rfc4408.txt   draft-ietf-spfbis-4408bis-05.txt 
Network Working Group M. Wong Network Working Group S. Kitterman
Request for Comments: 4408 W. Schlitt Internet-Draft Kitterman Technical Services
Sender Policy Framework (SPF) for Obsoletes: 4408 (if approved) August 18, 2012
Authorizing Use of Domains in E-Mail, Version 1 Intended status: Standards Track
Expires: February 19, 2013
Status of This Memo Sender Policy Framework (SPF) for Authorizing Use of Domains in Email,
Version 1
draft-ietf-spfbis-4408bis-05.txt
This memo defines an Experimental Protocol for the Internet Abstract
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 Email 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 "MAIL FROM" of a message or the domain given on
the SMTP HELO/EHLO commands. This document describes version 1 of
the Sender Policy Framework (SPF) protocol, whereby a domain can
explicitly authorize the hosts that are allowed to use its domain
name, and a receiving host can check such authorization.
Copyright (C) The Internet Society (2006). This document obsoletes RFC4408.
IESG Note Status of this Memo
The following documents (RFC 4405, RFC 4406, RFC 4407, and RFC 4408) This Internet-Draft is submitted in full conformance with the
are published simultaneously as Experimental RFCs, although there is provisions of BCP 78 and BCP 79.
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 Internet-Drafts are working documents of the Internet Engineering
and cautions the reader that there are serious open issues for each Task Force (IETF). Note that other groups may also distribute
approach and concerns about using them in tandem. The IESG believes working documents as Internet-Drafts. The list of current Internet-
that documenting the different approaches does less harm than not Drafts is at http://datatracker.ietf.org/drafts/current/.
documenting them.
Note that the Sender ID experiment may use DNS records that may have Internet-Drafts are draft documents valid for a maximum of six months
been created for the current SPF experiment or earlier versions in and may be updated, replaced, or obsoleted by other documents at any
this set of experiments. Depending on the content of the record, time. It is inappropriate to use Internet-Drafts as reference
this may mean that Sender-ID heuristics would be applied incorrectly material or to cite them other than as "work in progress."
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 This Internet-Draft will expire on February 19, 2013.
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.
Participants in the Sender-ID experiment need to be aware that the Copyright Notice
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 Copyright (c) 2012 IETF Trust and the persons identified as the
approaches during the two years following publication, in order that document authors. All rights reserved.
a community consensus can be reached in the future.
Abstract This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
E-mail on the Internet can be forged in a number of ways. In This document may contain material from IETF Documents or IETF
particular, existing protocols place no restriction on what a sending Contributions published or made publicly available before November
host can use as the reverse-path of a message or the domain given on 10, 2008. The person(s) controlling the copyright in some of this
the SMTP HELO/EHLO commands. This document describes version 1 of material may not have granted the IETF Trust the right to allow
the Sender Policy Framework (SPF) protocol, whereby a domain may modifications of such material outside the IETF Standards Process.
explicitly authorize the hosts that are allowed to use its domain Without obtaining an adequate license from the person(s) controlling
name, and a receiving host may check such authorization. the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents Table of Contents
1. Introduction ....................................................4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Protocol Status ............................................4 1.1. Protocol Status . . . . . . . . . . . . . . . . . . . . . 6
1.2. Terminology ................................................5 1.2. Experimental History . . . . . . . . . . . . . . . . . . . 7
2. Operation .......................................................5 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. The HELO Identity ..........................................5 1.3.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. The MAIL FROM Identity .....................................5 1.3.2. Imported Definitions . . . . . . . . . . . . . . . . . 7
2.3. Publishing Authorization ...................................6 1.3.3. Mail From Definition . . . . . . . . . . . . . . . . . 7
2.4. Checking Authorization .....................................6 1.3.4. HELO Definition . . . . . . . . . . . . . . . . . . . 8
2.5. Interpreting the Result ....................................7 1.3.5. Deprecated . . . . . . . . . . . . . . . . . . . . . . 8
2.5.1. None ................................................8 2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.2. Neutral .............................................8 2.1. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 9
2.5.3. Pass ................................................8 2.2. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . . 9
2.5.4. Fail ................................................8 2.3. Publishing Authorization . . . . . . . . . . . . . . . . . 9
2.5.5. SoftFail ............................................9 2.4. Checking Authorization . . . . . . . . . . . . . . . . . . 10
2.5.6. TempError ...........................................9 2.5. Interpreting the Result . . . . . . . . . . . . . . . . . 11
2.5.7. PermError ...........................................9 2.5.1. None . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. SPF Records .....................................................9 2.5.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. Publishing ................................................10 2.5.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.1. DNS Resource Record Types ..........................10 2.5.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.2. Multiple DNS Records ...............................11 2.5.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.3. Multiple Strings in a Single DNS record ............11 2.5.6. TempError . . . . . . . . . . . . . . . . . . . . . . 13
3.1.4. Record Size ........................................11 2.5.7. PermError . . . . . . . . . . . . . . . . . . . . . . 13
3.1.5. Wildcard Records ...................................11 3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. The check_host() Function ......................................12 3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . . 14
4.1. Arguments .................................................12 3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . . 14
4.2. Results ...................................................13 3.3. Multiple Strings in a Single DNS record . . . . . . . . . 15
4.3. Initial Processing ........................................13 3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 15
4.4. Record Lookup .............................................13 3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . . 15
4.5. Selecting Records .........................................13 4. The check_host() Function . . . . . . . . . . . . . . . . . . 17
4.6. Record Evaluation .........................................14 4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6.1. Term Evaluation ....................................14 4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6.2. Mechanisms .........................................15 4.3. Initial Processing . . . . . . . . . . . . . . . . . . . . 17
4.6.3. Modifiers ..........................................15 4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 18
4.7. Default Result ............................................16 4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 18
4.8. Domain Specification ......................................16 4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 18
5. Mechanism Definitions ..........................................16 4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 19
5.1. "all" .....................................................17 4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 19
5.2. "include" .................................................18 4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 20
5.3. "a" .......................................................19 4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 20
5.4. "mx" ......................................................20 4.7. Default Result . . . . . . . . . . . . . . . . . . . . . . 21
5.5. "ptr" .....................................................20 4.8. Domain Specification . . . . . . . . . . . . . . . . . . . 21
5.6. "ip4" and "ip6" ...........................................21 5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 22
5.7. "exists" ..................................................22 5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6. Modifier Definitions ...........................................22 5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. redirect: Redirected Query ................................23 5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2. exp: Explanation ..........................................23 5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7. The Received-SPF Header Field ..................................25 5.5. "ptr" (deprecated) . . . . . . . . . . . . . . . . . . . . 25
8. Macros .........................................................27 5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 27
8.1. Macro Definitions .........................................27 5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.2. Expansion Examples ........................................30 6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 29
9. Implications ...................................................31 6.1. redirect: Redirected Query . . . . . . . . . . . . . . . . 29
9.1. Sending Domains ...........................................31 6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . . 30
9.2. Mailing Lists .............................................32 7. Recording The Result . . . . . . . . . . . . . . . . . . . . . 32
9.3. Forwarding Services and Aliases ...........................32 7.1. The Received-SPF Header Field . . . . . . . . . . . . . . 32
9.4. Mail Services .............................................34 7.2. SPF Results in the Authentication-Results Header Field . . 34
9.5. MTA Relays ................................................34 8. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10. Security Considerations .......................................35 8.1. Macro Definitions . . . . . . . . . . . . . . . . . . . . 36
10.1. Processing Limits ........................................35 8.2. Expansion Examples . . . . . . . . . . . . . . . . . . . . 39
10.2. SPF-Authorized E-Mail May Contain Other False 9. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 41
Identities ...............................................37 9.1. Sending Domains . . . . . . . . . . . . . . . . . . . . . 41
10.3. Spoofed DNS and IP Data ..................................37 9.1.1. DNS Resource Considerations . . . . . . . . . . . . . 41
10.4. Cross-User Forgery .......................................37 9.1.2. Administrator's Considerations . . . . . . . . . . . . 42
10.5. Untrusted Information Sources ............................38 9.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . . 43
10.6. Privacy Exposure .........................................38 9.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 43
11. Contributors and Acknowledgements .............................38 9.2.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . 43
12. IANA Considerations ...........................................39 9.2.2. Forwarding Services and Aliases . . . . . . . . . . . 44
12.1. The SPF DNS Record Type ..................................39 9.2.3. Mail Services . . . . . . . . . . . . . . . . . . . . 46
12.2. The Received-SPF Mail Header Field .......................39 9.2.4. MTA Relays . . . . . . . . . . . . . . . . . . . . . . 46
13. References ....................................................39 9.3. Receivers . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. Normative References .....................................39 9.3.1. Policy For SPF Pass . . . . . . . . . . . . . . . . . 47
13.2. Informative References ...................................40 9.3.2. Policy For SPF Fail . . . . . . . . . . . . . . . . . 47
Appendix A. Collected ABNF .......................................42 9.3.3. Policy For SPF Permerror . . . . . . . . . . . . . . . 47
Appendix B. Extended Examples ....................................44 10. Security Considerations . . . . . . . . . . . . . . . . . . . 49
B.1. Simple Examples ..........................................44 10.1. Processing Limits . . . . . . . . . . . . . . . . . . . . 49
B.2. Multiple Domain Example ..................................45 10.2. SPF-Authorized Email May Contain Other False Identities . 49
B.3. DNSBL Style Example ......................................46 10.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . . 50
B.4. Multiple Requirements Example ............................46 10.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . . 50
10.5. Untrusted Information Sources . . . . . . . . . . . . . . 50
10.5.1. Recorded Results . . . . . . . . . . . . . . . . . . . 50
10.5.2. External Explanations . . . . . . . . . . . . . . . . 51
10.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . . 51
10.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . . 51
11. Contributors and Acknowledgements . . . . . . . . . . . . . . 52
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
12.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . . 53
12.2. The Received-SPF Mail Header Field . . . . . . . . . . . . 53
12.3. SPF Modifier Registration . . . . . . . . . . . . . . . . 53
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54
13.1. Normative References . . . . . . . . . . . . . . . . . . . 54
13.2. Informative References . . . . . . . . . . . . . . . . . . 55
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 57
Appendix B. Extended Examples . . . . . . . . . . . . . . . . . . 60
B.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 60
B.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 61
B.3. DNSBL Style Example . . . . . . . . . . . . . . . . . . . 62
B.4. Multiple Requirements Example . . . . . . . . . . . . . . 62
Appendix C. Change History . . . . . . . . . . . . . . . . . . . 63
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 66
1. Introduction 1. Introduction
The current E-Mail infrastructure has the property that any host The current email infrastructure has the property that any host
injecting mail into the mail system can identify itself as any domain injecting mail into the system can use any DNS domain name it wants
name it wants. Hosts can do this at a variety of levels: in in each of the various identifiers specified by [RFC5321] and
particular, the session, the envelope, and the mail headers. [RFC5322]. Although this feature is desirable in some circumstances,
Although this feature is desirable in some circumstances, it is a it is a major obstacle to reducing Unsolicited Bulk email (UBE, aka
major obstacle to reducing Unsolicited Bulk E-Mail (UBE, aka spam). spam). Furthermore, many domain owning ADMDs (ADministrative
Furthermore, many domain name holders are understandably concerned Management Domains, see [RFC5598]) are understandably concerned about
about the ease with which other entities may make use of their domain the ease with which other entities can make use of their domain
names, often with malicious intent. names, often with malicious intent.
This document defines a protocol by which domain owners may authorize This document defines a protocol by which ADMDs can authorize hosts
hosts to use their domain name in the "MAIL FROM" or "HELO" identity. to use their domain names in the "MAIL FROM" or "HELO" identities.
Compliant domain holders publish Sender Policy Framework (SPF) Compliant ADMDs publish Sender Policy Framework (SPF) records in DNS
records specifying which hosts are permitted to use their names, and specifying which hosts are permitted to use their names, and
compliant mail receivers use the published SPF records to test the compliant mail receivers use the published SPF records to test the
authorization of sending Mail Transfer Agents (MTAs) using a given authorization of sending Mail Transfer Agents (MTAs) using a given
"HELO" or "MAIL FROM" identity during a mail transaction. "HELO" or "MAIL FROM" identity during a mail transaction.
An additional benefit to mail receivers is that after the use of an An additional benefit to mail receivers is that after the use of an
identity is verified, local policy decisions about the mail can be 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. 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 This is advantageous because reputation of domain names is likely to
be more accurate than reputation of host IP addresses. Furthermore, be more accurate than reputation of host IP addresses. Furthermore,
if a claimed identity fails verification, local policy can take if a claimed identity fails verification, local policy can take
stronger action against such E-Mail, such as rejecting it. stronger action against such email, such as rejecting it.
1.1. Protocol Status 1.1. Protocol Status
SPF has been in development since the summer of 2003 and has seen SPF has been in development since the summer of 2003 and has seen
deployment beyond the developers beginning in December 2003. The deployment beyond the developers beginning in December 2003. The
design of SPF slowly evolved until the spring of 2004 and has since 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 stabilized. There have been quite a number of forms of SPF, some
written up as documents, some submitted as Internet Drafts, and many written up as documents, some submitted as Internet Drafts, and many
discussed and debated in development forums. discussed and debated in development forums. The protocol was
originally defined in [RFC4408], which this document replaces.
The goal of this document is to clearly document the protocol defined The goal of this work is to clearly document the protocol defined by
by earlier draft specifications of SPF as used in existing earlier draft specifications of SPF as used in existing
implementations. This conception of SPF is sometimes called "SPF implementations. This conception of SPF is sometimes called "SPF
Classic". It is understood that particular implementations and Classic". It is understood that particular implementations and
deployments may differ from, and build upon, this work. It is hoped deployments will differ from, and build upon, this work. It is hoped
that we have nonetheless captured the common understanding of SPF that we have nonetheless captured the common understanding of SPF
version 1. version 1.
1.2. Terminology 1.2. Experimental History
This document updates and replaces RFC 4408 that was part of a group
of simultaneously published Experimental RFCs (RFC 4405, RFC 4406,
RFC 4407, and RFC 4408) in 2006. At that time the IESG requested the
community observe the success or failure of the two approaches
documented in these RFCs during the two years following publication,
in order that a community consensus could be reached in the future.
SPF is widely deployed by large and small email providers alike.
There are multiple, interoperable implementations.
For SPF (as documented in RFC 4408) a careful effort was made to
collect and document lessons learned and errata during the two year
period. The errata list has been stable (no new submissions) and
only minor protocol lessons learned were identified. Resolution of
the IESG's experiment is documented in [RFC6686].
1.3. Terminology
1.3.1. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.3.2. Imported Definitions
The ABNF tokens "ALPHA", "DIGIT", and "SP" are defined in [RFC5234].
The token "local-part" is defined in [RFC5321].
"dot-atom", "quoted-string", "comment", "CFWS", "FWS", and "CRLF" are
defined in [RFC5322].
1.3.3. Mail From Definition
This document is concerned with the portion of a mail message This document is concerned with the portion of a mail message
commonly called "envelope sender", "return path", "reverse path", commonly called "envelope sender", "return path", "reverse path",
"bounce address", "2821 FROM", or "MAIL FROM". Since these terms are "bounce address", "5321 FROM", "MAIL FROM", or RFC5321.MailFrom.
either not well defined or often used casually, this document defines Since these terms are either not well defined or often used casually,
the "MAIL FROM" identity in Section 2.2. Note that other terms that this document uses "MAIL FROM" for consistency. This means the
may superficially look like the common terms, such as "reverse-path", RFC5321.MailFrom as defined in [RFC5598]. Note that other terms that
are used only with the defined meanings from normative documents. might superficially look like the common terms, such as "reverse-
path", are used only with the defined meanings from normative
documents.
2. Operation 1.3.4. HELO Definition
2.1. The HELO Identity This document also makes use of the HELO/EHLO identity. The "HELO"
identity derives from either the SMTP HELO or EHLO command (see
[RFC5321]). Since HELO and EHLO can, in many cases, be used
interchangeably, they are identified commonly as "HELO" in this
document. This means RFC5321.HELO/.EHLO as defined in [RFC5598].
These commands supply the identity of the SMTP client (sending host)
for the SMTP session.
The "HELO" identity derives from either the SMTP HELO or EHLO command 1.3.5. Deprecated
(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" There are [RFC4408] features that are marked "deprecated". In the
context of this document, deprecated means that senders SHOULD NOT
publish SPF records that make use of such features because they might
be removed entirely in future updates to the protocol. Such features
do, however, remain part of the SPF protocol and receiving systems
MUST support them unless this document explicitly says otherwise.
2. Operation
2.1. The "HELO" Identity
It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"
identity, but also separately check the "HELO" identity by applying identity, but also separately check the "HELO" identity by applying
the check_host() function (Section 4) to the "HELO" identity as the the check_host() function (Section 4) to the "HELO" identity as the
<sender>. <sender>. Checking "HELO" promotes consistency of results and can
reduce DNS resource usage. Additionally, since SPF records published
for "HELO" identities refer to a single host, when available, they
are a very reliable source of host authorization status.
