Network Working Group M. Wong
Internet-Draft W. Schlitt
Expires: June 30, 2005 December 30, 2004
Sender Policy Framework: Authorizing Use of Domains in E-MAIL
draft-schlitt-spf-classic-00
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on June 30, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
E-mail on the Internet can be forged in a number of ways. In
particular, existing protocols place no restriction in what a sending
host can use as the reverse-path of a message. This document
describes a protocol whereby a domain can explicitly authorize the
hosts that are allowed to use its domain name in a reverse-path, and
a way for receiving hosts to check such authorization.
Wong & Schlitt Expires June 30, 2005 [Page 1]
Internet-Draft Sender Policy Framework (SPF) December 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Protocol Status . . . . . . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 The HELO Identity . . . . . . . . . . . . . . . . . . . . 6
2.2 The MAIL FROM Identity . . . . . . . . . . . . . . . . . . 6
2.3 Publishing Authorization . . . . . . . . . . . . . . . . . 6
2.4 Checking Authorization . . . . . . . . . . . . . . . . . . 7
2.5 Interpreting the Result . . . . . . . . . . . . . . . . . 8
2.5.1 None . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5.2 Neutral . . . . . . . . . . . . . . . . . . . . . . . 8
2.5.3 Pass . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5.4 Fail . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5.5 SoftFail . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.6 TempError . . . . . . . . . . . . . . . . . . . . . . 9
2.5.7 PermError . . . . . . . . . . . . . . . . . . . . . . 9
3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Publishing . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1 DNS Resource Record Types . . . . . . . . . . . . . . 10
3.1.2 Multiple Records . . . . . . . . . . . . . . . . . . . 11
3.1.3 Multiple Strings . . . . . . . . . . . . . . . . . . . 11
3.1.4 Record Size . . . . . . . . . . . . . . . . . . . . . 11
3.1.5 Wildcard Records . . . . . . . . . . . . . . . . . . . 12
4. The check_host() Function . . . . . . . . . . . . . . . . . . 13
4.1 Arguments . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Initial Processing . . . . . . . . . . . . . . . . . . . . 13
4.4 Record Lookup . . . . . . . . . . . . . . . . . . . . . . 13
4.5 Selecting Records . . . . . . . . . . . . . . . . . . . . 14
4.6 Record Evaluation . . . . . . . . . . . . . . . . . . . . 14
4.6.1 Term Evaluation . . . . . . . . . . . . . . . . . . . 15
4.6.2 Mechanisms . . . . . . . . . . . . . . . . . . . . . . 15
4.6.3 Modifiers . . . . . . . . . . . . . . . . . . . . . . 16
4.7 Default Result . . . . . . . . . . . . . . . . . . . . . . 16
4.8 Domain Specification . . . . . . . . . . . . . . . . . . . 16
5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 17
5.1 "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2 "include" . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3 "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.4 "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.5 "ptr" . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.6 "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 21
5.7 "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 23
6.1 redirect: Redirected Query . . . . . . . . . . . . . . . . 23
6.2 exp: Explanation . . . . . . . . . . . . . . . . . . . . . 24
Wong & Schlitt Expires June 30, 2005 [Page 2]
Internet-Draft Sender Policy Framework (SPF) December 2004
7. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1 Processing Limits . . . . . . . . . . . . . . . . . . . . 26
7.2 The Received-SPF header . . . . . . . . . . . . . . . . . 27
8. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1 Macro definitions . . . . . . . . . . . . . . . . . . . . 30
8.2 Expansion Examples . . . . . . . . . . . . . . . . . . . . 33
9. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1 Sending Domains . . . . . . . . . . . . . . . . . . . . . 34
9.2 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 34
9.3 Forwarding Services and Aliases . . . . . . . . . . . . . 34
9.4 Mail Services . . . . . . . . . . . . . . . . . . . . . . 35
9.5 MTA Relays . . . . . . . . . . . . . . . . . . . . . . . . 35
10. Security Considerations . . . . . . . . . . . . . . . . . . 37
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 39
12. Contributors and Acknowledgements . . . . . . . . . . . . . 40
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
13.1 Normative References . . . . . . . . . . . . . . . . . . . . 41
13.2 Informative References . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 42
A. Collected ABNF . . . . . . . . . . . . . . . . . . . . . . . . 43
B. Extended Examples . . . . . . . . . . . . . . . . . . . . . . 45
B.1 Simple Examples . . . . . . . . . . . . . . . . . . . . . 45
B.2 Multiple Domain Example . . . . . . . . . . . . . . . . . 46
B.3 DNSBL Style Example . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . 48
Wong & Schlitt Expires June 30, 2005 [Page 3]
Internet-Draft Sender Policy Framework (SPF) December 2004
1. Introduction
The current e-mail infrastructure has the property that any host
injecting mail into the mail system can identify itself as any domain
name it wants. Hosts can do this at a variety of levels: in
particular, the session, the envelope, and the mail headers. While
this feature is desirable in some circumstances, it is a major
obstacle to reducing end-user unwanted e-mail (or "spam").
Furthermore, many domain name holders are understandably concerned
about the ease with which other entities may make use of their domain
names, often with intent to impersonate.
This document defines a protocol by which domain owners may authorize
hosts to use their domain name in the "MAIL FROM" or "HELO" identity.
Compliant domain holders publish SPF records about which hosts are
permitted to use their names, and compliant mail receivers use the
published SPF records to test the authorization of hosts using a
given "HELO" or "MAIL FROM" identity during a mail transaction.
An additional benefit to mail receivers is that when the use of an
identity is verified, then local policy decisions about the mail can
be made on the basis of the domain, rather than the host's IP
address. This is advantageous because reputation of domain names is
likely to be more accurate than reputation of host IP addresses.
Furthermore, if a claimed identity fails verification, then local
policy can take stronger action against such e-mail, such as
rejecting it.
1.1 Protocol Status
SPF has been in development since the Summer of 2003, and has seen
deployment beyond the developers beginning in December, 2003. The
design of SPF slowly evolved until the spring of 2004 and has since
stabilized. There have been quite a number of forms of SPF, some
written up as documents, some submitted as Internet Drafts, and many
discussed and debated in development forums.
The goal of this document is to clearly document the protocol defined
by earlier drafts specifications of SPF as used in existing
implementations. This conception of SPF is sometimes called "SPF
Classic". It is understood that particular implementations and
deployments may differ from, and build upon, this work. It is hoped
that we have nonetheless captured the common understanding of SPF
version 1.
1.2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
Wong & Schlitt Expires June 30, 2005 [Page 4]
Internet-Draft Sender Policy Framework (SPF) December 2004
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document is concerned with a portion of a mail message commonly
called "envelope sender", "return path", "reverse path", "bounce
address", "2821 FROM", or "MAIL FROM". Since these terms are either
not well defined, or often used casually, this document defines the
"MAIL FROM" identity in Section 2.2. Note that other terms, that may
superficially look like the common terms, such as "reverse-path", are
used only with the defined meanings from normative documents.
Wong & Schlitt Expires June 30, 2005 [Page 5]
Internet-Draft Sender Policy Framework (SPF) December 2004
2. Operation
2.1 The HELO Identity
The "HELO" identity derives from either the SMTP HELO or EHLO command
(see [RFC2821].) These commands supply the SMTP client (sender) 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 client must be prepared for the "HELO" identity to be
malformed.
SPF clients MAY check the "HELO" identity by calling the check_host()
function (Section 4) with the "HELO" identity as the <sender>. If
the HELO test returns a "fail", the overall result for the SMTP
session is "fail", and there is no need to test the "MAIL FROM"
identity.
2.2 The MAIL FROM Identity
The "MAIL FROM" identity derives from the SMTP MAIL command (see
[RFC2821].) This command supplies the "reverse-path" for a message,
which generally consists of the sender mailbox, and is the mailbox to
which notification messages are sent if there are problems delivering
the message.
[RFC2821] allows the reverse-path to be null (see Section 4.5.5.) In
this case, there is no explicit sender mailbox, and such a message
can be assumed to be a notification message from the mail system
itself. When the reverse-path is null, this document defines the
"MAIL FROM" identity to be the mailbox composed of the localpart
"postmaster" and the "HELO" identity
SPF clients MUST check the "MAIL FROM" identity unless HELO testing
produced a "fail". SPF clients check the "MAIL FROM" identity by
calling the check_host() function with the "MAIL FROM" identity as
the <sender>.
