RFC 7208
Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1
Pages: 64
Proposed Standard
→ Errata
Obsoletes: 4408
Updated by: 7372 8553 8616
Proposed Standard
→ Errata
Obsoletes: 4408
Updated by: 7372 8553 8616
Part 4 of 4 – Pages 50 to 64
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 ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet.15.2.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