2.2. The MAIL FROM Identity Note that requirements for the domain presented in the EHLO or HELO
command are not always clear to the sending party, and SPF verifiers
MUST be prepared for the "HELO" identity to be malformed or an IP
address literal. SPF checks can only be performed when the "HELO"
string is a valid fully qualified domain.
The "MAIL FROM" identity derives from the SMTP MAIL command (see 2.2. The "MAIL FROM" Identity
[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 SPF verifiers MUST check the ""MAIL FROM" identity if a completed
RFC 2821). In this case, there is no explicit sender mailbox, and "HELO" check has not reached a definitive policy result by applying
the check_host() function to the "MAIL FROM" identity as the
<sender>.
[RFC5321] allows the reverse-path to be null (see Section 4.5.5 in
[RFC5321]). In this case, there is no explicit sender mailbox, and
such a message can be assumed to be a notification message from the such a message can be assumed to be a notification message from the
mail system itself. When the reverse-path is null, this document mail system itself. When the reverse-path is null, this document
defines the "MAIL FROM" identity to be the mailbox composed of the defines the "MAIL FROM" identity to be the mailbox composed of the
localpart "postmaster" and the "HELO" identity (which may or may not local-part "postmaster" and the "HELO" identity (which might or might
have been checked separately before). 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 2.3. Publishing Authorization
An SPF-compliant domain MUST publish a valid SPF record as described 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 in Section 3. This record authorizes the use of the domain name in
the "HELO" and "MAIL FROM" identities by the MTAs it specifies. the "HELO" and "MAIL FROM" identities by the MTAs it specifies.
If domain owners choose to publish SPF records, it is RECOMMENDED SPF results can be used to make both positive (source is authorized)
that they end in "-all", or redirect to other records that do, so and negative (source is not authorized) determinations. If domain
that a definitive determination of authorization can be made. owners choose to publish SPF records and want to support receivers
making negative authorization determinations, then they MUST publish
records that end in "-all", or redirect to other records that do,
otherwise, no definitive determination of authorization can be made.
Potential issues and mitigations associated with negative
determinations are discussed in Section 9.
Domain holders may publish SPF records that explicitly authorize no Domain holders can publish SPF records that explicitly authorize no
hosts if mail should never originate using that domain. hosts if mail is not expected to originate using that domain.
When changing SPF records, care must be taken to ensure that there is 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 a transition period so that the old policy remains valid until all
legitimate E-Mail has been checked. legitimate email can reasonably expect to have been checked. This
can be as much as 30 days.
2.4. Checking Authorization 2.4. Checking Authorization
A mail receiver can perform a set of SPF checks for each mail message 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 it receives. An SPF check tests the authorization of a client host
to emit mail with a given identity. Typically, such checks are done to emit mail with a given identity. Typically, such checks are done
by a receiving MTA, but can be performed elsewhere in the mail by a receiving MTA, but can be performed elsewhere in the mail
processing chain so long as the required information is available and processing chain so long as the required information is available and
reliable. At least the "MAIL FROM" identity MUST be checked, but it reliable. At least the "MAIL FROM" identity MUST be checked, but it
is RECOMMENDED that the "HELO" identity also be checked beforehand. is RECOMMENDED that the "HELO" identity also be checked beforehand.
Without explicit approval of the domain owner, checking other Without explicit approval of the domain owner, checking other
identities against SPF version 1 records is NOT RECOMMENDED because identities against SPF version 1 records is NOT RECOMMENDED because
there are cases that are known to give incorrect results. For there are cases that are known to give incorrect results. For
example, almost all mailing lists rewrite the "MAIL FROM" identity example, almost all mailing lists rewrite the "MAIL FROM" identity
(see Section 9.2), but some do not change any other identities in the (see Section 9.2.1), but some do not change any other identities in
message. The scenario described in Section 9.3, sub-section 1.2, is the message. The scenario described in Section 9.2.2, sub-section
another example. Documents that define other identities should 1.2, is another example. Documents that define other identities will
define the method for explicit approval. have to define the method for explicit approval.
It is possible that mail receivers will use the SPF check as part of 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 a larger set of tests on incoming mail. The results of other tests
may influence whether or not a particular SPF check is performed. might influence whether or not a particular SPF check is performed.
For example, finding the sending host's IP address on a local white For example, finding the sending host's IP address on a local white
list may cause all other tests to be skipped and all mail from that list might cause all other tests to be skipped and all mail from that
host to be accepted. host to be accepted.
When a mail receiver decides to perform an SPF check, it MUST use a When a mail receiver decides to perform an SPF check, it MUST use a
correctly-implemented check_host() function (Section 4) evaluated correctly-implemented check_host() function (Section 4) evaluated
with the correct parameters. Although the test as a whole is with the correct parameters. Although the test as a whole is
optional, once it has been decided to perform a test it must be optional, once it has been decided to perform a test it must be
performed as specified so that the correct semantics are preserved performed as specified so that the correct semantics are preserved
between publisher and receiver. between publisher and receiver.
To make the test, the mail receiver MUST evaluate the check_host() To make the test, the mail receiver MUST evaluate the check_host()
function with the arguments set as follows: function with the arguments set as follows:
<ip> - the IP address of the SMTP client that is emitting the <ip> - the IP address of the SMTP client that is emitting the
mail, either IPv4 or IPv6. mail, either IPv4 or IPv6.
<domain> - the domain portion of the "MAIL FROM" or "HELO" identity. <domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
<sender> - 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. Note that the <domain> argument might not be a well-formed domain
For example, if the reverse-path was null, then the EHLO/HELO domain name. For example, if the reverse-path was null, then the EHLO/HELO
is used, with its associated problems (see Section 2.1). In these domain is used, with its associated problems (see Section 2.1). In
cases, check_host() is defined in Section 4.3 to return a "None" these cases, check_host() is defined in Section 4.3 to return a
result. "none" result.
Although invalid, malformed, or non-existent domains cause SPF checks Although invalid, malformed, or non-existent domains cause SPF checks
to return "None" because no SPF record can be found, it has long been 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, the policy of many MTAs to reject email from such domains, especially
especially in the case of invalid "MAIL FROM". In order to prevent in the case of invalid "MAIL FROM". Rejecting email will prevent one
the circumvention of SPF records, rejecting E-Mail from invalid method of circumventing of SPF records.
domains should be considered.
Implementations must take care to correctly extract the <domain> from Implementations MUST take care to correctly extract the <domain> from
the data given with the SMTP MAIL FROM command as many MTAs will the data given with the SMTP MAIL FROM command as many MTAs will
still accept such things as source routes (see [RFC2821], Appendix still accept such things as source routes (see [RFC5321], Appendix
C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]). C), the %-hack (see [RFC1123]), and bang paths (see [RFC1983]).
These archaic features have been maliciously used to bypass security These archaic features have been maliciously used to bypass security
systems. systems.
2.5. Interpreting the Result 2.5. Interpreting the Result
This section describes how software that performs the authorization This section describes how software that performs the authorization
should interpret the results of the check_host() function. The interprets the results of the check_host() function. The
authorization check SHOULD be performed during the processing of the authorization check SHOULD be performed during the processing of the
SMTP transaction that sends the mail. This allows errors to be SMTP transaction that sends the mail. This allows errors to be
returned directly to the sending MTA by way of SMTP replies. returned directly to the sending MTA by way of SMTP replies.
Performing the authorization after the SMTP transaction has finished Performing the authorization other than using the return-path and
may cause problems, such as the following: (1) It may be difficult to client address at the time of the MAIL command during the SMTP
accurately extract the required information from potentially transaction can cause problems, such as the following: (1) It might
deceptive headers; (2) legitimate E-Mail may fail because the be difficult to accurately extract the required information from
sender's policy may have since changed. potentially deceptive headers; (2) legitimate email might fail
because the sender's policy had since changed.
Generating non-delivery notifications to forged identities that have Generating non-delivery notifications to forged identities that have
failed the authorization check is generally abusive and against the failed the authorization check is a source of backscatter and SHOULD
explicit wishes of the identity owner. be avoided. [RFC3834] section 2 describes backscatter and the
problems it causes.
2.5.1. None 2.5.1. None
A result of "None" means that no records were published by the domain A result of "none" means either (a) no syntactically valid DNS domain
or that no checkable sender domain could be determined from the given name was extracted from the SMTP session that could be used as the
identity. The checking software cannot ascertain whether or not the one to be authorized, or (b) no TXT records were retrieved from the
client host is authorized. DNS that appeared to be intended for use by SPF verifiers.
2.5.2. Neutral 2.5.2. Neutral
The domain owner has explicitly stated that he cannot or does not The domain owner has explicitly stated that they cannot or do not
want to assert whether or not the IP address is authorized. A want to assert whether the IP address is authorized or not. A
"Neutral" result MUST be treated exactly like the "None" result; the "neutral" result MUST be treated exactly like the "none" result; the
distinction exists only for informational purposes. Treating distinction exists only for informational purposes. Treating
"Neutral" more harshly than "None" would discourage domain owners "neutral" more harshly than "none" would discourage domain owners
from testing the use of SPF records (see Section 9.1). from testing the use of SPF records (see Section 9.1).
2.5.3. Pass 2.5.3. Pass
A "Pass" result means that the client is authorized to inject mail A "pass" result means that the client is authorized to inject mail
with the given identity. The domain can now, in the sense of with the given identity. The domain can now, in the sense of
reputation, be considered responsible for sending the message. reputation, be considered responsible for sending the message.
Further policy checks can now proceed with confidence in the Further policy checks can now proceed with confidence in the
legitimate use of the identity. legitimate use of the identity.
2.5.4. Fail 2.5.4. Fail
A "Fail" result is an explicit statement that the client is not A "fail" result is an explicit statement that the client is not
authorized to use the domain in the given identity. The checking authorized to use the domain in the given identity. Disposition of
software can choose to mark the mail based on this or to reject the SPF fail messages is a matter of local policy. See Section 9.3 for
mail outright. considerations on developing local policy.
If the checking software chooses to reject the mail during the SMTP If the checking software chooses to reject the mail during the SMTP
transaction, then it SHOULD use an SMTP reply code of 550 (see transaction, then it SHOULD use an SMTP reply code of 550 (see
[RFC2821]) and, if supported, the 5.7.1 Delivery Status Notification [RFC5321]) and, if supported, the 5.7.1 enhanced status code (see
(DSN) code (see [RFC3464]), in addition to an appropriate reply text. [RFC3463]), in addition to an appropriate reply text. The
The check_host() function may return either a default explanation check_host() function will return either a default explanation string
string or one from the domain that published the SPF records (see or one from the domain that published the SPF records (see
Section 6.2). If the information does not originate with the Section 6.2). If the information does not originate with the
checking software, it should be made clear that the text is provided checking software, it should be made clear that the text is provided
by the sender's domain. For example: by the sender's domain. For example:
550-5.7.1 SPF MAIL FROM check failed: 550-5.7.1 SPF MAIL FROM check failed:
550-5.7.1 The domain example.com explains: 550-5.7.1 The domain example.com explains:
550 5.7.1 Please see http://www.example.com/mailpolicy.html 550 5.7.1 Please see http://www.example.com/mailpolicy.html
2.5.5. SoftFail If the checking software chooses not to reject the mail during the
SMTP transaction, then is SHOULD add a Received-SPF or
Authentication-Results header field (see Section 7) to communicate
this result to downstream message processors. While this is true for
all SPF results, it is of particular importance for "fail" results
since the message is explicitly not authorized by the domain owner.
A "SoftFail" result should be treated as somewhere between a "Fail" 2.5.5. Softfail
and a "Neutral". The domain believes the host is not authorized but
is not willing to make that strong of a statement. Receiving A "softfail" result ought to be treated as somewhere between "fail"
and "neutral"/"none". The domain believes the host is not authorized
but is not willing to make a strong policy statement. Receiving
software SHOULD NOT reject the message based solely on this result, software SHOULD NOT reject the message based solely on this result,
but MAY subject the message to closer scrutiny than normal. but MAY subject the message to closer scrutiny than normal.
The domain owner wants to discourage the use of this host and thus The domain owner wants to discourage the use of this host and thus
desires limited feedback when a "SoftFail" result occurs. For desires limited feedback when a "softfail" result occurs. For
example, the recipient's Mail User Agent (MUA) could highlight the example, the recipient's Mail User Agent (MUA) could highlight the
"SoftFail" status, or the receiving MTA could give the sender a "softfail" status, or the receiving MTA could give the sender a
message using a technique called "greylisting" whereby the MTA can message using greylisting, [RFC6647], with a note the first time the
issue an SMTP reply code of 451 (4.3.0 DSN code) with a note the message is received, but accept it on a later attempt based on
first time the message is received, but accept it the second time. receiver policy.
2.5.6. TempError 2.5.6. TempError
A "TempError" result means that the SPF client encountered a A "temperror" result means the SPF verifier encountered a transient
transient error while performing the check. Checking software can (DNS) error while performing the check. Checking software can choose
choose to accept or temporarily reject the message. If the message to accept or temporarily reject the message. If the message is
is rejected during the SMTP transaction for this reason, the software 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 SHOULD use an SMTP reply code of 451 and, if supported, the 4.4.3
code. enhanced status code. These errors can be caused by problems in
either the sender's or receiver's DNS software.
2.5.7. PermError 2.5.7. PermError
A "PermError" result means that the domain's published records could A "permerror" result means the domain's published records could not
not be correctly interpreted. This signals an error condition that be correctly interpreted. This signals an error condition that
requires manual intervention to be resolved, as opposed to the definitely requires manual intervention to be resolved. If the
TempError result. Be aware that if the domain owner uses macros message is rejected during the SMTP transaction for this reason, the
(Section 8), it is possible that this result is due to the checked software SHOULD use an SMTP reply code of 550 and, if supported, the
identities having an unexpected format. 5.5.2 enhanced status code. Be aware that if the domain owner uses
macros (Section 8), it is possible that this result is due to the
checked identities having an unexpected format.
3. SPF Records 3. SPF Records
An SPF record is a DNS Resource Record (RR) that declares which hosts An SPF record is a DNS TXT (type 16) Resource Record (RR) that
are, and are not, authorized to use a domain name for the "HELO" and declares which hosts are, and are not, authorized to use a domain
"MAIL FROM" identities. Loosely, the record partitions all hosts name for the "HELO" and "MAIL FROM" identities. Loosely, the record
into permitted and not-permitted sets (though some hosts might fall partitions all hosts into permitted and not-permitted sets (though
into neither category). some hosts might fall into neither category).
The SPF record is a single string of text. An example record is the The SPF record is a single string of text. An example record is the
following: following:
v=spf1 +mx a:colo.example.com/28 -all v=spf1 +mx a:colo.example.com/28 -all
This record has a version of "spf1" and three directives: "+mx", This record has a version of "spf1" and three directives: "+mx",
"a:colo.example.com/28" (the + is implied), and "-all". "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 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 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 records [RFC2782].
Section 3.1.1) is used.
The example above in Section 3 might be published via these lines in The example in Section 3 might be published via these lines in a
a domain zone file: domain zone file:
example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all" example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all"
smtp-out.example.com. TXT "v=spf1 a -all" smtp-out.example.com. TXT "v=spf1 a -all"
When publishing via TXT records, beware of other TXT records Since TXT records have multiple uses, beware of other TXT records
published there for other purposes. They may cause problems with published there for other purposes. They might cause problems with
size limits (see Section 3.1.4). size limits (see Section 3.4) and care MUST be taken to ensure only
SPF records are used for SPF processing.
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 Domains publishing records SHOULD try to keep the number of "include"
not optimal, but it is necessary because there are a number of DNS mechanisms and chained "redirect" modifiers to a minimum. Domains
server and resolver implementations in common use that cannot handle SHOULD also try to minimize the amount of other DNS information
the new RR type. The two-record-type scheme provides a forward path needed to evaluate a record. Section 4.6.4 and Section 9.1.1 provide
to the better solution of using an RR type reserved for this purpose. some suggestions on how to achieve this.
An SPF-compliant domain name SHOULD have SPF records of both RR 3.1. DNS Resource Records
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" SPF records MUST be published as type TXT [RFC1035]. The character
example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all" content of the record is encoded as [US-ASCII]. Use of alternate DNS
Example RRs in this document are shown with the TXT record type; RR types was supported in SPF's experimental phase, but has been
however, they could be published with the SPF type or with both discontinued. See Appendix A of [RFC6686] for further information.
types.
3.1.2. Multiple DNS Records 3.2. Multiple DNS Records
A domain name MUST NOT have multiple records that would cause an A domain name MUST NOT have multiple records that would cause an
authorization check to select more than one record. See Section 4.5 authorization check to select more than one record. See Section 4.5
for the selection rules. for the selection rules.
