Internet DRAFT - draft-homme-sieve-variables


Network Working Group                                        K. T. Homme
Updates: 3028
Document: draft-homme-sieve-variables-04.txt          University of Oslo
Expires Mar 15, 2005                                         15 Sep 2004

                      Sieve -- Variables Extension

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

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   In advanced filtering rule sets, it is useful to keep state or con-
   figuration details across rules.  This extension changes the interpo-
   lation of strings, adds an action to store data in variables, and
   supplies a new test so that the value of a string can be examined.

0.  Meta-information on this draft

   This information is intended to facilitate discussion.  It will be
   removed when this document leaves the Internet-Draft stage.

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0.1.  Discussion

   This draft is intended to be an extension to the Sieve mail filtering
   language, available from the RFC repository as

   This draft and the Sieve language itself are being discussed on the
   MTA Filters mailing list at <>.  Subscription
   requests can be sent to <> (send an
   email message with the word "subscribe" in the body).  More informa-
   tion on the mailing list along with a WWW archive of back messages is
   available at <>.

0.2.  Noted Changes

0.2.1.  Changes since -00

a)   allow generic time zone names, without requiring implementations to
     support it.  added a "${timezone}" variable so that the user can
     check if the implementation does support the time zone name he
     wants.  the default time zone was changed to localtime again.

b)   allow back references from :matches as well as :regex.

c)   added a section on implementation limits.

d)   clarified global scope so that it spans include.

e)   clarified that this draft only affects scripts which require "vari-

f)   changed modifiers into being tagged arguments for SET, added prece-
     dence table.

g)   added optional COMPARATOR to SET to solve the internationalisation
     problem with :lower etc.

h)   the name of the variable being SET is passed in a string to conform
     with overall Sieve grammar.  this string is explicitly disallowed
     from containing variable references.

0.2.2.  Changes since -01

a)   clarify that a character is a Unicode character.

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b)   added paragraph warning against relying on Sieve for virus checking
     to security section.

c)   added a paragraph defining constant string.

d)   added namespace to grammar.

e)   removed SETDATE.

f)   added wording and example requiring short-circuiting of test evalu-

0.2.3.  Changes since -02

a)   add references to Unicode and UTF-8, also more boilerplate

b)   fixed a meaningless example.

c)   changed term "numeric variables" to "numbered variables" to reduce
     the chance of it being interpreted as variables holding integer

d)   allow future extensions to access the raw string value.

e)   an unsuccessful match does NOT reset the numbered variables.

f)   added definition of "string :count"

g)   exceeding implementation limits on variable lengths should not make
     scripts abort.

0.2.4.  Changes since -02

a)   clarify short-circuiting.

b)   editorial changes.

0.3.  Open Issues

   This extension is in conflict with a MUST in [SIEVE] 2.4: "Tests MUST
   NOT have side effects."  This document therefore can't leave draft
   status until a revised Sieve specification has been accepted by the
   IESG.  No significant changes to this draft are foreseen before sub-
   mission as a proposed standard.

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1.  Introduction

   This is an extension to the Sieve language defined by [SIEVE].  It
   adds support for storing and referencing data in string variables.
   The mechanisms detailed in this document will only apply to Sieve
   scripts that include a require clause for the "variables" extension.
   The require clauses themselves are not affected by this extension.

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS] and [ABNF].  In this document, "character" means a
   [UNICODE] character, which may consist of multiple octets coded in

2.  Capability Identifier

   The capability string associated with the extension defined in this
   document is "variables".

3.  Interpretation of strings

   This extension changes the semantics of quoted-string, multi-line-
   literal and multi-line-dotstuff found in [SIEVE] to enable the inclu-
   sion of the value of variables.

   When a string is evaluated, substrings matching variable-ref SHALL be
   replaced by the value of variable-name.  Only one pass through the
   string SHALL be done.  Variable names are case insensitive, so "foo"
   and "FOO" refer to the same variable.  Unknown variables are replaced
   by the empty string.

      variable-ref        =  "${" variable-name "}"
      variable-name       =  num-variable / *namespace identifier
      namespace           =  identifier "."
      num-variable        =  1*DIGIT

      "&%${}!"     => unchanged, as the empty string is an illegal
      "${doh!}"    => unchanged, as "!" is illegal in identifiers

      The variable company holds the value "ACME".  No other variables
      are set.

      "${full}"    => the empty string
      "${company}" => "ACME"
      "${President, ${Company} Inc.}"

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                   => "${President, ACME Inc.}"

   The expanded string MUST use the variable values which are current
   when control reaches the statement the string is part of.

