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Urgent Action Against SPAM is Pending
Network Working Group M. Lentczner
Internet-Draft M. Wong
Expires: April 12, 2005 October 12, 2004
Sender Policy Framework: Authorizing Use of Domains in MAIL FROM
draft-lentczner-spf-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 April 12, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Protocol Status . . . . . . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 The Mail From Identity . . . . . . . . . . . . . . . . . . 6
2.2 Publishing Authorization . . . . . . . . . . . . . . . . . 6
2.3 Checking Authorization . . . . . . . . . . . . . . . . . . 6
2.4 Interpreting the Result . . . . . . . . . . . . . . . . . 7
2.4.1 Neutral . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.2 Pass . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.3 Fail . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.4 SoftFail . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.5 None . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.6 TempError . . . . . . . . . . . . . . . . . . . . . . 9
2.4.7 PermError . . . . . . . . . . . . . . . . . . . . . . 9
3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Publishing . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1 RR Types . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2 Multiple Records . . . . . . . . . . . . . . . . . . . 11
3.1.3 Additional Records . . . . . . . . . . . . . . . . . . 11
3.1.4 Multiple Strings . . . . . . . . . . . . . . . . . . . 11
3.1.5 Record Size . . . . . . . . . . . . . . . . . . . . . 12
3.1.6 Wildcard Records . . . . . . . . . . . . . . . . . . . 12
4. The check_host() Function . . . . . . . . . . . . . . . . . . 13
4.1 Arguments . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Initial Processing . . . . . . . . . . . . . . . . . . . . 14
4.4 Record Lookup . . . . . . . . . . . . . . . . . . . . . . 14
4.5 Selecting Records . . . . . . . . . . . . . . . . . . . . 14
4.6 Record Evaluation . . . . . . . . . . . . . . . . . . . . 15
4.6.1 Term Evaluation . . . . . . . . . . . . . . . . . . . 15
4.6.2 Mechanisms . . . . . . . . . . . . . . . . . . . . . . 16
4.6.3 Modifiers . . . . . . . . . . . . . . . . . . . . . . 16
4.7 Default result . . . . . . . . . . . . . . . . . . . . . . 17
4.8 Domain Spec . . . . . . . . . . . . . . . . . . . . . . . 17
5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 18
5.1 "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2 "include" . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3 "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.4 "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.5 "ptr" . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.6 "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 21
5.7 "exists" . . . . . . . . . . . . . . . . . . . . . . . . . 22
6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . . 23
6.1 redirect: Redirected Query . . . . . . . . . . . . . . . . 23
6.2 exp: Explanation . . . . . . . . . . . . . . . . . . . . . 24
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7. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1 Unrecognized Mechanisms and Modifiers . . . . . . . . . . 26
7.2 Processing Limits . . . . . . . . . . . . . . . . . . . . 26
8. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1 Macro definitions . . . . . . . . . . . . . . . . . . . . 28
8.2 Expansion Examples . . . . . . . . . . . . . . . . . . . . 31
9. Implications . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1 Sending Domains . . . . . . . . . . . . . . . . . . . . . 32
9.2 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 32
9.3 Forwarding Services . . . . . . . . . . . . . . . . . . . 32
9.4 Mail Services . . . . . . . . . . . . . . . . . . . . . . 33
9.5 MTA Relays . . . . . . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . 35
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 37
12. Contributors and Acknowledgements . . . . . . . . . . . . . 38
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
13.1 Normative References . . . . . . . . . . . . . . . . . . . . 39
13.2 Informative References . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 40
A. Collected ABNF . . . . . . . . . . . . . . . . . . . . . . . . 41
B. Extended Examples . . . . . . . . . . . . . . . . . . . . . . 43
B.1 Simple Examples . . . . . . . . . . . . . . . . . . . . . 43
B.2 Multiple Domain Example . . . . . . . . . . . . . . . . . 44
B.3 RBL Style Example . . . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . 46
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1. Introduction
The current e-mail infrastructure has the property that any host
injecting mail into the mail system can identify itself as any domain
name it wants. Hosts can do this at a variety of levels: in
particular, the session, the envelope, and the mail headers. While
this feature is desirable in some circumstances, it is a major
obstacle to reducing 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 hosts may be authorized by
domains to use the domain name in the envelope "Mail From" identity.
