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Internet Engineering Task Force (IETF)                          C. DabooRequest for Comments: 6609                                   Apple, Inc.Category: Standards Track                                       A. StoneISSN: 2070-1721                                              Serendipity                                                                May 2012Sieve Email Filtering: Include ExtensionAbstract   The Sieve Email Filtering "include" extension permits users to   include one Sieve script inside another.  This can make managing   large scripts or multiple sets of scripts much easier, and allows a   site and its users to build up libraries of scripts.  Users are able   to include their own personal scripts or site-wide scripts.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6609.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Daboo & Stone                Standards Track                    [Page 1]

RFC 6609                Sieve Extension: Include                May 2012Table of Contents1. Introduction and Overview .......................................22. Conventions Used in This Document ...............................23. Include Extension ...............................................33.1. General Considerations .....................................33.2. Control Structure "include" ................................43.3. Control Structure "return" .................................73.4. Interaction with the "variables" Extension .................83.4.1. Control Structure "global" ..........................83.4.2. Variables Namespace global .........................103.5. Interaction with Other Extensions .........................114. Security Considerations ........................................125. IANA Considerations ............................................126. References .....................................................136.1. Normative References ......................................136.2. Informative References ....................................13Appendix A. Acknowledgments .......................................141.  Introduction and Overview   It's convenient to be able to break Sieve [RFC5228] scripts down into   smaller components that can be reused in a variety of different   circumstances.  For example, users may want to have a default script   and a special 'vacation' script, the latter being activated when the   user goes on vacation.  In that case, the default actions should   continue to be run, but a vacation command should be executed first.   One option is to edit the default script to add or remove the   vacation command as needed.  Another is to have a vacation script   that simply has a vacation command and then includes the default   script.   This document defines the Sieve Email Filtering "include" extension,   which permits users to include one Sieve script inside another.2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   Conventions for notations are as in Sieve[RFC5228], Section 1.1.Daboo & Stone                Standards Track                    [Page 2]

RFC 6609                Sieve Extension: Include                May 2012   The following key phrases are used to describe scripts and script   execution:   script      a valid Sieve script.   script execution      an instance of a Sieve interpreter invoked for a given message      delivery, starting with the user's active script and continuing      through any included scripts until the final disposition of the      message (e.g., delivered, forwarded, discarded, rejected, etc.).   immediate script      the individual Sieve script file being executed.   including script      the individual Sieve script file that had an include statement      that included the immediate script.3.  Include Extension3.1.  General Considerations   Sieve implementations that implement the "include", "return", and   "global" commands described below have an identifier of "include" for   use with the capability mechanism.  If any of the "include",   "return", or "global" commands are used in a script, the "include"   capability MUST be listed in the "require" statement in that script.   Sieve implementations need to track the use of actions in included   scripts so that implicit "keep" behavior can be properly determined   based on whether any actions have executed in any script.   Sieve implementations are allowed to limit the total number of nested   included scripts, but MUST provide for a total of at least three   levels of nested scripts including the top-level script.  An error   MUST be generated either when the script is uploaded to the Sieve   repository, or when the script is executed, if any nesting limit is   exceeded.  If such an error is detected whilst processing a Sieve   script, an implicit "keep" action MUST be executed to prevent loss of   any messages.   Sieve implementations MUST NOT allow recursive script inclusion.   Both direct recursion, where script A includes script A (itself), and   indirect recursion, where script A includes script B which includes   script A once again, are prohibited.Daboo & Stone                Standards Track                    [Page 3]

RFC 6609                Sieve Extension: Include                May 2012   Sieve implementations MUST generate an error at execution time if an   included script is a recursive inclusion.  Implementations MUST NOT   generate errors for recursive includes at upload time, as this would   force an upload ordering requirement upon script authors and   generators.   Sieve implementations MUST generate an error at execution time if an   included script does not exist, except when the ":optional" parameter   is specified.  Implementations MUST NOT generate errors for scripts   missing at upload time, as this would force an upload ordering   requirement upon script authors and generators.   If the Sieve "variables" extension [RFC5229] is present, an issue   arises with the "scope" of variables defined in scripts that may   include each other.  For example, if a script defines the variable   "${status}" with one particular meaning or usage, and another defines   "${status}" with a different meaning, then if one script includes the   other there is an issue as to which "${status}" is being referenced.   To solve this problem, Sieve implementations MUST follow the scoping   rules defined inSection 3.4 and support the "global" command defined   there.3.2.  Control Structure "include"      Usage:  include [LOCATION] [":once"] [":optional"] <value: string>              LOCATION = ":personal" / ":global"   The "include" command takes an optional "location" parameter, an   optional ":once" parameter, an optional ":optional" parameter, and a   single string argument representing the name of the script to include   for processing at that point.  Implementations MUST restrict script   names according to ManageSieve[RFC5804], Section 1.6.  The script   name argument MUST be a constant string as defined in[RFC5229],   Section 3; implementations MUST NOT expand variables in the script   name argument.   The "location" parameter MUST default to ":personal" if not   specified.  The "location" parameter MUST NOT be specified more than   once.  The "location" has the following meanings:   :personal      Indicates that the named script is stored in the user's own      personal (private) Sieve repository.   :global      Indicates that the named script is stored in a site-wide Sieve      repository, accessible to all users of the Sieve system.Daboo & Stone                Standards Track                    [Page 4]

