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IMAP4 Binary Content Extension - RFC DRAFT Sender Policy Framework (SPF) |
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IMAP4 Binary Content Extension - RFC DRAFT Sender Policy Framework (SPF) |
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Get your own POP3 Email Address at Canada Emails.com
Urgent Action Against SPAM is Pending
Internet Draft
Category: Experimental Mark Lentczner
draft-mengwong-spf-00.txt Meng Weng Wong, pobox.com
Expires: July 2004 February 2004
Sender Policy Framework (SPF)
A Convention to Describe Hosts Authorized to Send SMTP Traffic
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026 except that the right to produce derivative
works is not granted.
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/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire in July 2004.
Abstract
Email address forgery is a problem on the Internet today. Domain
owners want to control the use of their names in email, but are
helpless because they lack the means. This document introduces a
language for domains to make email-related declarations in DNS. It
defines in detail one possible sender authentication scheme for
domains to describe the hosts from which they send mail. SMTP
receivers can use this scheme to detect sender forgery.
Table of Contents
TOC GOES HERE
1. Introduction
The intended audience for this document includes administrators of
the Domain Name System and developers of Mail Transfer Agents (MTAs),
Mail Delivery Agents (MDAs), and Mail User Agents (MUAs). They are
assumed to be familiar with the workings of SMTP and DNS. See
RFC2821 and RFC1034.
Forgery of domain names is a problem. Malicious entities often
falsify envelope and header addresses to make it harder to identify
the source of a message. When those addresses correspond to real
people and organizations, the victims of forgery suffer a cost in
bounce messages, tarnished reputations, and misdirected abuse
reports.
SPF is designed to fight email address forgery. It does this by
establishing a policy framework and an authentication scheme.
SPF defines a simple language. Domains can use that language to
describe the mail they send. SMTP receivers can use these
descriptions to evaluate messages.
SPF can implement a sender authentication scheme. In a sender
authentication scheme, a domain owner asserts that legitimate
messages from that domain must meet a certain description. Messages
which do not meet that description are not legitimate. These
assertions are made in machine-readable form.
SPF defines a specific sender authentication scheme based on the
designated sender model. A domain identifies certain hosts as
designated senders. Mail from those hosts is considered legitimate.
Mail from other hosts is not.
SPF publishes policy data in the DNS. DNS resolvers can cache SPF
data. Caching reduces lookup traffic. Sender domains do not have to
run new servers to advertise SPF information.
SPF is extensible. Multiple mechanisms can be defined. While other
sender authentication schemes can be expressed in SPF, the rest of
this document defines a designated sender scheme in detail.
1.1 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.
An SMTP client sends mail to an SMTP server. The SMTP server and
downstream systems comprise the SMTP receiver. The SMTP receiver
acts as an SPF client to an SPF publisher domain.
SPF processing may occur as early as the MAIL FROM stage of an SMTP
transaction or as late as the display stage in a Mail-User Agent.
For convenience, SMTP servers which accept, classify, discard, or
reject mail on the basis of SPF tests may be said to be speaking
"SMTP+SPF".
1.2 Designated Senders
Designated sender schemes are weaker than cryptographic schemes but
provide more assurance than the current SMTP model.
SPF defines a set of mechanisms which add up to a designated sender
scheme. First, domain owners designate legitimate outbound mail
servers in a compact, symbolic notation. SMTP receivers may query
sender domains using these mechanisms and decide the validity of a
given SMTP transaction while that transaction is ongoing, even before
any message data is passed. Alternatively, SPF tests can be
performed after SMTP time by an MDA or MUA. MTAs, MDAs, and MUAs may
choose to accept, classify, discard, or reject messages based on the
result of an SPF test.
SPF can be used to verify the sender of a message based on envelope
information available at SMTP time or according to the headers after
an SMTP transaction has completed.
2. SPF Records
Domains declare verifiable attributes that describe the mail they
send.
A domain's declarations are presented in an SPF record. The record
is a single string of text:
SPF-record = version *( 1*SP ( directive | modifier ) )
An example SPF 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".
2.1 Publishing
The SPF record is published in the DNS. The record is of type TXT.
The record is placed in the DNS tree at the level of the domain.
The TXT type was chosen for pragmatic reasons.
SPF clients ignore records which do not carry a recognized version
string. This document specifies the version string "v=spf1".
A domain MUST NOT return multiple records that begin with the word
"v=spf1". If more than one "v=spf1" record is returned, this
constitutes a syntax error and the result is "unknown".
An example SPF record is:
v=spf1 +mx +a:colo.example.com/28 -all
This might be published easily via this line in a domain file:
example.com. IN TXT "v=spf1 +mx +a:colo.example.com/28 -all"
In unusual situations, directives may require additional DNS
records. It is RECOMMENDED that these additional records be
published under the "_spf" subdomain. See Appendix B for examples.
2.2 Interpretation
SPF clients are applications that parse and interpret the SPF record
for a domain. Clients MUST correctly interpret the SPF record
according to the algorithm defined here.
2.2.1 Terms
This section defines important terms. They can be thought of as
variables in an SPF client. It is crucial that they be interpreted
correctly.
It is RECOMMENDED that the <responsible-sender> be drawn from the
envelope using this algorithm:
The <responsible-sender> comes from the domain name of the "MAIL
FROM" envelope sender. When the envelope sender has no domain, a
client MUST use the HELO domain instead. If the HELO argument does
not provide an FQDN, SPF processing terminates with "unknown".
