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Versions: (draft-kitterman-4408bis) 00 01 02 03 04 05 06 07 08 09 14 15 16 17 18 19 20 21 RFC 7208

Network Working Group                                       S. Kitterman
Internet-Draft                                                     Agari
Obsoletes: 4408 (if approved)                               July 5, 2012
Intended status: Standards Track
Expires: January 6, 2013

 Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,
                               Version 1


   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 Sender Policy Framework (SPF) protocol, whereby a domain can
   explicitly authorize the hosts that are allowed to use its domain
   name, and a receiving host can check such authorization.  This
   document obsoletes RFC4408.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 6, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Protocol Status  . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Experimental History . . . . . . . . . . . . . . . . . . .  6
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.2.  Imported Definitions . . . . . . . . . . . . . . . . .  6
       1.3.3.  Mail From Definition . . . . . . . . . . . . . . . . .  6
       1.3.4.  Deprecated . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  The HELO Identity  . . . . . . . . . . . . . . . . . . . .  8
     2.2.  The MAIL FROM Identity . . . . . . . . . . . . . . . . . .  8
     2.3.  Publishing Authorization . . . . . . . . . . . . . . . . .  8
     2.4.  Checking Authorization . . . . . . . . . . . . . . . . . .  9
     2.5.  Interpreting the Result  . . . . . . . . . . . . . . . . . 10
       2.5.1.  None . . . . . . . . . . . . . . . . . . . . . . . . . 10
       2.5.2.  Neutral  . . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.3.  Pass . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.4.  Fail . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.5.  Softfail . . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.6.  TempError  . . . . . . . . . . . . . . . . . . . . . . 12
       2.5.7.  PermError  . . . . . . . . . . . . . . . . . . . . . . 12
   3.  SPF Records  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.1.  Publishing . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.1.1.  DNS Resource Records . . . . . . . . . . . . . . . . . 13
       3.1.2.  Multiple DNS Records . . . . . . . . . . . . . . . . . 14
       3.1.3.  Multiple Strings in a Single DNS record  . . . . . . . 14
       3.1.4.  Record Size  . . . . . . . . . . . . . . . . . . . . . 14
       3.1.5.  Wildcard Records . . . . . . . . . . . . . . . . . . . 14
   4.  The check_host() Function  . . . . . . . . . . . . . . . . . . 16
     4.1.  Arguments  . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.2.  Results  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  Initial Processing . . . . . . . . . . . . . . . . . . . . 16
     4.4.  Record Lookup  . . . . . . . . . . . . . . . . . . . . . . 17
     4.5.  Selecting Records  . . . . . . . . . . . . . . . . . . . . 17
     4.6.  Record Evaluation  . . . . . . . . . . . . . . . . . . . . 17
       4.6.1.  Term Evaluation  . . . . . . . . . . . . . . . . . . . 18
       4.6.2.  Mechanisms . . . . . . . . . . . . . . . . . . . . . . 18
       4.6.3.  Modifiers  . . . . . . . . . . . . . . . . . . . . . . 19
       4.6.4.  Evaluation Limits  . . . . . . . . . . . . . . . . . . 19
     4.7.  Default Result . . . . . . . . . . . . . . . . . . . . . . 19
     4.8.  Domain Specification . . . . . . . . . . . . . . . . . . . 20
   5.  Mechanism Definitions  . . . . . . . . . . . . . . . . . . . . 21
     5.1.  "all"  . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     5.2.  "include"  . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.3.  "a"  . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     5.4.  "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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     5.5.  "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 24
     5.6.  "ip4" and "ip6"  . . . . . . . . . . . . . . . . . . . . . 25
     5.7.  "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 26
   6.  Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 27
     6.1.  redirect: Redirected Query . . . . . . . . . . . . . . . . 27
     6.2.  exp: Explanation . . . . . . . . . . . . . . . . . . . . . 28
   7.  The Received-SPF Header Field  . . . . . . . . . . . . . . . . 30
   8.  Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     8.1.  Macro Definitions  . . . . . . . . . . . . . . . . . . . . 32
     8.2.  Expansion Examples . . . . . . . . . . . . . . . . . . . . 35
   9.  Implications . . . . . . . . . . . . . . . . . . . . . . . . . 36
     9.1.  Sending Domains  . . . . . . . . . . . . . . . . . . . . . 36
       9.1.1.  DNS Resource Considerations  . . . . . . . . . . . . . 36
       9.1.2.  Administrator's Considerations . . . . . . . . . . . . 37
     9.2.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . 37
       9.2.1.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . 37
       9.2.2.  Forwarding Services and Aliases  . . . . . . . . . . . 37
     9.3.  Mail Services  . . . . . . . . . . . . . . . . . . . . . . 39
     9.4.  MTA Relays . . . . . . . . . . . . . . . . . . . . . . . . 40
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 41
     10.1. Processing Limits  . . . . . . . . . . . . . . . . . . . . 41
     10.2. SPF-Authorized Email May Contain Other False Identities  . 41
     10.3. Spoofed DNS and IP Data  . . . . . . . . . . . . . . . . . 42
     10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 42
     10.5. Untrusted Information Sources  . . . . . . . . . . . . . . 42
     10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 43
   11. Contributors and Acknowledgements  . . . . . . . . . . . . . . 44
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
     12.1. The SPF DNS Record Type  . . . . . . . . . . . . . . . . . 45
     12.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 45
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     13.2. Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 49
   Appendix B.  Extended Examples . . . . . . . . . . . . . . . . . . 52
     B.1.  Simple Examples  . . . . . . . . . . . . . . . . . . . . . 52
     B.2.  Multiple Domain Example  . . . . . . . . . . . . . . . . . 53
     B.3.  DNSBL Style Example  . . . . . . . . . . . . . . . . . . . 54
     B.4.  Multiple Requirements Example  . . . . . . . . . . . . . . 54
   Appendix C.  Change History  . . . . . . . . . . . . . . . . . . . 55
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 57