2.3 Publishing Authorization
An SPF compliant domain MUST publish a valid SPF record as described
in Section 3. This record authorizes the use of the domain name in
the "HELO" and/or "MAIL FROM" identity, by some sending MTAs, and not
by others.
It is RECOMMENDED that domains publish SPF records that end in
"-all", or redirect to other records that do, so that a definitive
determination of authorization can be made.
Wong & Schlitt Expires June 30, 2005 [Page 6]
Internet-Draft Sender Policy Framework (SPF) December 2004
Domain holders may publish SPF records that explicitly authorize no
hosts for domain names that shouldn't be used in sender mailboxes.
2.4 Checking Authorization
A mail receiver can perform an SPF compliant check for each mail
message it receives. This check tests the authorization of a client
host to inject mail with a given "MAIL FROM" identity. This check
MAY also be applied to the "HELO" identity. Typically, such checks
are done by a receiving MTA, but can be performed elsewhere in the
mail processing chain so long as the required information is
available. Checking other identities against SPF records is NOT
RECOMMENDED because there are cases that are known to give incorrect
results.
It is possible that mail receivers will use the SPF check as part of
a larger set of tests on incoming mail. The results of other tests
may influence whether or not a particular SPF check is performed.
For example, finding the sending host on a local white list may cause
all other tests to be skipped and all mail from that host to be
accepted.
When a mail receiver decides to perform an SPF check, it MUST
implement and evaluate the check_host() function (Section 4)
correctly. While the test as a whole is optional, once it has been
decided to perform a test it must be performed as specified so that
the correct semantics are preserved between publisher and receiver.
To make the test, the mail receiver MUST evaluate the check_host()
with the arguments set as follows:
<ip> - the IP address of the SMTP client that is injecting the
mail, either IPv4 or IPv6.
<domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
<sender> - the "MAIL FROM" or "HELO" identity.
Note that the <domain> argument may not be a well formed domain name.
For example, if the reverse-path was null, then the EHLO or HELO
domain is used. In a valid SMTP session, this can be an address
literal or entirely malformed. In these cases, check_host() is
defined in Section 4.3 to return a "None" result.
Care must be taken to correctly extract the <domain> from the
<sender> as many MTAs will still accept such things as source routes
(see [RFC2821] appendix C), the percent hack (see [RFC2162]) and bang
paths (see [RFC1983]). These archaic features have been maliciously
used to bypass security systems.
Wong & Schlitt Expires June 30, 2005 [Page 7]
Internet-Draft Sender Policy Framework (SPF) December 2004
Software SHOULD perform this authorization check during the
processing of the SMTP transaction that injects the mail. This
allows errors to be returned directly to the injecting server by way
of SMTP replies. Software can perform the check as early as the MAIL
command, though it may be easier to delay the check to some later
stage of the transaction.
Software can perform the authorization after the corresponding SMTP
transaction has completed. There are two problems with this
approach: 1) It may be difficult to accurately extract all the
required information such as client IP address and HELO domain name.
2) If the authorization fails, then generating a non-delivery
notification to the alleged sender is problematic due to the large
number of forged emails on the Internet today. Such an action would
go against the explicit wishes of the alleged sender.
2.5 Interpreting the Result
The check_host() function returns one of seven results. This section
describes how software that performs the authorization must interpret
the results. If the check is being performed during the SMTP mail
transaction, it also describes how to respond.
2.5.1 None
A result of None means that no records were published by the domain.
The checking software cannot ascertain if the client host is
authorized or not.
2.5.2 Neutral
The domain owner has explicitly stated that doesn't know whether the
IP is authorized or not. A Neutral result MUST be treated exactly
like the None result.
2.5.3 Pass
A Pass result means that the client is authorized to inject mail with
the given identity. Further policy checks, such as reputation, or
black and/or white listing, can now proceed with confidence in the
identity.
2.5.4 Fail
A Fail result is an explicit statement that the client is not
authorized to use the domain in the given identity. The checking
software can choose to mark the mail based on this, or to reject the
mail outright.
Wong & Schlitt Expires June 30, 2005 [Page 8]
Internet-Draft Sender Policy Framework (SPF) December 2004
If the checking software chooses to reject the mail during the SMTP
transaction, then it SHOULD use an SMTP reply code of 550 (see
[RFC2821]) and, if supported, the 5.7.1 DSN code (see [RFC2034]), in
addition to an appropriate message. The check_host() function may
return either a default explanation string, or one from the domain
that published the SPF records (see Section 6.2). If the information
doesn't originate with the checking software, it should be made clear
that text is not trusted. For example:
550-5.7.1 SPF MAIL FROM check failed:
550-5.7.1 The domain example.com explains:
550 5.7.1 Please see http://www.example.com/mailpolicy.html
2.5.5 SoftFail
A SoftFail result should be treated as somewhere between a Fail and a
Neutral. The domain believes the host isn't authorized but isn't
willing to make that strong of a statement. Receiving software
SHOULD NOT reject the message based on this result, but MAY subject
the message to closer scrutiny.
Since the domain has discouraged the use of this host, receivers MAY
try to inform either the sender or the recipient of the e-mail. As
examples, the recipient's MUA could highlight the SoftFail status.
Or the MTA could give the sender a message using a technique called
"greylisting" where by the MTA can issue an SMTP reply code of 451
(4.3.0 DSN code) with a note the first time the message was received,
but accept it the second time.
2.5.6 TempError
A TempError result means that the SPF client encountered a transient
error when performing the check. Checking software can choose to
accept or temporarily reject the message. If the message is rejected
during the SMTP transaction for this reason, the software SHOULD use
an SMTP reply code of 451 and, if supported, the 4.4.3 DSN code.
2.5.7 PermError
A PermError result means that the domain's published records couldn't
be correctly interpreted. Checking software SHOULD reject the
message. If rejecting during SMTP transaction time, it SHOULD use an
SMTP reply code of 550 and, if supported, the 5.5.2 DSN code.
Wong & Schlitt Expires June 30, 2005 [Page 9]
Internet-Draft Sender Policy Framework (SPF) December 2004
3. SPF Records
An SPF record declares which hosts are, and are not, authorized to
use a domain name for the "HELO" or "MAIL FROM" identity. Loosely,
the record partitions all hosts into permitted and not-permitted
sets. (Though some hosts might fall into neither category.)
The SPF record is a single string of text. An example record is:
v=spf1 +mx a:colo.example.com/28 -all
This record has a version of "v=spf1" and three directives: "+mx",
"a:colo.example.com/28" (the + is implied), and "-all".
3.1 Publishing
Domain owners wishing to be SPF compliant must publish SPF records
for the hosts that are used in both the MAIL FROM and HELO
identities. The SPF records are placed in the DNS tree at the host
name it pertains to, not a subdomain under it, such as is done with
SRV records. This is the same whether TXT RRs or SPF RRs are used.
The example above in Section 3 might be published easily via this
lines in a domain zone file:
example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
smtp-out.example.com. IN TXT "v=spf1 a -all"
When publishing via TXT records, beware of other TXT records
published there for other purposes. They may cause problems with
size limits (see Section 3.1.4.)
An SPF record published at the zone cut for the domain will be used
as a default for all subdomains within the zone (See Section 4.5.)
Domain owners SHOULD publish SPF records for hosts used for the HELO
and MAIL FROM identities instead of using the zone cut default
because the fallback requires additional DNS lookups. The zone cut
default does reduce the need to publish SPF records for non-email
related hosts, such as www.example.com.
3.1.1 DNS Resource Record Types
This document defines a new DNS Resource Record (RR) of type SPF,
type code to be determined. The format of this type is identical to
the TXT RR [RFC1035]. For either type, the character content of the
record is encoded as US-ASCII.
It is recognized that the current practice (using a TXT record) is
Wong & Schlitt Expires June 30, 2005 [Page 10]
Internet-Draft Sender Policy Framework (SPF) December 2004
not optimal, but it is necessary because there are a number of DNS
server and resolver implementations in common use that cannot handle
the new RR type. The two record type scheme provides a forward path
to the better solution of using a RR type reserved for this purpose.