If SPF processing occurs after SMTP time, the envelope sender may
be obtained from the Return-Path header. If the Return-Path header
has no domain, a client MAY try to extract the HELO domain from the
Received headers. If the headers do not yield useful envelope
information, SPF processing terminates with "unknown".
However, the <responsible-sender> address MAY be drawn from an
alternative source. For example, an MUA may find it more convenient
to extract the <responsible-sender> from the Return-Path header or
from the Sender: header.
The <current-domain> is initially drawn from the
<responsible-sender>. If the <responsible-sender> has no localpart,
clients MUST substitute the string "postmaster" for the localpart.
Recursive mechanisms such as Include and Redirect replace the initial
<current-domain> with another domain. However, they do not change
the value of the <responsible-sender>. See sections 4.2, 3.3, and
8.4.
2.2.2 Lookup
SPF clients perform a TXT type query in search of an SPF record.
Any number of records may be returned. Only the record which begins
with "v=spf1", followed by whitespace, is relevant to this document.
CNAME responses are followed as usual.
If no matching records are returned, an SPF client MUST assume that
the domain makes no SPF declarations. SPF processing MUST abort
and return "none".
If the domain does not exist (NXDOMAIN) an SPF client MAY return
either "fail" or "none".
Clients are NOT REQUIRED to fall back to parent domains. If a domain
has no SPF record, clients MUST NOT, on their own initiative,
substitute SPF data from a parent domain. For example, if no SPF
record is found for "workstation.example.com", clients MUST NOT
proceed automatically to query "example.com".
3. SPF Record Evaluation
An SPF client evaluates an SPF record and produces one of seven
results:
None: The domain does not publish SPF data.
Neutral (?): The SPF client MUST proceed as if a domain did not
publish SPF data. This result occurs if the domain explicitly
specifies a "?" value, or if processing "falls off the end" of
the SPF record.
Pass (+): the message meets the publishing domain's definition of
legitimacy. MTAs proceed to apply local policy and MAY accept or
reject the message accordingly.
Fail (-): the message does not meet a domain's definition of
legitimacy. MTAs MAY reject the message using a permanent
failure reply code. (Code 550 is RECOMMENDED. See RFC2821
section 7.1.)
Softfail (~): the message does not meet a domain's strict
definition of legitimacy, but the domain cannot confidently state
that the message is a forgery. MTAs SHOULD accept the message
but MAY subject it to a higher transaction cost, deeper scrutiny,
or an unfavourable score.
There are two error conditions, one temporary and one permanent.
Error: indicates an error during lookup; an MTA MAY reject the
message using a transient failure code, such as 450.
Unknown: indicates incomplete processing: an MTA MUST proceed as
if a domain did not publish SPF data.
When SPF-aware SMTP receivers accept a message, they SHOULD prepend a
Received-SPF header. See section 6.
SPF clients MUST use the algorithm described in this section
or its functional equivalent.
If an SPF client encounters a syntax error in an
SPF record, it must terminate processing and return a result
of "unknown".
3.1 Matching Version
An SPF record begins with a version section:
version = 'v=spf' version-number
version-number = non-zero-digit [ '.' ] *DIGIT
SPF clients MUST use only the records of the highest understood
version published by a domain and ignore all lower versions, unless
that version explicitly recognizes lower versioned responses.
For example, if an SPF client understands versions 1, 2 and 3, and
the DNS query results in records of version 1, 2 and 4, then only
the record with version 2 is used.
This specification describes version 1. If multiple "v=spf1" records
are returned, the SPF client MUST reject them all and act as if no
version 1 records were returned.
Minor version numbers are permitted, eg. "1.1".
SPF-like records of the form "v=spf1+ext" are not described by this
document.
3.2 SPF Directive Evaluation
There are two types of directives: mechanisms and modifiers. A given
mechanism type may always appear multiple times in a record.
Modifiers may be constrained to appear at most once per record,
depending on the definition of the modifier. Unknown mechanisms
cause processing to abort with the result "unknown". Unknown
modifiers are ignored by clients.
An SPF record contains an ordered list of mechanisms and modifiers:
SPF-record = version *( 1*SP ( [prefix] mechanism | modifier ) )
prefix = "+" / "-" / "~" / "?"
mechanism = 1*ALPHA [ ':' *VCHAR ] *[ '/' *DIGIT ]
modifier = 1*ALPHA '=' *VCHAR
Modifiers always contain an equals ('=') character.
Mechanisms usually contain ':' or '/' characters.
Directives that do not contain any of '=', ':', or '/' are
mechanisms.
Each directive 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
directive. If no directives remain, the default value is "-".
If it throws an exception, processing ends and the exception value is
returned (either "error" indicating a temporary failure, usually
DNS-related, or "unknown" indicating a syntax error or other
permanent failure resulting in incomplete processing.)
Mechanisms are described in Section 4 and 5.
A missing prefix in a directive is the same as a prefix of "+".
The possible prefixes are:
+ pass
- fail
~ softfail
? neutral
3.3 Default result
If none of the mechanisms match and there is no redirect modifier,
then the result of the SPF query is "neutral". If there is a
redirect modifier, the SPF client proceeds as defined in section 5.1.
Note that SPF records SHOULD always either use a redirect modifier or
an "all" mechanism to explicitly terminate processing.
For example:
v=spf1 +mx -all
v=spf1 +mx redirect=_spf.example.com
4. Mechanism Definitions
This section defines two types of mechanisms.