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1.  Introduction

   The current email 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.
   Although this feature is desirable in some circumstances, it is a
   major obstacle to reducing Unsolicited Bulk email (UBE, aka spam).
   Furthermore, many domain name holders are understandably concerned
   about the ease with which other entities can make use of their domain
   names, often with malicious intent.

   This document defines a protocol by which domain owners can authorize
   hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
   Compliant domain holders publish Sender Policy Framework (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 Mail Transfer Agents (MTAs) using a given
   "HELO" or "MAIL FROM" identity during a mail transaction.

   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 email, such as rejecting it.

1.1.  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 protocol was
   originally documented in [RFC4408], which this memo replaces.

   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 will differ from, and build upon, this work.  It is hoped
   that we have nonetheless captured the common understanding of SPF
   version 1.

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1.2.  Experimental History

   This document updates and replaces RFC 4408 that was part of a group
   of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,
   RFC 4407, and RFC 4408) in 2006.  At that time the IESG requested the
   community observe the success or failure of the two approaches
   documented in these RFCs during the two years following publication,
   in order that a community consensus can be reached in the future.

   SPF is widely deployed by large and small email providers alike.
   There are multiple, interoperable implementations.

   For SPF (as documented in RFC 4408) a careful effort was made to
   collect and document lessons learned and errata during the two year
   period.  The errata list has been stable (no new submissions) and
   only minor protocol lessons learned were identified.  Resolution of
   the IESG's experiment is documented in

1.3.  Terminology

1.3.1.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

1.3.2.  Imported Definitions

   The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].

   The token "local-part" is defined in [RFC5321].

   "dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are
   defined in [RFC5322]

1.3.3.  Mail From Definition

   This document is concerned with the portion of a mail message
   commonly called "envelope sender", "return path", "reverse path",
   "bounce address", "5321 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
   might superficially look like the common terms, such as "reverse-
   path", are used only with the defined meanings from normative

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1.3.4.  Deprecated

   There are several [RFC4408] features that are marked "deprecated".
   In the context of this document, deprecated means that senders SHOULD
   NOT publish SPF records that make use of the feature in question.
   Its use is NOT RECOMMENDED and it might be removed entirely in future
   updates to the protocol.  Such features do, however, remain part of
   the SPF protocol and receiving systems MUST support them unless this
   specification says otherwise.

