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yuuji@0 7 Network Working Group M. Gahrns
yuuji@0 8 Request for Comments: 2221 Microsoft
yuuji@0 9 Category: Standards Track October 1997
yuuji@0 10
yuuji@0 11
yuuji@0 12 IMAP4 Login Referrals
yuuji@0 13
yuuji@0 14 Status of this Memo
yuuji@0 15
yuuji@0 16 This document specifies an Internet standards track protocol for the
yuuji@0 17 Internet community, and requests discussion and suggestions for
yuuji@0 18 improvements. Please refer to the current edition of the "Internet
yuuji@0 19 Official Protocol Standards" (STD 1) for the standardization state
yuuji@0 20 and status of this protocol. Distribution of this memo is unlimited.
yuuji@0 21
yuuji@0 22 Copyright Notice
yuuji@0 23
yuuji@0 24 Copyright (C) The Internet Society (1997). All Rights Reserved.
yuuji@0 25
yuuji@0 26 1. Abstract
yuuji@0 27
yuuji@0 28 When dealing with large amounts of users and many IMAP4 [RFC-2060]
yuuji@0 29 servers, it is often necessary to move users from one IMAP4 server to
yuuji@0 30 another. For example, hardware failures or organizational changes
yuuji@0 31 may dictate such a move.
yuuji@0 32
yuuji@0 33 Login referrals allow clients to transparently connect to an
yuuji@0 34 alternate IMAP4 server, if their home IMAP4 server has changed.
yuuji@0 35
yuuji@0 36 A referral mechanism can provide efficiencies over the alternative
yuuji@0 37 'proxy method', in which the local IMAP4 server contacts the remote
yuuji@0 38 server on behalf of the client, and then transfers the data from the
yuuji@0 39 remote server to itself, and then on to the client. The referral
yuuji@0 40 mechanism's direct client connection to the remote server is often a
yuuji@0 41 more efficient use of bandwidth, and does not require the local
yuuji@0 42 server to impersonate the client when authenticating to the remote
yuuji@0 43 server.
yuuji@0 44
yuuji@0 45 2. Conventions used in this document
yuuji@0 46
yuuji@0 47 In examples, "C:" and "S:" indicate lines sent by the client and
yuuji@0 48 server respectively.
yuuji@0 49
yuuji@0 50 A home server, is an IMAP4 server that contains the user's inbox.
yuuji@0 51
yuuji@0 52 A remote server is a server that contains remote mailboxes.
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yuuji@0 58 Gahrns Standards Track [Page 1]
yuuji@0 59
yuuji@0 60 RFC 2221 IMAP4 Login Referrals October 1997
yuuji@0 61
yuuji@0 62
yuuji@0 63 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
yuuji@0 64 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
yuuji@0 65 document are to be interpreted as described in [RFC-2119].
yuuji@0 66
yuuji@0 67 3. Introduction and Overview
yuuji@0 68
yuuji@0 69 IMAP4 servers that support this extension MUST list the keyword
yuuji@0 70 LOGIN-REFERRALS in their CAPABILITY response. No client action is
yuuji@0 71 needed to invoke the LOGIN-REFERRALS capability in a server.
yuuji@0 72
yuuji@0 73 A LOGIN-REFERRALS capable IMAP4 server SHOULD NOT return a referral
yuuji@0 74 to a server that will return a referral. A client MUST NOT follow
yuuji@0 75 more than 10 levels of referral without consulting the user.