Compliant domain name 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 "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 has continuously evolved from them and is under active
development today. 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.
This document attempts to set down the common core of the SPF version
1 protocol, as implemented and deployed since about December, 2003.
This conception of SPF is sometimes called "SPF Classic". The goal
of this document is to be a stable reference that can be used for
experimenting with existing implementations and developing SPF
further. 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.
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1.2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document is concerned with 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 2.1. Note that other terms, that may
superficially look like the common terms, such as "reverse-path" or
"Return-Path" are used only with the defined meanings from normative
documents.
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2. Operation
2.1 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.
This document defines the "Mail From" identity to be mailbox portion
of the path of the reverse-path as defined in [RFC2821], Section
4.1.2. when it is non-null.
[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 domain supplied with the SMTP EHLO or HELO
command. Note that requirements for the domain presented in the EHLO
and HELO commands are not strict, and software must be prepared for a
"Mail From" identity so constructed to be ill formed.
Generally, software that performs the authorization checks described
below does so during a SMTP transaction, and so readily has the
information required at hand.
2.2 Publishing Authorization
An SPF compliant domain name MUST publish a valid SPF record as
described in Section 3. This record authorizes the use of the domain
name in the envelope "Mail From" identity, by some sending MTAs, and
not by others.
Domains SHOULD publish SPF records that end in "-all", or redirect to
other records that do, so that a definitive determination of
authorization can be made.
Domain holders may publish SPF records that explicitly authorize no
hosts for domain names that shouldn't be used in sender mailboxes.
2.3 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. Typically,
such checks are done by a receiving MTA, but can be performed
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elsewhere in the mail processing chain so long as the required
information is available.
It is expected 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 client host that is injecting the
mail
<domain> - the domain portion of the "Mail From" identity
<sender> - the "Mail From" 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, and that can be an address literal or entirely
malformed in a valid SMTP transaction. In these cases, check_host()
is defined in Section 4.3 to return a Fail result.
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 as such an action
would go against the explicit wishes of the alleged sender.
2.4 Interpreting the Result
The check_host() function returns one of seven results, some with
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additional information. 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.
2.4.1 Neutral
A Neutral result MUST be treated exactly like a None result.
2.4.2 Pass
A Pass result means that the client is authorized to inject mail with
the given "Mail From" identity. Further policy checks, such as
reputation, or black and/or white listing, can now proceed with
confidence based on the "Mail From" identity.
2.4.3 Fail
A Fail result is an explicit statement that the client is not
authorized to use the domain in the "Mail From" identity. The
checking software can choose to mark the mail based on this, or to
reject the mail outright.
If the checking software chooses to reject the mail during the SMTP
transaction, then it MUST use a 550 reply code with an appropriate
message. The Fail result includes a reason. The reason can be used
to construct an appropriate message. If the reason is "Not
Permitted", then an explanation string is also returned. This
explanation string comes from the domain that published the SPF
records and may contain a URL. Since that information doesn't
originate with the checking software, the checking software will want
to make it clear that text is not trusted. Example reply messages
for rejecting are:
550 SPF Mail From check failed: Malformed Domain
550 SPF Mail From check failed: Domain Does Not Exist
550-SPF Mail From check failed: Not Permitted
550-The domain example.com said:
550 Please see http://www.example.com/mailpolicy.html
2.4.4 SoftFail
A SoftFail result should be treated as somewhere between a Fail and a
Neutral. This value is used by domains as an intermediate state
during roll-out of publishing records. The domain believes the host
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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.
2.4.5 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.
2.4.6 TempError
A TempError result means that the receiving server 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
MUST use a 450 reply code.
