One document matched: draft-ietf-marid-protocol-00.txt
MARID Meng Weng Wong
Internet-Draft Mark Lentczner
Expires: January 9, 2005 July 11, 2004
The SPF Record Format and Test Protocol
draft-ietf-marid-protocol-00
Status of this Memo
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This Internet-Draft will expire on January 9, 2005.
Abstract
This document defines the format of a record that domains publish to
authorize a set of hosts to send mail on its behalf. This document
also defines the check_host() function that mail receivers evaluate
to see if a particular host is authorized to submit a particular
piece of mail. This document only describes the particulars of the
syntax and the algorithm of the function. See the MARID Core draft
(draft-ietf-marid-core) for how these are used in the context of
sending and receiving Internet mail.
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Table of Contents
1. Introduction
1.1 Earlier Work
1.2 Terminology
2. Permitted Sender Records
2.1 Publishing
2.1.1 DNS Record Type
2.1.2 Multiple Records
2.1.3 Additional Records
2.1.4 Multiple Strings
3 The check_host() Function
3.1 Arguments
3.2 Results
3.3 Initial Processing
3.4 Record Lookup
3.4.1 Matching Version
3.5. Record Evaluation
3.5.1 Term Evaluation
3.5.2 Mechanisms
3.5.3 Modifiers
3.6 Default result
4. Mechanism Definitions
4.1 "all"
4.2 "include"
4.3 "a"
4.4 "mx"
4.5 "ptr"
4.6 "ip4" and "ip6"
4.7 "exists"
5. Modifier Definitions
5.1 redirect: Redirected Query
5.2 exp: Explanation
6. Miscellaneous
6.1 Unrecognized Mechanisms and Modifiers
6.2 Processing Limits
7. Macros
7.1 Macro definitions
7.2 Expansion Examples
8. Security Considerations
9. IANA Considerations
9.1 Registration Template
10. Contributors and Acknowledgements
Normative References
Informative References
Authors' Addresses
Comments
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Appendix A. Collected ABNF
Appendix B. Extended Examples
B.1 Simple Examples
B.2 Multiple Domain Example
B.3 RBL Style Example
Intellectual Property Statement
Disclaimer of Validity
Copyright Statement
Acknowledgment
1. Introduction
Owners of domain names publish SPF records to make declarations about
hosts that are and are not permitted to use their domain names while
sending mail.
Mail receivers perform tests, which read published records to
determine how a domain's declarations apply to particular hosts in
during mail reception.
The format of the records, and the mechanics of using them to perform
the tests must be rigorously defined if domain owners and mail
receivers are to agree on what the declarations mean.
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 [RFC
2821] and [RFC 1034].
1.1 Earlier Work
This design is an evolution of work done under the name SPF. The
record format and test presented here has been intentionally designed
to be generally backward compatible with the currently deployed base
of records and code.
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 [RFC 2119].
2. Permitted Sender Records
Domains publish records to declare which hosts are and are not
permitted to use their names when sending mail. Loosely, the record
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partitions all hosts into permitted and not-permitted sets. (Though
some hosts might fall into other categories.)
The record is a single string of text:
record = version *( 1*SP ( directive / modifier ) ) *SP
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".
Mail receivers read these records to allow them to perform checks for
a given host in a given context. They do so by evaluating the
check_host() function (described below), and the directives of a
domain's record direct the evaluation.
2.1 Publishing
A domain publishes its record in DNS.
The record is placed in the DNS tree at the level of the domain name
it pertains to.
An example published record could be:
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"
2.1.1 DNS Record Type
The record type is a textual RR type to be allocated by the IANA for
this purpose.
However, because there is a large number of domains with these
records already deployed as TXT type records, and because there are a
number of DNS server and resolver implementations in common use that
cannot handle new RR types, the record type can be TXT.
Domains SHOULD publish records under both types. If a domain does
publish under both types, then they MUST have the same content.
Mail receivers SHOULD query for both types of records. If both are
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returned, then the new RR type MUST be preferred.
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
records and 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.