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2.  Operation

2.1.  The HELO Identity

   The "HELO" identity derives from either the SMTP HELO or EHLO command
   (see [RFC5321]).  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 emails are delivered with invalid HELO

   It is RECOMMENDED that SPF clients not only check 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
   [RFC5321]).  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.

   [RFC5321] allows the reverse-path to be null (see Section 4.5.5 in
   RFC 5321).  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 might or might
   not have been checked separately before).

   SPF clients MUST check the "MAIL FROM" identity if a completed "HELO"
   check has not reached a definitive policy result 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 can 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 email 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 9.2.1), but some do not change any other identities in
   the message.  The scenario described in Section 9.2.2, sub-section
   1.2, 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
   might influence whether or not a particular SPF check is performed.
   For example, finding the sending host's IP address on a local white
   list might 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.  Although 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 might 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" result.

   Although 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 email from such domains, especially
   in the case of invalid "MAIL FROM".  Rejecting email will prevent one
   method of circumventing of SPF records.

   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 [RFC5321], 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 MTA by way of SMTP replies.

   Performing the authorization after the SMTP transaction has finished
   can cause problems, such as the following: (1) It might be difficult
   to accurately extract the required information from potentially
   deceptive headers; (2) legitimate email might fail because the
   sender's policy had 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 or not the
   client host is authorized.

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2.5.2.  Neutral

   The domain owner has explicitly stated that he cannot or does not
   want to assert whether or not the IP address is authorized.  A
   "neutral" result MUST be treated exactly like the "none" result; the
   distinction exists only for informational purposes.  Treating
   "neutral" more harshly than "none" would 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
   [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see
   [RFC3463]), in addition to an appropriate reply text.  The
   check_host() function will return either a default explanation string
   or one from the domain that published the SPF records (see
   Section 6.2).  If the information does not 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 is not authorized but
   is not 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 thus
   desires limited feedback when a "softfail" result occurs.  For

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   example, the recipient's Mail User Agent (MUA) could highlight the
   "softfail" status, or the receiving MTA could give the sender a
   message using greylisting, [RFC6647], with a note the first time the
   message is received, but accept it on a later attempt based on
   receiver policy.

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
   enhanced status code.

2.5.7.  PermError

   A "permerror" result means that the domain's published records could
   not be correctly interpreted.  This signals an error condition that
   requires manual intervention to be resolved, as opposed to the
   temperror result.  If the message is rejected during the SMTP
   transaction for this reason, the software SHOULD use an SMTP reply
   code of 550 and, if supported, the 5.5.2 enhanced status code.  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 the

      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

   The example above in Section 3 might be published via these 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 might cause problems with
   size limits (see Section 3.1.4).

   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.  Section 9.1.1 provides some suggestions
   on how to achieve this.

3.1.1.  DNS Resource Records

   SPF records MUST be published as type TXT [RFC1035].  The character
   content of the record is encoded as [US-ASCII].  Use of alternate DNS
   RR types has been dropped.  See Appendix A of

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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 or 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..."

   TXT records containing multiple strings are useful in constructing
   records that would exceed the 255-byte maximum length of a string
   within a single TXT 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 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 long to
   fit in a single UDP packet MAY be silently ignored by SPF clients.

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 might 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 the following:

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       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 check_host() function 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 might 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.) or 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 type TXT only.

   If all DNS lookups that are made return a server failure (RCODE 2),
   or other error (RCODE other than 0 or 3), or time out, then
   check_host() exits immediately with the result "temperror".
   Alternatively, for a server failure (RCODE 2) result, check_host()
   MAY track failures and treat multiple failures within 24 hours for
   the same domain as "permerror".

   This alternative is intended to shorten the queue time of messages
   that cannot be accepted, by returning a permanent negative completion
   reply code to the client, instead of a transient one.  Saving queries
   is accomplished according to [RFC2308].

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

   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 "permerror".

   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".

   Implementations MAY choose to parse the entire record first and
   return "permerror" if the record is not syntactically well formed.

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   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
   Augmented Backus-Naur Form (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
                      ; where name is not any known modifier

   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 might 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 [RFC5234], 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, not match, or 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 as follows:

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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.  Although modifiers do
   not directly affect the evaluation of the record, the "redirect"
   modifier has an effect after all the mechanisms have been evaluated.