2.4.7 PermError
A PermError result means that the domain's published records couldn't
be correctly interpreted for this "Mail From" identity. Checking
software SHOULD reject the message. If rejecting during SMTP
transaction time, a 550 reply MUST be used.
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3. SPF Records
An SPF record declares which hosts are, and are not, authorized to
use a domain name for the "Mail From" identity. Loosely, the record
partitions all hosts into permitted and not-permitted sets. (Though
some hosts might fall into other categories.)
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", and "-all".
3.1 Publishing
A domain name's SPF record is published in DNS. The record is placed
in the DNS tree at the domain name it pertains to.
The previous example might be published easily via this line in a
domain zone file:
example.com. IN SPF "v=spf1 +mx +a:colo.example.com/28 -all"
Note: The record is published at the domain name to which it
pertains, not a name within the domain (such as is done with SRV
records.) When published using the SPF RR type (see below), this
poses no problems and was chosen as the clearest way to express the
declaration. When published via TXT records it is still published
directly at the domain name, even though other TXT records, for other
purposes may be published there.
3.1.1 RR Types
This document defines a new DNS RR type SPF, type code to be
determined. The format of this type is identical to the TXT RR
[RFC1035].
However, because there are a number of DNS server and resolver
implementations in common use that cannot handle new RR types, a
record can be published with type TXT.
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.
An SPF compliant check SHOULD lookup both types. Lookups can be
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performed serially or in parallel. If both types of records are
obtained for a domain, the SPF type MUST be used.
It is recognized that the current practice (using a TXT type record),
is not optimal, but a practical reality due to the state of deployed
software. The two record type scheme provides a forward path to the
better solution of using a RR type reserved for this purpose.
For either type, the character content of the record is encoded as
US-ASCII.
Example RRs in this document are shown with the SPF record type,
however they could also be published with a TXT type.
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 Additional Records
Some records contain directives that require additional SPF records.
It is suggested that those records be placed under an "_spf"
subdomain. See Appendix B for examples.
3.1.4 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:
SPF "v=spf1 .... first" "second string..."
MUST be treated as equivalent to
SPF "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.
Note: Some nameserver implementations will silently split long
strings in TXT records into several shorter strings.
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3.1.5 Record Size
The published SPF record for a given domain name SHOULD remain small
enough that the results of a query for it will fit within 512 octets.
This will keep even older DNS implementations from falling over to
TCP. Since the answer size is dependent on many things outside the
scope of this document, it is only possible to give this guideline:
If the combined length of the DNS name and the text of all the
records of a given type (TXT or SPF) is under 480 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.
3.1.6 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 SPF "v=spf1 +a:A.X.COM -all"
*.X.COM MX 10 A.X.COM
*.X.COM SPF "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 SPF "v=spf1 +a:A.X.COM -all"
*.A.X.COM MX 10 A.X.COM
*.A.X.COM SPF "v=spf1 +a:A.X.COM -all"
Notice that SPF records must be repeated twice for every name within
the domain: Once for the name, and once with a wildcard to cover the
tree under the name.
Use of wildcards is discouraged in general as they cause every name
under the domain to exist and queries against arbitrary names will
never return RCODE 3 (Name Error).
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4. The check_host() Function
The check_host() function fetches SPF records, parses them, and
interprets them to evaluate if a particular host is or is not
permitted to send mail with a given "Mail From" 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 three arguments:
<ip> - the IP address of the host under test
<domain> - the domain to check
<sender> - the full sending mailbox address
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).
Note: The IP address may be either IPv4 or IPv6.
4.2 Results
The function check_host() can result in one of seven results
described here. Based on the result, the action to be taken is
determined by and the local policies of the receiver. (see Section
2.4)
Results from interpreting valid records:
Neutral (?): published data is explicitly inconclusive
Pass (+): the <ip> is in the permitted set
Fail (-): the <ip> is in the not permitted set
SoftFail (~): the <ip> may be in the not permitted set, its use is
discouraged and the domain owner may move it to the not
permitted set in the future
Results from error conditions:
None - no published data
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TempError - transient error during DNS lookup or other processing
PermError - unrecoverable error during processing, such as an
error in the record format
If the result is "Fail", then an additional reason is returned. The
reason may be one of:
Not Permitted
Malformed Domain
Domain Does Not Exist
If the reason is "Not Permitted", then an explanation string is also
returned. The explanation string may be empty.
