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Versions: 00 01 02 03                                                   
MARID                                                            M. Wong
Internet-Draft                                              M. Lentczner
Expires: March 16, 2005                               September 15, 2004


              The SPF Record Format and Sender-ID Protocol
                      draft-ietf-marid-protocol-03

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on March 16, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document defines a protocol for the authorization of Internet
   hosts to use domain names in the sender mailbox of mail that those
   hosts send.  Authorization records are published in DNS for domain
   names that may be used as part of sender mailboxes.  Mail receivers
   then perform a check against those records to see if a client host
   submitting a piece of mail is actually authorized.  Since there are
   several different concepts of sender mailbox, this protocol is
   generic and can be applied to one or more of such "scopes".



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Publishing Domains . . . . . . . . . . . . . . . . . . . .  4
     1.2   Mail Receivers . . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Scopes . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.4   Earlier Work . . . . . . . . . . . . . . . . . . . . . . .  5
     1.5   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1   Publishing . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.1   RR Types . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.2   Version  . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.3   Multiple Records . . . . . . . . . . . . . . . . . . .  7
       2.1.4   Additional Records . . . . . . . . . . . . . . . . . .  7
       2.1.5   Multiple Strings . . . . . . . . . . . . . . . . . . .  7
       2.1.6   Record Size  . . . . . . . . . . . . . . . . . . . . .  8
       2.1.7   Wildcard Records . . . . . . . . . . . . . . . . . . .  8
       2.1.8   Minor Version  . . . . . . . . . . . . . . . . . . . .  9
   3.  The check_host() Function  . . . . . . . . . . . . . . . . . . 10
     3.1   Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2   Results  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.3   Initial Processing . . . . . . . . . . . . . . . . . . . . 11
     3.4   Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 11
     3.5   Selecting Records  . . . . . . . . . . . . . . . . . . . . 11
     3.6   Record Evaluation  . . . . . . . . . . . . . . . . . . . . 12
       3.6.1   Term Evaluation  . . . . . . . . . . . . . . . . . . . 12
       3.6.2   Mechanisms . . . . . . . . . . . . . . . . . . . . . . 13
       3.6.3   Modifiers  . . . . . . . . . . . . . . . . . . . . . . 14
     3.7   Default result . . . . . . . . . . . . . . . . . . . . . . 15
     3.8   Domain Spec  . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 16
     4.1   "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.2   "include"  . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.3   "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.4   "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.5   "ptr"  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.6   "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 20
     4.7   "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 22
     5.1   redirect: Redirected Query . . . . . . . . . . . . . . . . 22
     5.2   exp: Explanation . . . . . . . . . . . . . . . . . . . . . 23
   6.  Miscellaneous  . . . . . . . . . . . . . . . . . . . . . . . . 25
     6.1   Unrecognized Mechanisms and Modifiers  . . . . . . . . . . 25
     6.2   Processing Limits  . . . . . . . . . . . . . . . . . . . . 25
   7.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     7.1   Macro definitions  . . . . . . . . . . . . . . . . . . . . 27
     7.2   Expansion Examples . . . . . . . . . . . . . . . . . . . . 30
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31



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   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
     9.1   Registration Template  . . . . . . . . . . . . . . . . . . 32
   10.   Contributors and Acknowledgements  . . . . . . . . . . . . . 33
   11.   Comments . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 35
   12.1  Normative References . . . . . . . . . . . . . . . . . . . . 35
   12.2  Informative References . . . . . . . . . . . . . . . . . . . 35
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 36
   A.  Collected ABNF . . . . . . . . . . . . . . . . . . . . . . . . 37
   B.  Extended Examples  . . . . . . . . . . . . . . . . . . . . . . 39
     B.1   Simple Examples  . . . . . . . . . . . . . . . . . . . . . 39
     B.2   Multiple Domain Example  . . . . . . . . . . . . . . . . . 40
     B.3   RBL Style Example  . . . . . . . . . . . . . . . . . . . . 41
       Intellectual Property and Copyright Statements . . . . . . . . 42





































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1.  Introduction

   Mail on the Internet suffers from a lack of authorization mechanisms.
   In particular, a host injecting mail into the mail stream can send
   mail using almost any mailbox as a sender in the envelope and
   headers.  The Sender-ID protocol fills this void by enabling domains
   to authorize particular MTAs to send mail with their identity, and by
   enabling mail receivers to check these authorizations.

1.1  Publishing Domains

   A Sender-ID compliant domain name is one with a valid, published SPF
   record.  This record authorizes the use of the domain name, in one or
   more scopes (see below), by some sending MTAs, and not by others.

   A compliant domain SHOULD publish authorizations for every defined
   scope.

   Domain holders may publish SPF records that explicitly authorize no
   hosts for domain names that shouldn't be used in sender mailboxes.

1.2  Mail Receivers

   A mail receiver can perform a Sender-ID compliant check, on one or
   more scopes, for each mail message it receives.  Typically, these
   checks are done by a receiving MTA, but can be performed elsewhere in
   the mail processing chain so long as the required information is
   available.

   It is expected that mail receivers will use the Sender-ID checks as
   part of the a larger set of tests on incoming mail.  The results of
   other tests may influence whether or not a particular Sender-ID check
   is performed.  For example, finding the sending host on a local white
   list may cause all other tests to be skipped and all mail from that
   host to be accepted.

   When a mail receiver decides to perform a Sender-ID check, it MUST
   implement and evaluate the check_host() function (see below)
   correctly and follow the requirements of the particular scope under
   test.  While the tests as a whole or optional, once it has been
   decided to perform a test it must as performed specified so that the
   correct semantics are preserved between publisher and receiver.

1.3  Scopes

   There are several places in a mail transaction which involve the
   notion of a mail sender.  In particular, the [RFC2821] envelope has a
   reverse-path, and the [RFC2822] headers have "From:", "Sender:",



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   "Resent-From:" and "Resent-Sender:".  Since these identities have
   different semantics and processing characteristics (alone and in
   combination), Sender-ID defines different scopes.  Publishing domains
   can make authorizations about one or all scopes, and mail receivers
   can check one or more scopes.

   This document only defines the existence of two scopes: "mfrom" and
   "pra".  The details of these two scopes are defined in other
   documents: "mfrom" is defined in [Mailfrom], "pra" is defined in
   [PRA].

   Other scopes may be defined by future documents only.  There is no
   registry for scopes.  A scope definition must define what it
   identifies as the sending mailbox for a message, how to extract that
   information from a message, how to determine the initial arguments
   for the check_host() function, and what the compliant responses to
   the result are.  This ensure that domains with published records and
   mail receiver agree on the semantics of the scope.

1.4  Earlier Work

   This design is an evolution of work done under the name SPF.  The
   record format and test presented here has been intentionally designed
   to be generally backward compatible with the currently deployed base
   of records and code.