2.1.2 Multiple Records
A domain MUST NOT publish multiple records of the same type (TXT or
new RR) that begin with the version defined in this document
("v=spf1"). If a domain does, this will constitute permanent error.
2.1.3 Additional Records
Some records contain directives that require the creation of
additional DNS records. It is suggested that those records be placed
under an "_spf" subdomain. See Appendix B for examples.
2.1.4 Multiple Strings
Text DNS records (both TXT and text RR types) can be composed of more
than one string. If a published record contains multiple strings,
then record MUST be treated as if those strings are concatenated
together without adding spaces. For example:
TXT "v=spf1 .... first" "second string..."
MUST be treated as equivalent to
TXT "v=spf1 .... firstsecond string..."
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 text RR record.
It is STRONGLY ENCOURAGED that published records remain under 400
characters of text in order to keep DNS queries from falling over to
TCP. [[ This limit needs to be checked. ]]
Note: Many nameserver implementations will silently split long
strings in TXT records into several shorter strings.
3 The check_host() Function
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The check_host() function fetches published records, parses them, and
interprets them to evaluate if a particular host is or is not
permitted to send mail in a given context. Mail receivers that
perform this check MUST correctly implement the check_host() function
as described by the canonical algorithm defined here.
Implementations MAY use a different algorithm, so long as the results
are the same.
3.1 Arguments
The function check_host() take three arguments:
<ip> - the IP address of the host under test
<domain> - the domain to check
<sender> - the full sending mailbox address
See [MARID Core] for how these arguments are determined.
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 the same during recursive evaluations (see
"include" below)
Note: The IP address may be either v4 or v6.
3.2 Results
The function check_host() can result in one of seven results
described here. Possible actions to be taken based on these results
are decided by the checks a mail receiver is performing (see [MARID
Core]) and by the local policies of the receiver.
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
Results from error conditions:
None: no published data
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
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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.
3.3 Initial Processing
If the <domain> is not an FQDN, the 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.
3.4 Record Lookup
The record for <domain> is fetched. If the record is in a cache, and
has not expired, then it may simply be used. Otherwise, the record
must be fetched from DNS as follows:
In accordance with how the records are published, (see 2.1 above), a
DNS query needs to be made for the <domain> name.
If no matching records are returned, check_host() exits immediately
with the result "None".
If the domain does not exist (NXDOMAIN), check_host() exits
immediately with the result "Fail" and a reason of "Domain Does Not
Exist"
If the DNS lookup returns a server failure (SERVFAIL) or the query
times out, check_host() exits immediately with the result "TempError"
3.4.1 Matching Version
Only records which begin with a proper version section are
considered. The version section is defined as:
version = "v=spf1"
The version section is terminated either by a SP character, or the
end of the TXT record. Hence, a records that start "v=spf10", or
"v=spf1.0" are not considered records with proper versions.
If there is only one such record, it is used. If there are two such
records, one TXT and one new text RR, then the later MUST be used.
If two or more such records exist of the same type, then check_host()
exists immediately with the error "PermError".
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3.5. Record Evaluation
After the record has been retrieved, 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 well formed. See section
6.1.
3.5.1 Term Evaluation
There are two types of terms: 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 "PermError". Unknown
modifiers are ignored.
An record contains an ordered list of mechanisms and modifiers:
record = version *( 1*SP ( directive / modifier ) ) *SP
version = "v=spf1"
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = name [ ":" macro-string ] *[ "/" *DIGIT ]
modifier = name "=" macro-string
name = alpha *( alpha / digit / "-" / "_" / "." )
Mechanisms usually contain 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".
3.5.2 Mechanisms
Each mechanism is considered in turn from left to right.
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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
3.6.
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 5.2.
Specific mechanisms are described in Section 4.
3.5.3 Modifiers
Modifiers are either global or positional:
Global modifiers MAY appear anywhere in the record, but SHOULD
appear at the end, after all mechanisms and positional modifiers.
Positional modifiers apply only to the mechanism they follow. It
is a syntax error for a positional modifier to appear before the
first mechanism.
Modifiers of either type are also either singular or multiple:
Singular modifiers may appear only once in the record if they are
global, or once after each mechanism if they are positional.