3.1.3. Multiple Strings in a Single DNS record 3.3. Multiple Strings in a Single DNS record
As defined in [RFC1035] sections 3.3.14 and 3.3, a single text DNS 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 record can be composed of more than one string. If a published
string. If a published record contains multiple strings, then the record contains multiple character-strings, then the record MUST be
record MUST be treated as if those strings are concatenated together treated as if those strings are concatenated together without adding
without adding spaces. For example: spaces. For example:
IN TXT "v=spf1 .... first" "second string..." IN TXT "v=spf1 .... first" "second string..."
MUST be treated as equivalent to MUST be treated as equivalent to
IN TXT "v=spf1 .... firstsecond string..." IN TXT "v=spf1 .... firstsecond string..."
SPF or TXT records containing multiple strings are useful in TXT records containing multiple strings are useful in constructing
constructing records that would exceed the 255-byte maximum length of records that would exceed the 255-byte maximum length of a character-
a string within a single TXT or SPF RR record. string within a single TXT record.
3.1.4. Record Size 3.4. Record Size
The published SPF record for a given domain name SHOULD remain small 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. 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 This UDP limit is defined in [RFC1035] section 2.3.4. This will keep
TCP. Since the answer size is dependent on many things outside the even older DNS implementations from falling over to TCP. Since the
scope of this document, it is only possible to give this guideline: answer size is dependent on many things outside the scope of this
If the combined length of the DNS name and the text of all the document, it is only possible to give this guideline: If the combined
records of a given type (TXT or SPF) is under 450 characters, then length of the DNS name and the text of all the records of a given
DNS answers should fit in UDP packets. Note that when computing the type is under 450 characters, then DNS answers ought to fit in UDP
sizes for queries of the TXT format, one must take into account any packets. Note that when computing the sizes for queries of the TXT
other TXT records published at the domain name. Records that are too format, one must take into account any other TXT records published at
long to fit in a single UDP packet MAY be silently ignored by SPF the domain name. Records that are too long to fit in a single UDP
clients. packet could be silently ignored by SPF verifiers due to firewall and
other issues that cause DNS over TCP to be less reliable than DNS
over UDP.
3.1.5. Wildcard Records 3.5. Wildcard Records
Use of wildcard records for publishing is not recommended. Care must Use of wildcard records for publishing is NOT RECOMMENDED. Care must
be taken if wildcard records are used. If a domain publishes be taken if wildcard records are used. If a domain publishes
wildcard MX records, it may want to publish wildcard declarations, wildcard MX records, it might want to publish wildcard declarations,
subject to the same requirements and problems. In particular, the subject to the same requirements and problems. In particular, the
declaration must be repeated for any host that has any RR records at declaration MUST be repeated for any host that has any RR records at
all, and for subdomains thereof. For example, the example given in all, and for subdomains thereof. For example, the example given in
[RFC1034], Section 4.3.3, could be extended with the following: [RFC1034], Section 4.3.3, could be extended with the following:
X.COM. MX 10 A.X.COM X.COM. MX 10 A.X.COM
X.COM. TXT "v=spf1 a:A.X.COM -all" X.COM. TXT "v=spf1 a:A.X.COM -all"
*.X.COM. MX 10 A.X.COM *.X.COM. MX 10 A.X.COM
*.X.COM. TXT "v=spf1 a:A.X.COM -all" *.X.COM. TXT "v=spf1 a:A.X.COM -all"
A.X.COM. A 1.2.3.4 A.X.COM. A 1.2.3.4
A.X.COM. MX 10 A.X.COM A.X.COM. MX 10 A.X.COM
A.X.COM. TXT "v=spf1 a:A.X.COM -all" A.X.COM. TXT "v=spf1 a:A.X.COM -all"
*.A.X.COM. MX 10 A.X.COM *.A.X.COM. MX 10 A.X.COM
*.A.X.COM. TXT "v=spf1 a:A.X.COM -all" *.A.X.COM. TXT "v=spf1 a:A.X.COM -all"
Notice that SPF records must be repeated twice for every name within SPF records MUST be listed twice for every name within the domain:
the domain: once for the name, and once with a wildcard to cover the once for the name, and once with a wildcard to cover the tree under
tree under the name. the name in order to cover all domains in use in outgoing mail.
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 4. The check_host() Function
This description is not an API (Application Program Interface)
definition, but rather a function description used to illustrate the
algorithm. A compliant SPF implementation MUST do something
semantically equivalent to this description.
The check_host() function fetches SPF records, parses them, and The check_host() function fetches SPF records, parses them, and
interprets them to determine whether a particular host is or is not evaluates them to determine whether a particular host is or is not
permitted to send mail with a given identity. Mail receivers that permitted to send mail with a given identity. Mail receivers that
perform this check MUST correctly evaluate the check_host() function perform this check MUST correctly evaluate the check_host() function
as described here. as described here.
Implementations MAY use a different algorithm than the canonical Implementations MAY use a different algorithm than the canonical
algorithm defined here, so long as the results are the same in all algorithm defined here, so long as the results are the same in all
cases. cases.
4.1. Arguments 4.1. Arguments
skipping to change at page 13, line 12 skipping to change at page 17, line 40
information; initially, the domain portion of the "MAIL information; initially, the domain portion of the "MAIL
FROM" or "HELO" identity. FROM" or "HELO" identity.
<sender> - the "MAIL FROM" or "HELO" identity. <sender> - the "MAIL FROM" or "HELO" identity.
The domain portion of <sender> will usually be the same as the The domain portion of <sender> will usually be the same as the
<domain> argument when check_host() is initially evaluated. However, <domain> argument when check_host() is initially evaluated. However,
this will generally not be true for recursive evaluations (see this will generally not be true for recursive evaluations (see
Section 5.2 below). Section 5.2 below).
Actual implementations of the check_host() function may need
additional arguments.
4.2. Results 4.2. Results
The function check_host() can return one of several results described 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 in Section 2.5. Based on the result, the action to be taken is
determined by the local policies of the receiver. determined by the local policies of the receiver.
4.3. Initial Processing 4.3. Initial Processing
If the <domain> is malformed (label longer than 63 characters, zero- If the <domain> is malformed (e.g. label longer than 63 characters,
length label not at the end, etc.) or is not a fully qualified domain zero-length label not at the end, etc.) or is not a fully qualified
name, or if the DNS lookup returns "domain does not exist" (RCODE 3), domain name, or if the DNS lookup returns "domain does not exist"
check_host() immediately returns the result "None". (RCODE 3), check_host() immediately returns the result "none".
Properly formed domains are fully qualified email domains as
described in [RFC5321] Section 2.3.5. Internationalized domain names
MUST be encoded as A-labels, as described in Section 2.3 of
[RFC5890].
If the <sender> has no localpart, substitute the string "postmaster" If the <sender> has no local-part, substitute the string "postmaster"
for the localpart. for the local-part.
4.4. Record Lookup 4.4. Record Lookup
In accordance with how the records are published (see Section 3.1 In accordance with how the records are published (see Section 3
above), a DNS query needs to be made for the <domain> name, querying 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 for type TXT only.
looked up, the queries MAY be done in parallel.
If all DNS lookups that are made return a server failure (RCODE 2), 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 or other error (RCODE other than 0 or 3), or time out, then
check_host() exits immediately with the result "TempError". check_host() terminates immediately with the result "temperror".
Alternatively, for a server failure (RCODE 2) result, check_host()
MAY track failures and treat multiple failures within 24 hours for
the same domain as "permerror".
This alternative is intended to shorten the queue time of messages
that cannot be accepted, by returning a permanent negative completion
reply code to the client, instead of a transient one. [RFC2308]
suggests on an algorithm for doing such tracking and handling of
server failure codes.
4.5. Selecting Records 4.5. Selecting Records
Records begin with a version section: Records begin with a version section:
record = version terms *SP record = version terms *SP
version = "v=spf1" version = "v=spf1"
Starting with the set of records that were returned by the lookup, Starting with the set of records that were returned by the lookup,
record selection proceeds in two steps: discard records that do not begin with a version section of exactly
"v=spf1". Note that the version section is terminated either by an
1. Records that do not begin with a version section of exactly SP character or the end of the record. A record with a version
"v=spf1" are discarded. Note that the version section is section of "v=spf10" does not match and must be discarded.
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 If the resultant record set includes no records, check_host()
the domain makes no SPF declarations. SPF processing MUST stop and produces the "none" result. If the resultant record set includes
return "None". more than one record, check_host() produces the "permerror" result.
4.6. Record Evaluation 4.6. Record Evaluation
After one SPF record has been selected, the check_host() function After one SPF record has been selected, the check_host() function
parses and interprets it to find a result for the current test. If parses and interprets it to find a result for the current test. If
there are any syntax errors, check_host() returns immediately with there are any syntax errors, check_host() returns immediately with
the result "PermError". the result "permerror".
Implementations MAY choose to parse the entire record first and Implementations MAY choose to parse the entire record first and
return "PermError" if the record is not syntactically well formed. return "permerror" if the record is not syntactically well formed.
However, in all cases, any syntax errors anywhere in the record MUST However, in all cases, any syntax errors anywhere in the record MUST
be detected. be detected.
4.6.1. Term Evaluation 4.6.1. Term Evaluation
There are two types of terms: mechanisms and modifiers. A record There are two types of terms: mechanisms and modifiers. A record
contains an ordered list of these as specified in the following contains an ordered list of these as specified in the following
Augmented Backus-Naur Form (ABNF). Augmented Backus-Naur Form (ABNF).
terms = *( 1*SP ( directive / modifier ) ) terms = *( 1*SP ( directive / modifier ) )
directive = [ qualifier ] mechanism directive = [ qualifier ] mechanism
qualifier = "+" / "-" / "?" / "~" qualifier = "+" / "-" / "?" / "~"
mechanism = ( all / include mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists ) / A / MX / PTR / IP4 / IP6 / exists )
modifier = redirect / explanation / unknown-modifier modifier = redirect / explanation / unknown-modifier
unknown-modifier = name "=" macro-string unknown-modifier = name "=" macro-string
; where name is not any known modifier
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
Most mechanisms allow a ":" or "/" character after the name. Most mechanisms allow a ":" or "/" character after the name.
Modifiers always contain an equals ('=') character immediately after Modifiers always contain an equals ('=') character immediately after
the name, and before any ":" or "/" characters that may be part of the name, and before any ":" or "/" characters that might be part of
the macro-string. the macro-string.
Terms that do not contain any of "=", ":", or "/" are mechanisms, as Terms that do not contain any of "=", ":", or "/" are mechanisms, as
defined in Section 5. defined in Section 5.
As per the definition of the ABNF notation in [RFC4234], mechanism As per the definition of the ABNF notation in [RFC5234], mechanism
and modifier names are case-insensitive. and modifier names are case-insensitive.
4.6.2. Mechanisms 4.6.2. Mechanisms
Each mechanism is considered in turn from left to right. If there Each mechanism is considered in turn from left to right. If there
are no more mechanisms, the result is specified in Section 4.7. 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 When a mechanism is evaluated, one of three things can happen: it can
match, not match, or throw an exception. match, not match, or throw an exception.
If it matches, processing ends and the qualifier value is returned as If it matches, processing ends and the qualifier value is returned as
the result of that record. If it does not match, processing the result of that record. If it does not match, processing
continues with the next mechanism. If it throws an exception, continues with the next mechanism. If it returns an exception,
mechanism processing ends and the exception value is returned. mechanism processing ends and the exception value is returned.
The possible qualifiers, and the results they return are as follows: The possible qualifiers, and the results they cause check_host() to
return are as follows:
"+" Pass "+" pass
"-" Fail "-" fail
"~" SoftFail "~" softfail
"?" Neutral "?" neutral
The qualifier is optional and defaults to "+". The qualifier is optional and defaults to "+".
When a mechanism matches and the qualifier is "-", then a "Fail" When a mechanism matches and the qualifier is "-", then a "fail"
result is returned and the explanation string is computed as result is returned and the explanation string is computed as
described in Section 6.2. described in Section 6.2.
The specific mechanisms are described in Section 5. The specific mechanisms are described in Section 5.
4.6.3. Modifiers 4.6.3. Modifiers
Modifiers are not mechanisms: they do not return match or not-match. Modifiers are not mechanisms. They do not return match or not-match.
Instead they provide additional information. Although modifiers do Instead, they provide additional information. Although modifiers do
not directly affect the evaluation of the record, the "redirect" not directly affect the evaluation of the record, the "redirect"
modifier has an effect after all the mechanisms have been evaluated. modifier has an effect after all the mechanisms have been evaluated.
4.6.4. DNS Lookup Limits
SPF implementations MUST limit the number of mechanisms and modifiers
("terms") that cause any DNS query to at most 10 during SPF
evaluation. Specifically, the "include", "a", "mx", "ptr", and
"exists" mechanisms as well as the "redirect" modifier count against
this limit. The "all", "ip4", and "ip6" mechanisms do not count
against this limit. If this number is exceeded during a check, a
permerror MUST be returned. The "exp" modifier does not count
against this limit because the DNS lookup to fetch the explanation
string occurs after the SPF record evaluation has been completed.
When evaluating the "mx" and "ptr" mechanisms, or the %{p} macro,
there MUST be a limit of no more than 10 MX or PTR RRs looked up and
checked. If more than 10 "mx" or "ptr" records are returned for this
further lookup, a permerror MUST be returned. This limit is per
mechanism or macro in the record and in addition to the lookup limits
above.
MTAs or other processors SHOULD impose a limit on the maximum amount
of elapsed time to evaluate check_host(). Such a limit SHOULD allow
at least 20 seconds. If such a limit is exceeded, the result of
authorization SHOULD be "temperror".
4.7. Default Result 4.7. Default Result
If none of the mechanisms match and there is no "redirect" modifier, If none of the mechanisms match and there is no "redirect" modifier,
then the check_host() returns a result of "Neutral", just as if then the check_host() returns a result of "neutral", just as if
"?all" were specified as the last directive. If there is a "?all" were specified as the last directive. If there is a
"redirect" modifier, check_host() proceeds as defined in Section 6.1. "redirect" modifier, check_host() proceeds as defined in Section 6.1.
Note that records SHOULD always use either a "redirect" modifier or Note that records SHOULD always use either a "redirect" modifier or
an "all" mechanism to explicitly terminate processing. an "all" mechanism to explicitly terminate processing. Although the
latter has default (specifically "?all"), it aids debugging efforts
if it is explicitly included.
For example: For example:
v=spf1 +mx -all v=spf1 +mx -all
or or
v=spf1 +mx redirect=_spf.example.com v=spf1 +mx redirect=_spf.example.com
4.8. Domain Specification 4.8. Domain Specification
Several of these mechanisms and modifiers have a <domain-spec> Several of these mechanisms and modifiers have a domain-spec section.
section. The <domain-spec> string is macro expanded (see Section 8). The domain-spec string is subject to macro expansion (see Section 8).
The resulting string is the common presentation form of a fully- The resulting string is the common presentation form of a fully-
qualified DNS name: a series of labels separated by periods. This qualified DNS name: a series of labels separated by periods. This
domain is called the <target-name> in the rest of this document. 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 Note: The result of the macro expansion is not subject to any further
escaping. Hence, this facility cannot produce all characters that escaping. Hence, this facility cannot produce all characters that
are legal in a DNS label (e.g., the control characters). However, are legal in a DNS label (e.g., the control characters). However,
this facility is powerful enough to express legal host names and this facility is powerful enough to express legal host names and
common utility labels (such as "_spf") that are used in DNS. common utility labels (such as "_spf") that are used in DNS.
For several mechanisms, the <domain-spec> is optional. If it is not For several mechanisms, the <domain-spec> is optional. If it is not
provided, the <domain> is used as the <target-name>. provided, the <domain> is used as the <target-name>. Domain and
domain-spec are syntactically identical after macro expansion.
Domain is an input value for check_host() while domain-spec is
computed by check_host().
5. Mechanism Definitions 5. Mechanism Definitions
This section defines two types of mechanisms. This section defines two types of mechanisms.
Basic mechanisms contribute to the language framework. They do not Basic mechanisms contribute to the language framework. They do not
specify a particular type of authorization scheme. specify a particular type of authorization scheme.
all all
include include
Designated sender mechanisms are used to designate a set of <ip> Designated sender mechanisms are used to designate a set of <ip>
addresses as being permitted or not permitted to use the <domain> for addresses as being permitted or not permitted to use the <domain> for
sending mail. sending mail.
a a
mx mx
ptr ptr (deprecated)
ip4 ip4
ip6 ip6
exists exists
The following conventions apply to all mechanisms that perform a The following conventions apply to all mechanisms that perform a
comparison between <ip> and an IP address at any point: comparison between <ip> and an IP address at any point:
If no CIDR-length is given in the directive, then <ip> and the IP If no CIDR prefix length is given in the directive, then <ip> and the
address are compared for equality. (Here, CIDR is Classless Inter- IP address are compared for equality. (Here, CIDR is Classless
Domain Routing.) Inter-Domain Routing, described in [RFC4632].)
