Sender Policy Framework

In computing, Sender Policy Framework (SPF) allows software to identify messages that are or are not authorized to use the domain name in the SMTP HELO and MAIL FROM (Return-Path) commands, based on information published in a sender policy of the domain owner. Forged return paths are common in e-mail spam and result in backscatter. SPF is defined in RFC 4408.

Principles of operation

Normal SMTP allows any computer to send an e-mail claiming to be from anyone. Thus, it is easy for spammers to send e-mail from forged addresses. This makes it difficult to trace back to where the spam truly comes from, and easy for spammers to hide their true identity in order to avoid responsibility. Many believe that the ability for anyone to forge sender addresses (also known as Return-Paths) is a security flaw in modern SMTP, caused by an undesirable side-effect of the deprecation of source routes.

SPF allows the owner of an Internet domain to use special format of DNS SPF or TXT records to specify which machines are authorized to transmit e-mail for that domain. For example, the owner of the example.org domain can designate which machines are authorized to send e-mail whose sender e-mail address ends with "@example.org". Receivers checking SPF can reject messages from unauthorized machines before receiving the body of the message. Thus, principles of operations are quite similar to those of DNSBL, except that SPF leverages the authority delegation scheme of the real Domain Name System.

The sender address is transmitted at the beginning of the SMTP dialog. If the server rejects the sender, the unauthorized client should send a Bounce message to that address. If the server accepts the sender, and subsequently also accepts the recipient(s) and the body of the message, it should insert a Return-Path header in the message's body in order to save the sender address. While the address in the Return-Path often matches other originator addresses in the mail header like "From:" or "Sender:" this is not necessarily the case, and SPF does not prevent forgeries of these other addresses.

Spammers can send e-mail with an SPF PASS result if they have an account in a domain with a sender policy, or abuse a compromised system in this domain. However, doing so makes the spammer easier to trace and prosecute.

The main benefit of SPF is to people whose e-mail addresses are forged in the Return-Paths. They receive a large mass of unsolicited error messages and other auto-replies, making it difficult to use e-mail normally. If such people use SPF to specify their legitimate sending IPs with a FAIL result for all other IPs, then receivers checking SPF can reject forgeries, reducing the amount of back-scatter.

SPF has potential advantages beyond helping identify unwanted e-mail. In particular, if a sender provides SPF information, then receivers can use SPF PASS results in combination with a white list to identify known reliable senders. Scenarios like compromised systems and shared sending mailers limit this use.

FAIL and forwarding

SPF does not allow plain message forwarding. When a domain publishes an SPF FAIL policy, then legitimate mails sent to receivers forwarding their mail to third parties can be rejected and bounced if

This is a necessary and obvious feature of SPF – checks behind the "border" MTA (MX) of the receiver cannot work directly.

Publishers of SPF FAIL policies must accept this potential problem. They should test, e.g. with a SOFTFAIL policy, until they are satisfied with the results. See below for a list of alternatives to plain message forwarding.

HELO tests

For an empty Return-Path as used in error messages and other auto-replies, an SPF check of the HELO-identity is mandatory.

With a bogus HELO identity the result NONE would not help, but for valid host names SPF also protects the HELO identity. This SPF feature was always supported as an option for receivers, and later SPF drafts including the final specification recommend to check the HELO always.

This allows to white list sending mailers based on a HELO PASS, or to reject all mails after a HELO FAIL. It can also be used in reputation systems (any white or black list is a simple case of a reputation system).

Implementation

Thus, the key issue in SPF is the specification for the new DNS information that domains set and receivers use. The records are laid out like this (in typical DNS-syntax):

"v=" defines the version of SPF used. The following words provide mechanisms to use to determine if a domain is eligible to send mail. The "a" and "mx" specify the systems permitted to send messages for the given domain. The "-all" at the end specifies that, if the previous mechanisms did not match, the message should be rejected.

Mechanisms

Qualifiers

Modifiers

The modifiers allow for future extensions to the framework. To date only the two modifiers defined in the RFC 4408 have been widely deployed:

exp=some.example.com gives the name of a domain with a DNS TXT record (interpreted using SPF's macro language) to get an explanation for FAIL results – typically a URL which is added to the SMTP error code. This baroque feature is rarely used.

redirect=some.example.com can be used instead of the ALL-mechanism to link to the policy record of another domain. This modifier is easier to understand than the somewhat similar INCLUDE-mechanism.

Error handling

As soon as SPF implementations detect syntax errors in a sender policy they must abort the evaluation with result PERMERROR. Skipping erroneous mechanisms cannot work as expected, therefore include:bad.example and redirect=bad.example also cause a PERMERROR.

