阿兰达瓦卓玛抽烟照:RFC:5617 - DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)

Network Working Group                                          E. AllmanRequest for Comments: 5617                                Sendmail, Inc.Category: Standards Track                                      J. FentonCisco Systems, Inc.M. DelanyYahoo! Inc.J. LevineTaughannock NetworksAugust 2009DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)AbstractDomainKeys Identified Mail (DKIM) defines a domain-levelauthentication framework for email to permit verification of thesource and contents of messages.  This document specifies an adjunctmechanism to aid in assessing messages that do not contain a DKIMsignature for the domain used in the author's address.  It defines arecord that can advertise whether a domain signs its outgoing mail aswell as how other hosts can access that record.Status of This MemoThis document specifies an Internet standards track protocol for theInternet community, and requests discussion and suggestions forimprovements.  Please refer to the current edition of the "InternetOfficial Protocol Standards" (STD 1) for the standardization stateand status of this protocol.  Distribution of this memo is unlimited.Copyright NoticeCopyright (c) 2009 IETF Trust and the persons identified as thedocument authors.  All rights reserved.This document is subject to BCP 78 and the IETF Trust's LegalProvisions Relating to IETF Documents in effect on the date ofpublication of this document (http://trustee.ietf.org/license-info).Please review these documents carefully, as they describe your rightsand restrictions with respect to this document.Allman, et al.              Standards Track                     [Page 1]RFC 5617                          ADSP                       August 2009Table of Contents1. Introduction ....................................................32. Language and Terminology ........................................32.1. Terms Imported from the DKIM Signatures Specification ......32.2. Valid Signature ............................................42.3. Author Address .............................................42.4. Author Domain ..............................................42.5. Alleged Author .............................................42.6. Author Domain Signing Practices ............................42.7. Author Domain Signature ....................................43. Operation Overview ..............................................53.1. ADSP Applicability .........................................53.2. ADSP Usage .................................................63.3. ADSP Results ...............................................64. Detailed Description ............................................74.1. DNS Representation .........................................74.2. Publication of ADSP Records ................................74.2.1. Record Syntax .......................................74.3. ADSP Lookup Procedure ......................................95. IANA Considerations ............................................105.1. ADSP Specification Tag Registry ...........................105.2. ADSP Outbound Signing Practices Registry ..................115.3. Authentication-Results Method Registry Update .............115.4. Authentication-Results Result Registry Update .............116. Security Considerations ........................................136.1. ADSP Threat Model .........................................146.2. DNS Considerations ........................................146.3. DNS Wildcards .............................................156.4. Inappropriate Application of Author Domain Signatures .....157. References .....................................................167.1. Normative References ......................................167.2. Informative References ....................................16Appendix A.  Lookup Examples ......................................17A.1.  Domain and ADSP Exist .....................................17A.2.  Domain Exists, ADSP Does Not Exist ........................17A.3.  Domain Does Not Exist .....................................17Appendix B.  Usage Examples .......................................18B.1.  Single Location Domains ...................................18B.2.  Bulk Mailing Domains ......................................18B.3.  Bulk Mailing Domains with Discardable Mail ................19B.4.  Third-Party Senders .....................................19B.5.  Domains with Independent Users and Liberal Use Policies ...19B.6.  Non-Email Domains .......................................20Appendix C.  Acknowledgements .....................................20Allman, et al.              Standards Track                     [Page 2]RFC 5617                          ADSP                       August 20091.  IntroductionDomainKeys Identified Mail (DKIM) defines a mechanism by which emailmessages can be cryptographically signed, permitting a signing domainto claim responsibility for the introduction of a message into themail stream.  Message recipients can verify the signature by queryingthe Signer's domain directly to retrieve the appropriate public key,and thereby confirm that the message was attested to by a party inpossession of the private key for the signing domain.However, the legacy of the Internet is such that not all messageswill be signed, and the absence of a signature on a message is not ana priori indication of forgery.  