SIEVE Email Filtering Working C. Daboo Group January 20, 2006 Internet-Draft Expires: July 24, 2006 SIEVE Email Filtering: Spamtest and Virustest Extensions draft-ietf-sieve-spamtestbis-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 24, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract The SIEVE email filtering language "spamtest", "spamtestplus" and "virustest" extensions permit users to use simple, portable commands for spam and virus tests on email messages. Each extension provides a new test using matches against numeric 'scores'. It is the responsibility of the underlying SIEVE implementation to do the actual checks that result in values returned by the tests. Change History (to be removed prior to publication as an RFC) Daboo Expires July 24, 2006 [Page 1] Internet-Draft SIEVE Spamtest and Virustest January 2006 Changes from -01: 1. Changed ACAP reference to i18n-comparators draft. 2. Changed MUST in security section for virus checker updates to plain must. 3. Return string "untested" when :percent is used and no test has been done. 4. Remove MUST NOT for having both spamtestplus and spamtest capabilities present, and instead make it a SHOULD NOT. 5. Add text to state that implementations MUST return an error if spamtestplus is not present when :percent is used. 6. Tweak first para of security considerations to better reflect reality of testing. 7. Syntax -> Usage. 8. Updated references to 3028bis and 3431bis. Changes from -00: 1. Added description of how to check for untested when using :percent. 2. Changed requires item to 'spamtestplus'. 3. Changed text describing which requires item needs to be present. Changes from RFC3685: 1. Added ':percent' argument to spamtest. Daboo Expires July 24, 2006 [Page 2] Internet-Draft SIEVE Spamtest and Virustest January 2006 Table of Contents 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 4 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. General Considerations . . . . . . . . . . . . . . . . . . 5 3.2. Test spamtest . . . . . . . . . . . . . . . . . . . . . . 5 3.2.1. spamtest without :percent argument . . . . . . . . . . 6 3.2.2. spamtest with :percent argument . . . . . . . . . . . 6 3.3. Test virustest . . . . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5.1. spamtestplus registration . . . . . . . . . . . . . . . . 9 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Normative References . . . . . . . . . . . . . . . . . . . 10 6.2. Informative References . . . . . . . . . . . . . . . . . . 10 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Daboo Expires July 24, 2006 [Page 3] Internet-Draft SIEVE Spamtest and Virustest January 2006 1. Introduction and Overview SIEVE scripts are frequently being used to do spam and virus filtering based on either implicit script tests (e.g. tests for 'black-listed' senders directly encoded in the SIEVE script), or via testing messages modified by some external spam or virus checker that handled the message prior to SIEVE. The use of third-party spam and virus checker tools poses a problem since each tool has its own way of indicating the result of its checks. These usually take the form of a header added to the message, the content of which indicates the status using some syntax defined by the particular tool. Each user has to then create their own SIEVE scripts to match the contents of these headers to do filtering. This requires the script to stay in synchronisation with the third party tool as it gets updated or perhaps replaced with another. Thus scripts become tied to specific environments, and lose portability. The purpose of this document is to introduce two SIEVE tests that can be used to implement 'generic' tests for spam and viruses in messages processed via SIEVE scripts. These tests return a string containing a range of numeric values that indicate the severity of spam or viruses in a message, or a string that indicates the message has not passed through any spam or virus checking tools, or provides a direct indication of whether the message has been tested for spam or not. The spam and virus checks themselves are handled by the underlying SIEVE implementation in whatever manner is appropriate, and the implementation maps the results of these checks into the numeric ranges defined by the new tests. Thus a SIEVE implementation can have a spam test that implicitly checks for third-party spam tool headers and determines how those map into the spamtest numeric range. In order to do numeric comparisons against the returned strings, server implementations MUST also support the SIEVE relational [I-D.ietf-sieve-3431bis] extension, in addition to the extensions described here. All examples below assume the relational extension is present. 