Basic mechanisms contribute to the SPF language framework. They
do not specify a particular type of authentication scheme.
- all
- include
Designated sender mechanisms are used to discover the relationship of
the client IP address to the <responsible-sender>.
- a - ip4
- mx - ip6
- ptr - exists
Mechanisms either match, do not match, or throw an exception.
If they match, their prefix value is returned.
If they do not match, processing continues.
If they throw an exception, the exception value is returned.
Several of these mechanisms take an optional <domain-spec> argument.
If the <domain-spec> is present, then it is macro expanded (see
Section 7) and becomes the <target-name>. If the <domain-spec> is
not provided, the <current-domain> is used as the <target-name>.
CNAME responses are followed in the usual way.
4.1 'all'
all = 'all'
The 'all' mechanism is a test that always matches. It is used as the
rightmost mechanism in an SPF record to indicate the default result.
For example:
v=spf1 +mx +a -all
Mechanisms after "all" will never be tested.
4.2 'include'
include = 'include' ':' domain-spec
The 'include' mechanism triggers a recursive SPF query. The
domain-spec is expanded as per section 7. Then a new query is
launched using the resulting string as the <current-domain>. The
<responsible-sender> stays the same.
"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 an SPF client to, in effect, search the SPF records
for example.com and example.org for a "pass" result. Only if the
message were not permitted for either of those domains would the
result be "fail".
This mechanism matches when the new query result returns a pass, and
doesn't match when the result is fail. However, if the new query
returns none, unknown, or error, then processing of the entire SPF
query stops immediately and returns the unknown or error result.
pass => match, return the prefix value for "include"
fail => no match, continue processing
softfail => no match, continue processing
neutral => no match, continue processing
error => error, abort
unknown => unknown, abort
none => unknown, abort # see below
If the parent domain includes another domain, and that domain one day
loses its SPF record, it is better for the query to abort with
"unknown" than to continue on to a potential "-all".
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 canonical set of designated mailers 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".
4.3 Introducing Designated Sender Mechanisms
Designated sender schemes allow SMTP receivers to make policy
decisions on the basis of domain name rather than IP address.
These mechanisms allow a domain to declare that certain hosts send
mail from that domain. When an SPF client processes these
mechanisms, it tests to see if the <sending-host> matches.
Usually, the <sending-host> is the IP address of an SMTP client. The
SMTP receiver is the SPF client. The SPF lookup may also operate
after the SMTP transaction has terminated. In these cases the
<sending-host> may have to be extracted from the Received header or
some other meta-data about the message. Received headers can be
forged. Accurate analysis is possible. Still, be careful.
If mail is transferred between mail systems internal to an
organization, and that organization chooses to process SPF after such
transfers, then the <sending-host> should be the external host that
first transferred the mail into the organization's mail system.
When the <sending-host> is localhost, Designated Sender mechanisms
are not meaningful. Therefore, an SPF client immediately returns
"pass" without evaluating mechanisms.
The <sending-host> is required for these mechanisms. If it cannot
be determined, then these mechanisms cannot be tested, and "unknown"
is returned.
The following conventions apply to designated sender mechanisms:
If the optional <CIDR-length> is given, then only the upper
<CIDR-length> bits of each IP are compared to the <sending-host>.
If the SMTP connection is IPv6, read "AAAA lookup" for "A lookup",
except where "A" lookups are explicitly specified.
4.4 'a'
This mechanism matches if the <sending-host> is one of the
<target-name>'s IP addresses.
A = 'a' [ ':' domain-spec ] [ dual-cidr-length ]
The <sending-host> is compared to the IP address(es) of the
<target-name>. If any address matches, the mechanism matches.
4.5 'mx'
This mechanism matches if the <sending-host> is one of the MX hosts
for a domain name.
MX = 'mx' [ ':' domain-spec ] [ dual-cidr-length ]
SPF clients first perform an MX lookup on the <target-name>. SPF
clients then perform an A lookup on each MX name returned, in order
of MX priority. The <sending-host> 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,
SPF clients MUST NOT pretend the target is its single MX, and MUST
NOT default to an A lookup on the <target-name> directly. This
behaviour breaks with the legacy "implicit MX" rule. See RFC2821
Section 5. If such behaviour is desired, the publisher should
specify an "a" directive.
4.6 'ptr'
This mechanism tests if the <sending-host>'s name is within a
particular domain.
PTR = 'ptr' [ ':' domain-spec ]
First the <sending-host>'s name is looked up using this procedure:
perform a PTR lookup against the <sending-host>'s IP. For each
record returned, validate the host name by looking up its IP address.
If the <sending-host>'s 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 the
<sending-host>, or if the <target-name> and the <sending-host> 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.
4.7 'ip4' and 'ip6'
These mechanisms test if the <sending-host> falls into 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 ]
ip4-network = dotted-quad notation
ip6-network = conventional IPv6 notation
The <sending-host> is compared to the given network. If they match,
the mechanism matches.
If the cidr-length is omitted, the ip4-cidr-length is taken to be
"/32" and the ip6-cidr-length is taken to be "/128".
4.8 '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 7. 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.
SPF publishers can use this mechanism to specify arbitrarily complex
queries. For example, suppose example.com publishes the SPF 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.
5. Modifier Definitions
Only two standard modifiers are defined: Redirect and Exp. SPF
clients MUST support them both.
Modifiers are not mechanisms: they do not return match or no-match.
Instead they provide additional information or change the course of
SPF processing.