In fact, during early phases ofdeployment, it is very likely that most messages will remainunsigned.  However, some domains might decide to sign all of theiroutgoing mail, for example, to protect their brand names.  It mightbe desirable for such domains to be able to advertise that fact toother hosts.  This is the topic of Author Domain Signing Practices(ADSP).Hosts implementing this specification can inquire what Author DomainSigning Practices a domain advertises.  This inquiry is called anAuthor Domain Signing Practices check.The basic requirements for ADSP are given in [RFC5016].  Thisdocument refers extensively to [RFC4871] and assumes the reader isfamiliar with it.Requirements Notation:The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"in this document are to be interpreted as described in [RFC2119].2.  Language and Terminology2.1.  Terms Imported from the DKIM Signatures SpecificationSome terminology used herein is derived directly from [RFC4871].  Inseveral cases, references in that document to "Sender" have beenchanged to "Author" here, to emphasize the relationship to the Authoraddress(es) in the From: header field described in [RFC5322].Briefly,o  A "Signer" is the agent that signs a message, as defined inSection 2.1 of [RFC4871].Allman, et al.              Standards Track                     [Page 3]RFC 5617                          ADSP                       August 2009o  A "Local-part" is the part of an address preceding the @character, as defined in [RFC5322] and used in [RFC4871].2.2.  Valid SignatureA "Valid Signature" is any signature on a message that correctlyverifies using the procedure described in Section 6.1 of [RFC4871].2.3.  Author AddressAn "Author Address" is an email address in the From: header field ofa message [RFC5322].  If the From: header field contains multipleaddresses, the message has multiple Author Addresses.2.4.  Author DomainAn "Author Domain" is everything to the right of the "@" in an AuthorAddress (excluding the "@" itself).2.5.  Alleged AuthorAn "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 amachine-readable record published by the domain of an Alleged Authorthat includes statements about the domain's practices with respect tomail it sends with its domain in the From: line.2.7.  Author Domain SignatureAn "Author Domain Signature" is a Valid Signature in which the domainname of the DKIM signing entity, i.e., the d= tag in the DKIM-Signature header field, is the same as the domain name in the AuthorAddress.  Following [RFC5321], domain name comparisons are caseinsensitive.For example, if the From: line address is bob@domain.example, and themessage has a Valid Signature with the DKIM-Signature header fieldcontaining "d=domain.example", then the message has an Author DomainSignature.Allman, et al.              Standards Track                     [Page 4]RFC 5617                          ADSP                       August 20093.  Operation OverviewDomain owners publish ADSP information via a query mechanism such asthe Domain Name System; specific details are given in Section 4.1.Hosts can look up the ADSP information of the domain(s) specified bythe Author Address(es) as described in Section 4.3.  If a message hasmultiple Author Addresses, the ADSP lookups SHOULD be performedindependently on each address.  This document does not address theprocess a host might use to combine the lookup results.3.1.  ADSP ApplicabilityADSP as defined in this document is bound to DNS.  For this reason,ADSP is applicable only to Author Domains with appropriate DNSrecords (i.e., A, AAAA, and/or MX) indicating the possible use of thedomain for email.  The handling of other Author Domains is outsidethe scope of this document.  However, attackers may use such domainnames in a deliberate attempt to sidestep an organization's ADSPpolicy statements.  It is up to the ADSP checker implementation toreturn an appropriate error result for Author Domains outside thescope of ADSP.ADSP applies to specific domains, not domain subtrees.  If, forexample, an Author Address were user@domain.example, the AuthorDomain would be domain.example, and the applicable ADSP record wouldbe at _adsp._domainkey.domain.example.  An Author Address in asubdomain such as user@sub.domain.example would have a different ADSPrecord at _adsp._domainkey.sub.domain.example.  ADSP makes noconnection between a domain and its parent or child domains.Note:   If an organization wants to publish Author Domain SigningPractices for all the subdomains it uses, such as host namesof servers within the domain, it does so by creating ADSPrecords for every _adsp._domainkey..domain.example.Wildcards cannot be used (see Section 6.3.); however,suitable DNS management tools could automate creation of theADSP records.Note:   The results from DNS queries that are intended to validate adomain name unavoidably approximate the set of Author Domainsthat can appear in legitimate email.  For example, a DNS Arecord could belong to a device that does not even have anemail implementation.  It is up to the checker to decide whatdegree of approximation is acceptable.Allman, et al.              