GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc4408

Network Working Group M. Wong Request for Comments: 4408 W. Schlitt Category: Experimental April 2006

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

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

IESG Note

 The following documents  (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
 are published simultaneously as Experimental RFCs, although there is
 no general technical consensus and efforts to reconcile the two
 approaches have failed.  As such, these documents have not received
 full IETF review and are published "AS-IS" to document the different
 approaches as they were considered in the MARID working group.
 The IESG takes no position about which approach is to be preferred
 and cautions the reader that there are serious open issues for each
 approach and concerns about using them in tandem.  The IESG believes
 that documenting the different approaches does less harm than not
 documenting them.
 Note that the Sender ID experiment may use DNS records that may have
 been created for the current SPF experiment or earlier versions in
 this set of experiments.  Depending on the content of the record,
 this may mean that Sender-ID heuristics would be applied incorrectly
 to a message.  Depending on the actions associated by the recipient
 with those heuristics, the message may not be delivered or may be
 discarded on receipt.
 Participants relying on Sender ID experiment DNS records are warned
 that they may lose valid messages in this set of circumstances.
 aParticipants publishing SPF experiment DNS records should consider
 the advice given in section 3.4 of RFC 4406 and may wish to publish
 both v=spf1 and spf2.0 records to avoid the conflict.

Wong & Schlitt Experimental [Page 1] RFC 4408 Sender Policy Framework (SPF) April 2006

 Participants in the Sender-ID experiment need to be aware that the
 way Resent-* header fields are used will result in failure to receive
 legitimate email when interacting with standards-compliant systems
 (specifically automatic forwarders which comply with the standards by
 not adding Resent-* headers, and systems which comply with RFC 822
 but have not yet implemented RFC 2822 Resent-* semantics).  It would
 be inappropriate to advance Sender-ID on the standards track without
 resolving this interoperability problem.
 The community is invited to observe the success or failure of the two
 approaches during the two years following publication, in order that
 a community consensus can be reached in the future.

Abstract

 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 may
 explicitly authorize the hosts that are allowed to use its domain
 name, and a receiving host may check such authorization.

Table of Contents

 1. Introduction ....................................................4
    1.1. Protocol Status ............................................4
    1.2. Terminology ................................................5
 2. Operation .......................................................5
    2.1. The HELO Identity ..........................................5
    2.2. The MAIL FROM Identity .....................................5
    2.3. Publishing Authorization ...................................6
    2.4. Checking Authorization .....................................6
    2.5. Interpreting the Result ....................................7
         2.5.1. None ................................................8
         2.5.2. Neutral .............................................8
         2.5.3. Pass ................................................8
         2.5.4. Fail ................................................8
         2.5.5. SoftFail ............................................9
         2.5.6. TempError ...........................................9
         2.5.7. PermError ...........................................9
 3. SPF Records .....................................................9
    3.1. Publishing ................................................10
         3.1.1. DNS Resource Record Types ..........................10
         3.1.2. Multiple DNS Records ...............................11
         3.1.3. Multiple Strings in a Single DNS record ............11
         3.1.4. Record Size ........................................11
         3.1.5. Wildcard Records ...................................11

Wong & Schlitt Experimental [Page 2] RFC 4408 Sender Policy Framework (SPF) April 2006

 4. The check_host() Function ......................................12
    4.1. Arguments .................................................12
    4.2. Results ...................................................13
    4.3. Initial Processing ........................................13
    4.4. Record Lookup .............................................13
    4.5. Selecting Records .........................................13
    4.6. Record Evaluation .........................................14
         4.6.1. Term Evaluation ....................................14
         4.6.2. Mechanisms .........................................15
         4.6.3. Modifiers ..........................................15
    4.7. Default Result ............................................16
    4.8. Domain Specification ......................................16
 5. Mechanism Definitions ..........................................16
    5.1. "all" .....................................................17
    5.2. "include" .................................................18
    5.3. "a" .......................................................19
    5.4. "mx" ......................................................20
    5.5. "ptr" .....................................................20
    5.6. "ip4" and "ip6" ...........................................21
    5.7. "exists" ..................................................22
 6. Modifier Definitions ...........................................22
    6.1. redirect: Redirected Query ................................23
    6.2. exp: Explanation ..........................................23
 7. The Received-SPF Header Field ..................................25
 8. Macros .........................................................27
    8.1. Macro Definitions .........................................27
    8.2. Expansion Examples ........................................30
 9. Implications ...................................................31
    9.1. Sending Domains ...........................................31
    9.2. Mailing Lists .............................................32
    9.3. Forwarding Services and Aliases ...........................32
    9.4. Mail Services .............................................34
    9.5. MTA Relays ................................................34
 10. Security Considerations .......................................35
    10.1. Processing Limits ........................................35
    10.2. SPF-Authorized E-Mail May Contain Other False
          Identities ...............................................37
    10.3. Spoofed DNS and IP Data ..................................37
    10.4. Cross-User Forgery .......................................37
    10.5. Untrusted Information Sources ............................38
    10.6. Privacy Exposure .........................................38
 11. Contributors and Acknowledgements .............................38
 12. IANA Considerations ...........................................39
    12.1. The SPF DNS Record Type ..................................39
    12.2. The Received-SPF Mail Header Field .......................39
 13. References ....................................................39
    13.1. Normative References .....................................39
    13.2. Informative References ...................................40

Wong & Schlitt Experimental [Page 3] RFC 4408 Sender Policy Framework (SPF) April 2006

 Appendix A.  Collected ABNF .......................................42
 Appendix B.  Extended Examples ....................................44
    B.1.  Simple Examples ..........................................44
    B.2.  Multiple Domain Example ..................................45
    B.3.  DNSBL Style Example ......................................46
    B.4.  Multiple Requirements Example ............................46

1. Introduction

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

Wong & Schlitt Experimental [Page 4] RFC 4408 Sender Policy Framework (SPF) April 2006

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

1.2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 This document is concerned with the portion of a mail message
 commonly called "envelope sender", "return path", "reverse path",
 "bounce address", "2821 FROM", or "MAIL FROM".  Since these terms are
 either not well defined or often used casually, this document defines
 the "MAIL FROM" identity in Section 2.2.  Note that other terms that
 may superficially look like the common terms, such as "reverse-path",
 are used only with the defined meanings from normative documents.

