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Melnikov 3 Internet-Draft Isode Limited 4 Intended status: Standards Track July 5, 2009 5 Expires: January 6, 2010 7 Sieve Extension: Externally Stored Lists 8 draft-ietf-sieve-external-lists-00 10 Status of this Memo 12 This Internet-Draft is submitted to IETF in full conformance with the 13 provisions of BCP 78 and BCP 79. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This Internet-Draft will expire on January 6, 2010. 33 Copyright Notice 35 Copyright (c) 2009 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents in effect on the date of 40 publication of this document (http://trustee.ietf.org/license-info). 41 Please review these documents carefully, as they describe your rights 42 and restrictions with respect to this document. 44 Abstract 46 Sieve scripting language can be used for implementing of 47 whitelisting, blacklisting and personal distribution lists. 48 Currently this requires that all members of such lists be hardcoded 49 in the script itself. Whenever a member of such list is added or 50 deleted, the script needs to be updated and possibly uploaded to a 51 mail server. 53 This document defines a Sieve extension for accessing externally 54 stored mailing lists, i.e. list whose members are stored externally 55 to the script, for example in LDAP (RFC 4510), ACAP (RFC 2244) or a 56 relational database. 58 ToDo 60 o Need a way to advertise supported URI schemas in ManageSieve and 61 ihave. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 66 1.1. Conventions used in this document . . . . . . . . . . . . . . 3 68 2. Extlists extension . . . . . . . . . . . . . . . . . . . . . 3 69 2.1. Capability Identifier . . . . . . . . . . . . . . . . . . . . 3 70 2.2. :list tagged argument to the 'address', 'header' and 71 'envelope' tests . . . . . . . . . . . . . . . . . . . . . . 3 72 2.3. :list tagged argument to the 'redirect' action . . . . . . . 4 73 2.4. Syntax of an externally stored list name . . . . . . . . . . 4 74 2.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4 76 3. Security Considerations . . . . . . . . . . . . . . . . . . . 5 78 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 80 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 82 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 83 6.1. Normative References . . . . . . . . . . . . . . . . . . . . 6 84 6.2. Informative References . . . . . . . . . . . . . . . . . . . 7 86 Author's Address . . . . . . . . . . . . . . . . . . . . . . 7 88 1. Introduction 90 This document specified an extension to the Sieve language defined by 91 [Sieve] for checking membership in an externally stored list or for 92 sending messages to a list of recipients stored externally to the 93 Sieve script. 95 This extension adds a new tagged argument to the "header" and 96 "envelope" tests, and to the "redirect" action [Sieve]. 98 1.1. Conventions used in this document 100 Conventions for notations are as in [Sieve] section 1.1, including 101 the use of [ABNF]. 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 105 document are to be interpreted as described in [Kwds]. 107 2. Extlists extension 109 2.1. Capability Identifier 111 The capability string associated with the extension defined in this 112 document is "extlists". 114 2.2. :list tagged argument to the 'address', 'header' and 'envelope' 115 tests 117 Usage: address [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE] [:list] 118 119 121 Usage: header [COMPARATOR] [MATCH-TYPE] [:list] 122 123 125 Usage: envelope [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE] [:list] 126 127 129 The new ":list" tagged argument, if present, changes interpretation 130 of the "key-list" parameter to the 'address'/'header'/'envelope' test 131 to become a list of names of externally stored lists. E.g. the 132 "header" test with the ":list" parameter evaluates to true if the 133 value of any of the named headers, ignoring leading and trailing 134 whitespace, matches any member of one or more externally stored lists 135 of values with names specified in key-list. 137 See Section 2.4 for the detailed description of syntax used for 138 naming externally stored lists. 140 2.3. :list tagged argument to the 'redirect' action 142 Usage: redirect :list 144 The "redirect" action with the ":list" argument is used to send the 145 message to one or more email address stored in the externally stored 146 list 'ext-list-name'. This variant of the redirect command can be 147 used to implement a personal distribution list. 149 See Section 2.4 for the detailed description of syntax used for 150 naming externally stored lists. 152 2.4. Syntax of an externally stored list name 154 A name of an externally stored list is always an absolute URI. 155 Implementations might find URL such as [LDAP], [CardDAV] URL), or 156 [TAG-URI] to be useful for naming external lists. 158 The "tag" URI scheme can be used to represent opaque, but user 159 friendlier identifiers. Resolution of such identifiers is going to 160 be implementation specific and it can help in hiding the complexity 161 of an implementation from end users. For example, an implementation 162 can provide a web interface for managing lists of users stored in 163 LDAP. Requiring users to know generic LDAP URL syntax is not going 164 to be very practical, due to its complexity. However such 165 implementation can use a fixed tag URI prefix such as "tag: 166 example.com,:" (where can be, for example, a date 167 generated once on installation of the web interface and left 168 untouched upon upgrades) instead and the prefix doesn't even need to 169 be shown to end users. 