If a CIDR-length is specified, then only the specified number of If a CIDR prefix length is specified, then only the specified number
high-order bits of <ip> and the IP address are compared for equality. 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 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 <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 address, AAAA records are fetched. Even if the SMTP connection uses
via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513], Section IPv6, an IPv4-mapped IPv6 IP address (see [RFC4291], Section 2.5.5)
2.5.5) MUST still be considered an IPv4 address. MUST still be considered an IPv4 address and MUST be evaluated using
IPv4 mechanisms (i.e. "ip4" and "a").
Several mechanisms rely on information fetched from DNS. For these Several mechanisms rely on information fetched from the DNS. For
DNS queries, except where noted, if the DNS server returns an error these DNS queries, except where noted, if the DNS server returns an
(RCODE other than 0 or 3) or the query times out, the mechanism error (RCODE other than 0 or 3) or the query times out, the mechanism
throws the exception "TempError". If the server returns "domain does stops and the topmost check_host() returns "temperror". If the
not exist" (RCODE 3), then evaluation of the mechanism continues as server returns "domain does not exist" (RCODE 3), then evaluation of
if the server returned no error (RCODE 0) and zero answer records. the mechanism continues as if the server returned no error (RCODE 0)
and zero answer records.
5.1. "all" 5.1. "all"
all = "all" all = "all"
The "all" mechanism is a test that always matches. It is used as the The "all" mechanism is a test that always matches. It is used as the
rightmost mechanism in a record to provide an explicit default. rightmost mechanism in a record to provide an explicit default.
For example: For example:
v=spf1 a mx -all v=spf1 a mx -all
Mechanisms after "all" will never be tested. Any "redirect" modifier Mechanisms after "all" will never be tested. Mechanisms listed after
(Section 6.1) has no effect when there is an "all" mechanism. "all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be
ignored when there is an "all" mechanism in the record.
5.2. "include" 5.2. "include"
include = "include" ":" domain-spec include = "include" ":" domain-spec
The "include" mechanism triggers a recursive evaluation of The "include" mechanism triggers a recursive evaluation of
check_host(). The domain-spec is expanded as per Section 8. Then check_host().
check_host() is evaluated with the resulting string as the <domain>.
The <ip> and <sender> arguments remain the same as in the current 1. The domain-spec is expanded as per Section 8.
evaluation of check_host().
2. 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().
3. The recursive evaluation returns either match, not match, or an
error. If it matches, then the appropriate result for the
include: mechanism is used (e.g. include or +include gives a
"pass" result and -include gives "fail).
4. If there is no match, the parent check_host() resumes processing
as per the table below, with the previous value of <domain>
restored.
In hindsight, the name "include" was poorly chosen. Only the In hindsight, the name "include" was poorly chosen. Only the
evaluated result of the referenced SPF record is used, rather than evaluated result of the referenced SPF record is used, rather than
acting as if the referenced SPF record was literally included in the acting as if the referenced SPF record was literally included in the
first. For example, evaluating a "-all" directive in the referenced first. For example, evaluating a "-all" directive in the referenced
record does not terminate the overall processing and does not record does not terminate the overall processing and does not
necessarily result in an overall "Fail". (Better names for this necessarily result in an overall "fail". (Better names for this
mechanism would have been "if-pass", "on-pass", etc.) mechanism would have been "if-match", "on-match", etc.)
The "include" mechanism makes it possible for one domain to designate The "include" mechanism makes it possible for one domain to designate
multiple administratively-independent domains. For example, a vanity multiple administratively-independent domains. For example, a vanity
domain "example.net" might send mail using the servers of domain "example.net" might send mail using the servers of
administratively-independent domains example.com and example.org. administratively-independent domains example.com and example.org.
Example.net could say Example.net could say
IN TXT "v=spf1 include:example.com include:example.org -all" IN TXT "v=spf1 include:example.com include:example.org -all"
This would direct check_host() to, in effect, check the records of 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 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 were not permitted for either of those domains would the result be
"Fail". "fail".
Whether this mechanism matches, does not match, or throws an Whether this mechanism matches, does not match, or returns an
exception depends on the result of the recursive evaluation of exception depends on the result of the recursive evaluation of
check_host(): check_host():
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
| A recursive check_host() result | Causes the "include" mechanism | | A recursive check_host() result | Causes the "include" mechanism |
| of: | to: | | of: | to: |
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
| Pass | match | | pass | match |
| | | | | |
| Fail | not match | | fail | not match |
| | | | | |
| SoftFail | not match | | softfail | not match |
| | | | | |
| Neutral | not match | | neutral | not match |
| | | | | |
| TempError | throw TempError | | temperror | throw temperror |
| | | | | |
| PermError | throw PermError | | permerror | throw permerror |
| | | | | |
| None | throw PermError | | none | throw permerror |
+---------------------------------+---------------------------------+ +---------------------------------+---------------------------------+
The "include" mechanism is intended for crossing administrative The "include" mechanism is intended for crossing administrative
boundaries. Although it is possible to use includes to consolidate boundaries. For example, if example.com and example.org were managed
multiple domains that share the same set of designated hosts, domains by the same entity, and if the permitted set of hosts for both
are encouraged to use redirects where possible, and to minimize the domains was
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 "mx:example.com", it would be possible for example.org to specify
"include:example.com", but it would be preferable to specify "include:example.com", but it would be preferable to specify
"redirect=example.com" or even "mx:example.com". "redirect=example.com" or even "mx:example.com".
With the "include" mechanism an administratively external set of
hosts can be authorized, but determination of sender policy is still
a function of the original domain's SPF record (as determined by the
"all" mechanism in that record). The redirect modifier is more
suitable for consolidating both authorizations and policy into a
common set to be shared within an ADMD. Redirect is much more like a
common code element to be shared among records in a single ADMD. It
is possible to control both authorized hosts and policy for an
arbitrary number of domains from a single record.
5.3. "a" 5.3. "a"
This mechanism matches if <ip> is one of the <target-name>'s IP This mechanism matches if <ip> is one of the <target-name>'s IP
addresses. addresses.
A = "a" [ ":" domain-spec ] [ dual-cidr-length ] a = "a" [ ":" domain-spec ] [ dual-cidr-length ]
An address lookup is done on the <target-name>. The <ip> is compared An address lookup is done on the <target-name>. The <ip> is compared
to the returned address(es). If any address matches, the mechanism to the returned address(es). If any address matches, the mechanism
matches. matches.
5.4. "mx" 5.4. "mx"
This mechanism matches if <ip> is one of the MX hosts for a domain This mechanism matches if <ip> is one of the MX hosts for a domain
name. name.
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ] mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
check_host() first performs an MX lookup on the <target-name>. Then 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 it performs an address lookup on each MX name returned. The <ip> is
compared to each returned IP address. To prevent Denial of Service 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 (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 evaluation of an "mx" mechanism (see Section 10). If any address
matches, the mechanism matches. matches, the mechanism matches.
Note regarding implicit MXs: If the <target-name> has no MX records, 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 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 NOT default to an A or AAAA lookup on the <target-name> directly.
behavior breaks with the legacy "implicit MX" rule. See [RFC2821], This behavior diverges from the legacy "implicit MX" rule, (See
Section 5. If such behavior is desired, the publisher should specify [RFC5321], Section 5. If such behavior is desired, the publisher
an "a" directive. should specify an "a" directive).
5.5. "ptr" 5.5. "ptr" (deprecated)
This mechanism tests whether the DNS reverse-mapping for <ip> exists This mechanism tests whether the DNS reverse-mapping for <ip> exists
and correctly points to a domain name within a particular domain. and correctly points to a domain name within a particular domain.
This mechanism is deprecated and SHOULD NOT be used.
PTR = "ptr" [ ":" domain-spec ] ptr = "ptr" [ ":" domain-spec ]
First, the <ip>'s name is looked up using this procedure: perform a The <ip>'s name is looked up using this procedure:
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 1. Perform a DNS reverse-mapping for <ip>
sending-domain_names are found, use at most 10. for each name in 2. Look up the corresponding PTR record in "in-addr.arpa." if the
(sending-domain_names) { address is an IPv4 one and in "ip6.arpa." if it is an IPv6
IP_addresses := a_lookup(name); address.
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 3. For each record returned, validate the domain name by looking up
<target-name> domain. If any do, this mechanism matches. If no its IP address. To prevent DoS attacks, more than 10 PTR names
validated domain name can be found, or if none of the validated MUST NOT be looked up during the evaluation of a "ptr" mechanism
domain names end in the <target-name>, this mechanism fails to match. (see Section 4.6.4).
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 4. If <ip> is among the returned IP addresses, then that domain name
lookup, then that domain name is skipped and the search continues. is validated.
Check all validated domain names to see if they either match the
<target-name> domain or are a subdomain of the <target-name> domain.
If any do, this mechanism matches. If no validated domain name can
be found, or if none of the validated domain names match or are a
subdomain of 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: 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;
}
}
for each name in (validated-sending-domain_names) { for each name in (validated-sending-domain_names) {
if name ends in <domain-spec>, return match. if name ends in <domain-spec>, return match.
if name is <domain-spec>, return match. if name is <domain-spec>, return match.
} }
return no-match. return no-match.
This mechanism matches if the <target-name> is either an ancestor of This mechanism matches if the <target-name> is either a subdomain of
a validated domain name or if the <target-name> and a validated a validated domain name or if the <target-name> and a validated
domain name are the same. For example: "mail.example.com" is within domain name are the same. For example: "mail.example.com" is within
the domain "example.com", but "mail.bad-example.com" is not. the domain "example.com", but "mail.bad-example.com" is not.
Note: Use of this mechanism is discouraged because it is slow, it is Note: This mechanism has been deprecated because it is slow, it is
not as reliable as other mechanisms in cases of DNS errors, and it 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 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" records must be in place for the domain's hosts and the "ptr"
mechanism should be one of the last mechanisms checked. mechanism should be one of the last mechanisms checked. After many
years of SPF deployment experience it has been concluded it is
unnecessary and more reliable alternatives used instead. It is,
however, still in use and part of the SPF protocol, so compliant
check_host() implementations MUST support it.
5.6. "ip4" and "ip6" 5.6. "ip4" and "ip6"
These mechanisms test whether <ip> is contained within a given IP These mechanisms test whether <ip> is contained within a given IP
network. network.
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ] ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ] ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" 1*DIGIT ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = qnum "." qnum "." qnum "." qnum ip4-network = qnum "." qnum "." qnum "." qnum
qnum = DIGIT ; 0-9 qnum = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99 / %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 / "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249 / "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255 / "25" %x30-35 ; 250-255
; as per conventional dotted quad notation. e.g., 192.0.2.0 ; as per conventional dotted quad notation. e.g., 192.0.2.0
ip6-network = <as per [RFC 3513], section 2.2> ip6-network = <as per [RFC 4291], section 2.2>
; e.g., 2001:DB8::CD30 ; e.g., 2001:DB8::CD30
The <ip> is compared to the given network. If CIDR-length high-order The <ip> is compared to the given network. If CIDR prefix length
bits match, the mechanism matches. high-order bits match, the mechanism matches.
If ip4-cidr-length is omitted, it is taken to be "/32". If 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 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 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. notations. That is, use 192.0.2.0/24 instead of 192.0.2.
5.7. "exists" 5.7. "exists"
This mechanism is used to construct an arbitrary domain name that is This mechanism is used to construct an arbitrary domain name that is
used for a DNS A record query. It allows for complicated schemes used for a DNS A record query. It allows for complicated schemes
skipping to change at page 22, line 34 skipping to change at page 28, line 15
Domains can use this mechanism to specify arbitrarily complex Domains can use this mechanism to specify arbitrarily complex
queries. For example, suppose example.com publishes the record: queries. For example, suppose example.com publishes the record:
v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
The <target-name> might expand to The <target-name> might expand to
"1.2.0.192.someuser._spf.example.com". This makes fine-grained "1.2.0.192.someuser._spf.example.com". This makes fine-grained
decisions possible at the level of the user and client IP address. decisions possible at the level of the user and client IP address.
This mechanism enables queries that mimic the style of tests that This mechanism enables queries that mimic the style of tests that
existing anti-spam DNS blacklists (DNSBL) use. existing DNS white/black lists (DNSxLs) use, as described in
[RFC5782]. The query will either return NXDOMAIN (no match), any
valid answer (match), or an error.
6. Modifier Definitions 6. Modifier Definitions
Modifiers are name/value pairs that provide additional information. Modifiers are name/value pairs that provide additional information.
Modifiers always have an "=" separating the name and the value. Modifiers always have an "=" separating the name and the value.
The modifiers defined in this document ("redirect" and "exp") MAY The modifiers defined in this document ("redirect" and "exp") MAY
appear anywhere in the record, but SHOULD appear at the end, after appear anywhere in the record, but SHOULD appear at the end, after
all mechanisms. Ordering of these two modifiers does not matter. all mechanisms. Ordering of these two modifiers does not matter.
These two modifiers MUST NOT appear in a record more than once each. 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". If they do, then check_host() exits with a result of "permerror".
Unrecognized modifiers MUST be ignored no matter where in a record, Unrecognized modifiers MUST be ignored no matter where in a record,
or how often. This allows implementations of this document to or how often. This allows implementations of this document to
gracefully handle records with modifiers that are defined in other gracefully handle records with modifiers that are defined in other
specifications. specifications.
6.1. redirect: Redirected Query 6.1. redirect: Redirected Query
If all mechanisms fail to match, and a "redirect" modifier is The redirect modifier is intended for consolidating both
present, then processing proceeds as follows: authorizations and policy into a common set to be shared within a
single ADMD. Redirect is like a common code element to be shared
among records in a single ADMD. It is possible to control both
authorized hosts and policy for an arbitrary number of domains from a
single record.
redirect = "redirect" "=" domain-spec redirect = "redirect" "=" domain-spec
If all mechanisms fail to match, and a "redirect" modifier is
present, then processing proceeds as follows:
The domain-spec portion of the redirect section is expanded as per 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 macro rules in Section 8. Then check_host() is evaluated with
the resulting string as the <domain>. The <ip> and <sender> the resulting string as the <domain>. The <ip> and <sender>
arguments remain the same as current evaluation of check_host(). arguments remain the same as in the current evaluation of
check_host().
The result of this new evaluation of check_host() is then considered The result of this new evaluation of check_host() is then considered
the result of the current evaluation with the exception that if no 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 SPF record is found, or if the target-name is malformed, the result
is a "PermError" rather than "None". is a "permerror" rather than "none".
Note that the newly-queried domain may itself specify redirect Note that the newly-queried domain can itself specify redirect
processing. processing.
This facility is intended for use by organizations that wish to apply This facility is intended for use by organizations that wish to apply
the same record to multiple domains. For example: the same record to multiple domains. For example:
la.example.com. TXT "v=spf1 redirect=_spf.example.com" la.example.com. TXT "v=spf1 redirect=_spf.example.com"
ny.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" sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
_spf.example.com. TXT "v=spf1 mx:example.com -all" _spf.example.com. TXT "v=spf1 mx:example.com -all"
In this example, mail from any of the three domains is described by In this example, mail from any of the three domains is described by
the same record. This can be an administrative advantage. the same record. This can be an administrative advantage.
Note: In general, the domain "A" cannot reliably use a redirect to Note: In general, the domain "A" cannot reliably use a redirect to
another domain "B" not under the same administrative control. Since another domain "B" not under the same administrative control. Since
the <sender> stays the same, there is no guarantee that the record at the <sender> stays the same, there is no guarantee that the record at
domain "B" will correctly work for mailboxes in domain "A", domain "B" will correctly work for mailboxes in domain "A",
especially if domain "B" uses mechanisms involving localparts. An especially if domain "B" uses mechanisms involving local-parts. An
"include" directive may be more appropriate. "include" directive is generally be more appropriate.
For clarity, it is RECOMMENDED that any "redirect" modifier appear as For clarity, it is RECOMMENDED that any "redirect" modifier appear as
the very last term in a record. the very last term in a record.
6.2. exp: Explanation 6.2. exp: Explanation
explanation = "exp" "=" domain-spec explanation = "exp" "=" domain-spec
If check_host() results in a "Fail" due to a mechanism match (such as If check_host() results in a "fail" due to a mechanism match (such as
"-all"), and the "exp" modifier is present, then the explanation "-all"), and the "exp" modifier is present, then the explanation
string returned is computed as described below. If no "exp" modifier string returned is computed as described below. If no "exp" modifier
is present, then either a default explanation string or an empty is present, then either a default explanation string or an empty
explanation string may be returned. explanation string MUST be returned.
The <domain-spec> is macro expanded (see Section 8) and becomes the The domain-spec is macro expanded (see Section 8) and becomes the
<target-name>. The DNS TXT record for the <target-name> is fetched. <target-name>. The DNS TXT record for the <target-name> is fetched.
