Internet Engineering Task Force (IETF) S. Kitterman
Request for Comments: 7208 Kitterman Technical Services
Obsoletes: 4408 April 2014
Category: Standards Track
ISSN: 2070-1721
Sender Policy Framework (SPF)
for Authorizing Use of Domains in Email, Version 1
Abstract
Email on the Internet can be forged in a number of ways. In
particular, existing protocols place no restriction on what a sending
host can use as the "MAIL FROM" of a message or the domain given on
the SMTP HELO/EHLO commands. This document describes version 1 of
the Sender Policy Framework (SPF) protocol, whereby ADministrative
Management Domains (ADMDs) can explicitly authorize the hosts that
are allowed to use its domain names, and a receiving host can check
such authorization.
This document obsoletes RFC 4408.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7208.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2. Imported Definitions . . . . . . . . . . . . . . . . 5
1.1.3. MAIL FROM Definition . . . . . . . . . . . . . . . . 5
1.1.4. HELO Definition . . . . . . . . . . . . . . . . . . . 6
1.2. check_host() . . . . . . . . . . . . . . . . . . . . . . 6
2. Operational Overview . . . . . . . . . . . . . . . . . . . . 6
2.1. Publishing Authorization . . . . . . . . . . . . . . . . 6
2.2. Checking Authorization . . . . . . . . . . . . . . . . . 7
2.3. The "HELO" Identity . . . . . . . . . . . . . . . . . . . 8
2.4. The "MAIL FROM" Identity . . . . . . . . . . . . . . . . 8
2.5. Location of Checks . . . . . . . . . . . . . . . . . . . 9
2.6. Results of Evaluation . . . . . . . . . . . . . . . . . . 9
2.6.1. None . . . . . . . . . . . . . . . . . . . . . . . . 9
2.6.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . 10
2.6.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6.5. Softfail . . . . . . . . . . . . . . . . . . . . . . 10
2.6.6. Temperror . . . . . . . . . . . . . . . . . . . . . . 10
2.6.7. Permerror . . . . . . . . . . . . . . . . . . . . . . 10
3. SPF Records . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. DNS Resource Records . . . . . . . . . . . . . . . . . . 11
3.2. Multiple DNS Records . . . . . . . . . . . . . . . . . . 12
3.3. Multiple Strings in a Single DNS Record . . . . . . . . . 12
3.4. Record Size . . . . . . . . . . . . . . . . . . . . . . . 12
3.5. Wildcard Records . . . . . . . . . . . . . . . . . . . . 13
4. The check_host() Function . . . . . . . . . . . . . . . . . . 13
4.1. Arguments . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Initial Processing . . . . . . . . . . . . . . . . . . . 14
4.4. Record Lookup . . . . . . . . . . . . . . . . . . . . . . 15
4.5. Selecting Records . . . . . . . . . . . . . . . . . . . . 15
4.6. Record Evaluation . . . . . . . . . . . . . . . . . . . . 15
4.6.1. Term Evaluation . . . . . . . . . . . . . . . . . . . 15
4.6.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . 16
4.6.3. Modifiers . . . . . . . . . . . . . . . . . . . . . . 17
4.6.4. DNS Lookup Limits . . . . . . . . . . . . . . . . . . 17
4.7. Default Result . . . . . . . . . . . . . . . . . . . . . 18
4.8. Domain Specification . . . . . . . . . . . . . . . . . . 18
5. Mechanism Definitions . . . . . . . . . . . . . . . . . . . . 19
5.1. "all" . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2. "include" . . . . . . . . . . . . . . . . . . . . . . . . 20
5.3. "a" . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.4. "mx" . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.5. "ptr" (do not use) . . . . . . . . . . . . . . . . . . . 23
5.6. "ip4" and "ip6" . . . . . . . . . . . . . . . . . . . . . 24
5.7. "exists" . . . . . . . . . . . . . . . . . . . . . . . . 25
6. Modifier Definitions . . . . . . . . . . . . . . . . . . . . 25
6.1. redirect: Redirected Query . . . . . . . . . . . . . . . 26
6.2. exp: Explanation . . . . . . . . . . . . . . . . . . . . 27
7. Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. Formal Specification . . . . . . . . . . . . . . . . . . 28
7.2. Macro Definitions . . . . . . . . . . . . . . . . . . . . 29
7.3. Macro Processing Details . . . . . . . . . . . . . . . . 30
7.4. Expansion Examples . . . . . . . . . . . . . . . . . . . 32
8. Result Handling . . . . . . . . . . . . . . . . . . . . . . . 33
8.1. None . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.2. Neutral . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.3. Pass . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.4. Fail . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.5. Softfail . . . . . . . . . . . . . . . . . . . . . . . . 35
8.6. Temperror . . . . . . . . . . . . . . . . . . . . . . . . 35
8.7. Permerror . . . . . . . . . . . . . . . . . . . . . . . . 36
9. Recording the Result . . . . . . . . . . . . . . . . . . . . 36
9.1. The Received-SPF Header Field . . . . . . . . . . . . . . 37
9.2. SPF Results in the Authentication-Results Header Field . 39
10. Effects on Infrastructure . . . . . . . . . . . . . . . . . . 39
10.1. Sending Domains . . . . . . . . . . . . . . . . . . . . 39
10.1.1. DNS Resource Considerations . . . . . . . . . . . . 40
10.1.2. Administrator's Considerations . . . . . . . . . . . 40
10.1.3. Bounces . . . . . . . . . . . . . . . . . . . . . . 41
10.2. Receivers . . . . . . . . . . . . . . . . . . . . . . . 41
10.3. Mediators . . . . . . . . . . . . . . . . . . . . . . . 42
11. Security Considerations . . . . . . . . . . . . . . . . . . . 42
11.1. Processing Limits . . . . . . . . . . . . . . . . . . . 42
11.2. SPF-Authorized Email May Contain Other False Identities 44
11.3. Spoofed DNS and IP Data . . . . . . . . . . . . . . . . 44
11.4. Cross-User Forgery . . . . . . . . . . . . . . . . . . . 44
11.5. Untrusted Information Sources . . . . . . . . . . . . . 45
11.5.1. Recorded Results . . . . . . . . . . . . . . . . . . 45
11.5.2. External Explanations . . . . . . . . . . . . . . . 45
11.5.3. Macro Expansion . . . . . . . . . . . . . . . . . . 45
11.6. Privacy Exposure . . . . . . . . . . . . . . . . . . . . 46
11.7. Delivering Mail Producing a "Fail" Result . . . . . . . 46
12. Collected ABNF . . . . . . . . . . . . . . . . . . . . . . . 46
13. Contributors and Acknowledgements . . . . . . . . . . . . . . 48
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
14.1. The SPF DNS Record Type . . . . . . . . . . . . . . . . 49
14.2. The Received-SPF Mail Header Field . . . . . . . . . . . 49
14.3. SPF Modifier Registry . . . . . . . . . . . . . . . . . 50
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
15.1. Normative References . . . . . . . . . . . . . . . . . . 50
15.2. Informative References . . . . . . . . . . . . . . . . . 51
Appendix A. Extended Examples . . . . . . . . . . . . . . . . . 54
A.1. Simple Examples . . . . . . . . . . . . . . . . . . . . . 54
A.2. Multiple Domain Example . . . . . . . . . . . . . . . . . 56
A.3. DNS Blacklist (DNSBL) Style Example . . . . . . . . . . . 56
A.4. Multiple Requirements Example . . . . . . . . . . . . . . 57
Appendix B. Changes in Implementation Requirements from RFC 4408 57
Appendix C. Further Testing Advice . . . . . . . . . . . . . . . 58
Appendix D. SPF/Mediator Interactions . . . . . . . . . . . . . 58
D.1. Originating ADMDs . . . . . . . . . . . . . . . . . . . . 58
D.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 59
D.3. Receiving ADMDs . . . . . . . . . . . . . . . . . . . . . 60
Appendix E. Mail Services . . . . . . . . . . . . . . . . . . . 60
Appendix F. MTA Relays . . . . . . . . . . . . . . . . . . . . . 61
Appendix G. Local Policy Considerations . . . . . . . . . . . . 61
G.1. Policy for SPF Pass . . . . . . . . . . . . . . . . . . . 61
G.2. Policy for SPF Fail . . . . . . . . . . . . . . . . . . . 62
G.3. Policy for SPF Permerror . . . . . . . . . . . . . . . . 62
G.4. Policy for SPF Temperror . . . . . . . . . . . . . . . . 63
1. Introduction
The current email infrastructure has the property that any host
injecting mail into the system can use any DNS domain name it wants
in each of the various identifiers specified by [RFC5321] and
[RFC5322]. Although this feature is desirable in some circumstances,
it is a major obstacle to reducing Unsolicited Bulk Email (UBE, aka
spam). Furthermore, ADMDs (as described in [RFC5598]) are
understandably concerned about the ease with which other entities can
make use of their domain names, often with malicious intent.
This document defines a protocol by which ADMDs can authorize hosts
to use their domain names in the "MAIL FROM" or "HELO" identities.
Compliant ADMDs publish Sender Policy Framework (SPF) records in the
DNS specifying which hosts are permitted to use their names, and
compliant mail receivers use the published SPF records to test the
authorization of sending Mail Transfer Agents (MTAs) using a given
"HELO" or "MAIL FROM" identity during a mail transaction.
An additional benefit to mail receivers is that after the use of an
identity is verified, local policy decisions about the mail can be
made based on the sender's domain, rather than the host's IP address.
This is advantageous because reputation of domain names is likely to
be more accurate than reputation of host IP addresses since domains
are likely to be more stable over a longer period. Furthermore, if a
claimed identity fails verification, local policy can take stronger
action against such email, such as rejecting it.
1.1. Terminology
1.1.1. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.1.2. Imported Definitions
ABNF (Augmented Backus-Naur Form) ABNF is defined in [RFC5234], as
are the tokens "ALPHA", "DIGIT", and "SP" (space).
The tokens "Local-part", "Domain", and "Mailbox" are defined in
[RFC5321].
"dot-atom", "quoted-string", "comment", "CFWS" (comment folded white
space), "FWS" (folded white space), and "CRLF" (carriage-return/
line-feed) are defined in [RFC5322].
1.1.3. MAIL FROM Definition
This document is concerned with the identity of the sender of a mail
message, as referred to in [RFC5321]:
The transaction starts with a MAIL command that gives the sender
identification.
Since there are many other names for this identity, it is important
to choose a name that is:
1. commonly used
2. well defined
As such, throughout this document the term "MAIL FROM" will be used,
which is defined as the RFC5321.MailFrom (reverse-path) identity
described in [RFC5598].
1.1.4. HELO Definition
This document also makes use of the HELO/EHLO identity. The "HELO"
identity derives from either the SMTP HELO or EHLO command (see
[RFC5321]). Since HELO and EHLO can, in many cases, be used
interchangeably, they are identified commonly as "HELO" in this
document. This means RFC5321.HELO/.EHLO as defined in [RFC5598].
These commands supply the identity of the SMTP client (sending host)
for the SMTP session.
1.2. check_host()
Section 4 introduces an algorithm to evaluate an SPF policy against
an arriving email transaction. In an early implementation, this
algorithm was encoded in a function called check_host(). That name
is used in this document as symbolic of the SPF evaluation algorithm,
but of course implementers are not required to use this name.
2. Operational Overview
2.1. Publishing Authorization
An SPF-compliant domain publishes valid SPF records as described in
Section 3. These records authorize the use of the relevant domain
names in the "HELO" and "MAIL FROM" identities by the MTAs specified
therein.
SPF results can be used to make both positive (source is authorized)
and negative (source is not authorized) determinations. If ADMDs
choose to publish SPF records and want to support receivers making
negative authorization determinations, it is necessary for them to
publish records that end in "-all", or redirect to other records that
do; otherwise, no definitive determination of authorization can be
made. Potential issues and mitigations associated with negative
determinations are discussed in Section 10.
