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Versions: 00 01 02 03 04 05 RFC 5235

SIEVE Email Filtering Working                                   C. Daboo
Group                                                      July 13, 2006
Internet-Draft
Expires: January 14, 2007


        SIEVE Email Filtering: Spamtest and Virustest Extensions
                    draft-ietf-sieve-spamtestbis-05

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   This Internet-Draft will expire on January 14, 2007.

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 proper input to the tests.

Change History (to be removed prior to publication as an RFC)



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   Changes from -04:
   1.  Relaxed capability requirement so that both spamtest and
       spamtestplus can be present.
   2.  Tweaked text describing spam numeric value ranges.

   Changes from -03:
   1.  Clarified that there are two possible ways to test for not-spam.
   2.  Clarified that 'not tested for xxx' also implies 'SIEVE could not
       determine whether a test was done or not'.

   Changes from -02:
   1.  Changed formatting of tables.
   2.  Fixed missing 2119 definitions.
   3.  Moved reference to previous extension to informative.
   4.  Minor text improvements.
   5.  Fixed some single/double quote issues.
   6.  Reworded abstract, introduction and overview to use better SIEVE
       terminology when describing tests, commands and results.
   7.  Remove "untested" string result from ":percent" test.
   8.  Allow ":count" match type to be used for tested/untested checks.

   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.







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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  . . . . . . . . . . .  7
     3.3.  Test virustest . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  spamtest registration  . . . . . . . . . . . . . . . . . . 12
     5.2.  virustest registration . . . . . . . . . . . . . . . . . . 12
     5.3.  spamtestplus registration  . . . . . . . . . . . . . . . . 13
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16






























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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
   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
   [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.

   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



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   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 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 [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 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 [I-D.ietf-sieve-3431bis] ":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>




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   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 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:





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           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 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 the message was tested for spam or not, 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 in Section 3.1.




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   b.  a test without the ":percent" argument using the ":value" match
       type, testing for the normalized result 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 "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 [I-D.ietf-sieve-3431bis] ":count"
   match type can be used:

   Examples:









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           if spamtest :percent :count "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;
           }

   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:













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   +-----------+-------------------------------------------------------+
   | 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 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 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 a



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   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
   [I-D.ietf-sieve-3028bis] protocol, and these issues are discussed in
   [I-D.ietf-sieve-3028bis].


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 in [RFC3685]:

















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5.1.  spamtest registration

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: spamtest
       Capability keyword: spamtest
       Capability arguments: N/A
       Standards Track/IESG-approved experimental RFC number: this RFC
       Person and email address to contact for further information:

         Cyrus Daboo

         <mailto:cyrus@daboo.name>

       This information should be added to the list of sieve extensions
       given on http://www.iana.org/assignments/sieve-extensions.

5.2.  virustest registration

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: virustest
       Capability keyword: virustest
       Capability arguments: N/A
       Standards Track/IESG-approved experimental RFC number: this RFC
       Person and email address to contact for further information:

         Cyrus Daboo

         <mailto:cyrus@daboo.name>

       This information should be added to the list of sieve extensions
       given on http://www.iana.org/assignments/sieve-extensions.
















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5.3.  spamtestplus registration

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: spamtestplus
       Capability keyword: spamtestplus
       Capability arguments: :percent
       Standards Track/IESG-approved experimental RFC number: this RFC
       Person and email address to contact for further information:

         Cyrus Daboo

         <mailto:cyrus@daboo.name>

   This information should be added to the list of sieve extensions
   given on http://www.iana.org/assignments/sieve-extensions.


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-07 (work in progress),
              June 2006.

   [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.

6.2.  Informative References

   [I-D.newman-i18n-comparator]
              Newman, C., "Internet Application Protocol Collation
              Registry", draft-newman-i18n-comparator-12 (work in
              progress), June 2006.

   [RFC3685]  Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
              Extensions", RFC 3685, February 2004.






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Appendix 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.














































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Author's Address

   Cyrus Daboo

   Email: cyrus@daboo.name














































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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
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   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://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.


Disclaimer of Validity

   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 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.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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