An SPF compliant domain name SHOULD have SPF records of both RR
types. A compliant domain name MUST have a record of at least one
type. If a domain has records of both types, they MUST have
identical content. For example, instead of just publishing one
record as in Section 3.1 above, it is better to publish:
example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"
An SPF compliant check SHOULD lookup both types. Lookups can be
performed serially or in parallel. If both types of records are
obtained for a domain, the SPF type MUST be used.
Example RRs in this document are shown with the TXT record type,
however they could also be published with both RR types.
3.1.2 Multiple Records
A domain name MUST NOT have multiple records that would cause an
authorization check to select more than one record. See Section 4.5
for the selection rules.
3.1.3 Multiple Strings
A text DNS record (either TXT and SPF RR types) can be composed of
more than one string. If a published record contains multiple
strings, then the record MUST be treated as if those strings are
concatenated together without adding spaces. For example:
IN TXT "v=spf1 .... first" "second string..."
MUST be treated as equivalent to
IN TXT "v=spf1 .... firstsecond string..."
SPF or TXT records containing multiple strings are useful in order to
construct longer records which would otherwise exceed the maximum
length of a string within a TXT or SPF RR record.
3.1.4 Record Size
The published SPF record for a given domain name SHOULD remain small
enough that the results of a query for it will fit within 512 octets.
Wong & Schlitt Expires June 30, 2005 [Page 11]
Internet-Draft Sender Policy Framework (SPF) December 2004
This will keep even older DNS implementations from falling over to
TCP. Since the answer size is dependent on many things outside the
scope of this document, it is only possible to give this guideline:
If the combined length of the DNS name and the text of all the
records of a given type (TXT or SPF) is under 450 characters, then
DNS answers should fit in UDP packets. Note that when computing the
sizes for queries of the TXT format, one must take into account any
other TXT records published at the domain name. Records that are too
long to fit in a single UDP packet MAY be silently ignored.
3.1.5 Wildcard Records
Use of wildcard records for publishing is not recommended. Care must
be taken if wildcard records are used. If a domain publishes
wildcard MX records, it may want to publish wildcard declarations,
subject to the same requirements and problems. In particular, the
declaration must be repeated for any host that has any RR records at
all, and for subdomains thereof. For example, the example given in
[RFC1034], Section 4.3.3, could be extended with:
X.COM. MX 10 A.X.COM
X.COM. TXT "v=spf1 a:A.X.COM -all"
*.X.COM. MX 10 A.X.COM
*.X.COM. TXT "v=spf1 a:A.X.COM -all"
A.X.COM. A 1.2.3.4
A.X.COM. MX 10 A.X.COM
A.X.COM. TXT "v=spf1 a:A.X.COM -all"
*.A.X.COM. MX 10 A.X.COM
*.A.X.COM. TXT "v=spf1 a:A.X.COM -all"
Notice that SPF records must be repeated twice for every name within
the domain: Once for the name, and once with a wildcard to cover the
tree under the name.
Use of wildcards is discouraged in general as they cause every name
under the domain to exist and queries against arbitrary names will
never return RCODE 3 (Name Error).
Wong & Schlitt Expires June 30, 2005 [Page 12]
Internet-Draft Sender Policy Framework (SPF) December 2004
4. The check_host() Function
The check_host() function fetches SPF records, parses them, and
interprets them to evaluate if a particular host is or is not
permitted to send mail with a given identity. Mail receivers that
perform this check MUST correctly evaluate the check_host() function
as described here.
Implementations MAY use a different algorithm than the canonical
algorithm defined here, so long as the results are the same.
4.1 Arguments
The function check_host() takes these arguments:
<ip> - the IP address of the SMTP client that is injecting the
mail, either IPv4 or IPv6.
<domain> - the domain portion of the "MAIL FROM" or "HELO" identity.
<sender> - the "MAIL FROM" or "HELO" identity.
The domain portion of <sender> will usually be the same as the
<domain> argument when check_host() is initially evaluated. However,
it will generally not be true for recursive evaluations (see Section
5.2 below).
Actual implementations of the check_host() function will likely need
additional arguments.
4.2 Results
The function check_host() can result in one of seven results
described in Section 2.5. Based on the result, the action to be
taken is determined by the local policies of the receiver.
4.3 Initial Processing
If the <domain> is malformed or is not a fully qualified domain name,
check_host() immediately returns the result "None".
If the <sender> has no localpart, substitute the string "postmaster"
for the localpart.
4.4 Record Lookup
In accordance with how the records are published, see Section 3.1
above, a DNS query needs to be made for the <domain> name, querying
for either RR type TXT, SPF or both.
Wong & Schlitt Expires June 30, 2005 [Page 13]
Internet-Draft Sender Policy Framework (SPF) December 2004
If the DNS lookup returns a server failure (RCODE 2), or other error
(RCODE other than 0 or 3), or the query times out, check_host() exits
immediately with the result "TempError"
4.5 Selecting Records
Records begin with a version section:
record = version terms *SP
version = "v=spf1"
Starting with the set of records that were returned by the lookup,
record selection proceeds in two steps:
1. If any records of type SPF are in the set, then all records of
type TXT are discarded.
2. Records that do not begin with a version section of exactly
"v=spf1" are discarded. Note that the version section is
terminated either by a SP character or the end of the record. A
record with a version section of "v=spf10" does not match and
must be discarded.
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 for the <domain;>, the SPF client
MUST find the Zone Cut as defined in [RFC2181] section 6 and repeat
the above steps. The <domain>'s zone origin is then searched for SPF
records. If an SPF record is found at the zone origin, the <domain>
is set to the zone origin as if a "redirect" modifier was executed.
If no matching records are returned for either search, an SPF client
MUST assume that the domain makes no SPF declarations. SPF
processing MUST abort and return "None".
4.6 Record Evaluation
After one SPF record has been selected, the check_host() function
parses and interprets it to find a result for the current test. If
there are any syntax errors, check_host() returns immediately with
the result "PermError".
Implementations MAY choose to parse the entire record first and
return "PermError" if the record is not syntactically well formed.
However, in all cases, any syntax errors anywhere in the record MUST
be detected.
Wong & Schlitt Expires June 30, 2005 [Page 14]
Internet-Draft Sender Policy Framework (SPF) December 2004
4.6.1 Term Evaluation
There are two types of terms: mechanisms and modifiers. A record
contains an ordered list of these as specified in the following ABNF.
terms = *( 1*SP ( directive / modifier ) )
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists )
modifier = redirect / explanation / unknown-modifier
unknown-modifier = name "=" macro-string
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
Most mechanisms allow a ":" or "/" character after the name.
Modifiers always contain an equals ('=') character immediately after
the name, and before any ":" or "/" characters that may be part of
the macro-string.
Terms that do not contain any of "=", ":" or "/" are mechanisms.
As per the definition of the ABNF notation in [RFC2234], mechanism
and modifier names are case-insensitive.
4.6.2 Mechanisms
Each mechanism is considered in turn from left to right. If there
are no more mechanisms, the result is specified in Section 4.7.
When a mechanism is evaluated, one of three things can happen: it can
match, it can not match, or it can throw an exception.
If it matches, processing ends and the prefix value is returned as
the result of that record. If it does not match, processing
continues with the next mechanism. If it throws an exception,
mechanism processing ends and the exception value is returned.
The possible prefixes, and the results they return are:
"+" Pass
"-" Fail
"~" SoftFail
Wong & Schlitt Expires June 30, 2005 [Page 15]
Internet-Draft Sender Policy Framework (SPF) December 2004
"?" Neutral
The prefix is optional and defaults to "+".
When a mechanism matches, and the prefix is "-" so that a "Fail"
result is returned and the explanation string is computed as
described in Section 6.2.
Specific mechanisms are described in Section 5.
4.6.3 Modifiers
Modifiers are not mechanisms: they do not return match or not-match.
Instead they provide additional information. While modifiers do not
directly effect the evaluation of the record, the "redirect" modifier
has an effect after all the mechanisms have been evaluated.
4.7 Default Result
If none of the mechanisms match and there is no "redirect" modifier,
then the check_host() returns a result of "Neutral". If there is a
"redirect" modifier, check_host() proceeds as defined in Section 6.1.
Note that records SHOULD always either use a "redirect" modifier or
an "all" mechanism to explicitly terminate processing.
For example:
v=spf1 +mx -all
or
v=spf1 +mx redirect=_spf.example.com
4.8 Domain Specification
Several of these mechanisms and modifiers have a <domain-spec>
section. The <domain-spec> string is macro expanded (see Section 8).