2. Operation

2.1. The HELO Identity

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

2.2. The MAIL FROM Identity

 The "MAIL FROM" identity derives from the SMTP MAIL command (see
 [RFC2821]).  This command supplies the "reverse-path" for a message,
 which generally consists of the sender mailbox, and is the mailbox to
 which notification messages are to be sent if there are problems
 delivering the message.
 [RFC2821] allows the reverse-path to be null (see Section 4.5.5 in
 RFC 2821).  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

Wong & Schlitt Experimental [Page 5] RFC 4408 Sender Policy Framework (SPF) April 2006

 defines the "MAIL FROM" identity to be the mailbox composed of the
 localpart "postmaster" and the "HELO" identity (which may or may not
 have been checked separately before).
 SPF clients MUST check the "MAIL FROM" identity.  SPF clients check
 the "MAIL FROM" identity by applying the check_host() function to the
 "MAIL FROM" identity as the <sender>.

2.3. Publishing Authorization

 An SPF-compliant domain MUST publish a valid SPF record as described
 in Section 3.  This record authorizes the use of the domain name in
 the "HELO" and "MAIL FROM" identities by the MTAs it specifies.
 If domain owners choose to publish SPF records, it is RECOMMENDED
 that they end in "-all", or redirect to other records that do, so
 that a definitive determination of authorization can be made.
 Domain holders may publish SPF records that explicitly authorize no
 hosts if mail should never originate using that domain.
 When changing SPF records, care must be taken to ensure that there is
 a transition period so that the old policy remains valid until all
 legitimate E-Mail has been checked.

2.4. Checking Authorization

 A mail receiver can perform a set of SPF checks for each mail message
 it receives.  An SPF check tests the authorization of a client host
 to emit mail with a given identity.  Typically, such checks are done
 by a receiving MTA, but can be performed elsewhere in the mail
 processing chain so long as the required information is available and
 reliable.  At least the "MAIL FROM" identity MUST be checked, but it
 is RECOMMENDED that the "HELO" identity also be checked beforehand.
 Without explicit approval of the domain owner, checking other
 identities against SPF version 1 records is NOT RECOMMENDED because
 there are cases that are known to give incorrect results.  For
 example, almost all mailing lists rewrite the "MAIL FROM" identity
 (see Section 9.2), but some do not change any other identities in the
 message.  The scenario described in Section 9.3, 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
 may influence whether or not a particular SPF check is performed.
 For example, finding the sending host's IP address on a local white

Wong & Schlitt Experimental [Page 6] RFC 4408 Sender Policy Framework (SPF) April 2006

 list may cause all other tests to be skipped and all mail from that
 host to be accepted.
 When a mail receiver decides to perform an SPF check, it MUST use a
 correctly-implemented check_host() function (Section 4) evaluated
 with the correct parameters.  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.
 <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
 <sender> - the "MAIL FROM" or "HELO" identity.
 Note that the <domain> argument may not be a well-formed domain name.
 For example, if the reverse-path was null, then the EHLO/HELO domain
 is used, with its associated problems (see Section 2.1).  In these
 cases, check_host() is defined in Section 4.3 to return a "None"
 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 E-Mail from such domains,
 especially in the case of invalid "MAIL FROM".  In order to prevent
 the circumvention of SPF records, rejecting E-Mail from invalid
 domains should be considered.
 Implementations must take care to correctly extract the <domain> from
 the data given with the SMTP MAIL FROM command as many MTAs will
 still accept such things as source routes (see [RFC2821], Appendix
 C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
 These archaic features have been maliciously used to bypass security
 systems.

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.

Wong & Schlitt Experimental [Page 7] RFC 4408 Sender Policy Framework (SPF) April 2006

 Performing the authorization after the SMTP transaction has finished
 may cause problems, such as the following: (1) It may be difficult to
 accurately extract the required information from potentially
 deceptive headers; (2) legitimate E-Mail may fail because the
 sender's policy may have since changed.
 Generating non-delivery notifications to forged identities that have
 failed the authorization check is generally abusive and against the
 explicit wishes of the identity owner.

2.5.1. None

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

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
 [RFC2821]) and, if supported, the 5.7.1 Delivery Status Notification
 (DSN) code (see [RFC3464]), in addition to an appropriate reply text.
 The check_host() function may return either a default explanation
 string or one from the domain that published the SPF records (see
 Section 6.2).  If the information does not originate with the

Wong & Schlitt Experimental [Page 8] RFC 4408 Sender Policy Framework (SPF) April 2006

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

2.5.6. TempError

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

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).

Wong & Schlitt Experimental [Page 9] RFC 4408 Sender Policy Framework (SPF) April 2006

 The SPF record is a single string of text.  An example record is the
 following:
    v=spf1 +mx a:colo.example.com/28 -all
 This record has a version of "spf1" and three directives: "+mx",
 "a:colo.example.com/28" (the + is implied), and "-all".

3.1. Publishing

 Domain owners wishing to be SPF compliant must publish SPF records
 for the hosts that are used in the "MAIL FROM" and "HELO" identities.
 The SPF records are placed in the DNS tree at the host name it
 pertains to, not a subdomain under it, such as is done with SRV
 records.  This is the same whether the TXT or SPF RR type (see
 Section 3.1.1) is used.
 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 may cause problems with
 size limits (see Section 3.1.4).

3.1.1. DNS Resource Record Types

 This document defines a new DNS RR of type SPF, code 99.  The format
 of this type is identical to the TXT RR [RFC1035].  For either type,
 the character content of the record is encoded as [US-ASCII].
 It is recognized that the current practice (using a TXT record) is
 not optimal, but it is necessary because there are a number of DNS
 server and resolver implementations in common use that cannot handle
 the new RR type.  The two-record-type scheme provides a forward path
 to the better solution of using an RR type reserved for this purpose.
 An SPF-compliant domain name SHOULD have SPF records of both RR
 types.  A compliant domain name MUST have a record of at least one
 type.  If a domain has records of both types, they MUST have
 identical content.  For example, instead of publishing just one
 record as in Section 3.1 above, it is better to publish:
    example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
    example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"

Wong & Schlitt Experimental [Page 10] RFC 4408 Sender Policy Framework (SPF) April 2006

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

3.1.2. Multiple DNS Records

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

3.1.3. Multiple Strings in a Single DNS record

 As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS
 record (either TXT 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..."
 SPF or 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 or SPF RR record.