1.5  Terminology

   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].




















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2.  SPF Records

   SPF records to declare which hosts are, and are not, authorized to
   use a domain names for a given scope.  Loosely, the record partitions
   all hosts into permitted and not-permitted sets.  (Though some hosts
   might fall into other categories.)

   The SPF record is a single string of text.  An example record is:

      spf2.0/mfrom,pra +mx +a:colo.example.com/28 -all

   This record has a version of "spf2.0", a scope of "pra", and three
   directives: "+mx", "+a:colo.example.com/28", and "-all".

2.1  Publishing

   A domain name's SPF record is published in DNS.  The record is placed
   in the DNS tree at the domain name it pertains to.

   The previous example might be published easily via this line in a
   domain zone file:

      example.com.  IN SPF "spf2.0/mfrom,pra +mx +a:colo.example.com/28
      -all"

   Note: The record is published at the domain name to which it
   pertains, not a name within the domain (such as is done with SRV
   records.) When published with via the SPF RR type (see below), this
   poses no problems and was chosen as the clearest way to express the
   declaration.  When published via TXT records it is still published
   directly at the domain name, even though other TXT records, for other
   purposes may be published there.

2.1.1  RR Types

   This document defines a new DNS RR type SPF, type code to be
   determined.  The format of this type is identical to the TXT RR
   [RFC1035].

   However, because there are a number of DNS server and resolver
   implementations in common use that cannot handle new RR types, a
   record can be published with type TXT.

   A Sender-ID compliant domain name SHOULD have SPF records of both RR
   types.  A domain name MUST have a record of at least one type.  If a
   domain has records of both types, they MUST have identical content.

   A Sender-ID compliant check SHOULD lookup both types.  If both types



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   of records are returned for a domain, the SPF type MUST be used.

   It is recognized that the current practice (using a TXT type record),
   is not optimal, but a practical reality due to the state of deployed
   software.  The two record type scheme provides a forward path to the
   better solution of using a RR type reserved for this purpose.

   For either type, the character content of the record is encoded as
   US-ASCII.

   Example RRs in this document are shown with the SPF record type,
   however they could also be published with a TXT type.

2.1.2  Version

   Each record starts with a version section.  This version section
   contains a minor version field intended for future expansion.  This
   document only defines records with a version section that starts
   "spf2.0" (minor version of "0").

2.1.3  Multiple Records

   A domain name MUST NOT have multiple records that would cause an
   authorization check to select more than one record.  See Section 3.5
   for the selection rules.

   In particular, a domain name cannot publish two or more records for
   any given scope where the version section differs only in the
   ver-minor field,

2.1.4  Additional Records

   Some records contain directives that require additional SPF records.
   It is suggested that those records be placed under an "_spf"
   subdomain.  See Appendix B for examples.

2.1.5  Multiple Strings

   A Text DNS record (either TXT and SPF RR types) can be composed of
   more than one string.  If a published record contains multiple
   strings, then record MUST be treated as if those strings are
   concatenated together without adding spaces.  For example:

      SPF "spf2.0/mfrom,pra ....  first" "second string..."

   MUST be treated as equivalent to





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      SPF "spf2.0/mfrom,pra ....  firstsecond string..."

   SPF or TXT records containing multiple strings are useful in order to
   construct longer records which would otherwise exceed the maximum
   length of a string within a TXT or SPF RR record.

   Note: Some nameserver implementations will silently split long
   strings in TXT records into several shorter strings.

2.1.6  Record Size

   All the published SPF records for a given domain name SHOULD remain
   small enough that the results of a query for them will fit within 512
   octets.  This will keep even older DNS implementations from falling
   over to TCP.  Since the answer size is dependent on many things
   outside the scope of this document, it is only possible to give this
   guideline: If the combined length of the DNS name and the text of all
   the SPF records is under 480 characters, then DNS answers should fit
   in UDP packets.  Note that when computing the sizes for queries of
   the TXT format, one must take into account any other TXT records
   published at the domain name.

2.1.7  Wildcard Records

   Use of wildcard records for publishing is not recommended.  Care must
   be taken if wildcard records are used.  If a domain publishes
   wildcard MX records, it may want to publish wildcard declarations,
   subject to the same requirements and problems.  In particular, the
   declaration must be repeated for any host that has any RR records at
   all, and for subdomains thereof.  For example, the example given in
   [RFC1034], Section 4.3.3, could be extended with:

       X.COM           MX      10      A.X.COM
       X.COM           SPF     "spf2.0/mfrom,pra +a:A.X.COM -all"

       *.X.COM         MX      10      A.X.COM
       *.X.COM         SPF     "spf2.0/mfrom,pra +a:A.X.COM -all"

       A.X.COM         A       1.2.3.4
       A.X.COM         MX      10      A.X.COM
       A.X.COM         SPF     "spf2.0/mfrom,pra +a:A.X.COM -all"

       *.A.X.COM       MX      10      A.X.COM
       *.A.X.COM       SPF     "spf2.0/mfrom,pra +a:A.X.COM -all"

   Notice that the wildcard records must be repeated twice for every
   name within the domain: Once for the name, and once to cover the tree
   under the name.



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   Use of wildcards is discouraged in general as they cause every name
   under the domain to exist and queries against arbitrary names will
   never return RCODE 3 (Name Error).

2.1.8  Minor Version

   This document only specifies records with a minor version of "0".
   All published records MUST start with "spf2.0".

   Future versions of this document may define other minor versions to
   be used.








































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3.  The check_host() Function

   The check_host() function fetches SPF records, parses them, and
   interprets them to evaluate if a particular host is or is not
   permitted to send mail in a given context.  Mail receivers that
   perform this check MUST correctly implement the check_host() function
   as described by the canonical algorithm defined here.

   Implementations MAY use a different algorithm, so long as the results
   are the same.

3.1  Arguments

   The function check_host() take four arguments:

      <scope>  - the scope identifier
      <ip>     - the IP address of the host under test
      <domain> - the domain to check
      <sender> - the full sending mailbox address

   Each scope defines how these arguments are determined.  See the
   individual scope documents [Mailfrom] and [PRA]

   The domain portion of <sender> will usually be the same as the
   <domain> argument when check_host() is initially evaluated.  However,
   it will generally not be true for recursive evaluations (see Section
   4.2 below)

   Note: The IP address may be either IPv4 or IPv6.

3.2  Results

   The function check_host() can result in one of seven results
   described here.  Based on the result, the action to be taken is
   determined by the checks a mail receiver is performing (see
   definitions for each scope) and the local policies of the receiver.

