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Versions: 00 01 02 rfc4408                                  Experimental
                                                        IPR declarations
Network Working Group                                            M. Wong
Internet-Draft                                                W. Schlitt
Expires: December 8, 2005                                   June 6, 2005

Sender Policy Framework (SPF) for Authorizing Use of Domains in E-MAIL,
                               version 1

Status of this Memo

   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 becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 8, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   E-mail on the Internet can be forged in a number of ways.  In
   particular, existing protocols place no restriction on what a sending
   host can use as the reverse-path of a message or the domain given on
   the SMTP HELO/EHLO commands.  This document describes version 1 of
   the SPF protocol, whereby a domain may explicitly authorize the hosts
   that are allowed to use its domain name, and a receiving host may
   check such authorization.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  State of this draft  . . . . . . . . . . . . . . . . . . .  4
     1.2.  Protocol Status  . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  The HELO Identity  . . . . . . . . . . . . . . . . . . . .  6
     2.2.  The MAIL FROM Identity . . . . . . . . . . . . . . . . . .  6
     2.3.  Publishing Authorization . . . . . . . . . . . . . . . . .  6
     2.4.  Checking Authorization . . . . . . . . . . . . . . . . . .  7
     2.5.  Interpreting the Result  . . . . . . . . . . . . . . . . .  8
       2.5.1.  None . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.5.2.  Neutral  . . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.3.  Pass . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.4.  Fail . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.5.  SoftFail . . . . . . . . . . . . . . . . . . . . . . .  9
       2.5.6.  TempError  . . . . . . . . . . . . . . . . . . . . . . 10
       2.5.7.  PermError  . . . . . . . . . . . . . . . . . . . . . . 10
   3.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Publishing . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.1.1.  DNS Resource Record Types  . . . . . . . . . . . . . . 11
       3.1.2.  Multiple DNS Records . . . . . . . . . . . . . . . . . 12
       3.1.3.  Multiple Strings in a Single DNS record  . . . . . . . 12
       3.1.4.  Record Size  . . . . . . . . . . . . . . . . . . . . . 12
       3.1.5.  Wildcard Records . . . . . . . . . . . . . . . . . . . 13
   4.  The check_host() Function  . . . . . . . . . . . . . . . . . . 14
     4.1.  Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.2.  Results  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  Initial Processing . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 15
     4.5.  Selecting Records  . . . . . . . . . . . . . . . . . . . . 15
     4.6.  Record Evaluation  . . . . . . . . . . . . . . . . . . . . 15
       4.6.1.  Term Evaluation  . . . . . . . . . . . . . . . . . . . 16
       4.6.2.  Mechanisms . . . . . . . . . . . . . . . . . . . . . . 16
       4.6.3.  Modifiers  . . . . . . . . . . . . . . . . . . . . . . 17
     4.7.  Default Result . . . . . . . . . . . . . . . . . . . . . . 17
     4.8.  Domain Specification . . . . . . . . . . . . . . . . . . . 17
   5.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 19
     5.1.  "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.2.  "include"  . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.3.  "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     5.4.  "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     5.5.  "ptr"  . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.6.  "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 23
     5.7.  "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 24
   6.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 25
     6.1.  redirect: Redirected Query . . . . . . . . . . . . . . . . 25

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     6.2.  exp: Explanation . . . . . . . . . . . . . . . . . . . . . 26
   7.  The Received-SPF header field  . . . . . . . . . . . . . . . . 28
   8.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  Macro definitions  . . . . . . . . . . . . . . . . . . . . 30
     8.2.  Expansion Examples . . . . . . . . . . . . . . . . . . . . 33
   9.  Implications . . . . . . . . . . . . . . . . . . . . . . . . . 34
     9.1.  Sending Domains  . . . . . . . . . . . . . . . . . . . . . 34
     9.2.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 34
     9.3.  Forwarding Services and Aliases  . . . . . . . . . . . . . 34
     9.4.  Mail Services  . . . . . . . . . . . . . . . . . . . . . . 36
     9.5.  MTA Relays . . . . . . . . . . . . . . . . . . . . . . . . 37
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 38
     10.1. Processing Limits  . . . . . . . . . . . . . . . . . . . . 38
     10.2. SPF-Authorized E-Mail May Be UBE . . . . . . . . . . . . . 39
     10.3. Spoofed DNS and IP Data  . . . . . . . . . . . . . . . . . 40
     10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 40
     10.5. Untrusted Information Sources  . . . . . . . . . . . . . . 40
     10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 41
   11. Contributors and Acknowledgements  . . . . . . . . . . . . . . 42
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 43
     12.1. The SPF DNS Record Type  . . . . . . . . . . . . . . . . . 43
     12.2. The Received-SPF mail header . . . . . . . . . . . . . . . 43
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     13.2. Informative References . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 46
   Appendix B.  Extended Examples . . . . . . . . . . . . . . . . . . 48
     B.1.  Simple Examples  . . . . . . . . . . . . . . . . . . . . . 48
     B.2.  Multiple Domain Example  . . . . . . . . . . . . . . . . . 49
     B.3.  DNSBL Style Example  . . . . . . . . . . . . . . . . . . . 50
     B.4.  Multiple Requirements Example  . . . . . . . . . . . . . . 50
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 51
     C.1.  Changes in Version -02 . . . . . . . . . . . . . . . . . . 51
     C.2.  Changes in Version -01 . . . . . . . . . . . . . . . . . . 52
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 55
   Intellectual Property and Copyright Statements . . . . . . . . . . 56

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1.  Introduction

   The current e-mail infrastructure has the property that any host
   injecting mail into the mail system can identify itself as any domain
   name it wants.  Hosts can do this at a variety of levels: in
   particular, the session, the envelope, and the mail headers.  While
   this feature is desirable in some circumstances, it is a major
   obstacle to reducing Unsolicited Bulk E-mail (UBE, aka "spam").
   Furthermore, many domain name holders are understandably concerned
   about the ease with which other entities may make use of their domain
   names, often with malicious intent.

   This document defines a protocol by which domain owners may authorize
   hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
   Compliant domain holders publish SPF records specifying which hosts
   are permitted to use their names, and compliant mail receivers use
   the published SPF records to test the authorization of sending MTAs
   using a given "HELO" or "MAIL FROM" identity during a mail

   An additional benefit to mail receivers is that after the use of an
   identity is verified, local policy decisions about the mail can be
   made based on the sender's domain, rather than the host's IP address.
   This is advantageous because reputation of domain names is likely to
   be more accurate than reputation of host IP addresses.  Furthermore,
   if a claimed identity fails verification, local policy can take
   stronger action against such e-mail, such as rejecting it.

1.1.  State of this draft

   This draft version attempts to resolve all known issues and address
   all comments received from the IESG review of 2005/02/17, as well
   reviews from the namedroppers, ietf-smtp, ietf-822 and spf-discuss
   mailing lists both in January and in May.

   Please check the Change log in Appendix C before proposing changes,
   as it is possible that your idea has already been discussed.  Please
   post comments on the spf-discuss@v2.listbox.com mailing list or
   e-mail them directly to the author.

   I am sorry for the length of this I-D; I have not had time to make it

   RFC Editor Note: Please remove this section for the final publication
   of the document.  It has been inspired by

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1.2.  Protocol Status

   SPF has been in development since the Summer of 2003, and has seen
   deployment beyond the developers beginning in December, 2003.  The
   design of SPF slowly evolved until the spring of 2004 and has since
   stabilized.  There have been quite a number of forms of SPF, some
   written up as documents, some submitted as Internet Drafts, and many
   discussed and debated in development forums.

   The goal of this document is to clearly document the protocol defined
   by earlier draft specifications of SPF as used in existing
   implementations.  This conception of SPF is sometimes called "SPF
   Classic".  It is understood that particular implementations and
   deployments may differ from, and build upon, this work.  It is hoped
   that we have nonetheless captured the common understanding of SPF
   version 1.

1.3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document is concerned with the portion of a mail message
   commonly called "envelope sender", "return path", "reverse path",
   "bounce address", "2821 FROM", or "MAIL FROM".  Since these terms are
   either not well defined, or often used casually, this document
   defines the "MAIL FROM" identity in Section 2.2.  Note that other
   terms that may superficially look like the common terms, such as
   "reverse-path", are used only with the defined meanings from
   normative documents.

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2.  Operation

2.1.  The HELO Identity

   The "HELO" identity derives from either the SMTP HELO or EHLO command
   (see [RFC2821]).  These commands supply the SMTP client (sending
   host) for the SMTP session.  Note that requirements for the domain
   presented in the EHLO or HELO command are not always clear to the
   sending party, and SPF clients must be prepared for the "HELO"
   identity to be malformed or an IP address literal.  At the time of
   this writing, many legitimate e-mails are delivered with invalid HELO

   It is RECOMMENDED that SPF clients check not only the "MAIL FROM"
   identity, but also separately check the "HELO" identity by applying
   the check_host() function (Section 4) to the "HELO" identity as the

2.2.  The MAIL FROM Identity

   The "MAIL FROM" identity derives from the SMTP MAIL command (see
   [RFC2821]).  This command supplies the "reverse-path" for a message,
   which generally consists of the sender mailbox, and is the mailbox to
   which notification messages are to be sent if there are problems
   delivering the message.

   [RFC2821] allows the reverse-path to be null (see Section 4.5.5).  In
   this case, there is no explicit sender mailbox, and such a message
   can be assumed to be a notification message from the mail system
   itself.  When the reverse-path is null, this document defines the
   "MAIL FROM" identity to be the mailbox composed of the localpart
   "postmaster" and the "HELO" identity (which may or may not have been
   checked separately before).

   SPF clients MUST check the "MAIL FROM" identity.  SPF clients check
   the "MAIL FROM" identity by applying the check_host() function to the
   "MAIL FROM" identity as the <sender>.

2.3.  Publishing Authorization

   An SPF-compliant domain MUST publish a valid SPF record as described
   in Section 3.  This record authorizes the use of the domain name in
   the "HELO" and "MAIL FROM" identities by the MTAs it specifies.

   If domain owners choose to publish SPF records, it is RECOMMENDED
   that they end in "-all", or redirect to other records that do, so
   that a definitive determination of authorization can be made.

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   Domain holders may publish SPF records that explicitly authorize no
   hosts if mail should never originate using that domain.

   When changing SPF records, care must be taken to ensure that there is
   a transition period so that the old policy remains valid until all
   legitimate e-mail has been checked.

2.4.  Checking Authorization

   A mail receiver can perform a set of SPF checks for each mail message
   it receives.  An SPF check tests the authorization of a client host
   to emit mail with a given identity.  Typically, such 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 and
   reliable.  At least the "MAIL FROM" identity MUST be checked, but it
   is RECOMMENDED that the "HELO" identity also be checked beforehand.

   Without explicit approval of the domain owner, checking other
   identities against SPF version 1 records is NOT RECOMMENDED because
   there are cases that are known to give incorrect results.  For
   example, almost all mailing lists rewrite the "MAIL FROM" identity
   (see Section 9.2), but some do not change any other identities in the
   message.  The scenario described in Section is another
   example.  Documents that define other identities should define the
   method for explicit approval.

   It is possible that mail receivers will use the SPF check as part of
   a larger set of tests on incoming mail.  The results of other tests
   may influence whether or not a particular SPF check is performed.
   For example, finding the sending host's IP address 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 an SPF check, it MUST use a
   correctly-implemented check_host() function (Section 4) evaluated
   with the correct parameters.  While the test as a whole is optional,
   once it has been decided to perform a test it must be performed as
   specified so that the correct semantics are preserved between
   publisher and receiver.

   To make the test, the mail receiver MUST evaluate the check_host()
   function with the arguments set as follows:

   <ip>     - the IP address of the SMTP client that is emitting the
            mail, either IPv4 or IPv6.

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   <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.

   <sender> - the "MAIL FROM" or "HELO" identity.

   Note that the <domain> argument may not be a well-formed domain name.
   For example, if the reverse-path was null, then the EHLO/HELO domain
   is used, with its associated problems (see Section 2.1).  In these
   cases, check_host() is defined in Section 4.3 to return a "None"

   While invalid, malformed, or non-existent domains cause SPF checks to
   return "None" because no SPF record can be found, it has long been
   the policy of many MTAs to reject e-mail from such domains,
   especially in the case of invalid "MAIL FROM".  In order to prevent
   the circumvention of SPF records, rejecting e-mail from invalid
   domains should be considered.

   Implementations must take care to correctly extract the <domain> from
   the data given with the SMTP MAIL FROM command as many MTAs will
   still accept such things as source routes (see [RFC2821] appendix C),
   the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).  These
   archaic features have been maliciously used to bypass security

2.5.  Interpreting the Result

   This section describes how software that performs the authorization
   should interpret the results of the check_host() function.  The
   authorization check SHOULD be performed during the processing of the
   SMTP transaction that sends the mail.  This allows errors to be
   returned directly to the sending server by way of SMTP replies.

   Performing the authorization after the SMTP transaction has finished
   may cause problems, such as: 1) It may be difficult to accurately
   extract the required information from potentially deceptive headers.
   2) Legitimate e-mail may fail because the sender's policy may have
   since changed.

   Generating non-delivery notifications to forged identities that have
   failed the authorization check is generally abusive and against the
   explicit wishes of the identity owner.

2.5.1.  None

   A result of "None" means that no records were published by the
   domain, or that no checkable sender domain could be determined from
   the given identity.  The checking software cannot ascertain whether
   the client host is authorized or not.

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2.5.2.  Neutral

   The domain owner has explicitly stated that they cannot or do not
   want to assert whether the IP address is authorized or not.  A
   "Neutral" result MUST be treated exactly like the "None" result; the
   distinction exists only for informational purposes.  Treating
   "Neutral" more harshly than "None" will discourage domain owners from
   testing the use of SPF records (see Section 9.1).

2.5.3.  Pass

   A "Pass" result means that the client is authorized to inject mail
   with the given identity.  The domain can now, in the sense of
   reputation, be considered responsible for sending the message.
   Further policy checks can now proceed with confidence in the
   legitimate use of the identity.

2.5.4.  Fail

   A "Fail" result is an explicit statement that the client is not
   authorized to use the domain in the given identity.  The checking
   software can choose to mark the mail based on this, or to reject the
   mail outright.

   If the checking software chooses to reject the mail during the SMTP
   transaction, then it SHOULD use an SMTP reply code of 550 (see
   [RFC2821]) and, if supported, the 5.7.1 Delivery Status Notification
   (DSN) code (see [RFC3464]), in addition to an appropriate reply text.
   The check_host() function may return either a default explanation
   string, or one from the domain that published the SPF records (see
   Section 6.2).  If the information doesn't originate with the checking
   software, it should be made clear that the text is provided by the
   sender's domain.  For example:

       550-5.7.1 SPF MAIL FROM check failed:
       550-5.7.1 The domain example.com explains:
       550 5.7.1 Please see http://www.example.com/mailpolicy.html

2.5.5.  SoftFail

   A "SoftFail" result should be treated as somewhere between a "Fail"
   and a "Neutral".  The domain believes the host isn't authorized but
   isn't willing to make that strong of a statement.  Receiving software
   SHOULD NOT reject the message based solely on this result, but MAY
   subject the message to closer scrutiny than normal.

   The domain owner wants to discourage the use of this host and so they
   desire limited feedback when a "SoftFail" result occurs.  For

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   example, the recipient's MUA could highlight the "SoftFail" status,
   or the receiving MTA could give the sender a message using a
   technique called "greylisting" whereby the MTA can issue an SMTP
   reply code of 451 (4.3.0 DSN code) with a note the first time the
   message is received, but accept it the second time.