3.1.4. Record Size

 The published SPF record for a given domain name SHOULD remain small
 enough that the results of a query for it will fit within 512 octets.
 This will keep even older DNS implementations from falling over to
 TCP.  Since the answer size is dependent on many things outside the
 scope of this document, it is only possible to give this guideline:
 If the combined length of the DNS name and the text of all the
 records of a given type (TXT or SPF) is under 450 characters, then
 DNS answers should fit in UDP packets.  Note that when computing the
 sizes for queries of the TXT format, one must take into account any
 other TXT records published at the domain name.  Records that are too
 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 may want to publish wildcard declarations,

Wong & Schlitt Experimental [Page 11] RFC 4408 Sender Policy Framework (SPF) April 2006

 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:
     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       1.2.3.4
     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).

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
 cases.

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.

Wong & Schlitt Experimental [Page 12] RFC 4408 Sender Policy Framework (SPF) April 2006

 <sender> - the "MAIL FROM" or "HELO" identity.
 The domain portion of <sender> will usually be the same as the
 <domain> argument when check_host() is initially evaluated.  However,
 this will generally not be true for recursive evaluations (see
 Section 5.2 below).
 Actual implementations of the check_host() function may need
 additional arguments.

4.2. Results

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

4.3. Initial Processing

 If the <domain> is malformed (label longer than 63 characters, zero-
 length label not at the end, etc.) 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.

4.4. Record Lookup

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

4.5. Selecting Records

 Records begin with a version section:
 record           = version terms *SP
 version          = "v=spf1"
 Starting with the set of records that were returned by the lookup,
 record selection proceeds in two steps:

Wong & Schlitt Experimental [Page 13] RFC 4408 Sender Policy Framework (SPF) April 2006

 1. Records that do not begin with a version section of exactly
    "v=spf1" are discarded.  Note that the version section is
    terminated either by 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.
 2. If any records of type SPF are in the set, then all records of
    type TXT are discarded.
 After the above steps, there should be exactly one record remaining
 and evaluation can proceed.  If there are two or more records
 remaining, then check_host() exits immediately with the result of
 "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.
 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
 name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 Most mechanisms allow a ":" or "/" character after the name.

Wong & Schlitt Experimental [Page 14] RFC 4408 Sender Policy Framework (SPF) April 2006

 Modifiers always contain an equals ('=') character immediately after
 the name, and before any ":" or "/" characters that may be part of
 the macro-string.
 Terms that do not contain any of "=", ":", or "/" are mechanisms, as
 defined in Section 5.
 As per the definition of the ABNF notation in [RFC4234], 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:
    "+" 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.

Wong & Schlitt Experimental [Page 15] RFC 4408 Sender Policy Framework (SPF) April 2006

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.
 For example:
    v=spf1 +mx -all
 or
    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>.

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.
    all
    include
 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.

Wong & Schlitt Experimental [Page 16] RFC 4408 Sender Policy Framework (SPF) April 2006

    a
    mx
    ptr
    ip4
    ip6
    exists
 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 [RFC3513], Section
 2.5.5) MUST still be considered an IPv4 address.
 Several mechanisms rely on information fetched from DNS.  For these
 DNS queries, except where noted, if the DNS server returns an error
 (RCODE other than 0 or 3) or the query times out, the mechanism
 throws the exception "TempError".  If the server returns "domain does
 not exist" (RCODE 3), then evaluation of the mechanism continues as
 if the server returned no error (RCODE 0) and zero answer records.

5.1. "all"

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

Wong & Schlitt Experimental [Page 17] RFC 4408 Sender Policy Framework (SPF) April 2006

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
 "Fail".

Wong & Schlitt Experimental [Page 18] RFC 4408 Sender Policy Framework (SPF) April 2006

 Whether this mechanism matches, does not match, or throws an
 exception depends on the result of the recursive evaluation of
 check_host():
 +---------------------------------+---------------------------------+
 | 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
 addresses.
 A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
 An address lookup is done on the <target-name>.  The <ip> is compared
 to the returned address(es).  If any address matches, the mechanism
 matches.

Wong & Schlitt Experimental [Page 19] RFC 4408 Sender Policy Framework (SPF) April 2006

5.4. "mx"

 This mechanism matches if <ip> is one of the MX hosts for a domain
 name.
 MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 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 lookup on the <target-name> directly.  This
 behavior breaks with the legacy "implicit MX" rule.  See [RFC2821],
 Section 5.  If such behavior is desired, the publisher should specify
 an "a" directive.

5.5. "ptr"

 This mechanism tests whether the DNS reverse-mapping for <ip> exists
 and correctly points to a domain name within a particular domain.
 PTR              = "ptr"    [ ":" domain-spec ]
 First, the <ip>'s name is looked up using this procedure: perform a
 DNS reverse-mapping for <ip>, looking up the corresponding PTR record
 in "in-addr.arpa." if the address is an IPv4 one and in "ip6.arpa."
 if it is an IPv6 address.  For each record returned, validate the
 domain name by looking up its IP address.  To prevent DoS attacks,
 more than 10 PTR names MUST NOT be looked up during the evaluation of
 a "ptr" mechanism (see Section 10).  If <ip> is among the returned IP
 addresses, then that domain name is validated.  In pseudocode:
 sending-domain_names := ptr_lookup(sending-host_IP); if more than 10
 sending-domain_names are found, use at most 10.  for each name in
 (sending-domain_names) {
   IP_addresses := a_lookup(name);
   if the sending-domain_IP is one of the IP_addresses {
     validated-sending-domain_names += name;
   } }
 Check all validated domain names to see if they end in the
 <target-name> domain.  If any do, this mechanism matches.  If no
 validated domain name can be found, or if none of the validated

Wong & Schlitt Experimental [Page 20] RFC 4408 Sender Policy Framework (SPF) April 2006

 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.
 Pseudocode:
 for each name in (validated-sending-domain_names) {
   if name ends in <domain-spec>, return match.
   if name is <domain-spec>, return match.
 }
 return no-match.
 This mechanism matches if the <target-name> is either an ancestor of
 a validated domain name or if the <target-name> and a validated
 domain name are the same.  For example: "mail.example.com" is within
 the domain "example.com", but "mail.bad-example.com" is not.
 Note: Use of this mechanism is discouraged because it is slow, 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.