   Results from interpreting valid records:

      Neutral  (?): published data is explicitly inconclusive
      Pass     (+): the <ip> is in the permitted set
      Fail     (-): the <ip> is in the not permitted set
      SoftFail (~): the <ip> may be in the not permitted set, its use is
               discouraged and the domain owner may move it to the not
               permitted set in the future

   Results from error conditions:




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      None      - no published data
      TempError - transient error during DNS lookup or other processing
      PermError - unrecoverable error during processing, such as an
                error in the record format

   If the result is "Fail", then an additional reason is returned.  The
   reason may be one of:

      Not Permitted
      Malformed Domain
      Domain Does Not Exist

   If the reason is "Not Permitted", then an explanation string is also
   returned.  The explanation string may be empty.

3.3  Initial Processing

   If the <domain> is not an FQDN, the check_host() immediately returns
   the result "Fail" and a reason of "Malformed Domain".

   If the <sender> has no localpart, substitute the string "postmaster"
   for the localpart.

3.4  Record Lookup

   The records for <domain> are fetched.  If the records are in a cache,
   and has not expired, then they may simply be used.  Otherwise, the
   records must be fetched from DNS as follows:

   In accordance with how the records are published, (see Section 2.1
   above), a DNS query needs to be made for the <domain> name, querying
   for either RR type TXT, SPF or both.

   If the domain does not exist (RCODE 3), check_host() exits
   immediately with the result "Fail" and a reason of "Domain Does Not
   Exist"

   If the DNS lookup returns a server failure (RCODE 2), or other error
   (RCODE other than 0 or 3), or the query times out, check_host() exits
   immediately with the result "TempError"

3.5  Selecting Records

   Records begin with version and scope sections:






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   record      = version scope terms *SP
   version     = "spf2." ver-minor
   ver-minor   = 1*DIGIT
   scope       = "/" scope-id *( "," scope-id )
   scope-id    = "mfrom" / "pra" / name

   Starting with the set of records that were returned by the lookup,
   record selection proceeds in three steps:

   1.  If any records of type SPF are in the set, then all records of
       type TXT are discarded.
   2.  Records that do not begin with proper version and scope sections
       are discarded.  The version section contains a ver-minor field
       that is for backward compatible future extensions.  This field
       must be well-formed for a record to be retained, but is otherwise
       ignored.
   3.  Records that do not have a scope-id that matches <scope> are
       discarded.  Note that this is a complete string match on the
       scope-id tokens: If <scope> is "pra", then the record starting
       "spf2.0/mfrom,prattle,fubar" would be discarded, but a record
       starting "spf2.0/mfrom,pra,fubar" would be retained.

   After the above steps, there should be one record remaining and
   evaluation can proceed.  If there are no records remaining,
   check_host() exits immediately with the result "None".  If there are
   two or more records remaining, then check_host() exits immediately
   with the error "PermError".

3.6  Record Evaluation

   After one SPF record has been selected, the check_host() function
   parses and interprets it to find a result for the current test.  If
   at any point a syntax error is encountered, check_host() returns
   immediately with the result "PermError".

   Implementations MAY choose to parse the entire record first and
   return "PermError" if the record is not well formed.  See Section
   6.1.

3.6.1  Term Evaluation

   There are two types of terms: mechanisms and modifiers.  A given
   mechanism type may always appear multiple times in a record.
   Modifiers may be constrained to appear at most once per record,
   depending on the definition of the modifier.  Unknown mechanisms
   cause processing to abort with the result "PermError".  Unknown
   modifiers are ignored.




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   A record contains an ordered list of mechanisms and modifiers:

   terms       = *( 1*SP ( directive / modifier ) )

   directive   = [ prefix ] mechanism
   prefix      = "+" / "-" / "?" / "~"
   mechanism   = name [ ":" domain-spec ] *( "/" *DIGIT )
   modifier    = name "=" macro-string
   name        = alpha *( alpha / digit / "-" / "_" / "." )

   Most mechanisms allow a ":" or "/" character after the name.

   Modifiers always contain an equals ('=') character immediately after
   the name, and before any ":" or "/" characters that may be part of
   the macro-string.

   Terms that do not contain any of "=", ":" or "/" are mechanisms.

   Mechanism and modifier names are case-insensitive.  A mechanism
   "INCLUDE" is equivalent to "include".

3.6.2  Mechanisms

   Each mechanism is considered in turn from left to right.

   When a mechanism is evaluated, one of three things can happen: it can
   match, it can not match, or it can throw an exception.

   If it matches, processing ends and the prefix value is returned as
   the result of that record.  (The default prefix value is "+".)

   If it does not match, processing continues with the next mechanism.
   If no mechanisms remain, the default result is specified in Section
   3.7.

   If it throws an exception, mechanism processing ends and the
   exception value is returned.

   The possible prefixes, and the results they return are:
   "+" Pass
   "-" Fail
   "~" SoftFail
   "?" Neutral

   A missing prefix for a mechanism is the same as a prefix of "+".

   When a mechanism matches, and the prefix is "-" so that a "Fail"
   result is returned, the reason is Not Permitted, and the explanation



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   string is computed as described in Section 5.2.

   Specific mechanisms are described in Section 4.

3.6.3  Modifiers

   Modifiers are either global or positional:

      Global modifiers MAY appear anywhere in the record, but SHOULD
      appear at the end, after all mechanisms and positional modifiers.

      Positional modifiers apply only to the mechanism they follow.  It
      is a syntax error for a positional modifier to appear before the
      first mechanism.

   Modifiers of either type are also either singular or multiple:

      Singular modifiers may appear only once in the record if they are
      global, or once after each mechanism if they are positional.

      Multiple modifiers may appear multiple times in the record if they
      are global, or multiple times after each mechanism if they are
      positional.

   The definition of each specific modifier (see Section 5) states
   whether it is global or positional, and whether it is singular or
   multiple.  A modifier is not allowed to be defined as both global and
   positional.

   Ordering of modifiers does not matter, except:
   1)     positional modifiers must appear after the mechanism they
          affect and before any subsequent mechanisms.
   and 2) when a multiple modifier appears more than one time, the
          ordering of the appearances may be significant to the
          modifier.
   Other than these constraints, implementations MUST treat different
   orders of modifiers the same.  An intended side effect of these rules
   is modifiers cannot be defined that modify other modifiers.

   These rules allow an implementation to correctly preparse a record.
   Furthermore, they are crafted to allow the parsing algorithm to be
   stable, even when new modifiers are introduced.

   Modifiers which are unrecognized MUST be ignored.  This allows older
   implementations to handle records with modifiers that were defined
   after they were written.





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3.7  Default result

   If none of the mechanisms match and there is no redirect modifier,
   then the check_host() exits with a result of "Neutral".  If there is
   a redirect modifier, check_host() proceeds as defined in Section 5.1.

   Note that records SHOULD always either use a redirect modifier or an
   "all" mechanism to explicitly terminate processing.