4.3 Initial Processing
If the <domain> is not an fully qualified domain name, check_host()
immediately returns the result "Fail" and a reason of "Malformed
Domain".
If the <sender> has no localpart, substitute the string "postmaster"
for the localpart.
4.4 Record Lookup
The records for <domain> are fetched. If the records are in a cache,
and have not expired, then they may simply be used. Otherwise, the
records must be fetched from DNS as follows:
In accordance with how the records are published, (see Section 3.1
above), a DNS query needs to be made for the <domain> name, querying
for either RR type TXT, SPF or both.
If the domain does not exist (RCODE 3), check_host() exits
immediately with the result "Fail" and a reason of "Domain Does Not
Exist"
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"
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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 no records remaining,
check_host() exits immediately with the result "None". If there are
two or more records remaining, then check_host() exits immediately
with the error "PermError".
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
at any point a syntax error is encountered, 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.
Note: Unrecognized mechanisms are still syntactically well formed.
See Section 7.1.
4.6.1 Term Evaluation
There are two types of terms: mechanisms and modifiers. A given
mechanism type may appear multiple times in a record. A given
modifier may appear at most once per record. Unknown mechanisms
cause processing to abort with the result "PermError". Unknown
modifiers are ignored.
A record contains an ordered list of mechanisms and modifiers:
terms = *( 1*SP ( directive / modifier ) )
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists
/ unknown-mechanism )
modifier = name "=" macro-string
name = alpha *( alpha / digit / "-" / "_" / "." )
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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.
Mechanism and modifier names are case-insensitive. A mechanism
"INCLUDE" is equivalent to "include".
4.6.2 Mechanisms
Each mechanism is considered in turn from left to right.
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. (The default prefix value is "+".)
If it does not match, processing continues with the next mechanism.
If no mechanisms remain, the default result is specified in Section
4.7.
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
"?" Neutral
A missing prefix for a mechanism is the same as a prefix of "+".
When a mechanism matches, and the prefix is "-" so that a "Fail"
result is returned, the reason is Not Permitted, 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 key/value pairs that affect the evaluation of the
check_host() function.
The modifiers defined in this document ("redirect" and "exp") MAY
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appear anywhere in the record, but SHOULD appear at the end, after
all mechanisms. Ordering of these 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 "PermFail".
Unrecognized modifiers MUST be ignored no matter where in a record,
or how often. This allows implementations of this document to handle
records with modifiers that are defined in later versions.
4.7 Default result
If none of the mechanisms match and there is no "redirect" modifier,
then the check_host() exits with 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 Spec
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, for example, the space or control
characters. However, this facility is powerful enough to express
legal host names, and common utility labels (such as "_spf") that are
used in DNS.
For several mechanisms, the <domain-spec> is optional. If it is not
provided, the <domain> is used as the <target-name>.
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5. Mechanism Definitions
This section defines two types of mechanisms.
Basic mechanisms contribute to the language framework. They do not
specify a particular type of authorization scheme.
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
Other mechanisms may be defined in the future.
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.
Several mechanisms rely on information fetched from DNS. For these
DNS queries, if the DNS server returns an error (RCODE other than 0
or 3) or the query times out, the mechanism throws the exception
"TempError". If the server returns "domain does not exist" (RCODE
3), then evaluation of the mechanism continues as if the server
returned no error (RCODE 0) and zero answer records.
5.1 "all"
all = "all"
The "all" mechanism is a test that always matches. It is used as the
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rightmost mechanism in a record to provide an explicit default.
For example:
v=spf1 +mx +a -all
Mechanisms after "all" will never be tested. Any "redirect" modifier
(Section 6.1) has no effect when there is an "all" directive.