The resulting string is the common presentation form of a fully
qualified DNS name: A series of labels separated by periods. This
domain is called the <target-name> in the rest of this document.
Note: The result of the macro expansion is not subject to any further
escaping. Hence, this facility cannot produce all characters that
are legal in a DNS label (e.g. the control characters). However,
this facility is powerful enough to express legal host names, and
common utility labels (such as "_spf") that are used in DNS.
For several mechanisms, the <domain-spec> is optional. If it is not
provided, the <domain> is used as the <target-name>.
Wong & Schlitt Expires June 30, 2005 [Page 16]
Internet-Draft Sender Policy Framework (SPF) December 2004
5. Mechanism Definitions
This section defines two types of mechanisms.
Basic mechanisms contribute to the language framework. They do not
specify a particular type of authorization scheme.
all
include
Designated sender mechanisms are used to designate a set of <ip>
addresses as being permitted or not to use the <domain> for sending
mail.
a
mx
ptr
ip4
ip6
exists
The following conventions apply to all mechanisms that perform a
comparison between <ip> and an IP address at any point:
If no CIDR-length is given in the directive, then <ip> and the IP
address are compared for equality.
If a CIDR-length is specified, then only the specified number of
high-order bits of <ip> and the IP address are compared for equality.
When any mechanism fetches host addresses to compare with <ip>, when
<ip> is an IPv4 address, A records are fetched, when <ip> is an IPv6
address, AAAA records are fetched. Even if the SMTP connection is
via IPv6, an IPv4-mapped IPv6 IP address (see [RFC3513] section
2.5.5) MUST still be considered an IPv4 address.
Several mechanisms rely on information fetched from DNS. For these
DNS queries, except where noted, if the DNS server returns an error
(RCODE other than 0 or 3) or the query times out, the mechanism
throws the exception "TempError". If the server returns "domain does
not exist" (RCODE 3), then evaluation of the mechanism continues as
if the server returned no error (RCODE 0) and zero answer records.
5.1 "all"
all = "all"
The "all" mechanism is a test that always matches. It is used as the
Wong & Schlitt Expires June 30, 2005 [Page 17]
Internet-Draft Sender Policy Framework (SPF) December 2004
rightmost mechanism in a record to provide an explicit default.
For example:
v=spf1 a mx -all
Mechanisms after "all" will never be tested. Any "redirect" modifier
(Section 6.1) has no effect when there is an "all" mechanism.
5.2 "include"
include = "include" ":" domain-spec
The "include" mechanism triggers a recursive evaluation of
check_host(). The domain-spec is expanded as per Section 8. Then
check_host() is evaluated with the resulting string as the <domain>.
The <ip> and <sender> arguments remain the same as in the current
evaluation of check_host().
In hind sight, the name "include" was poorly chosen. Only the
evaluated results of the referenced SPF record is used, rather than
acting as if the referenced SPF record was literally included in the
first. Better names for this mechanism would have been something
like "on-pass" or "if-pass".
The "include" mechanism makes it possible for one domain to designate
multiple administratively independent domains. For example, a vanity
domain "example.net" might send mail using the servers of
administratively independent domains example.com and example.org.
Example.net could say
"v=spf1 include:example.com include:example.org -all".
That would direct check_host() to, in effect, check the records of
example.com and example.org for a "pass" result. Only if the host
were not permitted for either of those domains would the result be
"Fail".
Whether this mechanism matches or not, or throws an error depends on
the result of the recursive evaluation of check_host():
Wong & Schlitt Expires June 30, 2005 [Page 18]
Internet-Draft Sender Policy Framework (SPF) December 2004
+---------------------------------+---------------------------------+
| A recursive check_host() result | Causes the "include" mechanism |
| of: | to: |
+---------------------------------+---------------------------------+
| Pass | match |
| | |
| Fail | not match |
| | |
| SoftFail | not match |
| | |
| Neutral | not match |
| | |
| TempError | throw TempError |
| | |
| PermError | throw PermError |
| | |
| None | throw PermError |
+---------------------------------+---------------------------------+
The "include" mechanism is intended for crossing administrative
boundaries. While it is possible to use includes to consolidate
multiple domains that share the same set of designated hosts, domains
are encouraged to use redirects where possible, and to minimize the
number of includes within a single administrative domain. For
example, if example.com and example.org were managed by the same
entity, and if the permitted set of hosts for both domains were
"mx:example.com", it would be possible for example.org to specify
"include:example.com", but it would be preferable to specify
"redirect=example.com" or even "mx:example.com".
5.3 "a"
This mechanism matches if <ip> is one of the <target-name>'s IP
addresses.
A = "a" [ ":" domain-spec ] [ dual-cidr-length ]
An address lookup is done on the <target-name>. The <ip> is compared
to the returned address(es). If any address matches, the mechanism
matches.
5.4 "mx"
This mechanism matches if <ip> is one of the MX hosts for a domain
name.
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
Wong & Schlitt Expires June 30, 2005 [Page 19]
Internet-Draft Sender Policy Framework (SPF) December 2004
check_host() first performs an MX lookup on the <target-name>. Then
it performs an address lookup on each MX name returned. The <ip> is
compared to each returned IP address. To prevent DoS attacks, a
limit of 10 MX names MUST be enforced (see Section 10). If any
address matches, the mechanism matches.
Note regarding implicit MXes: If the <target-name> has no MX records,
check_host() MUST NOT pretend the target is its single MX, and MUST
NOT default to an A lookup on the <target-name> directly. This
behavior breaks with the legacy "implicit MX" rule. See [RFC2821]
Section 5. If such behavior is desired, the publisher should specify
an "a" directive.
5.5 "ptr"
This mechanism tests if the DNS reverse mapping for <ip> exists and
correctly points to a domain name within a particular domain.
PTR = "ptr" [ ":" domain-spec ]
First the <ip>'s name is looked up using this procedure: perform a
DNS reverse-mapping for <ip>, looking up the corresponding PTR record
in "in-addr.arpa." if the address is an IPv4 one and in "ip6.arpa."
if it is an IPv6 address. For each record returned, validate the
domain name by looking up its IP address. To prevent DoS attacks, a
limit of 10 PTR names MUST be enforced (see Section 10). If <ip> is
among the returned IP addresses, then that domain name is validated.
In pseudocode:
sending-domain_names := ptr_lookup(sending-host_IP);
if more than 10 sending-domain_names are found, use at most 10.
for each name in (sending-domain_names) {
IP_addresses := a_lookup(name);
if the sending-domain_IP is one of the IP_addresses {
validated-sending-domain_names += name;
}
}
Check all validated domain names to see if they end in the
<target-name> domain. If any do, this mechanism matches. If no
validated domain name can be found, or if none of the validated
domain names end in the <target-name>, this mechanism fails to match.
If a DNS error occurs while doing the PTR RR lookup, then this
mechanism fails to match. If a DNS error occurs while doing an A RR
lookup, then that domain name is skipped and the search continues.
Wong & Schlitt Expires June 30, 2005 [Page 20]
Internet-Draft Sender Policy Framework (SPF) December 2004
Pseudocode:
for each name in (validated-sending-domain_names) {
if name ends in <domain-spec>, return match.
if name is <domain-spec>, return match.
}
return no-match.
This mechanism matches if the <target-name> is either an ancestor of
a validated domain name, or if the <target-name> and a validated
domain name are the same. For example: "mail.example.com" is within
the domain "example.com", but "mail.bad-example.com" is not.
Note: Use of this mechanism is discouraged because it is slow, is not
as reliable as other mechanisms in cases of DNS errors and it places
a large burden on the arpa name servers. If used, proper PTR records
must be in place for the domain's hosts and the "ptr" mechanism
should be one of the last mechanisms checked.
5.6 "ip4" and "ip6"
These mechanisms test if <ip> is contained within a given IP network.
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = ; as per conventional dotted quad notation,
; e.g. 192.0.2.0
ip6-network = ; as per [RFC 3513], section 2.2,
; e.g. 2001:DB8::CD30
The <ip> is compared to the given network. If CIDR-length high-order
bits match, the mechanism matches.
If ip4-cidr-length is omitted it is taken to be "/32". If
ip6-cidr-length is omitted it is taken to be "/128". It is not
permitted to omit parts of the IP address instead of using CIDR
notations. That is, use 10.23.45.0/24 instead of 10.23.45.