RFC 6609                Sieve Extension: Include                May 2012   The ":once" parameter tells the interpreter only to include the named   script if it has not already been included at any other point during   script execution.  If the script has already been included,   processing continues immediately following the "include" command.   Implementations MUST NOT generate an error if an "include :once"   command names a script whose inclusion would be recursive; in this   case, the script MUST be considered previously included, and   therefore "include :once" will not include it again.   Note: It is RECOMMENDED that script authors and generators use the   ":once" parameter only when including a script that performs general   duties such as declaring global variables and making sanity checks of   the environment.   The ":optional" parameter indicates that the script may be missing.   Ordinarily, an implementation MUST generate an error during execution   if an "include" command specifies a script that does not exist.  When   ":optional" is specified, implementations MUST NOT generate an error   for a missing script, and MUST continue as if the "include" command   had not been present.   The included script MUST be a valid Sieve script.  Implementations   MUST validate that each script has its own "require" statements for   all optional capabilities used by that script.  The scope of a   "require" statement is the script in which it immediately appears,   and neither inherits nor passes on capabilities to other scripts   during the course of execution.   A "stop" command in an included script MUST stop all script   processing, including the processing of the scripts that include the   immediate one.  The "return" command (described below) stops   processing of the immediate script only, and allows the scripts that   include it to continue.   The "include" command MAY appear anywhere in a script where a control   structure is legal, and MAY be used within another control structure,   e.g., an "if" block.Daboo & Stone                Standards Track                    [Page 5]

RFC 6609                Sieve Extension: Include                May 2012   Examples:   The user has four scripts stored in their personal repository:   "default"      This is the default active script that includes several others.      require ["include"];      include :personal "always_allow";      include :global "spam_tests";      include :personal "spam_tests";      include :personal "mailing_lists";   Personal script "always_allow"      This script special-cases some correspondent email addresses and      makes sure any message containing those addresses is always kept.      if address :is "from" "boss@example.com"      {          keep;      }      elsif address :is "from" "ceo@example.com"      {          keep;      }   Personal script "spam_tests" (uses "reject" [RFC5429])      This script does some user-specific spam tests to catch spam      messages not caught by the site-wide spam tests.      require ["reject"];      if header :contains "Subject" "XXXX"      {          reject "Subject XXXX is unacceptable.";      }      elsif address :is "from" "money@example.com"      {          reject "Mail from this sender is unwelcome.";      }Daboo & Stone                Standards Track                    [Page 6]

RFC 6609                Sieve Extension: Include                May 2012   Personal script "mailing_lists"      This script looks for messages from different mailing lists and      files each into a mailbox specific to the mailing list.      require ["fileinto"];      if header :is "List-ID" "sieve.ietf.org"      {          fileinto "lists.sieve";      }      elsif header :is "List-ID" "ietf-imapext.imc.org"      {          fileinto "lists.imapext";      }   There is one script stored in the global repository:   Site script "spam_tests" (uses "reject" [RFC5429])      This script does some site-wide spam tests that any user at the      site can include in their own scripts at a suitable point.  The      script content is kept up to date by the site administrator.      require ["reject"];      if anyof (header :contains "Subject" "$$",                header :contains "Subject" "Make money")      {          reject "No thank you.";      }3.3.  Control Structure "return"      Usage:  return   The "return" command stops processing of the immediately included   script only and returns processing control to the script that   includes it.  If used in the main script (i.e., not in an included   script), it has the same effect as the "stop" command, including the   appropriate "keep" action if no other actions have been executed up   to that point.Daboo & Stone                Standards Track                    [Page 7]