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().
"include" 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():
+---------------------------------+---------------------------------+
| A recursive check_host() result | Causes the "include" mechanism |
| of: | to: |
+---------------------------------+---------------------------------+
| Pass | match |
| | |
| Fail | not match |
| | |
| SoftFail | not match |
| | |
| Neutral | not match |
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| | |
| 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 ]
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. 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.
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5.5 "ptr"
This mechanism tests if the DNS reverse mapping for <ip> exists and
validly 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.apra." if the address is an IPv4 one and "ip6.arpa." if
it is an IPv6 address. For each record returned, validate the host
name by looking up its IP address. If <ip> is among the returned IP
addresses, then that host name is validated. In pseudocode:
sending-host_names := ptr_lookup(sending-host_IP);
for each name in (sending-host_names) {
IP_addresses := a_lookup(name);
if the sending-host_IP is one of the IP_addresses {
validated_sending-host_names += name;
}
}
Check all validated hostnames to see if they end in the <target-name>
domain. If any do, this mechanism matches. If no validated hostname
can be found, or if none of the validated hostnames end in the
<target-name>, this mechanism fails to match.
Pseudocode:
for each name in (validated_sending-host_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 an ancestor of a
validated hostname, or if the <target-name> and a validated hostname
are the same. For example: "mail.example.com" is within the domain
"example.com", but "mail.bad-example.com" is not. If a validated
hostname is the <target-name>, a match results.
Note: This mechanism is not recommended. If a domain decides to use
it, it should make sure is has the proper PTR records in place for
its hosts.
5.6 "ip4" and "ip6"
These mechanisms test if <ip> is contained within a given IP network.
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IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
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".
5.7 "exists"
This mechanism is used to construct an arbitrary host 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
legal.
exists = "exists" ":" domain-spec
The domain-spec is expanded as per Section 8. The resulting domain
name is used for a DNS A 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 RBL lists use.
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6. Modifier Definitions
Modifiers are not mechanisms: they do not return match or no-match.
Instead they provide additional information or alter check_host()
processing.
While unrecognized mechanisms cause an immediate "PermError" abort,
unrecognized modifiers MUST be simply ignored. Modifiers therefore
provide a way to extend the record format in the future with backward
compatibility.
Only two modifiers are currently defined: "redirect" and "exp".
Implementations of check_host() MUST support them both.
There is one deprecated modifier: "default", which cannot be defined
by any future version of this document. Implementations MUST ignore
it.
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. SPF "v=spf1 redirect=_spf.example.com"
ny.example.com. SPF "v=spf1 redirect=_spf.example.com"
sf.example.com. SPF "v=spf1 redirect=_spf.example.com"
_spf.example.com. SPF "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, a domain A cannot reliably use a redirect to
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another domain B not under the same administrative control. Since
the <sender> stays the same, there is no guarantee that the record at
domain B will correctly work for 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 an empty explanation string is 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 an empty explanation string is returned.
The fetched TXT record's strings are concatenated with no spaces, and
then treated as a new macro-string which is macro-expanded. This
final result is the explanation string.
Software evaluating check_host() can use this string when the result
is "Fail" with a reason of "Not Permitted", 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 "%{d} 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.
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:
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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, explanations do
not propagate out. But during execution of a "redirect" modifier,
the explanation string from the target of the redirect is used.
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7. Miscellaneous
7.1 Unrecognized Mechanisms and Modifiers
New mechanisms can only be introduced by new versions of this
document.
Unrecognized mechanisms cause processing to abort: If, during
evaluation of a record, check_host() encounters a mechanism which it
does not understand, it terminates processing and returns
"PermError", without evaluating any further mechanisms. Mechanisms
listed before the unknown mechanism MUST, however, be evaluated.
For example, consider the record:
v=spf1 a mx ptr foo:_foo.%{d} -all
If during the evaluation of check_host(), any of the "a", "mx", or
"ptr" directives match, then check_host() would return a "Pass"
result. If none of those directives resulted in a match, then an
implementation that did not recognize the "foo" mechanism would
return "PermError". An implementation that did recognize the "foo"
mechanism would be able to perform an extended evaluation.