Standards Track                     [Page 5]RFC 5617                          ADSP                       August 20093.2.  ADSP UsageDepending on the Author Domain(s) and the signatures in a message, arecipient gets varying amounts of useful information from each ADSPlookup.o  If a message has no Valid Signature, the ADSP result is directlyrelevant to the message.o  If a message has an Author Domain Signature, ADSP provides nobenefit relative to that domain since the message is already knownto be compliant with any possible ADSP for that domain.o  If a message has a Valid Signature other than an Author DomainSignature, the receiver can use both the Signature and the ADSPresult in its evaluation of the message.3.3.  ADSP ResultsAn ADSP lookup for an Author Address produces one of four possibleresults:o  Messages from this domain might or might not have an Author DomainSignature.  This is the default if the domain exists in the DNSbut no ADSP record is found.o  All messages from this domain are signed with an Author DomainSignature.o  All messages from this domain are signed with an Author DomainSignature and are discardable, i.e., if a message arrives withouta valid Author Domain Signature, the domain encourages therecipient(s) to discard it.o  This domain is out of scope, i.e., the domain does not exist inthe DNS.An ADSP lookup could terminate without producing any result if a DNSlookup results in a temporary failure.Allman, et al.              Standards Track                     [Page 6]RFC 5617                          ADSP                       August 20094.  Detailed Description4.1.  DNS RepresentationADSP records are published using the DNS TXT resource record type.The RDATA for ADSP resource records is textual in format, withspecific syntax and semantics relating to their role in describingADSP.  The "Tag=Value List" syntax described in Section 3.2 of[RFC4871] is used, modified to use whitespace (WSP) rather thanfolding whitespace (FWS).  Records not in compliance with that syntaxor the syntax of individual tags described in Section 4.3 MUST beignored (considered equivalent to a NODATA result) for purposes ofADSP, although they MAY cause the logging of warning messages via anappropriate system logging mechanism.  If the RDATA contains multiplecharacter strings, the strings are logically concatenated with nodelimiters between the strings.Note:   ADSP changes the "Tag=Value List" syntax from [RFC4871] touse WSP rather than FWS in its DNS records.Domains MUST NOT publish ADSP records with wildcard names.  Wildcardswithin a domain publishing ADSP records pose a particular problem, asdiscussed in more detail in Section 6.3.4.2.  Publication of ADSP RecordsADSP is intended to apply to all mail sent using the domain namestring of an Alleged Author.4.2.1.  Record SyntaxADSP records use the "tag=value" syntax described in Section 3.2 of[RFC4871], modified to use WSP rather than FWS.  Every ADSP recordMUST start with an outbound signing-practices tag, so the first fourcharacters of the record are lowercase "dkim", followed by optionalwhitespace and "=".Tags used in ADSP records are described below.  Unrecognized tagsMUST be ignored.  In the ABNF below, the WSP token is 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.Allman, et al.              Standards Track                     [Page 7]RFC 5617                          ADSP                       August 2009all       All mail from the domain is signed with an AuthorDomain Signature.discardableAll mail from the domain is signed with anAuthor Domain Signature.  Furthermore, if amessage arrives without a valid Author DomainSignature due to modification in transit,submission via a path without access to asigning key, or any other reason, the domainencourages the recipient(s) to discard it.Any other values are treated as "unknown".ABNF:; Copyright (c) 2009 IETF Trust and the persons identified as; authors of the code.  All rights reserved.; Redistribution and use in source and binary forms, with or without; modification, are permitted provided that the following conditions; are met:; - Redistributions of source code must retain the above copyright;   notice, this list of conditions and the following disclaimer.; - Redistributions in binary form must reproduce the above copyright;   notice, this list of conditions and the following disclaimer in;   the documentation and/or other materials provided with the;   distribution.; - Neither the name of Internet Society, IETF or IETF Trust, nor the;   names of specific contributors, may be used to endorse or promote;   products derived from this software without specific prior;   written permission.; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS; 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT; LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS; FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE; COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,; INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES; (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR; SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION);  HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN; CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR;  OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,; EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.Allman, et al.              