While unrecognized mechanisms cause an immediate "unknown" abort,
unrecognized modifiers are simply ignored.
Modifiers therefore provide an easy way to extend the SPF protocol.
5.1 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 7. The resulting string
is a new domain that is now queried: The <current-domain>
is set to this new domain, and the new domain's SPF record
is fetched and processed. Note that <responsible-sender> does not
change.
The result of this new query is then considered the result of
original query.
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 SPF 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 SPF record. This can be an administrative advantage.
Note: in general, a domain A cannot reliably use a redirect to
another domain B not under the same administrative control. Since
the <responsible-sender> stays the same, there is no guarantee that
the SPF directives 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.
Only one redirect modifier may appear per SPF record. The modifier
does not have to appear at the end; it MAY appear anywhere in the
record. However, for clarity it is RECOMMENDED that redirect
modifiers appear after mechanisms.
5.2 exp: Explanation
The argument to the explanation modifier is a domain-spec to be TXT
queried. The result of the TXT query is a macro-string that is
macro-expanded. If SPF processing results in a rejection, the
expanded result SHOULD be shown to the sender in the SMTP reject
message. This string allows the publishing domain to communicate
further information via the SMTP receiver to legitimate senders in
the form of a short message or URL.
Only one exp modifier may appear per SPF record.
explanation = 'exp' '=' domain-spec
When an SPF client performs a query, and the result is anything other
than pass, then the explanation string, if present, SHOULD be
presented to the SMTP client after macro expansion. See section 7.
Suppose example.com has this SPF record
v=spf1 mx -all exp=explain._spf.%{d}
Here are some examples of the explanation TXT record at
explain._spf.example.com:
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
-- SMTP sender's IP address
See http://%{d}/why.html?s=%{S}&i=%{I}&h=%{H}
-- a complicated example that constructs a URL with
-- most of the parameters of the failed message so that
-- a web page can be generated with instructions
If multiple explanation TXT records are returned, they are
concatenated in the order they were received. Use of multiple TXT
records is discouraged as DNS does not guarantee order.
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.
6. Miscellaneous
6.1 Unrecognized Mechanisms and Modifiers
Future extensions to this standard may introduce new mechanisms and
modifiers.
Unrecognized mechanisms cause processing to abort: if, during
evaluation of an SPF record, an SPF client encounters a mechanism
which it does not understand or which it cannot properly evaluate
(due perhaps to insufficient information about the message at
evaluation time), then it terminates processing and returns
"unknown", without evaluating any further mechanisms. Mechanisms
listed before the unknown mechanism MUST, however, be evaluated.
For example, given the record
v=spf1 a mx ptr domainkeys:_dk.%{d} -all
messages that match the "a", "mx", or "ptr" mechanisms would return a
"pass" result. An SPF client that did not recognize the mechanism
"domainkeys" would return "unknown". An SPF client that was
domainkeys-aware would be able to perform extended evaluation. If
the message matched the domainkeys test, it would pass; if it did
not, evaluation would proceed to "-all" and return "fail".
"domainkeys" is an example of an unknown extension mechanism that
could be defined in future versions of this standard. It is NOT
defined by this proposal.
Unrecognized modifiers are ignored: if an SPF client encounters
modifiers which it does not recognize, it MUST ignore them and
continue processing. Modifiers always contain an "=" sign.
Unrecognized mechanisms are preserved in the Received-SPF header.
See section 6.3.
6.2 Processing Limits
During processing, an SPF client may perform additional SPF
subqueries due to the Include mechanism and the Redirect modifier.
SPF clients must be prepared to handle records that are set up
incorrectly or maliciously. SPF clients MUST perform loop detection,
limit SPF recursion, or both. If an SPF client chooses to limit
recursion depth, then at least a total of 20 redirects and includes
SHOULD be supported. (This number should be enough for even the most
complicated configurations.)
If a loop is detected, or if more than 20 subqueries are triggered,
an SPF client MAY abort the lookup and return the result "unknown".
Regular non-recursive lookups due to mechanisms like "a" and "mx" or
due to modifiers like "exp" do not count toward this total.
6.3 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 MUST use the "Received-SPF" header defined here. This
information is intended for the recipient. (Information intended for
the sender of the e-mail is described in Section 5.2, Explanation.)
The header SHOULD be prepended to existing headers. It MUST appear
above any other Received-SPF headers in the message. The header has
the format:
header = 'Received-SPF:' 1*SP result [ 1*SP '(' comment ')' ]
*( 1*SP key-value-pair )
result = 'pass' / 'fail' / 'error' / 'softfail' / 'neutral' /
'none' / 'unknown' / unknown-mechanisms
unknown-mechanisms = 'unknown' *( 1*SP [prefix] mechanism )
key-value-pair = 1*VCHAR '=' *(WSP / VCHAR) ';'
comment = [ smtp-receiver-hostname ': ' comment-string ]
The comment-string should convey supporting information for the
result (such as <responsible-sender> and <current-domain>).
If processing was aborted due to unrecognized mechanisms, the
Received-SPF header SHOULD show the unrecognized mechanisms after
the "unknown" word.