5.7 "exists"
This mechanism is used to construct an arbitrary domain name that is
used for a DNS A record query. It allows for complicated schemes
involving arbitrary parts of the mail envelope to determine what is
Wong & Schlitt Expires June 30, 2005 [Page 21]
Internet-Draft Sender Policy Framework (SPF) December 2004
permitted.
exists = "exists" ":" domain-spec
The domain-spec is expanded as per Section 8. The resulting domain
name is used for a DNS A RR lookup. If any A record is returned,
this mechanism matches. The lookup type is 'A' even when the
connection type is IPv6.
Domains can use this mechanism to specify arbitrarily complex
queries. For example, suppose example.com publishes the record:
v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
The <target-name> might expand to
"1.2.0.192.someuser._spf.example.com". This makes fine-grained
decisions possible at the level of the user and client IP address.
This mechanism enables queries that mimic the style of tests that
existing DNSBL lists use.
Wong & Schlitt Expires June 30, 2005 [Page 22]
Internet-Draft Sender Policy Framework (SPF) December 2004
6. Modifier Definitions
Modifiers are name/value pairs that provide additional information.
Modifiers always have an "=" separating the name and the value.
The modifiers defined in this document ("redirect" and "exp") MAY
appear anywhere in the record, but SHOULD appear at the end, after
all mechanisms. Ordering of these two modifiers does not matter.
These modifiers MUST NOT appear in a record more than once each. If
they do, then check_host() exits with a result of "PermError".
Unrecognized modifiers SHOULD be ignored no matter where in a record,
nor how often. This allows implementations of this document to
handle records with modifiers that are defined in other
specifications.
6.1 redirect: Redirected Query
If all mechanisms fail to match, and a "redirect" modifier is
present, then processing proceeds as follows.
redirect = "redirect" "=" domain-spec
The domain-spec portion of the redirect section is expanded as per
the macro rules in Section 8. Then check_host() is evaluated with
the resulting string as the <domain>. The <ip> and <sender>
arguments remain the same as current evaluation of check_host().
The result of this new evaluation of check_host() is then considered
the result of the current evaluation.
Note that the newly queried domain may itself specify redirect
processing.
This facility is intended for use by organizations that wish to apply
the same record to multiple domains. For example:
la.example.com. TXT "v=spf1 redirect=_spf.example.com"
ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
_spf.example.com. TXT "v=spf1 mx:example.com -all"
In this example, mail from any of the three domains is described by
the same record. This can be an administrative advantage.
Note: In general, the domain "A" cannot reliably use a redirect to
another domain "B" not under the same administrative control. Since
the <sender> stays the same, there is no guarantee that the record at
Wong & Schlitt Expires June 30, 2005 [Page 23]
Internet-Draft Sender Policy Framework (SPF) December 2004
domain "B" will correctly work for addresses in domain "A",
especially if domain "B" uses mechanisms involving localparts. An
"include" directive may be more appropriate.
For clarity it is RECOMMENDED that any "redirect" modifier appear as
the very last term in a record.
6.2 exp: Explanation
explanation = "exp" "=" domain-spec
If check_host() results in a "Fail" due to a mechanism match (such as
"-all"), and the "exp" modifier is present, then the explanation
string returned is computed as described below. If no "exp" modifier
is present, then either a default explanation string or an empty
explanation string may be returned.
The <domain-spec> is macro expanded (see Section 8) and becomes the
<target-name>. The DNS TXT record for the <target-name> is fetched.
If <domain-spec> is empty, or there are any processing errors (any
RCODE other than 0), or if no records are returned, or if more than
one record is returned, then proceed as if no exp modifier was given.
The fetched TXT record's strings are concatenated with no spaces, and
then treated as an <explain-string> which is macro-expanded. This
final result is the explanation string.
Software evaluating check_host() can use this string to communicate
information from the publishing domain in the form of a short message
or URL. Software should make it clear that the explanation string
comes from a third party. For example, it can prepend the macro
string "%{o} explains: " to the explanation.
Implementations MAY limit the length of the resulting explanation
string to allow for other protocol constraints and/or reasonable
processing limits. The SPF client SHOULD make it clear when an
explanation string is coming from a third party, such as shown in
Section 2.5.4.
Suppose example.com has this record
v=spf1 mx -all exp=explain._spf.%{d}
Here are some examples of possible explanation TXT records at
explain._spf.example.com:
Wong & Schlitt Expires June 30, 2005 [Page 24]
Internet-Draft Sender Policy Framework (SPF) December 2004
Example.com mail should only be sent by its own servers.
-- a simple, constant message
%{i} is not one of %{d}'s designated mail servers.
-- a message with a little more info, including the IP address
that failed the check
See http://%{d}/why.html?s=%{S}&i=%{I}
-- a complicated example that constructs a URL with the
arguments to check_host() so that a web page can be
generated with detailed, custom instructions
Note: During recursion into an "include" mechanism, exp= modifiers do
not propagate out. In contrast, during execution of a "redirect"
modifier, the explanation string from the target of the redirect is
used.
Wong & Schlitt Expires June 30, 2005 [Page 25]
Internet-Draft Sender Policy Framework (SPF) December 2004
7. Miscellaneous
7.1 Processing Limits
During processing, an evaluation of check_host() may require
additional evaluations of check_host() due to the "include" mechanism
and/or the "redirect" modifier.
In order to prevent Denial-of-Service (DoS) attacks, the total number
of DNS lookups must be limited. The subject of a DoS attack can be
either the SPF client directly, the domain owner of the claimed
sender, or some third party domain that is referenced in the SPF
record.
Of these, the case of a third party referenced in the SPF record is
the easiest for a DoS attack to effectively exploit. For example, a
malicious person could create an SPF record with many references to a
victim domain, send many e-mails to different SPF clients and the SPF
clients would create a DoS attack. In effect, the SPF clients are
being used to amplify the attacker's bandwidth by using fewer bytes
in the SMTP session than is generated by the DNS queries. Using SPF
clients also allows the attacker to hide the true source of the
attack.
As a result, limits that may seem reasonable for an individual mail
server can still allow an unreasonable amount of bandwidth
amplification. Therefore the processing limits need to be quite
small.
SPF implementations MUST limit the number of mechanism that do DNS
lookups to at most 10, if this number is exceeded, a PermError MUST
be returned. The mechanisms that count against this limit are
"include", "a", "mx", "ptr", "exists" and the "redirect" modifier.
The "all", "ip4" and "ip6" mechanisms do not require DNS lookups and
therefore do not count against this limit. The "exp" modifier
requires a DNS lookup, but it is not counted as it is used only in
the case of errors.
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 implementation 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.
Wong & Schlitt Expires June 30, 2005 [Page 26]
Internet-Draft Sender Policy Framework (SPF) December 2004
Implementations must be prepared to handle records that are set up
incorrectly or maliciously.
MTAs or other processors MAY also impose a limit on the maximum
amount of elapsed time to evaluate check_host(). Such a limit SHOULD
allow at least 20 seconds. If such a limit is exceeded, the result
of authentication SHOULD be "TempError".
Domains publishing records SHOULD try to keep the number of "include"
mechanisms and chained "redirect" modifiers to a minimum. Domains
SHOULD also try to minimize the amount of other DNS information
needed to evaluate a record. This can be done by choosing directives
that require less DNS information and placing lower cost mechanisms
earlier in the SPF record.
For example, consider a domain set up as:
example.com. IN MX 10 mx.example.com.
mx.example.com. IN A 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" only requires the A records.
Evaluating for "c.example.com" requires none.
However, there may be administrative considerations: Using "a" over
"ip4" allows hosts to be renumbered easily. Using "mx" over "a"
allows the set of mail hosts to be changed easily.
7.2 The Received-SPF header
It is RECOMMENDED that SMTP receivers record the result of SPF
processing in the message headers. If an SMTP receiver chooses to do
so, it SHOULD use the "Received-SPF" header defined here. This
information is intended for the recipient. (Information intended for
the sender described in Section 6.2, Explanation.)