171 2.5. Examples 173 Example 1: 174 require ["extlists"]; 176 # Submission from list members is sent to all members 177 if allof (envelope :detail :list "to" 178 "tag:example.com,2009-05-28:mylist", 179 header :contains :list "from" 180 "tag:example.com,2009-05-28:mylist") { 181 redirect :list "tag:example.com,2009-05-28:mylist"; 182 } 184 3. Security Considerations 186 Security considerations related to the "envelope"/"header" tests and 187 "redirect" action discussed in [Sieve] also apply to this document. 189 A failure to retrieve data due to the server storing the external 190 list membership being down or otherwise inaccessible may alter the 191 result of Sieve processing. So implementations SHOULD treat a 192 temporary failure to retrieve or verify external list membership in 193 the same manner as a temporary failure to retrieve a Sieve script. 194 For example, if the Sieve script is stored in the Lightweight 195 Directory Access Protocol (LDAP) and the script can't be retrieved 196 when a message is processed, then the agent performing Sieve 197 processing can, for example, assume that the script doesn't exist or 198 delay message delivery until the script can be retrieved 199 successfully. External list memberships should be treated as if they 200 are a part of the script itself, so a temporary failure to retrieve 201 them should be handled in the same way as a temporary failure to 202 retrieve the Sieve script itself. 204 Protocols/APIs used to retrieve/verify external list membership MUST 205 provide at least the same level of confidentiality as protocols/APIs 206 used to retrieve Sieve scripts. For example, if Sieve scripts are 207 retrieved using LDAP secured with Transport Layer Security (TLS) 208 encryption, then the protocol used to retrieve external list 209 membership must use a comparable mechanism for providing connection 210 confidentiality. In particular, the protocol used to retrieve 211 external list membership must not be lacking encryption. 213 Implementations of this extensions should keep in mind that matching 214 values against an externally stored list can be IO and/or CPU 215 intensive. This can be used to deny service to the mailserver and/or 216 to servers providing access to externally stored mailing lists. A 217 naive implementation, such as the one that tries to retrieve content 218 of the whole list to perform matching can make this worse. But note 219 that many protocols that can be used for accessing externally stored 220 lists support flexible searching facilities that can be used to 221 minimize network traffic and load on the directory service. For 222 example LDAP allows for search filters. 224 Many organizations support external lists with thousands of 225 recipients. In order to avoid mailbombs, when redirecting a message 226 to an externally stored mailing list, implementations SHOULD enforce 227 limits on the number of recipients and/or on domains to which such 228 recipients belong. 230 4. IANA Considerations 232 The following template specifies the IANA registration of the notify 233 Sieve extension specified in this document: 235 To: iana@iana.org 236 Subject: Registration of new Sieve extension 237 Capability name: extlists 238 Description: adds the ':list' tagged argument to 'address', 'header' 239 and 'envelope' tests, and to the 'redirect' action. The ':list' 240 argument changes address/header/envelope test to match values against 241 values stored in one or more externally stored list. The ':list' 242 argument to the redirect action changes the redirect action to 243 forward the message to email addresses stored in the externally 244 stored list. 245 RFC number: this RFC 246 Contact address: 247 The Sieve discussion list 249 This information should be added to the list of sieve extensions 250 given on http://www.iana.org/assignments/sieve-extensions. 252 5. Acknowledgements 254 Thanks to Alexandros Vellis, Barry Leiba, Nigel Swinson, Kjetil 255 Torgrim Homme, Dave Cridland, Cyrus Daboo, Pete Resnick for ideas, 256 comments and suggestions. 258 6. References 260 6.1. Normative References 262 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 263 Specifications: ABNF", RFC 5234, January 2008. 265 [Kwds] Bradner, S., "Key words for use in RFCs to Indicate 266 Requirement Levels", RFC 2119, March 1997. 268 [Sieve] Guenther, P. and T. Showalter, "Sieve: An Email Filtering 269 Language", RFC 5228, January 2008. 271 [URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 272 Resource Identifier (URI): Generic Syntax", STD 66, 273 RFC 3986, January 2005. 275 6.2. Informative References 277 [ACAP] Newman, C. and J. Myers, "ACAP -- Application 278 Configuration Access Protocol", RFC 2244, November 1997. 280 [CardDAV] Daboo, C., "vCard Extensions to WebDAV (CardDAV)", work in 281 progress, draft-daboo-carddav, May 2007. 283 [LDAP] Zeilenga, K., "Lightweight Directory Access Protocol 284 (LDAP): Technical Specification Road Map", RFC 4510, 285 June 2006. 287 [TAG-URI] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme", 288 RFC 4151, October 2005. 290 Author's Address 292 Alexey Melnikov 293 Isode Limited 294 5 Castle Business Village 295 36 Station Road 296 Hampton, Middlesex TW12 2BX 297 UK 299 Email: Alexey.Melnikov@isode.com