4.6.4.  Evaluation Limits

   SPF implementations MUST limit the number of mechanisms and modifiers
   ("terms") that do DNS lookups to at most 10 per during SPF record
   evaluation.  Specifically, the "include", "a", "mx", "ptr", and
   "exists" mechanisms as well as the "redirect" modifier count against
   this limit.  The "all", "ip4", and "ip6" mechanisms do not count
   against this limit.  If this number is exceeded during a check, a
   permerror MUST be returned.  The "exp" modifier does not count
   against this limit because the DNS lookup to fetch the explanation
   string occurs after the SPF record evaluatation has been completed.

   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
   checked.  If more than 10 "mx" or "ptr" records are returned for this
   further lookup, a permerror MUST be returned.  This limit is per
   mechanism or macro in the record and in addition to the lookup limits

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 use either a "redirect" modifier or
   an "all" mechanism to explicitly terminate processing.

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   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
   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.

      ptr (deprecated)

   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.  (Here, CIDR is Classless Inter-
   Domain Routing.)

   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 [RFC4291], Section
   2.5.5) MUST still be considered an IPv4 address and MUST be evaluated
   using IPv4 mechanisms (i.e. "ip4" and "a").

   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"

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

   For example:

      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
   exception depends on the result of the recursive evaluation of

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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.  Although 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 was
   "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 Denial of Service
   (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 MXs: 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 or AAAA lookup on the <target-name> directly.
   This behavior breaks with the legacy "implicit MX" rule.  See
   [RFC5321], Section 5.  If such behavior is desired, the publisher
   should specify an "a" directive.

5.5.  "ptr" (deprecated)

   This mechanism tests whether the DNS reverse-mapping for <ip> exists
   and correctly points to a domain name within a particular domain.
   This mechanism is deprecated and SHOULD NOT be used.

   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: This mechanism has been deprecated because it is slow, it 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.  After many
   yaers of SPF deployment experience it has been concluded it is
   unnecessary and more reliable alternatives used instead.

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 4291], 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

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   permitted to omit parts of the IP address instead of using CIDR
   notations.  That is, use instead of 192.0.2.

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 in the current evaluation of

   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 can 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.

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   Note: In general, the domain "A" cannot reliably use a redirect to
   another domain "B" not under the same administrative control.  Since
   the <sender> stays the same, there is no guarantee that the record at
   domain "B" will correctly work for mailboxes in domain "A",
   especially if domain "B" uses mechanisms involving localparts.  An
   "include" directive is generally 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 MUST 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 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 [RFC5321]
   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:

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

   Here are some examples of possible explanation TXT records at

      "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 information, 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 header.  If an SMTP receiver chooses to do
   so, it SHOULD use the "Received-SPF" header field 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 field is a trace field (see [RFC5322] Section
   3.6.7) and SHOULD be prepended to the existing header, above the
   Received: field that is generated by the SMTP receiver.  It MUST
   appear above all other Received-SPF fields in the message.  The
   header field has the following format:

   header-field     = "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 / name

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

   dot-atom         = <unquoted word as per [RFC5322]>
   quoted-string    = <quoted string as per [RFC5322]>
   comment          = <comment string as per [RFC5322]>
   CFWS             = <comment or folding white space as per [RFC5322]>
   FWS              = <folding white space as per [RFC5322]>
   CRLF             = <standard end-of-line token as per [RFC2532]>

   The header field 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 "client-ip", "helo", and, if the

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   "MAIL FROM" identity was checked, "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.

   SPF clients MUST make sure that the Received-SPF header field 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 are the

   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 the term.

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

   toplabel         = ( *alphanum ALPHA *alphanum ) /
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
                      ; LDH rule plus additional TLD restrictions
                      ; (see [RFC3696], Section 2 for background)
   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" / "v"
   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> (deprecated)
      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
      h = HELO/EHLO domain

   The following macro letters are allowed only 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 the following:

      *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 might contain empty strings.  Older
   implementations of SPF prohibit trailing dots in domain names, so
   trailing dots should not be published by domain owners, although they
   must be accepted by implementations conforming to this document.
   Macros MAY specify delimiter characters that 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 email
   address with a localpart, an "@" character, and a domain.  The "l"
   macro expands to just the localpart.  The "o" macro expands to just
   the domain part.  Note that these values remain the same during
   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 [RFC4291], 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.  This macro is deprecated
   and SHOULD NOT be 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
   can be different from 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 1,
   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 lowercased equivalents, and
   are then URL escaped.  URL escaping must be performed for characters
   not in the "unreserved" set, which is defined in [RFC3986].