ADMDs that wish to declare that no hosts are authorized to use their
DNS domain names in the HELO or MAIL FROM commands during SMTP
sessions can publish SPF records that say so for domain names that
are neither used in the domain part of email addresses nor expected
to originate mail.
When changing SPF records, care has to be taken to ensure that there
is a transition period so that the old policy remains valid until all
legitimate email can reasonably expect to have been checked.
[RFC5321], Section 4.5.4.1 discusses how long a message might be in
transit. While offline checks are possible, the closer to the
original transmission time checks are performed, the more likely they
are to get an SPF result that matches the sending ADMD intent at the
time the message was sent.
2.2. Checking Authorization
A mail receiver can perform a set of SPF checks for each mail message
it receives. An SPF check tests the authorization of a client host
to emit mail with a given identity. Typically, such checks are done
by a receiving MTA, but can be performed elsewhere in the mail
processing chain so long as the required information is available and
reliable. The "MAIL FROM" and "HELO" identities are checked as
described in Sections 2.4 and 2.3, respectively.
Without explicit approval of the publishing ADMD, checking other
identities against SPF version 1 records is NOT RECOMMENDED because
there are cases that are known to give incorrect results. For
example, almost all mailing lists rewrite the "MAIL FROM" identity
(see Section 10.3), but some do not change any other identities in
the message. Documents that define other identities will have to
define the method for explicit approval.
It is possible that mail receivers will use the SPF check as part of
a larger set of tests on incoming mail. The results of other tests
might influence whether or not a particular SPF check is performed.
For example, finding the sending host's IP address on a local
whitelist might cause all other tests to be skipped and all mail from
that host to be accepted.
When a mail receiver decides to perform an SPF check, it has to use a
correctly implemented check_host() function (Section 4) evaluated
with the correct parameters. Although the test as a whole is
optional, once it has been decided to perform a test it has to be
performed as specified so that the correct semantics are preserved
between publisher and receiver.
To make the test, the mail receiver MUST evaluate the check_host()
function with the arguments described in Section 4.1.
Although invalid, malformed, or non-existent domains cause SPF checks
to return "none" because no SPF record can be found, it has long been
the policy of many MTAs to reject email from such domains, especially
in the case of invalid "MAIL FROM". Rejecting email will prevent one
method of circumventing of SPF records.
Implementations have to take care to correctly extract the <domain>
from the data given with the SMTP MAIL FROM command as many MTAs will
still accept such things as source routes (see Appendix C of
[RFC5321]), the %-hack (see [RFC1123]), and bang paths (see
[RFC1983]). These archaic features have been maliciously used to
bypass security systems.
2.3. The "HELO" Identity
It is RECOMMENDED that SPF verifiers not only check the "MAIL FROM"
identity but also separately check the "HELO" identity by applying
the check_host() function (Section 4) to the "HELO" identity as the
<sender>. Checking "HELO" promotes consistency of results and can
reduce DNS resource usage. If a conclusive determination about the
message can be made based on a check of "HELO", then the use of DNS
resources to process the typically more complex "MAIL FROM" can be
avoided. Additionally, since SPF records published for "HELO"
identities refer to a single host, when available, they are a very
reliable source of host authorization status. Checking "HELO" before
"MAIL FROM" is the RECOMMENDED sequence if both are checked.
Note that requirements for the domain presented in the EHLO or HELO
command are not always clear to the sending party, and SPF verifiers
have to be prepared for the identity to be an IP address literal (see
[RFC5321], Section 4.1.3) or simply be malformed. This SPF check can
only be performed when the "HELO" string is a valid, multi-label
domain name.
2.4. The "MAIL FROM" Identity
SPF verifiers MUST check the "MAIL FROM" identity if a "HELO" check
either has not been performed or has not reached a definitive policy
result by applying the check_host() function to the "MAIL FROM"
identity as the <sender>.
[RFC5321] allows the reverse-path to be null (see Section 4.5.5 in
[RFC5321]). In this case, there is no explicit sender mailbox, and
such a message can be assumed to be a notification message from the
mail system itself. When the reverse-path is null, this document
defines the "MAIL FROM" identity to be the mailbox composed of the
local-part "postmaster" and the "HELO" identity (which might or might
not have been checked separately before).
2.5. Location of Checks
The authorization check SHOULD be performed during the processing of
the SMTP transaction that receives the mail. This reduces the
complexity of determining the correct IP address to use as an input
to check_host() and allows errors to be returned directly to the
sending MTA by way of SMTP replies. Appendix D of [RFC7001] provides
a more thorough discussion of this topic.
The authorization check is performed during the SMTP transaction at
the time of the MAIL command, and uses the MAIL FROM value and the
client IP address. Performing the check at later times or with other
input can cause problems such as the following:
o It might be difficult to accurately extract the required
information from potentially deceptive headers.
o Legitimate email might fail the authorization check because the
sender's policy has since changed.
Generating non-delivery notifications to forged identities that have
failed the authorization check often constitutes backscatter, i.e.,
nuisance rejection notices that are not actionable. Operators are
strongly advised to avoid such practices. Section 2 of [RFC3834]
describes backscatter and the problems it causes.
2.6. Results of Evaluation
Section 4 defines check_host(), a model function definition that uses
the inputs defined above and the sender's policy published in the DNS
to reach a conclusion about client authorization. An SPF verifier
implements something semantically equivalent to the function defined
there.
This section enumerates and briefly defines the possible outputs of
that function. Note, however, that the protocol establishes no
normative requirements for handling any particular result.
Discussion of handling options for each result can be found in
Section 8.
2.6.1. None
A result of "none" means either (a) no syntactically valid DNS domain
name was extracted from the SMTP session that could be used as the
one to be authorized, or (b) no SPF records were retrieved from the
DNS.
2.6.2. Neutral
A "neutral" result means the ADMD has explicitly stated that it is
not asserting whether the IP address is authorized.
2.6.3. Pass
A "pass" result is an explicit statement that the client is
authorized to inject mail with the given identity.
2.6.4. Fail
A "fail" result is an explicit statement that the client is not
authorized to use the domain in the given identity.
2.6.5. Softfail
A "softfail" result is a weak statement by the publishing ADMD that
the host is probably not authorized. It has not published a
stronger, more definitive policy that results in a "fail".
2.6.6. Temperror
A "temperror" result means the SPF verifier encountered a transient
(generally DNS) error while performing the check. A later retry may
succeed without further DNS operator action.
2.6.7. Permerror
A "permerror" result means the domain's published records could not
be correctly interpreted. This signals an error condition that
definitely requires DNS operator intervention to be resolved.
3. SPF Records
An SPF record is a DNS record that declares which hosts are, and are
not, authorized to use a domain name for the "HELO" and "MAIL FROM"
identities. Loosely, the record partitions hosts into permitted and
not-permitted sets (though some hosts might fall into neither
category).
The SPF record is expressed as a single string of text found in the
RDATA of a single DNS TXT resource record; multiple SPF records are
not permitted for the same owner name. The record format and the
process for selecting records are described below in Section 4. An
example record is the following:
v=spf1 +mx a:colo.example.com/28 -all
This record has a version of "spf1" and three directives: "+mx",
"a:colo.example.com/28" (the "+" is implied), and "-all".
Each SPF record is placed in the DNS tree at the owner name it
pertains to, not in a subdomain under the owner name. This is
similar to how SRV records [RFC2782] are done.
The example in this section might be published via these lines in a
domain zone file:
example.com. TXT "v=spf1 +mx a:colo.example.com/28 -all"
Since TXT records have multiple uses, beware of other TXT records
published there for other purposes. They might cause problems with
size limits (see Section 3.4), and care has to be taken to ensure
that only SPF records are used for SPF processing.
ADMDs publishing SPF records ought to keep the amount of DNS
information needed to evaluate a record to a minimum. Sections 4.6.4
and 10.1.1 provide some suggestions about "include" mechanisms and
chained "redirect" modifiers.
3.1. DNS Resource Records
SPF records MUST be published as a DNS TXT (type 16) Resource Record
(RR) [RFC1035] only. The character content of the record is encoded
as [US-ASCII]. Use of alternative DNS RR types was supported in
SPF's experimental phase but has been discontinued.
In 2003, when SPF was first being developed, the requirements for
assignment of a new DNS RR type were considerably more stringent than
they are now. Additionally, support for easy deployment of new DNS
RR types was not widely deployed in DNS servers and provisioning
systems. As a result, developers of SPF found it easier and more
practical to use the TXT RR type for SPF records.
In its review of [RFC4408], the SPFbis working group concluded that
its dual RR type transition model was fundamentally flawed since it
contained no common RR type that implementers were required to serve
and required to check. Many alternatives were considered to resolve
this issue, but ultimately the working group concluded that
significant migration to the SPF RR type in the foreseeable future
was very unlikely and that the best solution for resolving this
interoperability issue was to drop support for the SPF RR type from
SPF version 1. See Appendix A of [RFC6686] for further information.
The circumstances surrounding SPF's initial deployment a decade ago
are unique. If a future update to SPF were developed that did not
reuse existing SPF records, it could use the SPF RR type. SPF's use
of the TXT RR type for structured data should in no way be taken as
precedent for future protocol designers. Further discussion of
design considerations when using new DNS RR types can be found in
[RFC5507].
3.2. Multiple DNS Records
A domain name MUST NOT have multiple records that would cause an
authorization check to select more than one record. See Section 4.5
for the selection rules.
3.3. Multiple Strings in a Single DNS Record
As defined in [RFC1035], Sections 3.3 and 3.3.14, a single text DNS
record can be composed of more than one string. If a published
record contains multiple character-strings, then the record MUST be
treated as if those strings are concatenated together without adding
spaces. For example:
IN TXT "v=spf1 .... first" "second string..."
is equivalent to:
IN TXT "v=spf1 .... firstsecond string..."
TXT records containing multiple strings are useful in constructing
records that would exceed the 255-octet maximum length of a
character-string within a single TXT record.
3.4. Record Size
The published SPF record for a given domain name SHOULD remain small
enough that the results of a query for it will fit within 512 octets.
Otherwise, there is a possibility of exceeding a DNS protocol limit.
This UDP limit is defined in [RFC1035], Section 2.3.4, although it
was raised by [RFC2671]. Staying below 512 octets ought to prevent
older DNS implementations from failing over to TCP and will work with
UDP in the absence of EDNS0 [RFC6891] support. Since the answer size
is dependent on many things outside the scope of this document, it is
only possible to give this guideline: If the size of the DNS message,
the combined length of the DNS name and the text of all the records
of a given type is under 450 octets, then DNS answers ought to fit in
UDP packets. Records that are too long to fit in a single UDP packet
could be silently ignored by SPF verifiers due to firewall and other
issues that interfere with the operation of DNS over TCP or using
ENDS0.
Note that when computing the sizes for replies to queries of the TXT
format, one has to take into account any other TXT records published
at the domain name. Similarly, the sizes for replies to all queries
related to SPF have to be evaluated to fit in a single 512-octet UDP
packet (i.e., DNS message size limited to 450 octets).
3.5. Wildcard Records
Use of wildcard records for publishing is discouraged, and care has
to be taken if they are used. If a zone includes wildcard MX
records, it might want to publish wildcard declarations, subject to
the same requirements and problems. In particular, the declaration
MUST be repeated for any host that has any RR records at all, and for
subdomains thereof. Consider the example in [RFC1034],
Section 4.3.3. Based on that, we can do the following:
EXAMPLE.COM. MX 10 A.EXAMPLE.COM
EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all"
*.EXAMPLE.COM. MX 10 A.EXAMPLE.COM
*.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all"
A.EXAMPLE.COM. A 203.0.113.1
A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM
A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all"
*.A.EXAMPLE.COM. MX 10 A.EXAMPLE.COM
*.A.EXAMPLE.COM. TXT "v=spf1 a:A.EXAMPLE.COM -all"
SPF records have to be listed twice for every name within the zone:
once for the name, and once with a wildcard to cover the tree under
the name, in order to cover all domains in use in outgoing mail.