Another safety guard is the maximum of ten mechanisms querying DNS, i.e. any mechanism except from IP4, IP6, and ALL. Implementations can abort the evaluation with result SOFTERROR when it takes too long or a DNS query times out, but they must return PERMERROR if the policy directly or indirectly needs more than ten queries for mechanisms. Any redirect= also counts towards this processing limit.

A typical SPF HELO policy v=spf1 a -all needs three DNS queries: (1) TXT, (2) SPF, and (3) A or AAAA. This last query counts as the first mechanism towards the limit (10). In this example it is also the last, because ALL needs no DNS lookup.

Caveats

SPF avoids abuses of domain names; it does not validate that a given e-mail actually comes from the claimed user. Users within a given domain can forge each others' address, if permitted by the signature process. In addition, domains that share a mail sender (e.g. with virtual hosting) can forge each others' domain even if dire policies are applied.

SPF FAIL rejection

SPF FAIL policies can be an effective but dangerous tool. Some publishers of SPF policies try to avoid the dangers by using SOFTFAIL (designed for limited testing periods) instead of FAIL. But SOFTFAIL can be even more dangerous than FAIL with receivers rejecting FAIL and accepting SOFTFAIL tagged as potential junk.

A reject in a forwarding scenario is a clean decision. The forwarder will send an error message to the address in the Return-Path. Typically the error message (bounce) contains an explanation of the SMTP error and the failing (forwarded to) address. The original sender can then send his mail again directly to this address bypassing the forwarder, a crude emulation of the normal SMTP error code 551 user not local.

However, an accepted SOFTFAIL tagged as potential junk could be deleted by the final recipient. This is a user who has arranged his forwarding in a way that cannot work with SPF; this user could also be careless with checking his potential junk and simply delete it.

The same line of argument also suggests that receivers should take the SPF recommendation to reject SPF FAIL seriously where possible. Accepting SPF FAIL results as potential junk can be more dangerous than simply rejecting FAILed mails. While senders with an SPF FAIL can be expected to know what it means, the same is obviously not the case for a receiver arranging his forwarding in a way that cannot work with SPF.

Checkpoints

Checking SPF behind the "border" MTA (MX) is not impossible; the relevant info is usually noted in a Received timestamp line added by one of the MXs of the receiver. But at this time it is too late to reject SPF FAIL; the checking entity can essentially only delete FAILing mail.

Experts should be able to get this right, but it is no "plug and play" situation; therefore the SPF specification recommends to check SPF only at the "border" (MX) in the SMTP session, not later. Later SPF checks can also have unexpected results if the publishers of sender policies do not plan modifications of their policy carefully with regard to DNS cache expiration.

DoS attack

An Internet draft¹ discussed concerns related to the scale of an SPF answer leading to network exploits as a means to corrupt the DNS. This issue is also covered in the security considerations of the SPF RFC. The SPF project did a detailed analysis ² of this draft and claimed that SPF does not pose any unique threat of DNS DoS.

History

The idea to limit by IP address who could send mail using a given sender domain dates back to 1997³. The SPF concept was presented at YAPC and OSCON (O'Reilly Open Source Convention) in 2003, in a short paper titled "Repudiating Mail-From" written by Paul Vixie in 2002. Other predecessors were "Reverse MX" by Hadmut Danisch, and "Designated Mailer Protocol" by Gordon Fecyk.

In June 2003, Meng Weng Wong merged the RMX and DMP specifications⁴ and solicited suggestions from other programmers. Over the next six months, a large number of changes were made and a large community had started working on SPF.

Originally SPF stood for Sender Permitted From and was sometimes also called SMTP+SPF, but it was changed to Sender Policy Framework in February 2004.

In early 2004, the IETF created the MARID working group and tried to use SPF and Microsoft's CallerID proposal as the basis for what is now known as Sender ID.

After the collapse of MARID the SPF community returned to the original "classic" version of SPF. In July 2005 this version of the specification was approved by the IESG as an IETF experiment, inviting the community to observe SPF during the two years following publication. On April 28, 2006, the SPF RFC was published as experimental RFC 4408.

Controversy

In 2004, Steven M. Bellovin wrote a long e-mail that discusses his concerns with SPF.⁵ Some of these include:

Deployment

In a survey published in 2007, 5% of the .com and .net domains had some kind of SPF policy – this might include overall useless policies like v=spf1 ?all.⁸ In April 2007, BITS, a division of the Financial Services Roundtable, published e-mail security recommendations for its members including SPF deployment.⁹

In 2008 the Messaging Anti-Abuse Working Group (MAAWG) published a paper about email-authentication covering SPF, Sender ID, and DKIM.¹⁰. In their "Sender Best Communication Practices" the MAAWG stated: "At the very least, senders should incorporate SPF records for their mailing domains".¹¹

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