Example headers generated by mybox.example.org:
Received-SPF: pass (mybox.example.org: domain of
myname@example.com designates 192.0.2.1 as
permitted sender)
receiver=mybox.example.org;
client-ip=192.0.2.1;
envelope-from=<myname@example.com>;
helo=foo.example.com;
Received-SPF: fail (mybox.example.org: domain of
myname@example.com does not designate
192.0.2.1 as permitted sender)
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 does not
designate 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 designated permitted sender hosts)
Received-SPF: unknown -extension:foo (mybox.example.org: domain
of myname@example.com uses
mechanism not recognized by
this client)
Received-SPF: error (mybox.example.org: error in processing
during lookup of myname@example.com: DNS
timeout)
SPF clients may append zero or more of the following key-value-pairs
at their discretion:
receiver the hostname of the SPF client
client-ip the IP address of the SMTP client
envelope-from the envelope sender address
helo the hostname 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-".
7. Macros
7.1 Macro definitions
Certain directives perform macro interpolation on their arguments.
macro-string = *( macro-char / VCHAR )
macro-char = ( '%{' ALPHA *DIGIT [ 'r' ] *delim '}' )
/ '%%'
/ '%_'
/ '%-'
A literal '%' is expressed by '%%'.
%_ expands to a single " " space.
%- expands to a URL-encoded space, viz. "%20".
The following macro letters are expanded:
l = local-part of responsible-sender
s = responsible-sender
o = responsible-domain
d = current-domain
i = SMTP client IP
t = current timestamp in UTC epoch seconds notation
p = SMTP client domain name
v = client IP version string: "in-addr" for ipv4 or "ip6" for ipv6
h = HELO/EHLO domain
The uppercase versions of those macros are URL-encoded as well.
A '%' character not followed by a '{', '%', '-', or '_' character
MUST be interpreted as a literal. SPF publishers 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
Your spam volume has increased by 581%%
Legal modifiers are
*DIGIT ; one or more digits
'r' ; reverse value, splitting on dots by default
The DIGIT modifier indicates the number of right-hand parts to use
after optional reversal. The modifier MUST be nonzero. If DIGIT
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.
The 'r' modifier 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 and the 'r' modifiers split a string into parts. By
default, strings are split on "." dots. Modifiers may be followed by
one or more splitting characters which are used instead of the ".".
Splitting characters MUST be non-alphanumeric. Parts are always
rejoined using "." and not the original splitting characters.
For the "l" and "s" macros: when the local-part has no length, the
string "postmaster" is substituted.
The "p" macro expands to the validated domain name of the SMTP
client. The validation procedure is described in section 5.4. If
there are no validated domain names, the word "unknown" is
substituted. If multiple validated domain names exist, the first one
returned in the PTR result is chosen.
The "s" macro expands to the sender email address: a localpart, an @
sign, and a domain. The "o" macro is the domain part of the "s".
They remain the same during a recursive "include" or "redirect"
subquery.
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 are URL escaped.
URL encoding is described in RFC2396.
7.2 Expansion Examples
The <responsible-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
%{p} mx.example.org
%{p2} example.org
%{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
%{p2}.trusted-domains.example.net
example.org.trusted-domains.example.net
IPv6:
%{ir}.example.org 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.example.org
8. Conformance Definitions
The following sections define SPF conformance applied to various
entities.
8.1 Introduction
In an SMTP+SPF transaction, there are three primary entites:
- the envelope sender domain
- the sending host (the SMTP client)
- the SMTP receiver
The conformance status of each entity has implications for the
transaction as a whole.
8.2 Conformance with regard to sender domains
For a domain to be considered SPF-conformant, its authoritative DNS
servers are REQUIRED to publish an SPF record for that domain.
Domains which do not publish SPF data SHALL NOT be deemed
SPF-conformant.
If foo.example.com claims SPF compliance, it must have a record of
the form:
foo.example.com IN TXT "v=spf1 ..."
A domain also SHOULD publish policy records for each of its
designated servers to accommodate the MAIL FROM: <>.
8.3 Conformance with regard to sending e-mail systems
To be considered SPF-conformant, an SMTP sending host MUST resolve a
"pass" for all the SPF-conformant domains for which it sends mail.
When an SMTP host sends a message delivery status notification
message, it MAY use the null envelope sender:
MAIL FROM: <>
The sender host's HELO/EHLO command string MUST include the Fully
Qualified Domain Name of the sender host, and an SPF record MUST
exist for that FQDN for the host to be considered SPF-conformant.
For example: in a transaction with
HELO mx01.example.com
MAIL FROM: <>
an SMTP+SPF receiver will perform an SPF query of the form
mx01.example.com TXT
and expect a result such as
"v=spf1 ptr:example.com -all"
or
"v=spf1 a -all"
8.4 Conformance with regard to receiving e-mail systems
To describe itself as SPF-conformant, an SMTP receiver MUST perform
SPF tests.
SPF tests need not be performed while an SMTP transaction is ongoing:
if the MDA performs the test, that is sufficient. A server need not
reject a message; but if it does not, it SHOULD add a Received-SPF
header. If a server rejects a message, it SHOULD include any
<explanation> provided by the SPF publisher.
If a receiver system has a choice of testing the envelope sender (as
recorded in the Return-Path header) versus the message headers (as
recorded in Sender or From), the envelope is recommended.
Receiver systems SHOULD exclude special recipients such as
postmaster@ and abuse@ from SPF processing. See RFC2142.
SPF is only one component in a policy engine. An SPF-conformant SMTP
receiver is NOT REQUIRED to perform SPF tests on messages whose
dispositions have already been decided on the basis of other policy.
Example 1: if an SMTP receiver requires that sender domains must
possess MX or A records, and rejects transactions where they do
not, then SPF tests are moot.