Standards Track                     [Page 8]RFC 5617                          ADSP                       August 2009adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP("unknown" / "all" / "discardable" /x-adsp-dkim-tag)x-adsp-dkim-tag = hyphenated-word   ; for future extension; hyphenated-word is defined in RFC 48714.3.  ADSP Lookup ProcedureHosts doing an ADSP lookup MUST produce a result that is semanticallyequivalent to applying the following steps in the order listed below.In practice, these steps can be performed in parallel in order toimprove performance.  However, implementations SHOULD avoid doingunnecessary DNS lookups.For the purposes of this section, a "valid ADSP record" is one thatis both syntactically and semantically correct; in particular, itmatches the ABNF for a "tag-list" and starts with a valid "dkim" tag.Check Domain Scope:An ADSP checker implementation MUST determine whether a givenAuthor Domain is within the scope for ADSP.  Given the backgroundin Section 3.1, the checker MUST decide which degree ofapproximation is acceptable.  The checker MUST return anappropriate error result for Author Domains that are outside thescope of ADSP.The host MUST perform a DNS query for a record corresponding tothe Author Domain (with no prefix).  The type of the query can beof any type, since this step is only to determine if the domainitself exists in DNS.  This query MAY be done in parallel with thequery to fetch the named ADSP Record.  If the result of this queryis that the Author Domain does not exist in the DNS (often calledan NXDOMAIN error, rcode=3 in [RFC1035]), the algorithm MUSTterminate with an error indicating that the domain is out ofscope.  Note that a result with rcode=0 but no records (oftencalled NODATA) is not the same as NXDOMAIN.NON-NORMATIVE DISCUSSION: Any resource record type could beused for this query since the existence of a resource record ofany type will prevent an NXDOMAIN error.  MX is a reasonablechoice for this purpose because this record type is thought tobe the most common for domains used in email, and willtherefore produce a result that can be more readily cached thana negative result.If the domain does exist, the checker MAY make more extensivechecks to verify the existence of the domain, such as the onesdescribed in Section 5 of [RFC5321].  If those checks indicateAllman, et al.              Standards Track                     [Page 9]RFC 5617                          ADSP                       August 2009that the Author Domain does not exist for mail, e.g., the domainhas no MX, A, or AAAA record, the checker SHOULD terminate with anerror indicating that the domain is out of scope.Fetch Named ADSP Record:The host MUST query DNS for a TXT record corresponding to theAuthor Domain prefixed by "_adsp._domainkey." (note the trailingdot).If the result of this query is a NOERROR response (rcode=0 in[RFC1035]) with an answer that is a single record that is a validADSP record, use that record, and the algorithm terminates.If the result of the query is NXDOMAIN or NOERROR with zerorecords, there is no ADSP record.  If the result of the querycontains more than one record, or a record that is not a validADSP 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 SMTPerror indicating a temporary failure.See Appendix A for examples of ADSP lookup.5.  IANA ConsiderationsADSP adds the following namespaces to the IANA registry.  In allcases, new values are assigned only for values that have beendocumented in a published RFC after IETF Review, as specified in[RFC5226].5.1.  ADSP Specification Tag RegistryAn ADSP record provides for a list of specification tags.  IANA hasestablished the ADSP Specification Tag Registry for specificationtags that can be used in ADSP fields.The initial entry in the registry is:+------+-----------------+| TYPE | REFERENCE       |+------+-----------------+| dkim | (RFC 5617)      |+------+-----------------+ADSP Specification Tag Registry Initial ValuesAllman, et al.              Standards Track                    [Page 10]RFC 5617                          ADSP                       August 20095.2.  ADSP Outbound Signing Practices RegistryThe "dkim=" tag specification, defined in Section 4.2.1, provides fora value specifying Outbound Signing Practices.  IANA has establishedthe ADSP Outbound Signing Practices Registry for Outbound SigningPractices.The initial entries in the registry comprise:+-------------+-----------------+| TYPE        | REFERENCE       |+-------------+-----------------+| unknown     | (RFC 5617)      || all         | (RFC 5617)      || discardable | (RFC 5617)      |+-------------+-----------------+ADSP Outbound Signing Practices Registry Initial Values5.3.  Authentication-Results Method Registry UpdateIANA has added the following to the Email Authentication Method NameRegistry:Method: dkim-adspDefined In:  RFC 5617ptype:  headerproperty:  fromvalue:  contents of the [RFC5322] From: header field, with commentsremoved5.4.  Authentication-Results Result Registry UpdateIANA has added the following in the Email Authentication Result NameRegistry:Code:     noneExisting/New Code:  existingDefined In:  [RFC5451]Auth Method:  dkim-adsp (added)Allman, et al.              