Sieve Email Filtering: Delivering to Special-Use Mailboxes
draft-ietf-extra-sieve-special-use-03
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8579.
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Author | Stephan Bosch | ||
Last updated | 2018-09-26 (Latest revision 2018-09-05) | ||
Replaces | draft-bosch-sieve-special-use | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Reviews | |||
Additional resources | Mailing list discussion | ||
Stream | WG state | In WG Last Call | |
Document shepherd | (None) | ||
IESG | IESG state | Became RFC 8579 (Proposed Standard) | |
Consensus boilerplate | Unknown | ||
Telechat date | (None) | ||
Responsible AD | (None) | ||
Send notices to | (None) |
draft-ietf-extra-sieve-special-use-03
EXTRA S. Bosch Internet-Draft Dovecot Oy Intended status: Standards Track September 5, 2018 Expires: March 9, 2019 Sieve Email Filtering: Delivering to Special-Use Mailboxes draft-ietf-extra-sieve-special-use-03 Abstract The SPECIAL-USE capability of the IMAP protocol (RFC 6154) allows clients to identify special-use mailboxes; e.g., where draft or sent messages should be put. This simplifies client configuration. In contrast, the Sieve mail filtering language (RFC 5228) currently has no such capability. This memo defines a Sieve extension that fills this gap: it adds a test for checking whether a special-use attribute is assigned for a particular mailbox or any mailbox, and it adds the ability to file messages into an anonymous mailbox that has a particular special-use attribute assigned. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on March 9, 2019. Copyright Notice Copyright (c) 2018 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 Bosch Expires March 9, 2019 [Page 1] Internet-Draft Sieve: Special-Use Mailboxes September 2018 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Test "specialuse_exists" . . . . . . . . . . . . . . . . . . 3 4. ":specialuse" Argument to "fileinto" Command . . . . . . . . 4 4.1. Mailboxes Created Implicitly by the "fileinto" Command . 5 5. Sieve Capability Strings . . . . . . . . . . . . . . . . . . 6 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction Commonly, several mailboxes in an IMAP message store [IMAP] have a special use; e.g. it is where the user's draft messages are stored, where a copy of sent messages are kept, or it is where spam messages are filed automatically at delivery. The SPECIAL-USE capability [SPECIAL-USE] of the IMAP protocol defines mailbox attributes that identify these special mailboxes explicitly to the client. This way, client configuration is simplified significantly. Using the CREATE- SPECIAL-USE capability [SPECIAL-USE], IMAP clients can also configure these attributes dynamically based on user preference. Unlike the IMAP protocol, the Sieve mail filtering language [SIEVE] currently cannot freely access these special-use mailbox attributes. Particularly, the Sieve interpreter cannot find an anonymous mailbox that has a particular special-use attribute assigned. This would be very useful for example to find the user's Spam mailbox at delivery. In Sieve, limited access to the special-use attributes is provided using the "mboxmetadata" extension [SIEVE-MAILBOX], which allows testing for the presence of a special-use attribute in the "/private/ specialuse" IMAP METADATA [IMAP-METADATA] entry of a mailbox. Still, not all implementers will be willing to add the complexity of the IMAP METADATA capability, just to provide access to special-use attributes to the Sieve interpreter. Bosch Expires March 9, 2019 [Page 2] Internet-Draft Sieve: Special-Use Mailboxes September 2018 This document defines an extension to the Sieve mail filtering language that adds the ability to freely access mailbox special-use attributes. It adds a test called "specialuse_exists" that checks whether a special-use attribute is assigned for a particular mailbox or - if omitted - any of the user's personal mailboxes. It also adds the ability to file messages into an anonymous personal mailbox that has a particular special-use attribute assigned using a ":specialuse" argument for the "fileinto" command [SIEVE]. 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 [KEYWORDS]. Conventions for notations are as in [SIEVE] Section 1.1, including use of the "Usage:" label for the definition of action and tagged arguments syntax. 3. Test "specialuse_exists" Usage: specialuse_exists [<mailbox: string>] <special-use-flags: string-list> If the "mailbox" string argument is omitted, the "specialuse_exists" test yields true if all of the following statements are true for each of the special-use flags listed in the "special-use-flags" argument: a. at least one mailbox exists in the user's personal namespace [NAMESPACE] that has that particular special-use flag assigned, and b. that mailbox allows the user in whose context the Sieve script runs to "deliver" messages into it. If the "mailbox" argument is specified, the "specialuse_exists" test yields true if all of the following statements are true: a. the indicated mailbox exists, b. that mailbox allows the user in whose context the Sieve script runs to "deliver" messages into it, and c. that mailbox has all of the special-use flags listed in the "special-use-flags" argument assigned to it. Bosch Expires March 9, 2019 [Page 3] Internet-Draft Sieve: Special-Use Mailboxes September 2018 Refer to the specification of the "mailboxexists" test in Section 3.1 of RFC 5490 [SIEVE-MAILBOX] for a definition of when "delivery" of messages into a mailbox is deemed possible. 