   For example:

      spf2.0/mfrom,pra +mx -all
   or
      spf2.0/mfrom,pra +mx redirect=_spf.example.com

3.8  Domain Spec

   Several of these mechanisms and modifiers have a <domain-spec>
   section.  The <domain-spec> string is macro expanded (see Section 7).
   The resulting string is the common presentation form of a fully
   qualified DNS name: A series of labels separated by periods.  This
   domain is called the <target-name> in the rest of this document.

   Note: The result of the macro expansion is not subject to any further
   escaping.  Hence, this facility cannot produce all characters that
   are legal in a DNS label, for example, the space or control
   characters.  However, this facility is powerful enough to express
   legal host names, and common utility labels (such as "_spf") that are
   used in DNS.

   For mechanisms, the <domain-spec> is optional.  If it is not
   provided, the <domain> is used as the <target-name>.



















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4.  Mechanism Definitions

   This section defines two types of mechanisms.

   Basic mechanisms contribute to the language framework.  They do not
   specify a particular type of authorization scheme.

      all
      include

   Designated sender mechanisms are used to designate a set of <ip>
   address as being permitted or not to use the <domain> for sending
   mail.

      a
      mx
      ptr
      ip4
      ip6
      exists

   Other mechanisms may be defined in the future.

   Mechanisms either match, do not match, or throw an exception.  If
   they match, their prefix value is returned.  If they do not match,
   processing continues.  If they throw an exception, the exception
   value is returned.

   The following conventions apply to all mechanisms that perform an
   comparison between <ip> and an IP address at any point:

   If no CIDR-length is given in the directive, then <ip> and the IP
   address are compared for equality.

   If a CIDR-length is specified, then only the specified number of
   high-order bits of <ip> and the IP address are compared for equality.

   When any mechanisms fetches host addresses to compare with <ip>, when
   <ip> is an IPv4 address, A records are fetched, when <ip> is an IPv6
   address, AAAA records are fetched.

   Several mechanisms rely on information fetched from DNS.  While
   fetching that information and DNS server returns an error (RCODE
   other than 0 or 3) or the query times out, the mechanism throws the
   exception "TempError".  Should the server return domain does not
   exist (RCODE 3), then evaluation of the mechanism continues with an
   empty set of records.




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4.1  "all"

   all = "all"

   The "all" mechanism is a test that always matches.  It is used as the
   rightmost mechanism in a record to provide an explicit default.

   For example:
      spf2.0/mfrom,pra +mx +a -all

   Mechanisms after "all" will never be tested.

4.2  "include"

   include = "include" ":" domain-spec

   The "include" mechanism triggers a recursive evaluation of
   check_host().  The domain-spec is expanded as per Section 7.  Then
   check_host() is evaluated with the resulting string as the <domain>.
   The <scope>, <ip> and <sender> arguments remain the same as current
   evaluation of check_host().

   "Include" makes it possible for one domain to designate multiple
   administratively independent domains.

   For example, a vanity domain "example.net" might send mail using the
   servers of administratively independent domains example.com and
   example.org.

   Example.net could say

      "spf2.0/mfrom,pra include:example.com include:example.org -all".

   That would direct check_host() to, in effect, check the records of
   example.com and example.org for a "pass" result.  Only if the host
   were not permitted for either of those domains would the result be
   "Fail".

   Whether this mechanism matches or not, or throws an error depends on
   the result of the recursive evaluation of check_host():











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   +---------------------------------+---------------------------------+
   | A recursive check_host() result | Causes the include mechanism    |
   | of:                             | to:                             |
   +---------------------------------+---------------------------------+
   | Pass                            | match                           |
   |                                 |                                 |
   | Fail                            | not match                       |
   |                                 |                                 |
   | SoftFail                        | not match                       |
   |                                 |                                 |
   | Neutral                         | not match                       |
   |                                 |                                 |
   | TempError                       | throw TempError                 |
   |                                 |                                 |
   | PermError                       | throw PermError                 |
   |                                 |                                 |
   | None                            | throw PermError                 |
   +---------------------------------+---------------------------------+

   The Include mechanism is intended for crossing administrative
   boundaries.  While it is possible to use includes to consolidate
   multiple domains that share the same set of designated hosts, domains
   are encouraged to use redirects where possible, and to minimize the
   number of includes within a single administrative domain.  For
   example, if example.com and example.org were managed by the same
   entity, and if the permitted set of hosts for both domains were
   "mx:example.com", it would be possible for example.org to specify
   "include:example.com", but it would be preferable to specify
   "redirect=example.com" or even "mx:example.com".

4.3  "a"

   This mechanism matches if <ip> is one of the <target-name>'s IP
   addresses.

   A = "a" [ ":" domain-spec ] [ dual-cidr-length ]

   An address lookup is done on the <target-name>.  The <ip> is compared
   to the returned address(es).  If any address matches, the mechanism
   matches.

4.4  "mx"

   This mechanism matches if <ip> is one of the MX hosts for a domain
   name.

   MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]




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   check_host() first performs an MX lookup on the <target-name>.  Then
   perform an address lookup on each MX name returned, in order of MX
   priority.  The <ip> is compared to each returned IP address.  If any
   address matches, the mechanism matches.

   Note Regarding Implicit MXes: If the <target-name> has no MX records,
   check_host() MUST NOT pretend the target is its single MX, and MUST
   NOT default to an A lookup on the <target-name> directly.  This
   behavior breaks with the legacy "implicit MX" rule.  See [RFC2821]
   Section 5.  If such behavior is desired, the publisher should specify
   an "a" directive.

4.5  "ptr"

   This mechanism tests if the DNS reverse mapping for <ip> exists and
   validly points to a domain name within a particular domain.

   PTR = "ptr" [ ":" domain-spec ]

   First the <ip>'s name is looked up using this procedure: perform a
   DNS reverse-mapping for <ip>, looking up the corresponding PTR record
   in "in-addr.apra.".  For each record returned, validate the host name
   by looking up its IP address.  If <ip> is among the returned IP
   addresses, then that host name is validated.  In pseudocode:

   sending-host_names := ptr_lookup(sending-host_IP);
   for each name in (sending-host_names) {
     IP_addresses := a_lookup(name);
     if the sending-host_IP is one of the IP_addresses {
       validated_sending-host_names += name;
     }
   }

   Check all validated hostnames to see if they end in the <target-name>
   domain.  If any do, this mechanism matches.  If no validated hostname
   can be found, or if none of the validated hostnames end in the
   <target-name>, this mechanism fails to match.

   Pseudocode:

   for each name in (validated_sending-host_names) {
     if name ends in <domain-spec>, return match.
     if name is <domain-spec>, return match.
   }
   return no-match.

   This mechanism matches if the <target-name> is an ancestor of a
   validated hostname, or if the <target-name> and a validated hostname;



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   are the same.  For example: "mail.example.com" is within the domain
   "example.com", but "mail.bad-example.com" is not.  If a validated
   hostname is the <target-name>, a match results.