Standards Track                    [Page 11]RFC 5617                          ADSP                       August 2009Meaning:  No DKIM Author Domain Signing Practices (ADSP) record waspublished.Code:     passExisting/New Code:  existingDefined In:  [RFC5451]Auth Method:  dkim-adsp (added)Meaning:  This message had an Author Domain Signature that wasvalidated.  (An ADSP check is not strictly required to beperformed for this result since a valid Author DomainSignature satisfies all possible ADSP policies.)Code:     unknownExisting/New Code:  newDefined In:  RFC 5617Auth Method:  dkim-adspMeaning:  No valid Author Domain Signature was found on the messageand the published ADSP was "unknown".Code:     failExisting/New Code:  existingDefined In:  [RFC5451]Auth Method:  dkim-adsp (added)Meaning:  No valid Author Domain Signature was found on the messageand the published ADSP was "all".Code:     discardExisting/New Code:  newDefined In:  RFC 5617Auth Method:  dkim-adspMeaning:  No valid Author Domain Signature was found on the messageand the published ADSP was "discardable".Allman, et al.              Standards Track                    [Page 12]RFC 5617                          ADSP                       August 2009Code:     nxdomainExisting/New Code:  newDefined In:  RFC 5617Auth Method:  dkim-adspMeaning:  Evaluating the ADSP for the Author's DNS domain indicatedthat the Author's DNS domain does not exist.Code:     temperrorExisting/New Code:  existingDefined In:  [RFC5451]Auth Method:  dkim-adsp (added)Meaning:  An ADSP record could not be retrieved due to some errorthat is likely transient in nature, such as a temporary DNSerror.  A later attempt may produce a final result.Code:     permerrorExisting/New Code:  existingDefined In:  [RFC5451]Auth Method:  dkim-adsp (added)Meaning:  An ADSP record could not be retrieved due to some errorthat is likely not transient in nature, such as a permanentDNS error.  A later attempt is unlikely to produce a finalresult.6.  Security ConsiderationsSecurity considerations in the ADSP are mostly related to attempts onthe part of malicious senders to represent themselves as Authors forwhom they are not authorized to send mail, often in an attempt todefraud either the recipient or an Alleged Author.Additional security considerations regarding Author Domain SigningPractices are found in the DKIM threat analysis [RFC4686].Allman, et al.              Standards Track                    [Page 13]RFC 5617                          ADSP                       August 20096.1.  ADSP Threat ModelEmail recipients often have a core set of content Authors that theyalready trust.  Common examples include financial institutions withwhich they have an existing relationship and Internet web transactionsites with which they conduct business.Email abuse often seeks to exploit a legitimate email Author's name-recognition among recipients by using the Author's domain name in theFrom: header field.  Especially since many popular Mail User Agents(MUAs) do not display the Author's email address, there is noempirical evidence of the extent that this particular unauthorizeduse of a domain name contributes to recipient deception or thateliminating it will have significant effect.However, closing this potential exploit could facilitate some typesof optimized processing by receive-side message filtering engines,since it could permit them to maintain higher-confidence assertionsabout From: header field uses of a domain when the occurrence isauthorized.Unauthorized uses of domain names occur elsewhere in messages, as dounauthorized uses of organizations' names.  These attacks are outsidethe scope of this specification.ADSP does not provide any benefit -- nor, indeed, have any effect atall -- unless an external system acts upon the verdict, either bytreating the message differently during the delivery process or byshowing some indicator to the end recipient.  Such a system is out ofscope for this specification.ADSP checkers may perform multiple DNS lookups per Alleged AuthorDomain.  Since these lookups are driven by domain names in emailmessage headers of possibly fraudulent email, legitimate ADSPcheckers can become participants in traffic multiplication attacks ondomains that appear in fraudulent email.6.2.  DNS ConsiderationsAn attacker might attack the DNS infrastructure in an attempt toimpersonate ADSP records to influence a receiver's decision on how itwill handle mail.  However, such an attacker is more likely to attackat a higher level, e.g., redirecting A or MX record lookups in orderto capture traffic that was legitimately intended for the targetdomain.  These DNS security issues are addressed by DNSSEC [RFC4033].Allman, et al.              Standards Track                    [Page 14]RFC 5617                          ADSP                       August 2009Because ADSP operates within the framework of the legacy emailsystem, the default result in the absence of an ADSP record is thatthe domain does not sign all of its messages.  It is thereforeimportant that the ADSP clients distinguish a DNS failure such as"SERVFAIL" from other DNS errors so that appropriate actions can betaken.6.3.  