . . . . . . . . . 22
     D.10. Changes since -ietf-dkim-ssp-00  . . . . . . . . . . . . . 23
     D.11. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 24
     D.12. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 24
     D.13. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

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1.  Introduction

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

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

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

   The basic requirements for 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 [RFC5322].
   Briefly,

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

   o  A "Local-part" is the part of an address preceding the @
      character, as defined in [RFC5322] and used in [RFC4871].

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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 [RFC5322].  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 checked.

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 Domain Signature

   An "Author Domain Signature" is a Valid Signature in which the domain
   name of the DKIM signing entity, the d= tag in the DKIM-Signature
   header field, is the same as the domain name in the Author Address.
   Following [RFC5321], domain name comparisons are case insensitive.

   For example, if the From: line address is bob@domain.example, and the
   message has a Valid Signature, with the DKIM-Signature header field
   containing "d=domain.example", then the message has an Author Domain
   Signature.

3.  Operation Overview

   Domain owners publish 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 ADSP information of the domain(s) specified by
   the Author Address(es) as described in Section 4.3.  If a message has

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   multiple Author Addresses the ADSP lookups SHOULD be performed
   independently on each address.  This document does not address the
   process a host might use to combine the lookup results.

3.1.  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 checker implementation
   to return an appropriate error result for Author Domains outside the
   scope of ADSP.

   ADSP applies to specific domains, not domain subtrees.  If, for
   example, an Author Address were user@domain.example, the Author
   Domain would be domain.example, and the applicable ADSP record would
   be at _adsp._domainkey.domain.example.  An Author Address in a
   subdomain such as user@sub.domain.example would have a different ADSP
   record at _adsp._domainkey.sub.domain.example.  ADSP makes no
   connection between a domain and its parent or child domains.

   Note:   If an organization wants to publish Author Domain Signing
      Practices for all the subdomains it uses, such as host names of
      servers within the domain, it does so by creating ADSP records for
      every _adsp._domainkey.<sub>.domain.example.  Wildcards cannot be
      used (see Section 6.3.); however, suitable DNS management tools
      could automate creation of the ADSP records.

   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 checker 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 ADSP
   lookup.

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

   o  If a message has an Author Domain Signature, ADSP provides no
      benefit relative to that domain since the message is already known

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      to be compliant with any possible ADSP for that domain.

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

3.3.  ADSP Results

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

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

   o  All messages from this domain are signed with an Author Domain
      Signature.

   o  All messages from this domain are signed with an Author Domain
      Signature and discardable, i.e., if a message arrives without a
      valid Author Domain Signature, the domain encourages the
      recipient(s) to discard it.

   o  This domain is out of scope, i.e., the domain does not exist in
      the DNS.

   An ADSP lookup could terminate without producing any result if a DNS
   lookup results in a temporary failure.

4.  Detailed Description

4.1.  DNS Representation

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

   The RDATA for ADSP resource records is textual in format, with
   specific syntax and semantics relating to their role in describing
   ADSP.  The "Tag=Value List" syntax described in section 3.2 of
   [RFC4871] is used, modified to use WSP rather than FWS.  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 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.

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   Note:   ADSP changes the "Tag=Value List" syntax from [RFC4871] to
      use WSP rather than FWS in its DNS records.

   Note:   Domains MUST NOT publish ADSP records with wildcard names.
      Wildcards within a domain publishing ADSP records pose a
      particular problem, as discussed in more detail in Section 6.3.

4.2.  Publication of ADSP Records

   ADSP is intended to apply to all mail sent using the domain name
   string of an Alleged Author.

4.2.1.  Record Syntax

   ADSP records use the "tag=value" syntax described in section 3.2 of
   [RFC4871], modified to use WSP rather than FWS.  Every ADSP record
   MUST start with an outbound signing practices tag, so the first four
   characters of the record are lower case "dkim", followed by optional
   whitespace and "=".

   Tags used in ADSP records are described below.  Unrecognized tags
   MUST be ignored.  In the ABNF below, the WSP token, and 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 Domain
         Signature.

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

      Any other values are treated as "unknown".

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

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4.3.  ADSP Lookup Procedure

   Hosts doing an ADSP lookup MUST produce a result that is semantically
   equivalent to applying the following steps in the order listed below.
   In practice, 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 ADSP record" is one that is
   both syntactically and semantically correct; in particular, it
   matches the ABNF for a "tag-list" and starts with a valid "dkim" tag.