4. The check_host() Function
This description is not an application programming interface
definition, but rather a function description used to illustrate the
algorithm. A compliant SPF implementation MUST produce results
semantically equivalent to this description.
The check_host() function fetches SPF records, parses them, and
evaluates them to determine whether a particular host is or is not
permitted to send mail with a given identity. Receiving ADMDs that
perform this check MUST correctly evaluate the check_host() function
as described here.
Implementations MAY use a different algorithm than the canonical
algorithm defined here, so long as the results are the same in all
cases.
4.1. Arguments
The check_host() function takes these arguments:
<ip> - the IP address of the SMTP client that is emitting
the mail, either IPv4 or IPv6.
<domain> - the domain that provides the sought-after authorization
information; initially, the domain portion of the
"MAIL FROM" or "HELO" identity.
<sender> - the "MAIL FROM" or "HELO" identity.
For recursive evaluations, the domain portion of <sender> might not
be the same as the <domain> argument when check_host() is initially
evaluated. In most other cases it will be the same (see Section 5.2
below). The overall DNS lookup limit for SPF terms described below
in Section 4.6.4 must be tracked as a single global limit for all
evaluations, not just for a single instance of a recursive
evaluation.
Note that the <domain> argument might not be a well-formed domain
name. For example, if the reverse-path was null, then the EHLO/HELO
domain is used, with its associated problems (see Section 2.3). In
these cases, check_host() is defined in Section 4.3 to return a
"none" result.
4.2. Results
The check_host() function can return one of several results described
in Section 2.6. Based on the result, the action to be taken is
determined by the local policies of the receiver. This is discussed
in Section 8.
4.3. Initial Processing
If the <domain> is malformed (e.g., label longer than 63 characters,
zero-length label not at the end, etc.) or is not a multi-label
domain name, or if the DNS lookup returns "Name Error" (RCODE 3, also
known as "NXDOMAIN" [RFC2308]), check_host() immediately returns the
result "none". DNS RCODEs are defined in [RFC1035]. Properly formed
domains are fully qualified domains as defined in [RFC1983]. That
is, in the DNS they are implicitly qualified relative to the root
(see Section 3.1 of [RFC1034]). Internationalized domain names MUST
be encoded as A-labels, as described in Section 2.3 of [RFC5890].
If the <sender> has no local-part, substitute the string "postmaster"
for the local-part.
4.4. Record Lookup
In accordance with how the records are published (see Section 3
above), a DNS query needs to be made for the <domain> name, querying
for type TXT only.
If the DNS lookup returns a server failure (RCODE 2) or some other
error (RCODE other than 0 or 3), or if the lookup times out, then
check_host() terminates immediately with the result "temperror".
4.5. Selecting Records
Records begin with a version section:
record = version terms *SP
version = "v=spf1"
Starting with the set of records that were returned by the lookup,
discard records that do not begin with a version section of exactly
"v=spf1". Note that the version section is terminated by either an
SP character or the end of the record. As an example, a record with
a version section of "v=spf10" does not match and is discarded.
If the resultant record set includes no records, check_host()
produces the "none" result. If the resultant record set includes
more than one record, check_host() produces the "permerror" result.
4.6. Record Evaluation
The check_host() function parses and interprets the SPF record to
find a result for the current test. The syntax of the record is
validated first, and if there are any syntax errors anywhere in the
record, check_host() returns immediately with the result "permerror",
without further interpretation or evaluation.
4.6.1. Term Evaluation
There are two types of terms: mechanisms (defined in Section 5) and
modifiers (defined in Section 6). A record contains an ordered list
of these as specified in the following Augmented Backus-Naur Form
(ABNF).
terms = *( 1*SP ( directive / modifier ) )
directive = [ qualifier ] mechanism
qualifier = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ a / mx / ptr / ip4 / ip6 / exists )
modifier = redirect / explanation / unknown-modifier
unknown-modifier = name "=" macro-string
; where name is not any known modifier
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
Most mechanisms allow a ":" or "/" character after the name.
Modifiers always contain an equals ('=') character immediately after
the name, and before any ":" or "/" characters that might be part of
the macro-string.
Terms that do not contain any of "=", ":", or "/" are mechanisms, as
defined in Section 5.
As per the definition of the ABNF notation in [RFC5234], mechanism
and modifier names are case-insensitive.
4.6.2. Mechanisms
Each mechanism is considered in turn from left to right. If there
are no more mechanisms, the result is the default result as described
in Section 4.7.
When a mechanism is evaluated, one of three things can happen: it can
match, not match, or return an exception.
If it matches, processing ends and the qualifier value is returned as
the result of that record. If it does not match, processing
continues with the next mechanism. If it returns an exception,
mechanism processing ends and the exception value is returned.
The possible qualifiers, and the results they cause check_host() to
return, are as follows:
"+" pass
"-" fail
"~" softfail
"?" neutral
The qualifier is optional and defaults to "+".
When a mechanism matches and the qualifier is "-", then a "fail"
result is returned and the explanation string is computed as
described in Section 6.2.
The specific mechanisms are described in Section 5.
4.6.3. Modifiers
Modifiers are not mechanisms. They do not return match or not-match.
Instead, they provide additional information. Although modifiers do
not directly affect the evaluation of the record, the "redirect"
modifier has an effect after all the mechanisms have been evaluated.
4.6.4. DNS Lookup Limits
Some mechanisms and modifiers (collectively, "terms") cause DNS
queries at the time of evaluation, and some do not. The following
terms cause DNS queries: the "include", "a", "mx", "ptr", and
"exists" mechanisms, and the "redirect" modifier. SPF
implementations MUST limit the total number of those terms to 10
during SPF evaluation, to avoid unreasonable load on the DNS. If
this limit is exceeded, the implementation MUST return "permerror".
The other terms -- the "all", "ip4", and "ip6" mechanisms, and the
"exp" modifier -- do not cause DNS queries at the time of SPF
evaluation (the "exp" modifier only causes a lookup at a later time),
and their use is not subject to this limit.
When evaluating the "mx" mechanism, the number of "MX" resource
records queried is included in the overall limit of 10 mechanisms/
modifiers that cause DNS lookups as described above. In addition to
that limit, the evaluation of each "MX" record MUST NOT result in
querying more than 10 address records -- either "A" or "AAAA"
resource records. If this limit is exceeded, the "mx" mechanism MUST
produce a "permerror" result.
When evaluating the "ptr" mechanism or the %{p} macro, the number of
"PTR" resource records queried is included in the overall limit of 10
mechanisms/modifiers that cause DNS lookups as described above. In
addition to that limit, the evaluation of each "PTR" record MUST NOT
result in querying more than 10 address records -- either "A" or
"AAAA" resource records. If this limit is exceeded, all records
other than the first 10 MUST be ignored.
The reason for the disparity is that the set of and contents of the
MX record are under control of the publishing ADMD, while the set of
and contents of PTR records are under control of the owner of the IP
address actually making the connection.
These limits are per mechanism or macro in the record, and are in
addition to the lookup limits specified above.
MTAs or other processors SHOULD impose a limit on the maximum amount
of elapsed time to evaluate check_host(). Such a limit SHOULD allow
at least 20 seconds. If such a limit is exceeded, the result of
authorization SHOULD be "temperror".
As described at the end of Section 11.1, there may be cases where it
is useful to limit the number of "terms" for which DNS queries return
either a positive answer (RCODE 0) with an answer count of 0, or a
"Name Error" (RCODE 3) answer. These are sometimes collectively
referred to as "void lookups". SPF implementations SHOULD limit
"void lookups" to two. An implementation MAY choose to make such a
limit configurable. In this case, a default of two is RECOMMENDED.
Exceeding the limit produces a "permerror" result.
4.7. Default Result
If none of the mechanisms match and there is no "redirect" modifier,
then the check_host() returns a result of "neutral", just as if
"?all" were specified as the last directive. If there is a
"redirect" modifier, check_host() proceeds as defined in Section 6.1.
It is better to use either a "redirect" modifier or an "all"
mechanism to explicitly terminate processing. Although there is an
implicit "?all" at the end of every record that is not explicitly
terminated, it aids debugging efforts when it is explicitly provided.
For example:
v=spf1 +mx -all
or
v=spf1 +mx redirect=_spf.example.com
4.8. Domain Specification
Several of these mechanisms and modifiers have a <domain-spec>
section. The <domain-spec> string is subject to macro expansion (see
Section 7). The resulting string is the common presentation form of
a fully qualified DNS name: a series of labels separated by periods.
This domain is called the <target-name> in the rest of this document.
Note: The result of the macro expansion is not subject to any further
escaping. Hence, this facility cannot produce all characters that
are legal in a DNS label (e.g., the control characters). However,
this facility is powerful enough to express legal host names and
common utility labels (such as "_spf") that are used in DNS.
For several mechanisms, the <domain-spec> is optional. If it is not
provided, the <domain> from the check_host() arguments (see
Section 4.1) is used as the <target-name>. "domain" and <domain-
spec> are syntactically identical after macro expansion. "domain" is
an input value for check_host(), while <domain-spec> is computed by
check_host().
The result of evaluating check_host() with a syntactically invalid
domain is undefined.
Note: This document and its predecessors make no provisions for
defining correct handling of a syntactically invalid <domain-spec>
(which might be the result of macro expansion), per [RFC1035].
Examples include names with empty labels, such as "foo..example.com",
and labels that are longer than 63 characters. Some implementations
choose to treat such errors as no-match not-match and therefore ignore such
names, while others return a "permerror" exception.
5. Mechanism Definitions
This section defines two types of mechanisms: basic language
framework mechanisms and designated sender mechanisms.
Basic mechanisms contribute to the language framework. They do not
specify a particular type of authorization scheme. The basic
mechanisms are as follows:
all
include
Designated sender mechanisms are used to identify a set of <ip>
addresses as being permitted or not permitted to use the <domain> for
sending mail. The designated sender mechanisms are as follows:
a
mx
ptr (do not use)
ip4
ip6
exists
The following conventions apply to all mechanisms that perform a
comparison between <ip> and an IP address at any point:
If no CIDR prefix length is given in the directive, then <ip> and the
IP address are compared for equality. (Here, CIDR is Classless
Inter-Domain Routing, described in [RFC4632].)
If a CIDR prefix length is specified, then only the specified number
of high-order bits of <ip> and the IP address are compared for
equality.
When any mechanism fetches host addresses to compare with <ip>, when
<ip> is an IPv4, "A" records are fetched; when <ip> is an IPv6
address, "AAAA" records are fetched. SPF implementations on IPv6
servers need to handle both "AAAA" and "A" records, for clients on
IPv4-mapped IPv6 addresses [RFC4291]. IPv4 <ip> addresses are only
listed in an SPF record using the "ip4" mechanism.
Several mechanisms rely on information fetched from the DNS. For
these DNS queries, except where noted, if the DNS server returns an
error (RCODE other than 0 or 3) or the query times out, the mechanism
stops and the topmost check_host() returns "temperror". If the
server returns "Name Error" (RCODE 3), then evaluation of the
mechanism continues as if the server returned no error (RCODE 0) and
zero answer records.
5.1. "all"
all = "all"
The "all" mechanism is a test that always matches. It is used as the
rightmost mechanism in a record to provide an explicit default.
For example:
v=spf1 a mx -all
Mechanisms after "all" will never be tested. Mechanisms listed after
"all" MUST be ignored. Any "redirect" modifier (Section 6.1) MUST be
ignored when there is an "all" mechanism in the record, regardless of
the relative ordering of the terms.
5.2. "include"
include = "include" ":" domain-spec
The "include" mechanism triggers a recursive evaluation of
check_host().
1. The <domain-spec> is expanded as per Section 7.
2. check_host() is evaluated with the resulting string as the
<domain>. The <ip> and <sender> arguments remain the same as in
the current evaluation of check_host().