Example 2: if an SMTP receiver expects messages from a trusted
client, such as a secondary MX for its own domain, then SPF tests
are not needed.
Example 3: if an SMTP receiver is considering a transaction which
does not yield a fully-qualified domain name in either the MAIL
FROM sender or the HELO command, SPF tests are not appropriate, and
the disposition of the message should be decided on the basis of
other policy.
8.5 Conformance with regard to a particular SMTP transaction
An email message during delivery is conformant if SPF evaluation
results in a "pass", "neutral", or "softfail".
8.6 Conformance with regard to an email-sending user
An email-sending user is conformant if all of her outbound mail is
sent through a designated mailer for her sender domain.
8.7 Rejection of non-SPF conformant email
Mail from a domain SHOULD NOT be automatically treated as suspect
just because the domain doesn't publish SPF records.
8.8 Rejection of SPF conformant email
An SPF email system MAY choose to reject or discard email on the
basis of local policy. SPF is one component in an overall
email-policy engine. SPF merely makes it possible for policy
decisions to be made with confidence at the sender-domain level. The
actual policy decisions are outside the scope of this document.
8.9 Recommendations
If a domain contains subdomains or hostnames that have A or MX
records, those subdomains and hostnames SHOULD publish SPF records as
well. If they do not, they remain at risk of forgery.
Domain names used in a legitimate envelope sender SHOULD possess MX
records.
This proposal recommends the deprecation of the legacy "implicit MX"
rule defined in RFC2821 section 5. Domains that want to receive mail
SHOULD always define explicit MX records.
8.10 Changes to Existing Semantics
8.10.1 The Return-Path is now also a Responsible Sender
From RFC2821:
The <reverse-path> portion of the first or only argument contains
the source mailbox (between "<" and ">" brackets), which can be
used to report errors (see section 4.2 for a discussion of error
reporting).
When SPF is used to authenticate the return-path, the domain in the
source mailbox is now also the party responsible for sending the
message.
This semantic change is justified by the desire to control joe-jobs.
Joe jobs are a distributed denial of service attack against a given
address executed by forging messages using a victim sender address
and sending them to thousands of recipients. Inevitably, some of
those delivery attempts fail, and bounce messages are generated to
the victim sender address. These unwanted bounce messages can end up
crippling the victim mailbox. SPF gives these potential victims a
way to protect their mailboxes. With SPF, senders can now control
the use of their address in the return-path.
9. Applicability Statement
This section discusses deployment considerations.
9.1 Adoption by disreputable domains
It is trivial for a domain to publish "+all" to allow all. A
disreputable domain could then send unwanted email from any host.
This is a common objection to SPF.
SPF provides a value-neutral framework for sender authentication and
accreditation. Senders use that framework to make assertions
regarding the quality of messages.
It is the responsibility of the SMTP receiver to evaluate those
assertions. First the SMTP receiver discards obvious forgeries.
Then it evaluates the remaining messages according to the reputation
of the sender and any accompanying accreditation assertions.
Whether a given domain is reputable or not is a decision that belongs
to an SMTP receiver. Policy decisions regarding particular messages
are outside the scope of SPF.
9.2 Limitations
In an SPF-conformant environment, envelope sender forgery is limited
to the local domain. Unless per-user macros are used, it is possible
for one user to forge another user's address within that domain.
However, the organization responsible for the domain presumably has
the wherewithal to follow an audit trail.
9.3 Phased Rollout
At an adopting domain, adoption of SPF could occur in phases. A
domain might move through these phases by changing its default
response type from "neutral" to "softfail" to "fail".
The phases are characterized by different levels of awareness
among the domain's userbase, and different levels of strictness on
the part of SPF-conformant receivers.
When a sufficient majority of its users are SPF-conformant, a
domain SHOULD change its default to "fail". That sends a signal to
SPF-conformant mail receivers that non-conformant messages SHOULD
be rejected. Setting "-all" protects the users from account fraud
and joe-jobbing.
Messages that explicitly fail SPF with a "fail" SHOULD be rejected.
9.4 Verbatim Forwarding
SPF changes the model of email from a store-and-forward system to an
end-to-end system. Verbatim email forwarding suffers accordingly.
In verbatim forwarding, an intermediate forwarding host rewrites the
envelope recipient, but leaves the envelope sender untouched.
Forwarding hosts will need to rewrite the envelope sender as well to
maintain SPF conformance.
The rewritten envelope sender can take a variety of forms, depending
on whether bounces should be forwarded back to the original sender.
If bounces are unimportant, the rewritten sender could be as simple
as <nobody@example.com>. If bounces are important, forwarders can
use envelope encapsulation with a MAC nonce, or rewrite the address
into a database-backed cookie.
A DNS whitelist (or its domain-name equivalent, a Right-Hand-Side
Whitelist (RHSWL)) of well-known trusted forwarders can help ease the
transition.
9.5 Per-user exemptions
The "exists" mechanism can be used to exempt certain users from the
SPF requirements that apply to the rest of the domain.
10. Security Considerations
SPF depends on DNS. A malicious attacker could poison a target's DNS
cache with spoofed DNS data.
SPF assumes the client IP address is true. A malicious attacker
could spoof TCP sequences to make mail appear to come from a
designated host. If this happens, an "echo" command could be
introduced to ESMTP to require a simple challenge/response
confirmation.
SPF clients need to limit themselves to 20 includes and redirects to
avoid attacks.
11. IANA Considerations
The registry of standard mechanism and modifier names may be turned
over to IANA for management.