   Check Domain Scope:   An ADSP checker implementation MUST determine
      whether a given Author Domain is within scope for ADSP.  Given the
      background in Section 3.1 the checker MUST decide which degree of
      approximation is acceptable.  The checker MUST return an
      appropriate error result for Author Domains that are outside the
      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 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, rcode=3 in [RFC1035]), the algorithm MUST
      terminate with an error indicating that the domain is out of
      scope.  Note that a result with rcode=0 but no records (often
      called NODATA) is not the same as NXDOMAIN.

         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 because this record type is thought to
         be the most common for domains used in e-mail, and will
         therefore produce a result which can be more readily cached
         than a negative result.

      If the domain does exist, the checker MAY make more extensive
      checks to verify the existence of the domain, such as the ones
      described in Section 5 of [RFC5321].  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 checker SHOULD terminate with an
      error indicating that the domain is out of scope.

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

      If the result of this query is a NOERROR response (rcode=0 in
      [RFC1035]) with an answer which is a single record that is a valid
      ADSP record, use that record, and the algorithm terminates.

      If the result of the query is NXDOMAIN or NOERROR with zero
      records, there is no ADSP record.  If the result of the query
      contains more than one record, or a record that is not a valid
      ADSP record, the ADSP result is undefined.

      If a query results in a "SERVFAIL" error response (rcode=2 in
      [RFC1035]), the algorithm terminates without returning a result;
      possible actions include queuing the message or returning an SMTP
      error indicating a temporary failure.

   See Appendix A for examples of ADSP Lookup.

5.  IANA Considerations

   ADSP adds the following namespaces to the IANA registry.  In all
   cases, new values are assigned only for values that have been
   documented in a published RFC after IETF Review as specified in
   [RFC5226].

5.1.  ADSP Specification Tag Registry

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

   The initial entry in the registry is:
                         +------+-----------------+
                         | TYPE | REFERENCE       |
                         +------+-----------------+
                         | dkim | (this document) |
                         +------+-----------------+
   ADSP Specification Tag Registry Initial Values

5.2.  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 ADSP
   Outbound Signing Practices Registry for Outbound Signing Practices.

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   The initial entries in the registry comprise:
                     +-------------+-----------------+
                     | TYPE        | REFERENCE       |
                     +-------------+-----------------+
                     | unknown     | (this document) |
                     | all         | (this document) |
                     | discardable | (this document) |
                     +-------------+-----------------+
   ADSP Outbound Signing Practices Registry Initial Values

5.3.  Authentication-Results Method Registry Update

   IANA is requested to add the following to the Email Authentication
   Method Name Registry:

   Method:  dkim-adsp

   Defined In:  this memo

   ptype:  header

   property:  from

   value:  Contents of the [RFC5322] From: header field, with comments
      removed

5.4.  Authentication-Results Result Registry Update

   IANA is requested to add or update the following in the Email
   Authentication Result Name Registry:

   Code:  none

   Existing/New Code:  existing

   Defined In:  [RFC5451]

   Auth Method:  dkim-adsp (added)

   Meaning:  No DKIM author domain signing practises (ADSP) record was
      published.

   Code:  pass

   Existing/New Code:  existing

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   Defined In:  [RFC5451]

   Auth Method:  dkim-adsp (added)

   Meaning:  This message had an Author Domain Domain Signature that was
      validated.  (An ADSP check is not strictly required to be
      performed for this result, since a valid Author Domain Signature
      satisfies all possible ADSP policies.)

   Code:  unknown

   Existing/New Code:  new

   Defined In:  this memo

   Auth Method:  dkim-adsp

   Meaning:  No valid Author Domain Signature was found on the message
      and the published ADSP was "unknown".

   Code:  fail

   Existing/New Code:  existing

   Defined In:  [RFC5451]

   Auth Method:  dkim-adsp (added)

   Meaning:  No valid Author Domain Signature was found on the message
      and the published ADSP was "all".

   Code:  discard

   Existing/New Code:  new

   Defined In:  this memo

   Auth Method:  dkim-adsp

   Meaning:  No valid Author Domain Signature was found on the message
      and the published ADSP was "discardable".

   Code:  nxdomain

   Existing/New Code:  new

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   Defined In:  this memo

   Auth Method:  dkim-adsp

   Meaning:  Evaluating the ADSP for the Author's DNS domain indicated
      that the Author's DNS domain does not exist.