RFC 6609                Sieve Extension: Include                May 20123.4.  Interaction with the "variables" Extension   In order to avoid problems of variables in an included script   "overwriting" those from the script that includes it, this   specification requires that all variables defined in a script MUST be   kept "private" to the immediate script by default -- that is, they   are not "visible" to other scripts.  This ensures that two script   authors cannot inadvertently cause problems by choosing the same name   for a variable.   However, sometimes there is a need to make a variable defined in one   script available to others.  This specification defines the new   command "global" to declare that a variable is shared among scripts.   Effectively, two namespaces are defined: one local to the immediate   script, and another shared among all scripts.  Implementations MUST   allow a non-global variable to have the same name as a global   variable but have no interaction between them.3.4.1.  Control Structure "global"      Usage:  global <value: string-list>   The "global" command accepts a string list argument that defines one   or more names of variables to be stored in the global variable space.   Each name MUST be a constant string and conform to the syntax of   variable-name as defined in the "variables" extension document[RFC5229], Section 3.  Match variables cannot be specified, and   namespace prefixes are not allowed.  An invalid name MUST be detected   as a syntax error.   The "global" command is only available when the script has both   "include" and "variables" in its require line.  If the "global"   command appears when only "include" or only "variables" has been   required, an error MUST be generated when the script is uploaded.   If a "global" command is given the name of a variable that has   previously been defined in the immediate script with "set", an error   MUST be generated either when the script is uploaded or at execution   time.   If a "global" command lists a variable that has not been defined in   the "global" namespace, the name of the variable is now marked as   global, and any subsequent "set" command will set the value of the   variable in global scope.Daboo & Stone                Standards Track                    [Page 8]

RFC 6609                Sieve Extension: Include                May 2012   A variable has global scope in all scripts that have declared it with   the "global" command.  If a script uses that variable name without   declaring it global, the name specifies a separate, non-global   variable within that script.   Interpretation of a string containing a variable marked as global,   but without any value set, SHALL behave as any other access to an   unknown variable, as specified in the "variables" extension document[RFC5229], Section 3 (i.e., evaluates to an empty string).   Example:   The active script      The included script may contain repetitive code that is      effectively a subroutine that can be factored out.  In this      script, the test that matches last will leave its value in the      test_mailbox variable, and the top-level script will file the      message into that mailbox.  If no tests matched, the message will      be implicitly kept in the INBOX.      require ["fileinto", "include", "variables", "relational"];      global "test";      global "test_mailbox";      set "test" "$$";      include "subject_tests";      set "test" "Make money";      include "subject_tests";      if string :count "eq" "${test_mailbox}" "1"      {          fileinto "${test_mailbox}";          stop;      }Daboo & Stone                Standards Track                    [Page 9]

RFC 6609                Sieve Extension: Include                May 2012   Personal script "subject_tests"      This script performs a number of tests against the message, sets      the global test_mailbox variable with a folder to file the message      into, and then falls back to the top-level script.      require ["include", "variables"];      global ["test", "test_mailbox"];      if header :contains "Subject" "${test}"      {          set "test_mailbox" "spam-${test}";      }3.4.2.  Variables Namespace global   In addition to the "global" command, this document defines the   variables namespace "global", in accordance with the "variables"   extension document[RFC5229], Section 3.  The "global" namespace has   no sub-namespaces (e.g., 'set "global.data.from" "me@example.com";'   is not allowed).  The variable-name part MUST be a valid identifier   (e.g., 'set "global.12" "value";' is not valid because "12" is not a   valid identifier).   Note that the "variables" extension document[RFC5229], Section 3   suggests that extensions should define a namespace that is the same   as its capability string (in this case, "include" rather than   "global").  Nevertheless, references to the "global" namespace   without a prior require statement for the "include" extension MUST   cause an error.   Example:      require ["variables", "include"];      set "global.i_am_on_vacation" "1";   Variables declared global and variables accessed via the "global"   namespace MUST each be one and the same.  In the following example   script, we see the variable "i_am_on_vacation" used in a "global"   command, and again with the "global" namespace.  Consider these as   two syntaxes with identical meaning.Daboo & Stone                Standards Track                   [Page 10]