The Received-SPF header is a trace field (See [RFC2822] section
3.6.7) and SHOULD be prepended to existing headers, above the
Received: header that is generated by the SMTP receiver. It MUST
appear above any other Received-SPF headers in the message. The
header has the format:
Wong & Schlitt Expires June 30, 2005 [Page 27]
Internet-Draft Sender Policy Framework (SPF) December 2004
header = "Received-SPF:" [CFWS] result [CFWS]
[ key-value-list ]
result = "Pass" / "Fail" / "TempError" / "SoftFail" /
"Neutral" / "None" / "PermError"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"]
key-value-pair = name [CFWS] "=" ( dot-atom / quoted-string )
dot-atom = ; unquoted word as per [RFC2822]
quoted-string = ; quoted string as per [RFC2822]
CFWS = ; comment or folding white space as per [RFC2822]
The <comment-string> should convey supporting information for the
result, such as <ip>, <sender> and <domain>.
SPF clients may append zero or more of the following key-value-pairs
at their discretion:
receiver the host name of the SPF client
client-ip the IP address of the SMTP client
envelope-from the envelope sender address
helo the host name given in the HELO or EHLO command
mechanism the mechanism that matched (if no mechanisms matched,
substitute the word "default".)
problem if an error was returned, details about the error
Other key-value pairs may be defined by SPF clients. Until a new key
name becomes widely accepted, new key names should start with "x-".
SPF clients MUST make sure that the Received-SPF header does not
contain invalid characters, is excessively long, or contain malicious
data that has been provided by the sender.
Examples of various header styles that could be generated:
Received-SPF: Pass (mybox.example.org: domain of
myname@example.com designates 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;
Wong & Schlitt Expires June 30, 2005 [Page 28]
Internet-Draft Sender Policy Framework (SPF) December 2004
Received-SPF: Fail (mybox.example.org: domain of
myname@example.com does not designate
192.0.2.1 as permitted sender)
receiver=mybox.example.org;
client-ip=192.0.2.1;
envelope-from=<myname@example.com>;
helo=foo.example.com;
Received-SPF: SoftFail (mybox.example.org: domain of
transitioning myname@example.com discourages
use of 192.0.2.1 as permitted sender)
Received-SPF: Neutral (mybox.example.org: 192.0.2.1 is neither
permitted nor denied by domain of
myname@example.com)
Received-SPF: None (mybox.example.org: myname@example.com does
not designate permitted sender hosts)
Received-SPF: PermError (mybox.example.org: domain
of myname@example.com used an invalid
SPF mechanism)
Received-SPF: TempError (mybox.example.org: error in processing
during lookup of myname@example.com: DNS
timeout)
Wong & Schlitt Expires June 30, 2005 [Page 29]
Internet-Draft Sender Policy Framework (SPF) December 2004
8. Macros
8.1 Macro definitions
Many mechanisms and modifiers perform macro interpolation on part of
the term.
domain-spec = macro-string domain-end
domain-end = ( "." toplabel ) / macro-expand
toplabel = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
; LDH rule (See [RFC3696])
alphanum = ALPHA / DIGIT
macro-string = *( macro-expand / macro-literal )
explain-string = *( macro-string / SP )
macro-expand = ( "%{" macro-letter transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t"
transformer = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
A literal "%" is expressed by "%%".
"%_" expands to a single " " space.
"%-" expands to a URL-encoded space, viz. "%20".
The following macro letters are expanded in term arguments:
s = <sender>
l = local-part of <sender>
o = domain of <sender>
d = <domain>
i = <ip>
p = the validated domain name of <ip>
v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is
ipv6
h = HELO/EHLO domain
The following macro letters are only allowed in "exp" text:
c = SMTP client IP (easily readable format)
r = domain name of host performing the check
Wong & Schlitt Expires June 30, 2005 [Page 30]
Internet-Draft Sender Policy Framework (SPF) December 2004
t = current timestamp
A '%' character not followed by a '{', '%', '-', or '_' character is
a syntax error. So,
-exists:%(ir).sbl.spamhaus.org
is incorrect and will cause check_host() to return a "PermError".
Instead, say
-exists:%{ir}.sbl.spamhaus.org
Optional transformers are:
*DIGIT : zero or more digits
'r' : reverse value, splitting on dots by default
If transformers or delimiters are provided, the replacement value for
a macro letter is split into parts. After performing any reversal
operation and/or removal of left-hand parts, the parts are rejoined
using "." and not the original splitting characters.
By default, strings are split on "." (dots). Note that no special
treatment is given to leading, trailing or consecutive delimiters,
and so the list of parts may contain empty strings. Macros may
specify delimiter characters which are used instead of ".".
The 'r' transformer indicates a reversal operation: if the client IP
address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
and the macro %{ir} would expand to "1.2.0.192".
The DIGIT transformer indicates the number of right-hand parts to
use, after optional reversal. If a DIGIT is specified, the value
MUST be nonzero. If no DIGITs are specified, or if the value
specifies more parts than are available, all the available parts are
used. If the DIGIT was 5, and only 3 parts were available, the macro
interpreter would pretend the DIGIT was 3. Implementations MUST
support at least a value of 128, as that is the maximum number of
labels in a domain name.
The "s" macro expands to the <sender> argument. It is an e-mail
address with a localpart, an "@" character, and a domain. The "l"
macro expands to just the localpart. The "o" macro expands to just
the domain part. Note that these values remain the same during
recursive and chained evaluations due to "include" and/or "redirect".
Note also that if the original <sender> had no localpart, the
localpart was set to "postmaster" in initial processing (see Section
4.3).
For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format.
Wong & Schlitt Expires June 30, 2005 [Page 31]
Internet-Draft Sender Policy Framework (SPF) December 2004
For IPv6 addresses, the "i" macro expands to a dot-format address; it
is intended for use in %{ir}. The "c" macro may expand to any of the
hexadecimal colon-format addresses specified in [RFC3513] section
2.2. It is intended for humans to read.
The "p" macro expands to the validated domain name of <ip>. The
procedure for finding the validated domain name is defined in Section
5.5. If the <domain> is present in the list of validated domains, it
SHOULD be used. Otherwise, if a subdomain of the <domain> is
present, it SHOULD be used. Otherwise, any name from the list may be
used. If there are no validated domain name or if a DNS error
occurs, the string "unknown" is used.
The "r" macro expands to the name of the receiving MTA. This SHOULD
be a fully qualified domain name, but if one does not exist (as when
the checking is done by a script) or if policy restrictions dictate
otherwise, the word "unknown" should be substituted. The domain name
may be different than the name found in the MX record that the client
MTA used to locate the receiving MTA.
The "t" macro expands to the decimal representation of the
approximate number of seconds since the Epoch (Midnight, January 1st,
1970, UTC). This is the same value as returned by the POSIX time()
function in most standards compliant libraries.
When the result of macro expansion is used in a domain name query, if
the expanded domain name exceeds 253 characters (the maximum length
of a domain name), the left side is truncated to fit, by removing
successive subdomains until the total length does not exceed 253
characters.
Uppercased macros expand exactly as their lower case equivalents, and
are then URL escaped. URL escaping for the non-uric characters is
described in [RFC2396].
Note: Domains should avoid using the "s", "l", "o" or "h" macros in
conjunction with any mechanism directive. While these macros are
powerful and allow per-user records to be published, they severely
limit the ability of implementations to cache results of check_host()
and they reduce the effectiveness of DNS caches.
Implementations should be aware that if no directive processed during
the evaluation of check_host() contains an "s", "l", "o" or "h"
macro, then the results of the evaluation can be cached on the basis
of <domain> and <ip> alone for as long as the shortest TTL of all the
DNS records involved.
Wong & Schlitt Expires June 30, 2005 [Page 32]
Internet-Draft Sender Policy Framework (SPF) December 2004
8.2 Expansion Examples
The <sender> is strong-bad@email.example.com.
The IPv4 SMTP client IP is 192.0.2.3.
The IPv6 SMTP client IP is 5f05:2000:80ad:5800::1.
The PTR domain name of the client IP is mx.example.org.
macro expansion
------- ----------------------------
%{s} strong-bad@email.example.com
%{o} email.example.com
%{d} email.example.com
%{d4} email.example.com
%{d3} email.example.com
%{d2} example.com
%{d1} com
%{dr} com.example.email
%{d2r} example.email
%{l} strong-bad
%{l-} strong.bad
%{lr} strong-bad
%{lr-} bad.strong
%{l1r-} strong
macro-string expansion
--------------------------------------------------------------------
%{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com
%{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com
%{lr-}.lp.%{ir}.%{v}._spf.%{d2}
bad.strong.lp.3.2.0.192.in-addr._spf.example.com
%{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
3.2.0.192.in-addr.strong.lp._spf.example.com
%{d2}.trusted-domains.example.net
example.com.trusted-domains.example.net
IPv6:
%{ir}.%{v}._spf.%{d2} 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.