4. ":specialuse" Argument to "fileinto" Command Usage: fileinto [:specialuse <special-use-flag: string>] <mailbox: string> Normally, the "fileinto" command delivers the message in the mailbox specified using its positional mailbox argument. However, if the optional ":specialuse" argument is also specified, the "fileinto" command first checks whether a mailbox exists in the user's personal namespace [NAMESPACE] with the specified special-use flag assigned to it. If that is the case, that special-use mailbox is used for delivery instead. If there is no such mailbox or if the specified special-use flag is unknown to the implementation in general, the "fileinto" action proceeds as it would without the ":specialuse" argument. Summarizing, if the ":specialuse" argument is specified, the fileinto command deals with two mailboxes that may or may not exist: o An anonymous special-use mailbox in the user's personal namespace, which has at least the special-use flag specified with the ":specialuse" argument assigned to it. o The default mailbox named by the positional string argument of the "fileinto" command, which is used when the special-use mailbox is not found. The special-use flag specified with the ":specialuse" argument MUST conform to the "use-attr" syntax described in Section 6 of RFC6154 [SIEVE-MAILBOX]. Implementations SHOULD handle an invalid special- use flag in the same way as an invalid mailbox name is handled. The string parameter of the ":specialuse" argument is not a constant string, which means that variable substitutions are allowed when the "variables" extension [VARIABLES] is active. In that case, the syntax of the special-use flag is only verified at runtime. If neither the special-use mailbox nor the default mailbox exists, the "fileinto" action MUST proceed exactly as it does in case the ":specialuse" is argument is absent and the mailbox named by its positional argument does not exist. The various options for handling this situation are described in Section 4.1 of RFC5228 [SIEVE]. More than one mailbox in the user's personal namespace can have a particular special-use flag assigned. If one of those mailboxes is Bosch Expires March 9, 2019 [Page 4] Internet-Draft Sieve: Special-Use Mailboxes September 2018 in fact the default mailbox named by the positional string argument of the "fileinto" command, that mailbox MUST be used for delivery. If the default mailbox is not one of the options, the mailbox that is chosen for delivery is implementation-defined. However, while the set of mailboxes to which the involved special-use flags are assigned remains unchanged, implementations SHOULD ensure that the mailbox choice is made consistently, so that the same mailbox is used every time. Conversely, the chosen mailbox MAY change once the special-use flag assignments that are relevant for the mailbox choice are changed (usually by user interaction). If delivery to the special-use mailbox fails for reasons not relating to its existence, the Sieve interpreter MUST NOT subsequently attempt delivery in the indicated default mailbox as a fall-back. Instead, it MUST proceed exactly as it does in case the ":specialuse" argument is absent and delivery to the mailbox named by its positional argument fails. This prevents the situation where messages are unexpectedly spread over two mailboxes in case transient or intermittent delivery failures occur. 4.1. Mailboxes Created Implicitly by the "fileinto" Command Before attempting to deliver the message into the specified mailbox, the "fileinto" command may implicitly create the mailbox if it does not exist (see Section 4.1 of RFC5228 [SIEVE]). This optional behavior can be requested explicitly using the "mailbox" extension [SIEVE-MAILBOX], which adds the optional ":create" argument to the "fileinto" command. If the ":create" argument is specified with "fileinto", it instructs the Sieve interpreter to unconditionally create the specified mailbox if needed. Note that the ":create" argument has no effect when the implicit creation of mailboxes for delivery is the default behavior. When the ":specialuse" argument is present, this behavior does not change: the Sieve interpreter will implicitly create the specified default mailbox if needed. This need arises when both the special- use mailbox and the default mailbox are not found. If the server implementation supports the CREATE-SPECIAL-USE capability [SPECIAL-USE] for IMAP (i.e., it allows assigning special- use flags to new mailboxes) it SHOULD assign the special-use flag specified with the ":specialuse" argument to the newly created mailbox. Bosch Expires March 9, 2019 [Page 5] Internet-Draft Sieve: Special-Use Mailboxes September 2018 5. Sieve Capability Strings A Sieve implementation that defines the "specialuse_exists" test and the ":specialuse" argument for the "fileinto" command will advertise the capability string "special-use". 