RFC 6609                Sieve Extension: Include                May 2012   Example:      require ["variables", "include", "vacation"];      global "i_am_on_vacation";      set "global.i_am_on_vacation" "1";      if string :is "${i_am_on_vacation}" "1"      {          vacation "It's true, I am on vacation.";      }3.5.  Interaction with Other Extensions   When "include" is used with the "editheader" extension [RFC5293], any   changes made to headers in a script MUST be propagated both to and   from included scripts.  By way of example, if a script deletes one   header and adds another, then includes a second script, the included   script MUST NOT see the removed header, and MUST see the added   header.  Likewise, if the included script adds or removes a header,   upon returning to the including script, subsequent actions MUST see   the added headers and MUST NOT see the removed headers.   When "include" is used with the MIME extension [RFC5703]   "foreverypart" control structure, the included script MUST be   presented with the current MIME part as though it were the entire   message.  A script SHALL NOT have any special control over the   control structure it was included from.  The "break" command in an   included script is not valid on its own and may not terminate a   "foreverypart" iteration in another script.  The included script can   use "return" to transfer control back to the including script.  A   global variable can be used to convey results to the including   script.  A "stop" in an included script, even within a "foreverypart"   loop, still halts all script execution, perSection 3.2.   When "include" is used with the "reject" extension [RFC5429], calling   "reject" or "ereject" at any time sets the reject action on the   message, and continues script execution.  Apropos of the MIME   extension, if an included script sees only a portion of the message   and calls a reject, it is the entire message and not the single MIME   part that carries the rejection.Daboo & Stone                Standards Track                   [Page 11]

RFC 6609                Sieve Extension: Include                May 20124.  Security Considerations   Sieve implementations MUST ensure adequate security for the global   script repository to prevent unauthorized changes to global scripts.   For example, a site policy might enable only certain users with   administrative privileges to modify the global scripts.  Sites are   advised against allowing all users to have write access to the sites'   global scripts.   Sieve implementations MUST ensure that script names are checked for   validity and proper permissions prior to inclusion, in order to   prevent a malicious user from gaining access to files accessible to   the mail server software that should not be accessible to the user.   Sieve implementations MUST ensure that script names are safe for use   with their storage system.  An error MUST be generated either when   the script is uploaded or at execution time for a script including a   name that could be used as a vector to attack the storage system.  By   way of example, the following include commands should be considered   hostile: 'include "./../..//etc/passwd"', 'include "foo$(`rm   star`)"'.   Beyond these, the "include" extension does not raise any security   considerations that are not discussed in the base Sieve [RFC5228]   document and the "variables" extension document [RFC5229].5.  IANA Considerations   The following template specifies the IANA registration of the Sieve   extension specified in this document:      To: iana@iana.org      Subject: Registration of new Sieve extension      Capability name: include      Description:     adds the "include" command to execute other Sieve                       scripts, the "return" action from an included                       script, and the "global" command and "global"                       variables namespace to access variables shared                       among included scripts.      RFC number:      this RFC      Contact address: the Sieve discussion list <sieve@ietf.org>   This information has been added to IANA's "Sieve Extensions" registry   (http://www.iana.org).Daboo & Stone                Standards Track                   [Page 12]

RFC 6609                Sieve Extension: Include                May 20126.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC5228]  Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An Email              Filtering Language",RFC 5228, January 2008.   [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",RFC 5229, January 2008.   [RFC5804]  Melnikov, A., Ed., and T. Martin, "A Protocol for Remotely              Managing Sieve Scripts",RFC 5804, July 2010.6.2.  Informative References   [RFC5293]  Degener, J. and P. Guenther, "Sieve Email Filtering:              Editheader Extension",RFC 5293, August 2008.   [RFC5429]  Stone, A., Ed., "Sieve Email Filtering: Reject and              Extended Reject Extensions",RFC 5429, March 2009.   [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part              Tests, Iteration, Extraction, Replacement, and Enclosure",RFC 5703, October 2009.Daboo & Stone                Standards Track                   [Page 13]

RFC 6609                Sieve Extension: Include                May 2012Appendix A.  Acknowledgments   Thanks to Stephan Bosch, Ned Freed, Arnt Gulbrandsen, Tony Hansen,   Kjetil Torgrim Homme, Jeffrey Hutzelman, Barry Leiba, Alexey   Melnikov, Ken Murchison, Marc Mutz, and Rob Siemborski, for comments   and corrections.Authors' Addresses   Cyrus Daboo   Apple Inc.   1 Infinite Loop   Cupertino, CA  95014   USA   EMail: cyrus@daboo.name   URI:http://www.apple.com/   Aaron Stone   Serendipity   1817 California St. #104   San Francisco, CA  94109   USA   EMail: aaron@serendipity.cxDaboo & Stone                Standards Track                   [Page 14]