   Strings where no variable substitutions take place are referred to as
   constant strings.  Future extensions may specify that passing non-
   constant strings as arguments to its actions or tests is an error.

   Future extensions may make internal state available through vari-
   ables.  These variables SHOULD be put in a namespace with the same
   name as its capability string.  Notice that the user can not specify
   a namespace when setting variables with SET.

   Tests or actions in future extensions may need to access the unex-
   panded version of the string argument and, e.g., do the expansion
   after setting variables in its namespace.  The design of the imple-
   mentation should allow this.

3.1.  Quoting

   The semantics of quoting using backslash are not changed: backslash
   quoting is resolved before doing variable substitution.

      "${fo\o}"  => ${foo}  => the expansion of variable foo.
      "${fo\\o}" => ${fo\o} => illegal identifier => left verbatim.
      "\${foo}"  => ${foo}  => the expansion of variable foo.
      "\\${foo}" => \${foo} => a backslash character followed by the
                               expansion of variable foo.

   If it is required to include a character sequence such as "${beep}"
   verbatim in a text literal, the user can define a variable to circum-
   vent expansion to the empty string.

      set "dollar" "$";
      set "text" "regarding ${dollar}{beep}";

3.2.  Numbered variables

   The decimal value of the numbered variable name will index the list
   of matching strings from the most recently evaluated successful match
   of type ":matches" or ":regex".  The list is empty if no match has
   been successful.

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   For ":matches", the list will contain one string for each wildcard
   ("?" and "*") in the match pattern.  Each string holds what the cor-
   responding wildcard expands to, possibly the empty string.  The wild-
   cards expand greedily.

   For ":regex", the list will contain the strings corresponding to the
   group operators.  The groups are ordered by the position of the open-
   ing parenthesis, from left to right.

   The first string in the list has index 1.  If the index is out of
   range, the empty string will be substituted.  Index 0 returns the
   number of strings in the list as a decimal number.

   The interpreter MUST short-circuit tests, ie. not perform more tests
   than necessary to find the result.  Evaluation order MUST be left to
   right.  If a test has two or more list arguments, the implementation
   is free to choose which to iterate over first.


      require [ "fileinto", "regex", "variables" ];

      if header :regex "List-ID" "<(.*)@" {
          fileinto "lists.${1}"; stop;

      # this is equivalent to the above:
      if header :matches "List-ID" "*<*@*" {
          fileinto "lists.${2}"; stop;

      if address :matches [ "To", "Cc" ] "coyote@**.com" {
          # ${0} is always "2", and ${2} is always the empty string.
          fileinto "business.${1}"; stop;
      } else {
          # control can't reach this block if any match was
          # successful, so ${0} is always "0" here.

      if anyof (true, address :domain :matches "To" "*.com") {
          # second test is never evaluated, so ${0} is still "0"

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4.  Action set

   Syntax:   set [MODIFIER] [COMPARATOR] <name: string> <value: string>

   The "set" action stores the specified value in the variable identi-
   fied by name.  The name MUST be a constant string and conform to the
   syntax of identifier.  An illegal name MUST cause a syntax error.

   The default comparator is "i;ascii-casemap".  The comparator only
   affects the result when certain modifiers are used.

   All variables have global scope: they are visible until processing
   stops.  Variable names are case insensitive.

      set "honorific"  "Mr";
      set "first_name" "Wile";
      set "last_name"  "Coyote";
      set "vacation" text:
      Dear ${HONORIFIC} ${last_name},
      I'm out, please leave a message after the meep.

   "set" does not affect the implicit keep.

4.1.  Modifiers

   Modifiers are applied on a value before it is stored in the variable.
   Modifier names are case insensitive.  Unknown modifiers MUST yield a
   syntax error.  More than one modifier can be specified, in which case
   they are applied according to this precedence list, highest value

                        Precedence     Modifier
                            1          :length
                            2          :lowerfirst
                            3          :lower

   If two or more modifiers of the same precedence are used, they can be

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   applied in any order.

      set "a" "juMBlEd lETteRS";             => "juMBlEd lETteRS"
      set :length "b" "${a}";                => "15"
      set :lower "b" "${a}";                 => "jumbled letters"
      set :upperfirst "b" "${a}";            => "JuMBlEd lETteRS"
      set :upperfirst :lower "b" "${a}";     => "Jumbled letters"

4.1.1.  Modifier ":length"

   The value is the decimal number of characters in the expansion, con-
   verted to a string.