5.6. "ip4" and "ip6"

 These mechanisms test whether <ip> is contained within a given IP
 network.
 IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 ip4-cidr-length  = "/" 1*DIGIT
 ip6-cidr-length  = "/" 1*DIGIT
 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., 192.0.2.0
 ip6-network      = <as per [RFC 3513], section 2.2>
          ; e.g., 2001:DB8::CD30
 The <ip> is compared to the given network.  If CIDR-length high-order
 bits match, the mechanism matches.

Wong & Schlitt Experimental [Page 21] RFC 4408 Sender Policy Framework (SPF) April 2006

 If ip4-cidr-length is omitted, it is taken to be "/32".  If
 ip6-cidr-length is omitted, it is taken to be "/128".  It is not
 permitted to omit parts of the IP address instead of using CIDR
 notations.  That is, use 192.0.2.0/24 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
 permitted.
 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
 "1.2.0.192.someuser._spf.example.com".  This makes fine-grained
 decisions possible at the level of the user and client IP address.
 This mechanism enables queries that mimic the style of tests that
 existing anti-spam DNS blacklists (DNSBL) use.

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
 specifications.

Wong & Schlitt Experimental [Page 22] RFC 4408 Sender Policy Framework (SPF) April 2006

6.1. redirect: Redirected Query

 If all mechanisms fail to match, and a "redirect" modifier is
 present, then processing proceeds as follows:
 redirect         = "redirect" "=" domain-spec
 The domain-spec portion of the redirect section is expanded as per
 the macro rules in Section 8.  Then check_host() is evaluated with
 the resulting string as the <domain>.  The <ip> and <sender>
 arguments remain the same as current evaluation of check_host().
 The result of this new evaluation of check_host() is then considered
 the result of the current evaluation with the exception that if no
 SPF record is found, or if the target-name is malformed, the result
 is a "PermError" rather than "None".
 Note that the newly-queried domain may itself specify redirect
 processing.
 This facility is intended for use by organizations that wish to apply
 the same record to multiple domains.  For example:
   la.example.com. TXT "v=spf1 redirect=_spf.example.com"
   ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
   sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
 _spf.example.com. TXT "v=spf1 mx:example.com -all"
 In this example, mail from any of the three domains is described by
 the same record.  This can be an administrative advantage.
 Note: In general, the domain "A" cannot reliably use a redirect to
 another domain "B" not under the same administrative control.  Since
 the <sender> stays the same, there is no guarantee that the record at
 domain "B" will correctly work for mailboxes in domain "A",
 especially if domain "B" uses mechanisms involving localparts.  An
 "include" directive may be more appropriate.
 For clarity, it is RECOMMENDED that any "redirect" modifier appear as
 the very last term in a record.

6.2. exp: Explanation

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

Wong & Schlitt Experimental [Page 23] RFC 4408 Sender Policy Framework (SPF) April 2006

 is present, then either a default explanation string or an empty
 explanation string may be returned.
 The <domain-spec> is macro expanded (see Section 8) and becomes the
 <target-name>.  The DNS TXT record for the <target-name> is fetched.
 If <domain-spec> is empty, or there are any DNS processing errors
 (any RCODE other than 0), or if no records are returned, or if more
 than one record is returned, or if there are syntax errors in the
 explanation string, then proceed as if no exp modifier was given.
 The fetched TXT record's strings are concatenated with no spaces, and
 then treated as an <explain-string>, which is macro-expanded.  This
 final result is the explanation string.  Implementations MAY limit
 the length of the resulting explanation string to allow for other
 protocol constraints and/or reasonable processing limits.  Since the
 explanation string is intended for an SMTP response and [RFC2821]
 Section 2.4 says that responses are in [US-ASCII], the explanation
 string is also limited to US-ASCII.
 Software evaluating check_host() can use this string to communicate
 information from the publishing domain in the form of a short message
 or URL.  Software SHOULD make it clear that the explanation string
 comes from a third party.  For example, it can prepend the macro
 string "%{o} explains: " to the explanation, such as shown in Section
 2.5.4.
 Suppose example.com has this record:
    v=spf1 mx -all exp=explain._spf.%{d}
 Here are some examples of possible explanation TXT records at
 explain._spf.example.com:
    "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

Wong & Schlitt Experimental [Page 24] RFC 4408 Sender Policy Framework (SPF) April 2006

 a "redirect" modifier, an exp= modifier from the original domain MUST
 NOT be used.

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 [RFC2822] 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 / "x-" name / name
 identity         = "mailfrom"   ; for the "MAIL FROM" identity
                    / "helo"     ; for the "HELO" identity
                    / name       ; other identities
 dot-atom         = <unquoted word as per [RFC2822]>
 quoted-string    = <quoted string as per [RFC2822]>
 comment          = <comment string as per [RFC2822]>
 CFWS             = <comment or folding white space as per [RFC2822]>
 FWS              = <folding white space as per [RFC2822]>
 CRLF             = <standard end-of-line token as per [RFC2822]>
 The header field SHOULD include a "(...)" style <comment> after the
 result, conveying supporting information for the result, such as
 <ip>, <sender>, and <domain>.

Wong & Schlitt Experimental [Page 25] RFC 4408 Sender Policy Framework (SPF) April 2006

 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
 "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
                rule
 Other keys may be defined by SPF clients.  Until a new key name
 becomes widely accepted, new key names should start with "x-".
 SPF clients MUST make sure that the Received-SPF header 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
 following:
 Received-SPF: Pass (mybox.example.org: domain of
  myname@example.com designates 192.0.2.1 as permitted sender)
     receiver=mybox.example.org; client-ip=192.0.2.1;
     envelope-from=<myname@example.com>; helo=foo.example.com;
 Received-SPF: Fail (mybox.example.org: domain of
                   myname@example.com does not designate
                   192.0.2.1 as permitted sender)
                   identity=mailfrom; client-ip=192.0.2.1;
                   envelope-from=<myname@example.com>;

Wong & Schlitt Experimental [Page 26] RFC 4408 Sender Policy Framework (SPF) April 2006

8. Macros

8.1. Macro Definitions

 Many mechanisms and modifiers perform macro expansion on part of 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)
 alphanum         = ALPHA / DIGIT
 explain-string   = *( macro-string / SP )
 macro-string     = *( macro-expand / macro-literal )
 macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                    / "%%" / "%_" / "%-"
 macro-literal    = %x21-24 / %x26-7E
                    ; visible characters except "%"
 macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                    "c" / "r" / "t"
 transformers     = *DIGIT [ "r" ]
 delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 A literal "%" is expressed by "%%".
    "%_" expands to a single " " space.
    "%-" expands to a URL-encoded space, viz., "%20".
 The following macro letters are expanded in term arguments:
    s = <sender>
    l = local-part of <sender>
    o = domain of <sender>
    d = <domain>
    i = <ip>
    p = the validated domain name of <ip>
    v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
    h = HELO/EHLO domain