   Note: This mechanism is not recommended.  If a domain decides to use
   it, it should make sure is has the proper PTR records in place for
   its hosts.

4.6  "ip4" and "ip6"

   These mechanisms test if <ip> is contained within a given IP network.

   IP4             = "ip4" ":" ip4-network [ ip4-cidr-length ]
   IP6             = "ip6" ":" ip6-network [ ip6-cidr-length ]
   ip4-cidr-length = "/" 1*DIGIT
   ip6-cidr-length = "/" 1*DIGIT

   ip4-network     = as per conventional dotted quad notation,
                     e.g. 192.0.2.0
   ip6-network     = as per [RFC 3513], section 2.2,
                     e.g. 2001:DB8::CD30

   The <ip> is compared to the given network.  If CIDR-length high-order
   bits match, the mechanism matches.

   If ip4-cidr-length is omitted it is taken to be "/32".  If
   ip6-cidr-length is omitted it is taken to be "/128".

4.7  "exists"

   This mechanism is used to construct an arbitrary host name that is
   used for a DNS A record query.  It allows for complicated schemes
   involving arbitrary parts of the mail envelope to determine what is
   legal.

   exists = "exists" ":" domain-spec

   The domain-spec is expanded as per Section 7.  The resulting domain
   name is used for a DNS A lookup.  If any A record is returned, this
   mechanism matches.  The lookup type is 'A' even when the connection
   type is IPv6.

   Domains can use this mechanism to specify arbitrarily complex
   queries.  For example, suppose example.com publishes the record:

      spf2.0/mfrom,pra exists:%{ir}.%{l1r+-}._spf.%{d} -all

   The target-name might expand to



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   "1.2.0.192.someuser._spf.example.com".  This makes fine-grained
   decisions possible at the level of the user and client IP address.

   This mechanism enable queries that mimic the style of tests that
   existing RBL lists use.














































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5.  Modifier Definitions

   Modifiers are not mechanisms: they do not return match or no-match.
   Instead they provide additional information or alter check_host()
   processing.  Global modifiers affect the entire record, whereas
   positional modifiers only affect the preceding mechanism in the
   record.

   While unrecognized mechanisms cause an immediate "PermError" abort,
   unrecognized modifiers MUST be simply ignored.  Modifiers therefore
   provide an way to extend the record format in the future with
   backward compatibility.

   Only two modifiers are currently defined: "redirect" and "exp".
   Implementations of check_host() MUST support them both.

   This document reserves two modifiers for future definition:
   "accredit" and "match_subdomains".  Until these are defined,
   implementations SHOULD ignore them.  There is also one deprecated
   modifier: "default".  Implementations MUST ignore it.

5.1  redirect: Redirected Query

   The "redirect" modifier is global and singular.

   If all mechanisms fail to match, and a redirect modifier is present,
   then processing proceeds as follows.

   redirect = "redirect" "=" domain-spec

   The domain-spec portion of the redirect section is expanded as per
   the macro rules in Section 7.  Then check_host() is evaluated with
   the resulting string as the <domain>.  The <scope>, <ip> and <sender>
   arguments remain the same as current evaluation of check_host().

   The result of this new evaluation of check_host() is then considered
   the result of current evaluation.

   Note that the newly queried domain may itself specify redirect
   processing.

   This facility is intended for use by organizations that wish to apply
   the same record to multiple domains.  For example:

     la.example.com. SPF "spf2.0/mfrom,pra redirect=_spf.example.com"
     ny.example.com. SPF "spf2.0/mfrom,pra redirect=_spf.example.com"
     sf.example.com. SPF "spf2.0/mfrom,pra redirect=_spf.example.com"
   _spf.example.com. SPF "spf2.0/mfrom,pra mx:example.com -all"



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   In this example, mail from any of the three domains is described by
   the same record.  This can be an administrative advantage.

   Note: In general, a domain A cannot reliably use a redirect to
   another domain B not under the same administrative control.  Since
   the <sender> stays the same, there is no guarantee that the record at
   domain B will correctly work for addresses in domain A, especially if
   domain B uses mechanisms involving localparts.  An "include"
   directive may be more appropriate.

   For clarity it is RECOMMENDED that any redirect modifier appear as
   the very last term in a record.

5.2  exp: Explanation

   The "exp" modifier is global and singular.

   explanation = "exp" "=" domain-spec

   If check_host() results in a "Fail" due to a mechanism match (such as
   "-all"), and the exp modifier is present, then the explanation string
   returned is computed as described below.  If no exp modifier is
   present, then an empty explanation string is returned.

   The <domain-spec> is macro expanded (see Section 7) and becomes the
   <target-name>.  The DNS TXT record for the <target-name> is fetched
   either from a cache or via a query to DNS.

   If <domain-spec> is empty, or there are any processing errors (any
   RCODE other than 0), or if no records are returned, or if more than
   one record is returned, then an empty explanation string is returned.

   The fetched TXT record's strings are concatenated with no spaces, and
   then treated as a new macro-string which is macro-expanded.  This
   final result is the explanation string.

   Software evaluating check_host() can use this string when the result
   is "Fail" with a reason of "Not Permitted", to communicate
   information from the publishing domain in the form of a short message
   or URL.  Software should make it clear that the explanation string
   comes from a third party.  For example, it can prepend the macro
   string "%{d} explains: " to the explanation.

   Implementations MAY limit the length of the resulting explanation
   string to allow for other protocol constraints and/or reasonable
   processing limits.

   Suppose example.com has this record



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      spf2.0/mfrom,pra mx -all exp=explain._spf.%{d}

   Here are some examples of possible explanation TXT records at
   explain._spf.example.com:

      Example.com mail should only be sent by its own servers.

         -- a simple, constant message

      %{i} is not one of %{d}'s designated mail servers.

         -- a message with a little more info, including the IP address
            that failed the check

      See http://%{d}/why.html?s=%{S}&i=%{I}

         -- a complicated example that constructs a URL with the
            parameters check_host() so that a web page can be generated
            with detailed, custom instructions

   Note: During recursion into an Include mechanism, explanations do not
   propagate out.  But during execution of a Redirect modifier, the
   explanation string from the target of the redirect is used.




























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6.  Miscellaneous

6.1  Unrecognized Mechanisms and Modifiers

   New mechanisms can only be introduced by new versions of this
   document.

   Unrecognized mechanisms cause processing to abort: If, during
   evaluation of a record, check_host() encounters a mechanism which it
   does not understand, it terminates processing and returns
   "PermError", without evaluating any further mechanisms.  Mechanisms
   listed before the unknown mechanism MUST, however, be evaluated.

