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rfc:rfc6609

Internet Engineering Task Force (IETF) C. Daboo Request for Comments: 6609 Apple, Inc. Category: Standards Track A. Stone ISSN: 2070-1721 Serendipity

                                                              May 2012
              Sieve Email Filtering: Include Extension

Abstract

 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 in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://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 to BCP 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 2012

Table of Contents

 1. Introduction and Overview .......................................2
 2. Conventions Used in This Document ...............................2
 3. Include Extension ...............................................3
    3.1. General Considerations .....................................3
    3.2. Control Structure "include" ................................4
    3.3. Control Structure "return" .................................7
    3.4. Interaction with the "variables" Extension .................8
         3.4.1. Control Structure "global" ..........................8
         3.4.2. Variables Namespace global .........................10
    3.5. Interaction with Other Extensions .........................11
 4. Security Considerations ........................................12
 5. IANA Considerations ............................................12
 6. References .....................................................13
    6.1. Normative References ......................................13
    6.2. Informative References ....................................13
 Appendix A. Acknowledgments .......................................14

1. 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 Extension

3.1. 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 in Section 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 2012

3.4. 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, per Section 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 2012

4. 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 2012

6. References

6.1. 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 2012

Appendix 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.cx

Daboo & Stone Standards Track [Page 14]

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