3. The recursive evaluation returns match, not-match, or an error.
4. If it returns match, then the appropriate result for the
"include" mechanism is used (e.g., include or +include produces a
"pass" result and -include produces "fail").
5. If it returns no match not-match or an error, the parent check_host()
resumes processing as per the table below, with the previous
value of <domain> restored.
In hindsight, the name "include" was poorly chosen. Only the
evaluated result of the referenced SPF record is used, rather than
literally including the mechanisms of the referenced record in the
first. For example, evaluating a "-all" directive in the referenced
record does not terminate the overall processing and does not
necessarily result in an overall "fail". (Better names for this
mechanism would have been "if-match", "on-match", etc.)
The "include" mechanism makes it possible for one domain to designate
multiple administratively independent domains. For example, a vanity
domain "example.net" might send mail using the servers of
administratively independent domains example.com and example.org.
Example.net could say
IN TXT "v=spf1 include:example.com include:example.org -all"
This would direct check_host() to, in effect, check the records of
example.com and example.org for a "pass" result. Only if the host
were not permitted for either of those domains would the result be
"fail".
Whether this mechanism matches, does not match, or returns an
exception depends on the result of the recursive evaluation of
check_host():
+---------------------------------+---------------------------------+
| A recursive check_host() result | Causes the "include" mechanism |
| of: | to: |
+---------------------------------+---------------------------------+
| pass | match |
| | |
| fail | not match |
| | |
| softfail | not match |
| | |
| neutral | not match |
| | |
| temperror | return temperror |
| | |
| permerror | return permerror |
| | |
| none | return permerror |
+---------------------------------+---------------------------------+
The "include" mechanism is intended for crossing administrative
boundaries. When remaining within one administrative authority,
"include" is usually not the best choice. For example, if
example.com and example.org were managed by the same entity, and if
the permitted set of hosts for both domains was "mx:example.com", it
would be possible for example.org to specify "include:example.com",
but it would be preferable to specify "redirect=example.com" or even
"mx:example.com".
With the "include" mechanism, an administratively external set of
hosts can be authorized, but determination of sender policy is still
a function of the original domain's SPF record (as determined by the
"all" mechanism in that record). The "redirect" modifier is more
suitable for consolidating both authorizations and policy into a
common set to be shared within an ADMD. Redirect is much more like a
common code element to be shared among records in a single ADMD. It
is possible to control both authorized hosts and policy for an
arbitrary number of domains from a single record.
5.3. "a"
This mechanism matches if <ip> is one of the <target-name>'s IP
addresses. For clarity, this means the "a" mechanism also matches
AAAA records.
a = "a" [ ":" domain-spec ] [ dual-cidr-length ]
An address lookup is done on the <target-name> using the type of
lookup (A or AAAA) appropriate for the connection type (IPv4 or
IPv6). The <ip> is compared to the returned address(es). If any
address matches, the mechanism matches.
5.4. "mx"
This mechanism matches if <ip> is one of the MX hosts for a domain
name.
mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
check_host() first performs an MX lookup on the <target-name>. Then
it performs an address lookup on each MX name returned. The <ip> is
compared to each returned IP address. To prevent denial-of-service
(DoS) attacks, the processing limits defined in Section 4.6.4 MUST be
followed. If the MX lookup limit is exceeded, then "permerror" is
returned and the evaluation is terminated. If any address matches,
the mechanism matches.
Note regarding implicit MXes: If the <target-name> has no MX record,
check_host() MUST NOT apply the implicit MX rules of [RFC5321] by
querying for an A or AAAA record for the same name.
5.5. "ptr" (do not use)
This mechanism tests whether the DNS reverse-mapping for <ip> exists
and correctly points to a domain name within a particular domain.
This mechanism SHOULD NOT be published. See the note at the end of
this section for more information.
ptr = "ptr" [ ":" domain-spec ]
The <ip>'s name is looked up using this procedure:
o Perform a DNS reverse-mapping for <ip>: Look up the corresponding
PTR record in "in-addr.arpa." if the address is an IPv4 address
and in "ip6.arpa." if it is an IPv6 address.
o For each record returned, validate the domain name by looking up
its IP addresses. To prevent DoS attacks, the PTR processing
limits defined in Section 4.6.4 MUST be applied. If they are
exceeded, processing is terminated and the mechanism does not
match.
o If <ip> is among the returned IP addresses, then that domain name
is validated.
Check all validated domain names to see if they either match the
<target-name> domain or are a subdomain of the <target-name> domain.
If any do, this mechanism matches. If no validated domain name can
be found, or if none of the validated domain names match or are a
subdomain of the <target-name>, this mechanism fails to match. If a
DNS error occurs while doing the PTR RR lookup, then this mechanism
fails to match. If a DNS error occurs while doing an A RR lookup,
then that domain name is skipped and the search continues.
This mechanism matches if
o the <target-name> is a subdomain of a validated domain name, or
o the <target-name> and a validated domain name are the same.
For example, "mail.example.com" is within the domain "example.com",
but "mail.bad-example.com" is not.
Note: This mechanism is slow, it is not as reliable as other
mechanisms in cases of DNS errors, and it places a large burden on
the .arpa name servers. If used, proper PTR records have to be in
place for the domain's hosts and the "ptr" mechanism SHOULD be one of
the last mechanisms checked. After many years of SPF deployment
experience, it has been concluded that it is unnecessary and more
reliable alternatives should be used instead. It is, however, still
in use as part of the SPF protocol, so compliant check_host()
implementations MUST support it.
5.6. "ip4" and "ip6"
These mechanisms test whether <ip> is contained within a given IP
network.
ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32
ip6-cidr-length = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = qnum "." qnum "." qnum "." qnum
qnum = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
; as per conventional dotted-quad notation, e.g., 192.0.2.0
ip6-network = <as per Section 2.2 of [RFC4291]>
; e.g., 2001:db8::cd30
The <ip> is compared to the given network. If CIDR prefix length
high-order bits match, the mechanism matches.
If ip4-cidr-length is omitted, it is taken to be "/32". If
ip6-cidr-length is omitted, it is taken to be "/128". It is not
permitted to omit parts of the IP address instead of using CIDR
notations. That is, use 192.0.2.0/24 instead of 192.0.2.
5.7. "exists"
This mechanism is used to construct an arbitrary domain name that is
used for a DNS A record query. It allows for complicated schemes
involving arbitrary parts of the mail envelope to determine what is
permitted.
exists = "exists" ":" domain-spec
The <domain-spec> is expanded as per Section 7. The resulting domain
name is used for a DNS A RR lookup (even when the connection type is
IPv6). If any A record is returned, this mechanism matches.
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.
6. Modifier Definitions
Modifiers are name/value pairs that provide additional information.
Modifiers always have an "=" separating the name and the value.
The modifiers defined in this document ("redirect" and "exp") SHOULD
appear at the end of the record, after all mechanisms, though
syntactically they can appear anywhere in the record. Ordering of
these two modifiers does not matter. These two modifiers MUST NOT
appear in a record more than once each. If they do, then
check_host() exits with a result of "permerror".
Unrecognized modifiers MUST be ignored no matter where, or how often,
they appear in a record. This allows implementations conforming to
this document to gracefully handle records with modifiers that are
defined in other specifications.
6.1. redirect: Redirected Query
The "redirect" modifier is intended for consolidating both
authorizations and policy into a common set to be shared within a
single ADMD. It is possible to control both authorized hosts and
policy for an arbitrary number of domains from a single record.
redirect = "redirect" "=" domain-spec
If all mechanisms fail to match, and a "redirect" modifier is
present, then processing proceeds as follows:
The <domain-spec> portion of the redirect section is expanded as per
the 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 in the current evaluation of
check_host().
The result of this new evaluation of check_host() is then considered
the result of the current evaluation with the exception that if no
SPF record is found, or if the <target-name> is malformed, the result
is a "permerror" rather than "none".
Note that the newly queried domain can itself specify redirect
processing.
This facility is intended for use by organizations that wish to apply
the same record to multiple domains. For example:
la.example.com. TXT "v=spf1 redirect=_spf.example.com"
ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
_spf.example.com. TXT "v=spf1 mx:example.com -all"
In this example, mail from any of the three domains is described by
the same record. This can be an administrative advantage.
Note: In general, the domain "A" cannot reliably use a redirect to
another domain "B" not under the same administrative control. Since
the <sender> stays the same, there is no guarantee that the record at
domain "B" will correctly work for mailboxes in domain "A",
especially if domain "B" uses mechanisms involving local-parts. An
"include" directive will generally be more appropriate.
For clarity, any "redirect" modifier SHOULD appear as the very last
term in a record. Any "redirect" modifier MUST be ignored if there
is an "all" mechanism anywhere in the record.
6.2. exp: Explanation
explanation = "exp" "=" domain-spec
If check_host() results in a "fail" due to a mechanism match (such as
"-all"), and the "exp" modifier is present, then the explanation
string returned is computed as described below. If no "exp" modifier
is present, then either a default explanation string or an empty
explanation string MUST be returned to the calling application.
The <domain-spec> is macro expanded (see Section 7) and becomes the
<target-name>. The DNS TXT RRset for the <target-name> is fetched.
If there are any DNS processing errors (any RCODE other than 0), or
if no records are returned, or if more than one record is returned,
or if there are syntax errors in the explanation string, then proceed
as if no "exp" modifier was given.
The fetched TXT record's strings are concatenated with no spaces, and
then treated as an explain-string, which is macro-expanded. This
final result is the explanation string. Implementations MAY limit
the length of the resulting explanation string to allow for other
protocol constraints and/or reasonable processing limits. Since the
explanation string is intended for an SMTP response and Section 2.4
of [RFC5321] says that responses are in [US-ASCII], the explanation
string MUST be limited to [US-ASCII].
Software evaluating check_host() can use this string to communicate
information from the publishing domain in the form of a short message
or URL. Software SHOULD make it clear that the explanation string
comes from a third party. For example, it can prepend the macro
string "%{o} explains: " to the explanation, as shown in the example
in Section 8.4.
Suppose example.com has this record:
v=spf1 mx -all exp=explain._spf.%{d}
Here are some examples of possible explanation TXT records at
explain._spf.example.com:
"Mail from example.com 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 information, including the
IP address that failed the check
"See http://%{d}/why.html?s=%{S}&i=%{I}"
-- a complicated example that constructs a URL with the
arguments to check_host() so that a web page can be
generated with detailed, custom instructions
Note: During recursion into an "include" mechanism, an "exp" modifier
from the <target-name> MUST NOT be used. In contrast, when executing
a "redirect" modifier, an "exp" modifier from the original domain
MUST NOT be used. This is because "include" is meant to cross
administrative boundaries and the explanation provided should be the
one from the receiving ADMD, while "redirect" is meant to operate as
a tool to consolidate policy records within an ADMD so the redirected
explanation is the one that ought to have priority.
7. Macros
When evaluating an SPF policy record, certain character sequences are
intended to be replaced by parameters of the message or of the
connection. These character sequences are referred to as "macros".
7.1. Formal Specification
The ABNF description for a macro is as follows:
domain-spec = macro-string domain-end
domain-end = ( "." toplabel [ "." ] ) / macro-expand
toplabel = ( *alphanum ALPHA *alphanum ) /
( 1*alphanum "-" *( alphanum / "-" ) alphanum )
alphanum = ALPHA / DIGIT
explain-string = *( macro-string / SP )
macro-string = *( macro-expand / macro-literal )
macro-expand = ( "%{" macro-letter transformers *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t" / "v"
transformers = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
The "toplabel" construction is subject to the letter-digit-hyphen
(LDH) rule plus additional top-level domain (TLD) restrictions. See
Section 2 of [RFC3696] for background.
Some special cases:
o A literal "%" is expressed by "%%".
o "%_" expands to a single " " space.
o "%-" expands to a URL-encoded space, viz., "%20".
