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Versions: (draft-homme-sieve-variables) 00 01 02 03 04 05 06 07 08 RFC 5229

Network Working Group                                        K. T. Homme
Updates: 3028
Document: draft-ietf-sieve-variables-08.txt           University of Oslo
Expires Jun 18, 2006                                         18 Dec 2005



                       Sieve Extension: Variables



Status of this Memo


   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3978.  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.

   Distribution of this memo is unlimited.


Abstract

   In advanced mail filtering rule sets, it is useful to keep state or
   configuration details across rules.  This document updates the Sieve
   filtering language (RFC 3028) with an extension to support variables.
   The extension changes the interpretation 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.



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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.


0.1.  Discussion

   This draft is intended to be an extension to the Sieve mail filtering
   language, available from the RFC repository as
   <ftp://ftp.ietf.org/rfc/rfc3028.txt>.

   This draft and the Sieve language itself are being discussed on the
   MTA Filters mailing list at <ietf-mta-filters@imc.org>.  Subscription
   requests can be sent to <ietf-mta-filters-request@imc.org> (send an
   mail message with the word "subscribe" in the body).  More
   information on the mailing list along with a WWW archive of back
   messages is available at <http://www.imc.org/ietf-mta-filters/>.


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
     "variables".

f)   changed modifiers into being tagged arguments for SET, added
     precedence 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.




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0.2.2.  Changes since -01

a)   clarify that a character is a Unicode character.

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
     evaluation.


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
     values.

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 -03

a)   clarify short-circuiting.

b)   editorial changes.


0.2.5.  Changes since -04

a)   the wildcards in :matches was changed from greedy to non-greedy to
     better support "principle of least surprise".  added example to



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     illustrate the difference.

b)   add definition of "variable"; clarify grammar is based on [SIEVE];
     clarify role of namespaces; add informative references for [REGEX]
     and [SPAMTEST]; add normative reference for [RELATIONAL]

c)   the use of unsupported numbered variables must be flagged as a
     syntax error by implementations.


0.2.6.  Changes since -00 (WG series)

a)   added example for string test

b)   moved introductory text for MODIFIER from 5.1 into 5.0

c)   added Syntax line for MODIFIER.

d)   added comment to an example showing that the non-greedy "*" still
     matches everything due to implicit anchors.

e)   added example of expansion of string with unbalanced braces.

f)   updated reference to [SPAMTEST].


0.2.7.  Changes since -01

a)   moved References from appendix into the document itself.

b)   added example of SET with a comparator.

c)   changed "highest value" to the less ambiguous "largest value".

d)   updated reference to [UTF-8].

e)   allow numbered variables in namespaces.

f)   change ${0} to mean the complete match.


0.2.8.  Changes since -02

a)   explicitly state compatibility with actions in base spec.

b)   "numbered variables" are now called "match variables".





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c)   clarify definition of "match variable".

d)   it's not the whole namespace which should match the extension
     keyword, only the first component.

e)   allow level 2 and above of the namespace specification to be all-
     digit.

f)   combining :upper and :lower etc. is now a syntax error.

g)   allow SET to set variables in namespaces if the extension allows
     it.

0.2.9.  Changes since -03

a)   added two new modifiers, ":quoteregex" and ":quotewildcard".

b)   added wording about security implications of silent truncation.

0.2.10.  Changes since -04

a)   fix buggy markup and add missing modifier to syntax description

b)   changed two "syntax error" (which really weren't) into just
     "error".

c)   changed "Syntax:" into "Usage:" to mirror [SIEVE] convention.

d)   removed description of regex interaction and :quoteregex

e)   added note to clarify that ${0010} is the same as ${10}.

f)   changed name of document to align better with other extensions
     (uses same format at 3431 and 3894)

0.2.11.  Changes since -05

a)   removed "open issues" section.

b)   updated [RELATIONAL] reference

0.2.12.  Changes since -06

a)   updated abstract to mention what this document extends.

b)   changed default scoping behaviour in anticipation of "include"
     extension.




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c)   updated reference to RFC 2234.

d)   clarified whitespace stripping behaviour for "string" test.