Wong & Schlitt Experimental [Page 27] RFC 4408 Sender Policy Framework (SPF) April 2006

 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
 A '%' character not followed by a '{', '%', '-', or '_' character is
 a syntax error.  So
  1. exists:%(ir).sbl.spamhaus.example.org
 is incorrect and will cause check_host() to return a "PermError".
 Instead, say
  1. exists:%{ir}.sbl.spamhaus.example.org
 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 may 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 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
 and the macro %{ir} would expand to "1.2.0.192".
 The DIGIT transformer indicates the number of right-hand parts to
 use, after optional reversal.  If a DIGIT is specified, the value
 MUST be nonzero.  If no DIGITs are specified, or if the value
 specifies more parts than are available, all the available parts are
 used.  If the DIGIT was 5, and only 3 parts were available, the macro
 interpreter would pretend the DIGIT was 3.  Implementations MUST
 support at least a value of 128, as that is the maximum number of
 labels in a domain name.

Wong & Schlitt Experimental [Page 28] RFC 4408 Sender Policy Framework (SPF) April 2006

 The "s" macro expands to the <sender> argument.  It is an E-Mail
 address with a localpart, an "@" character, and a domain.  The "l"
 macro expands to just the localpart.  The "o" macro expands to just
 the domain part.  Note that these values remain the same during
 recursive and chained evaluations due to "include" and/or "redirect".
 Note also that if the original <sender> had no localpart, the
 localpart was set to "postmaster" in initial processing (see Section
 4.3).
 For IPv4 addresses, both the "i" and "c" macros expand to the
 standard dotted-quad format.
 For IPv6 addresses, the "i" macro expands to a dot-format address; it
 is intended for use in %{ir}.  The "c" macro may expand to any of the
 hexadecimal colon-format addresses specified in [RFC3513], Section
 2.2.  It is intended for humans to read.
 The "p" macro expands to the validated domain name of <ip>.  The
 procedure for finding the validated domain name is defined in Section
 5.5.  If the <domain> is present in the list of validated domains, it
 SHOULD be used.  Otherwise, if a subdomain of the <domain> is
 present, it SHOULD be used.  Otherwise, any name from the list may be
 used.  If there are no validated domain names or if a DNS error
 occurs, the string "unknown" is used.
 The "r" macro expands to the name of the receiving MTA.  This SHOULD
 be a fully qualified domain name, but if one does not exist (as when
 the checking is done by a MUA) or if policy restrictions dictate
 otherwise, the word "unknown" SHOULD be substituted.  The domain name
 may be different 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
 characters.
 Uppercased macros expand exactly as their lowercased equivalents, and
 are then URL escaped.  URL escaping must be performed for characters
 not in the "uric" set, which is defined in [RFC3986].

Wong & Schlitt Experimental [Page 29] RFC 4408 Sender Policy Framework (SPF) April 2006

 Note: Care must be taken so that macro expansion for legitimate
 E-Mail does not exceed the 63-character limit on DNS labels.  The
 localpart of E-Mail addresses, in particular, can have more than 63
 characters between dots.
 Note: Domains should avoid using the "s", "l", "o", or "h" macros in
 conjunction with any mechanism directive.  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.
 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 192.0.2.3.
    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

Wong & Schlitt Experimental [Page 30] RFC 4408 Sender Policy Framework (SPF) April 2006

 macro-string                                               expansion
 --------------------------------------------------------------------
 %{ir}.%{v}._spf.%{d2}             3.2.0.192.in-addr._spf.example.com
 %{lr-}.lp._spf.%{d2}                  bad.strong.lp._spf.example.com
 %{lr-}.lp.%{ir}.%{v}._spf.%{d2}
                     bad.strong.lp.3.2.0.192.in-addr._spf.example.com
 %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
                         3.2.0.192.in-addr.strong.lp._spf.example.com
 %{d2}.trusted-domains.example.net
                              example.com.trusted-domains.example.net
 IPv6:
 %{ir}.%{v}._spf.%{d2}                               1.0.B.C.0.0.0.0.
 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.ip6._spf.example.com

9. Implications

 This section outlines the major implications that adoption of this
 document will have on various entities involved in Internet E-Mail.
 It is intended to make clear to the reader where this document
 knowingly affects the operation of such entities.  This section is
 not a "how-to" manual, or a "best practices" document, and it is not
 a comprehensive list of what such entities should do in light of this
 document.
 This section is non-normative.

9.1. Sending Domains

 Domains that wish to be compliant with this specification will need
 to determine the list of hosts that they allow to use their domain
 name in the "HELO" and "MAIL FROM" identities.  It is recognized that
 forming such a list is not just a simple technical exercise, but
 involves policy decisions with both technical and administrative
 considerations.
 It can be helpful to publish records that include a "tracking
 exists:" mechanism.  By looking at the name server logs, a rough list
 may then be generated.  For example:
    v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all

Wong & Schlitt Experimental [Page 31] RFC 4408 Sender Policy Framework (SPF) April 2006

9.2. Mailing Lists

 Mailing lists must be aware of how they re-inject mail that is sent
 to the list.  Mailing lists MUST comply with the requirements in
 [RFC2821], Section 3.10, and [RFC1123], Section 5.3.6, that say that
 the reverse-path MUST be changed to be the mailbox of a person or
 other entity who administers the list.  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 may or may
 not encounter problems depending on how access to the list is
 restricted.  Such lists that are entirely internal to a domain (only
 people in the domain can send to or receive from the list) are not
 affected.

9.3. Forwarding Services and Aliases

 Forwarding services take mail that is received at a mailbox and
 direct it to some external mailbox.  At the time of this writing, the
 near-universal practice of such services is to use the original "MAIL
 FROM" of a message when re-injecting it for delivery to the external
 mailbox.  [RFC1123] and [RFC2821] describe this action as an "alias"
 rather than a "mail list".  This means 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
 authorization.
 There are three places that techniques can be used to ameliorate this
 problem.
 1. The beginning, when E-Mail is first sent.
     1. "Neutral" results could be given for IP addresses that may be
        forwarders, instead of "Fail" results.  For example:
           "v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all"
        This would cause a lookup on an anti-spam DNS blacklist
        (DNSBL) and cause a result of "Fail" only for E-Mail coming
        from listed sources.  All other E-Mail, including E-Mail sent
        through forwarders, would receive a "Neutral" result.  By
        checking the DNSBL after the known good sources, problems with
        incorrect listing on the DNSBL are greatly reduced.

