Sender Policy Framework

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In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). SPF allows software to identify and reject forged addresses in the SMTP MAIL FROM (Return-Path), a typical nuisance in e-mail spam. SPF is defined in Experimental RFC 4408.

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[edit] Principles of Operation

Normal SMTP allows any computer to send e-mail claiming to be from anyone. Thus, it's 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 appear to be senders the receiver would ordinarily trust. Many believe that the ability for anyone to forge Return-Path addresses is a security flaw in modern SMTP, caused by an undesirable side-effect of the deprecation of source routes.

Further information: Sender Rewriting Scheme (SRS)

SPF allows the owner of an Internet domain to use special format of DNS 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 e-mail address in the Return-Path ends with "@example.org". Receivers checking SPF can then treat differently (e.g. mark or reject) any e-mail that claims to come from that domain, but fails in a check against the IPs listed in the sender policy of this domain.

SPF protects the address in the Return-Path, that is the address to which bounces would be sent if the mail is not delivered. 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 spam 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.

[edit] FAIL and forwarding

If 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 (1) the forwarder doesn't rewrite the Return-Path unlike mailing lists, (2) the next hop doesn't white list the forwarder, and (3) this hop checks SPF. This is a necessary and obvious feature of SPF - checks behind the "border" MTA (MX) of the receiver can't work directly.

Publishers of SPF FAIL policies must accept this potential problem; they cannot expect that all receivers change their forwarding arrangements, i.e. clear at least one of the three critical conditions.

A technique called Sender Rewriting Scheme (SRS) is a way for mail forwarding services to avoid this problem.

[edit] 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. Actually it checks an artificial postmaster@mail.example.org address for a HELO or EHLO mail.example.org.

With a bogus HELO identity the result NONE won't help, but for valid host names SPF also protects the HELO identity. This SPF feature was always supported as 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).

[edit] Implementation

Implementing SPF has two parts:

  • Domains identify the machines authorized to send e-mail on their behalf. Domains do this by adding an additional record to their existing DNS information.
  • Receivers can request and use SPF information. They use ordinary DNS queries, which are typically cached to enhance performance. Receivers then interpret the SPF information as specified and act upon the result.

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):

example.org. IN TXT "v=spf1 a mx -all"

"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.

[edit] Mechanisms

Eight mechanisms are defined:

ALL Matches always, used for a default result like -all for all IPs not matched by prior mechanisms.
A If the domain name has an A (or AAAA for IPv6 ) record corresponding to the sender's address, it will match. (That is, the mail comes directly from the domain name.)
IP4 If the sender is in a given IPv4 range, match.
IP6 If the sender is in a given IPv6 range, match.
MX If the domain name has an MX record resolving to the sender's address, it will match. (That is, the mail comes from one of the domain's mail servers)
PTR If the Forward Confirmed reverse DNS domain of the sending IP ending in the domain name.
EXISTS If the given domain resolves, match (no matter the address it resolves to). This is rarely used, along with the SPF macro language it offers more complex matches like DNSBL-queries.
INCLUDE If the included (a misnomer) policy passes the test this mechanism matches. This is typically used to include policies of more than one ISP.

[edit] Qualifiers

Each mechanism can be combined with one of four qualifiers:

  • + for a PASS result, this can be omitted, +mx is the same as mx.
  • ? for a NEUTRAL result interpreted like NONE (no policy).
  • ~ for SOFTFAIL, a debugging aid between NEUTRAL and FAIL.
  • - for FAIL, the mail should be rejected (see below).

[edit] Modifiers

The modifiers allow for future extensions of the framework. So far only the two modifiers defined in the RFC 4408 are widely deployed.

exp=some.example.com gives the name of a domain with a DNS TXT record, which is interpreted using SPF's macro language to get an explanation for FAIL results, typically an URL, 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.

[edit] 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's also the last, because ALL needs no DNS lookup.

[edit] Caveats

SPF normally only validates the domain of the envelope sender (in the Return-Path). Domains that share mail senders (e.g. with virtual hosting) can forge each others' domain. SPF does not validate that a given e-mail actually comes from the claimed user, because it operates at the network level.

[edit] 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 arguments 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.

[edit] 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's 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's 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 don't plan modifications of their policy carefully with regard to DNS cache expiration.

[edit] DoS attack

A 2006 IETF Internet-Draft, SPF DoS Exploitation, discusses 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.

[edit] History

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 specifications 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. On April 28th, 2006, the SPF RFC was published as RFC 4408.

[edit] Controversy

Steven M. Bellovin has written a long e-mail dated January 5, 2004, discussing some of his concerns with SPF. Some of these include:

  • SPF uses TXT records in DNS, which are supposed to be free-form text with no semantics attached. SPF proponents readily acknowledge that it would be better to have records specifically designated for SPF, but this choice was made to enable rapid implementation of SPF. In July 2005, IANA assigned the Resource Record type 99 to SPF. SPF publishers may publish both record types and SPF checkers may check for either types. It will likely take many years before all DNS software fully supports this new record.
  • As of the time he wrote his message, there was no consensus that this is the right way to go. Some major e-mail service providers have not bought into this scheme. Unless and until they do, it doesn't help much, either for their customers (who make up a substantial proportion of the user population) or for everyone else (since their addresses could be forged). It's worth noting that since this concern was raised, among others AOL has embraced SPF.
  • Bellovin's strongest concerns involve the underlying assumptions of SPF (SPF's "semantic model"). When using SPF, the SPF DNS records determine how a sender is allowed to send. That means that the owner of the domain will control how senders are allowed to send. People who use "portable" e-mail addresses (such as e-mail addresses created by professional organizations) will be required to use the domain owner's SMTP sender, which may not currently even exist. Organizations providing these "portable" addresses could, however, create their own Mail Submission Agents (MSAs) (RFC 4409) or offer VPNs. Besides SPF only ties the SMTP Return-Path to permitted MSAs; users are still free to use their RFC 2822 addresses elsewhere.

Jonathan de Boyne Pollard has written a separate diatribe against SPF that discusses alleged SPF's conflicts with mail RFCs, its ability to force ISPs to force their customers to use mail in particular ways, and the forwarding issue.

[edit] Deployment

Despite its limitations, many people have decided that the pros of SPF outweigh its cons and have begun implementing SPF. Anti-spam products such as SpamAssassin version 3.0.0 implement SPF. Many mail transfer agents (MTAs) support SPF directly such as Wildcat, Microsoft Exchange and SmarterMail, or have patches/plug-ins available that support SPF, including Postfix, Sendmail, Exim, and Qmail. Many prominent domains decided to post SPF data for their domains as of mid-2004, including Amazon, AOL, EBay, Google, GMX, Hotmail, Microsoft, and W3C.

In a survey published 2007 5% of the .com and .net domains had some kind of SPF policy - this might include pretty useless policies like v=spf1 ?all [1].

[edit] See also

[edit] External links

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