7.2. Macro Definitions
The following macro letters are expanded in term arguments:
s = <sender>
l = local-part of <sender>
o = domain of <sender>
d = <domain>
i = <ip>
p = the validated domain name of <ip> (do not use)
v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
h = HELO/EHLO domain
<domain>, <sender>, and <ip> are defined in Section 4.1.
The following macro letters are allowed only in "exp" text:
c = SMTP client IP (easily readable format)
r = domain name of host performing the check
t = current timestamp
7.3. Macro Processing Details
A '%' character not followed by a '{', '%', '-', or '_' character is
a syntax error. So:
-exists:%(ir).sbl.example.org
is incorrect and will cause check_host() to yield a "permerror".
Instead, the following is legal:
-exists:%{ir}.sbl.example.org
Optional transformers are the following:
*DIGIT = zero or more digits
'r' = reverse value, splitting on dots by default
If transformers or delimiters are provided, the replacement value for
a macro letter is split into parts separated by one or more of the
specified delimiter characters. After performing any reversal
operation and/or removal of left-hand parts, the parts are rejoined
using "." and not the original splitting characters.
By default, strings are split on "." (dots). Note that no special
treatment is given to leading, trailing, or consecutive delimiters in
input strings, and so the list of parts might contain empty strings.
Some older implementations of SPF prohibit trailing dots in domain
names, so trailing dots SHOULD NOT be published, although they MUST
be accepted by implementations conforming to this document. Macros
can specify delimiter characters that are used instead of ".".
The "r" transformer indicates a reversal operation: if the client IP
address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1"
and the macro %{ir} would expand to "1.2.0.192".
The DIGIT transformer indicates the number of right-hand parts to
use, after optional reversal. If a DIGIT is specified, the value
MUST be nonzero. If no DIGITs are specified, or if the value
specifies more parts than are available, all the available parts are
used. If the DIGIT was 5, and only 3 parts were available, the macro
interpreter would pretend the DIGIT was 3. Implementations MUST
support at least a value of 127, as that is the maximum number of
labels in a domain name (less the zero-length label at the end).
The "s" macro expands to the <sender> argument. It is an email
address with a local-part, an "@" character, and a domain. The "l"
macro expands to just the local-part. The "o" macro expands to just
the domain part. Note that these values remain the same during
recursive and chained evaluations due to "include" and/or "redirect".
Note also that if the original <sender> had no local-part, the
local-part was set to "postmaster" in initial processing (see
Section 4.3).
For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format.
For IPv6 addresses, the "i" macro expands to a dot-format address; it
is intended for use in %{ir}. The "c" macro can expand to any of the
hexadecimal colon-format addresses specified in Section 2.2 of
[RFC4291]. It is intended for humans to read.
The "p" macro expands to the validated domain name of <ip>. The
procedure for finding the validated domain name is defined in
Section 5.5. If the <domain> is present in the list of validated
domains, it SHOULD be used. Otherwise, if a subdomain of the
<domain> is present, it SHOULD be used. Otherwise, any name from the
list can be used. If there are no validated domain names or if a DNS
error occurs, the string "unknown" is used.
This macro SHOULD NOT be published (see Section 5.5 for the
discussion).
The "h" macro expands to the parameter that was provided to the SMTP
server via the HELO or EHLO SMTP verb. For sessions where that verb
was provided more than once, the most recent instance is used.
The "r" macro expands to the name of the receiving MTA. This SHOULD
be a fully qualified domain name, but if one does not exist (as when
the checking is done by a Mail User Agent (MUA)) or if policy
restrictions dictate otherwise, the word "unknown" SHOULD be
substituted. The domain name can be different from the name found in
the MX record that the client MTA used to locate the receiving MTA.
The "t" macro expands to the decimal representation of the
approximate number of seconds since the Epoch (Midnight, January 1,
1970, UTC) at the time of the evaluation. This is the same value as
the value that is returned by the Portable Operating System Interface
(POSIX) time() function in most standards-compliant libraries.
When the result of macro expansion is used in a domain name query, if
the expanded domain name exceeds 253 characters (the maximum length
of a domain name in this format), the left side is truncated to fit,
by removing successive domain labels (and their following dots) until
the total length does not exceed 253 characters.
Uppercase macros expand exactly as their lowercase equivalents, and
are then URL escaped. URL escaping MUST be performed for characters
not in the "unreserved" set, which is defined in [RFC3986].
Care has to be taken by the sending ADMD so that macro expansion for
legitimate email does not exceed the 63-character limit on DNS
labels. The local-part of email addresses, in particular, can have
more than 63 characters between dots.
To minimize DNS lookup resource requirements, it is better if sending
ADMDs avoid using the "s", "l", "o", or "h" macros in conjunction
with any mechanism directive. Although 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.
If no directive processed during the evaluation of check_host()
contains an "s", "l", "o", or "h" macro, then the results of the
evaluation can be cached on the basis of <domain> and <ip> alone for
as long as the DNS record involved with the shortest Time to Live
(TTL) has not expired.
7.4. 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 2001:db8::cb01.
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.b.c.0.0.0.0.
0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6._spf.example.com
8. Result Handling
This section provides guidance for SPF verifier operators in response
to the various possible outputs of check_host() on a message.
Definitions of SPF results are presented in Section 2.6; this section
provides more detail on each for use in developing local policy for
message handling.
Every operating environment is different. There are some receivers
for whom strict adherence to SPF is appropriate, and definitive
treatment of messages that are evaluated to be explicitly
unauthorized ("fail" and sometimes "softfail") is the norm. There
are others for which the "false negative" cases are more of a
concern. This concern is typically handled by merely recording the
result in the header and allowing the message to pass on for
additional processing. There are still others where SPF is one of
several inputs to the message-handling decision. As such, there is
no comprehensive normative requirement for message handling in
response to any particular result. This section is provided to
present a complete picture of the likely cause of each result and,
where available, the experience gained during experimental
deployment.
There are essentially two classes of handling choices:
o Handling within the SMTP session that attempted to deliver the
message, such as by returning a permanent SMTP error (rejection)
or temporary SMTP error ("try again later");
o Permitting the message to pass (a successful SMTP reply code) and
adding an additional header field that indicates the result
returned by check_host() and other salient details; this is
discussed in more detail in Section 9.
8.1. None
With a "none" result, the SPF verifier has no information at all
about the authorization or lack thereof of the client to use the
checked identity or identities. The check_host() function completed
without errors but was not able to reach any conclusion.
8.2. Neutral
A "neutral" result indicates that although a policy for the identity
was discovered, there is no definite assertion (positive or negative)
about the client.
A "neutral" result MUST be treated exactly like the "none" result;
the distinction exists only for informational purposes. Treating
"neutral" more harshly than "none" would discourage ADMDs from
testing the use of SPF records (see Section 10.1).
8.3. Pass
A "pass" result means the client is authorized to inject mail with
the given identity. The domain can now, in the sense of reputation,
be considered responsible for sending the message. Further policy
checks can now proceed with confidence in the legitimate use of the
identity. This is further discussed in Appendix G.1.
8.4. Fail
A "fail" result is an explicit statement that the client is not
authorized to use the domain in the given identity. Disposition of
SPF fail messages is a matter of local policy. See Appendix G.2 for
considerations on developing local policy.
If the checking software chooses to reject the mail during the SMTP
transaction, then it SHOULD use an SMTP reply code of 550 (see
[RFC5321]) and, if supported, the 5.7.1 enhanced status code (see
[RFC3463], Section 3.8), in addition to an appropriate reply text.
The check_host() function will return either a default explanation
string or one from the domain that published the SPF records (see
Section 6.2). If the information does not originate with the
checking software, it is good to make it clear that the text is
provided by the sender's domain. For example:
550 5.7.1 SPF MAIL FROM check failed:
550 5.7.1 The domain example.com explains:
550 5.7.1 Please see http://www.example.com/mailpolicy.html
If the checking software chooses not to reject the mail during the
SMTP transaction, then it SHOULD add a Received-SPF or
Authentication-Results header field (see Section 9) to communicate
this result to downstream message processors. While this is true for
all SPF results, it is of particular importance for "fail" results
since the message is explicitly not authorized by the ADMD.
8.5. Softfail
A "softfail" result ought to be treated as somewhere between "fail"
and "neutral"/"none". The ADMD believes the host is not authorized
but is not willing to make a strong policy statement. Receiving
software SHOULD NOT reject the message based solely on this result,
but MAY subject the message to closer scrutiny than normal.
The ADMD wants to discourage the use of this host and thus desires
limited feedback when a "softfail" result occurs. For example, the
recipient's MUA could highlight the "softfail" status, or the
receiving MTA could give the sender a message using greylisting
[RFC6647], with a note the first time the message is received, but
accept it on a later attempt based on receiver policy.
8.6. Temperror
A "temperror" result means the SPF verifier encountered a transient
(generally DNS) error while performing the check. Checking software
can choose to accept or temporarily reject the message. If the
message is rejected during the SMTP transaction for this reason, the
software SHOULD use an SMTP reply code of 451 and, if supported, the
4.4.3 enhanced status code (see Section 3.5 of [RFC3463]). These
errors can be caused by problems in either the sender's or receiver's
DNS software. See Appendix G.4 for considerations on developing
local policy.
8.7. Permerror
A "permerror" result means the domain's published records could not
be correctly interpreted. This signals an error condition that
definitely requires DNS operator intervention to be resolved. If the
message is rejected during the SMTP transaction for this reason, the
software SHOULD use an SMTP reply code of 550 and, if supported, the
5.5.2 enhanced status code (see [RFC3463], Section 3.6). Be aware
that if the ADMD uses macros (Section 7), it is possible that this
result is due to the checked identities having an unexpected format.
It is also possible that this result is generated by certain SPF
verifiers due to the input arguments having an unexpected format; see
Section 4.8. See Appendix G.3 for considerations on developing local
policy.
9. Recording the Result
To provide downstream agents, such as MUAs, with the information they
might need in terms of evaluating or representing the apparent safety
of the message content, it is RECOMMENDED that SMTP receivers record
the result of SPF processing in the message header. For SPF verifier
operators that choose to record SPF results in the header of the
message for processing by internal filters or MUAs, two methods are
presented: Section 9.1 defines the Received-SPF field, which is the
results field originally defined for SPF use. Section 9.2 discusses
the Authentication-Results header field [RFC7001], which was
specified more recently and is designed for use by SPF and other
authentication methods.
Both are in common use, and hence both are included here. However,
it is important to note that they were designed to serve slightly
different purposes. Received-SPF is intended to include enough
information to enable reconstruction of the SPF evaluation of the
message, while Authentication-Results is designed only to relay the
result itself and related output details of likely use to end users
(e.g., what property of the message was actually authenticated and
what it contained), leaving reconstructive work to the purview of
system logs and the Received field contents. Also, Received-SPF
relies on compliance of agents within the receiving ADMD to adhere to
the header field ordering rules of [RFC5321] and [RFC5322], while
Authentication-Results includes some provisions to protect against
non-compliant implementations.
An SPF verifier operator could choose to use both to serve different
downstream agents. In such cases, care needs to be taken to ensure
that both fields are conveying the same details, or unexpected
results can occur.
9.1. The Received-SPF Header Field
The Received-SPF header field is a trace field (see [RFC5322],
Section 3.6.7) and SHOULD be prepended to the existing header, above
the Received: field that is generated by the SMTP receiver. It MUST
appear above all other Received-SPF fields in the message. The
header field has the following format:
header-field = "Received-SPF:" [CFWS] result FWS [comment FWS]
[ key-value-list ] CRLF
result = "pass" / "fail" / "softfail" / "neutral" /
"none" / "temperror" / "permerror"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"]
key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string )
key = "client-ip" / "envelope-from" / "helo" /
"problem" / "receiver" / "identity" /
"mechanism" / name
identity = "mailfrom" ; for the "MAIL FROM" identity
/ "helo" ; for the "HELO" identity
/ name ; other identities
dot-atom = <unquoted word as per [RFC5322]>
quoted-string = <quoted string as per [RFC5322]>
comment = <comment string as per [RFC5322]>
CFWS = <comment or folding white space as per [RFC5322]>
FWS = <folding white space as per [RFC5322]>
CRLF = <standard end-of-line token as per [RFC5322]>
The header field SHOULD include a "(...)" style comment after the
result, conveying supporting information for the result, such as
<ip>, <sender>, and <domain>.