Multiple modifiers may appear multiple times in the record if they
are global, or multiple times after each mechanism if they are
positional.
The definition of each specific modifier (see Section 5) states
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whether it is global or positional, and whether it is singular or
multiple. A modifier is not allowed to be defined as both global and
positional.
Ordering of modifiers does not matter, except:
1) positional modifiers must appear after the mechanism they
affect and before any subsequent mechanisms.
and 2) when a multiple modifier appears more than one time, the
ordering of the appearances may be significant.
Other than these constraints, implementations MUST treat different
orders of modifiers the same. An intended side effect of these rules
is modifiers cannot be defined that modify other modifiers.
These rules allow an implementation to correctly preparse a record.
Furthermore, they are crafted to allow the parsing algorithm to be
stable, even when new modifiers are introduced.
Modifiers which are unrecognized MUST be ignored. This allows older
implementations to handle records with modifiers that were defined
after they were written.
3.6 Default result
If none of the mechanisms match and there is no redirect modifier,
then the check_host() exists with a result of "Neutral". If there is
a redirect modifier, check_host() proceeds as defined in section 5.1.
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. 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>
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address as being permitted or not to use the <domain> for sending
mail.
a ip4
mx ip6
ptr exists
Other mechanisms may be defined in the future.
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 <domain> is used as the <target-name>.
The following conventions apply to all mechanisms that perform an
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 mechanisms fetchs 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.
4.1 "all"
all = "all"
The "all" mechanism is a test that always matches. It is used as the
rightmost mechanism in a record to provide an explicit default.
For example:
v=spf1 +mx +a -all
Mechanisms after "all" will never be tested.
4.2 "include"
include = "include" ":" domain-spec
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The "include" mechanism triggers a recursive evaluation of
check_host(). The domain-spec is expanded as per section 7. 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().
"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".
This mechanism matches when the recursive check_host() result returns
a "Pass", and doesn't match when the result is "Fail", "SoftFail", or
"Neutral". If the result is "TempError", "PermError" or "None", then
processing of the current check_host() stops immediately and returns
either "TempError" in the first case, or "PermError" in the second
two..
recursive include
check_host() mechanism
result result processing
--------- -- ---------------- -----------------------------------
Pass => match, return based on prefix of "include"
Fail => no match, continue processing
SoftFail => no match, continue processing
Neutral => no match, continue processing
TempError => throw TempError, abort processing, return TempError
PermError => throw PermError, abort processing, return PermError
None => throw PermError, abort processing, return 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
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"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 "a"
This mechanism matches if <ip> is one of the <target-name>'s IP
addresses.
A = "a" [ ":" domain-spec ] [ dual-cidr-length ]
The <ip> is compared to the IP address(es) of the <target-name>. If
any address matches, the mechanism matches.
4.4 "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
perform an A lookup on each MX name returned, in order of MX
priority. 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 [RFC 2821]
Section 5. If such behavior is desired, the publisher should specify
an "a" directive.
4.5 "ptr"
This mechanism tests if <ip>'s name is within a particular domain.
PTR = "ptr" [ ":" domain-spec ]
First the <ip>'s name is looked up using this procedure: perform a
PTR lookup against <ip>. 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 {
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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
name of <ip>, or if the <target-name> and the name of <ip> 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 represents a burden on the reverse DNS tree.
Therefore, it should be used only as a last resort, if domain policy
cannot be expressed using alternative mechanisms. If a domain
decides to use it, it should ensure it has the proper PTR records in
place for its hosts.
4.6 "ip4" and "ip6"
These mechanisms test if <ip> 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 = as per conventional dotted quad notation,
e.g. 192.0.2.0
ip6-network = as per [RFC 3513], section 2.2,
e.g. 12AB:0:0::CD30
The <ip> 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".
Note: These mechanisms do not require any additional DNS lookups, and
represent the least burden on the DNS. They are recommended.
4.7 "exists"
This mechanism is used to construct an arbitrary host name that is
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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.
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 enable queries that mimic the style of tests that
existing RBL lists use.