12. Contributors and Acknowledgements
SPF owes a debt of parentage to RMX (by Hadmut Danisch in 2003) and to
DMP (by Gordon Fecyk in 2003). It traces its ancestry farther back
through "Repudiating Mail-From" by Paul Vixie in 2002 to a suggestion
by Jim Miller in 1998.
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 following individuals who
contributed to, critically reviewed, or otherwise furthered the
development of this specification. The authors wish to apologize in
advance for any omissions.
Alain Knaff, Alain Nakache, Alan DeKok, Alan Hodgson, Alex Rosen,
Alex Van Den Bogaerdt, Andreas Kreuzinger, Andrew Boling, Andrew
Rose, Andrew Sweger, Andy Bakun, Andy Lester, Andy Smith, Anne
Mitchell, Anthony Campbell, Anthony Howe, Aredridel, Arik Baratz,
Arlie Davis, Ask Bjoern Hansen, B. Gingery, B. Johannessen, Benjamin
Franz, Bill Cole, Bill Landry, Bill Rockefeller, Bob Atkinson, Bob
Greene, Bob Miller, Bob Proulx, Bob Tanner, Brad Templeton, Bradley
Cloete, Brian Coloney, Brian Hatch, Brian Nelson, Bryan Campbell,
Bryce Jasmer, Carl Hutzler, Casey West, Carsten Kuckuk, Catherine
Hampton, Chris Drake, Christophe Saout, Christopher Chan, Chuck
Wolber, Clifford Hammerschmidt, Colin Whittaker, Cyrus Mehta, Dan
Boresjo, Dan Nadir, Dan Willett Sr., Daniel Mack, Daniel
Roethlisberger, Dave Lugo, David A. Wheeler, David Brodbeck, David
Saez, Dennis Carr, der Mouse, Derek J. Balling, Dirk Van Mieghem, Don
Andrews, Don Koch, Dr. Ernst Molitor, Dustin Trammell, Edward Ned
Harvey, Eric Allman, Eric Helfgott, Eric S. Raymond, Erik Corry,
Ernesto Baschny, Fridrik Skulason, G. Roderick Singleton, George
Mitchell, George Schlossnagle, George Webb, Gerald Oskoboiny, Graham
Barr, Graham Murray, Greg Connor, Greg Wooledge, Guillaume Filion,
Gustav Foseid, Hans Dieter Pearcey, Harry Katz, Harry Palmer, Henrik
Edlund, Howard Lee Harkness, Izzy Kindred, Jameel Akari, James
Couzens, Jan Wildeboer, Jason Buchanan, Jasper Wallace, Jeremy Doupe,
Jeremy Pullicino, Jesus Duarte, Jim Lyon, Jim Popovitch, Jim Ramsay,
Joe Zasada, John A. Martin, John Capo, John Payne, John R Levine,
John Warren, Jon Bertrand, Jon Loeliger, Jonathan Steinert, Jordan
Pollack, Julian Mehnle, Justin Mason, Justo Alonso, Karl J. Smith,
Kelson Vibber, Kenn Humborg, Kenneth Porter, Kevin Reed, Kitt
Diebold, Klaus Alexander Seistrup, Larry Smith, Lee Carver, Lee
Maguire, Loic Prylli, Marc Alaia, Marc Hudson, Mark Anderson, Mark
Delany, Mark Farver, Mark Foster, Mark Jason Dominus, Mark Jeftovic,
Mark Shewmaker, Mark Tranchant, Mark, system administrator at
Asarian-host.org, Marrandy, Martin H. Sluka, Marty Lyons, Matt Perry,
Matt Sergeant, Matthew Barr, Matthew Mucker, Matthew Walker, Mike
Batchelor, Neil Schwartzman, Nick Phillips, Nicolas Bougues, Nils
Puhlmann, Odhiambo Washington, Oliver Gerler, Paul Blair, Paul Vixie,
Paul Wouters, Paul Howarth, Peter Baumann, Peter Viertel, Peter
Westlake, Phil Howard, Phil Karn, Phil White, Philip Hazel, Philip
Tucker, Philipp Morger, Phillip Hallam-Baker, R. Scott Perry, Rahul
Tongia, Ray Everett-Church, Raymond S Brand, Rich Puhek, Richard
George, Richard Hansen, Richard Kay, Richard Pitt, Rick Stewart, Rik
Van Riel, Rob Kaper, Robert Spier, Rolf E. Sonneveld, Ronald
F. Guilmette, Roy Badami, Russell Kroll, Sam Horrocks, Sam Norris,
Sanford Whiteman, Scott Nelson, Scott Savarese, Sean Comeau, Seth
Breidbart, Seth Goodman, Seun Osewa, Shevek, Steven Bellovin, Steven
G. Willis, Steven W. Orr, Stuart Schneider, Suresh Ramasubramanian,
Ted Cabeen, Terence Way, Theo Van Dinter, Thomas Harold, Thomas
R. Stephenson, Thor Kooda, Tim Freeman, Tim Gladding, Tim Wilde, Tom
Allison, Tracy Martin, Tugrul Galatali, Ty Lammy, Vidar Holen, Viktor
Duchovni, Vivien M., Walt Mankowski, Wayne Schlitt, Wechsler, Weldon
Whipple, Will Lowe, Will Senn, William Astle, Yakov Shafranovich, Zan
Hecht, and Zolta'N Za'Mbori.
The folks on the SPAM-L mailing list
The folks on the ASRG mailing list and in the IETF.