4.1.2.  Case modifiers

   These modifiers change the letters of the text from upper to lower
   case or vice versa.  The implementation MUST support US-ASCII, but is
   not required to handle the entire Unicode repertoire.  The comparator
   specified SHOULD be consulted to establish which locale to use.  Modifier ":upper"

   All lower case letters are converted to their upper case counterpart.  Modifier ":lower"

   All upper case letters are converted to their lower case counterpart.  Modifier ":upperfirst"

   The first character of the string is converted to upper case if it is
   a letter and set in lower case.  The rest of the string is left
   unchanged.  Modifier ":lowerfirst"

   The first character of the string is converted to lower case if it is
   a letter and set in upper case.  The rest of the string is left

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5.  Test string

   Syntax: string [MATCH-TYPE] [COMPARATOR]
           <source: string-list> <key-list: string-list>

   The "string" test evaluates to true if any of the source strings
   matches any key.  The type of match defaults to ":is".

   The "relational" extension adds a match type called ":count".  The
   count of a single string is 0 if it is the empty string, or 1 other-
   wise.  The count of a string list is the sum of the counts of the
   member strings.

6.  Implementation Limits

   An implementation of this draft MUST support at least 128 distinct
   variables.  The supported length of variable names MUST be at least
   32 characters.  Each variable MUST be able to hold at least 4000
   characters.  Attempts to set the variable to a value larger than what
   the implementation supports SHOULD be reported as an error at com-
   pile-time if possible.  If the attempt is discovered during run-time,
   the value SHOULD be truncated and it MUST NOT be treated as an error.

   Numbered variables ${1} through ${9} MUST be supported.  References
   to higher indices than the implementation supports should be treated
   as a syntax error which MUST be discovered at compile-time.

7.  Security Considerations

   When numbered variables are used, and the author of the script isn't
   careful, strings can contain arbitrary values controlled by the
   sender of the e-mail.

   The introduction of variables makes advanced decision making easier
   to write, but since no looping construct is provided, all Sieve
   scripts will terminate orderly.

   Sieve filtering should not be relied on as a security measure against
   hostile e-mail messages.  Sieve is designed to do simple, mostly
   static tests, and is not suitable for use as a spam or virus checker,
   where the perpetrator has a motivation to vary the format of the
   email in order to avoid filtering rules.

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8.  IANA Considerations

   The following template specifies the IANA registration of the vari-
   ables Sieve extension specified in this document:

   Subject: Registration of new Sieve extension

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

      Kjetil Torgrim Homme
      University of Oslo
      Pb 1080, Blindern
      NO-0316 OSLO


   This information should be added to the list of sieve extensions
   given on

9.  Acknowledgments

   Thanks to Jutta Degener, Ned Freed, Lawrence Greenfield, Peder Stray
   and Nigel Swinson for valuable feedback.

10.  Author's Address

   Kjetil T. Homme
   University of Oslo
   PO Box 1080
   0316 Oslo, Norway

   Phone: +47 9366 0091

Appendix A.  Normative References

     [ABNF]     D. Crocker, Ed., "Augmented BNF for Syntax Specifica-
                tions: ABNF", Internet Mail Consortium, RFC 2234, Novem-
                ber 1997

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     [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", Harvard University, RFC 2119, March

     [SIEVE]    Showalter, T., "Sieve: A Mail Filtering Language", Mira-
                point, RFC 3028, January 2001.

     [UNICODE]  The Unicode Consortium, "The Unicode Standard -- World-
                wide Character Encoding -- Version 1.0", Addison-Wesley,
                Volume 1, 1991, Volume 2, 1992.

     [UTF-8]    Yergeau, F., "UTF-8, a transformation format of Unicode
                and ISO 10646", RFC 2044, October 1996.

Appendix B.  Intellectual Property Rights Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to per-
   tain 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; neither does it represent that it has made
   any effort to identify any such rights.  Information on the IETF's
   procedures with respect to rights in standards-track and standards-
   related documentation can be found in BCP-11.  Copies of claims of
   rights made available for publication 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 implementors or users of this specification can be obtained from
   the IETF Secretariat.

Appendix C.  Full Copyright Statement

   Copyright (C) The Internet Society 2003. All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this doc-
   ument itself may not be modified in any way, such as by removing the
   copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of develop-
   ing Internet standards in which case the procedures for copyrights
   defined in the Internet Standards process must be followed, or as

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   required to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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