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Network Working Group                                           C. DabooRequest for Comments: 5235                                  January 2008Obsoletes:3685Category: Standards TrackSieve Email Filtering: Spamtest and Virustest ExtensionsStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.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 proper input to the tests.Table of Contents1. Introduction and Overview .......................................22. Conventions Used in This Document ...............................23. Sieve Extensions ................................................33.1. General Considerations .....................................33.2. Test spamtest ..............................................33.2.1. spamtest without :percent Argument ..................43.2.2. spamtest with :percent Argument .....................53.3. Test virustest .............................................74. Security Considerations .........................................95. IANA Considerations .............................................95.1. spamtest Registration ......................................95.2. virustest Registration ....................................105.3. spamtestplus Registration .................................106. References .....................................................106.1. Normative References ......................................106.2. Informative References ....................................11Appendix A. Acknowledgments .......................................12Appendix B. Important Changes sinceRFC 3685 ......................12Daboo                       Standards Track                     [Page 1]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 20081.  Introduction and Overview   Sieve scripts are frequently being used to do spam and virus   filtering either based on 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   synchronization 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.  The spam and virus checks themselves   are handled by the underlying Sieve implementation in whatever manner   is appropriate, so that the Sieve spam and virus test commands can be   used in a portable way.   In order to do numeric comparisons against the returned strings,   server implementations MUST also support the Sieve relational   [RFC5231] 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[RFC5228] Section 1.1.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [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.   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.Daboo                       Standards Track                     [Page 2]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 20083.  Sieve Extensions3.1.  General Considerations   The "spamtest" and "virustest" tests described below evaluate the   results of implementation-specific spam and virus checks in a   portable way.  The implementation may, for example, check for third-   party spam tool headers and determine how those map into the way the   test commands are used.  To do this, the underlying Sieve   implementation provides a normalized result string as one of the   inputs to each test command.  The normalized result string is   considered to be the value on the left-hand side of the test, and the   comparison values given in the test command are considered to be on   the right-hand side.   The normalized result starts with a digit string, with its numeric   value within the range of values used by the specific test,   indicating the severity of spam or viruses in a message or whether   any tests were done at all.  This may optionally be followed by a   space (%x20) character and arbitrary text, or in one specific case a   single keyword is returned.  The numeric value can be compared to   specific values using the Sieve relational [RFC5231] extension in   conjunction with the "i;ascii-numeric" comparator [RFC4790], which   will test for the presence of a numeric value at the start of the   string, ignoring any additional text in the string.  The optional   text can be used to carry implementation-specific details about the   tests and descriptive comments about the result.  Tests can be done   using standard string comparators against this text if it helps to   refine behavior; however, this will break portability of the script   as the text will likely be specific to a particular implementation.   In addition, the Sieve relational [RFC5231] ":count" match type can   be used to determine if the underlying implementation actually did a   test.  If the underlying spam or virus test was done, the ":count" of   the normalized result will return the numeric value "1", whilst if   the test was not done, or the Sieve implementation could not   determine if a test was done or not done, the ":count" value will be   "0" (zero).3.2.  Test spamtest           Usage:    spamtest [":percent"] [COMPARATOR] [MATCH-TYPE]                     <value: string>   Sieve implementations that implement the "spamtest" test use an   identifier of either "spamtest" or "spamtestplus" for use with the   capability mechanism.Daboo                       Standards Track                     [Page 3]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008   If the ":percent" argument is not used with any spamtest test, then   one or both of "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 normalized spamtest   result matches the value.  The type of match is specified by the   optional match 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 normalized result string provided for the left-hand side of the   test starts with a numeric value in the range "0" (zero) through   "10", with meanings summarized below:   +----------+--------------------------------------------------------+   | spamtest | interpretation                                         |   | value    |                                                        |   +----------+--------------------------------------------------------+   | 0        | message was not tested for spam, or Sieve could not    |   |          | determine whether any test was done                    |   |          |                                                        |   | 1        | message was tested and is clear of spam                |   |          |                                                        |   | 2 - 9    | message was tested and may contain spam; a higher      |   |          | number indicates a greater likelihood of spam          |   |          |                                                        |   | 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"];Daboo                       Standards Track                     [Page 4]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008           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 normalized result 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   normalized result string provided for the left-hand side of the test   starts with a numeric value in the range "0" (zero) through "100",   with meanings summarized below:   +----------+-------------------------------------------------------+   | spamtest | interpretation                                        |   | value    |                                                       |   +----------+-------------------------------------------------------+   | 0        | message was tested and is clear of spam, or was not   |   |          | tested for spam, or Sieve could not determine whether |   |          | any test was done                                     |   |          |                                                       |   | 1 - 99   | message was tested and may contain spam; a higher     |   |          | percentage indicates a greater likelihood of spam     |   |          |                                                       |   | 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 or not the message was tested for spam, two   options can be used:   a.  a test with or without the ":percent" argument and ":count" match       type, testing for the value "0" as described inSection 3.1.   