5. Modifier Definitions
Modifiers are not mechanisms: they do not return match or no-match.
Instead they provide additional information or alter check_host()
processing. Global modifiers affect the entire record, whereas
positional modifiers only affect the preceeding mechanism in the
record.
While unrecognized mechanisms cause an immediate "PermError" abort,
unrecognized modifiers MUST be simply ignored. Modifiers therefore
provide an 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.
This document reserves two modifiers for future definition:
"accredit" and "match_subdomains". Until these are defined,
implementations SHOULD ignore them. There is also one deprecated
modifier: "default". Implementations MUST ignore it.
5.1 redirect: Redirected Query
The "redirect" modifier is global and singular.
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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. 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 current evaluation.
Note that the newly queried domain may itself specify redirect
processing.
This facility is intended for use by organizations that wish to apply
the same record to multiple domains. For example:
la.example.com. TXT "v=spf1 redirect=_spf.example.com"
ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
_spf.example.com. TXT "v=spf1 mx:example.com -all"
In this example, mail from any of the three domains is described by
the same record. This can be an administrative advantage.
Note: In general, a domain A cannot reliably use a redirect to
another domain B not under the same administrative control. Since
the <sender> stays the same, there is no guarantee that the record at
domain B will correctly work for 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.
5.2 exp: Explanation
The "exp" modifier is global and singular.
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.
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The argument to the explanation modifier is a domain-spec. The DNS
TXT record for that domain is fetched either from a cache or via a
query to DNS. If there are any processing errors, temporary (such as
SERVFAIL) or permanent (such as NXDOMAIN), or if no records are
returned, or if more than one record is returned, then the
explanation string empty with no further computation.
The fetched TXT record's strings are concatenated with no spaces, and
then treated as a macro-string that 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. See [MARID Core].
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:
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 parameters 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.
6. Miscellaneous
6.1 Unrecognized Mechanisms and Modifiers
Future extensions to this standard may introduce new mechanisms and
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modifiers.
Unrecognized mechanisms cause processing to abort: If, during
evaluation of a record, check_host() encounters a mechanism which it
does not understand, then 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 future versions of this standard. 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.
6.2 Processing Limits
During processing, an 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 evaluation limit of an implementation is
reached, check_host() MUST abort processing and return the result
"PermError".
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".
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Domains publishing records SHOULD try 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 TXT "v=spf1 +mx:example.com -all"
b.example.com. IN TXT "v=spf1 +a:mx.example.com -all"
c.example.com. IN TXT "v=spf1 +ip4:192.0.2.1 -all"
Evaluating check_host() for the domain "a.example.com" requires the
MX records for "example.com", and then the A for records for the
listed hosts. Evaluating for "b.example.com" only requires the A
records. Evaluating for "c.example.com" requries none.
However, there may be administrative considerations: Using "a" over
"ip4" allows hosts to be renumbered easily. Using "mx" over "a"
allows the set of mail hosts to be changed easily.
7. Macros
7.1 Macro definitions
Certain terms perform macro interpolation on their arguments.
macro-string = *( macro-char / VCHAR )
macro-char = ( "%{" ALPHA transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
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>
v = the string "in-addr" for if <ip> is ipv4,
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or "ip6" if <ip> is ipv6
The following macro letters are only allowed in "exp" text:
c = SMTP client IP (in easily readable format)
r = domain name of host performing the check
t = current timestamp in UTC epoch seconds notation
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
Your spam volume has increased by 581%%
Legal optional transformers are:
*DIGIT ; zero or more digits
'r' ; reverse value, splitting on dots by default
If transformers or delimiters are provided, the macro strings are
split into parts. After performing any reversal operation or removal
of left-hand parts, the parts are rejoined using "." and not the
original splitting characters.
By default, strings are split on "." (dots). 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 email
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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 a
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
3.3).
For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format.
For IPv6 addresses, the "i" macro expands to 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 [RFC 3513] section
2.2. It is intended for humans to read.
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.
Any unrecognized macro letters are expanded as the string "unknown".
There are two deprecated macro letters: "p" and "h". These are
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 equiavalents,
and are then URL escaped. URL escaping is described in [RFC 2396].