6. Examples The following example saves the message in the mailbox where messages deemed to be junk mail are held. This mailbox is identified using the "\Junk" special-use attribute. If no mailbox has this attribute assigned, the message is filed into the mailbox named "Spam". If the mailbox named "Spam" does not exist either, the result of this Sieve script is implementation-dependent: e.g., it may trigger an error or the mailbox may be created implicitly. require "fileinto"; require "special-use"; fileinto :specialuse "\\Junk" "Spam"; The following very similar example explicitly handles the case in which neither a "\Junk" special-use mailbox nor the "Spam" mailbox exist. In that case, a mailbox called "Spam" is created, and the message is stored there. Additionally, the "\Junk" special-use attribute may be assigned to it. require "fileinto"; require "special-use"; require "mailbox"; fileinto :specialuse "\\Junk" :create "Spam"; The following example is used in a Sieve script that is triggered from an IMAP event, rather than at message delivery [IMAPSIEVE]. This Sieve script redirects messages to an automated recipient that processes junk mail, if those messages are copied or moved into a mailbox that has the "\Junk" special-use attribute assigned. Bosch Expires March 9, 2019 [Page 6] Internet-Draft Sieve: Special-Use Mailboxes September 2018 require "imapsieve"; require "special-use"; require "environment"; require "variables"; if environment :contains "imap.mailbox" "*" { set "mailbox" "${1}"; } if allof( environment "imap.cause" "COPY", specialuse_exists "${mailbox}" "\\Junk") { redirect "spam-report@example.org"; } 7. Security Considerations Security considerations are discussed in [SIEVE], [VARIABLES], and [SPECIAL-USE]. It is believed that this extension does not introduce any additional security concerns. Note that this specification explicitly restricts the anonymous special-use mailbox to the user's personal namespace. First, this avoids the need to search the entire mail storage for mailboxes that have a particular special-use flag assigned. This could put undue load on the system, while shared special-use mailboxes are deemed of limited use with the currently defined special-use flags. Secondly, it prevents security concerns with shared mailboxes that have special-use flags assigned that apply to all users. Searching the entire mail storage for special-use mailboxes could lead to messages unexpectedly or even maliciously being filed to shared mailboxes. This restriction could be lifted for particular future special-use flags, but such new flags should have a clear application for shared mailboxes and the security concerns should be considered carefully. 8. IANA Considerations The following template specifies the IANA registration of the Sieve extension specified in this document: Bosch Expires March 9, 2019 [Page 7] Internet-Draft Sieve: Special-Use Mailboxes September 2018 To: iana@iana.org Subject: Registration of new Sieve extension Capability name: special-use Description: adds a test for checking whether an IMAP special-use attribute is assigned for a particular mailbox or any mailbox, and it adds the ability to file messages into an anonymous mailbox that has a particular IMAP special-use attribute assigned. RFC number: this RFC Contact address: Sieve mailing list <sieve@ietf.org> This information should be added to the list of sieve extensions given on http://www.iana.org/assignments/sieve-extensions. 9. Acknowledgements Thanks to Stan Kalisch, Barry Leiba, Alexey Melnikov, Ken Murchison, and Ned Freed for reviews and suggestions. Thanks to the authors of RFC5490 [SIEVE-MAILBOX] from which some descriptive text is borrowed in this document. 10. References 10.1. Normative References [IMAP-METADATA] Daboo, C., "The IMAP METADATA Extension", RFC 5464, DOI 10.17487/RFC5464, February 2009, <http://www.rfc-editor.org/info/rfc5464>. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [NAMESPACE] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, DOI 10.17487/RFC2342, May 1998, <https://www.rfc- editor.org/info/rfc2342>. [SIEVE] Guenther, P. and T. Showalter, "Sieve: An Email Filtering Language", RFC 5228, January 2008. Bosch Expires March 9, 2019 [Page 8] Internet-Draft Sieve: Special-Use Mailboxes September 2018 [SIEVE-MAILBOX] Melnikov, A., "The Sieve Mail-Filtering Language -- Extensions for Checking Mailbox Status and Accessing Mailbox Metadata", RFC 5490, March 2009. [SPECIAL-USE] Leiba, B. and J. Nicolson, "IMAP LIST Extension for Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154, March 2011, <http://www.rfc-editor.org/info/rfc6154>. [VARIABLES] Homme, K., "Sieve Email Filtering: Variables Extension", RFC 5229, January 2008. 10.2. Informative References [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, <http://www.rfc-editor.org/info/rfc3501>. [IMAPSIEVE] Leiba, B., "Support for Internet Message Access Protocol (IMAP) Events in Sieve", RFC 6785, DOI 10.17487/RFC6785, November 2012, <http://www.rfc-editor.org/info/rfc6785>. Author's Address Stephan Bosch Dovecot Oy Lars Sonckin Kaari 12 Espoo 02600 Finland Email: stephan.bosch@dovecot.fi Bosch Expires March 9, 2019 [Page 9]