Wong & Schlitt Experimental [Page 32] RFC 4408 Sender Policy Framework (SPF) April 2006

     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
        E-Mail would otherwise be rejected as not coming from a known
        good source.
        Note that due to the 63-character limit for domain labels,
        this approach only works reliably if the localpart signature
        scheme is guaranteed either to only produce localparts with a
        maximum of 63 characters or to gracefully handle truncated
        localparts.
     3. Similarly, a specialized DNS server could be set up that will
        rate-limit the E-Mail 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:
               "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
 2.  The middle, when E-Mail is forwarded.
     1. Forwarding services can solve the problem by rewriting the
        "MAIL FROM" to be in their own domain.  This means that mail
        bounced from the external mailbox will have to be re-bounced
        by the forwarding service.  Various schemes to do this exist
        though they vary widely in complexity and resource
        requirements on the part of the forwarding service.
     2. Several popular MTAs can be forced from "alias" semantics to
        "mailing list" semantics by configuring an additional alias
        with "owner-" prepended to the original alias name (e.g., an
        alias of "friends: george@example.com, fred@example.org" would
        need another alias of the form "owner-friends:  localowner").

Wong & Schlitt Experimental [Page 33] RFC 4408 Sender Policy Framework (SPF) April 2006

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

9.4. Mail Services

 Service providers that offer mail services to third-party domains,
 such as sending of bulk mail, may want to adjust their setup in light
 of the authorization check described in this document.  If the "MAIL
 FROM" identity used for such E-Mail uses the domain of the service
 provider, then the provider needs only to ensure that 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
 several options for the third-party domain to authorize the service
 provider's MTAs to send mail on its behalf.  For mail service
 providers, such as ISPs, that have a wide variety of customers using
 the same MTA, steps should be taken to prevent cross-customer forgery
 (see Section 10.4).

9.5. MTA Relays

 The authorization check generally precludes the use of arbitrary MTA
 relays between sender and receiver of an E-Mail message.
 Within an organization, MTA relays can be effectively deployed.
 However, for purposes of this document, such relays are effectively
 transparent.  The SPF authorization check is a check between border
 MTAs of different domains.
 For mail senders, this means that published SPF records must
 authorize any MTAs that actually send across the Internet.  Usually,
 these are just the border MTAs as internal MTAs simply forward mail
 to these MTAs for delivery.

Wong & Schlitt Experimental [Page 34] RFC 4408 Sender Policy Framework (SPF) April 2006

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

10. Security Considerations

10.1. Processing Limits

 As with most aspects of E-Mail, there are a number of ways that
 malicious parties could use the protocol as an avenue for a
 Denial-of-Service (DoS) attack.  The processing limits outlined here
 are designed to prevent attacks such as the following:
 o  A malicious party could create an SPF record with many references
    to a victim's domain and send many E-Mails to different SPF
    clients; those SPF clients would then create a DoS attack.  In
    effect, the SPF clients are being used to amplify the attacker's
    bandwidth by using fewer bytes in the SMTP session than are used
    by the DNS queries.  Using SPF clients also allows the attacker to
    hide the true source of the attack.
 o  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 may seem reasonable for an individual mail server can
 still allow an unreasonable amount of bandwidth amplification.
 Therefore, the processing limits need to be quite low.
 SPF implementations MUST limit the number of mechanisms and modifiers
 that do DNS lookups to at most 10 per SPF check, including any
 lookups caused by the use of the "include" mechanism or the

Wong & Schlitt Experimental [Page 35] RFC 4408 Sender Policy Framework (SPF) April 2006

 "redirect" modifier.  If this number is exceeded during a check, a
 PermError MUST be returned.  The "include", "a", "mx", "ptr", and
 "exists" mechanisms as well as the "redirect" modifier do count
 against this limit.  The "all", "ip4", and "ip6" mechanisms do not
 require DNS lookups and therefore do not count against this limit.
 The "exp" modifier does not count against this limit because the DNS
 lookup to fetch the explanation string occurs after the SPF record
 has been evaluated.
 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.
 SPF implementations SHOULD limit the total amount of data obtained
 from the DNS queries.  For example, when DNS over TCP or EDNS0 are
 available, there may need to be an explicit limit to how much data
 will be accepted to prevent excessive bandwidth usage or memory usage
 and DoS attacks.
 MTAs or other processors MAY also impose a limit on the maximum
 amount of elapsed time to evaluate check_host().  Such a limit SHOULD
 allow at least 20 seconds.  If such a limit is exceeded, the result
 of authorization SHOULD be "TempError".
 Domains publishing records SHOULD try to keep the number of "include"
 mechanisms and chained "redirect" modifiers to a minimum.  Domains
 SHOULD also try to minimize the amount of other DNS information
 needed to evaluate a record.  This can be done by choosing directives
 that require less DNS information and placing lower-cost mechanisms
 earlier in the SPF record.
 For example, consider a domain set up as follows:
 example.com.      IN MX   10 mx.example.com.
 mx.example.com.   IN A    192.0.2.1
 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 ip4:192.0.2.1 -all"
 Evaluating check_host() for the domain "a.example.com" requires the
 MX records for "example.com", and then the A records for the listed
 hosts.  Evaluating for "b.example.com" requires only the A records.
 Evaluating for "c.example.com" requires none.
 However, there may be administrative considerations: using "a" over
 "ip4" allows hosts to be renumbered easily.  Using "mx" over "a"
 allows the set of mail hosts to be changed easily.

Wong & Schlitt Experimental [Page 36] RFC 4408 Sender Policy Framework (SPF) April 2006

10.2. SPF-Authorized E-Mail 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
 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 may be lulled into a false sense of security.