5.d.a.0.8.0.0.0.2.5.0.f.5.ip6._spf.example.com
Wong & Schlitt Expires June 30, 2005 [Page 33]
Internet-Draft Sender Policy Framework (SPF) December 2004
9. Implications
This section outlines the major implications that adoption of this
document will have on various entities involved in Internet e-mail.
It is intended to make clear to the reader where this document
knowingly affects the operation of such entities. This section is
not a "how-to" manual, nor a "best practices" document, and is not a
comprehensive list of what such entities should do in light of this
document.
This section is non-normative.
9.1 Sending Domains
Domains that wish to be compliant with this specification will need
to determine the list of hosts that they allow to use their domain
name in the "HELO" and "MAIL FROM" identities. It is recognized that
forming such a list is not just a simple technical exercise, but
involves policy decisions with both technical and administrative
considerations.
It can be helpful to publish records that include a "tracking
exists:" mechanism. By looking at the name server logs, an
incompletely list may be generated. For example:
v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
9.2 Mailing Lists
Mailing lists must be aware of how they re-inject mail that is sent
to the list. Mailing lists MUST comply with the requirement in
[RFC2821] Section 3.10 and [RFC1123] Section 5.3.6 that say that the
reverse-path MUST be changed to be the address of a person or other
entity who administers the list. While the reasons for changing the
reverse-path are many and long standing, SPF adds enforcement to this
requirement.
In practice, almost all mailing list software in use already complies
with this requirement. Mailing lists that do not comply, may or may
not encounter problems depending on how access to the list is
restricted. Such lists that are entirely internal to a domain (only
people in the domain can send to or receive from the list) are not
affected.
9.3 Forwarding Services and Aliases
Forwarding services take mail that is received at a mailbox and
direct it to some external mailbox. At the time of this writing, the
near-universal practice of such services is to use the original MAIL
Wong & Schlitt Expires June 30, 2005 [Page 34]
Internet-Draft Sender Policy Framework (SPF) December 2004
FROM of a message when re-injecting it for delivery to the external
mailbox. [RFC1123] and [RFC2821] describe this action as an "alias"
rather than a "mail list". This means the external mailbox's MTA
sees all such mail in a connection from a host of the forwarding
service, and so the "MAIL FROM" identity will not in general pass
authorization.
There are several possible ways that this authorization failure can
be ameliorated. If the owner of the external mailbox wishes to trust
the forwarding service, they can direct the external mailbox's MTA to
skip such tests when the client host belongs to the forwarding
service. Tests against some other identity may also be used to
override the test against the "MAIL FROM" identity.
For larger domains, it may not be possible to have a complete or
accurate list of forwarding services used by the owners of the
domain's mailboxes. In such cases, white lists of generally
recognized forwarding services could be employed.
Forwarding services can also solve the problem by using MAIL FROM
that contain their own domain. This means that mail bounced from the
external mailbox will have to be re-bounced by the forwarding
service. Various schemes to do this exist though they vary widely in
complexity and resource requirements on the part of the forwarding
service. Several popular MTAs can change "alias" semantics to
"mailing list" semantics by including an adding another alias with
"owner-" added to the beginning of the alias name. (e.g. an alias
of "friends: george@example.com, fred@example.org" would need another
alias of the form "owner-friends: localowner")
9.4 Mail Services
Entities that offer mail services to other domains such as sending of
bulk mail will may have to alter their mail in light of the
authorization check in this document. If the MAIL FROM used for such
e-mail uses the domain of the mail service provider, then the
provider needs only to ensure that their sending host is authorized
by their own SPF record, if any.
If the MAIL FROM does not use the mail service provider's domain,
then extra care must be taken. The SPF record format has several
options for authorizing the sending MTAs of another domain (the
service provider's)
9.5 MTA Relays
The authorization check generally precludes the use of arbitrary MTA
relays between sender and receiver of an e-mail message.
Wong & Schlitt Expires June 30, 2005 [Page 35]
Internet-Draft Sender Policy Framework (SPF) December 2004
Within an organization, MTA relays can be effectively deployed.
However, for purposes of this document, such relays are effectively
invisible. The "MAIL FROM" identity authorization check is a check
between border MTAs.
For mail senders, this means that published SPF records must
authorize any MTAs that actually send across the Internet. Usually,
these are just the border MTAs as internal MTAs simply forward mail
to these MTAs for delivery.
Mail receivers will generally want to perform the authorization check
at the border MTAs. This allows mail that fails to be rejected
during the SMTP session rather than bounced. Internal MTAs then do
not perform the authorization test. To perform the authorization
test other than at the border, the host that first transferred the
message to the organization must be determined, which can be
difficult to extract from headers. Testing other than at the border
is not recommended.
Wong & Schlitt Expires June 30, 2005 [Page 36]
Internet-Draft Sender Policy Framework (SPF) December 2004
10. Security Considerations
The "MAIL FROM" and "HELO" identity authorizations must not be
construed to provide more assurance than it does. It is entirely
possible for a malicious sender to inject a message using their own
domain in the identities used by SPF, to have that domain's SPF
record authorize the sending host, and yet the message content can
easily claim other identities in the headers. Unless the user, or
the MUA takes care to note that the authorized identity does not
match the other, more commonly presented identities (such as the
From: header), the user may be lulled into a false sense of security.
There are two aspects of this protocol that malicious parties could
exploit to undermine the validity of the check_host() function:
The evaluation of check_host() relies heavily on DNS. A malicious
attacker could attack the DNS infrastructure and cause
check_host() to see spoofed DNS data, and then return incorrect
results. This could include returning "Pass" for an <ip> value
where the actual domain's record would evaluate to "Fail". See
[RFC3833] for a description of the DNS weaknesses.
The client IP address, <ip>, is assumed to be correct. A
malicious attacker could spoof TCP sequence numbers to make mail
appear to come from a permitted host for a domain that the
attacker is impersonating.
As with most aspects of mail, there are a number of ways that
malicious parties could use the protocol as an avenue of a
Denial-of-Service (DoS) attack. The processing limits outlined in
Section 7.1 are designed to prevent attacks such as:
Malicious parties could create SPF records that make many
references to the target's domain and then send large volume mail
to other SPF clients that use this SPF record. These legitimate
machines would then present a DNS load on the target as they
fetched the relevant DNS references.
While implementations of check_host() need to limit the number of
DNS lookups, malicious domains could publish records exceed these
limits in an attempt to waste computation effort at their targets
when they send them mail. Malicious domains could also design SPF
records that cause excessive memory or CPU usage.
Malicious parties could send large volume mail purporting to come
from the intended target to a wide variety of legitimate mail
hosts. These legitimate machines would then present a DNS load on
the target as they fetched the relevant records.
When the authorization check fails, an explanation string may be
included in the reject response. Both the sender and the rejecting
Wong & Schlitt Expires June 30, 2005 [Page 37]
Internet-Draft Sender Policy Framework (SPF) December 2004
receiver need to be aware that the explanation was determined by the
publisher of the SPF record checked and, in general, not the
receiver. The explanation may contain URLs that may be malicious,
offensive and/or have misleading text. This is probably less of a
concern than it may initially seem since such messages are returned
to the sender, and the source is the SPF record published by the
domain in the identity claimed by that very sender. To put it
another way, the only people who see malicious explanation strings
are people whose messages claim to be from domains that publish such
strings in their SPF records.
SPF uses information supplied by third parties, such as the HELO
domain name, the MAIL FROM and SPF records. This information is then
sent to the receiver in the Received-SPF: mail headers and possibly
returned to the client MTA in the form of an SMTP rejection message.
This information must be checked for invalid characters and
excessively long lines.
Wong & Schlitt Expires June 30, 2005 [Page 38]
Internet-Draft Sender Policy Framework (SPF) December 2004
11. IANA Considerations
The IANA needs to assign a new Resource Record Type and Qtype from
the DNS Parameters Registry for the SPF RR type.