If <domain-spec> is empty, or there are any DNS processing errors If there are any DNS processing errors (any RCODE other than 0), or
(any RCODE other than 0), or if no records are returned, or if more if no records are returned, or if more than one record is returned,
than one record is returned, or if there are syntax errors in the or if there are syntax errors in the explanation string, then proceed
explanation string, then proceed as if no exp modifier was given. as if no exp modifier was given.
The fetched TXT record's strings are concatenated with no spaces, and The fetched TXT record's strings are concatenated with no spaces, and
then treated as an <explain-string>, which is macro-expanded. This then treated as an explain-string, which is macro-expanded. This
final result is the explanation string. Implementations MAY limit final result is the explanation string. Implementations MAY limit
the length of the resulting explanation string to allow for other the length of the resulting explanation string to allow for other
protocol constraints and/or reasonable processing limits. Since the protocol constraints and/or reasonable processing limits. Since the
explanation string is intended for an SMTP response and [RFC2821] explanation string is intended for an SMTP response and [RFC5321]
Section 2.4 says that responses are in [US-ASCII], the explanation Section 2.4 says that responses are in [US-ASCII], the explanation
string is also limited to US-ASCII. string MUST be limited to US-ASCII.
Software evaluating check_host() can use this string to communicate Software evaluating check_host() can use this string to communicate
information from the publishing domain in the form of a short message information from the publishing domain in the form of a short message
or URL. Software SHOULD make it clear that the explanation string or URL. Software SHOULD make it clear that the explanation string
comes from a third party. For example, it can prepend the macro comes from a third party. For example, it can prepend the macro
string "%{o} explains: " to the explanation, such as shown in Section string "%{o} explains: " to the explanation, such as shown in
2.5.4. Section 2.5.4.
Suppose example.com has this record: Suppose example.com has this record:
v=spf1 mx -all exp=explain._spf.%{d} v=spf1 mx -all exp=explain._spf.%{d}
Here are some examples of possible explanation TXT records at Here are some examples of possible explanation TXT records at
explain._spf.example.com: explain._spf.example.com:
"Mail from example.com should only be sent by its own servers." "Mail from example.com should only be sent by its own servers."
-- a simple, constant message -- a simple, constant message
skipping to change at page 25, line 7 skipping to change at page 32, line 5
"See http://%{d}/why.html?s=%{S}&i=%{I}" "See http://%{d}/why.html?s=%{S}&i=%{I}"
-- a complicated example that constructs a URL with the -- a complicated example that constructs a URL with the
arguments to check_host() so that a web page can be arguments to check_host() so that a web page can be
generated with detailed, custom instructions generated with detailed, custom instructions
Note: During recursion into an "include" mechanism, an exp= modifier Note: During recursion into an "include" mechanism, an exp= modifier
from the <target-name> MUST NOT be used. In contrast, when executing from the <target-name> MUST NOT be used. In contrast, when executing
a "redirect" modifier, an exp= modifier from the original domain MUST a "redirect" modifier, an exp= modifier from the original domain MUST
NOT be used. NOT be used.
7. The Received-SPF Header Field 7. Recording The Result
It is RECOMMENDED that SMTP receivers record the result of SPF It is RECOMMENDED that SMTP receivers record the result of SPF
processing in the message header. If an SMTP receiver chooses to do processing in the message header. There are two methods for doing
so, it SHOULD use the "Received-SPF" header field defined here for this: the Received-SPF header field defined here and the more generic
each identity that was checked. This information is intended for the Authentication-Results header field defined in [RFC5451]. Because
recipient. (Information intended for the sender is described in these fields are generally used within a receiving ADMD, it is a
Section 6.2, Explanation.) local policy choice which to include. In general, the more broadly
applicable Authentication-Results header field ought to be used, but
it SHOULD be used in such a way that it conveys the same information
that the verifier would have provided in a Received-SPF header field
if that had been used.
The Received-SPF header field is a trace field (see [RFC2822] Section If an SMTP receiver chooses to do so, it SHOULD use one of these
header fields 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.)
7.1. The Received-SPF Header Field
The Received-SPF header field is a trace field (see [RFC5322] Section
3.6.7) and SHOULD be prepended to the existing header, above the 3.6.7) and SHOULD be prepended to the existing header, above the
Received: field that is generated by the SMTP receiver. It MUST Received: field that is generated by the SMTP receiver. It MUST
appear above all other Received-SPF fields in the message. The appear above all other Received-SPF fields in the message. The
header field has the following format: header field has the following format:
header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] header-field = "Received-SPF:" [CFWS] result FWS [comment FWS]
[ key-value-list ] CRLF [ key-value-list ] CRLF
result = "Pass" / "Fail" / "SoftFail" / "Neutral" / result = "pass" / "fail" / "softfail" / "neutral" /
"None" / "TempError" / "PermError" "none" / "temperror" / "permerror"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"] [";"]
key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string )
key = "client-ip" / "envelope-from" / "helo" / key = "client-ip" / "envelope-from" / "helo" /
"problem" / "receiver" / "identity" / "problem" / "receiver" / "identity" /
mechanism / "x-" name / name mechanism / name
identity = "mailfrom" ; for the "MAIL FROM" identity identity = "mailfrom" ; for the "MAIL FROM" identity
/ "helo" ; for the "HELO" identity / "helo" ; for the "HELO" identity
/ name ; other identities / name ; other identities
dot-atom = <unquoted word as per [RFC2822]> dot-atom = <unquoted word as per [RFC5322]>
quoted-string = <quoted string as per [RFC2822]> quoted-string = <quoted string as per [RFC5322]>
comment = <comment string as per [RFC2822]> comment = <comment string as per [RFC5322]>
CFWS = <comment or folding white space as per [RFC2822]> CFWS = <comment or folding white space as per [RFC5322]>
FWS = <folding white space as per [RFC2822]> FWS = <folding white space as per [RFC5322]>
CRLF = <standard end-of-line token as per [RFC2822]> CRLF = <standard end-of-line token as per [RFC2532]>
The header field SHOULD include a "(...)" style <comment> after the The header field SHOULD include a "(...)" style comment after the
result, conveying supporting information for the result, such as result, conveying supporting information for the result, such as
<ip>, <sender>, and <domain>. <ip>, <sender>, and <domain>.
The following key-value pairs are designed for later machine parsing. The following key-value pairs are designed for later machine parsing.
SPF clients SHOULD give enough information so that the SPF results SPF verifiers SHOULD give enough information so that the SPF results
can be verified. That is, at least "client-ip", "helo", and, if the can be verified. That is, at least "client-ip", "helo", and, if the
"MAIL FROM" identity was checked, "envelope-from". "MAIL FROM" identity was checked, "envelope-from".
client-ip the IP address of the SMTP client client-ip the IP address of the SMTP client
envelope-from the envelope sender mailbox envelope-from the envelope sender mailbox
helo the host name given in the HELO or EHLO command helo the host name given in the HELO or EHLO command
mechanism the mechanism that matched (if no mechanisms matched, mechanism the mechanism that matched (if no mechanisms matched,
substitute the word "default") substitute the word "default")
problem if an error was returned, details about the error problem if an error was returned, details about the error
receiver the host name of the SPF verifier
receiver the host name of the SPF client
identity the identity that was checked; see the <identity> ABNF identity the identity that was checked; see the <identity> ABNF
rule rule
Other keys may be defined by SPF clients. Until a new key name Other keys MAY be defined by SPF verifiers.
becomes widely accepted, new key names should start with "x-".
SPF clients MUST make sure that the Received-SPF header field does SPF verifiers MUST make sure that the Received-SPF header field does
not contain invalid characters, is not excessively long, and does not not contain invalid characters, is not excessively long (See
contain malicious data that has been provided by the sender. [RFC5322] Section 2.1.1), and does not contain malicious data that
has been provided by the sender.
Examples of various header styles that could be generated are the Examples of various header field styles that could be generated are
following: the following:
Received-SPF: Pass (mybox.example.org: domain of Received-SPF: pass (mybox.example.org: domain of
myname@example.com designates 192.0.2.1 as permitted sender) myname@example.com designates 192.0.2.1 as permitted sender)
receiver=mybox.example.org; client-ip=192.0.2.1; receiver=mybox.example.org; client-ip=192.0.2.1;
envelope-from=<myname@example.com>; helo=foo.example.com; envelope-from="myname@example.com"; helo=foo.example.com;
Received-SPF: Fail (mybox.example.org: domain of Received-SPF: fail (mybox.example.org: domain of
myname@example.com does not designate myname@example.com does not designate
192.0.2.1 as permitted sender) 192.0.2.1 as permitted sender)
identity=mailfrom; client-ip=192.0.2.1; identity=mailfrom; client-ip=192.0.2.1;
envelope-from=<myname@example.com>; envelope-from="myname@example.com";
7.2. SPF Results in the Authentication-Results Header Field
As mentioned in Section 7, the Authentication-Results header field is
designed to communicate lists of tests a border MTA did and their
results. The specified elements of the field provide less
information than the SPF-Received field:
Authentication-Results: myhost.example.org; spf=pass
smtp.mailfrom=example.net
Received-SPF: pass (myhost.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;
It is, however, possible to add CFWS in the "reason" part of an
Authentication-Results header field and provide the equivalent
information. Receivers SHOULD include the same information they
would have provided if they had used the Received-SPF field.
The reason SHOULD include a key-value-list with keys provinding
information normally included in a Received-SPF header field that is
not already part of the Authentication-Results header field. That
is, at least "client-ip", "helo", and, if the "MAIL FROM" identity
was checked, "envelope-from". Authentication-Results header fields
can contain results for more than one authentication, so one field
can provide results for both an "MAIL FROM" check and an "HELO"
check.
reasonspec = <reason per [RFC5451]>
A suitably enhanced Authentication-Results header field might look
like (for a "MAIL FROM" check in this example):
Authentication-Results: myhost.example.org; spf=pass
reason="client-ip=192.0.2.1; smtp.helo=foo.example.com"
smtp.mailfrom=user@example.net
8. Macros 8. Macros
8.1. Macro Definitions 8.1. Macro Definitions
Many mechanisms and modifiers perform macro expansion on part of the Many mechanisms and modifiers perform macro expansion on a term.
term.
domain-spec = macro-string domain-end domain-spec = macro-string domain-end
domain-end = ( "." toplabel [ "." ] ) / macro-expand domain-end = ( "." toplabel [ "." ] ) / macro-expand
toplabel = ( *alphanum ALPHA *alphanum ) / toplabel = ( *alphanum ALPHA *alphanum ) /
( 1*alphanum "-" *( alphanum / "-" ) alphanum ) ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
; LDH rule plus additional TLD restrictions ; LDH rule plus additional TLD restrictions
; (see [RFC3696], Section 2) ; (see [RFC3696], Section 2 for background)
alphanum = ALPHA / DIGIT alphanum = ALPHA / DIGIT
explain-string = *( macro-string / SP ) explain-string = *( macro-string / SP )
macro-string = *( macro-expand / macro-literal ) macro-string = *( macro-expand / macro-literal )
macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) macro-expand = ( "%{" macro-letter transformers *delimiter "}" )
/ "%%" / "%_" / "%-" / "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E macro-literal = %x21-24 / %x26-7E
; visible characters except "%" ; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t" "c" / "r" / "t" / "v"
transformers = *DIGIT [ "r" ] transformers = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
A literal "%" is expressed by "%%". A literal "%" is expressed by "%%".
"%_" expands to a single " " space. "%_" expands to a single " " space.
"%-" expands to a URL-encoded space, viz., "%20". "%-" expands to a URL-encoded space, viz., "%20".
The following macro letters are expanded in term arguments: The following macro letters are expanded in term arguments:
s = <sender> s = <sender>
l = local-part of <sender> l = local-part of <sender>
o = domain of <sender> o = domain of <sender>
d = <domain> d = <domain>
i = <ip> i = <ip>
p = the validated domain name of <ip> p = the validated domain name of <ip> (deprecated)
v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6 v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
h = HELO/EHLO domain h = HELO/EHLO domain
The following macro letters are allowed only in "exp" text: The following macro letters are allowed only in "exp" text:
c = SMTP client IP (easily readable format) c = SMTP client IP (easily readable format)
r = domain name of host performing the check r = domain name of host performing the check
t = current timestamp t = current timestamp
A '%' character not followed by a '{', '%', '-', or '_' character is A '%' character not followed by a '{', '%', '-', or '_' character is
a syntax error. So a syntax error. So
-exists:%(ir).sbl.spamhaus.example.org -exists:%(ir).sbl.spamhaus.example.org
is incorrect and will cause check_host() to yield a "permerror".
is incorrect and will cause check_host() to return a "PermError".
Instead, say Instead, say
-exists:%{ir}.sbl.spamhaus.example.org -exists:%{ir}.sbl.spamhaus.example.org
Optional transformers are the following: Optional transformers are the following:
*DIGIT = zero or more digits *DIGIT = zero or more digits
'r' = reverse value, splitting on dots by default 'r' = reverse value, splitting on dots by default
If transformers or delimiters are provided, the replacement value for If transformers or delimiters are provided, the replacement value for
a macro letter is split into parts. After performing any reversal a macro letter is split into parts. After performing any reversal
operation and/or removal of left-hand parts, the parts are rejoined operation and/or removal of left-hand parts, the parts are rejoined
skipping to change at page 28, line 31 skipping to change at page 37, line 22
*DIGIT = zero or more digits *DIGIT = zero or more digits
'r' = reverse value, splitting on dots by default 'r' = reverse value, splitting on dots by default
If transformers or delimiters are provided, the replacement value for If transformers or delimiters are provided, the replacement value for
a macro letter is split into parts. After performing any reversal a macro letter is split into parts. After performing any reversal
operation and/or removal of left-hand parts, the parts are rejoined operation and/or removal of left-hand parts, the parts are rejoined
using "." and not the original splitting characters. using "." and not the original splitting characters.
By default, strings are split on "." (dots). Note that no special By default, strings are split on "." (dots). Note that no special
treatment is given to leading, trailing, or consecutive delimiters, treatment is given to leading, trailing, or consecutive delimiters in
and so the list of parts may contain empty strings. Older input strings, and so the list of parts might contain empty strings.
implementations of SPF prohibit trailing dots in domain names, so Some older implementations of SPF prohibit trailing dots in domain
trailing dots should not be published by domain owners, although they names, so trailing dots SHOULD NOT be published by domain owners,
must be accepted by implementations conforming to this document. although they MUST be accepted by implementations conforming to this
Macros may specify delimiter characters that are used instead of ".". document. Macros MAY specify delimiter characters that are used
instead of ".".
The 'r' transformer indicates a reversal operation: if the client IP 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" 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". and the macro %{ir} would expand to "1.2.0.192".
The DIGIT transformer indicates the number of right-hand parts to The DIGIT transformer indicates the number of right-hand parts to
use, after optional reversal. If a DIGIT is specified, the value use, after optional reversal. If a DIGIT is specified, the value
MUST be nonzero. If no DIGITs are specified, or if 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 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 used. If the DIGIT was 5, and only 3 parts were available, the macro
interpreter would pretend the DIGIT was 3. Implementations MUST interpreter would pretend the DIGIT was 3. Implementations MUST
support at least a value of 128, as that is the maximum number of support at least a value of 128, as that is the maximum number of
labels in a domain name. labels in a domain name.
The "s" macro expands to the <sender> argument. It is an E-Mail The "s" macro expands to the <sender> argument. It is an email
address with a localpart, an "@" character, and a domain. The "l" address with a local-part, an "@" character, and a domain. The "l"
macro expands to just the localpart. The "o" macro expands to just macro expands to just the local-part. The "o" macro expands to just
the domain part. Note that these values remain the same during the domain part. Note that these values remain the same during
recursive and chained evaluations due to "include" and/or "redirect". recursive and chained evaluations due to "include" and/or "redirect".
Note also that if the original <sender> had no localpart, the Note also that if the original <sender> had no local-part, the local-
localpart was set to "postmaster" in initial processing (see Section part was set to "postmaster" in initial processing (see Section 4.3).
4.3).
For IPv4 addresses, both the "i" and "c" macros expand to the For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format. standard dotted-quad format.
For IPv6 addresses, the "i" macro expands to a dot-format address; it 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 is intended for use in %{ir}. The "c" macro MAY expand to any of the
hexadecimal colon-format addresses specified in [RFC3513], Section hexadecimal colon-format addresses specified in [RFC4291], Section
2.2. It is intended for humans to read. 2.2. It is intended for humans to read.
The "p" macro expands to the validated domain name of <ip>. The The "p" macro expands to the validated domain name of <ip>. The
procedure for finding the validated domain name is defined in Section procedure for finding the validated domain name is defined in
5.5. If the <domain> is present in the list of validated domains, it Section 5.5. If the <domain> is present in the list of validated
SHOULD be used. Otherwise, if a subdomain of the <domain> is domains, it SHOULD be used. Otherwise, if a subdomain of the
present, it SHOULD be used. Otherwise, any name from the list may be <domain> is present, it SHOULD be used. Otherwise, any name from the
used. If there are no validated domain names or if a DNS error list MAY be used. If there are no validated domain names or if a DNS
occurs, the string "unknown" is used. error occurs, the string "unknown" is used. This macro is deprecated
and SHOULD NOT be used.