2. Conventions Used in This Document Conventions for notations are as in [I-D.ietf-sieve-3028bis] section 1.1, including use of [RFC2119]. The term 'spam' is used in this document to refer to unsolicited or unwanted email messages. This document does not attempt to define what exactly constitutes spam, or how it should be identified, or what actions should be taken when detected. Daboo Expires July 24, 2006 [Page 4] Internet-Draft SIEVE Spamtest and Virustest January 2006 The term 'virus' is used in this document to refer to any type of message whose content can cause malicious damage. This document does not attempt to define what exactly constitutes a virus, or how it should be identified, or what actions should be taken when detected. 3. SIEVE Extensions 3.1. General Considerations The "spamtest" and "virustest" tests described below can both return a string that starts with a numeric value, followed by an optional space (%x20) character and optional arbitrary text. The numeric value can be compared to specific values using the SIEVE relational [I-D.ietf-sieve-3431bis] extension in conjunction with the "i;ascii- numeric" comparator [I-D.newman-i18n-comparator], which will test for the presence of a numeric value at the start of the string, ignoring any additional text in the string. The additional text can be used to carry implementation specific details about the tests performed and descriptive comments about the result. Tests can be done using standard string comparators against this text if it helps to refine behaviour, however this will break portability of the script as the text will likely be specific to a particular implementation. 3.2. Test spamtest Usage: spamtest [":percent"] [COMPARATOR] [MATCH-TYPE] SIEVE implementations that implement the "spamtest" test use an identifier of either "spamtest" or "spamtestplus" for use with the capability mechanism. If the ":percent" argument is not used with any spamtest test, then one of either the "spamtest" or "spamtestplus" capability identifiers MUST be present. If the ":percent" argument is used with any spamtest test, then the "spamtestplus" capability identifier MUST be present. SIEVE implementations MUST return an error if the ":percent" argument is used and "spamtestplus" is not specified. In the interests of brevity and clarity, scripts SHOULD NOT specify both "spamtestplus" and "spamtest" capability identifiers together. The "spamtest" test evaluates to true if the spamtest result matches the value. The type of match is specified by the optional match Daboo Expires July 24, 2006 [Page 5] Internet-Draft SIEVE Spamtest and Virustest January 2006 argument, which defaults to ":is" if not specified. 3.2.1. spamtest without :percent argument When the ":percent" argument is not present in the "spamtest" test, the result of the test is a string starting with a numeric value in the range "0" (zero) through "10", with meanings summarised below: spamtest interpretation value 0 message was not tested for spam 1 message was tested and is clear of spam 2 - 9 message was tested and has a varying likelihood of containing spam in increasing order 10 message was tested and definitely contains spam The underlying SIEVE implementation will map whatever spam check is done into this numeric range, as appropriate. Examples: require ["spamtest", "fileinto", "relational", "comparator-i;ascii-numeric"]; if spamtest :value "eq" :comparator "i;ascii-numeric" "0" { fileinto "INBOX.unclassified"; } elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3" { fileinto "INBOX.spam-trap"; } In this example, any message that has not passed through a spam check tool will be filed into the mailbox "INBOX.unclassified". Any message with a spamtest value greater than or equal to "3" is filed into a mailbox called "INBOX.spam-trap" in the user's mailstore. 3.2.2. spamtest with :percent argument When the ":percent" argument is present in the "spamtest" test, the result of the test is a string starting with a numeric value in the range "0" (zero) through "100", with meanings summarised below, or the string "untested" for the case where the message was not tested for spam (corresponding to the "0" value returned when the ":percent" argument is not used): Daboo Expires July 24, 2006 [Page 6] Internet-Draft SIEVE Spamtest and Virustest January 2006 spamtest interpretation value "untested" message was not tested for spam 0 message was tested and is clear of spam 1 - 99 message was tested and has a varying likelihood of containing spam in increasing order 100 message was tested and definitely contains spam The underlying SIEVE implementation will map whatever spam check is done into the numeric range, as appropriate. To determine whether the message was tested for spam or not, the preferred solution is to use the test without the ":percent" argument, testing for the value "0" as described in Section 3.2.1. Examples: require ["spamtestplus", "fileinto", "relational", "comparator-i;ascii-numeric"]; if spamtest :value "eq" :comparator "i;ascii-numeric" "0" { fileinto "INBOX.unclassified"; } elsif spamtest :percent :value "ge" :comparator "i;ascii-numeric" "30" { fileinto "INBOX.spam-trap"; } In this example, any message that has not passed through a spam check tool will be filed into the mailbox "INBOX.unclassified". Any message with a spamtest percentage value greater than or equal to "30" is filed into a mailbox called "INBOX.spam-trap" in the user's mailstore. 