10.3. Spoofed DNS and IP Data

 There are two aspects of this protocol that malicious parties could
 exploit to undermine the validity of the check_host() function:
 o  The evaluation of check_host() relies heavily on DNS.  A malicious
    attacker could attack the DNS infrastructure and cause
    check_host() to see spoofed DNS data, and then return incorrect
    results.  This could include returning "Pass" for an <ip> value
    where the actual domain's record would evaluate to "Fail".  See
    [RFC3833] for a description of DNS weaknesses.
 o  The client IP address, <ip>, is assumed to be correct.  A
    malicious attacker could spoof TCP sequence numbers to make mail
    appear to come from a permitted host for a domain that the
    attacker is impersonating.

10.4. Cross-User Forgery

 By definition, SPF policies just map domain names to sets of
 authorized MTAs, not whole E-Mail addresses to sets of authorized
 users.  Although the "l" macro (Section 8) provides a limited way to
 define individual sets of authorized MTAs for specific E-Mail
 addresses, it is generally impossible to verify, through SPF, the use
 of specific E-Mail addresses by individual users of the same MTA.
 It is up to mail services and their MTAs to directly prevent
 cross-user forgery: based on SMTP AUTH ([RFC2554]), users should be
 restricted to using only those E-Mail addresses that are actually
 under their control (see [RFC4409], Section 6.1).  Another means to
 verify the identity of individual users is message cryptography such
 as PGP ([RFC2440]) or S/MIME ([RFC3851]).

Wong & Schlitt Experimental [Page 37] RFC 4408 Sender Policy Framework (SPF) April 2006

10.5. Untrusted Information Sources

 SPF uses information supplied by third parties, such as the "HELO"
 domain name, the "MAIL FROM" address, and SPF records.  This
 information is then passed to the receiver in the Received-SPF: trace
 fields and possibly returned to the client MTA in the form of an SMTP
 rejection message.  This information must be checked for invalid
 characters and excessively long lines.
 When the authorization check fails, an explanation string may be
 included in the reject response.  Both the sender and the rejecting
 receiver need to be aware that the explanation was determined by the
 publisher of the SPF record checked and, in general, not the
 receiver.  The explanation may contain malicious URLs, or it may be
 offensive or misleading.
 This is probably less of a concern than it may initially seem since
 such messages are returned to the sender, and the explanation strings
 come from the sender policy published by the domain in the identity
 claimed by that very sender.  As long as the DSN is not redirected to
 someone other than the actual sender, the only people who see
 malicious explanation strings are people whose messages claim to be
 from domains that publish such strings in their SPF records.  In
 practice, DSNs can be misdirected, such as when an MTA accepts an
 E-Mail and then later generates a DSN to a forged address, or when an
 E-Mail forwarder does not direct the DSN back to the original sender.

10.6. Privacy Exposure

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

11. Contributors and Acknowledgements

 This document is largely based on the work of Meng Weng Wong and Mark
 Lentczner.  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 and Mark.
 This design owes a debt of parentage to [RMX] by Hadmut Danisch and
 to [DMP] by Gordon Fecyk.  The idea of using a DNS record to check
 the legitimacy of an E-Mail address traces its ancestry further back
 through messages on the namedroppers mailing list by Paul Vixie

Wong & Schlitt Experimental [Page 38] RFC 4408 Sender Policy Framework (SPF) April 2006

 [Vixie] (based on suggestion by Jim Miller) and by David Green
 [Green].
 Philip Gladstone contributed the concept of macros to the
 specification, multiplying the expressiveness of the language and
 making per-user and per-IP lookups possible.
 The authors would also like to thank the literally hundreds of
 individuals who have participated in the development of this design.
 They are far too numerous to name, but they include the following:
    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.

12. IANA Considerations

12.1. The SPF DNS Record Type

 The IANA has assigned a new Resource Record Type and Qtype from the
 DNS Parameters Registry for the SPF RR type with code 99.

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 ([RFC2822])
    Status: Experimental
    Author/Change controller: IETF
    Specification document(s): RFC 4408
    Related information:
    Requesting SPF Council review of any proposed changes and
    additions to this field are recommended.  For information about
    the SPF Council see http://www.openspf.org/Council

13. References

13.1. Normative References

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

Wong & Schlitt Experimental [Page 39] RFC 4408 Sender Policy Framework (SPF) April 2006

 [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.
 [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
            April 2001.
 [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
            2001.
 [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
            for Delivery Status Notifications", RFC 3464, January
            2003.
 [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
            (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 4234, October 2005.
 [US-ASCII] 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.

13.2 Informative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1983]  Malkin, G., "Internet Users' Glossary", RFC 1983, August
            1996.
 [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
            "OpenPGP Message Format", RFC 2440, November 1998.

Wong & Schlitt Experimental [Page 40] RFC 4408 Sender Policy Framework (SPF) April 2006

 [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
            RFC 2554, March 1999.
 [RFC3696]  Klensin, J., "Application Techniques for Checking and
            Transformation of Names", RFC 3696, February 2004.
 [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
            Name System (DNS)", RFC 3833, August 2004.
 [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
            Extensions (S/MIME) Version 3.1 Message Specification",
            RFC 3851, July 2004.
 [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            RFC 4409, April 2006.
 [RMX]      Danish, H., "The RMX DNS RR Type for light weight sender
            authentication", Work In Progress
 [DMP]      Fecyk, G., "Designated Mailers Protocol", Work In Progress
 [Vixie]    Vixie, P., "Repudiating MAIL FROM", 2002.
 [Green]    Green, D., "Domain-Authorized SMTP Mail", 2002.

Wong & Schlitt Experimental [Page 41] RFC 4408 Sender Policy Framework (SPF) April 2006

Appendix A. Collected ABNF

 This section is normative and any discrepancies with the ABNF
 fragments in the preceding text are to be resolved in favor of this
 grammar.
 See [RFC4234] for ABNF notation.  Please note that as per this ABNF
 definition, literal text strings (those in quotes) are case-
 insensitive.  Hence, "mx" matches "mx", "MX", "mX", and "Mx".
 record           = version terms *SP
 version          = "v=spf1"
 terms            = *( 1*SP ( directive / modifier ) )
 directive        = [ qualifier ] mechanism
 qualifier        = "+" / "-" / "?" / "~"
 mechanism        = ( all / include
                    / A / MX / PTR / IP4 / IP6 / exists )
 all              = "all"
 include          = "include"  ":" domain-spec
 A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
 MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
 PTR              = "ptr"    [ ":" domain-spec ]
 IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
 IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
 exists           = "exists"   ":" domain-spec
 modifier         = redirect / explanation / unknown-modifier
 redirect         = "redirect" "=" domain-spec
 explanation      = "exp" "=" domain-spec
 unknown-modifier = name "=" macro-string
 ip4-cidr-length  = "/" 1*DIGIT
 ip6-cidr-length  = "/" 1*DIGIT
 dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
 ip4-network      = qnum "." qnum "." qnum "." qnum
 qnum             = DIGIT                 ; 0-9
                    / %x31-39 DIGIT       ; 10-99
                    / "1" 2DIGIT          ; 100-199
                    / "2" %x30-34 DIGIT   ; 200-249
                    / "25" %x30-35        ; 250-255
           ; conventional dotted quad notation.  e.g., 192.0.2.0
 ip6-network      = <as per [RFC 3513], section 2.2>
           ; e.g., 2001:DB8::CD30