2.5.6.  TempError

   A "TempError" result means that the SPF client encountered a
   transient error while performing the check.  Checking software can
   choose to accept or temporarily reject the message.  If the message
   is rejected during the SMTP transaction for this reason, the software
   SHOULD use an SMTP reply code of 451 and, if supported, the 4.4.3 DSN

2.5.7.  PermError

   A "PermError" result means that the domain's published records
   couldn't be correctly interpreted.  This signals an error condition
   that requires manual intervention to be resolved, as opposed to the
   TempError result.  Be aware that if the domain owner uses macros
   (Section 8), it is possible that this result is due to the checked
   identities having an unexpected format.

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3.  SPF Records

   An SPF record is a DNS Resource Record (RR) that declares which hosts
   are, and are not, authorized to use a domain name for the "HELO" and
   "MAIL FROM" identities.  Loosely, the record partitions all hosts
   into permitted and not-permitted sets.  (Though some hosts might fall
   into neither category.)

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

      v=spf1 +mx a:colo.example.com/28 -all

   This record has a version of "spf1" and three directives: "+mx",
   "a:colo.example.com/28" (the + is implied), and "-all".

3.1.  Publishing

   Domain owners wishing to be SPF compliant must publish SPF records
   for the hosts that are used in the "MAIL FROM" and "HELO" identities.
   The SPF records are placed in the DNS tree at the host name it
   pertains to, not a subdomain under it, such as is done with SRV
   records.  This is the same whether the TXT or SPF RR type is used.

   The example above in Section 3 might be published via this lines in a
   domain zone file:

      example.com.          TXT "v=spf1 +mx a:colo.example.com/28 -all"
      smtp-out.example.com. TXT "v=spf1 a -all"

   When publishing via TXT records, beware of other TXT records
   published there for other purposes.  They may cause problems with
   size limits (see Section 3.1.4).

3.1.1.  DNS Resource Record Types

   This document defines a new DNS RR of type SPF, type code to be
   determined.  The format of this type is identical to the TXT RR
   [RFC1035].  For either type, the character content of the record is
   encoded as [US-ASCII].

   RFC Editor Note: Please add the DNS RR type code once it has been
   allocated by the IANA.

   It is recognized that the current practice (using a TXT record) is
   not optimal, but it is necessary because there are a number of DNS
   server and resolver implementations in common use that cannot handle
   the new RR type.  The two-record-type scheme provides a forward path
   to the better solution of using an RR type reserved for this purpose.

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   An SPF-compliant domain name SHOULD have SPF records of both RR
   types.  A compliant domain name MUST have a record of at least one
   type.  If a domain has records of both types, they MUST have
   identical content.  For example, instead of just publishing one
   record as in Section 3.1 above, it is better to publish:

      example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
      example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"

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

3.1.2.  Multiple DNS Records

   A domain name MUST NOT have multiple records that would cause an
   authorization check to select more than one record.  See Section 4.5
   for the selection rules.

3.1.3.  Multiple Strings in a Single DNS record

   As defined in [RFC1035] sections 3.3.14 and 3.3, a single 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 the
   record MUST be treated as if those strings are concatenated together
   without adding spaces.  For example:

      IN TXT "v=spf1 .... first" "second string..."

   MUST be treated as equivalent to

      IN TXT "v=spf1 .... firstsecond string..."

   SPF or TXT records containing multiple strings are useful in order to
   construct records which would exceed the 255 byte maximum length of a
   string within a single TXT or SPF RR record.

3.1.4.  Record Size

   The published SPF record for a given domain name SHOULD remain small
   enough that the results of a query for it 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
   records of a given type (TXT or SPF) is under 450 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.  Records that are too

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   long to fit in a single UDP packet MAY be silently ignored by SPF

3.1.5.  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.          TXT     "v=spf1 a:A.X.COM -all"

       *.X.COM.        MX      10      A.X.COM
       *.X.COM.        TXT     "v=spf1 a:A.X.COM -all"

       A.X.COM.        A
       A.X.COM.        MX      10      A.X.COM
       A.X.COM.        TXT     "v=spf1 a:A.X.COM -all"

       *.A.X.COM.      MX      10      A.X.COM
       *.A.X.COM.      TXT     "v=spf1 a:A.X.COM -all"

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

   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).

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4.  The check_host() Function

   The check_host() function fetches SPF records, parses them, and
   interprets them to determine whether a particular host is or is not
   permitted to send mail with a given identity.  Mail receivers that
   perform this check MUST correctly evaluate the check_host() function
   as described here.

   Implementations MAY use a different algorithm than the canonical
   algorithm defined here, so long as the results are the same in all

4.1.  Arguments

   The function check_host() takes these arguments:

   <ip>     - the IP address of the SMTP client that is emitting the
            mail, either IPv4 or IPv6.

   <domain> - the domain that provides the sought-after authorization
            information; initially the domain portion of the "MAIL FROM"
            or "HELO" identity.

   <sender> - the "MAIL FROM" or "HELO" identity.

   The domain portion of <sender> will usually be the same as the
   <domain> argument when check_host() is initially evaluated.  However,
   this will generally not be true for recursive evaluations (see
   Section 5.2 below).

   Actual implementations of the check_host() function may need
   additional arguments.

4.2.  Results

   The function check_host() can return one of several results described
   in Section 2.5.  Based on the result, the action to be taken is
   determined by the local policies of the receiver.

4.3.  Initial Processing

   If the <domain> is malformed (label longer than 63 characters, zero
   length label not at the end, etc.), is not a fully qualified domain
   name, or if the DNS lookup returns "domain does not exist" (RCODE 3),
   check_host() immediately returns the result "None".

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

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4.4.  Record Lookup

   In accordance with how the records are published, see Section 3.1
   above, a DNS query needs to be made for the <domain> name, querying
   for either RR type TXT, SPF, or both.  If both SPF and TXT RRs are
   looked up, the queries MAY be done in parallel.

   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".

4.5.  Selecting Records

   Records begin with a version section:

   record           = version terms *SP
   version          = "v=spf1"

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

   1.  Records that do not begin with a version section of exactly
       "v=spf1" are discarded.  Note that the version section is
       terminated either by a SP character or the end of the record.  A
       record with a version section of "v=spf10" does not match and
       must be discarded.

   2.  If there are both SPF and TXT records in the set and if they are
       not all identical, return a "PermError".

   3.  If any records of type SPF are in the set, then all records of
       type TXT are discarded.

   After the above steps, there should be exactly one record remaining
   and evaluation can proceed.  If there are two or more records
   remaining, then check_host() exits immediately with the result of

   If no matching records are returned, an SPF client MUST assume that
   the domain makes no SPF declarations.  SPF processing MUST stop and
   return "None".

4.6.  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
   there are any syntax errors, check_host() returns immediately with
   the result "PermError".

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   Implementations MAY choose to parse the entire record first and
   return "PermError" if the record is not syntactically well formed.
   However, in all cases, any syntax errors anywhere in the record MUST
   be detected.

4.6.1.  Term Evaluation

   There are two types of terms: mechanisms and modifiers.  A record
   contains an ordered list of these as specified in the following ABNF.

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

   directive        = [ qualifier ] mechanism
   qualifier        = "+" / "-" / "?" / "~"
   mechanism        = ( all / include
                      / A / MX / PTR / IP4 / IP6 / exists )
   modifier         = redirect / explanation / unknown-modifier
   unknown-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, as
   defined in Section 5.

   As per the definition of the ABNF notation in [I-D.crocker-abnf-
   rfc2234bis], mechanism and modifier names are case-insensitive.

4.6.2.  Mechanisms

   Each mechanism is considered in turn from left to right.  If there
   are no more mechanisms, the result is specified in Section 4.7.

   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 qualifier value is returned as
   the result of that record.  If it does not match, processing
   continues with the next mechanism.  If it throws an exception,
   mechanism processing ends and the exception value is returned.

   The possible qualifiers, and the results they return are:

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      "+" Pass
      "-" Fail
      "~" SoftFail
      "?" Neutral

   The qualifier is optional and defaults to "+".