The folks on the spf-discuss mailing list.
The folks on the mailing list that shall not be named.
The folks on #perl.
Appendix A. Collected ABNF for SPF records
See RFC2234 for ABNF notation.
SPF-record = version *( 1*SP ( [prefix] mechanism | modifier ) )
version = 'v=spf' 1*digit
prefix = ( "+" / "-" / "?" / "~" )
modifier = redirect / explanation / unknown-modifier
redirect = 'redirect' '=' domain-spec
explanation = 'exp' '=' domain-spec
unknown-modifier = 1*ALPHA '=' *VCHAR
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists
/ extension )
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' ':' macro-string
extension = *alphanum [ ':' *VCHAR ]
ip4-network = as in RFC2373 [15], e.g. 192.0.2.0
ip6-network = as in RFC2373 [15], e.g. 12AB:0:0:CD30
domain-spec = domain-name / macro-string
domain-name = domain-part *( '.' domain-part ) [ '.' ]
domain-part = as defined in RFC1034
dual-cidr-length = [ ip4-cidr-length ] [ '/' ip6-cidr-length ]
ip4-cidr-length = [ '/' 1*DIGIT ]
ip6-cidr-length = [ '/' 1*DIGIT ]
macro-string = *( macro-char / VCHAR )
macro-char = ( '%{' alpha *digit [ 'r' ] *delim '}' )
/ '%%' / '%_' / '%-'
alphanum = alpha | digit
delim = '.' | '-' | '+' | ',' | '|' | '_' | '='
Appendix B. Extended Examples
These examples are based on the following DNS set up:
; 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 192.168.1.10
A 192.168.1.11
amy A 192.168.1.65
bob A 192.168.1.66
mail-a A 192.168.1.129
mail-b A 192.168.1.130
www CNAME example.com.
; The reverse IP for that domain
$ORIGIN 1.168.192.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.
; A related domain
$ORIGIN example.org
@ MX 10 mail-c
mail-c A 192.168.2.140
; The reverse IP for that domain
$ORIGIN 2.168.192.in-addr.arpa.
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.
Appendix B.1 Simple Example
If <current-domain> is "example.com", then this describes the
effect various possible SPF records for example.com would have
on various <sending-hosts>
"v=spf1 +all"
-- any mail message passes
"v=spf1 a -all"
-- sending hosts 192.168.1.10 and 192.168.1.11 pass
"v=spf1 a:example.org -all"
-- no sending hosts pass since example.org has no A records
"v=spf1 mx -all"
-- sending hosts 192.168.1.129 and 192.168.1.130 pass
"v=spf1 mx:example.org -all"
-- sending host 192.168.2.140 passes
"v=spf1 mx mx:example.org -all"
-- sending hosts 192.168.1.129, 192.168.1.130,
and 192.168.2.140 pass
"v=spf1 mx/24 mx:example.org/24 -all"
-- any sending host in 192.168.1.0/24 or 192.168.2.0/24 passes
"v=spf1 ptr -all"
-- sending host 192.168.1.65 passes
(reverse IP is valid and in example.com)
-- sending host 192.168.2.140 fails
(reverse IP is valid, but not in example.com)
-- sending host 10.0.0.4 fails
(reverse IP is not valid)
"v=spf1 ip4:192.168.1.128/25 -all"
-- sending host 192.168.1.65 fails
-- sending host 192.168.1.129 passes
Appendix B.2 Multiple Domain Examples
These examples show the effect of related SPF records:
example.org: "v=spf1 include:example.com include:example.net -all"
This SPF 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.
1.example.org: "v=spf1 redirect=example.org"
2.example.org: "v=spf1 redirect=example.org"
3.example.org: "v=spf1 redirect=example.org"
These SPF records allow a set of domains that all use the same
mail system to make use of that mail system's SPF record. In this
way, only the mail system's SPF record needs to updated when
the mail setup changes. These domains' SPF records never have to
change.
Appendix 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
Then consider these SPF records
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}"
This describes users at example.com who send mail from arbitrary
servers, and users who send mail from a set of personal servers.
Normative References
[1] [RFC2396]
Informative References
(These references will be properly cited in a later version of the
draft.)
[1] [RFC1034]
[2] [RFC1464]
[3] [RFC2119]
[4] [RFC2142]
[5] [RFC2234] ABNF
[6] [RFC2373]
[7] [RFC2505]
[8] [RFC2821]
[9] [RFC2822]
[10] Danisch, Hadmut.
"The RMX DNS RR Type for light weight sender authentication",
http://www.danisch.de/work/security/antispam.html, October 2003,
Work In Progress.
[11] Fecyk, Gordon. "Designated Mailers Protocol", December 2003,
Work In Progress. http://www.pan-am.ca/dmp/
[12] Wong, M.W., "Sender Rewriting Scheme", Work In Progress,
http://spf.pobox.com/srs/
[13] DeKok, Alan. The LMAP discussion document.
[14] Vixie, Paul. "Repudiating Mail-From".
http://ops.ietf.org/lists/namedroppers/namedroppers.2002/msg00658.html
[15] http://nospam.couchpotato.net/
Authors
Meng Weng Wong
Singapore
mengwong+spf@pobox.com
Mark Lentczner
1209 Villa Street
Mountain View, CA 94041
United States of America
markl@glyphic.com
Comments on this draft are welcome on the spf-discuss mailing list.
To subscribe to the mailing list, please see
http://spf.pobox.com/mailinglist.html
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