0.2.13.  Changes since -07

a)   Replaced reference to Unicode with reference to ISO 10646 and made
     it informational rather than normative.

b)   Updated [ABNF] since it has been published.

c)   Removed the use of comparator with SET to affect case folding.
     Restrict case modifiers to US ASCII.

d)   Mention in abstract that this draft updates RFC 3028.

e)   Clarify that match variables contain unmodified extracts from the
     source value.

f)   Include "INBOX" in all mailbox names for consistency.


1.  Introduction

   This is an extension to the Sieve language defined by [SIEVE].  It
   adds support for storing and referencing named data.  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].  The grammar builds on the grammar of
   [SIEVE].  In this document, "character" means a character from the
   ISO 10646 coded character set [ISO10646], which may consist of
   multiple octets coded in [UTF-8], and "variable" is a named reference
   to data stored or read back using the mechanisms of this extension.


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



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   inclusion 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        =  "${" [namespace] variable-name "}"
      namespace           =  identifier "." *sub-namespace
      sub-namespace       =  variable-name "."
      variable-name       =  num-variable / identifier
      num-variable        =  1*DIGIT

   Examples:
      "&%${}!"     => unchanged, as the empty string is an illegal
                      identifier
      "${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"
      "${BAD${Company}" => "${BADACME"
      "${President, ${Company} Inc.}"
                        => "${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.

   Namespaces are meant for future extensions which make internal state
   available through variables.  These variables SHOULD be put in a
   namespace whose first component is the same as its capability string.
   Such extensions SHOULD state which, if any, of the variables in its
   namespace are modifiable with the "set" action.

   References to namespaces without a prior require statement for the
   relevant extension MUST cause an error.

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



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3.1.  Quoting

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

   Examples:
      "${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
   circumvent expansion to the empty string.

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


3.2.  Match variables

   A "match variable" has a name consisting only of decimal digits and
   has no namespace component.

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

       Note: Extra leading zeroes are allowed and ignored.

   The list will contain one string for each wildcard ("?" and "*") in
   the match pattern.  Each string holds the substring from the source
   value that the corresponding wildcard expands to, possibly the empty
   string.  The wildcards match as little as possible (non-greedy
   matching).

   The first string in the list has index 1.  If the index is out of
   range, the empty string will be substituted.  Index 0 contains the
   matched part of the source value.

   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.

   An extension describing a new match type (e.g., [REGEX]) MAY specify



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   that match variables are set as a side effect when the match type is
   used in a script which has enabled the "variables" extension.

   Example:

      require ["fileinto", "variables"];

      if header :matches "List-ID" "*<*@*" {
          fileinto "INBOX.lists.${2}"; stop;
      }

      # Imagine the header
      # Subject: [acme-users] [fwd] version 1.0 is out
      if header :matches "Subject" "[*] *" {
          # ${1} will hold "acme-users",
          # ${2} will hold "[fwd] version 1.0 is out"
          fileinfo "INBOX.lists.${1}"; stop;
      }

      # Imagine the header
      # To: coyote@ACME.Example.COM
      if address :matches ["To", "Cc"] ["coyote@**.com",
              "wile@**.com"] {
          # ${0} is the matching address
          # ${1} is always the empty string
          # ${2} is part of the domain name ("ACME.Example")
          fileinto "INBOX.business.${2}"; stop;
      } else {
          # Control wouldn't reach this block if any match was
          # successful, so no match variables are set at this
          # point.
      }

      if anyof (true, address :domain :matches "To" "*.com") {
          # The second test is never evaluated, so there are
          # still no match variables set.
          stop;
      }


4.  Action set

   Usage:    set [MODIFIER] <name: string> <value: string>

   The "set" action stores the specified value in the variable
   identified by name.  The name MUST be a constant string and conform
   to the syntax of variable-name.  Match variables can not be set.  A
   namespace can not be used unless an extension explicitly allows its



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   use in "set".  An invalid name MUST be detected as a syntax error.

   Modifiers are applied on a value before it is stored in the variable.
   See next section for details.

   Variables are only visible to the currently running script.  Note:
   Future extensions may provide different scoping rules for variables.

   Variable names are case insensitive.

   Example:
      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.  It is compatible with all
   actions defined in [SIEVE].