   When a mechanism matches and the qualifier is "-", then a "Fail"
   result is returned and the explanation string is computed as
   described in Section 6.2.

   The specific mechanisms are described in Section 5.

4.6.3.  Modifiers

   Modifiers are not mechanisms: they do not return match or not-match.
   Instead they provide additional information.  While modifiers do not
   directly affect the evaluation of the record, the "redirect" modifier
   has an effect after all the mechanisms have been evaluated.

4.7.  Default Result

   If none of the mechanisms match and there is no "redirect" modifier,
   then the check_host() returns a result of "Neutral", just as if
   "?all" were specified as the last directive.  If there is a
   "redirect" modifier, check_host() proceeds as defined in Section 6.1.

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

   For example:

      v=spf1 +mx -all
      v=spf1 +mx redirect=_spf.example.com

4.8.  Domain Specification

   Several of these mechanisms and modifiers have a <domain-spec>
   section.  The <domain-spec> string is macro expanded (see Section 8).
   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 (e.g. the control characters).  However,
   this facility is powerful enough to express legal host names, and

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   common utility labels (such as "_spf") that are used in DNS.

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

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5.  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.


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


   The following conventions apply to all mechanisms that perform a
   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 mechanism 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.  Even if the SMTP connection is
   via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513] section
   2.5.5) MUST still be considered an IPv4 address.

   Several mechanisms rely on information fetched from DNS.  For these
   DNS queries, except where noted, if the DNS server returns an error
   (RCODE other than 0 or 3) or the query times out, the mechanism
   throws the exception "TempError".  If the server returns "domain does
   not exist" (RCODE 3), then evaluation of the mechanism continues as
   if the server returned no error (RCODE 0) and zero answer records.

5.1.  "all"

   all              = "all"

   The "all" mechanism is a test that always matches.  It is used as the

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   rightmost mechanism in a record to provide an explicit default.

   For example:

      v=spf1 a mx -all

   Mechanisms after "all" will never be tested.  Any "redirect" modifier
   (Section 6.1) has no effect when there is an "all" mechanism.

5.2.  "include"

   include          = "include"  ":" domain-spec

   The "include" mechanism triggers a recursive evaluation of
   check_host().  The domain-spec is expanded as per Section 8.  Then
   check_host() is evaluated with the resulting string as the <domain>.
   The <ip> and <sender> arguments remain the same as in the current
   evaluation of check_host().

   In hindsight, the name "include" was poorly chosen.  Only the
   evaluated result of the referenced SPF record is used, rather than
   acting as if the referenced SPF record was literally included in the
   first.  For example, evaluating a "-all" directive in the referenced
   record does not terminate the overall processing and does not
   necessarily result in an overall "Fail".  (Better names for this
   mechanism would have been "if-pass", "on-pass", etc.)

   The "include" mechanism 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

      IN TXT "v=spf1 include:example.com include:example.org -all"

   This 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

   Whether this mechanism matches, does not match, 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".

5.3.  "a"

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

   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

5.4.  "mx"

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

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

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   check_host() first performs an MX lookup on the <target-name>.  Then
   it performs an address lookup on each MX name returned.  The <ip> is
   compared to each returned IP address.  To prevent DoS attacks, more
   than 10 MX names MUST NOT be looked up during the evaluation of an
   "mx" mechanism (see Section 10).  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.

5.5.  "ptr"

   This mechanism tests whether the DNS reverse mapping for <ip> exists
   and correctly 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.arpa." if the address is an IPv4 one and in "ip6.arpa."
   if it is an IPv6 address.  For each record returned, validate the
   domain name by looking up its IP address.  To prevent DoS attacks,
   more than 10 PTR names MUST NOT be looked up during the evaluation of
   a "ptr" mechanism (see Section 10).  If <ip> is among the returned IP
   addresses, then that domain name is validated.  In pseudocode:

   sending-domain_names := ptr_lookup(sending-host_IP);
   if more than 10 sending-domain_names are found, use at most 10.
   for each name in (sending-domain_names) {
     IP_addresses := a_lookup(name);
     if the sending-domain_IP is one of the IP_addresses {
       validated-sending-domain_names += name;

   Check all validated domain names to see if they end in the
   <target-name> domain.  If any do, this mechanism matches.  If no
   validated domain name can be found, or if none of the validated
   domain names end in the <target-name>, this mechanism fails to match.
   If a DNS error occurs while doing the PTR RR lookup, then this
   mechanism fails to match.  If a DNS error occurs while doing an A RR
   lookup, then that domain name is skipped and the search continues.

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   for each name in (validated-sending-domain_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 either an ancestor of
   a validated domain name, or if the <target-name> and a validated
   domain name are the same.  For example: "mail.example.com" is within
   the domain "example.com", but "mail.bad-example.com" is not.

   Note: Use of this mechanism is discouraged because it is slow, is not
   as reliable as other mechanisms in cases of DNS errors and it places
   a large burden on the arpa name servers.  If used, proper PTR records
   must be in place for the domain's hosts and the "ptr" mechanism
   should be one of the last mechanisms checked.

5.6.  "ip4" and "ip6"

   These mechanisms test whether <ip> is contained within a given IP

   IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
   IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]

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

   ip4-network      = qnum "." qnum "." qnum "." qnum
   qnum             = DIGIT                 ; 0-9
                      / %x31-39 DIGIT       ; 10-99
                      / "1" 2DIGIT          ; 100-199
                      / "2" %x30-34 DIGIT   ; 200-249
                      / "25" %x30-35        ; 250-255
             ; as per conventional dotted quad notation.  e.g.
   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".  It is not
   permitted to omit parts of the IP address instead of using CIDR
   notations.  That is, use instead of 192.0.2.

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5.7.  "exists"

   This mechanism is used to construct an arbitrary domain 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

   exists           = "exists"   ":" domain-spec

   The domain-spec is expanded as per Section 8.  The resulting domain
   name is used for a DNS A RR 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:

      v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all

   The <target-name> might expand to
   "".  This makes fine-grained
   decisions possible at the level of the user and client IP address.

   This mechanism enables queries that mimic the style of tests that
   existing anti-spam DNS blacklists (DNSBL) use.

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6.  Modifier Definitions

   Modifiers are name/value pairs that provide additional information.
   Modifiers always have an "=" separating the name and the value.

   The modifiers defined in this document ("redirect" and "exp") MAY
   appear anywhere in the record, but SHOULD appear at the end, after
   all mechanisms.  Ordering of these two modifiers does not matter.
   These two modifiers MUST NOT appear in a record more than once each.
   If they do, then check_host() exits with a result of "PermError".

   Unrecognized modifiers MUST be ignored no matter where in a record,
   or how often.  This allows implementations of this document to
   gracefully handle records with modifiers that are defined in other

6.1.  redirect: Redirected Query

   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 8.  Then check_host() is evaluated with
   the resulting string as the <domain>.  The <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 the current evaluation with the exception that if no
   SPF record is found, or if the target-name is malformed, the result
   is a "PermError" rather than "None".

   Note that the newly-queried domain may itself specify redirect

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

     la.example.com. TXT "v=spf1 redirect=_spf.example.com"
     ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
     sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
   _spf.example.com. TXT "v=spf1 mx:example.com -all"

   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, the domain "A" cannot reliably use a redirect to

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   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 mailboxes 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.

6.2.  exp: Explanation

   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 either a default explanation string or an empty
   explanation string may be returned.

   The <domain-spec> is macro expanded (see Section 8) and becomes the
   <target-name>.  The DNS TXT record for the <target-name> is fetched.

   If <domain-spec> is empty, or there are any DNS processing errors
   (any RCODE other than 0), or if no records are returned, or if more
   than one record is returned, or if there are syntax errors in the
   explanation string, then proceed as if no exp modifier was given.

   The fetched TXT record's strings are concatenated with no spaces, and
   then treated as an <explain-string> which is macro-expanded.  This
   final result is the explanation string.  Implementations MAY limit
   the length of the resulting explanation string to allow for other
   protocol constraints and/or reasonable processing limits.  Since the
   explanation string is intended for an SMTP response and [RFC2821]
   section 2.4 says that responses are in [US-ASCII], the explanation
   string is also limited to US-ASCII.

   Software evaluating check_host() can use this string 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 "%{o} explains: " to the explanation, such as shown in
   Section 2.5.4.

   Suppose example.com has this record:

      v=spf1 mx -all exp=explain._spf.%{d}

   Here are some examples of possible explanation TXT records at

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      "Mail from example.com 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
            arguments to check_host() so that a web page can be
            generated with detailed, custom instructions

   Note: During recursion into an "include" mechanism, an exp= modifier
   from the <target-name> MUST NOT be used.  In contrast, when executing
   a "redirect" modifier, an exp= modifier from the original domain MUST
   NOT be used.

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7.  The Received-SPF header field

   It is RECOMMENDED that SMTP receivers record the result of SPF
   processing in the message headers.  If an SMTP receiver chooses to do
   so, it SHOULD use the "Received-SPF" header defined here for each
   identity that was checked.  This information is intended for the
   recipient.  (Information intended for the sender is described in
   Section 6.2, Explanation.)

   The Received-SPF header is a trace field (see [RFC2822] section
   3.6.7) and SHOULD be prepended to existing headers, above the
   Received: header that is generated by the SMTP receiver.  It MUST
   appear above any other Received-SPF headers in the message.  The
   header has the format:

   header           = "Received-SPF:" [CFWS] result FWS [comment FWS]
                      [ key-value-list ] CRLF

   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
                      "None" / "TempError" / "PermError"

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )

   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )

   key              = "client-ip" / "envelope-from" / "helo" /
                      "problem" / "receiver" / "identity" /
                       mechanism / "x-" name / name

   identity         = "mailfrom"   ; for the "MAIL FROM" identity
                      / "helo"     ; for the "HELO" identity
                      / name       ; other identities

   dot-atom         = <unquoted word as per [RFC2822]>
   quoted-string    = <quoted string as per [RFC2822]>
   comment          = <comment string as per [RFC2822]>
   CFWS             = <comment or folding white space as per [RFC2822]>
   FWS              = <folding white space as per [RFC2822]>
   CRLF             = <standard end-of-line token as per [RFC2822]>

   The header SHOULD include a "(...)" style <comment> after the result,
   conveying supporting information for the result, such as <ip>,
   <sender> and <domain>.

   The following key-value pairs are designed for later machine parsing.
   SPF clients SHOULD give enough information so that the SPF results
   can be verified.  That is, at least the "client-ip", "helo", and, if

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   the "MAIL FROM" identity was checked, the "envelope-from".

   client-ip      the IP address of the SMTP client

   envelope-from  the envelope sender mailbox

   helo           the host name given in the HELO or EHLO command

   mechanism      the mechanism that matched (if no mechanisms matched,
                  substitute the word "default".)

   problem        if an error was returned, details about the error

   receiver       the host name of the SPF client

   identity       the identity that was checked, see the <identity> ABNF

   Other keys may be defined by SPF clients.  Until a new key name
   becomes widely accepted, new key names should start with "x-".

   SPF clients MUST make sure that the Received-SPF header does not
   contain invalid characters, is not excessively long, and does not
   contain malicious data that has been provided by the sender.

   Examples of various header styles that could be generated:

   Received-SPF: Pass (mybox.example.org: domain of
    myname@example.com designates as permitted sender)
       receiver=mybox.example.org; client-ip=;
       envelope-from=<myname@example.com>; helo=foo.example.com;

   Received-SPF: Fail (mybox.example.org: domain of
                     myname@example.com does not designate
            as permitted sender)
                     identity=mailfrom; client-ip=;

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8.  Macros

8.1.  Macro definitions

   Many mechanisms and modifiers perform macro expansion on part of the

   domain-spec      = macro-string domain-end
   domain-end       = ( "." toplabel ) / macro-expand

   toplabel         = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
                      ; LDH rule (See [RFC3696])
   alphanum         = ALPHA / DIGIT

   explain-string   = *( macro-string / SP )

   macro-string     = *( macro-expand / macro-literal )
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                      / "%%" / "%_" / "%-"
   macro-literal    = %x21-24 / %x26-7E
                      ; visible characters except "%"
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                      "c" / "r" / "t"
   transformers     = *DIGIT [ "r" ]
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

   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 domain name of <ip>
      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
      h = HELO/EHLO domain

   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

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   A '%' character not followed by a '{', '%', '-', or '_' character is
   a syntax error.  So,


   is incorrect and will cause check_host() to return a "PermError".
   Instead, say


   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 ".".

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

   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 e-mail
   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
   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 4.3).

   For IPv4 addresses, both the "i" and "c" macros expand to the

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   standard dotted-quad format.

   For IPv6 addresses, the "i" macro expands to a 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 domain name of <ip>.  The
   procedure for finding the validated domain name is defined in
   Section 5.5.  If the <domain> is present in the list of validated
   domains, it SHOULD be used.  Otherwise, if a subdomain of the
   <domain> is present, it SHOULD be used.  Otherwise, any name from the
   list may be used.  If there are no validated domain names or if a DNS
   error occurs, 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 MUA) 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.

   The "t" macro expands to the decimal representation of the
   approximate number of seconds since the Epoch (Midnight, January 1st,
   1970, UTC).  This is the same value as is returned by the POSIX
   time() function in most standards-compliant libraries.

   When the result of macro expansion is used in a domain name query, if
   the expanded domain name exceeds 253 characters (the maximum length
   of a domain name), the left side is truncated to fit, by removing
   successive domain labels until the total length does not exceed 253

   Uppercased macros expand exactly as their lower case equivalents, and
   are then URL escaped.  URL escaping must be performed for characters
   not in the "uric" set, which is defined in [RFC3986].

   Note: Care must be taken so that macro expansion for legitimate
   e-mail does not exceed the 63 character limit on DNS labels.  The
   localpart of e-mail addresses, in particular, can have more than 63
   characters between dots.

   Note: Domains should avoid using the "s", "l", "o", or "h" 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.

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   Implementations should be aware that if no directive processed during
   the evaluation of check_host() contains an "s", "l", "o" or "h"
   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.

8.2.  Expansion Examples

      The <sender> is strong-bad@email.example.com.
      The IPv4 SMTP client IP is
      The IPv6 SMTP client IP is 2001:DB8::CB01.
      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
   %{lr-}.lp._spf.%{d2}                  bad.strong.lp._spf.example.com




   %{ir}.%{v}._spf.%{d2}                               1.0.B.C.

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9.  Implications

   This section outlines the major implications that adoption of this
   document will have on various entities involved in Internet e-mail.
   It is intended to make clear to the reader where this document
   knowingly affects the operation of such entities.  This section is
   not a "how-to" manual, nor a "best practices" document, and is not a
   comprehensive list of what such entities should do in light of this

   This section is non-normative.

9.1.  Sending Domains

   Domains that wish to be compliant with this specification will need
   to determine the list of hosts that they allow to use their domain
   name in the "HELO" and "MAIL FROM" identities.  It is recognized that
   forming such a list is not just a simple technical exercise, but
   involves policy decisions with both technical and administrative

   It can be helpful to publish records that include a "tracking
   exists:" mechanism.  By looking at the name server logs, a rough list
   may then be generated.  For example:

      v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all

9.2.  Mailing Lists

   Mailing lists must be aware of how they re-inject mail that is sent
   to the list.  Mailing lists MUST comply with the requirements in
   [RFC2821] Section 3.10 and [RFC1123] Section 5.3.6 that say that the
   reverse-path MUST be changed to be the mailbox of a person or other
   entity who administers the list.  While the reasons for changing the
   reverse-path are many and long standing, SPF adds enforcement to this

   In practice, almost all mailing list software in use already complies
   with this requirement.  Mailing lists that do not comply may or may
   not encounter problems depending on how access to the list is
   restricted.  Such lists that are entirely internal to a domain (only
   people in the domain can send to or receive from the list) are not

9.3.  Forwarding Services and Aliases

   Forwarding services take mail that is received at a mailbox and
   direct it to some external mailbox.  At the time of this writing, the

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   near-universal practice of such services is to use the original "MAIL
   FROM" of a message when re-injecting it for delivery to the external
   mailbox.  [RFC1123] and [RFC2821] describe this action as an "alias"
   rather than a "mail list".  This means the external mailbox's MTA
   sees all such mail in a connection from a host of the forwarding
   service, and so the "MAIL FROM" identity will not, in general, pass

   There are three places that techniques can be used to ameliorate this

   1.  The beginning, when e-mail is first sent.

       1.  "Neutral" results could be given for IP addresses that may be
           forwarders, instead of "Fail" results.  For example:

              "v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all"

           This would cause a lookup on an anti-spam DNS blocklist
           (DNSBL) and cause a result of "Fail" only for e-mail coming
           from listed sources.  All other e-mail, including e-mail sent
           through forwarders, would receive a "Neutral" result.  By
           checking the DNSBL after the known good sources, problems
           with incorrect listing on the DNSBL are greatly reduced.

       2.  The "MAIL FROM" identity could have additional information in
           the localpart that cryptographically identifies the mail as
           coming from an authorized source.  In this case, such an SPF
           record could be used:

              "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"

           Then, a specialized DNS server can be set up to serve the
           _spf_verify subdomain which validates the localpart.  While
           this requires an extra DNS lookup, this only happens when the
           e-mail would otherwise be rejected as not coming from a known
           good source.

           Note that due to the 63 character limit for domain labels,
           this approach only works reliably if the localpart signature
           scheme is guaranteed either to only produce localparts with a
           maximum of 63 characters or to gracefully handle truncated

       3.  Similarly, a specialized DNS server could be set up that will
           rate-limit the e-mail coming from unexpected IP addresses.

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              "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"

       4.  SPF allows the creation of per-user policies for special
           cases.  For example, the following SPF record and appropriate
           wildcard DNS records can be used:

              "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"

   2.  The middle, when e-mail is forwarded.

       1.  Forwarding services can solve the problem by rewriting the
           "MAIL FROM" to be in their own domain.  This means that mail
           bounced from the external mailbox will have to be re-bounced
           by the forwarding service.  Various schemes to do this exist
           though they vary widely in complexity and resource
           requirements on the part of the forwarding service.

       2.  Several popular MTAs can be forced from "alias" semantics to
           "mailing list" semantics by configuring an additional alias
           with "owner-" prepended to the original alias name (e.g. an
           alias of "friends: george@example.com, fred@example.org"
           would need another alias of the form "owner-friends:

   3.  The end, when e-mail is received.

       1.  If the owner of the external mailbox wishes to trust the
           forwarding service, they can direct the external mailbox's
           MTA to skip SPF tests when the client host belongs to the
           forwarding service.

       2.  Tests against other identities, such as the "HELO" identity,
           may be used to override a failed test against the "MAIL FROM"

       3.  For larger domains, it may not be possible to have a complete
           or accurate list of forwarding services used by the owners of
           the domain's mailboxes.  In such cases, whitelists of
           generally-recognized forwarding services could be employed.

9.4.  Mail Services

   Service providers that offer mail services to third-party domains,
   such as sending of bulk mail, may have to adjust their setup in light
   of the authorization check described in this document.  If the "MAIL
   FROM" identity used for such e-mail uses the domain of the service
   provider, then the provider needs only to ensure that their sending
   host is authorized by their own SPF record, if any.

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   If the "MAIL FROM" identity does not use the mail service provider's
   domain, then extra care must be taken.  The SPF record format has
   several options for the third party domain to authorize the service
   provider's MTAs to send mail on its behalf.  For mail service
   providers, such as ISPs, that have a wide variety of customers using
   the same MTA, steps should be taken to prevent cross-customer forgery
   (see Section 10.4).

9.5.  MTA Relays

   The authorization check generally precludes the use of arbitrary MTA
   relays between sender and receiver of an e-mail message.

   Within an organization, MTA relays can be effectively deployed.
   However, for purposes of this document, such relays are effectively
   transparent.  The SPF authorization check is a check between border
   MTAs of different domains.

   For mail senders, this means that published SPF records must
   authorize any MTAs that actually send across the Internet.  Usually,
   these are just the border MTAs as internal MTAs simply forward mail
   to these MTAs for delivery.

   Mail receivers will generally want to perform the authorization check
   at the border MTAs, specifically including all secondary MXes.  This
   allows mail that fails to be rejected during the SMTP session rather
   than bounced.  Internal MTAs then do not perform the authorization
   test.  To perform the authorization test other than at the border,
   the host that first transferred the message to the organization must
   be determined, which can be difficult to extract from headers.
   Testing other than at the border is not recommended.

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10.  Security Considerations

10.1.  Processing Limits

   As with most aspects of e-mail, there are a number of ways that
   malicious parties could use the protocol as an avenue for a Denial-
   of-Service (DoS) attack.  The processing limits outlined here are
   designed to prevent attacks such as:

   o  A malicious party could create an SPF record with many references
      to a victim's domain and send many e-mails to different SPF
      clients; those SPF clients would then create a DoS attack.  In
      effect, the SPF clients are being used to amplify the attacker's
      bandwidth by using fewer bytes in the SMTP session than are used
      by the DNS queries.  Using SPF clients also allows the attacker to
      hide the true source of the attack.

   o  While implementations of check_host() are supposed to limit the
      number of DNS lookups, malicious domains could publish records
      that exceed these limits in an attempt to waste computation effort
      at their targets when they send them mail.  Malicious domains
      could also design SPF records that cause particular
      implementations to use excessive memory or CPU usage, or to
      trigger bugs.

   o  Malicious parties could send a large volume of 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.

   Of these, the case of a third party referenced in the SPF record is
   the easiest for a DoS attack to effectively exploit.  As a result,
   limits that may seem reasonable for an individual mail server can
   still allow an unreasonable amount of bandwidth amplification.
   Therefore the processing limits need to be quite low.

   SPF implementations MUST limit the number of mechanisms and modifiers
   that do DNS lookups to at most 10 per SPF check, including any
   lookups caused by the use of the "include" mechanism or the
   "redirect" modifier.  If this number is exceeded during a check, a
   PermError MUST be returned.  The "include", "a", "mx", "ptr", and
   "exists" mechanisms as well as the "redirect" modifier do count
   against this limit.  The "all", "ip4" and "ip6" mechanisms do not
   require DNS lookups and therefore do not count against this limit.
   The "exp" modifier does not count against this limit because the DNS
   lookup to fetch the explanation string occurs after the SPF record
   has been evaluated.

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   When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
   there MUST be a limit of no more than 10 MX or PTR RRs looked up and

   SPF implementations SHOULD limit the total amount of data obtained
   from the DNS queries.  For example, when DNS over TCP or EDNS0 are
   available, there may need to be an explicit limit to how much data
   will be accepted to prevent excessive bandwidth usage or memory
   usage, and DoS attacks.

   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 authorization SHOULD be "TempError".

   Domains publishing records SHOULD try to keep the number of "include"
   mechanisms 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 and placing lower-cost mechanisms
   earlier in the SPF record.

   For example, consider a domain set up as:

   example.com.      IN MX   10 mx.example.com.
   mx.example.com.   IN A
   a.example.com.    IN TXT  "v=spf1 mx:example.com -all"
   b.example.com.    IN TXT  "v=spf1 a:mx.example.com -all"
   c.example.com.    IN TXT  "v=spf1 ip4: -all"

   Evaluating check_host() for the domain "a.example.com" requires the
   MX records for "example.com", and then the A 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.

10.2.  SPF-Authorized E-Mail May Be UBE

   The "MAIL FROM" and "HELO" identity authorizations must not be
   construed to provide more assurance than they do.  It is entirely
   possible for a malicious sender to inject a message using their own
   domain in the identities used by SPF, to have that domain's SPF
   record authorize the sending host, and yet the message content can
   easily claim other identities in the headers.  Unless the user or the
   MUA takes care to note that the authorized identity does not match

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   the other more commonly-presented identities (such as the From:
   header), the user may be lulled into a false sense of security.

10.3.  Spoofed DNS and IP Data

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

   o  The evaluation of check_host() relies heavily on DNS.  A malicious
      attacker could attack the DNS infrastructure and cause
      check_host() to see spoofed DNS data, and then return incorrect
      results.  This could include returning "Pass" for an <ip> value
      where the actual domain's record would evaluate to "Fail".  See
      [RFC3833] for a description of the DNS weaknesses.

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

10.4.  Cross-User Forgery

   By definition, SPF policies just map domain names to sets of
   authorized MTAs, not whole e-mail addresses to sets of authorized
   users.  Although the "l" macro (Section 8) provides a limited way to
   define individual sets of authorized MTAs for specific e-mail
   addresses, it is generally impossible to verify, through SPF, the use
   of specific e-mail addresses by individual users of the same MTA.

   It is up to mail services and their MTAs to directly prevent cross-
   user forgery: based on SMTP AUTH ([RFC2554]), users should be
   restricted to using only those e-mail addresses that are actually
   under their control (see [I-D.gellens-submit-bis] section 6.1).
   Another means to verify the identity of individual users is message
   cryptography such as PGP ([RFC2440]) or S/MIME ([RFC3851]).

10.5.  Untrusted Information Sources

   SPF uses information supplied by third parties, such as the "HELO"
   domain name, the "MAIL FROM" address, and SPF records.  This
   information is then passed to the receiver in the Received-SPF: mail
   headers and possibly returned to the client MTA in the form of an
   SMTP rejection message.  This information must be checked for invalid
   characters and excessively long lines.

   When the authorization check fails, an explanation string may be
   included in the reject response.  Both the sender and the rejecting
   receiver need to be aware that the explanation was determined by the

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   publisher of the SPF record checked and, in general, not the
   receiver.  The explanation may contain malicious URLs, or it may be
   offensive or misleading.

   This is probably less of a concern than it may initially seem since
   such messages are returned to the sender, and the explanation strings
   come from the sender policy published by the domain in the identity
   claimed by that very sender.  As long as the DSN is not redirected to
   someone other than the actual sender, the only people who see
   malicious explanation strings are people whose messages claim to be
   from domains that publish such strings in their SPF records.  In
   practice DSNs can be misdirected, such as when an MTA accepts an
   e-mail and then later generates a DSN to a forged address, or when an
   e-mail forwarder does not direct the DSN back to the original sender.

10.6.  Privacy Exposure

   Checking SPF records causes DNS queries to be sent to the domain
   owner.  These DNS queries, especially if they are caused by the
   "exists" mechanism, can contain information about who is sending
   e-mail and likely to which MTA the e-mail is being sent to.  This can
   introduce some privacy concerns, which may be more or less of an
   issue depending on local laws and the relationship between the domain
   owner and the person sending the e-mail.

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11.  Contributors and Acknowledgements

   This document is largely based on the work of Meng Weng Wong and Mark
   Lentczner.  While, as this section acknowledges, many people have
   contributed to this document, a very large portion of the writing and
   editing are due to Meng and Mark.

   This design owes a debt of parentage to [RMX] by Hadmut Danisch and
   to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
   the legitimacy of an e-mail address traces its ancestry farther back
   through messages on the namedroppers mailing list by Paul Vixie
   [Vixie] (based on suggestion by Jim Miller) and by David Green

   Philip Gladstone contributed the concept of 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.
   They are far too numerous to name, but they include:

      The folks on the spf-discuss mailing list.
      The folks on the SPAM-L mailing list.
      The folks on the IRTF ASRG mailing list.
      The folks on the IETF MARID mailing list.
      The folks on #perl.

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12.  IANA Considerations

12.1.  The SPF DNS Record Type

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

12.2.  The Received-SPF mail header

   Per [RFC3864], the "Received-SPF:" header field is added to the IANA
   Permanent Message Header Field Registry.  The following is the
   registration template:

      Header field name: Received-SPF
      Applicable protocol: mail ([RFC2822])
      Status: standard
      (Note to RFC Editor: Replace the status with the final
      determination by the IESG)
      Author/Change controller: IETF
      Specification document(s): this Internet Draft
      (Note to RFC Editor: Replace this with RFC YYYY (RFC number of
      this spec))
      Related information:
      Requesting SPF Council review of any proposed changes and
      additions to this field is recommended.  For information about SPF
      Council see http://spf.mehnle.net/

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13.  References

13.1  Normative References

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

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

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

              Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", draft-crocker-abnf-rfc2234bis-00
              (work in progress), March 2005.

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

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

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

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

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

              American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange, X3.4", 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

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13.2  Informative References

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

   [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983,
              August 1996.

   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.

              Gellens, R. and J. Klensin, "Message Submission for Mail",
              draft-gellens-submit-bis-02 (work in progress),
              April 2005.

   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
              RFC 2554, March 1999.

   [RFC3696]  Klensin, J., "Application Techniques for Checking and
              Transformation of Names", RFC 3696, February 2004.

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, August 2004.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 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.

              Work In Progress

   [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.

   [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.

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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

   See [I-D.crocker-abnf-rfc2234bis] for ABNF notation.  Please note
   that as per this ABNF definition, literal text strings (those in
   quotes) are case-insensitive.  Hence, "mx" matches "mx", "MX", "mX"
   and "Mx".

   record           = version terms *SP
   version          = "v=spf1"

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

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

   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

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

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

   ip4-network      = qnum "." qnum "." qnum "." qnum
   qnum             = DIGIT                 ; 0-9
                      / %x31-39 DIGIT       ; 10-99
                      / "1" 2DIGIT          ; 100-199
                      / "2" %x30-34 DIGIT   ; 200-249
                      / "25" %x30-35        ; 250-255
             ; conventional dotted quad notation.  e.g.
   ip6-network      = <as per [RFC 3513], section 2.2>
             ; e.g. 2001:DB8::CD30

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   domain-spec      = macro-string domain-end
   domain-end       = ( "." toplabel ) / macro-expand
   toplabel         = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
                      ; LDH rule (See [RFC3696])
   alphanum         = ALPHA / DIGIT

   explain-string   = *( macro-string / SP )

   macro-string     = *( macro-expand / macro-literal )
   macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                      / "%%" / "%_" / "%-"
   macro-literal    = %x21-24 / %x26-7E
                      ; visible characters except "%"
   macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                      "c" / "r" / "t"
   transformers     = *DIGIT [ "r" ]
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

   name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )

   header           = "Received-SPF:" [CFWS] result FWS [comment FWS]
                      [ key-value-list ] CRLF

   result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
                      "None" / "TempError" / "PermError"

   key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )

   key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )

   key              = "client-ip" / "envelope-from" / "helo" /
                      "problem" / "receiver" / "identity" /
                       mechanism / "x-" name / name

   identity         = "mailfrom"   ; for the "MAIL FROM" identity
                      / "helo"     ; for the "HELO" identity
                      / name       ; other identities

   dot-atom         = <unquoted word as per [RFC2822]>
   quoted-string    = <quoted string as per [RFC2822]>
   comment          = <comment string as per [RFC2822]>
   CFWS             = <comment or folding white space as per [RFC2822]>
   FWS              = <folding white space as per [RFC2822]>
   CRLF             = <standard end-of-line token as per [RFC2822]>

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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
   amy         A
   bob         A
   mail-a      A
   mail-b      A
   www         CNAME example.com.

   ; A related domain
   $ORIGIN example.org.
   @           MX  10 mail-c
   mail-c      A

   ; 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".

   v=spf1 +all
      -- any <ip> passes

   v=spf1 a -all
      -- hosts and pass

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   v=spf1 a:example.org -all
      -- no sending hosts pass since example.org has no A records

   v=spf1 mx -all
      -- sending hosts and pass

   v=spf1 mx:example.org -all
      -- sending host passes

   v=spf1 mx mx:example.org -all
      -- sending hosts,, and pass

   v=spf1 mx/30 mx:example.org/30 -all
      -- any sending host in or passes

   v=spf1 ptr -all
      -- sending host passes (reverse DNS is valid and is in
      -- sending host fails (reverse DNS is valid, but not
         in example.com)
      -- sending host fails (reverse IP is not valid)

   v=spf1 ip4: -all
      -- sending host fails
      -- sending host passes

B.2.  Multiple Domain Example

   These examples show the effect of related records:

      example.org: "v=spf1 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's and example.net's
   designated servers.

      la.example.org: "v=spf1 redirect=example.org"
      ny.example.org: "v=spf1 redirect=example.org"
      sf.example.org: "v=spf1 redirect=example.org"

   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 be updated when the mail setup
   changes.  These domains' records never have to change.

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B.3.  DNSBL Style Example

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

   $ORIGIN _spf.example.com.
   mary.mobile-users                   A
   fred.mobile-users                   A     A     A

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


   v=spf1 mx


   v=spf1 exists:%{l1r+}.%{d}


   v=spf1 exists:%{ir}.%{l1r+}.%{d}

B.4.  Multiple Requirements Example

   Say that your sender policy requires that both the IP address is
   within a certain range and that the reverse DNS for the IP matches.
   This can be done several ways, including:

   example.com.           SPF  ( "v=spf1 "
                                 "-include:ip4._spf.%{d} "
                                 "-include:ptr._spf.%{d} "
                                 "+all" )
   ip4._spf.example.com.  SPF  "v=spf1 -ip4: +all"
   ptr._spf.example.com.  SPF  "v=spf1 -ptr +all"

   This example shows how the "-include" mechanism can be useful, how an
   SPF record that ends in "+all" can be very restrictive and the use of
   De Morgan's Law.

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Appendix C.  Change Log

   RFC Editor Note: This section is to be removed during the final
   publication of the document.

C.1.  Changes in Version -02

   o  The abstract notes that SPF-classic covers both the HELO and MAIL
      FROM identities. (ietf-822 review)

   o  In section 2.3 "Publishing Authorization", it now makes it clear
      that publishing is optional. (ietf-smtp review)

   o  The definition of the "SoftFail" result have been recast from
      Receiver Policy to Sender Policy.

   o  The definitions of Neutral, Pass and PermError have been updated/
      clarified to more correctly reflect the semantics of

   o  A note to the RFC editor was made indicating that the SPF DNS RR
      type number should be added to the draft once the IANA has made an

   o  The ip4-network ABNF has been fixed to give the ABNF of the
      dotted-quad format, rather than just using words to explain it.

   o  The ABNF for the Received-SPF header now shows that it ends with a
      CRLF. (ietf-822 review)

   o  The new, optional, "scope" keyword-value pair has been renamed to

   o  The "exp=" modifier no longer counts toward the DoS DNS lookup

   o  In section 10.5 "Untrusted Information Sources", the explanation
      about explanation strings going to only the sender has been fixed
      to note that, in some cases, it can go to other people. (ietf-822

   o  Sections 3.1.2 and 3.1.3 were updated to make the distinction
      between "multiple TXT RRs" and "multiple strings within a TXT"
      clearer. (ietf-822 review)

   o  A normative reference to US-ASCII has been added.

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   o  Text describing how to lookup and process the SPF records has been
      removed from section 3.1.1 "DNS Resource Record Types" and merged
      into similar text in sections 4.4 "Record Lookup" and 4.5
      "Selecting Records"

   o  Section 4.5 "Selecting Records" has been updated to give an
      algorithm that says to return a PermError when it discovers that
      SPF and TXT records don't match.

   o  In section 6.1 "redirect: Redirected Query", the semantics have
      been changed to specify a result of PermError instead of None in
      cases where the target domain does not have any SPF records.  It
      makes no sense to return None, that is "no SPF records found",
      when SPF records were found.

   o  In section 6.2 "exp: Explanation", it is explained that the record
      must be in US-ASCII due to requirements of RFC2821.

   o  In section 6.2 "exp: Explanation", the duplicate warning about
      source being from a third party was deleted.

   o  A note has been added to section warning about domain
      labels being over 63 characters.

   o  The "prefix" ABNF rule was renamed to "qualifier" to reflect the
      semantics of the rule, rather than the syntax.

C.2.  Changes in Version -01

   o  IETF boilerplate was updated to BCP 79.

   o  A version number was added to the title.  (IESG review)

   o  Many grammatical, typographical and spelling errors were
      corrected, along with rephrasing sentences to make the intent and
      meaning clearer.

   o  Sections have been re-ordered in so that they conform to the
      instructions2authors.txt document.  All required sections and
      arrangements are included, and only the "Security Considerations"
      section is not in the suggested order.  Since the Security
      Considerations is such an important part of the spec, it has been
      moved before the Acknowledgement section.

   o  The HELO identity checking has been changed from "MAY" to

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   o  The e-mail receiver policy definition on how to handle HELO
      checking was removed.  It was copied incorrectly from
      draft-mengwong-spf-01, changing its meaning.

   o  A note was added that when changing SPF records, there needs to be
      a transitional period to prevent incorrect results.

   o  The RECOMMENDATION not to use other identities with version 1 SPF
      records has been clarified.  Example cases where checking other
      identities will cause incorrect results have been cited.  (IESG

   o  The "zone cut" method of determining if there is an SPF record at
      the top of the zone has been removed.  It wasn't implemented very
      often and could not always be easily done.  (IESG/namedroppers'

   o  A note was added that receivers should consider rejecting e-mail
      for non-existent domains in order to prevent circumvention of SPF
      policies.  This is due to the remove of "zone cuts".
      (namedroppers' review)

   o  The RECOMMENDATION to perform SPF checks during the SMTP session
      has been clarified and strengthened.

   o  Note added about the consequences of treating "Neutral" results
      worse than "None".

   o  The suggested e-mail receiver policy when a "PermError" is
      encountered has been changed to be, effectively, the same
      semantics as were in draft-mengwong-spf-01.  (MAAWG review)

   o  ABNF cleaned up to pass Bill Fenner's checker and not just the one
      at http://www.apps.ietf.org/abnf.html

   o  A few host names/IP addresses were fixed to use appropriate ones
      for I-Ds.

   o  A definition of what to should be done if there are syntax errors
      in the explanation string was added.  (E.g. use the default.)

   o  Section 10 "Security Considerations" has been broken up into
      subsections and reorganized.

   o  Section 7.1 "Process Limits" has been merged into the similar
      language in the "Security Considerations" section.

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   o  The ABNF for the Received-SPF e-mail header has been made to be
      more compatible with draft-mengwong-spf-01.  It was fixed to
      require whitespace when needed and to show where the suggested
      comment should be added to the header.

   o  The IANA Considerations section now has the required information
      to document the Received-SPF header.

   o  A new, optional, "scope" keyword has added to the Received-SPF

   o  The non-normative Section 9.3 "Forwarding Services and Aliases"
      has been expanded to more thoroughly cover the subject.

   o  New Security Considerations sections on "Privacy Exposure" and
      "Cross-User Forgery" have been added.

   o  A new example of an SPF policy with a non-obvious implementation
      has been added.

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Authors' Addresses

   Meng Weng Wong

   Email: mengwong+spf@pobox.com
   URI:   http://spf.pobox.com/

   Wayne Schlitt
   4615 Meredeth #9
   Lincoln Nebraska, NE  68506
   United States of America

   Email: wayne@schlitt.net
   URI:   http://www.schlitt.net/spf/

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Intellectual Property Statement

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Copyright Statement

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   Funding for the RFC Editor function is currently provided by the
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