   Code:  temperror

   Existing/New Code:  existing

   Defined In:  [RFC5451]

   Auth Method:  dkim-adsp (added)

   Meaning:  An ADSP record could not be retrieved due to some error
      that is likely transient in nature, such as a temporary DNS error.
      A later attempt may produce a final result.

   Code:  permerror

   Existing/New Code:  existing

   Defined In:  [RFC5451]

   Auth Method:  dkim-adsp (added)

   Meaning:  An ADSP record could not be retrieved due to some error
      that is likely not transient in nature, such as a permanent DNS
      error.  A later attempt is unlikely to produce a final result.

6.  Security Considerations

   Security considerations in the 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.  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.

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   Email abuse often seeks to exploit a legitimate email author's name-
   recognition among recipients, by using 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.

   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.

   ADSP checkers may perform multiple DNS lookups per Alleged Author
   Domain.  Since these lookups are driven by domain names in email
   message headers of possibly fraudulent email, legitimate ADSP
   checkers can become participants in traffic multiplication attacks on
   domains that appear in fraudulent email.

6.2.  DNS Considerations

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

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6.3.  DNS Wildcards

   DNS wildcards (described in [RFC4592]) that exist in the DNS
   hierarchy at or above the domain being checked interfere with the
   ability to verify the scope of the ADSP check described in
   Section 4.3.  For example, a wildcard record for *.domain.example
   makes all subdomains such as foo.domain.example exist in the DNS.
   Domains that intend to make active use of ADSP by publishing a
   practice other than Unknown are advised to avoid the use of wildcards
   in their hierarchy.

   If a domain contains wildcards, then any name that matches the
   wildcard can appear to be a valid mail domain eligible for ADSP.  But
   the "_adsp._domainkey." prefix on ADSP records does not allow
   publication of wildcard records that cover ADSP records without also
   covering non-ADSP records, nor publication of wildcard records that
   cover non-ADSP records without also covering ADSP records.  A domain
   that uses ADSP practices other than unknown SHOULD NOT publish
   wildcard records.

6.4.  Inappropriate Application of Author Domain Signatures

   In one model of DKIM usage, a domain signs messages that are in
   transit through their system.  Since any signature whose domain
   matches the Author Domain is by definition an Author Domain
   Signature, it would be unwise to sign mail whose Author Domain is the
   signer's domain if the mail is not known to meet the domain's
   standards for an Author Domain Signature.

   One such use case is where a domain might apply such a signature is
   following application of an Authentication-Results header field as
   described in Section 7.1 of [RFC5451].  This problem can be easily
   avoided either by not applying a signature that might be confused
   with an Author Domain Signature or by applying a signature from some
   other domain, such as a subdomain of the Author Domain.

7.  References

7.1.  References - Normative

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

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

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.

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              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, July 2006.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

7.2.  References - Informative

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

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

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

Appendix A.  Lookup Examples

   Assume the example domain publishes these DNS records: (In these
   examples, the numbers in parentheses are comments to help identify
   the records, not part of the records themselves.)

   aaa.example                  A     192.0.2.1        (1)
   _adsp._domainkey.aaa.example TXT   "dkim=all"       (2)

   bbb.example                  MX 10 mail.bbb.example (3)
   mail.bbb.example             A     192.0.2.2        (4)

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A.1.  Domain and ADSP exist

   A mail message contains this From: header line:

   From: bob@aaa.example (Bob the Author)

   The ADSP Lookup first identifies the Author Address bob@aaa.example
   and the Author Domain aaa.example.  It does an MX DNS query for
   aaa.example, and gets back a NOERROR result with no DNS records.
   (There's no MX record, but since record (1) exists, the name exists
   in the DNS.)  Since that query didn't return an error, the Lookup
   proceeds to a TXT DNS query for _adsp._domainkey.aaa.example, which
   returns record (2).  Since this is a valid DKIM record, the result is
   that all messages from this domain are signed.

A.2.  Domain exists, ADSP does not exist

   A mail message contains this From: header line:

   From: alice@bbb.example (Old-fashioned Alice)

   The ADSP Lookup first identifies the Author Address alice@bbb.example
   and the Author Domain bbb.example.  It does an MX DNS query for
   bbb.example, and gets back record (3).  Since that query didn't
   return an error, it then proceeds to a TXT DNS query for
   _adsp._domainkey.bbb.example, which returns NXDOMAIN.  Since the
   domain exists but there is no ADSP record, ADSP returns the default
   unknown result: messages may or may not have an author domain
   signature.