DNS WildcardsDNS wildcards (described in [RFC4592]) that exist in the DNShierarchy at or above the domain being checked interfere with theability to verify the scope of the ADSP check described inSection 4.3.  For example, a wildcard record for *.domain.examplemakes all subdomains such as foo.domain.example exist in the DNS.Domains that intend to make active use of ADSP by publishing apractice other than unknown are advised to avoid the use of wildcardsin their hierarchy.If a domain contains wildcards, then any name that matches thewildcard can appear to be a valid mail domain eligible for ADSP.  Butthe "_adsp._domainkey." prefix on ADSP records does not allowpublication of wildcard records that cover ADSP records without alsocovering non-ADSP records, nor does it allow publication of wildcardrecords that cover non-ADSP records without also covering ADSPrecords.  A domain that uses ADSP practices other than unknown SHOULDNOT publish wildcard records.6.4.  Inappropriate Application of Author Domain SignaturesIn one model of DKIM usage, a domain signs messages that are intransit through their system.  Since any signature whose domainmatches the Author Domain is, by definition, an Author DomainSignature, it would be unwise to sign mail whose Author Domain is theSigner's domain if the mail is not known to meet the domain'sstandards for an Author Domain Signature.One such use case is where a domain might apply such a signaturefollowing application of an Authentication-Results header field asdescribed in Section 7.1 of [RFC5451].  This problem can be easilyavoided either by not applying a signature that might be confusedwith an Author Domain Signature or by applying a signature from someother domain, such as a subdomain of the Author Domain.Allman, et al.              Standards Track                    [Page 15]RFC 5617                          ADSP                       August 20097.  References7.1.  Normative References[RFC1035]  Mockapetris, P., "Domain names - implementation andspecification", STD 13, RFC 1035, November 1987.[RFC2119]  Bradner, S., "Key words for use in RFCs to IndicateRequirement Levels", BCP 14, RFC 2119, March 1997.[RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.Rose, "DNS Security Introduction and Requirements",RFC 4033, March 2005.[RFC4592]  Lewis, E., "The Role of Wildcards in the Domain NameSystem", 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 anIANA Considerations Section in RFCs", BCP 26, RFC 5226,May 2008.[RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for SyntaxSpecifications: 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 IndicatingMessage Authentication Status", RFC 5451, April 2009.7.2.  Informative References[RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeysIdentified 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.Allman, et al.              Standards Track                    [Page 16]RFC 5617                          ADSP                       August 2009Appendix A.  Lookup ExamplesAssume the example domain publishes these DNS records (in theseexamples, the numbers in parentheses are comments to help identifythe 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)A.1.  Domain and ADSP ExistA mail message contains this From: header line:From: bob@aaa.example (Bob the Author)The ADSP lookup first identifies the Author Address bob@aaa.exampleand the Author Domain aaa.example.  It does an MX DNS query foraaa.example and gets back a NOERROR result with no DNS records.(There's no MX record, but since record (1) exists, the name existsin the DNS.)  Since that query didn't return an error, the lookupproceeds to a TXT DNS query for _adsp._domainkey.aaa.example, whichreturns record (2).  Since this is a valid ADSP record, the result isthat all messages from this domain are signed.A.2.  Domain Exists, ADSP Does Not ExistA mail message contains this From: header line:From: alice@bbb.example (Old-fashioned Alice)The ADSP lookup first identifies the Author Address alice@bbb.exampleand the Author Domain bbb.example.  It does an MX DNS query forbbb.example and gets back record (3).  Since that query didn't returnan error, it then proceeds to a TXT DNS query for_adsp._domainkey.bbb.example, which returns NXDOMAIN.  Since thedomain exists but there is no ADSP record, ADSP returns the defaultunknown result: messages may or may not have an author domainsignature.A.3.  Domain Does Not ExistA mail message contains this From: header line:From: frank@ccc.example (Unreliable Frank)Allman, et al.              Standards Track                    [Page 17]RFC 5617                          ADSP                       August 2009The ADSP lookup first identifies the Author Address frank@ccc.exampleand the Author Domain ccc.example.  It does an MX DNS query forccc.example and gets back an NXDOMAIN result since there are norecords at all for ccc.example.  The lookup terminates with theresult that the domain does not exist in the DNS and so is out ofscope.Appendix B.  Usage ExamplesThese examples are intended to illustrate typical uses of ADSP.  