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yuuji@0 77 A LOGIN-REFERRALS response code MUST contain as an argument a valid
yuuji@0 78 IMAP server URL as defined in [IMAP-URL].
yuuji@0 79
yuuji@0 80 A home server referral consists of either a tagged NO or OK, or an
yuuji@0 81 untagged BYE response that contains a LOGIN-REFERRALS response code.
yuuji@0 82
yuuji@0 83 Example: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/] Remote Server
yuuji@0 84
yuuji@0 85 NOTE: user;AUTH=* is specified as required by [IMAP-URL] to avoid a
yuuji@0 86 client falling back to anonymous login.
yuuji@0 87
yuuji@0 88 4. Home Server Referrals
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yuuji@0 90 A home server referral may be returned in response to an AUTHENTICATE
yuuji@0 91 or LOGIN command, or it may appear in the connection startup banner.
yuuji@0 92 If a server returns a home server referral in a tagged NO response,
yuuji@0 93 that server does not contain any mailboxes that are accessible to the
yuuji@0 94 user. If a server returns a home server referral in a tagged OK
yuuji@0 95 response, it indicates that the user's personal mailboxes are
yuuji@0 96 elsewhere, but the server contains public mailboxes which are
yuuji@0 97 readable by the user. After receiving a home server referral, the
yuuji@0 98 client can not make any assumptions as to whether this was a
yuuji@0 99 permanent or temporary move of the user.
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yuuji@0 101 4.1. LOGIN and AUTHENTICATE Referrals
yuuji@0 102
yuuji@0 103 An IMAP4 server MAY respond to a LOGIN or AUTHENTICATE command with a
yuuji@0 104 home server referral if it wishes to direct the user to another IMAP4
yuuji@0 105 server.
yuuji@0 106
yuuji@0 107 Example: C: A001 LOGIN MIKE PASSWORD
yuuji@0 108 S: A001 NO [REFERRAL IMAP://MIKE@SERVER2/] Specified user
yuuji@0 109 is invalid on this server. Try SERVER2.
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yuuji@0 113
yuuji@0 114 Gahrns Standards Track [Page 2]
yuuji@0 115
yuuji@0 116 RFC 2221 IMAP4 Login Referrals October 1997
yuuji@0 117
yuuji@0 118
yuuji@0 119 Example: C: A001 LOGIN MATTHEW PASSWORD
yuuji@0 120 S: A001 OK [REFERRAL IMAP://MATTHEW@SERVER2/] Specified
yuuji@0 121 user's personal mailboxes located on Server2, but
yuuji@0 122 public mailboxes are available.
yuuji@0 123
yuuji@0 124 Example: C: A001 AUTHENTICATE GSSAPI
yuuji@0 125 <authentication exchange>
yuuji@0 126 S: A001 NO [REFERRAL IMAP://user;AUTH=GSSAPI@SERVER2/]
yuuji@0 127 Specified user is invalid on this server. Try
yuuji@0 128 SERVER2.
yuuji@0 129
yuuji@0 130 4.2. BYE at connection startup referral
yuuji@0 131
yuuji@0 132 An IMAP4 server MAY respond with an untagged BYE and a REFERRAL
yuuji@0 133 response code that contains an IMAP URL to a home server if it is not
yuuji@0 134 willing to accept connections and wishes to direct the client to
yuuji@0 135 another IMAP4 server.
yuuji@0 136
yuuji@0 137 Example: S: * BYE [REFERRAL IMAP://user;AUTH=*@SERVER2/] Server not
yuuji@0 138 accepting connections. Try SERVER2
yuuji@0 139
yuuji@0 140 5. Formal Syntax
yuuji@0 141
yuuji@0 142 The following syntax specification uses the augmented Backus-Naur
yuuji@0 143 Form (BNF) as described in [ABNF].
yuuji@0 144
yuuji@0 145 This amends the "resp_text_code" element of the IMAP4 grammar
yuuji@0 146 described in [RFC-2060]
yuuji@0 147
yuuji@0 148 resp_text_code =/ "REFERRAL" SPACE <imapurl>
yuuji@0 149 ; See [IMAP-URL] for definition of <imapurl>
yuuji@0 150 ; See [RFC-2060] for base definition of resp_text_code
yuuji@0 151
yuuji@0 152 6. Security Considerations
yuuji@0 153
yuuji@0 154 The IMAP4 login referral mechanism makes use of IMAP URLs, and as
yuuji@0 155 such, have the same security considerations as general internet URLs
yuuji@0 156 [RFC-1738], and in particular IMAP URLs [IMAP-URL].
yuuji@0 157
yuuji@0 158 A server MUST NOT give a login referral if authentication for that
yuuji@0 159 user fails. This is to avoid revealing information about the user's
yuuji@0 160 account to an unauthorized user.
yuuji@0 161
yuuji@0 162 With the LOGIN-REFERRALS capability, it is potentially easier to
yuuji@0 163 write a rogue 'password catching' server that collects login data and
yuuji@0 164 then refers the client to their actual IMAP4 server. Although
yuuji@0 165 referrals reduce the effort to write such a server, the referral
yuuji@0 166 response makes detection of the intrusion easier.