b.  a test without the ":percent" argument using the ":value" match       type, testing for the normalized result value "0" as described inSection 3.2.1.Daboo                       Standards Track                     [Page 5]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008   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 "eq"                          :comparator "i;ascii-numeric" "0"           {               fileinto "INBOX.not-spam";           }           elsif spamtest :percent :value "lt"                          :comparator "i;ascii-numeric" "37"           {               fileinto "INBOX.spam-trap";           }           else           {               discard;           }   In this example, any message that has not passed through a spam check   tool will be filed into the mailbox "INBOX.unclassified".  Any   message that is classified as definitely not containing spam   (normalized result value "0") will be filed into the mailbox   "INBOX.not-spam".  Any message with a normalized result value less   than "37" is filed into a mailbox called "INBOX.spam-trap" in the   user's mailstore.  Any other normalized result value will result in   the message being discarded.   Alternatively, the Sieve relational [RFC5231] ":count" match type can   be used:   Examples:           if spamtest :percent :count "eq"                       :comparator "i;ascii-numeric" "0"           {               fileinto "INBOX.unclassified";           }Daboo                       Standards Track                     [Page 6]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008           elsif spamtest :percent :value "eq"                          :comparator "i;ascii-numeric" "0"           {               fileinto "INBOX.not-spam";           }           elsif spamtest :percent :value "lt"                          :comparator "i;ascii-numeric" "37"           {               fileinto "INBOX.spam-trap";           }           else           {               discard;           }   This example will result in exactly the same behavior as the previous   one.3.3.  Test virustest           Usage:    virustest [COMPARATOR] [MATCH-TYPE]                     <value: string>   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 normalized result   string matches the value.  The type of match is specified by the   optional match argument, which defaults to ":is" if not specified.   The normalized result string provided for the left side of the test   starts with a numeric value in the range "0" (zero) through "5", with   meanings summarized below:Daboo                       Standards Track                     [Page 7]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008   +-----------+-------------------------------------------------------+   | virustest | interpretation                                        |   | value     |                                                       |   +-----------+-------------------------------------------------------+   | 0         | message was not tested for viruses, or Sieve could    |   |           | not determine whether any test was done               |   |           |                                                       |   | 1         | message was tested and contains no known viruses      |   |           |                                                       |   | 2         | message was tested and contained a known virus that   |   |           | was replaced with harmless content                    |   |           |                                                       |   | 3         | message was tested and contained a known virus that   |   |           | was "cured" such that it is now harmless              |   |           |                                                       |   | 4         | message was tested and possibly contains a known      |   |           | virus                                                 |   |           |                                                       |   | 5         | message was tested and definitely 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 categorized 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 normalized result value equal to "4" is filed into aDaboo                       Standards Track                     [Page 8]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008   mailbox called "INBOX.quarantine" in the user's mailstore.  Any   message with a normalized result value equal to "5" is discarded   (removed) and not delivered to the user's mailstore.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   [RFC5228] protocol, and these issues are discussed in [RFC5228].5.  IANA Considerations   The following templates specify the IANA registration of the Sieve   extensions specified in this document.  The registrations for   "spamtest" and "virustest" replace those from [RFC3685]:5.1.  spamtest Registration      To: iana@iana.org      Subject: Registration of new Sieve extension      Capability name: spamtest      Description:     Provides a test to check for varying likelihood of                       an email message being spam.      RFC number:RFC 5235      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>   This information has been added to the list of Sieve extensions given   onhttp://www.iana.org/assignments/sieve-extensions.Daboo                       Standards Track                     [Page 9]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 20085.2.  virustest Registration      To: iana@iana.org      Subject: Registration of new Sieve extension      Capability name: virustest      Description:     Provides a test to check for varying likelihood of                       there being malicious content in an email message.      RFC number:RFC 5235      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>   This information has been added to the list of Sieve extensions given   onhttp://www.iana.org/assignments/sieve-extensions.5.3.  spamtestplus Registration      To: iana@iana.org      Subject: Registration of new Sieve extension      Capability name: spamtestplus      Description:     Provides a test to check for varying likelihood of                       an email message being spam, possibly using a                       percentage range.      RFC number:RFC 5235      Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>   This information has been added to the list of Sieve extensions given   onhttp://www.iana.org/assignments/sieve-extensions.6.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet              Application Protocol Collation Registry",RFC 4790, March              2007.   [RFC5228]  Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An Email              Filtering Language",RFC 5228, January 2008.   [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:              Relational Extension",RFC 5231, January 2008.Daboo                       Standards Track                    [Page 10]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 20086.2.  Informative References   [RFC3685]  Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest              Extensions",RFC 3685, February 2004.Daboo                       Standards Track                    [Page 11]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008Appendix A.  Acknowledgments   Thanks to Mark E. Mallett, Tony Hansen, Jutta Degener, Ned Freed,   Ashish Gawarikar, Alexey Melnikov, Nigel Swinson, and Aaron Stone for   comments and corrections.Appendix B.  Important Changes sinceRFC 3685   Listed below are some of the major changes from the previous   specification [RFC3685], which this one supersedes.   1. A ":percent" argument has been added to the "spamtest" test adding      a new 0-100 numerical range for test results.   2. A "spamtestplus" requires item has been added to indicate the      presence of this extension in scripts.   3. The "count" match type from [RFC5231] can now be used to determine      whether or not a message was tested.   4. Clarified that "test not done" also means "Sieve system could not      determine if a test was done".Author's Address   Cyrus Daboo   EMail: cyrus@daboo.nameDaboo                       Standards Track                    [Page 12]

RFC 5235        Sieve: Spamtest and Virustest Extensions    January 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   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 inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Daboo                       Standards Track                    [Page 13]