The "r" macro expands to the name of the receiving MTA. This SHOULD 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 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 the checking is done by a MUA) or if policy restrictions dictate
otherwise, the word "unknown" SHOULD be substituted. The domain name otherwise, the word "unknown" SHOULD be substituted. The domain name
may be different from the name found in the MX record that the client can be different from the name found in the MX record that the client
MTA used to locate the receiving MTA. MTA used to locate the receiving MTA.
The "t" macro expands to the decimal representation of the The "t" macro expands to the decimal representation of the
approximate number of seconds since the Epoch (Midnight, January 1, approximate number of seconds since the Epoch (Midnight, January 1,
1970, UTC). This is the same value as is returned by the POSIX 1970, UTC) at the time of the evaluation. This is the same value as
time() function in most standards-compliant libraries. 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 When the result of macro expansion is used in a domain name query, if
the expanded domain name exceeds 253 characters (the maximum length the expanded domain name exceeds 253 characters (the maximum length
of a domain name), the left side is truncated to fit, by removing of a domain name), the left side is truncated to fit, by removing
successive domain labels until the total length does not exceed 253 successive domain labels (and their following dots) until the total
characters. length does not exceed 253 characters.
Uppercased macros expand exactly as their lowercased equivalents, and Uppercased macros expand exactly as their lowercased equivalents, and
are then URL escaped. URL escaping must be performed for characters are then URL escaped. URL escaping must be performed for characters
not in the "uric" set, which is defined in [RFC3986]. not in the "unreserved" set, which is defined in [RFC3986].
Note: Care must be taken so that macro expansion for legitimate Note: Care must be taken so that macro expansion for legitimate email
E-Mail does not exceed the 63-character limit on DNS labels. The does not exceed the 63-character limit on DNS labels. The local-part
localpart of E-Mail addresses, in particular, can have more than 63 of email addresses, in particular, can have more than 63 characters
characters between dots. between dots.
Note: Domains should avoid using the "s", "l", "o", or "h" macros in Note: Domains SHOULD avoid using the "s", "l", "o", or "h" macros in
conjunction with any mechanism directive. Although these macros are conjunction with any mechanism directive. Although these macros are
powerful and allow per-user records to be published, they severely powerful and allow per-user records to be published, they severely
limit the ability of implementations to cache results of check_host() limit the ability of implementations to cache results of check_host()
and they reduce the effectiveness of DNS caches. and they reduce the effectiveness of DNS caches.
Implementations should be aware that if no directive processed during Note: If no directive processed during the evaluation of check_host()
the evaluation of check_host() contains an "s", "l", "o", or "h" contains an "s", "l", "o", or "h" macro, then the results of the
macro, then the results of the evaluation can be cached on the basis evaluation can be cached on the basis of <domain> and <ip> alone for
of <domain> and <ip> alone for as long as the shortest Time To Live as long as the shortest Time To Live (TTL) of all the DNS records
(TTL) of all the DNS records involved. involved.
8.2. Expansion Examples 8.2. Expansion Examples
The <sender> is strong-bad@email.example.com. The <sender> is strong-bad@email.example.com.
The IPv4 SMTP client IP is 192.0.2.3. The IPv4 SMTP client IP is 192.0.2.3.
The IPv6 SMTP client IP is 2001:DB8::CB01. The IPv6 SMTP client IP is 2001:DB8::CB01.
The PTR domain name of the client IP is mx.example.org. The PTR domain name of the client IP is mx.example.org.
macro expansion macro expansion
------- ---------------------------- ------- ----------------------------
skipping to change at page 31, line 20 skipping to change at page 40, line 4
bad.strong.lp.3.2.0.192.in-addr._spf.example.com bad.strong.lp.3.2.0.192.in-addr._spf.example.com
%{ir}.%{v}.%{l1r-}.lp._spf.%{d2} %{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
3.2.0.192.in-addr.strong.lp._spf.example.com 3.2.0.192.in-addr.strong.lp._spf.example.com
%{d2}.trusted-domains.example.net %{d2}.trusted-domains.example.net
example.com.trusted-domains.example.net example.com.trusted-domains.example.net
IPv6: IPv6:
%{ir}.%{v}._spf.%{d2} 1.0.B.C.0.0.0.0. %{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 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 9. Implications
This section outlines the major implications that adoption of this This section outlines the major implications that adoption of this
document will have on various entities involved in Internet E-Mail. document will have on various entities involved in Internet email.
It is intended to make clear to the reader where this document It is intended to make clear to the reader where this document
knowingly affects the operation of such entities. This section is knowingly affects the operation of such entities. This section is
not a "how-to" manual, or a "best practices" document, and it is not 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 a comprehensive list of what such entities should do in light of this
document. document.
This section is non-normative. This section is non-normative. [RFC5598] describes the Internet
email architecture. This section is organized based on the different
segments of the architecture.
9.1. Sending Domains 9.1. Sending Domains
Domains that wish to be compliant with this specification will need Originating ADMDs (ADministrative Management Domains - [RFC5598]
to determine the list of hosts that they allow to use their domain Section 2.2.1 and Section 2.3) that wish to be compliant with this
name in the "HELO" and "MAIL FROM" identities. It is recognized that specification will need to determine the list of relays ([RFC5598]
forming such a list is not just a simple technical exercise, but Section 2.2.2) that they allow to use their domain name in the "HELO"
involves policy decisions with both technical and administrative and "MAIL FROM" identities when relaying to other ADMDs. It is
considerations. 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 9.1.1. DNS Resource Considerations
Minimizing the DNS resources required for SPF lookups can be done by
choosing directives that require less DNS information and by placing
lower-cost mechanisms earlier in the SPF record.
+----------+--------+-----------------+
| term | cost | limit |
+----------+--------+-----------------+
| ip4/ip6 | 0 | - |
| a | 1 | 10 |
| include | 1 | 10 |
| redirect | 1 | 10 |
| exists | 1 | 10 |
| mx | 1 + N* | 10 and N* <= 10 |
| ptr/%{p} | 1 + N* | 10 and N* <= 10 |
| all | 0 | 1 |
+----------+--------+-----------------+
* N is the number of RRs found during each term evaluation
Section 4.6.4 specifies the limits receivers have to use. It is
essential to publish records that do not exceed these requirements.
It is also required to carefully weight the cost and the
maintainability of licit solutions.
For example, consider a domain set up as follows:
example.com. IN MX 10 mx.example.com.
IN MX 20 mx2.example.com.
mx.example.com. IN A 192.0.2.1
mx2.example.com. IN A 192.0.2.129
Assume the administrative point is to authorize (pass) mx and mx2
while failing every other host. Compare the following solutions:
Best record:
example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all"
Good record:
$ORIGIN example.com.
@ IN TXT "v=spf1 a:authorized_spf.example.com -all"
authorized_spf IN A 192.0.2.1
IN A 192.0.2.129
Expensive record:
example.com. IN TXT "v=spf1 mx:example.com -all"
Wasteful, bad record:
example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all"
9.1.2. Administrator's Considerations
There might be administrative considerations: using "a" over "ip4" or
"ip6" allows hosts to be renumbered easily. Using "mx" over "a"
allows the set of mail hosts to be changed easily. Unless such
changes are common, it is better to use the less resource intensive
mechanisms like "ip4" and "ip6" over "a" or "a" or "mx".
In some specific cases, standard advice on record content is
appropriate. Publishing SPF records for domains that send no mail is
a well established best practice. The record for a domain that sends
no mail is:
www.example.com. IN TXT "v=spf1 -all"
Publishing SPF records for individual hosts is also best practice.
The hostname is generally the identity used in the 5321.HELO/.EHLO
command. In the case of messages with a null 5321.MailFrom, this is
used as the domain for 5321.MailFrom SPF checks, in addition to being
used in 5321.HELO/.EHLO based SPF checks. The standard SPF record
for an individual host that is involved in mail processing is:
relay.example.com. IN TXT "v=spf1 a -all"
Validating correct deployment is difficult. [RFC6652] describes one
mechanism for soliciting feedback on SPF failures. Another approach
that can be helpful to publish records that include a "tracking
exists:" mechanism. By looking at the name server logs, a rough list exists:" mechanism. By looking at the name server logs, a rough list
may then be generated. For example: can then be generated. For example:
v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
9.2. Mailing Lists Regardless of the method used, understanding the ADMD's outbound mail
architecture is essential to effective deployment.
9.1.3. Bounces
As explained in Section 1.3.3, [RFC5321] allows the reverse-path to
be null, which is typical of some Delivery Status Notification
[RFC3464], commonly called email bounces. In this case the only
entity available for performing an SPF check is the "HELO" identity
defined in Section 1.3.4. SPF functionality is enhanced by
administrators ensuring this identity is set correctly and has an
appropriate SPF record. It is normal to have the HELO identity set
to hostname instead of domain. Zone file generation for significant
numbers of hosts can be consolidated using the redirect modifier and
scripted for initial deployment. Specific deployment advice is given
above in Section Section 9.1.2.
9.2. Mediators
Broadly speaking, there are two types of mediating ADMDs that can
affect SPF deployment of other ADMDs: mailing lists (see [RFC5598]
Section 5.3) and ReSenders ([RFC5598] Section 5.2).
9.2.1. Mailing Lists
Mailing lists must be aware of how they re-inject mail that is sent 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 to the list. Mailing lists MUST comply with the requirements in
[RFC2821], Section 3.10, and [RFC1123], Section 5.3.6, that say that [RFC5321], Section 3.10, and [RFC1123], Section 5.3.6, that say that
the reverse-path MUST be changed to be the mailbox of a person or the reverse-path MUST be changed to be the mailbox of a person or
other entity who administers the list. Whereas the reasons for other entity who administers the list. Whereas the reasons for
changing the reverse-path are many and long-standing, SPF adds changing the reverse-path are many and long-standing, SPF adds
enforcement to this requirement. enforcement to this requirement.
In practice, almost all mailing list software in use already complies In practice, almost all mailing list software in use already complies
with this requirement. Mailing lists that do not comply may or may with this requirement. Mailing lists that do not comply might
not encounter problems depending on how access to the list is encounter problems depending on how access to the list is restricted.
restricted. Such lists that are entirely internal to a domain (only Such lists that are entirely internal to a domain (only people in the
people in the domain can send to or receive from the list) are not domain can send to or receive from the list) are not affected.
affected.
9.3. Forwarding Services and Aliases 9.2.2. Forwarding Services and Aliases
Forwarding services take mail that is received at a mailbox and Forwarding services take mail that is received at a mailbox and
direct it to some external mailbox. At the time of this writing, the 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 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 FROM" of a message when re-injecting it for delivery to the external
mailbox. [RFC1123] and [RFC2821] describe this action as an "alias" mailbox. [RFC1123] and [RFC5321] describe this action as an "alias"
rather than a "mail list". This means that the external mailbox's rather than a "mail list". This means the external mailbox's MTA
MTA sees all such mail in a connection from a host of the forwarding 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 service, and so the "MAIL FROM" identity will not, in general, pass
authorization. authorization.
There are three places that techniques can be used to ameliorate this There are three places that techniques can be used to ameliorate this
problem. problem.
1. The beginning, when E-Mail is first sent. 1. The beginning, when email is first sent (Originating ADMDs).
1. "Neutral" results could be given for IP addresses that may be 1. "Neutral" results could be given for IP addresses that might
forwarders, instead of "Fail" results. For example: be forwarders, instead of "fail" results. For example:
"v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all" "v=spf1 mx -exists:%{ir}.sbl.spamhaus.example.org ?all"
This would cause a lookup on an anti-spam DNS blacklist This would cause a lookup on an anti-spam DNS blacklist
(DNSBL) and cause a result of "Fail" only for E-Mail coming (DNSBL) and cause a result of "fail" only for email coming
from listed sources. All other E-Mail, including E-Mail sent from listed sources. All other email, including email sent
through forwarders, would receive a "Neutral" result. By through forwarders, would receive a "neutral" result. By
checking the DNSBL after the known good sources, problems with checking the DNSBL after the known good sources, problems
incorrect listing on the DNSBL are greatly reduced. with incorrect listing on the DNSBL are greatly reduced.
2. The "MAIL FROM" identity could have additional information in 2. The "MAIL FROM" identity could have additional information in
the localpart that cryptographically identifies the mail as the local-part that cryptographically identifies the mail as
coming from an authorized source. In this case, such an SPF coming from an authorized source. In this case, such an SPF
record could be used: record could be used:
"v=spf1 mx exists:%{l}._spf_verify.%{d} -all" "v=spf1 mx exists:%{l}._spf_verify.%{d} -all"
Then, a specialized DNS server can be set up to serve the Then, a specialized DNS server can be set up to serve the
_spf_verify subdomain that validates the localpart. Although _spf_verify subdomain that validates the local-part.
this requires an extra DNS lookup, this happens only when the Although this requires an extra DNS lookup, this happens only
E-Mail would otherwise be rejected as not coming from a known when the email would otherwise be rejected as not coming from
good source. a known good source.
Note that due to the 63-character limit for domain labels, Note that due to the 63-character limit for domain labels,
this approach only works reliably if the localpart signature this approach only works reliably if the local-part signature
scheme is guaranteed either to only produce localparts with a scheme is guaranteed either to only produce local-parts with
maximum of 63 characters or to gracefully handle truncated a maximum of 63 characters or to gracefully handle truncated
localparts. local-parts.
3. Similarly, a specialized DNS server could be set up that will 3. Similarly, a specialized DNS server could be set up that will
rate-limit the E-Mail coming from unexpected IP addresses. rate-limit the email coming from unexpected IP addresses.
"v=spf1 mx exists:%{ir}._spf_rate.%{d} -all" "v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"
4. SPF allows the creation of per-user policies for special 4. SPF allows the creation of per-user policies for special
cases. For example, the following SPF record and appropriate cases. For example, the following SPF record and appropriate
wildcard DNS records can be used: wildcard DNS records can be used:
"v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}" "v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
2. The middle, when E-Mail is forwarded. 2. The middle, when email is forwarded (Mediating ADMDs).
1. Forwarding services can solve the problem by rewriting the 1. Forwarding services can solve the problem by rewriting the
"MAIL FROM" to be in their own domain. This means that mail "MAIL FROM" to be in their own domain. This means mail
bounced from the external mailbox will have to be re-bounced rejected from the external mailbox will have to be forwarded
by the forwarding service. Various schemes to do this exist back to the original sender by the forwarding service.
though they vary widely in complexity and resource Various schemes to do this exist though they vary widely in
requirements on the part of the forwarding service. complexity and resource requirements on the part of the
forwarding service.
2. Several popular MTAs can be forced from "alias" semantics to 2. Several popular MTAs can be forced from "alias" semantics to
"mailing list" semantics by configuring an additional alias "mailing list" semantics by configuring an additional alias
with "owner-" prepended to the original alias name (e.g., an with "owner-" prepended to the original alias name (e.g., an
alias of "friends: george@example.com, fred@example.org" would alias of "friends: george@example.com, fred@example.org"
need another alias of the form "owner-friends: localowner"). would need another alias of the form "owner-friends:
localowner").
3. The end, when E-Mail is received. 3. The end, when email is received (Receiving ADMDs).
1. If the owner of the external mailbox wishes to trust the 1. If the owner of the external mailbox wishes to trust the
forwarding service, he can direct the external mailbox's MTA forwarding service, he can direct the external mailbox's MTA
to skip SPF tests when the client host belongs to the to skip SPF tests when the client host belongs to the
forwarding service. forwarding service.
2. Tests against other identities, such as the "HELO" identity, 2. Tests against other identities, such as the "HELO" identity,
may be used to override a failed test against the "MAIL FROM" MAY be used to override a failed test against the "MAIL FROM"
identity. identity.
3. For larger domains, it may not be possible to have a complete 3. For larger domains, it might not be possible to have a
or accurate list of forwarding services used by the owners of complete or accurate list of forwarding services used by the
the domain's mailboxes. In such cases, whitelists of owners of the domain's mailboxes. In such cases, whitelists
generally-recognized forwarding services could be employed. of generally-recognized forwarding services could be
employed.
9.4. Mail Services 9.2.3. Mail Services
Service providers that offer mail services to third-party domains, MSPs (Mail Service Providers - [RFC5598] Section 2.3) that offer mail
such as sending of bulk mail, may want to adjust their setup in light services to third-party domains, such as sending of bulk mail, might
of the authorization check described in this document. If the "MAIL want to adjust their configurations in light of the authorization
FROM" identity used for such E-Mail uses the domain of the service check described in this document. If the domain part of the "MAIL
provider, then the provider needs only to ensure that its sending FROM" identity used for such email uses the domain of one of the MSPs
host is authorized by its own SPF record, if any. domain, 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 If the "MAIL FROM" identity does not use the MSP's domain, then extra
domain, then extra care must be taken. The SPF record format has care must be taken. The SPF record format has several options for
several options for the third-party domain to authorize the service the third-party domain to authorize the service provider's MTAs to
provider's MTAs to send mail on its behalf. For mail service send mail on its behalf. For MSPs, such as ISPs, that have a wide
providers, such as ISPs, that have a wide variety of customers using variety of customers using the same MTA, steps should be taken to
the same MTA, steps should be taken to prevent cross-customer forgery prevent cross-customer forgery (see Section 10.4).