Theyare not intended to be exhaustive or to apply to every domain's ormail system's individual situation.Domain managers are advised to consider the ways that mail processingcan modify messages in ways that will invalidate an existing DKIMsignature, such as mailing lists, courtesy forwarders, and otherpaths that could add or modify headers, or modify the message body.If the modifications invalidate the DKIM signature, recipient hostswill consider the mail not to have an Author Domain Signature, eventhough the signature was present when the mail was originally sent.B.1.  Single Location DomainsOne common mail system configuration handles all of a domain's users'incoming and outgoing mail through a single Mail Transport Agent(MTA) or group of MTAs.  With this configuration, the MTA(s) can beconfigured to sign outgoing mail with an Author Domain Signature.In this situation, it might be appropriate to publish an ADSP recordfor the domain containing "all", depending on whether the users alsosend mail through other paths that do not apply an Author DomainSignature.  Such paths could include MTAs at hotels or hotspotnetworks used by travelling users, web sites that provide "mail anarticle" features, user messages sent through mailing lists, orthird-party mail clients that support multiple user identities.B.2.  Bulk Mailing DomainsAnother common configuration uses a domain solely for bulk orbroadcast mail, with no individual human users -- again, typicallysending all the mail through a single MTA or group of MTAs that canapply an Author Domain Signature.  In this case, the domain'smanagement can be confident that all of its outgoing mail will besent through the signing MTA(s).  Lacking individual users, thedomain is unlikely to participate in mailing lists, but could stillsend mail through other paths that might invalidate signatures.Allman, et al.              Standards Track                    [Page 18]RFC 5617                          ADSP                       August 2009Domain owners often use specialist mailing providers to send theirbulk mail.  In this case, the mailing provider needs access to asuitable signing key in order to apply an Author Domain Signature.One possible route would be for the domain owner to generate the keyand give it to the mailing provider.  Another would be for the domainto delegate a subdomain to the mailing provider, for example,bigbank.example might delegate email.bigbank.example to such aprovider; in this case, the provider can generate the keys and DKIMDNS records itself and use the subdomain in the Author Address in themail.Regardless of the DNS and key management strategy chosen, whoevermaintains the DKIM records for the domain could also install an ADSPrecord containing "all".B.3.  Bulk Mailing Domains with Discardable MailIn some cases, a domain might sign all of its outgoing mail with anAuthor Domain Signature and prefer that recipient systems discardmail without a valid Author Domain Signature in order to avoid havingits mail confused with mail sent from sources that do not apply anAuthor Domain Signature.  (In the case of domains with tightlycontrolled outgoing mail, this latter kind of mail is sometimesloosely called "forgeries".)  In such cases, it might be appropriateto publish an ADSP record containing "discardable".  Note that adomain SHOULD NOT publish a "discardable" record if it wishes tomaximize the likelihood that mail from the domain is delivered, sinceit could cause some fraction of the mail the domain sends to bediscarded.B.4.  Third-Party SendersAnother common use case is for a third party to enter into anagreement whereby that third party will send bulk or other mail onbehalf of a designated Author or Author Domain, using that domain inthe [RFC5322] From: or other headers.  Due to the many and variedcomplexities of such agreements, third-party signing is not addressedin this specification.B.5.  Domains with Independent Users and Liberal Use PoliciesWhen a domain has independent users and its usage policy does notexplicitly restrict them to sending mail only from designated mailservers (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 likelyresult in significant breakage because independent users are likelyto send mail from the external paths enumerated in Appendix B.1.Allman, et al.              Standards Track                    [Page 19]RFC 5617                          ADSP                       August 2009B.6.  Non-Email DomainsIf a domain sends no mail at all, it can safely publish a"discardable" ADSP record, since any mail with an Author Address inthe domain is a forgery.Appendix C.  AcknowledgementsThis document greatly benefited from comments by Steve Atkins, JonCallas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, EllenSiegel, Michael Thomas, and Wietse Venema.Allman, et al.              Standards Track                    [Page 20]RFC 5617                          ADSP                       August 2009Authors' AddressesEric AllmanSendmail, Inc.6475 Christie Ave, Suite 350Emeryville, CA  94608Phone: +1 510 594 5501EMail: eric+dkim@sendmail.orgJim FentonCisco Systems, Inc.170 W. Tasman DriveSan Jose, CA  95134-1706Phone: +1 408 526 5914EMail: fenton@cisco.comMark DelanyYahoo! Inc.701 First AvenueSunnyvale, CA  94089Phone: +1 408 349 6831EMail: markd+dkim@yahoo-inc.comJohn LevineTaughannock NetworksPO Box 727Trumansburg, NY  14886Phone: +1 831 480 2300EMail: standards@taugh.comURI:   http://www.taugh.comAllman, et al.              Standards Track                    [Page 21]