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Versions: 00 01 02 03 04 05 RFC 5235

SIEVE Email Filtering Working                                   C. Daboo
Group                                                       ISAMET, Inc.
Internet-Draft                                          January 19, 2005
Expires: July 20, 2005

        SIEVE Email Filtering: Spamtest and Virustest Extensions
                    draft-ietf-sieve-spamtestbis-00

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   RFC 3668.

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   This Internet-Draft will expire on July 20, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The SIEVE email filtering language "spamtest", "spamtestpercent" 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.


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

   Changes from RFC3685:
   1.  Added ':percent' argument to spamtest.

Table of Contents

   1.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  SIEVE Extensions . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1   General Considerations . . . . . . . . . . . . . . . . . .  4
     3.2   Test spamtest  . . . . . . . . . . . . . . . . . . . . . .  4
       3.2.1   spamtest without :percent argument . . . . . . . . . .  4
       3.2.2   spamtest with :percent argument  . . . . . . . . . . .  5
     3.3   Test virustest . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     5.1   spamtestpercent registration . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.1   Normative References . . . . . . . . . . . . . . . . . . . .  8
   6.2   Informative References . . . . . . . . . . . . . . . . . . .  8
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10














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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
   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.  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
   [RFC3431] 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 [RFC3028] 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.

   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 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
   [RFC3431] extension in conjunction with the "i;ascii-numeric"
   comparator [RFC2244], 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

           Syntax:   spamtest [":percent"] [COMPARATOR] [MATCH-TYPE]
                     <value: string>

   SIEVE implementations that implement the "spamtest" test have an
   identifier of "spamtest" for use with the capability mechanism.  If
   the ":percent" argument is used with any spamtest test, then the
   capability idenitifier "spamtestpercent" MUST be present, and the
   "spamtest" capability MUST NOT be present.

   The "spamtest" test evaluates to true if the 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 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


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                 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 "-1" (minus one) through "100", with meanings summarised below:

           spamtest    interpretation
           value

                 -1         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 this numeric range, as appropriate.

   Examples:




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           require ["spamtestpercent", "fileinto",
                    "relational", "comparator-i;ascii-numeric"];

           if spamtest :percent :value "eq"
                       :comparator "i;ascii-numeric" "-1"
           {
               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 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

           Syntax:   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 virustest result
   matches the value.  The type of match is specified by the optional
   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


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   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.

4.  Security Considerations

   SIEVE implementations SHOULD ensure that "spamtest" and "virustest"
   tests can only occur for messages that have gone through a legitimate
   spam or virus check process.  If such checks rely on the addition of
   special headers to messages, it is the responsibility of the
   implementation to ensure that such headers cannot be spoofed by the
   sender, to 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
   [RFC3028] protocol, and these issues are discussed in [RFC3028].


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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  spamtestpercent registration

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

       Capability name: spamtestpercent
       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
         ISAMET, Inc.
         5001 Baum Blvd., Suite 650,
         Pittsburgh, PA 15213
         U.S.A.

         <mailto:daboo@isamet.com>

   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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3028]  Showalter, T., "Sieve: A Mail Filtering Language", RFC
              3028, January 2001.

   [RFC3431]  Segmuller, W., "Sieve Extension: Relational Tests", RFC
              3431, December 2002.

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

6.2  Informative References

   [RFC2244]  Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.


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Author's Address

   Cyrus Daboo
   ISAMET, Inc.
   5001 Baum Blvd.
   Suite 650
   Pittsburgh, PA  15213
   US

   EMail: daboo@isamet.com

Appendix A.  Acknowledgments

   Thanks to Tony Hansen, Jutta Degener, Ned Freed, Ashish Gawarikar and
   Nigel Swinson for comments and corrections.


















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