The following key-value pairs are designed for later machine parsing.
SPF verifiers SHOULD give enough information so that the SPF results
can be verified -- that is, at least "client-ip", "helo", and, if the
"MAIL FROM" identity was checked, "envelope-from".
client-ip the IP address of the SMTP client
envelope-from the envelope sender mailbox
helo the host name given in the HELO or EHLO command
mechanism the mechanism that matched (if no mechanisms matched,
substitute the word "default")
problem if an error was returned, details about the error
receiver the host name of the SPF verifier
identity the identity that was checked; see the <identity>
ABNF rule
Other keys MAY be defined by SPF verifiers.
SPF verifiers MUST make sure that the Received-SPF header field does
not contain invalid characters, is not excessively long (see
[RFC5322], Section 2.1.1), and does not contain malicious data that
has been provided by the sender.
Examples of various header field styles that could be generated are
the following:
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)
identity=mailfrom; client-ip=192.0.2.1;
envelope-from="myname@example.com";
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;
mechanism=ip4:192.0.2.1; envelope-from="myname@example.com";
helo=foo.example.com;
9.2. SPF Results in the Authentication-Results Header Field
As mentioned in Section 9, the Authentication-Results header field is
designed to communicate lists of tests a border MTA did and their
results. The specified elements of the field provide less
information than the Received-SPF field:
Authentication-Results: myhost.example.org; spf=pass
smtp.mailfrom=example.net
Received-SPF: pass (myhost.example.org: domain of
myname@example.com designates 192.0.2.1 as permitted sender)
receiver=mybox.example.org; client-ip=192.0.2.1;
envelope-from="myname@example.com"; helo=foo.example.com;
It is, however, possible to add CFWS in the "reason" part of an
Authentication-Results header field and provide the equivalent
information, if desired.
As an example, an expanded Authentication-Results header field might
look like (for a "MAIL FROM" check in this example):
Authentication-Results: myhost.example.org; spf=pass
reason="client-ip=192.0.2.1; smtp.helo=foo.example.com"
smtp.mailfrom=user@example.net
10. Effects on Infrastructure
This section outlines the major implications that adoption of this
protocol will have on various entities involved in Internet email.
It is intended to make clear to the reader where this protocol
knowingly affects the operation of such entities. This section is
not a "how-to" manual, or a "best practices" document, and it is not
a comprehensive list of what such entities ought to do in light of
this specification.
This section provides operational advice and instruction only. It is
non-normative.
[RFC5598] describes the Internet email architecture. This section is
organized based on the different segments of the architecture.
10.1. Sending Domains
Originating ADMDs (ADministrative Management Domains --
Sections 2.2.1 and 2.3 of [RFC5598]) that wish to be compliant with
this specification will need to determine the list of relays
([RFC5598], Section 2.2.2) that they allow to use their domain name
in the "HELO" and "MAIL FROM" identities when relaying to other
ADMDs. It is recognized that forming such a list is not just a
simple technical exercise, but involves policy decisions with both
technical and administrative considerations.
10.1.1. DNS Resource Considerations
Minimizing the DNS resources needed for SPF lookups can be done by
choosing directives that require less DNS information and by placing
lower-cost mechanisms earlier in the SPF record.
Section 4.6.4 specifies the limits receivers have to use. It is
essential to publish records that do not exceed these requirements.
It is also required to carefully weigh the cost and the
maintainability of licit solutions.
For example, consider a domain set up as follows:
example.com. IN MX 10 mx.example.com.
IN MX 20 mx2.example.com.
mx.example.com. IN A 192.0.2.1
mx2.example.com. IN A 192.0.2.129
Assume the administrative point is to authorize (pass) mx and mx2
while failing every other host. Compare the following solutions:
Best record:
example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:192.0.2.129 -all"
Good record:
$ORIGIN example.com.
@ IN TXT "v=spf1 a:authorized-spf.example.com -all"
authorized-spf IN A 192.0.2.1
IN A 192.0.2.129
Expensive record:
example.com. IN TXT "v=spf1 mx:example.com -all"
Wasteful, bad record:
example.com. IN TXT "v=spf1 ip4:192.0.2.0/24 mx -all"
10.1.2. Administrator's Considerations
There might be administrative considerations: using "a" over "ip4" or
"ip6" allows hosts to be renumbered easily at the cost of a DNS query
per receiver. Using "mx" over "a" allows the set of mail hosts to be
changed easily. Unless such changes are common, it is better to use
the less resource-intensive mechanisms like "ip4" and "ip6" over "a"
or "a" over "mx".
In some specific cases, standard advice on record content is
appropriate. Publishing SPF records for domains that send no mail is
a well-established best practice. The record for a domain that sends
no mail is:
www.example.com. IN TXT "v=spf1 -all"
Publishing SPF records for individual hosts is also best practice.
The host name is generally the identity used in the 5321.HELO/.EHLO
command. In the case of messages with a null 5321.MailFrom, this is
used as the domain for 5321.MailFrom SPF checks, in addition to being
used in 5321.HELO/.EHLO-based SPF checks. The standard SPF record
for an individual host that is involved in mail processing is:
relay.example.com. IN TXT "v=spf1 a -all"
Validating correct deployment is difficult. [RFC6652] describes one
mechanism for soliciting feedback on SPF failures. Another
suggestion can be found in Appendix C.
Regardless of the method used, understanding the ADMD's outbound mail
architecture is essential to effective deployment.
10.1.3. Bounces
As explained in Section 2.4, [RFC5321] allows the MAIL FROM to be
null, which is typical of some Delivery Status Notifications
[RFC3464], commonly called email bounces. In this case, the only
entity available for performing an SPF check is the "HELO" identity
defined in Section 1.1.4. SPF functionality is enhanced by
administrators ensuring this identity is set correctly and has an
appropriate SPF record. It is normal to have the "HELO" identity set
to the host name instead of the domain. Zone file generation for
significant numbers of hosts can be consolidated using the "redirect"
modifier and scripted for initial deployment. Specific deployment
advice is given above in Section 10.1.2.
10.2. Receivers
SPF results can be used in combination with other methods to
determine the final local disposition (either positive or negative)
of a message. It can also be considered dispositive on its own.
An attempt to have one organization (sender) direct the email-
handling policies of another (receiver) is inherently challenging and
often controversial. As stated elsewhere in this document, there is
no comprehensive normative requirement for specific handling of a
message based on SPF results. The information presented in Section 8
and in Appendix G is offered for receiver consideration when forming
local handling policies.
The primary considerations are that SPF might return "pass" for mail
that is ultimately harmful (e.g., spammers that arrange for SPF to
pass using disposable domain names, or virus or spam outbreaks from
within trusted sources), and might also return "fail" for mail that
is ultimately legitimate (e.g., legitimate mail that has traversed a
mail alias). It is important to take both of these cases under
consideration when establishing local handling policy.
10.3. Mediators
Mediators are a type of User Actor [RFC5598]. That is, a mediator
takes 'delivery' of a message and posts a 'submission' of a new
message. The mediator can make the newly posted message be as
similar to or as different from the original message as they wish.
Examples include mailing lists (see Section 5.3 of [RFC5598]) and
ReSenders (Section 5.2 of [RFC5598]). This is discussed in
[RFC5321], Section 3.9. For the operation of SPF, the essential
concern is the email address in the 5321.MailFrom command for the new
message.
Because SPF evaluation is based on the IP address of the "last"
sending SMTP server, the address of the mediator will be used, rather
than the address of the SMTP server that sent the message to the
mediator. Some mediators retain the email address from the original
message, while some use a new address.
If the address is the same as for the original message, and the
original message had an associated SPF record, then the SPF
evaluation will fail unless mitigations such as those described in
Appendix D are used.
11. Security Considerations
11.1. Processing Limits
As with most aspects of email, there are a number of ways that
malicious parties could use the protocol as an avenue for a DoS
attack. The processing limits outlined in Section 4.6.4 are designed
to prevent attacks such as the following:
o A malicious party could create an SPF record with many references
to a victim's domain and send many emails to different SPF
verifiers; those SPF verifiers would then create a DoS attack. In
effect, the SPF verifiers are being used to amplify the attacker's
bandwidth by using fewer octets in the SMTP session than are used
by the DNS queries. Using SPF verifiers also allows the attacker
to hide the true source of the attack. This potential attack is
based on large volumes of mail being transmitted.
o Whereas implementations of check_host() are supposed to limit the
number of DNS lookups, malicious domains could publish records
that exceed these limits in an attempt to waste computation effort
at their targets when they send them mail. Malicious domains
could also design SPF records that cause particular
implementations to use excessive memory or CPU or to trigger bugs.
If a receiver is configured to accept mail with an SPF result of
"temperror", such an attack might result in mail that would
otherwise have been rejected due to an SPF "fail" result being
accepted. This potential attack is based on specially crafted SPF
records being used to exhaust DNS resources of the victim.
o Malicious parties could send a large volume of 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.
o Malicious parties could, in theory, use SPF records as a vehicle
for DNS lookup amplification for a DoS attack. In this scenario,
the attacker publishes an SPF record in its own DNS that uses "a"
and "mx" mechanisms directed toward the intended victim, e.g.,
"a:example.com a:foo.example.com a:bar.example.com ..." and then
distributes mail with a MAIL FROM value including its own domain
in large volume to a wide variety of destinations. Any such
destination operating an SPF verifier will begin querying all of
the names associated with the "a" mechanisms in that record. The
names used in the record needn't exist for the attack to be
effective. Operational experience since the publication of
[RFC4408] suggests that mitigation of this class of attack can be
accomplished with minimal impact on the deployed base by having
the verifier abort processing and return "permerror"
(Section 2.6.7) as soon as more than two "void lookups" have been
encountered (defined in Section 4.6.4).
Of these, the case of a third party referenced in the SPF record is
the easiest for a DoS attack to effectively exploit. As a result,
limits that might seem reasonable for an individual mail server can
still allow an unreasonable amount of bandwidth amplification.
Therefore, the processing limits need to be quite low.
11.2. SPF-Authorized Email May Contain Other False Identities
The "MAIL FROM" and "HELO" identity authorizations do not provide
assurance about the authorization/authenticity of other identities
used in the message. It is entirely possible for a malicious sender
to inject a message using his own domain in the identities used by
SPF and have that domain's SPF record authorize the sending host, and
yet the message can easily list other identities in its header.
Unless the user or the MUA takes care to note that the authorized
identity does not match the other more commonly presented identities
(such as the From: header field), the user might be lulled into a
false sense of security.
11.3. Spoofed DNS and IP Data
There are two aspects of this protocol that malicious parties could
exploit to undermine the validity of the check_host() function:
o The evaluation of check_host() relies heavily on DNS. A malicious
attacker could attack the DNS infrastructure and cause
check_host() to see spoofed DNS data, and then return incorrect
results. This could include returning "pass" for an <ip> value
where the actual domain's record would evaluate to "fail". See
[RFC3833] for a description of DNS weaknesses, and see [RFC4033]
for a countermeasure.
o The client IP address, <ip>, is assumed to be correct. In a
modern, correctly configured system, the risk of this not being
true is nil.
11.4. Cross-User Forgery
By definition, SPF policies just map domain names to sets of
authorized MTAs, not whole email addresses to sets of authorized
users. Although the "l" macro (Section 7) provides a limited way to
define individual sets of authorized MTAs for specific email
addresses, it is generally impossible to verify, through SPF, the use
of specific email addresses by individual users of the same MTA.