   For example, consider the record:

      spf2.0/mfrom,pra a mx ptr foo:_foo.%{d} -all

   If during the evaluation of check_host(), any of the "a", "mx", or
   "ptr" directives match, then check_host() would return a "Pass"
   result.  If none of those directives resulted in a match, then an
   implementation that did not recognize the "foo" mechanism would
   return "PermError".  An implementation that did recognize the "foo"
   mechanism would be able to perform an extended evaluation.

   Note: "foo" is an example of an unknown extension mechanism that
   could be defined in the future.  It is NOT defined by this proposal.

   New modifiers can be introduced by registering them with the IANA, or
   in new versions of this document

   Unrecognized modifiers are ignored: if an implementation encounters
   modifiers which it does not recognize, it MUST ignore them.

6.2  Processing Limits

   During processing, an evaluation of check_host() may require
   additional evaluations of check_host() due to the Include mechanism
   and/or the Redirect modifier.

   Implementations must be prepared to handle records that are set up
   incorrectly or maliciously.  Implementations MUST perform loop
   detection, limit additional evaluations, or both.  If an
   implementation chooses to limit additional evaluations, then at least
   a total of 10 evaluations of check_host() for a single query MUST be
   supported.  (This number should be enough for even the most
   complicated configurations.)

   If a loop is detected, or evaluation limit of an implementation is



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   reached, check_host() MUST abort processing and return the result
   "PermError".

   MTAs or other processors MAY also impose a limit on the maximum
   amount of elapsed time to evaluate check_host().  Such a limit SHOULD
   allow at least 20 seconds.  If such a limit is exceeded, the result
   of authentication SHOULD be "TempError".

   Domains publishing records SHOULD try keep the number include
   directives, and chained redirect modifiers to a minimum.  Domains
   SHOULD also try to minimize the amount of other DNS information
   needed to evaluate a record.  This can be done by choosing directives
   that require less DNS information.

   For example, consider a domain set up as:

   example.com.      IN MX   10 mx.example.com.
   mx.example.com.   IN A    192.0.2.1
   a.example.com.    IN SPF  "spf2.0/mfrom,pra +mx:example.com -all"
   b.example.com.    IN SPF  "spf2.0/mfrom,pra +a:mx.example.com -all"
   c.example.com.    IN SPF  "spf2.0/mfrom,pra +ip4:192.0.2.1 -all"

   Evaluating check_host() for the domain "a.example.com" requires the
   MX records for "example.com", and then the A for records for the
   listed hosts.  Evaluating for "b.example.com" only requires the A
   records.  Evaluating for "c.example.com" requires none.

   However, there may be administrative considerations: Using "a" over
   "ip4" allows hosts to be renumbered easily.  Using "mx" over "a"
   allows the set of mail hosts to be changed easily.





















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7.  Macros

7.1  Macro definitions

   Certain terms perform macro interpolation on their arguments.

   domain-spec  = *( macro-char / v-char-dm )
   macro-string = *( macro-char / v-char-ms )
   macro-char   = ( "%{" ALPHA transformer *delimiter "}" )
                  / "%%" / "%_" / "%-"
   transformer  = *DIGIT [ "r" ]
   delimiter    = "." / "-" / "+" / "," / "/" / "_" / "="
   v-char-dm   = %x21-24 / %x26-2E / %x30-7E
                 ; visible characters except "%" and "/"
   v-char-ms   = %x21-24 / %x26-7E
                 ; visible characters except "%"

   A literal "%" is expressed by "%%".
      "%_" expands to a single " " space.
      "%-" expands to a URL-encoded space, viz.  "%20".

   The following macro letters are expanded in term arguments:


      s  = <sender>
      l  = local-part of <sender>
      o  = domain of <sender>
      d  = <domain>
      i  = <ip>
      p  = the validated host name of <ip>
      v  = the string "in-addr" for if <ip> is ipv4, or "ip6" if <ip> is
         ipv6

   The following macro letters are only allowed in "exp" text:


      c  = SMTP client IP (easily readable format)
      r  = domain name of host performing the check
      t  = current timestamp in UTC epoch seconds notation

   The uppercase versions of all these macros are URL-encoded.

   A '%' character not followed by a '{', '%', '-', or '_' character
   MUST be interpreted as a literal.  Domains SHOULD NOT rely on this
   feature; they MUST escape % literals.  For example, an explanation
   TXT record
      Your spam volume has increased by 581%
   is incorrect.  Instead, say



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      Your spam volume has increased by 581%%

   All other legal visible characters are simply expanded to themselves.
   Note that the two different macro contexts, domain-spec, and
   macro-string allow slightly different sets of legal visible
   characters, v-char-dm and v-char-ms respectively.

   Legal optional transformers are:


      *DIGIT : zero or more digits
      'r'    : reverse value, splitting on dots by default

   If transformers or delimiters are provided, the replacement value for
   a macro letter is split into parts.  After performing any reversal
   operation and/or removal of left-hand parts, the parts are rejoined
   using "." and not the original splitting characters.

   By default, strings are split on "." (dots).  Note that no special
   treatment is given to leading, trailing or consecutive delimiters,
   and so the list of parts may contain empty strings.  Macros may
   specify delimiter characters which are used instead of ".".
   Delimiters MUST be one or more of the characters:

   "." / "-" / "+" / "," / "/" / "_" / "="

   The 'r' transformer indicates a reversal operation: if the client IP
   address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
   and the macro %{ir} would expand to "1.2.0.192".

   The DIGIT transformer indicates the number of right-hand parts to
   use, after optional reversal.  If a DIGIT is specified, the value
   MUST be nonzero.  If no DIGITs are specified, or if the value
   specifies more parts than are available, all the available parts are
   used.  If the DIGIT was 5, and only 3 parts were available, the macro
   interpreter would pretend the DIGIT was 3.  Implementations MUST
   support at least a value of 128, as that is the maximum number of
   labels in a domain name.

   The "s" macro expands to the <sender> argument.  It is an email
   address with a localpart, an "@" character, and a domain.  The "l"
   macro expands to just the localpart.  The "o" macro expands to just
   the domain part.  Note that these values remain the same during a
   recursive and chained evaluations due to "include" and/or "redirect".
   Note also that if the original <sender> had no localpart, the
   localpart was set to "postmaster" in initial processing (see Section
   3.3).




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   For IPv4 addresses, both the "i" and "c" macros expand to the
   standard dotted-quad format.

   For IPv6 addresses, the "i" macro expands to dot-format address; it
   is intended for use in %{ir}.  The "c" macro may expand to any of the
   hexadecimal colon-format addresses specified in [RFC3513] section
   2.2.  It is intended for humans to read.

   The "p" macro expands to the validated host name of <ip>.  The
   procedure for finding the validated host names is defined in Section
   4.5.  If that procedure produces more than one validated host name,
   any name from the list may be used.  If that procedure produces no
   validate host name the string "unknown" is used.