Wong & Schlitt Expires June 30, 2005 [Page 39]
Internet-Draft Sender Policy Framework (SPF) December 2004
12. Contributors and Acknowledgements
This document is largely based on the work of Meng Weng Wong and Mark
Lentczner. Mark is not listed as an author by his request. While,
as this section acknowledges, many people have contributed to this
document, a very large portion of the writing and editing are due to
Meng and Mark.
This design owes a debt of parentage to [RMX] by Hadmut Danisch and
to [DMP] by Gordon Fecyk. The idea of using a DNS record to check
the legitimacy of an e-mail address traces its ancestry farther back
through messages on the namedroppers mailing list by Paul Vixie
[Vixie] (based on suggestion by Jim Miller) and by David Green
[Green].
Philip Gladstone contributed macros to the specification, multiplying
the expressiveness of the language and making per-user and per-IP
lookups possible.
The authors would also like to thank the literally hundreds of
individuals who have participated in the development of this design.
There are far too numerous to name, but they include:
The folks on the spf-discuss mailing list.
The folks on the SPAM-L mailing list.
The folks on the IRTF ASRG mailing list.
The folks on the IETF MARID mailing list.
The folks on #perl.
Wong & Schlitt Expires June 30, 2005 [Page 40]
Internet-Draft Sender Policy Framework (SPF) December 2004
13. References
13.1 Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
13.2 Informative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, August
1996.
[RFC2162] Allocchio, C., "MaXIM-11 - Mapping between X.400 /
Internet mail and Mail-11 mail", RFC 2162, January 1998.
[RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, February 2004.
Wong & Schlitt Expires June 30, 2005 [Page 41]
Internet-Draft Sender Policy Framework (SPF) December 2004
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004.
[RMX] Danish, H., "The RMX DNS RR Type for light weight sender
authentication", October 2003.
Work In Progress
[DMP] Fecyk, G., "Designated Mailers Protocol", December 2003.
Work In Progress
[Vixie] Vixie, P., "Repudiating MAIL FROM", 2002.
[Green] Green, D., "Domain-Authorized SMTP Mail", 2002.
Authors' Addresses
Meng Weng Wong
Singapore
EMail: mengwong+spf@pobox.com
URI: http://spf.pobox.com/
Wayne Schlitt
4615 Meredeth #9
Lincoln Nebraska, NE 68506
United States of America
EMail: wayne@schlitt.net
URI: http://www.schlitt.net/spf/
Wong & Schlitt Expires June 30, 2005 [Page 42]
Internet-Draft Sender Policy Framework (SPF) December 2004
Appendix A. Collected ABNF
This section is normative and any discrepancies with the ABNF
fragments in the preceding text are to be resolved in favor of this
grammar.
See [RFC2234] for ABNF notation. Please note that as per this ABNF
definition, literal text strings (those in quotes) are
case-insensitive. Hence, "mx" matches "mx", "MX", "mX" and "Mx".
record = version terms *SP
version = "v=spf1"
terms = *( 1*SP ( directive / modifier ) )
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists )
all = "all"
include = "include" ":" domain-spec
A = "a" [ ":" domain-spec ] [ dual-cidr-length ]
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
PTR = "ptr" [ ":" domain-spec ]
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
exists = "exists" ":" domain-spec
modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = ; as per conventional dotted quad notation,
; e.g. 192.0.2.0
ip6-network = ; as per [RFC 3513], section 2.2,
; e.g. 2001:DB8::CD30
domain-spec = macro-string domain-end
domain-end = ( "." toplabel ) / macro-expand
toplabel = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
; LDH rule (See [RFC3696])
alphanum = ALPHA / DIGIT
Wong & Schlitt Expires June 30, 2005 [Page 43]
Internet-Draft Sender Policy Framework (SPF) December 2004
macro-string = *( macro-expand / macro-literal )
explain-string = *( macro-string / SP )
macro-expand = ( "%{" macro-letter transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t"
transformer = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header = "Received-SPF:" [CFWS] result [CFWS]
[ key-value-list ]
result = "Pass" / "Fail" / "TempError" / "SoftFail" /
"Neutral" / "None" / "PermError"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"]
key-value-pair = name [CFWS] "=" ( dot-atom / quoted-string )
dot-atom = ; unquoted word as per [RFC2822]
quoted-string = ; quoted string as per [RFC2822]
CFWS = ; comment or folding white space as per [RFC2822]
Wong & Schlitt Expires June 30, 2005 [Page 44]
Internet-Draft Sender Policy Framework (SPF) December 2004
Appendix B. Extended Examples
These examples are based on the following DNS setup:
; A domain with two mail servers, two hosts
; and two servers at the domain name
$ORIGIN example.com.
@ MX 10 mail-a
MX 20 mail-b
A 192.0.2.10
A 192.0.2.11
amy A 192.0.2.65
bob A 192.0.2.66
mail-a A 192.0.2.129
mail-b A 192.0.2.130
www CNAME example.com.
; A related domain
$ORIGIN example.org
@ MX 10 mail-c
mail-c A 192.0.2.140
; The reverse IP for those addresses
$ORIGIN 2.0.192.in-addr.arpa.
10 PTR example.com.
11 PTR example.com.
65 PTR amy.example.com.
66 PTR bob.example.com.
129 PTR mail-a.example.com.
130 PTR mail-b.example.com.
140 PTR mail-c.example.org.
; A rogue reverse IP domain that claims to be
; something it's not
$ORIGIN 0.0.10.in-addr.arpa.
4 PTR bob.example.com.
B.1 Simple Examples
These examples show various possible published records for
example.com and which values if <ip> would cause check_host() to
return "Pass". Note that <domain> is "example.com".
Wong & Schlitt Expires June 30, 2005 [Page 45]
Internet-Draft Sender Policy Framework (SPF) December 2004
v=spf1 +all
-- any <ip> passes
v=spf1 a -all
-- hosts 192.0.2.10 and 192.0.2.11 pass
v=spf1 a:example.org -all
-- no sending hosts pass since example.org has no A records
v=spf1 mx -all
-- sending hosts 192.0.2.129 and 192.0.2.130 pass
v=spf1 mx:example.org -all
-- sending host 192.0.2.140 passes
v=spf1 mx mx:example.org -all
-- sending hosts 192.0.2.129, 192.0.2.130, and 192.0.2.140 pass
v=spf1 mx/30 mx:example.org/30 -all
-- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
v=spf1 ptr -all
-- sending host 192.0.2.65 passes (reverse IP is valid and in
example.com)
-- sending host 192.0.2.140 fails (reverse IP is valid, but not in
example.com)
-- sending host 10.0.0.4 fails (reverse IP is not valid)
v=spf1 ip4:192.0.2.128/28 -all
-- sending host 192.0.2.65 fails
-- sending host 192.0.2.129 passes
B.2 Multiple Domain Example
These examples show the effect of related records:
example.org: "v=spf1 include:example.com include:example.net -all"
This record would be used if mail from example.org actually came
through servers at example.com and example.net. Example.org's
designated servers are the union of example.com and example.net's
designated servers.
la.example.org: "v=spf1 redirect=example.org"
ny.example.org: "v=spf1 redirect=example.org"
sf.example.org: "v=spf1 redirect=example.org"
These records allow a set of domains that all use the same mail
Wong & Schlitt Expires June 30, 2005 [Page 46]
Internet-Draft Sender Policy Framework (SPF) December 2004
system to make use of that mail system's record. In this way, only
the mail system's record needs to updated when the mail setup
changes. These domains' records never have to change.
B.3 DNSBL Style Example
Imagine that, in addition to the domain records listed above, there
are these:
$Origin _spf.example.com.
mary.mobile-users A 127.0.0.2
fred.mobile-users A 127.0.0.2
15.15.168.192.joel.remote-users A 127.0.0.2
16.15.168.192.joel.remote-users A 127.0.0.2
The following records describe users at example.com who mail from
arbitrary servers, or who mail from personal servers.
example.com:
v=spf1 mx
include:mobile-users._spf.%{d}
include:remote-users._spf.%{d}
-all
mobile-users._spf.example.com:
v=spf1 exists:%{l1r+}.%{d}
remote-users._spf.example.com:
v=spf1 exists:%{ir}.%{l1r+}.%{d}
Wong & Schlitt Expires June 30, 2005 [Page 47]
Internet-Draft Sender Policy Framework (SPF) December 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Wong & Schlitt Expires June 30, 2005 [Page 48]
