| Sender Policy Framework |
Article Index for Sender |
Website Links For Sender |
Information AboutSender Policy Framework |
| CATEGORIES ABOUT SENDER POLICY FRAMEWORK | |
| spam filtering | |
| e-mail authentication | |
| internet protocols | |
|
PRINCIPLES OF OPERATION Normal SMTP allows any computer to send an 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 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''. 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 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 exploits 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 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. 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. 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 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 Implementing SPF has two parts:
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. Mechanisms Eight ''mechanisms'' are defined: Qualifiers Each ''mechanism'' can be combined with one of four ''qualifiers'':
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''. 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. 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. 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. 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. 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. The SPF project did a detailed analysis of this draft and concluded that SPF does not pose any unique threat of DNS DoS. What remains overlooked in this conclusion is that although there is a limit of 10 SPF mechanisms, each mechanism may invoke 10 queries targeting a victim for a total of 100 transactions per name resolved. In addition, the local-part macro can be employed to randomize subsequent queries, where none of the spammers resources are consumed. Any and all such traffic then represents an infinite gain DNS amplification attack. The spammer is able to spam and stage a completely free attack. The magnitude of this possible exploit is not common in other protocols. 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. 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:
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.
Deployment Despite its limitations, many people have decided that the pros of SPF outweigh its cons and have begun implementing SPF. Anti-spam software such as , Sendmail , Exim , and Qmail . Many prominent domains decided to post SPF data for their domains as of mid-2004, including , 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 overall useless policies like v=spf1 ?all.http://www.onlamp.com/pub/a/onlamp/2007/01/11/dns-extensions.html SEE ALSO
EXTERNAL LINKS
|
|
|