   The "r" macro expands to the name of the receiving MTA.  This SHOULD
   be a fully qualified domain name, but if one does not exist (as when
   the checking is done by a script) or if policy restrictions dictate
   otherwise, the word "unknown" SHOULD be substituted.  The domain name
   MAY be different than the name found in the MX record that the client
   MTA used to locate the receiving MTA.

   Any unrecognized macro letters are expanded as the string "unknown".
   There is one deprecated macro letter: "h".  It is expanded as the
   string "deprecated".

   When the result of macro expansion is used in a domain name query, if
   the expanded domain name exceeds 255 characters (the maximum length
   of a domain name), the left side is truncated to fit, by removing
   successive subdomains until the total length falls below 255
   characters.

   Uppercased macros expand exactly as their lower case equivalents, and
   are then URL escaped.  URL escaping is described in [RFC2396].

   Note: Domains should avoid using the "s", "l" or "o" macros in
   conjunction with any mechanism directive.  While these macros are
   powerful and allow per-user records to be published, they severely
   limit the ability of implementations to cache results of check_host()
   and they reduce the effectiveness of DNS caches.

   Implementations should be aware that if no directive processed during
   the evaluation of check_host() contains an "s", "l", or "o" macro,
   then the results of the evaluation can be cached on the basis of
   <domain> and <ip> alone for as long as the shortest TTL of all the
   DNS records involved.






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7.2  Expansion Examples

      The <sender> is strong-bad@email.example.com.
      The IPv4 SMTP client IP is 192.0.2.3.
      The IPv6 SMTP client IP is 5f05:2000:80ad:5800::1.
      The PTR domain name of the client IP is mx.example.org.

   macro                       expansion
   -------  ----------------------------
   %{s}     strong-bad@email.example.com
   %{o}                email.example.com
   %{d}                email.example.com
   %{d4}               email.example.com
   %{d3}               email.example.com
   %{d2}                     example.com
   %{d1}                             com
   %{dr}               com.example.email
   %{d2r}                  example.email
   %{l}                       strong-bad
   %{l-}                      strong.bad
   %{lr}                      strong-bad
   %{lr-}                     bad.strong
   %{l1r-}                        strong


   macro-string                                               expansion
   --------------------------------------------------------------------
   %{ir}.%{v}._spf.%{d2}             3.2.0.192.in-addr._spf.example.com
   %{lr-}.lp._spf.%{d2}                  bad.strong.lp._spf.example.com

   %{lr-}.lp.%{ir}.%{v}._spf.%{d2}
                       bad.strong.lp.3.2.0.192.in-addr._spf.example.com

   %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
                           3.2.0.192.in-addr.strong.lp._spf.example.com

   %{d2}.trusted-domains.example.net
                                example.com.trusted-domains.example.net

   IPv6:
   %{ir}.%{v}._spf.%{d2}         1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.
                         5.d.a.0.8.0.0.0.2.5.0.f.5.ip6._spf.example.com









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8.  Security Considerations

   There are two aspects of this protocol that malicious parties could
   exploit to undermine the validity of the check_host() function:

   The evaluation of check_host() relies heavily on DNS.  A malicious
   attacker could poison a target's DNS cache with spoofed DNS data, and
   cause check_host() to return incorrect results, including "Pass" for
   an <ip> value where the actual domain's record would evaluate to
   "Fail".

   The client IP address, <ip>, is assumed to be correct true.  A
   malicious attacker could spoof TCP sequences to make mail appear to
   come from a permitted host for a domain that the attacker is
   impersonating.

   As with most aspects of mail, there are a number of ways that
   malicious parties could use the protocol as an avenue of a
   distributed denial of service attack:

   While implementations of check_host() need to limit the number of
   includes and redirects and/or check for loops, malicious domains
   could publish records that exercise or exceed these limits in an
   attempt to waste computation effort at their targets when they send
   them mail.

   Malicious parties could send large volume mail purporting to come
   from the intended target to a wide variety of legitimate mail hosts.
   These legitimate machines would then present a DNS load on the target
   as they fetched the relevant records.

   While these distributed denial of service attacks are possible, they
   seem more convoluted to mount, and have less of an impact, than other
   simpler attacks.

   The exp modifier allows the domain being checked to provide a text
   message should the check_host() function fail.  Explicit provisions
   in the macro facility support domains including URLs in this message.
   Since this message is eventually shown to a user, that user needs to
   take care, as with all URLs, in deciding to follow the URL.











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9.  IANA Considerations

   The IANA needs to assign a new Resource Record Type and Qtype from
   the DNS Parameters Registry for the SPF RR type.

   The IANA will need to maintain one registry in support of this
   specification.  The registry consists of the modifiers as described
   in Section 3.6.3 and Section 5

   [[Missing application review policy]]

9.1  Registration Template

   To: ietf-types@iana.org
   Subject: Registration of SPF Modifier XXX

   Modifier name:

   Type: (Global or Positional)

   Appearance: (Single or Multiple)

   Security considerations:

   Interoperability considerations:

   Published specification:

   Person & email address to contact for further information:

   Author/Change controller:

   (Any other information that the author deems interesting may be added
   below this line.)

   [[This template needs to be reviewed]]















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10.  Contributors and Acknowledgements

   This design owes a debt of parentage to RMX (by Hadmut Danisch in
   2003) and to DMP (by Gordon Fecyk in 2003).  It traces its ancestry
   farther back through "Repudiating Mail-From" by Paul Vixie in 2002 to
   a suggestion by Jim Miller in 1998.

   Philip Gladstone contributed macros to the specification, multiplying
   the expressiveness of the language and making per-user and per-IP
   lookups possible.

   The authors would also like to thank the literally hundreds of
   individuals who have participated in the development of this design.
   There are far too numerous to name, but they include:

      The folks on the SPAM-L mailing list.
      The folks on the ASRG and MARID/MXCOMP mailing lists.
      The folks on the spf-discuss mailing list.
      The folks on the mailing list that shall not be named.
      The folks on #perl.































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11.  Comments

   Comments on this draft are welcome.  In the interests of openness,
   rather than contacting the authors directly, please post to either:

      the ietf-mxcomp mailing list
      http://www.imc.org/ietf-mxcomp/index.html

   or

      the spf-discuss mailing list.
      http://spf.pobox.com/mailinglist.html







































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12.  References

12.1  Normative References

   [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.

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

12.2  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

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

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

   [RFC3668]  Bradner, S., "Intellectual Property Rights in IETF
              Technology", BCP 79, RFC 3668, February 2004.

   [Mailfrom]
              Lentczner, M. and M. Wong, "Authorizing Use of Domains in
              MAIL FROM", draft-ietf-marid-mailfrom-00 (work in
              progress), September 2004.

   [PRA]      Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              draft-ietf-marid-core-03 (work in progress), August 2004.