Wong & Schlitt Experimental [Page 42] RFC 4408 Sender Policy Framework (SPF) April 2006

 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)
 alphanum         = ALPHA / DIGIT
 explain-string   = *( macro-string / SP )
 macro-string     = *( macro-expand / macro-literal )
 macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                    / "%%" / "%_" / "%-"
 macro-literal    = %x21-24 / %x26-7E
                    ; visible characters except "%"
 macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                    "c" / "r" / "t"
 transformers     = *DIGIT [ "r" ]
 delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
 name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
 header-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 / "x-" name / name
 identity         = "mailfrom"   ; for the "MAIL FROM" identity
                    / "helo"     ; for the "HELO" identity
                    / name       ; other identities
 dot-atom         = <unquoted word as per [RFC2822]>
 quoted-string    = <quoted string as per [RFC2822]>
 comment          = <comment string as per [RFC2822]>
 CFWS             = <comment or folding white space as per [RFC2822]>
 FWS              = <folding white space as per [RFC2822]>
 CRLF             = <standard end-of-line token as per [RFC2822]>

Wong & Schlitt Experimental [Page 43] RFC 4408 Sender Policy Framework (SPF) April 2006

Appendix B. Extended Examples

 These examples are based on the following DNS setup:
 ; A domain with two mail servers, two hosts
 ; and two servers at the domain name
 $ORIGIN example.com.
 @           MX  10 mail-a
             MX  20 mail-b
             A   192.0.2.10
             A   192.0.2.11
 amy         A   192.0.2.65
 bob         A   192.0.2.66
 mail-a      A   192.0.2.129
 mail-b      A   192.0.2.130
 www         CNAME example.com.
 ; A related domain
 $ORIGIN example.org.
 @           MX  10 mail-c
 mail-c      A   192.0.2.140
 ; The reverse IP for those addresses
 $ORIGIN 2.0.192.in-addr.arpa.
 10          PTR example.com.
 11          PTR example.com.
 65          PTR amy.example.com.
 66          PTR bob.example.com.
 129         PTR mail-a.example.com.
 130         PTR mail-b.example.com.
 140         PTR mail-c.example.org.
 ; A rogue reverse IP domain that claims to be
 ; something it's not
 $ORIGIN 0.0.10.in-addr.arpa.
 4           PTR bob.example.com.

B.1. Simple Examples

 These examples show various possible published records for
 example.com and which values if <ip> would cause check_host() to
 return "Pass".  Note that <domain> is "example.com".
 v=spf1 +all
    -- any <ip> passes
 v=spf1 a -all
    -- hosts 192.0.2.10 and 192.0.2.11 pass

Wong & Schlitt Experimental [Page 44] RFC 4408 Sender Policy Framework (SPF) April 2006

 v=spf1 a:example.org -all
    -- no sending hosts pass since example.org has no A records
 v=spf1 mx -all
    -- sending hosts 192.0.2.129 and 192.0.2.130 pass
 v=spf1 mx:example.org -all
    -- sending host 192.0.2.140 passes
 v=spf1 mx mx:example.org -all
    -- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass
 v=spf1 mx/30 mx:example.org/30 -all
    -- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
 v=spf1 ptr -all
    -- sending host 192.0.2.65 passes (reverse DNS is valid and is in
       example.com)
    -- sending host 192.0.2.140 fails (reverse DNS is valid, but not
       in example.com)
    -- sending host 10.0.0.4 fails (reverse IP is not valid)
 v=spf1 ip4:192.0.2.128/28 -all
    -- sending host 192.0.2.65 fails
    -- sending host 192.0.2.129 passes

B.2. Multiple Domain Example

 These examples show the effect of related records:
    example.org: "v=spf1 include:example.com include:example.net -all"
 This record would be used if mail from example.org actually came
 through servers at example.com and example.net.  Example.org's
 designated servers are the union of example.com's and example.net's
 designated servers.
    la.example.org: "v=spf1 redirect=example.org"
    ny.example.org: "v=spf1 redirect=example.org"
    sf.example.org: "v=spf1 redirect=example.org"
 These records allow a set of domains that all use the same mail
 system to make use of that mail system's record.  In this way, only
 the mail system's record needs to be updated when the mail setup
 changes.  These domains' records never have to change.

Wong & Schlitt Experimental [Page 45] RFC 4408 Sender Policy Framework (SPF) April 2006

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
 127.0.0.2 fred.mobile-users                   A 127.0.0.2
 15.15.168.192.joel.remote-users     A 127.0.0.2
 16.15.168.192.joel.remote-users     A 127.0.0.2
 The following records describe users at example.com who mail from
 arbitrary servers, or who mail from personal servers.
 example.com:
 v=spf1 mx
        include:mobile-users._spf.%{d}
        include:remote-users._spf.%{d}
        -all
 mobile-users._spf.example.com:
 v=spf1 exists:%{l1r+}.%{d}
 remote-users._spf.example.com:
 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 -ip4:192.0.2.0/24 +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.

Wong & Schlitt Experimental [Page 46] RFC 4408 Sender Policy Framework (SPF) April 2006

Authors' Addresses

 Meng Weng Wong
 Singapore
 EMail: mengwong+spf@pobox.com
 Wayne Schlitt
 4615 Meredeth #9
 Lincoln Nebraska, NE  68506
 United States of America
 EMail: wayne@schlitt.net
 URI:   http://www.schlitt.net/spf/

Wong & Schlitt Experimental [Page 47] RFC 4408 Sender Policy Framework (SPF) April 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Wong & Schlitt Experimental [Page 48]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4408.txt · Last modified: 2006/04/27 22:48 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki