Network Working Group K. T. Homme Updates: 3028 Document: draft-homme-sieve-variables-03.txt University of Oslo Expires Feb 6, 2005 6 Aug 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. 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. Homme [Page 1] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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- 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. Homme [Page 2] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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. 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. 0.3. Open Issues None known. 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] Homme [Page 3] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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". 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 Homme [Page 4] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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. 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 "${beep}" verbatim in a text literal, the user can define a variable to circum- vent expansion to the empty string. Example: set "dollar" "$"; set "text" "regarding ${dollar}{beep}"; 3.2. Numbered variables The decimal value of the numbered variable name will index the list of matching strings from the most recently evaluated successful match of type ":matches" or ":regex". The list is empty if no match has been successful. For ":matches", the list will contain one string for each wildcard ("?" and "*") in the match pattern. Each string holds what the cor- responding wildcard expands to, possibly the empty string. The wild- cards expand greedily. For ":regex", the list will contain the strings corresponding to the group operators. The groups are ordered by the position of the open- ing parenthesis, from left to right. The first string in the list has index 1. If the index is out of range, the empty string will be substituted. Index 0 returns the number of strings in the list as a decimal number. Homme [Page 5] Internet Draft Sieve -- Variables Extension 6 Aug 2004 The interpreter MUST short-circuit tests, ie. not perform more tests than necessary to find the result. Example: require [ "fileinto", "regex", "variables" ]; if header :regex "List-ID" "<(.*)@" { fileinto "lists.${1}"; stop; } # this is equivalent to the above: if header :matches "List-ID" "*<*@*" { fileinto "lists.${2}"; stop; } if 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. stop; } if anyof (true, address :domain :matches "To" "*.com") { # second test is never evaluated, so ${0} is still "0" stop; } 4. Action set Syntax: set [MODIFIER] [COMPARATOR] 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"; Homme [Page 6] Internet Draft Sieve -- Variables Extension 6 Aug 2004 set "last_name" "Coyote"; set "vacation" text: Dear ${HONORIFIC} ${last_name}, I'm out, please leave a message after the meep. . ; "set" does not affect the implicit keep. 4.1. Modifiers Modifiers are applied on value before it is stored in the variable. Modifier names are case insensitive. Unknown modifiers MUST yield a syntax error. More than one modifier can be specified, in which case they are applied according to this precedence list, highest value first: Precedence Modifier ----------------------------- 1 :length ----------------------------- 2 :lowerfirst :upperfirst ----------------------------- 3 :lower :upper If two or more modifiers of the same precedence are used, they can be applied in any order. Examples: set "var" "juMBlEd lETteRS"; => "juMBlEd lETteRS" set :length "var" "${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. Homme [Page 7] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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] 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. Homme [Page 8] Internet Draft Sieve -- Variables Extension 6 Aug 2004 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, strings can contain arbitrary val- ues 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. IANA Considerations The following template specifies the IANA registration of the vari- ables 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 Standards Track/IESG-approved experimental RFC number: this RFC Person and email address to contact for further information: Kjetil Torgrim Homme University of Oslo Homme [Page 9] Internet Draft Sieve -- Variables Extension 6 Aug 2004 Pb 1080, Blindern NO-0316 OSLO E-mail: 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 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 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, Novem- ber 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. [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. Homme [Page 10] Internet Draft Sieve -- Variables Extension 6 Aug 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 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. 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