It is up to mail services and their MTAs to directly prevent
cross-user forgery: based on SMTP AUTH ([RFC4954]), users have to be
restricted to using only those email addresses that are actually
under their control (see Section 6.1 of [RFC6409]). Another means to
verify the identity of individual users is message cryptography, such
as Pretty Good Privacy (PGP) ([RFC4880]) or S/MIME ([RFC5751]).
11.5. Untrusted Information Sources
An SPF-compliant receiver gathers information from the SMTP commands
it receives and from the published DNS records of the sending domain
holder (e.g., "HELO" domain name, the "MAIL FROM" address from the
envelope, and SPF DNS records published by the domain holder). These
parameters are not validated in the SMTP process.
All of these pieces of information are generated by actors outside of
the authority of the receiver, and thus are not guaranteed to be
accurate or legitimate.
11.5.1. Recorded Results
This information, passed to the receiver in the Received-SPF: or
Authentication-Results: trace fields, can be returned to the client
MTA as an SMTP rejection message. If such an SMTP rejection message
is generated, the information from the trace fields has to be checked
for such problems as invalid characters and excessively long lines.
11.5.2. External Explanations
When the authorization check fails, an explanation string could be
included in the reject response. Both the sender and the rejecting
receiver need to be aware that the explanation was determined by the
publisher of the SPF record checked and, in general, not the
receiver. The explanation can contain malicious URLs, or it might be
offensive or misleading.
Explanations returned to sender domains due to "exp" modifiers
(Section 6.2) were generated by the sender policy published by the
domain holders themselves. As long as messages are only returned
with non-delivery notifications ([RFC3464]) to domains publishing the
explanation strings from their own DNS SPF records, the only affected
parties are the original publishers of the domain's SPF records.
In practice, such non-delivery notifications can be misdirected, such
as when an MTA accepts an email and only later generates the
notification to a forged address, or when an email forwarder does not
direct the bounce back to the original sender.
11.5.3. Macro Expansion
Macros (Section 7) allow senders to inject arbitrary text (any non-
null [US-ASCII] character) into receiver DNS queries. It is
necessary to be prepared for hostile or unexpected content.
11.6. Privacy Exposure
Checking SPF records causes DNS queries to be sent to the domain
owner. These DNS queries, especially if they are caused by the
"exists" mechanism, can contain information about who is sending
email and likely to which MTA the email is being sent. This can
introduce some privacy concerns, which are more or less of an issue
depending on local laws and the relationship between the ADMD and the
person sending the email.
11.7. Delivering Mail Producing a "Fail" Result
Operators that choose to deliver mail for which SPF produces a "fail"
result need to understand that they are admitting content that is
explicitly not authorized by the purported sender. While there are
known failure modes that can be considered "false negatives", the
distinct choice to admit those messages increases end-user exposure
to likely harm. This is especially true for domains belonging to
known good actors that are typically well-behaved; unauthorized mail
from those sources might well be subjected to much higher skepticism
and content analysis.
SPF does not, however, include the capacity to distinguish good
actors from bad ones, nor does it handle the concept of known actors
versus unknown ones. Those notions are out of scope for this
specification.
12. 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 [RFC5234] for ABNF notation. Please note that as per this ABNF
definition, literal text strings (those in quotes) are case-
insensitive. Hence, "mx" matches "mx", "MX", "mX", and "Mx".
record = version terms *SP
version = "v=spf1"
terms = *( 1*SP ( directive / modifier ) )
directive = [ qualifier ] mechanism
qualifier = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ a / mx / ptr / ip4 / ip6 / exists )
all = "all"
include = "include" ":" domain-spec
a = "a" [ ":" domain-spec ] [ dual-cidr-length ]
mx = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
ptr = "ptr" [ ":" domain-spec ]
ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
exists = "exists" ":" domain-spec
modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
; where name is not any known modifier
ip4-cidr-length = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32
ip6-cidr-length = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = qnum "." qnum "." qnum "." qnum
qnum = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
; conventional dotted-quad notation, e.g., 192.0.2.0
ip6-network = <as per Section 2.2 of [RFC4291]>
; e.g., 2001:db8::cd30
domain-spec = macro-string domain-end
domain-end = ( "." toplabel [ "." ] ) / macro-expand
toplabel = ( *alphanum ALPHA *alphanum ) /
( 1*alphanum "-" *( alphanum / "-" ) alphanum )
; LDH rule plus additional TLD restrictions
; (see Section 2 of [RFC3696] for background)
alphanum = ALPHA / DIGIT
explain-string = *( macro-string / SP )
macro-string = *( macro-expand / macro-literal )
macro-expand = ( "%{" macro-letter transformers *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t" / "v"
transformers = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header-field = "Received-SPF:" [CFWS] result FWS [comment FWS]
[ key-value-list ] CRLF
result = "pass" / "fail" / "softfail" / "neutral" /
"none" / "temperror" / "permerror"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair )
[";"]
key-value-pair = key [CFWS] "=" ( dot-atom / quoted-string )
key = "client-ip" / "envelope-from" / "helo" /
"problem" / "receiver" / "identity" /
"mechanism" / name
identity = "mailfrom" ; for the "MAIL FROM" identity
/ "helo" ; for the "HELO" identity
/ name ; other identities
sender = Mailbox
ip = ip4-network / ip6-network
ALPHA = <A-Z / a-z as per [RFC5234]>
DIGIT = <0-9 as per [RFC5234]>
SP = <space character as per [RFC5234]>
dot-atom = <unquoted word as per [RFC5322]>
quoted-string = <quoted string as per [RFC5322]>
comment = <comment string as per [RFC5322]>
CFWS = <comment or folding white space as per [RFC5322]>
FWS = <folding white space as per [RFC5322]>
CRLF = <standard end-of-line token as per [RFC5322]>
13. Contributors and Acknowledgements
This document is largely based on the work of Meng Weng Wong, Mark
Lentczner, and Wayne Schlitt. Although, as this section
acknowledges, many people have contributed to this document, a very
large portion of the writing and editing is due to Meng, Mark, and
Wayne.
This design owes a debt of parentage to [RMX] by Hadmut Danisch and
to [DMP] by Gordon Fecyk. The idea of using a DNS record to check
the legitimacy of an email address traces its ancestry further back
through messages on the namedroppers mailing list by Paul Vixie
[Vixie] (based on suggestion by Jim Miller) and by David Green
[Green].
Philip Gladstone contributed the concept of macros to the
specification, multiplying the expressiveness of the language and
making per-user and per-IP lookups possible.
The authors of both this document and [RFC4408] would also like to
thank the literally hundreds of individuals who have participated in
the development of this design. They are far too numerous to name,
but they include the following:
The participants in the SPFbis working group.
The folks on the spf-discuss mailing list.
The folks on the SPAM-L mailing list.
The folks on the IRTF ASRG mailing list.
The folks on the IETF MARID mailing list.
The folks on #perl.
14. IANA Considerations
14.1. The SPF DNS Record Type
Per [RFC4408], the IANA assigned the Resource Record Type and Qtype
from the "Domain Name System (DNS) Parameters" registry for the SPF
RR type with code 99. The format of this type is identical to the
TXT RR [RFC1035]. The character content of the record is encoded as
[US-ASCII].
Studies have shown that RRTYPE 99 has not seen any substantial use,
and in fact its existence and mechanism defined in [RFC4408] have led
to some interoperability issues. Accordingly, its use is no longer
appropriate for SPF version 1; implementations are not to use it.
IANA has updated the "Resource Record (RR) TYPEs" registry to
indicate that this document is the reference document for that
RRTYPE.
14.2. The Received-SPF Mail Header Field
Per [RFC3864], the "Received-SPF:" header field is added to the IANA
"Permanent Message Header Field Names" registry. The following is
the registration template:
Header field name: Received-SPF
Applicable protocol: mail ([RFC5322])
Status: standard
Author/Change controller: IETF
Specification document(s): RFC 7208
14.3. SPF Modifier Registry
IANA has changed the reference for the "exp" and "redirect" modifiers
in the "Modifier Names" registry, under Sender Policy Framework
Parameters, from [RFC4408] to this document. Their status is
unchanged.
15. References
15.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
3463, January 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, September 2013.
[US-ASCII]
American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange, X3.4", 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
15.2. Informative References
[BATV] Levine, J., Crocker, D., Silberman, S., and T. Finch,
"Bounce Address Tag Validation (BATV)", Work in Progress,
May 2008.
[DMP] Fecyk, G., "Designated Mailers Protocol", Work in
Progress, May 2004.
[Green] Green, D., "Domain-Authorized SMTP Mail", June 2002,
<http://www.mhonarc.org/archive/html/ietf-asrg/2003-03/
msg01525.html>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1983] Malkin, G., "Internet Users' Glossary", RFC 1983, August
1996.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, January
2003.
[RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, February 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", RFC
4408, April 2006.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5507] IAB, Faltstrom, P., Austein, R., and P. Koch, "Design
Choices When Expanding the DNS", RFC 5507, April 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An
Applicability Statement for SMTP", RFC 6647, June 2012.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
[RFC6652] Kitterman, S., "Sender Policy Framework (SPF)
Authentication Failure Reporting Using the Abuse Reporting
Format", RFC 6652, June 2012.
[RFC6686] Kucherawy, M., "Resolution of the Sender Policy Framework
(SPF) and Sender ID Experiments", RFC 6686, July 2012.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[RMX] Danisch, H., "The RMX DNS RR and method for lightweight
SMTP sender authorization", Work in Progress, May 2004.
[Vixie] Vixie, P., "Repudiating MAIL FROM", 2002,
<http://marc.info/?l=namedroppers&m=102298170127004&w=4>.
Appendix A. Extended Examples
These examples are based on the following DNS setup:
; A domain with two mail servers, two hosts, and two servers
; at the domain name
$ORIGIN example.com.
@ MX 10 mail-a
MX 20 mail-b
A 192.0.2.10
A 192.0.2.11
amy A 192.0.2.65
bob A 192.0.2.66
mail-a A 192.0.2.129
mail-b A 192.0.2.130
www CNAME example.com.
; A related domain
$ORIGIN example.org.
@ MX 10 mail-c
mail-c A 192.0.2.140
; The reverse IP for those addresses
$ORIGIN 2.0.192.in-addr.arpa.
10 PTR example.com.
11 PTR example.com.
65 PTR amy.example.com.
66 PTR bob.example.com.
129 PTR mail-a.example.com.
130 PTR mail-b.example.com.
140 PTR mail-c.example.org.
; A rogue reverse IP domain that claims to be
; something it's not
$ORIGIN 0.0.10.in-addr.arpa.
4 PTR bob.example.com.
A.1. Simple Examples
These examples show various possible published records for
example.com and which values of <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/30 mx:example.org/30 -all
-- any sending host in 192.0.2.128/30 or 192.0.2.140/30 passes
v=spf1 ptr -all
-- sending host 192.0.2.65 passes (reverse DNS is valid and is
in example.com)
-- sending host 192.0.2.140 fails (reverse DNS 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
A.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's 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 be updated when the mail setup
changes. These domains' records never have to change.
A.3. DNS Blacklist (DNSBL) Style Example
Imagine that, in addition to the domain records listed above, there
are these (see [RFC5782]):
$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}
A.4. Multiple Requirements Example
Say that your sender policy requires both that the IP address is
within a certain range and that the reverse DNS for the IP matches.
This can be done several ways, including the following:
example.com. SPF ( "v=spf1 "
"-include:ip4._spf.%{d} "
"-include:ptr._spf.%{d} "
"+all" )
ip4._spf.example.com. SPF "v=spf1 -ip4:192.0.2.0/24 +all"
ptr._spf.example.com. SPF "v=spf1 -ptr +all"
This example shows how the "-include" mechanism can be useful, how an
SPF record that ends in "+all" can be very restrictive, and the use
of De Morgan's Law.