A.3.  Domain does not exist

   A mail message contains this From: header line:

   From: frank@ccc.example (Unreliable Frank)

   The ADSP Lookup first identifies the Author Address frank@ccc.example
   and the Author Domain ccc.example.  It does an MX DNS query for
   ccc.example, and gets back an NXDOMAIN result since there are no
   records at all for ccc.example.  The lookup terminates with the
   result that the domain does not exist in the DNS and so is out of
   scope.

Appendix B.  Usage Examples

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

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   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 Domain
   Signature, even though the signature was present when the mail was
   originally sent.

B.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 Domain Signature.

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

B.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 Domain 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 Domain 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.

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   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".

B.3.  Bulk Mailing Domains with Discardable Mail

   In some cases, a domain might sign all of its outgoing mail with an
   Author Domain Signature, but prefer that recipient systems discard
   mail without a valid Author Domain Signature to avoid confusion from
   mail sent from sources that do not apply an Author Domain Signature.
   (In the case of domains with tightly controlled outgoing mail, this
   latter kind of mail is sometimes loosely called "forgeries".)  In
   that case, it might be appropriate to publish an 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.

B.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 RFC5322 From: or other headers.  Due to the many and varied
   complexities of such agreements, third party signing is not addressed
   in this specification.

B.5.  Domains with Independent Users and Liberal Use Policies

   When a domain has independent users and its usage policy does not
   explicitly restrict them to sending mail only from designated mail
   servers (e.g. many ISP domains and even some corporate domains), then
   it is only appropriate to publish an ADSP record containing
   "unknown".  Publishing either "all" or "discardable" will likely
   result in significant breakage because independent users are likely
   to send mail from the external paths enumerated in Appendix B.1.

B.6.  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 C.  Acknowledgements

   This document greatly benefited from comments by Steve Atkins, Jon
   Callas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, Ellen

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   Siegel, Michael Thomas, and Wietse Venema.

Appendix D.  Change Log

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

D.1.  Changes since -ietf-dkim-09

   o  Change author signature to use d=.

   o  Change all "author signature" to "author domain signature".

   o  Add authentication results for IANA per Murray K.

   o  Minor editorial clarifications.

D.2.  Changes since -ietf-dkim-08

   o  Simplify and clarify interpretation of d= and i=.

   o  Add note pointing out that i= usage conflicts with normal usage,
      and suggest workaround.

   o  Add note pointing out that you can mechanically add ADSP records
      for all the subdomains you use.

   o  in Section 3.2 fix text to say that only an Author Signature
      counts.

D.3.  Changes since -ietf-dkim-07

   Clarify that ADSP records use WSP rather than FWS in 4.1 and 4.2.1.

D.4.  Changes since -ietf-dkim-06

   Minor editorial changes suggested by AD:

   o  expand DKIM in title

   o  clarify that there's no subdomain matching in Section 3.1

   o  ADSP lookup can terminate without a result if the DNS lookup fails

   o  random dkim= values are treated as unknown

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   o  in 4.2 note WSP not FWS

   o  in 4.3 note that NODATA is not NXDOMAIN

   o  add new Appendix A with lookup examples

   Also address Tony's nits in
   http://mipassoc.org/pipermail/ietf-dkim/2008q3/010720.html.  Make the
   examples consistently use the .example domain.

D.5.  Changes since -ietf-dkim-05

   Minor editorial nits: define NOERROR, SERVFAIL, NXDOMAIN as rfc1035
   rcodes, change some punctuation, IANA section change IETF Consensus
   to the new IETF Review.

D.6.  Changes since -ietf-dkim-04

   o  Require dkim at the front of each record.

   o  Disparage wildcard records.

   o  Changed ABNF use of whitespace from FWS back to WSP, dkim-base is
      wrong.

   o  RFC 2434 -> 5226, make ref to 4686 informational since it's not
      standards track.

   o  Improve examples with material from Ellen.

D.7.  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.

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   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.

D.8.  Changes since -ietf-dkim-02

   o  Merge in more text from ADSP draft.

   o  Phrase actions as host's rather than checker.

   o  Explanatory description of i= matching.

   o  Lookup procedure consistently refers to one 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

D.9.  Changes since -ietf-dkim-ssp-01

   o  Reworded introduction for clarity.

   o  Various definition clarifications.

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   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).

D.10.  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.

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   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.

D.11.  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.

D.12.  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.

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   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.

D.13.  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

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   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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