   Note: Care must be taken so that macro expansion for legitimate email
   does not exceed the 63-character limit on DNS labels.  The localpart
   of email 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.  Although 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 Time To Live
   (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 email.
   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, or a "best practices" document, and it is not
   a comprehensive list of what such entities should do in light of this

   This section is non-normative.  [RFC5598] describes the Internet
   email architecture.  This section is organized based on the different
   segments of the architecture.

9.1.  Sending Domains

   Originating ADMDs (ADministrative Management Domains - [RFC5598]
   Section 2.2.1 and Section 2.3) that wish to be compliant with this
   specification will need to determine the list of relays ([RFC5598]
   Section 2.2.2) that they allow to use their domain name in the "HELO"
   and "MAIL FROM" identities when relaying to other ADMDs.  It is
   recognized that forming such a list is not just a simple technical
   exercise, but involves policy decisions with both technical and
   administrative considerations.

9.1.1.  DNS Resource Considerations

   Minimizing the DNS resources required for SPF lookups 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 follows:

   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" requires only the A records.
   Evaluating for "c.example.com" requires none.

   Section 4.6.4 specifies the limits receivers have to use.  It is
   essential to publish records that do not exceed these requirements.
   As an example, if you have more than 10 MX records, do not use the
   'MX" mechanism to describe them in your SPF record.

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9.1.2.  Administrator's Considerations

   There might be administrative considerations: using "a" over "ip4" or
   "ip6" allows hosts to be renumbered easily.  Using "mx" over "a"
   allows the set of mail hosts to be changed easily.  Unless such
   changes are common, it is better to use the less resource intensive
   mechanisms like "ip4" and "ip6".

   Validating correct deployment is difficult.  [RFC6652] describes one
   mechanism for soliciting feedback on SPF failures.  Another approach
   that can be helpful to publish records that include a "tracking
   exists:" mechanism.  By looking at the name server logs, a rough list
   can then be generated.  For example:

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

   Regardless of the method used, understanding the ADMD's outbound mail
   architecture is essential to effective deployment.

9.2.  Mediators

   Broadly speaking, there are two types of mediating ADMDs that can
   affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]
   Section 5.3) and ReSenders ([RFC5598] Section 5.2).

9.2.1.  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
   [RFC5321], 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.  Whereas the reasons for
   changing the reverse-path are many and long-standing, SPF adds
   enforcement to this requirement.

   In practice, almost all mailing list software in use already complies
   with this requirement.  Mailing lists that do not comply might
   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 affected.

9.2.2.  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
   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 [RFC5321] describe this action as an "alias"

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   rather than a "mail list".  This means that 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 email is first sent (Originating ADMDs).

       1.  "Neutral" results could be given for IP addresses that might
           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 blacklist
           (DNSBL) and cause a result of "fail" only for email coming
           from listed sources.  All other email, including email 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 that validates the localpart.  Although
           this requires an extra DNS lookup, this happens only when the
           email 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 email coming from unexpected IP addresses.

              "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:

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

   2.  The middle, when email is forwarded (Mediating ADMDs).

       1.  Forwarding services can solve the problem by rewriting the
           "MAIL FROM" to be in their own domain.  This means that mail
           rejected from the external mailbox will have to be forwarded
           back to the original sender 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 email is received (Receiving ADMDs).

       1.  If the owner of the external mailbox wishes to trust the
           forwarding service, he 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 might 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

9.3.  Mail Services

   MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail
   services to third-party domains, such as sending of bulk mail, might
   want to adjust their setup in light of the authorization check
   described in this document.  If the "MAIL FROM" identity used for
   such email uses the domain of the service provider, then the provider
   needs only to ensure that its sending host is authorized by its own
   SPF record, if any.

   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

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   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).