   [RMX]      Danish, H., "The RMX DNS RR Type for light weight sender
              authentication", October 2003.

              Work In Progress

   [DMP]      Fecyk, G., "Designated Mailers Protocol", December 2003.



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              Work In Progress

   [Vixie]    Vixie, P., "Repudiating Mail-From", 2002.


Authors' Addresses

   Meng Weng Wong
   Singapore

   EMail: mengwong+spf@pobox.com


   Mark Lentczner
   1209 Villa Street
   Mountain View, CA  94041
   United States of America

   EMail: markl@glyphic.com
   URI:   http://www.ozonehouse.com/mark/































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Appendix A.  Collected ABNF

   This section is normative and any discrepancies with the ABNF
   fragments in the preceding text are to be resolved in favor of this
   grammar.

   See [RFC2234] for ABNF notation.

   record      = version scope terms *SP

   version     = "spf2." ver-minor
   ver-minor   = 1*DIGIT
   scope       = "/" scope-id *[ "," scope-id ]
   scope-id    = "mfrom" / "pra" / name

   terms       = *( 1*SP ( directive / modifier ) )

   directive   = [ prefix ] mechanism
   prefix      = "+" / "-" / "?" / "~"
   mechanism   = ( all / include
                  / A / MX / PTR / IP4 / IP6 / exists
                  / unknown-mechanism )

   all          = "all"
   include      = "include"  ":" domain-spec
   A            = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
   MX           = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
   PTR          = "ptr"    [ ":" domain-spec ]
   IP4          = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
   IP6          = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
   exists       = "exists"   ":" domain-spec

   unknown-mechanism = name [ ":" macro-string ]

   modifier    = redirect / explanation / unknown-modifier
   redirect    = "redirect" "=" domain-spec
   explanation = "exp" "=" domain-spec
   unknown-modifier = name "=" macro-string


   ip4-network  = as per conventional dotted quad notation,
                  e.g. 192.0.2.0
   ip6-network  = as per [RFC 3513], section 2.2,
                  e.g. 2001:DB8::CD30

   dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
   ip4-cidr-length  = "/" 1*DIGIT
   ip6-cidr-length  = "/" 1*DIGIT



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   domain-spec  = *( macro-char / v-char-dm )
   macro-string = *( macro-char / v-char-ms )
   macro-char   = ( "%{" ALPHA transformer *delimiter "}" )
                  / "%%" / "%_" / "%-"
   transformer  = *DIGIT [ "r" ]

   name        = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
   delimiter   = "." / "-" / "+" / "," / "/" / "_" / "="
   v-char-dm   = %x21-24 / %x26-2E / %x30-7E
                 ; visible characters except "%" and "/"
   v-char-ms   = %x21-24 / %x26-7E
                 ; visible characters except "%"







































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Appendix B.  Extended Examples

   These examples are based on the following DNS setup:

   ; A domain with two mail servers, two hosts
   ; and two servers at the domain name

   $ORIGIN example.com.
   @           MX  10 mail-a
               MX  20 mail-b
               A   192.0.2.10
               A   192.0.2.11
   amy         A   192.0.2.65
   bob         A   192.0.2.66
   mail-a      A   192.0.2.129
   mail-b      A   192.0.2.130
   www         CNAME example.com.

   ; A related domain

   $ORIGIN example.org
   @           MX  10 mail-c
   mail-c      A   192.0.2.140

   ; The reverse IP for those addresses

   $ORIGIN 2.0.192.in-addr.arpa.
   10          PTR example.com.
   11          PTR example.com.
   65          PTR amy.example.com.
   66          PTR bob.example.com.
   129         PTR mail-a.example.com.
   130         PTR mail-b.example.com.
   140         PTR mail-c.example.org.

   ; A rogue reverse IP domain that claims to be
   ; something it's not

   $ORIGIN 0.0.10.in-addr.arpa.
   4           PTR bob.example.com.


B.1  Simple Examples

   These examples show various possible published records for
   example.com and which values if <ip> would cause check_host() to
   return "Pass".  Note that <domain> is "example.com".




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   spf2.0/mfrom,pra +all
      -- any <ip> passes

   spf2.0/mfrom,pra a -all
      -- hosts 192.0.2.10 and 192.0.2.11 pass

   spf2.0/mfrom,pra a:example.org -all
      -- no sending hosts pass since example.org has no A records

   spf2.0/mfrom,pra mx -all
      -- sending hosts 192.0.2.129 and 192.0.2.130 pass

   spf2.0/mfrom,pra mx:example.org -all
      -- sending host 192.0.2.140 passes

   spf2.0/mfrom,pra mx mx:example.org -all
      -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass

   spf2.0/mfrom,pra mx/30 mx:example.org/30 -all
      -- any sending host in 192.0.2.128/30 or 192.168.2.140/30 passes

   spf2.0/mfrom,pra ptr -all
      -- sending host 192.0.2.65 passes (reverse IP is valid and in
      example.com)
      -- sending host 192.0.2.140 fails (reverse IP is valid, but not in
      example.com)
      -- sending host 10.0.0.4 fails (reverse IP is not valid)

   spf2.0/mfrom,pra ip4:192.0.2.128/28 -all
      -- sending host 192.0.2.65 fails
      -- sending host 192.0.2.129 passes

B.2  Multiple Domain Example

   These examples show the effect of related records:

      example.org: "spf2.0/mfrom,pra include:example.com
      include:example.net -all"

   This record would be used if mail from example.org actually came
   through servers at example.com and example.net.  Example.org's
   designated servers are the union of example.com and example.net's
   designated servers.

      la.example.org: "spf2.0/mfrom,pra redirect=example.org"
      ny.example.org: "spf2.0/mfrom,pra redirect=example.org"
      sf.example.org: "spf2.0/mfrom,pra redirect=example.org"




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   These records allow a set of domains that all use the same mail
   system to make use of that mail system's record.  In this way, only
   the mail system's record needs to updated when the mail setup
   changes.  These domains' records never have to change.

B.3  RBL Style Example

   Imagine that, in addition to the domain records listed above, there
   are these:

   $Origin _spf.example.com.
   mary.mobile-users                   A 127.0.0.2
   fred.mobile-users                   A 127.0.0.2
   15.15.168.192.joel.remote-users     A 127.0.0.2
   16.15.168.192.joel.remote-users     A 127.0.0.2

   The following records describe users at example.com who mail from
   arbitrary servers, or who mail from personal servers.

   example.com:

   spf2.0/mfrom,pra mx
          include:mobile-users._spf.%{d}
          include:remote-users._spf.%{d}
          -all

   mobile-users._spf.example.com:

   spf2.0/mfrom,pra exists:%{l1r+}.%{d}

   remote-users._spf.example.com:

   spf2.0/mfrom,pra exists:%{ir}.%{l1r+}.%{d}


















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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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