Note: Domains should avoid, where possible, 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
<domain> and <ip> alone for as long as the shortest TTL of all the
DNS records involved.
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7.2 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
8. Security Considerations
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
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attacker could poison a target's DNS cache with spoofed DNS data, and
cause check_host() to return incorrect results, including "Pass" for
an <ip> value where the actual domain's record would evaluate to
"Fail".
The client IP address, <ip>, is assumed to be true and 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
includes and redirects and/or check for loops, malicious domains
could published 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.
9. IANA Considerations
The IANA will need to maintain one registry in support of this
specification. The registry consists of the modifiers as described
in section 3.5.3 and section 5.
[[Missing application review policy]]
9.1 Registration Template
To: ietf-types@iana.org
Subject: Registration of SPF Modifier XXX
Modifier name:
Type: (Global or Positional)
Appearance: (Single or Multiple)
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Security considerations:
Interoperability considerations:
Published specification:
Person & email address to contact for further information:
Author/Change controller:
(Any other information that the author deems interesting may be added
below this line.)
[[This template needs to be reviewed]]
10. Contributors and Acknowledgements
This design 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 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 and MARID/MXCOMP mailing lists.
The folks on the spf-discuss mailing list.
The folks on the mailing list that shall not be named.
The folks on #perl and #spf.
Normative References
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", RFC 1034, November 1987.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2234] Crocker, D.H. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC 2396] Berners-Lee, T., R.T. Fielding and L. Masinter,
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"Uniform Resource Identifiers (URI): Generic Syntax",
RFC 2396, August 1998.
[RFC 3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
Informative References
[RFC 2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC 3668] Bradner, S., "Intellectual Property Rights in IETF
Technology", RFC 3668, February 2004.
[MARID Core] Lyon, J. and M. Wong,
"MTA Authentication Records in DNS", Work In Progress,
to be published as draft-ietf-marid-core-02.txt,
expected July 2004.
[RMX] Danisch, Hadmut., "The RMX DNS RR Type for light weight
sender authentication", Work In Progress,
http://www.danisch.de/work/security/antispam.html,
October 2003.
[DMP] Fecyk, Gordon., "Designated Mailers Protocol",
http://www.pan-am.ca/dmp/, Work In Progress,
December 2003.
[Vixie] Vixie, Paul., "Repudiating Mail-From",
http://ops.ietf.org/lists/namedroppers/
namedroppers.2002/msg00658.html
Authors' Addresses
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
Comments on this draft are welcome. In the interests of openness,
rather than contacting the authors directly, please post to either:
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the ietf-mxcomp mailing list
http://www.imc.org/ietf-mxcomp/index.html
or
the spf-discuss mailing list.
http://spf.pobox.com/mailinglist.html
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 [RFC 2234] for ABNF notation.
record = version *( 1*SP ( directive / modifier ) ) *SP
version = "v=spf1"
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 ] *[ "/" *DIGIT ]
modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
domain-spec = domain-name / macro-string
domain-name = domain-part *( "." domain-part ) [ "." ]
domain-part = as defined in [RFC 1034]
ip4-network = as per conventional dotted quad notation,
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e.g. 192.0.2.0
ip6-network = as per [RFC 3513], section 2.2,
e.g. 12AB:0:0::CD30
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 transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
transformer = [ *DIGIT ] [ "r" ]
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
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.
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130 PTR mail-b.example.com.
140 PTR mail-c.example.org.
; A rogue reverse IP domain that claims to be
; something it's not
$ORIGIN 0.0.10.in-addr.arpa.
4 PTR bob.example.com.
B.1 Simple Examples
These examples show various possible published records for
example.com and which values if <ip> would cause check_host() to
return "Pass". Note that <domain> is "example.com".
v=spf1 +all
-- any <ip> passes
v=spf1 a -all
-- hosts 192.0.2.10 and 192.0.2.11 pass
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/8 mx:example.org/8 -all
-- any sending host in 192.0.2.128/24 or 192.168.2.136/8 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
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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
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}"
Intellectual Property Statement
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The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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