4.1.  Modifiers

   Usage:    ":lower" / ":upper" / ":lowerfirst" / ":upperfirst" /
             ":quotewildcard" / ":length"

   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, largest value
   first:


















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                     +--------------------------------+
                     | Precedence     Modifier        |
                     +--------------------------------+
                     |     40         :lower          |
                     |                :upper          |
                     +--------------------------------+
                     |     30         :lowerfirst     |
                     |                :upperfirst     |
                     +--------------------------------+
                     |     20         :quotewildcard  |
                     +--------------------------------+
                     |     10         :length         |
                     +--------------------------------+

   It is an error to use two or more modifiers of the same precedence in
   a single "set" action.

   Examples:
      # The value assigned to the variable is printed after the arrow
      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"
      set :quotewildcard "b" "Rock*";        => "Rock\*"


4.1.1.  Modifier ":length"

   The value is the decimal number of characters in the expansion,
   converted to a string.


4.1.2.  Modifier ":quotewildcard"

   This modifier adds the necessary quoting to ensure that the expanded
   text will only match a literal occurence if used as a parameter to
   :matches.  Every character with special meaning ("*", "?"  and "\")
   is prefixed with "\" in the expansion.

4.1.3.  Case modifiers

   These modifiers change the letters of the text from upper to lower
   case or vice versa.  Characters other than "A"-"Z", "a"-"z" from US-
   ASCII are left unchanged.






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4.1.3.1.  Modifier ":upper"

   All lower case letters are converted to their upper case counterpart.


4.1.3.2.  Modifier ":lower"

   All upper case letters are converted to their lower case counterpart.


4.1.3.3.  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.


4.1.3.4.  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
   unchanged.


5.  Test string

   Usage:    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".

   In the "string" test, both source and key-list are taken from the
   script, not the message, and whitespace stripping MUST NOT be done
   unless the script explicitly requests this through some future
   mechanism.

   Example:
      set "state" "${state} pending";
      if string :matches " ${state} " "* pending *" {
          # the above test always succeeds
      }

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




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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
   compile-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.

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


7.  Security Considerations

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

   Since values stored by "set" which exceed implementation limits are
   silently truncated, it's not appropriate to store large structures
   with security implications in variables.

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

   Sieve filtering should not be relied on as a security measure against
   hostile 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 mail in
   order to avoid filtering rules.  See also [SPAMTEST].


8.  IANA Considerations

   The following template specifies the IANA registration of the
   variables Sieve extension specified in this document:

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

   Capability name: variables
   Capability keyword: variables
   Capability arguments: N/A



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   Standards Track/IESG-approved experimental RFC number:
           this RFC
   Person and email address to contact for further information:
           Kjetil Torgrim Homme
           kjetilho@ifi.uio.no

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

9.  Acknowledgments

   Thanks to Cyrus Daboo, Jutta Degener, Ned Freed, Lawrence Greenfield,
   Jeffrey Hutzelman, Mark E. Mallett, Alexey Melnikov, 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
   E-mail: kjetilho@ifi.uio.no


11.  References


11.1.  Normative references


     [ABNF]       Crocker, D. and Overell, P., "Augmented BNF for Syntax
                  Specifications: ABNF", RFC 4234, October 2005.

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

     [RELATIONAL] Leiba, B. and Segmuller, W., "Sieve Extension:
                  Relational Tests", Work in Progress, draft-ietf-
                  sieve-3431bis-XX.txt

     [SIEVE]      Guenther, P. and Showalter, T., "Sieve: An Email
                  Filtering Language", Work in Progress, draft-ietf-
                  sieve-3028bis-XX.txt





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     [UTF-8]      Yergeau, F., "UTF-8, a transformation format of
                  Unicode and ISO 10646", RFC 3629, November 2003.


11.2.  Informative References


     [ISO10646]   ISO/IEC, "Information Technology - Universal Multiple-
                  Octet Coded Character Set (UCS) - Part 1: Architecture
                  and Basic Multilingual Plane", May 1993, with
                  amendments.

     [REGEX]      Murchison, K., "Sieve Email Filtering -- Regular
                  Expression Extension", Work in Progress.

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


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
   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; 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 (2005).

   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.

   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,



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   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 in BCP 78 and BCP 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 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.


Acknowledgement

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

















Homme                                                          [Page 16]


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