Note: "foo" is an example of an unknown extension mechanism that
could be defined in the future. It is NOT defined by this proposal.
Unrecognized modifiers are ignored: if an implementation encounters
modifiers which it does not recognize, it MUST ignore them.
7.2 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.
Implementations must be prepared to handle records that are set up
incorrectly or maliciously. Implementations MUST perform loop
detection, limit additional evaluations, or both. If an
implementation chooses to limit additional evaluations, then at least
a total of 10 evaluations of check_host() for a single query MUST be
supported. (This number should be enough for even the most
complicated configurations.)
If a loop is detected, or the evaluation limit of an implementation
is reached, check_host() MUST abort processing and return the result
"PermError".
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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"
directives 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.
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 SPF "v=spf1 +mx:example.com -all"
b.example.com. IN SPF "v=spf1 +a:mx.example.com -all"
c.example.com. IN SPF "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.
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8. Macros
8.1 Macro definitions
Many mechanisms and modifiers perform macro interpolation on part of
the term.
domain-spec = *( macro-expand / macro-literal )
macro-string = *( macro-expand / macro-literal / "/" )
macro-expand = ( "%{" ALPHA transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-2E / %x30-7E
; visible characters except "%" and "/"
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 host name of <ip>
v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is
ipv6
The following macro letters are only allowed in "exp" text:
c = SMTP client IP (easily readable format)
r = domain name of host performing the check
t = current timestamp
The uppercase versions of all these macros are URL-encoded.
A '%' character not followed by a '{', '%', '-', or '_' character
MUST be interpreted as a literal. Domains SHOULD NOT rely on this
feature; they MUST escape % literals. For example, an explanation
TXT record
Your spam volume has increased by 581%
is incorrect. Instead, say
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Your spam volume has increased by 581%%
All other legal visible characters are simply expanded to themselves.
Note that the two different macro contexts, domain-spec, and
macro-string allow slightly different sets of legal visible
characters. In particular, macro-string allows the slash character.
Legal 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 ".".
Delimiters MUST be one or more of the characters:
"." / "-" / "+" / "," / "/" / "_" / "="
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).
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For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format.
For IPv6 addresses, the "i" macro expands to a dot-format address; it
is intended for use in %{ir}. The "c" macro may expand to any of the
hexadecimal colon-format addresses specified in [RFC3513] section
2.2. It is intended for humans to read.
The "p" macro expands to the validated host name of <ip>. The
procedure for finding the validated host names is defined in Section
5.5. If that procedure produces more than one validated host name,
any name from the list may be used. If that procedure produces no
validate host name 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 number of
seconds since the Epoch (Midnight, January 1st, 1970, UTC). This is
the same value as returned by the time() function in most standards
compliant libraries.
Any unrecognized macro letters are expanded as the string "unknown".
There is one deprecated macro letter: "h". It is expanded as the
string "deprecated".
When the result of macro expansion is used in a domain name query, if
the expanded domain name exceeds 255 characters (the maximum length
of a domain name), the left side is truncated to fit, by removing
successive subdomains until the total length falls below 255
characters.
Uppercased macros expand exactly as their lower case equivalents, and
are then URL escaped. URL escaping is described in [RFC2396].
Note: Domains should avoid using the "s", "l" or "o" 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", or "o" macro,
then the results of the evaluation can be cached on the basis of
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<domain> and <ip> alone for as long as the shortest TTL of all the
DNS records involved.
8.2 Expansion Examples
The <sender> is strong-bad@email.example.com.
The IPv4 SMTP client IP is 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
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9. Implications
This section outlines the major implications that adoption of this
document will have on various entities involved in Internet e-mail.