9.4.  MTA Relays

   Relays are described in [RFC5598] Section 2.2.2.  The authorization
   check generally precludes the use of arbitrary MTA relays between
   sender and receiver of an email 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 ADMDs.

   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.

   The receiving ADMD will generally want to perform the authorization
   check at the boundary MTAs, specifically including all secondary MXs.
   This allows mail that fails to be rejected during the SMTP session
   rather than sending a failure report.  Internal MTAs then do not
   perform the authorization test.  To perform the authorization test
   other than at the boundary, the host that first transferred the
   message to the organization must be determined, which can be
   difficult to extract from the message header.  Testing other than at
   the boundary is likely to produce unreliable results.

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10.  Security Considerations

10.1.  Processing Limits

   As with most aspects of email, 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 in
   Section 4.6.4 are designed to prevent attacks such as the following:

   o  A malicious party could create an SPF record with many references
      to a victim's domain and send many emails 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  Whereas 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 might 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.

   MTAs or other processors SHOULD 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".

10.2.  SPF-Authorized Email May Contain Other False Identities

   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 his own
   domain in the identities used by SPF, to have that domain's SPF
   record authorize the sending host, and yet the message can easily

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   list other identities in its header.  Unless the user or the MUA
   takes care to note that the authorized identity does not match the
   other more commonly-presented identities (such as the From: header
   field), the user might 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 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 email 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 email
   addresses, it is generally impossible to verify, through SPF, the use
   of specific email 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 ([RFC4954]), users should be
   restricted to using only those email addresses that are actually
   under their control (see [RFC6409], Section 6.1).  Another means to
   verify the identity of individual users is message cryptography such
   as PGP ([RFC4880]) or S/MIME ([RFC5751]).

10.5.  Untrusted Information Sources

   An SPF compliant receiver gathers information from the SMTP commands
   it receives and from the published DNS records of the sending domain
   holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the
   envelope, and SPF DNS records published by the domain holder).

   This information, passed to the receiver in the Received-SPF: trace
   fields, may be returned to the client MTA as an SMTP rejection
   message.  If such an SMTP rejection message is generated, the

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   information from the trace fields must be checked for such problems
   as invalid characters and excessively long lines.

   When the authorization check fails, an explanation string could be
   included in the reject response.  Both the sender and the rejecting
   receiver need to be aware that the explanation was determined by the
   publisher of the SPF record checked and, in general, not the
   receiver.  The explanation can contain malicious URLs, or it might be
   offensive or misleading.

   Explanations returned to sender domains due to exp modifiers,
   (Section 6.2), were generated by the sender policy published by the
   domain holders themselves.  As long as messages are only returned
   with non-delivery notification to domains publishing the explanation
   strings from their own DNS SPF records, the only affected parties are
   the original publishers of the domain's SPF records.

   In practice, such non-delivery notifications can be misdirected, such
   as when an MTA accepts an email and only later generates the
   notification to a forged address, or when an email forwarder does not
   direct the bounce 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
   email and likely to which MTA the email is being sent.  This can
   introduce some privacy concerns, which are more or less of an issue
   depending on local laws and the relationship between the domain owner
   and the person sending the email.

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11.  Contributors and Acknowledgements

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

   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 email address traces its ancestry further 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 of bothe this document and [RFC4408] 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 following:

      The participants in the SPFbis working group.
      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

   Per [RFC4408], the IANA assigned the Resource Record Type and Qtype
   from the DNS Parameters Registry for the SPF RR type with code 99.
   The format of this type is identical to the TXT RR [RFC1035].  The
   character content of the record is encoded as [US-ASCII].  Use of
   this record type is obsolete for SPF Version 1.

   IANA is requested to add (to be changed to "has added" upon
   publication) an annotation to the SPF RRTYPE saying "(OBSOLETE - use
   TXT)" in the DNS Parameters registry.

12.2.  The Received-SPF Mail Header Field

   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 ([RFC5322])
      Status: Standards Track
      Author/Change controller: IETF
      Specification document(s): [RFC4408], RFC XXXX

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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.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, January 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.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

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

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

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

              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

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

              Work In Progress

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

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

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

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [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.

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

   [RFC6647]  Kucherawy, M. and D. Crocker, "Email Greylisting: An
              Applicability Statement for SMTP", RFC 6647, June 2012.

