Network Working Group K. T. Homme Updates: 3028 Document: draft-ietf-sieve-variables-03.txt University of Oslo Expires Oct 28, 2005 28 Apr 2005 Sieve Mail Filtering Language: Variables Extension 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 filtering rule sets, it is useful to keep state or configuration details across rules. This 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. Homme [Page 1]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 email 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. Homme [Page 2]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 Homme [Page 3]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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". Homme [Page 4]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 a now syntax error. g) allow SET to set variables in namespaces if the extension allows it. 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 submission as a proposed standard. 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 [UNICODE] character, 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". Homme [Page 5]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 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" "${President, ${Company} Inc.}" => "${President, ACME Inc.}" "${BAD${Company}"=>"${BADACME" The expanded string MUST use the variable values which are current when control reaches the statement the string is part of. Strings where no variable substitutions take place are referred to as constant strings. Future extensions may specify that passing non- constant strings as arguments to its actions or tests is an error. Namespaces are meant for future extensions which make internal state available through variables. These variables SHOULD be put in a namespace whose first component is the same as its capability string. Such extensions SHOULD state which, if any, of the variables in its namespace are modifiable with the "set" action. References to namespaces without a prior require statement for the relevant extension MUST cause a syntax error. Homme [Page 6]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 Tests or actions in future extensions may need to access the unexpanded version of the string argument and, e.g., do the expansion after setting variables in its namespace. The design of the implementation should allow this. 3.1. Quoting The semantics of quoting using backslash are not changed: backslash quoting is resolved before doing variable substitution. Examples: "${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 ${dollar}{beep}"; 3.2. Match variables A "match variable" has a name consisting only of decimal digits and has no namespace component. The decimal value of the match variable name will index the list of matching strings from the most recently evaluated successful match of type ":matches" or ":regex" (see [REGEX]). The list is empty if no match has been successful. For ":matches", the list will contain one string for each wildcard ("?" and "*") in the match pattern. Each string holds what the corresponding wildcard expands to, possibly the empty string. The wildcards match as little as possible (non-greedy matching). For ":regex", the list will contain the strings corresponding to the group operators. The groups are ordered by the position of the opening parenthesis, from left to right. Note that in regular expressions, expansions match as much as possible (greedy matching). The first string in the list has index 1. If the index is out of Homme [Page 7]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 range, the empty string will be substituted. Index 0 contains the matched part of the source value. The interpreter MUST short-circuit tests, ie. not perform more tests than necessary to find the result. Evaluation order MUST be left to right. If a test has two or more list arguments, the implementation is free to choose which to iterate over first. Example: require ["fileinto", "regex", "variables"]; if header :regex "List-ID" "<(.*)@" { fileinto "lists.${1}"; stop; } # This usually gives the same result as the above if header :matches "List-ID" "*<*@*" { fileinto "lists.${2}"; stop; } # Imagine the header # Subject: [acme-users] [fwd] version 1.0 is out if header :regex "Subject" "^[(.*)] (.*)$" { # ${1} will hold "acme-users] [fwd" stop; } if header :matches "Subject" "[*] *" { # ${1} will hold "acme-users", # ${2} will hold "[fwd] version 1.0 is out" fileinfo "lists.${1}"; stop; } if address :matches ["To", "Cc"] ["coyote@**.com", "wile@**.com"] { # ${0} is the matching address. # ${1} is always the empty string. fileinto "business.${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; Homme [Page 8]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 } 4. Action set Syntax: set [MODIFIER] [COMPARATOR] <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 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. 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} ${last_name}, I'm out, please leave a message after the meep. . ; "set" does not affect the implicit keep. It is compatible with all actions defined in [SIEVE]. 4.1. Modifiers Syntax: ":lower" / ":upper" / ":lowerfirst" / ":upperfirst" / ":length" Modifier names are case insensitive. Unknown modifiers MUST yield a syntax error. More than one modifier can be specified, in which case they are applied according to this precedence list, largest value first: Homme [Page 9]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 +-----------------------------+ | Precedence Modifier | +-----------------------------+ | 30 :lower | | :upper | +-----------------------------+ | 20 :lowerfirst | | :upperfirst | +-----------------------------+ | 10 :length | +-----------------------------+ Using two or more modifiers of the same precedence is a syntax error. Examples: # The value assigned to the variable is printed after the arrow set "a" "juMBlEd lETteRS"; => "juMBlEd lETteRS" set :length "b" "${a}"; => "15" set :lower "b" "${a}"; => "jumbled letters" set :lower :comparator "i;octet" "b" "${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, converted 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. Homme [Page 10]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 source strings matches any key. The type of match defaults to ":is". 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. 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. Homme [Page 11]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 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 in an orderly manner. 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. 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 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 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 Cyrus Daboo, Jutta Degener, Ned Freed, Lawrence Greenfield, Mark E. Mallett, Alexey Melnikov, Peder Stray and Nigel Swinson for valuable feedback. Homme [Page 12]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 2234, November 1997 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RELATIONAL] Segmuller, W., "Sieve Extension: Relational Tests", RFC 3431, December 2002 [SIEVE] Showalter, T., "Sieve: A Mail Filtering Language", RFC 3028, January 2001. [UNICODE] The Unicode Consortium, "The Unicode Standard -- Worldwide 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 3629, November 2003. 11.2. Informative References [REGEX] K. Murchison, "Sieve Email Filtering -- Regular Expression Extension", Work in Progress. [SPAMTEST] C. Daboo, "SIEVE Email Filtering: Spamtest and VirusTest Extensions", RFC 3685, February 2004 Homme [Page 13]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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, 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 Homme [Page 14]
Internet Draft Sieve -- Variables Extension 28 Apr 2005 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 15]