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yuuji@0 170 Gahrns Standards Track [Page 3]
yuuji@0 171
yuuji@0 172 RFC 2221 IMAP4 Login Referrals October 1997
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yuuji@0 175 7. References
yuuji@0 176
yuuji@0 177 [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
yuuji@0 178 4rev1", RFC 2060, December 1996.
yuuji@0 179
yuuji@0 180 [IMAP-URL], Newman, C., "IMAP URL Scheme", RFC 2192, Innosoft,
yuuji@0 181 September 1997.
yuuji@0 182
yuuji@0 183 [RFC-1738], Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
yuuji@0 184 Resource Locators (URL)", RFC 1738, December 1994.
yuuji@0 185
yuuji@0 186 [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
yuuji@0 187 Requirement Levels", RFC 2119, March 1997.
yuuji@0 188
yuuji@0 189 [ABNF], DRUMS working group, Dave Crocker Editor, "Augmented BNF for
yuuji@0 190 Syntax Specifications: ABNF", Work in Progress.
yuuji@0 191
yuuji@0 192 8. Acknowledgments
yuuji@0 193
yuuji@0 194 Many valuable suggestions were received from private discussions and
yuuji@0 195 the IMAP4 mailing list. In particular, Raymond Cheng, Mark Crispin,
yuuji@0 196 Mark Keasling Chris Newman and Larry Osterman made significant
yuuji@0 197 contributions to this document.
yuuji@0 198
yuuji@0 199 9. Author's Address
yuuji@0 200
yuuji@0 201 Mike Gahrns
yuuji@0 202 Microsoft
yuuji@0 203 One Microsoft Way
yuuji@0 204 Redmond, WA, 98072
yuuji@0 205
yuuji@0 206 Phone: (206) 936-9833
yuuji@0 207 EMail: mikega@microsoft.com
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yuuji@0 226 Gahrns Standards Track [Page 4]
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yuuji@0 228 RFC 2221 IMAP4 Login Referrals October 1997
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yuuji@0 231 10. Full Copyright Statement
yuuji@0 232
yuuji@0 233 Copyright (C) The Internet Society (1997). All Rights Reserved.
yuuji@0 234
yuuji@0 235 This document and translations of it may be copied and furnished to
yuuji@0 236 others, and derivative works that comment on or otherwise explain it
yuuji@0 237 or assist in its implmentation may be prepared, copied, published
yuuji@0 238 andand distributed, in whole or in part, without restriction of any
yuuji@0 239 kind, provided that the above copyright notice and this paragraph are
yuuji@0 240 included on all such copies and derivative works. However, this
yuuji@0 241 document itself may not be modified in any way, such as by removing
yuuji@0 242 the copyright notice or references to the Internet Society or other
yuuji@0 243 Internet organizations, except as needed for the purpose of
yuuji@0 244 developing Internet standards in which case the procedures for
yuuji@0 245 copyrights defined in the Internet Standards process must be
yuuji@0 246 followed, or as required to translate it into languages other than
yuuji@0 247 English.
yuuji@0 248
yuuji@0 249 The limited permissions granted above are perpetual and will not be
yuuji@0 250 revoked by the Internet Society or its successors or assigns.
yuuji@0 251
yuuji@0 252 This document and the information contained herein is provided on an
yuuji@0 253 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
yuuji@0 254 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
yuuji@0 255 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
yuuji@0 256 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
yuuji@0 257 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
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yuuji@0 282 Gahrns Standards Track [Page 5]
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