Appendix B. Changes in Implementation Requirements from RFC 4408
The modifications to implementation requirements from [RFC4408] are
all either (a) corrections to errors in [RFC4408] or (b) additional
documentation based on consensus of operational experience acquired
since the publication of [RFC4408].
o Use of DNS RR type SPF (99) has been removed from the protocol;
see [RFC6686] for background.
o A new DNS-related processing limit based on "void lookups" has
been added (Section 4.6.4).
o Use of the ptr mechanism and the %p macro has been strongly
discouraged (Sections 5.5 and 7.2). The ptr mechanism and the %p
macro remain part of the protocol because they were found to be in
use, but records ought to be updated to avoid them.
o Use of the "Authentication-Results" header field [RFC7001] as a
possible alternative to use of the "Received-SPF" header field is
discussed (Section 9.2).
o There have been a number of minor corrections to the ABNF to make
it more clear and correct (Section 12). SPF library implementers
should give the revised ABNF a careful review to determine if
implementation changes are needed.
o Use of X- fields in the ABNF has been removed; see [RFC6648] for
background.
o Ambiguity about how to deal with invalid <domain-spec> after macro
expansion has been documented. Depending on one specific behavior
has to be avoided (Section 4.8).
o General operational information has been updated and expanded
based on eight years of post-[RFC4408] operations experience. See
Section 10 and Appendices D through G below.
o Security considerations have been reviewed and updated
(Section 11).
Appendix C. Further Testing Advice
Another approach that can be helpful is to publish records that
include a "tracking exists:" mechanism. By looking at the name
server logs, a rough list can then be generated. For example:
v=spf1 exists:_h.%{h}._l.%{l}._o.%{o}._i.%{i}._spf.%{d} ?all
This associated macro expansion would cause the sending HELO domain,
local-part of the sending email address, domain part of the sending
email address, and the IP address from which the connection was
received to be embedded in an SPF query and logged in the sender's
DNS logs.
This approach, which has been used since very early in the SPF
project, allows senders to unilaterally collect data to evaluate the
correctness of their SPF records. Unlike newer feedback mechanisms,
it does not require any special cooperation from SPF verifiers. A
similar example, one of the earliest SPF records published, can still
be found as of this writing at altavista.net.
Appendix D. SPF/Mediator Interactions
There are three places that techniques can be used to ameliorate
unintended SPF failures with mediators.
D.1. Originating ADMDs
The beginning, when email is first sent:
o "Neutral" results could be given for IP addresses that might be
forwarders, instead of "fail" results based on a list of known
reliable forwarders. For example:
"v=spf1 mx ?exists:%{ir}.whitelist.example.org -all"
This would cause a lookup on a DNS White List (DNSWL) and cause a
result of "fail" only for email not coming from either the
domain's mx host(s) (SPF pass) or whitelisted sources (SPF
neutral). This, in effect, outsources an element of sender policy
to the maintainer of the whitelist.
o The "MAIL FROM" identity could have additional information in the
local-part that cryptographically identifies the mail as coming
from an authorized source. In this case, an SPF record such as
the following could be used:
"v=spf1 mx exists:%{l}._spf_verify.%{d} -all"
Then, a specialized DNS server can be set up to serve the
_spf_verify subdomain that validates the local-part. Although
this requires an extra DNS lookup, this happens only when the
email would otherwise be rejected as not coming from a known good
source.
Note that due to the 63-character limit for domain labels, this
approach only works reliably if the local-part signature scheme is
guaranteed to either only produce local-parts with a maximum of
63 characters or gracefully handle truncated local-parts. The
method used to secure the local-part is a local implementation
issue; it need not be standard. An example of one way to do it
can be found in [BATV].
o Similarly, a specialized DNS server could be set up that will
rate-limit the email coming from unexpected IP addresses.
"v=spf1 mx exists:%{ir}._spf_rate.%{d} -all"
o SPF allows the creation of per-user policies for special cases.
For example, the following SPF record and appropriate wildcard DNS
records can be used:
"v=spf1 mx redirect=%{l1r+}._at_.%{o}._spf.%{d}"
D.2. Mediators
The middle, when email is forwarded:
o Mediators can solve the problem by rewriting the "MAIL FROM" to be
in their own domain. This means mail rejected from the external
mailbox will have to be forwarded back to the original sender by
the forwarding service. Various schemes to do this exist, though
they vary widely in complexity and resource requirements on the
part of the mediator.
o Several popular MTAs can be forced from "alias" semantics to
"mailing list" semantics by configuring an additional alias with
"owner-" prepended to the original alias name (e.g., an alias of
"friends: george@example.com, fred@example.org" would need another
alias of the form "owner-friends: localowner").
o Mediators could reject mail that would "fail" SPF if forwarded
using an SMTP reply code of 551, User not local (see Section 3.4
of [RFC5321]) to communicate the correct target address to resend
the mail to.
D.3. Receiving ADMDs
The end, when email is received:
o If the owner of the external mailbox wishes to trust the mediator,
he can direct the external mailbox's MTA to skip SPF tests when
the client host belongs to the mediator.
o Tests against other identities, such as the "HELO" identity, can
be used to override a failed test against the "MAIL FROM"
identity.
o For larger domains, it might not be possible to have a complete or
accurate list of forwarding services used by the owners of the
domain's mailboxes. In such cases, whitelists of generally
recognized forwarding services could be employed.
Appendix E. Mail Services
MSPs (Mail Service Providers -- Section 2.3 of [RFC5598]) that offer
mail services to third-party domains, such as the sending of bulk
mail, might want to adjust their configurations in light of the
authorization check described in this document. If the domain part
of the "MAIL FROM" identity used for such email uses one of the MSP's
domains, then the provider needs only to ensure that its sending host
is authorized by its own SPF record, if any.
If the "MAIL FROM" identity does not use the MSP's domain, then extra
care has to be taken. The SPF record format has several options for
the third-party domain to authorize the service provider's MTAs to
send mail on its behalf. For MSPs, such as ISPs, that have a wide
variety of customers using the same MTA, steps are required to
mitigate the risk of cross-customer forgery (see Section 11.4).
Appendix F. MTA Relays
Relays are described in [RFC5598], Section 2.2.2. The authorization
check generally precludes the use of arbitrary MTA relays between the
sender and receiver of an email message.
Within an organization, MTA relays can be effectively deployed.
However, for the purposes of this document, such relays are
effectively transparent. The SPF authorization check is a check
between border MTAs of different ADMDs.
For mail senders, this means published SPF records have to authorize
any MTAs that actually send across the Internet. Usually, these are
just the border MTAs as internal MTAs simply forward mail to these
MTAs for relaying.
The receiving ADMD will generally want to perform the authorization
check at the boundary MTAs, including all secondary MXs. Internal
MTAs (including MTAs that might serve as both boundary MTAs and
internal relays from secondary MXs when they are processing the
relayed mail stream) then do not perform the authorization test. To
perform the authorization test other than at the boundary, the host
that first transferred the message to the receiving ADMD has to be
determined, which can be difficult to extract from the message header
because (a) header fields can be forged or malformed, and (b) there's
no standard way to encode that information such that it can be
reliably extracted. Testing other than at the boundary is likely to
produce unreliable results. This is described further in Appendix D
of [RFC7001].
Appendix G. Local Policy Considerations
SPF results can be used in combination with other methods to
determine the final local disposition (either positive or negative)
of a message. It can also be considered dispositive on its own.
G.1. Policy for SPF Pass
SPF "pass" results can be used in combination with "whitelists" of
known "good" domains to bypass some or all additional pre-delivery
email checks. Exactly which checks and how to determine appropriate
whitelist entries have to be based on local conditions and
requirements.
G.2. Policy for SPF Fail
SPF "fail" results can be used to reject messages during the SMTP
transaction based on either "MAIL FROM" or "HELO" identity results.
This reduces resource requirements for various content-filtering
methods and conserves bandwidth since rejection can be done before
the SMTP content is transferred. It also gives immediate feedback to
the sender, who might then be able to resolve the issue. Due to some
of the issues described in this section (Appendix G), SPF-based
rejection does present some risk of rejecting legitimate email when
rejecting email based on "MAIL FROM" results.
SPF "fail" results can alternately be used as one input into a larger
set of evaluations that might, based on a combination of SPF "fail"
results with other evaluation techniques, result in the email being
marked negatively in some way (this might be via delivery to a
special spam folder, modifying subject lines, or other locally
determined means). Developing the details of such an approach has to
be based on local conditions and requirements. Using SPF results in
this way does not have the advantages of resource conservation and
immediate feedback to the sender associated with SMTP rejection, but
could produce fewer undesirable rejections in a well-designed system.
Such an approach might result in email that was not authorized by the
sending ADMD being unknowingly delivered to end users.
Either general approach can be used, as they both leave a clear
disposition of emails; either they are delivered in some manner or
the sender is notified of the failure. Other dispositions such as
"dropping" or deleting email after acceptance are inappropriate
because they leave uncertainty and reduce the overall reliability and
utility of email across the Internet.
G.3. Policy for SPF Permerror
The "permerror" result (see Section 2.6.7) indicates that the SPF
processing module at the receiver determined that the retrieved SPF
policy record could not be interpreted. This gives no true
indication about the authorized use of the data found in the
envelope.
As with all results, implementers have a choice to make regarding
what to do with a message that yields this result. SMTP allows only
a few basic options.
Rejection of the message is an option, in that it is the one thing a
receiver can do to draw attention to the difficulty encountered while
protecting itself from messages that do not have a definite SPF
result of some kind. However, if the SPF implementation is defective
and returns spurious "permerror" results, only the sender is actively
notified of the defect (in the form of rejected mail), and not the
receiver making use of SPF.
The less intrusive handling choice is to deliver the message, perhaps
with some kind of annotation of the difficulty encountered and/or
logging of a similar nature. However, this will not be desirable to
SPF verifier operators that wish to implement SPF checking as
strictly as possible, nor is this sort of passive reporting of
problems typically effective.
There is of course the option of placing this choice in the hands of
the SPF verifier operator rather than the implementer since this kind
of choice is often a matter of local policy rather than a condition
with a universal solution, but this adds one more piece of complexity
to an already non-trivial environment.
Both implementers and SPF verifier operators need to be cautious of
all choices and outcomes when handling SPF results.
G.4. Policy for SPF Temperror
The "temperror" result (see Section 2.6.6) indicates that the SPF
processing module at the receiver could not retrieve an SPF policy
record due to a (probably) transient condition. This gives no true
indication about the authorized use of the data found in the
envelope.
As with all results, implementers have a choice to make regarding
what to do with a message that yields this result. SMTP allows only
a few basic options.
Deferring the message is an option, in that it is the one thing a
receiver can do to draw attention to the difficulty encountered while
protecting itself from messages that do not have a definite SPF
result of some kind. However, if the SPF implementation is defective
and returns spurious "temperror" results, only the sender is actively
notified of the defect (in the form of mail rejected after it times
out of the sending queue), and not the receiver making use of SPF.
Because of long queue lifetimes, it is possible that mail will be
repeatedly deferred for several days, and so any awareness that the
sender may have regarding a problem could be quite delayed. If
"temperrors" persist for multiple delivery attempts, it might be
preferable to treat the error as permanent and reduce the amount of
time the message is in transit.
The less intrusive handling choice is to deliver the message, perhaps
with some kind of annotation of the difficulty encountered and/or
logging of a similar nature. However, this will not be desirable to
SPF verifier operators that wish to implement SPF checking as
strictly as possible, nor is this sort of passive reporting of
problems typically effective.
There is of course the option of placing this choice in the hands of
the SPF verifier operator rather than the implementer since this kind
of choice is often a matter of local policy rather than a condition
with a universal solution, but this adds one more piece of complexity
to an already non-trivial environment.
Both implementers and SPF verifier operators need to be cautious of
all choices and outcomes when handling SPF results.
Author's Address
Scott Kitterman
Kitterman Technical Services
3611 Scheel Dr.
Ellicott City, MD 21042
United States of America
EMail: scott@kitterman.com