Network Working Group K. T. Homme Updates: 3028 Document: draft-homme-sieve-variables-02.txt University of Oslo Expires June 11, 2003 11 Dec 2003 Sieve -- Variables Extension Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. 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 mate- rial 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 To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Distribution of this memo is unlimited. Abstract 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. 0.1. Discussion This draft is intended to be an extension to the Sieve mail filtering language, available from the RFC repository as Homme [Page 1]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 <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 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 <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 "vari- ables". 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. 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- ation. Homme [Page 2]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 g) added open issues d and e. 0.3. Open Issues a) resolved. b) this extension is particularily useful if fileinto creates new folders on demand. [SIEVE] doesn't prohibit this, and currently some implementations will create new folders automatically, others won't. c) resolved. d) the EDITHEADER draft includes an action that needs the unexpanded string to be passed to the procedure, since the action first per- forms matching which may influence numeric variable references in the argument. this is can be seen as a layering violation, and the variable draft should state explicitly whether such extensions are possible. e) the numeric variables are causing many headaches since they may change spontaneously when running tests or even _during_ actions. an alternative approach is SETMATCH ["var1", "var2", "var3"] which stores the first three match components into the listed variables. (an empty string as a variable name means skip storing that match.) this approach makes REPLACEHEADER less powerful. a related question is what happens when a match fails. the draft currently says that the numeric variables are reset, but this may be inconvenient. 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 which 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]. In this document, "character" means a Unicode character, which may consist of multiple octets coded in UTF-8. 2. Capability Identifier The capability string associated with the extension defined in this document is "variables". Homme [Page 3]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 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. The syntax follows [ABNF]. variable-ref = "${" variable-name "}" variable-name = num-variable / *namespace identifier namespace = identifier "." num-variable = 1*DIGIT When the 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. Unknown variables are replaced by the empty string. 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" "${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. 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. 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. Homme [Page 4]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 If it is required to include a character sequence such as "${beep}" verbatim in a text literal, the user can define a variable to circum- vent expansion to the empty string. Example: set dollar "$"; set text "regarding ${dollar}{beep}"; 3.2. Numeric variables The decimal value of the numeric variable name will index the list of matching strings from the most recently evaluated match of type ":matches" or ":regex". The list is empty if the match was unsuc- cessful. For ":matches", the list will contain one string for each wildcard in the match pattern. Each string holds what the corresponding wildcard expands to, possibly the empty string. The wildcards expand greed- ily. For ":regex", the list will contain the strings corresponding to the group operators. The groups are ordered by the position of the open- ing parenthesis, from left to right. The first string in the list has index 1. If the index is out of range, the empty string will be substituted. Index 0 returns the number of strings in the list. The interpreter MUST short-circuit tests, ie. not perform more tests than necessary to find the result. Example: require [ "fileinto", "regex", "variables" ]; if header :regex "List-ID" "<(.*)@" { fileinto "lists.${1}"; stop; } # this is equivalent to the above: if header :matches "List-ID" "*<*@*" { fileinto "lists.${2}"; stop; } if header :matches [ "To", "Cc" ] "coyote@**.com" { # ${0} is always "2", and ${2} is always the empty string. fileinto "business.${1}"; stop; } else { # ${0} is always "0" stop; } if anyof (true, address :is :domain "To" "example.com") { # second test is never evaluated, so ${0} is always "0" Homme [Page 5]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 stop; } 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. Example: set "honorific" "Mr"; set "first_name" "Wile"; set "last_name" "Coyote"; set "vacation" text: Dear ${HONORIFIC} ${last_name}, I'm out, please leave a message after the meep. . ; "set" does not affect the implicit keep. 4.1. Modifiers Modifiers are applied on value before it is stored in the variable. Modifier names are case insensitive. Unknown modifiers MUST yield a syntax error. More than one modifier can be specified, in which case they are applied according to this precedence list, highest value first: Precedence Modifier ----------------------------- 1 :length ----------------------------- 2 :lowerfirst :upperfirst ----------------------------- 3 :lower :upper If two or more modifiers of the same precedence are used, they can be applied in any order. Homme [Page 6]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 Examples: set "var" "juMBlEd lETteRS"; => "juMBlEd lETteRS" set :length "var" "${var}"; => "15" set :lower "var" "${var}"; => "jumbled letters" set :upperfirst "var" "${var}"; => "JuMBlEd lETteRS" set :upperfirst :lower "var" "${var}"; => "Jumbled letters" 4.1.1. Modifier ":length" The value is the decimal number of letters 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. 4.1.2.1. Modifier ":upper" All lower case letters are converted to their upper case counterpart. 4.1.2.2. Modifier ":lower" All upper case letters are converted to their lower case counterpart. 4.1.2.3. 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. 4.1.2.4. 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 Syntax: string [MATCH-TYPE] [COMPARATOR] <source: string-list> <key-list: string-list> The "string" test evaluates to true if any of the strings matches any key. The type of match defaults to ":is". Homme [Page 7]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 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 MUST be treated as an error. Numeric variables ${1} through ${9} MUST be supported. Referencing higher indices than is supported is a syntax error which MUST be dis- covered at compile-time. If the string matching a wildcard or a regex group operator exceeds the maximum variable size, the implemen- tation SHOULD truncate it and MUST NOT treat it as an error. 7. Security Considerations When combined with the regex extension, strings can contain arbitrary values controlled by the sender of the e-mail if the author of the script isn't careful. 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. 8. Acknowledgments Thanks to Jutta Degener, Ned Freed, Lawrence Greenfield, Peder Stray and Nigel Swinson for valuable feedback. 9. 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 Appendix A. Normative References [ABNF] D. Crocker, Ed., "Augmented BNF for Syntax Specifica- tions: ABNF", Internet Mail Consortium, RFC 2234, Homme [Page 8]
Internet Draft Sieve -- Variables Extension 11 Dec 2003 November 1997 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", Harvard University, RFC 2119, March 1997. [SIEVE] Showalter, T., "Sieve: A Mail Filtering Language", Mira- point, RFC 3028, January 2001. Appendix B. 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 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 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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 MER- CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Homme [Page 9]