(see Section 10.4).
9.5. MTA Relays 9.2.4. MTA Relays
The authorization check generally precludes the use of arbitrary MTA Relays are described in [RFC5598] Section 2.2.2. The authorization
relays between sender and receiver of an E-Mail message. check generally precludes the use of arbitrary MTA relays between
sender and receiver of an email message.
Within an organization, MTA relays can be effectively deployed. Within an organization, MTA relays can be effectively deployed.
However, for purposes of this document, such relays are effectively However, for purposes of this document, such relays are effectively
transparent. The SPF authorization check is a check between border transparent. The SPF authorization check is a check between border
MTAs of different domains. MTAs of different ADMDs.
For mail senders, this means that published SPF records must For mail senders, this means that published SPF records must
authorize any MTAs that actually send across the Internet. Usually, authorize any MTAs that actually send across the Internet. Usually,
these are just the border MTAs as internal MTAs simply forward mail these are just the border MTAs as internal MTAs simply forward mail
to these MTAs for delivery. to these MTAs for relaying.
Mail receivers will generally want to perform the authorization check The receiving ADMD will generally want to perform the authorization
at the border MTAs, specifically including all secondary MXs. This check at the boundary MTAs, including all secondary MXs. Internal
allows mail that fails to be rejected during the SMTP session rather MTAs (including MTAs that might serve both as boundary MTAs and
than bounced. Internal MTAs then do not perform the authorization internal relays from secondary MXs when they are processing the
test. To perform the authorization test other than at the border, relayed mail stream) then do not perform the authorization test. To
the host that first transferred the message to the organization must perform the authorization test other than at the boundary, the host
be determined, which can be difficult to extract from the message that first transferred the message to the receiving ADMD must be
header. Testing other than at the border is not recommended. determined, which can be difficult to extract from the message header
because (a) header fields can be forged or malformed, and (b) there's
no standard way to encode that information such that it can be
reliably extracted. Testing other than at the boundary is likely to
produce unreliable results.
9.3. Receivers
SPF results can be used in combination with other methods to
determine the final local disposition (either positive or negative of
a message. It can also be considered dispositive on it's own.
9.3.1. Policy For SPF Pass
SPF pass results can be used in combination with "white lists" of
known "good" domains to bypass some or all additional pre-delivery
email checks. Exactly which checks and how to determine appropriate
white list entries has to be based on local conditions and
requirements.
9.3.2. Policy For SPF Fail
SPF fail results can be used to reject messages during the SMTP
transaction based on either "MAIL FROM" or "HELO" identity results.
This reduces resource requirements for various content filtering
methods and conserves bandwidth since rejection can be done before
the SMTP content is transferred. It also gives immediate feedback to
the sender who might then be able to resolve the issue. Due to some
of the issues described above in this section (Section 9), SPF based
rejection does present some risk of rejecting legitimate email when
rejecting based on "MAIL FROM" results.
SPF fail results can also be used as one input into a larger set of
evaluations which might, based on the overall evaluation result in
the email being marked negatively in some way (this might be via
delivery to a special spam folder, modifying subject lines, or other
locally determined means). Developing the details of such an
approach have to be based on local conditions and requirements.
Using SPF results in this way does not have the advantages of
resource conservation and immediate feedback to the sender associated
with SMTP rejection, but could produce fewer undesireable rejections
in a well designed system.
Either general approach can be used as they both leave a clear
disposition of emails. They are either delivered in some manner or
the sender is notified of the failure. Other dispositions such as
"dropping" or deleting email after acceptance are inappropriate
because they leave uncertainty and reduce the overall reliabilility
and utility of email across the Internet.
9.3.3. Policy For SPF Permerror
The "permerror" result (see Section 2.5.7) indicates the SPF
processing module at the receiver determined that the retrieved SPF
policy record could not be interpreted. This gives no true
indication about the authorized use of the data found in the
envelope.
As with all results, implementers have a choice to make regarding
what to do with a message that yields this result. SMTP allows only
a few basic options.
Rejection of the message is an option, in that it is the one thing a
receiver can do to draw attention to the difficulty encountered while
protecting itself from messages that do not have a definite SPF
result of some kind. However, if the SPF implementation is defective
and returns spurious "permerror" results, only the sender is actively
notified of the defect (in the form of rejected mail), and not the
receiver making use of SPF.
The less intrusive handling choice is to deliver the message, perhaps
with some kind of annotation of the difficulty encountered and/or
logging of a similar nature. However, this will not be desirable to
operators that wish to implement SPF checking as strictly as
possible, nor is this sort of passive problem reporting typically
effective.
There is of course the option placing this choice in the hands of the
operator rather than the implementer since this kind of choice is
often a matter of local policy rather than a condition with a
universal solution, but this adds one more piece of complexity to an
already non-trivial environment.
Both implementers and operators need to be cautious of all choices
and outcomes when handling SPF results.
10. Security Considerations 10. Security Considerations
10.1. Processing Limits 10.1. Processing Limits
As with most aspects of E-Mail, there are a number of ways that As with most aspects of email, there are a number of ways that
malicious parties could use the protocol as an avenue for a malicious parties could use the protocol as an avenue for a
Denial-of-Service (DoS) attack. The processing limits outlined here Denial-of-Service (DoS) attack. The processing limits outlined in
are designed to prevent attacks such as the following: Section 4.6.4 are designed to prevent attacks such as the following:
o A malicious party could create an SPF record with many references 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 to a victim's domain and send many emails to different SPF
clients; those SPF clients would then create a DoS attack. In verifiers; those SPF verifiers would then create a DoS attack. In
effect, the SPF clients are being used to amplify the attacker's effect, the SPF verifiers are being used to amplify the attacker's
bandwidth by using fewer bytes in the SMTP session than are used bandwidth by using fewer bytes in the SMTP session than are used
by the DNS queries. Using SPF clients also allows the attacker to by the DNS queries. Using SPF clients also allows the attacker to
hide the true source of the attack. hide the true source of the attack.
o Whereas implementations of check_host() are supposed to limit the o Whereas implementations of check_host() are supposed to limit the
number of DNS lookups, malicious domains could publish records number of DNS lookups, malicious domains could publish records
that exceed these limits in an attempt to waste computation effort that exceed these limits in an attempt to waste computation effort
at their targets when they send them mail. Malicious domains at their targets when they send them mail. Malicious domains
could also design SPF records that cause particular could also design SPF records that cause particular
implementations to use excessive memory or CPU usage, or to implementations to use excessive memory or CPU usage, or to
trigger bugs. trigger bugs.
o Malicious parties could send a large volume of mail purporting to o Malicious parties could send a large volume of mail purporting to
come from the intended target to a wide variety of legitimate mail come from the intended target to a wide variety of legitimate mail
hosts. These legitimate machines would then present a DNS load on hosts. These legitimate machines would then present a DNS load on
the target as they fetched the relevant records. the target as they fetched the relevant records.
Of these, the case of a third party referenced in the SPF record is 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, the easiest for a DoS attack to effectively exploit. As a result,
limits that may seem reasonable for an individual mail server can limits that might seem reasonable for an individual mail server can
still allow an unreasonable amount of bandwidth amplification. still allow an unreasonable amount of bandwidth amplification.
Therefore, the processing limits need to be quite low. Therefore, the processing limits need to be quite low.
SPF implementations MUST limit the number of mechanisms and modifiers 10.2. SPF-Authorized Email May Contain Other False Identities
that do DNS lookups to at most 10 per SPF check, including any
lookups caused by the use of the "include" mechanism or the
"redirect" modifier. If this number is exceeded during a check, a
PermError MUST be returned. The "include", "a", "mx", "ptr", and
"exists" mechanisms as well as the "redirect" modifier do count
against this limit. The "all", "ip4", and "ip6" mechanisms do not
require DNS lookups and therefore do not count against this limit.
The "exp" modifier does not count against this limit because the DNS
lookup to fetch the explanation string occurs after the SPF record
has been evaluated.
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.
10.2. SPF-Authorized E-Mail May Contain Other False Identities
The "MAIL FROM" and "HELO" identity authorizations must not be The "MAIL FROM" and "HELO" identity authorizations must not be
construed to provide more assurance than they do. It is entirely construed to provide more assurance than they do. It is entirely
possible for a malicious sender to inject a message using his own 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 domain in the identities used by SPF, to have that domain's SPF
record authorize the sending host, and yet the message can easily record authorize the sending host, and yet the message can easily
list other identities in its header. Unless the user or the MUA list other identities in its header. Unless the user or the MUA
takes care to note that the authorized identity does not match the takes care to note that the authorized identity does not match the
other more commonly-presented identities (such as the From: header other more commonly-presented identities (such as the From: header
field), the user may be lulled into a false sense of security. field), the user might be lulled into a false sense of security.
10.3. Spoofed DNS and IP Data 10.3. Spoofed DNS and IP Data
There are two aspects of this protocol that malicious parties could There are two aspects of this protocol that malicious parties could
exploit to undermine the validity of the check_host() function: exploit to undermine the validity of the check_host() function:
o The evaluation of check_host() relies heavily on DNS. A malicious o The evaluation of check_host() relies heavily on DNS. A malicious
attacker could attack the DNS infrastructure and cause attacker could attack the DNS infrastructure and cause
check_host() to see spoofed DNS data, and then return incorrect check_host() to see spoofed DNS data, and then return incorrect
results. This could include returning "Pass" for an <ip> value results. This could include returning "pass" for an <ip> value
where the actual domain's record would evaluate to "Fail". See where the actual domain's record would evaluate to "fail". See
[RFC3833] for a description of DNS weaknesses. [RFC3833] for a description of DNS weaknesses.
o The client IP address, <ip>, is assumed to be correct. A o The client IP address, <ip>, is assumed to be correct. In a
malicious attacker could spoof TCP sequence numbers to make mail modern, correctly configured system the risk of this not being
appear to come from a permitted host for a domain that the true is nil.
attacker is impersonating.
10.4. Cross-User Forgery 10.4. Cross-User Forgery
By definition, SPF policies just map domain names to sets of By definition, SPF policies just map domain names to sets of
authorized MTAs, not whole E-Mail addresses to sets of authorized authorized MTAs, not whole email addresses to sets of authorized
users. Although the "l" macro (Section 8) provides a limited way to users. Although the "l" macro (Section 8) provides a limited way to
define individual sets of authorized MTAs for specific E-Mail define individual sets of authorized MTAs for specific email
addresses, it is generally impossible to verify, through SPF, the use addresses, it is generally impossible to verify, through SPF, the use
of specific E-Mail addresses by individual users of the same MTA. of specific email addresses by individual users of the same MTA.
It is up to mail services and their MTAs to directly prevent It is up to mail services and their MTAs to directly prevent
cross-user forgery: based on SMTP AUTH ([RFC2554]), users should be cross-user forgery: based on SMTP AUTH ([RFC4954]), users should be
restricted to using only those E-Mail addresses that are actually restricted to using only those email addresses that are actually
under their control (see [RFC4409], Section 6.1). Another means to under their control (see [RFC6409], Section 6.1). Another means to
verify the identity of individual users is message cryptography such verify the identity of individual users is message cryptography such
as PGP ([RFC2440]) or S/MIME ([RFC3851]). as PGP ([RFC4880]) or S/MIME ([RFC5751]).
10.5. Untrusted Information Sources 10.5. Untrusted Information Sources
SPF uses information supplied by third parties, such as the "HELO" An SPF compliant receiver gathers information from the SMTP commands
domain name, the "MAIL FROM" address, and SPF records. This it receives and from the published DNS records of the sending domain
information is then passed to the receiver in the Received-SPF: trace holder, (e.g., "HELO" domain name, the "MAIL FROM" address from the
fields and possibly returned to the client MTA in the form of an SMTP envelope, and SPF DNS records published by the domain holder).
rejection message. This information must be checked for invalid
characters and excessively long lines.
When the authorization check fails, an explanation string may be 10.5.1. Recorded Results
This information, passed to the receiver in the Received-SPF: or
Authentication-Results: trace fields, may be returned to the client
MTA as an SMTP rejection message. If such an SMTP rejection message
is generated, the information from the trace fields must be checked
for such problems as invalid characters and excessively long lines.
10.5.2. External Explanations
When the authorization check fails, an explanation string could be
included in the reject response. Both the sender and the rejecting included in the reject response. Both the sender and the rejecting
receiver need to be aware that the explanation was determined by the receiver need to be aware that the explanation was determined by the
publisher of the SPF record checked and, in general, not the publisher of the SPF record checked and, in general, not the
receiver. The explanation may contain malicious URLs, or it may be receiver. The explanation can contain malicious URLs, or it might be
offensive or misleading. offensive or misleading.
This is probably less of a concern than it may initially seem since Explanations returned to sender domains due to "exp" modifiers,
such messages are returned to the sender, and the explanation strings (Section 6.2), were generated by the sender policy published by the
come from the sender policy published by the domain in the identity domain holders themselves. As long as messages are only returned
claimed by that very sender. As long as the DSN is not redirected to with non-delivery notification ([RFC3464]) to domains publishing the
someone other than the actual sender, the only people who see explanation strings from their own DNS SPF records, the only affected
malicious explanation strings are people whose messages claim to be parties are the original publishers of the domain's SPF records.
from domains that publish such strings in their SPF records. In
practice, DSNs can be misdirected, such as when an MTA accepts an In practice, such non-delivery notifications can be misdirected, such
E-Mail and then later generates a DSN to a forged address, or when an as when an MTA accepts an email and only later generates the
E-Mail forwarder does not direct the DSN back to the original sender. notification to a forged address, or when an email forwarder does not
direct the bounce back to the original sender.
10.5.3. Macro Expansion
Macros (Section 8) allow senders to inject arbitrary text (any non-
null [US-ASCII] character) into receiver DNS queries. It is necesary
to be prepared for hostile or unexpected content.
10.6. Privacy Exposure 10.6. Privacy Exposure
Checking SPF records causes DNS queries to be sent to the domain Checking SPF records causes DNS queries to be sent to the domain
owner. These DNS queries, especially if they are caused by the owner. These DNS queries, especially if they are caused by the
"exists" mechanism, can contain information about who is sending "exists" mechanism, can contain information about who is sending
E-Mail and likely to which MTA the E-Mail is being sent. This can email and likely to which MTA the email is being sent. This can
introduce some privacy concerns, which may be more or less of an introduce some privacy concerns, which are more or less of an issue
issue depending on local laws and the relationship between the domain depending on local laws and the relationship between the domain owner
owner and the person sending the E-Mail. and the person sending the email.
11. Contributors and Acknowledgements 11. Contributors and Acknowledgements
This document is largely based on the work of Meng Weng Wong and Mark This document is largely based on the work of Meng Weng Wong, Mark
Lentczner. Although, as this section acknowledges, many people have Lentczner, and Wayne Schlitt. Although, as this section
contributed to this document, a very large portion of the writing and acknowledges, many people have contributed to this document, a very
editing are due to Meng and Mark. large portion of the writing and editing are due to Meng, Mark, and
Wayne.
This design owes a debt of parentage to [RMX] by Hadmut Danisch and This design owes a debt of parentage to [RMX] by Hadmut Danisch and
to [DMP] by Gordon Fecyk. The idea of using a DNS record to check 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 the legitimacy of an email address traces its ancestry further back
through messages on the namedroppers mailing list by Paul Vixie through messages on the namedroppers mailing list by Paul Vixie
[Vixie] (based on suggestion by Jim Miller) and by David Green [Vixie] (based on suggestion by Jim Miller) and by David Green
[Green]. [Green].
Philip Gladstone contributed the concept of macros to the Philip Gladstone contributed the concept of macros to the
specification, multiplying the expressiveness of the language and specification, multiplying the expressiveness of the language and
making per-user and per-IP lookups possible. making per-user and per-IP lookups possible.
The authors would also like to thank the literally hundreds of The authors of both this document and [RFC4408] would also like to
individuals who have participated in the development of this design. thank the literally hundreds of individuals who have participated in
They are far too numerous to name, but they include the following: the development of this design. They are far too numerous to name,
but they include the following:
The participants in the SPFbis working group.
The folks on the spf-discuss mailing list. The folks on the spf-discuss mailing list.
The folks on the SPAM-L mailing list. The folks on the SPAM-L mailing list.
The folks on the IRTF ASRG mailing list. The folks on the IRTF ASRG mailing list.
The folks on the IETF MARID mailing list. The folks on the IETF MARID mailing list.