It is intended to make clear to the reader where this document
knowingly affects the operation of such entities. This section is
not a "how-to" manual, nor a "best practices" document, and is not a
comprehensive list of what such entities should do in light of this
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 "Mail From" identity. It is recognized that forming such
a list is not just a simple technical exercise, but involves policy
decisions with both technical and administrative considerations.
9.2 Mailing Lists
Mailing lists must be aware of how they re-inject mail that is sent
to the list. If the list re-injects mail with the same reverse-path
that the mail had when it was received, then that mail may fail the
authorization tests defined in this document. In particular, they
will fail when the domain of the reverse-path publishes SPF records
for the "Mail From" identity, those records do not authorize the
mailing list host, and a receiver of the mailing list performs the
authorization test.
Almost all mailing list software in use for public mailing lists uses
a reverse-path with the mailing list's own domain so that the
software can receive mail bounces and assist in the administration of
the list. Lists that use such software, configured to operate this
way will require only one modest change in light of this document:
The mailing list host needs to be authorized by the mailing list
domain's own SPF record, if the domain publishes one.
Mailing lists based on simple alias expansion, or other software that
doesn't manage bounces directly, 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
Forwarding services take mail that is received at a mailbox and
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direct it to some external mailbox. At the time of this writing, the
near-universal practice of such services is to use the original
reverse-path of a message when re-injecting it for delivery to the
external mailbox. 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 could also skirt the issue by using reverse-paths
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.
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 reverse-path 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 reverse-path 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.
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
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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.
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10. Security Considerations
The "Mail From" identity authorization must not be construed to
provide more assurance than it does. It is entirely possible for a
malicious sender to inject a message with a reverse-path that uses
their own domain, 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 "Mail From" 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 sequences 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
distributed denial of service attack:
While implementations of check_host() need to limit the number of
"include" and "redirect" terms and/or check for loops, malicious
domains could publish records that exercise or exceed these limits
in an attempt to waste computation effort at their targets when
they send them mail.
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.
While these distributed denial of service attacks are possible,
they seem more convoluted to mount, and have less of an impact,
than other simpler attacks.
When the authorization check fails with the code "Not Permitted", an
explanation string may be included in the reject response. Both the
sender and the rejecting receiver need to be aware that the
explanation was determined by the publisher of the SPF record
checked, and is in general not the receiver. The explanation may
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contain URLs that may be malicious, and/or offensive or misleading
text. This is probably less of a concern than it may seem since such
messages are returned to the sender, and their source is the SPF
record published by the domain in the "Mail From" 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.
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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.
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12. Contributors and Acknowledgements
This design owes a debt of parentage to [RMX] by Hadmut Danisch and
to [DMP] by Gordon Fecyk. The idea of using a DNS record to check
the legitimacy of an email address traces its ancestry 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 SPAM-L mailing list.
The folks on the ASRG mailing list.
The folks on the spf-discuss mailing list.
The folks on #perl.
The folks in the MARID working group and on the MXCOMP mailing
list.
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13. References
13.1 Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 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.
[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.
[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.
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Authors' Addresses
Mark Lentczner
1209 Villa Street
Mountain View, CA 94041
United States of America
EMail: markl@glyphic.com
URI: http://www.ozonehouse.com/mark/
Meng Weng Wong
Singapore
EMail: mengwong+spf@pobox.com
URI: http://spf.pobox.com/
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Appendix A. Collected ABNF
This section is normative and any discrepancies with the ABNF
fragments in the preceding text are to be resolved in favor of this
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
/ unknown-mechanism )
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
unknown-mechanism = name [ ":" macro-string ]
modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
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
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
domain-spec = *( macro-expand / macro-literal )
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macro-string = *( macro-expand / macro-literal / "/" )
macro-expand = ( "%{" ALPHA transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-2E / %x30-7E
; visible characters except "%" and "/"
transformer = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
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Appendix B. Extended Examples
These examples are based on the following DNS setup:
; A domain with two mail servers, two hosts
; and two servers at the domain name
$ORIGIN example.com.
@ MX 10 mail-a
MX 20 mail-b
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".
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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
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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 RBL 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}
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