3.3. Test virustest Usage: virustest [COMPARATOR] [MATCH-TYPE] SIEVE implementations that implement the "virustest" test have an identifier of "virustest" for use with the capability mechanism. The "virustest" test evaluates to true if the virustest result matches the value. The type of match is specified by the optional Daboo Expires July 24, 2006 [Page 7] Internet-Draft SIEVE Spamtest and Virustest January 2006 match argument, which defaults to ":is" if not specified. The virustest result is a string starting with a numeric value in the range "0" (zero) through "5", with meanings summarised below: virustest interpretation value 0 message was not tested for viruses 1 message was tested and contains no known viruses 2 message was tested and contained a known virus which was replaced with harmless content 3 message was tested and contained a known virus which was "cured" such that it is now harmless 4 message was tested and possibly contains a known virus 5 message was tested and definately contains a known virus The underlying SIEVE implementation will map whatever virus checks are done into this numeric range, as appropriate. If the message has not been categorised by any virus checking tools, then the virustest result is "0". Example: require ["virustest", "fileinto", "relational", "comparator-i;ascii-numeric"]; if virustest :value "eq" :comparator "i;ascii-numeric" "0" { fileinto "INBOX.unclassified"; } if virustest :value "eq" :comparator "i;ascii-numeric" "4" { fileinto "INBOX.quarantine"; } elsif virustest :value "eq" :comparator "i;ascii-numeric" "5" { discard; } In this example, any message that has not passed through a virus check tool will be filed into the mailbox "INBOX.unclassified". Any message with a virustest value equal to "4" is filed into a mailbox called "INBOX.quarantine" in the user's mailstore. Any message with a virustest value equal to "5" is discarded (removed) and not delivered to the user's mailstore. Daboo Expires July 24, 2006 [Page 8] Internet-Draft SIEVE Spamtest and Virustest January 2006 4. Security Considerations SIEVE implementations SHOULD ensure that "spamtest" and "virustest" tests only report spam and virus test results for messages that actually have gone through a legitimate spam or virus check process. In particular, if such checks rely on the addition and subsequent checking of private header fields, it is the responsibility of the implementation to ensure that such headers cannot be spoofed by the sender or intermediary and thereby prevent the implementation from being tricked into returning the wrong result for the test. Server administrators must ensure that the virus checking tools are kept up to date, to provide reasonable protection for users using the "virustest" test. Users should be made aware of the fact that the "virustest" test does not provide a 100% reliable way to remove all viruses, and they should continue to exercise caution when dealing with messages of unknown content and origin. Beyond that, the "spamtest" and "virustest" extensions do not raise any security considerations that are not present in the base [I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in [I-D.ietf-sieve-3028bis]. 5. IANA Considerations The following template specifies the IANA registration of the Sieve extensions specified in this document, that are not already registered in [RFC3685]: 5.1. spamtestplus registration To: iana@iana.org Subject: Registration of new Sieve extension Capability name: spamtestplus Capability keyword: spamtest Capability arguments: :percent Standards Track/IESG-approved experimental RFC number: this RFC Person and email address to contact for further information: Cyrus Daboo This information should be added to the list of sieve extensions given on http://www.iana.org/assignments/sieve-extensions. Daboo Expires July 24, 2006 [Page 9] Internet-Draft SIEVE Spamtest and Virustest January 2006 6. References 6.1. Normative References [I-D.ietf-sieve-3028bis] Showalter, T. and P. Guenther, "Sieve: An Email Filtering Language", draft-ietf-sieve-3028bis-05 (work in progress), November 2005. [I-D.ietf-sieve-3431bis] Segmuller, W. and B. Leiba, "Sieve Extension: Relational Tests", draft-ietf-sieve-3431bis-04 (work in progress), December 2005. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest Extensions", RFC 3685, February 2004. 6.2. Informative References [I-D.newman-i18n-comparator] Newman, C., "Internet Application Protocol Collation Registry", draft-newman-i18n-comparator-05 (work in progress), September 2005. Appendix A. Acknowledgments Thanks to Tony Hansen, Jutta Degener, Ned Freed, Ashish Gawarikar, Alexey Melnikov and Nigel Swinson for comments and corrections. Daboo Expires July 24, 2006 [Page 10] Internet-Draft SIEVE Spamtest and Virustest January 2006 Author's Address Cyrus Daboo Email: cyrus@daboo.name Daboo Expires July 24, 2006 [Page 11] Internet-Draft SIEVE Spamtest and Virustest January 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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