The folks on #perl. The folks on #perl.
12. IANA Considerations 12. IANA Considerations
12.1. The SPF DNS Record Type 12.1. The SPF DNS Record Type
The IANA has assigned a new Resource Record Type and Qtype from the Per [RFC4408], the IANA assigned the Resource Record Type and Qtype
DNS Parameters Registry for the SPF RR type with code 99. from the DNS Parameters Registry for the SPF RR type with code 99.
The format of this type is identical to the TXT RR [RFC1035]. The
character content of the record is encoded as [US-ASCII]. Use of
this record type is obsolete for SPF Version 1.
IANA is requested to add an annotation to the SPF RRTYPE saying
"(OBSOLETE - use TXT)" in the DNS Parameters registry.
[NOTE TO RFC EDITOR: (to be changed to " ... has added ..." upon
publication)]
12.2. The Received-SPF Mail Header Field 12.2. The Received-SPF Mail Header Field
Per [RFC3864], the "Received-SPF:" header field is added to the IANA Per [RFC3864], the "Received-SPF:" header field is added to the IANA
Permanent Message Header Field Registry. The following is the Permanent Message Header Field Registry. The following is the
registration template: registration template:
Header field name: Received-SPF Header field name: Received-SPF
Applicable protocol: mail ([RFC2822]) Applicable protocol: mail ([RFC5322])
Status: Experimental Status: Standards Track
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): RFC 4408 Specification document(s): RFC XXXX
Related information: [NOTE TO RFC EDITOR: (this document)]
Requesting SPF Council review of any proposed changes and
additions to this field are recommended. For information about 12.3. SPF Modifier Registration
the SPF Council see http://www.openspf.org/Council
[RFC6652] created a new SPF Modifier Registration. IANA is requested
to change the reference for the exp and redirect modifiers from
[RFC4408] to this document. Their status should not be changed.
13. References 13. References
13.1. Normative References 13.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
April 2001. RFC 3463, January 2003.
[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 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC Resource Identifier (URI): Generic Syntax", STD 66,
3986, January 2005. RFC 3986, January 2005.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Specifications: ABNF", RFC 4234, October 2005. Architecture", RFC 4291, February 2006.
[US-ASCII] American National Standards Institute (formerly United [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[US-ASCII]
American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for States of America Standards Institute), "USA Code for
Information Interchange, X3.4", 1968. Information Interchange, X3.4", 1968.
ANSI X3.4-1968 has been replaced by newer versions with slight ANSI X3.4-1968 has been replaced by newer versions with
modifications, but the 1968 version remains definitive for slight modifications, but the 1968 version remains
the Internet. definitive for the Internet.
13.2 Informative References 13.2. Informative References
[DMP] Fecyk, G., "Designated Mailers Protocol".
Work In Progress
[Green] Green, D., "Domain-Authorized SMTP Mail", 2002.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, August [RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983,
1996. August 1996.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
"OpenPGP Message Format", RFC 2440, November 1998. NCACHE)", RFC 2308, March 1998.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication", [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
RFC 2554, March 1999. specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
[RFC3696] Klensin, J., "Application Techniques for Checking and [RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, February 2004. Transformation of Names", RFC 3696, February 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004. Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail [RFC3834] Moore, K., "Recommendations for Automatic Responses to
Extensions (S/MIME) Version 3.1 Message Specification", Electronic Mail", RFC 3834, August 2004.
RFC 3851, July 2004.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
RFC 4409, April 2006. for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
[RMX] Danish, H., "The RMX DNS RR Type for light weight sender [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
authentication", Work In Progress (CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[DMP] Fecyk, G., "Designated Mailers Protocol", Work In Progress [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[Vixie] Vixie, P., "Repudiating MAIL FROM", 2002. [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[Green] Green, D., "Domain-Authorized SMTP Mail", 2002. [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An
Applicability Statement for SMTP", RFC 6647, June 2012.
[RFC6652] Kitterman, S., "Sender Policy Framework (SPF)
Authentication Failure Reporting Using the Abuse Reporting
Format", RFC 6652, June 2012.
[RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework
(SPF) and Sender ID Experiments", RFC 6686, July 2012.
[RMX] Danisch, H., "The RMX DNS RR Type for light weight sender
authentication".
Work In Progress
[Vixie] Vixie, P., "Repudiating MAIL FROM", 2002.
Appendix A. Collected ABNF Appendix A. Collected ABNF
This section is normative and any discrepancies with the ABNF This section is normative and any discrepancies with the ABNF
fragments in the preceding text are to be resolved in favor of this fragments in the preceding text are to be resolved in favor of this
grammar. grammar.
See [RFC4234] for ABNF notation. Please note that as per this ABNF See [RFC5234] for ABNF notation. Please note that as per this ABNF
definition, literal text strings (those in quotes) are case- definition, literal text strings (those in quotes) are case-
insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx". insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx".
record = version terms *SP record = version terms *SP
version = "v=spf1" version = "v=spf1"
terms = *( 1*SP ( directive / modifier ) ) terms = *( 1*SP ( directive / modifier ) )
directive = [ qualifier ] mechanism directive = [ qualifier ] mechanism
qualifier = "+" / "-" / "?" / "~" qualifier = "+" / "-" / "?" / "~"
skipping to change at page 42, line 38 skipping to change at page 57, line 38
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ] MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
PTR = "ptr" [ ":" domain-spec ] PTR = "ptr" [ ":" domain-spec ]
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ] IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ] IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
exists = "exists" ":" domain-spec exists = "exists" ":" domain-spec
modifier = redirect / explanation / unknown-modifier modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string unknown-modifier = name "=" macro-string
; where name is not any known modifier
ip4-cidr-length = "/" 1*DIGIT ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ] dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = qnum "." qnum "." qnum "." qnum ip4-network = qnum "." qnum "." qnum "." qnum
qnum = DIGIT ; 0-9 qnum = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99 / %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 / "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249 / "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255 / "25" %x30-35 ; 250-255
; conventional dotted quad notation. e.g., 192.0.2.0 ; conventional dotted quad notation. e.g., 192.0.2.0
ip6-network = <as per [RFC 3513], section 2.2> ip6-network = <as per [RFC 4291], section 2.2>
; e.g., 2001:DB8::CD30 ; e.g., 2001:DB8::CD30
domain-spec = macro-string domain-end domain-spec = macro-string domain-end
domain-end = ( "." toplabel [ "." ] ) / macro-expand domain-end = ( "." toplabel [ "." ] ) / macro-expand
toplabel = ( *alphanum ALPHA *alphanum ) / toplabel = ( *alphanum ALPHA *alphanum ) /
( 1*alphanum "-" *( alphanum / "-" ) alphanum ) ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
; LDH rule plus additional TLD restrictions ; LDH rule plus additional TLD restrictions
; (see [RFC3696], Section 2) ; (see [RFC3696], Section 2 for background)
alphanum = ALPHA / DIGIT alphanum = ALPHA / DIGIT
explain-string = *( macro-string / SP ) explain-string = *( macro-string / SP )
macro-string = *( macro-expand / macro-literal ) macro-string = *( macro-expand / macro-literal )
macro-expand = ( "%{" macro-letter transformers *delimiter "}" ) macro-expand = ( "%{" macro-letter transformers *delimiter "}" )
/ "%%" / "%_" / "%-" / "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E macro-literal = %x21-24 / %x26-7E
; visible characters except "%" ; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" / macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t" "c" / "r" / "t" / "v"
transformers = *DIGIT [ "r" ] transformers = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." ) name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header-field = "Received-SPF:" [CFWS] result FWS [comment FWS] header-field = "Received-SPF:" [CFWS] result FWS [comment FWS]
[ key-value-list ] CRLF [ key-value-list ] CRLF
result = "Pass" / "Fail" / "SoftFail" / "Neutral" / result = "pass" / "fail" / "softfail" / "neutral" /
"None" / "TempError" / "PermError" "none" / "temperror" / "permerror"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"] [";"]
key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string ) key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string )
key = "client-ip" / "envelope-from" / "helo" / key = "client-ip" / "envelope-from" / "helo" /
"problem" / "receiver" / "identity" / "problem" / "receiver" / identity /
mechanism / "x-" name / name mechanism / name
identity = "mailfrom" ; for the "MAIL FROM" identity identity = "mailfrom" ; for the "MAIL FROM" identity
/ "helo" ; for the "HELO" identity / "helo" ; for the "HELO" identity
/ name ; other identities / name ; other identities
dot-atom = <unquoted word as per [RFC2822]> ALPHA = <A-Z / a-z as per [RFC5234]>
quoted-string = <quoted string as per [RFC2822]> DIGIT = <0-9 as per [RFC5234]>
comment = <comment string as per [RFC2822]> SP = <space character as per [RFC5234]>
CFWS = <comment or folding white space as per [RFC2822]> domain = <fully qualified domain as per [RFC5321]>
FWS = <folding white space as per [RFC2822]> dot-atom = <unquoted word as per [RFC5322]>
CRLF = <standard end-of-line token as per [RFC2822]> quoted-string = <quoted string as per [RFC5322]>
comment = <comment string as per [RFC5322]>
CFWS = <comment or folding white space as per [RFC5322]>
FWS = <folding white space as per [RFC5322]>
CRLF = <standard end-of-line token as per [RFC5322]>
authserv-id = <authserv-id per [RFC5451]>
reasonspec = <reason per [RFC5451]>
Appendix B. Extended Examples Appendix B. Extended Examples
These examples are based on the following DNS setup: These examples are based on the following DNS setup:
; A domain with two mail servers, two hosts ; A domain with two mail servers, two hosts
; and two servers at the domain name ; and two servers at the domain name
$ORIGIN example.com. $ORIGIN example.com.
@ MX 10 mail-a @ MX 10 mail-a
MX 20 mail-b MX 20 mail-b
skipping to change at page 44, line 46 skipping to change at page 60, line 46
; A rogue reverse IP domain that claims to be ; A rogue reverse IP domain that claims to be
; something it's not ; something it's not
$ORIGIN 0.0.10.in-addr.arpa. $ORIGIN 0.0.10.in-addr.arpa.
4 PTR bob.example.com. 4 PTR bob.example.com.
B.1. Simple Examples B.1. Simple Examples
These examples show various possible published records for These examples show various possible published records for
example.com and which values if <ip> would cause check_host() to example.com and which values if <ip> would cause check_host() to
return "Pass". Note that <domain> is "example.com". return "pass". Note that <domain> is "example.com".
v=spf1 +all v=spf1 +all
-- any <ip> passes -- any <ip> passes
v=spf1 a -all v=spf1 a -all
-- hosts 192.0.2.10 and 192.0.2.11 pass -- hosts 192.0.2.10 and 192.0.2.11 pass
v=spf1 a:example.org -all v=spf1 a:example.org -all
-- no sending hosts pass since example.org has no A records -- no sending hosts pass since example.org has no A records
v=spf1 mx -all v=spf1 mx -all
-- sending hosts 192.0.2.129 and 192.0.2.130 pass -- sending hosts 192.0.2.129 and 192.0.2.130 pass
v=spf1 mx:example.org -all v=spf1 mx:example.org -all
-- sending host 192.0.2.140 passes -- sending host 192.0.2.140 passes
v=spf1 mx mx:example.org -all v=spf1 mx mx:example.org -all
skipping to change at page 46, line 10 skipping to change at page 62, line 12
These records allow a set of domains that all use the same mail 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 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 the mail system's record needs to be updated when the mail setup
changes. These domains' records never have to change. changes. These domains' records never have to change.
B.3. DNSBL Style Example B.3. DNSBL Style Example
Imagine that, in addition to the domain records listed above, there Imagine that, in addition to the domain records listed above, there
are these: are these:
$ORIGIN _spf.example.com. mary.mobile-users A $ORIGIN _spf.example.com.
127.0.0.2 fred.mobile-users A 127.0.0.2 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 15.15.168.192.joel.remote-users A 127.0.0.2
16.15.168.192.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 The following records describe users at example.com who mail from
arbitrary servers, or who mail from personal servers. arbitrary servers, or who mail from personal servers.
example.com: example.com:
v=spf1 mx v=spf1 mx
include:mobile-users._spf.%{d} include:mobile-users._spf.%{d}
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"-include:ip4._spf.%{d} " "-include:ip4._spf.%{d} "
"-include:ptr._spf.%{d} " "-include:ptr._spf.%{d} "
"+all" ) "+all" )
ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all" ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all"
ptr._spf.example.com. SPF "v=spf1 -ptr +all" ptr._spf.example.com. SPF "v=spf1 -ptr +all"
This example shows how the "-include" mechanism can be useful, how an 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 SPF record that ends in "+all" can be very restrictive, and the use
of De Morgan's Law. of De Morgan's Law.
Authors' Addresses Appendix C. Change History
Meng Weng Wong Changes since RFC 4408 (to be removed prior to publication)
Singapore
EMail: mengwong+spf@pobox.com Moved to standards track
Wayne Schlitt Authors updated
4615 Meredeth #9
Lincoln Nebraska, NE 68506
United States of America
EMail: wayne@schlitt.net IESG Note regarding experimental use replaced with discussion of
URI: http://www.schlitt.net/spf/ results
Full Copyright Statement Process errata:
Copyright (C) The Internet Society (2006). Add %v macro to ABNF grammar
This document is subject to the rights, licenses and restrictions Replace "uric" by "unreserved"
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 Recommend an SMTP reply code for optional permerror rejections
"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 Correct syntax in Received-SPF examples
The IETF takes no position regarding the validity or scope of any Fix unknown-modifier clause is too greedy in ABNF
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 Correct use of empty domain-spec on exp modifier
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 Fix minor typo errata
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 Convert to spfbis working group draft,
draft-ietf-spfbis-4408bis-00
Funding for the RFC Editor function is provided by the IETF Addressed Ticket #1, RFC 4408 Section 2.5.6 - Temporary errors by
Administrative Support Activity (IASA). giving the option to turn repeated SERVFAIL into permerror and
adding RFC 2308 reference.
Clarified text about IPv4 mapped addresses to resolve test suite
ambiguity
Clarified ambiguity about result when more than 10 "mx" or "ptr"
records are returned for lookup to specify permerror. This
resolves one of the test suite ambiguities
Made all references to result codes lower case per issue #7
Adjusted section 2.2 Requirement to check mail from per issue #15
Added missing "v" element in macro-letter in the collected ABNF
per issue #16 - section 8.1 was already fixed in the pre-WG draft
Marked ptr and "p" macro deprecated/SHOULD NOT use per issue #27
Expunged lower case may from the draft per issue #8
Expunged "x-" name as an obsolete concept
Updated obslete references: RFC2821 to RFC5321, RFC2822 to
RFC5322, and RFC4234 to RFC5234
Refer to RFC6647 to describe greylisting instead of trying to
describe it directly.
Updated informative references to the current versions.
Added definition for deprecated since there are questions.
Start to rework section 9 with some RFC5598 terms.
Added mention of RFC 6552 feedback reports in section 9.
Added draft-ietf-spfbis-experiment as an informational reference.
Drop Type SPF.
Try and clarify informational nature of RFC3696
Fix ABNF nits and add missing definitions per Bill's ABNF checker.
Make DNS lookup time limit SHOULD instead of MAY.
Reorganize and clarify processing limits. Move hard limits to new
section 4.6.4, Evaluation Limits. Move advice to non-normative
section 9.
Removed paragraph in section 10.1 about limiting total data
volumes as it is unused (and removable per the charter) and serves
no purpose (it isn't something that actually can be implemented in
any reasonable way).
Added text and figures from Alessandro Vesely in section 9.1 to
better explain DNS resource limits.
Multiple editorial fixes from Murray Kucherawy's review.
Also based on Murray's review, reworked SMTP identity definitions
and made RFC 5598 a normative reference instead of informative.
This is a downref that will have to be mentioned in the last call.
Added RFC 3834 as an informative reference about backscatter.
Added IDN requirements and normative reference to RFC 5890 to deal
with the question "like DKIM did it.:
Added informative reference to RFC 4632 for CIDR and use CIDR
prefix length instead of CIDR-length to match its terminology.
Added RFC 5782 informative reference on DNSxLs to support
improving the exists description.
Added text on creating a Authentication-Results header field that
matches the Received-SPF header field information and added a
normative reference to RFC 5451.
Added informative reference to RFC 2782 due to SRV mention.
Added informative reference to RFC 3464 due to DSN mention.
Added informative reference to RFC 5617 for it's DNS wildcard use.
Added informative reference to RFC 5782 to enhance the explanation
of how the exists mechanism works. Clarified the intended match/
no-match method.
Added new sections on Receiver policy for SPF pass, fail, and
permerror.
Added new section 9 discussion on treatment of bounces and the
significance of HELO records.
Added request to IANA to update the SPF modifier registry.
Author's Address
Scott Kitterman
Kitterman Technical Services
3611 Scheel Dr
Ellicott City, MD 21042
United States of America
Email: scott@kitterman.com
 End of changes. 291 change blocks. 
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