   [RFC6652]  Kitterman, S., "Sender Policy Framework (SPF)
              Authentication Failure Reporting Using the Abuse Reporting

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              Format", RFC 6652, June 2012.

   [RMX]      Danisch, H., "The RMX DNS RR Type for light weight sender

              Work In Progress

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

              Kucherawy, M., "Resolution of The SPF and Sender ID
              Experiments", draft-ietf-spfbis-experiment (work in
              progress), 2012.

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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 [RFC5234] 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
                      ; where name is not any known modifier

   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 4291], section 2.2>
            ; e.g., 2001:DB8::CD30

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

   toplabel         = ( *alphanum ALPHA *alphanum ) /
                      ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
                      ; LDH rule plus additional TLD restrictions
                      ; (see [RFC3696], Section 2 for background)
   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" / "v"
   transformers     = *DIGIT [ "r" ]
   delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

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

   header-field     = "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 / name

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

   ALPHA            = <A-Z / a-z as per [RFC5234]>
   DIGIT            = <0-9 as per [RFC5234]>
   SP               = <space character as per [RFC5234]>
   dot-atom         = <unquoted word as per [RFC5322]>
   quoted-string    = <quoted string as per [RFC5322]>
   comment          = <comment string as per [RFC5322]>

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   CFWS             = <comment or folding white space as per [RFC5322]>
   FWS              = <folding white space as per [RFC5322]>
   CRLF             = <standard end-of-line token as per [RFC5322]>

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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

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      --  any <ip> passes

   v=spf1 a -all
      --  hosts and pass

   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

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

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

   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 History

   Changes since RFC 4408 (to be removed prior to publication)

      Moved to standards track

      Authors updated

      IESG Note regarding experimental use replaced with discussion of

      Process errata:

      Add %v macro to ABNF grammar

      Replace "uric" by "unreserved"

      Recommend an SMTP reply code for optional permerror rejections

      Correct syntax in Received-SPF examples

      Fix unknown-modifier clause is too greedy in ABNF

      Correct use of empty domain-spec on exp modifier

      Fix minor typo errata

      Convert to spfbis working group draft,

      Addressed Ticket #1, RFC 4408 Section 2.5.6 - Temporary errors by
      giving the option to turn repeated SERVFAIL into permerror and
      adding RFC 2308 reference.

      Clarified text about IPv4 mapped addresses to resolve test suite

      Clarified ambiguity about result when more than 10 "mx" or "ptr"
      records are returned for lookup to specify permerror.  This
      resolves one of the test suite ambiguities

      Made all references to result codes lower case per issue #7

      Adjusted section 2.2 Requirement to check mail from per issue #15

      Added missing "v" element in macro-letter in the collected ABNF
      per issue #16 - section 8.1 was already fixed in the pre-WG draft

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      Marked ptr and "p" macro deprecated/SHOULD NOT use per issue #27

      Expunged lower case may from the draft per issue #8

      Expunged "x-" name as an obsolete concept

      Updated obslete references: RFC2821 to RFC5321, RFC2822 to
      RFC5322, and RFC4234 to RFC5234

      Refer to RFC6647 to describe greylisting instead of trying to
      describe it directly.

      Updated informative references to the current versions.

      Added definition for deprecated since there are questions.

      Start to rework section 9 with some RFC5598 terms.

      Added mention of RFC 6552 feedback reports in section 9.

      Added draft-ietf-spfbis-experiment as an informational reference.

      Drop Type SPF.

      Try and clarify informational nature of RFC3696

      Fix ABNF nits and add missing definitions per Bill's ABNF checker.

      Make DNS lookup time limit SHOULD instead of MAY.

      Reorganize and clarify processing limits.  Move hard limits to new
      section 4.6.4, Evaluation Limits.  Move advice to non-normative
      section 9.

      Removed paragraph in section 10.1 about limiting total data
      volumes as it is unused (and removable per the charter) and serves
      no purpose (it isn't something that actually can be implemented in
      any reasonable way).

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Author's Address

   Scott Kitterman
   3611 Scheel Dr
   Ellicott City, MD  21042
   United States of America

   Email: scott@kitterman.com

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