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Network Working Group                                           M. GahrnsRequest for Comments: 2221                                      MicrosoftCategory: Standards Track                                    October 1997IMAP4 Login ReferralsStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1997).  All Rights Reserved.1. Abstract   When dealing with large amounts of users and many IMAP4 [RFC-2060]   servers, it is often necessary to move users from one IMAP4 server to   another.  For example, hardware failures or organizational changes   may dictate such a move.   Login referrals allow clients to transparently connect to an   alternate IMAP4 server, if their home IMAP4 server has changed.   A referral mechanism can provide efficiencies over the alternative   'proxy method', in which the local IMAP4 server contacts the remote   server on behalf of the client, and then transfers the data from the   remote server to itself, and then on to the client.  The referral   mechanism's direct client connection to the remote server is often a   more efficient use of bandwidth, and does not require the local   server to impersonate the client when authenticating to the remote   server.2. Conventions used in this document   In examples, "C:" and "S:" indicate lines sent by the client and   server respectively.   A home server, is an IMAP4 server that contains the user's inbox.   A remote server is a server that contains remote mailboxes.Gahrns                      Standards Track                     [Page 1]

RFC 2221                 IMAP4 Login Referrals              October 1997   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 [RFC-2119].3. Introduction and Overview   IMAP4 servers that support this extension MUST list the keyword   LOGIN-REFERRALS in their CAPABILITY response.  No client action is   needed to invoke the LOGIN-REFERRALS capability in a server.   A LOGIN-REFERRALS capable IMAP4 server SHOULD NOT return a referral   to a server that will return a referral. A client MUST NOT follow   more than 10 levels of referral without consulting the user.   A LOGIN-REFERRALS response code MUST contain as an argument a valid   IMAP server URL as defined in [IMAP-URL].   A home server referral consists of either a tagged NO or OK, or an   untagged BYE response that contains a LOGIN-REFERRALS response code.   Example: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/] Remote Server   NOTE: user;AUTH=* is specified as required by [IMAP-URL] to avoid a   client falling back to anonymous login.4. Home Server Referrals   A home server referral may be returned in response to an AUTHENTICATE   or LOGIN command, or it may appear in the connection startup banner.   If a server returns a home server referral in a tagged NO response,   that server does not contain any mailboxes that are accessible to the   user.  If a server returns a home server referral in a tagged OK   response, it indicates that the user's personal mailboxes are   elsewhere, but the server contains public mailboxes which are   readable by the user.  After receiving a home server referral, the   client can not make any assumptions as to whether this was a   permanent or temporary move of the user.4.1.  LOGIN and AUTHENTICATE Referrals   An IMAP4 server MAY respond to a LOGIN or AUTHENTICATE command with a   home server referral if it wishes to direct the user to another IMAP4   server.   Example:  C: A001 LOGIN MIKE PASSWORD             S: A001 NO [REFERRAL IMAP://MIKE@SERVER2/] Specified user                     is invalid on this server. Try SERVER2.Gahrns                      Standards Track                     [Page 2]

RFC 2221                 IMAP4 Login Referrals              October 1997   Example:  C: A001 LOGIN MATTHEW PASSWORD             S: A001 OK [REFERRAL IMAP://MATTHEW@SERVER2/] Specified                     user's personal mailboxes located on Server2, but                     public mailboxes are available.   Example:  C: A001 AUTHENTICATE GSSAPI             <authentication exchange>             S: A001 NO [REFERRAL IMAP://user;AUTH=GSSAPI@SERVER2/]                     Specified user is invalid on this server. Try                     SERVER2.4.2. BYE at connection startup referral   An IMAP4 server MAY respond with an untagged BYE and a REFERRAL   response code that contains an IMAP URL to a home server if it is not   willing to accept connections and wishes to direct the client to   another IMAP4 server.   Example:  S: * BYE [REFERRAL IMAP://user;AUTH=*@SERVER2/] Server not                  accepting connections.  Try SERVER25. Formal Syntax   The following syntax specification uses the augmented Backus-Naur   Form (BNF) as described in [ABNF].   This amends the "resp_text_code" element of the IMAP4 grammar   described in [RFC-2060]   resp_text_code =/ "REFERRAL" SPACE <imapurl>      ; See [IMAP-URL] for definition of <imapurl>      ; See [RFC-2060] for base definition of resp_text_code6. Security Considerations   The IMAP4 login referral mechanism makes use of IMAP URLs, and as   such, have the same security considerations as general internet URLs   [RFC-1738], and in particular IMAP URLs [IMAP-URL].   A server MUST NOT give a login referral if authentication for that   user fails. This is to avoid revealing information about the user's   account to an unauthorized user.   With the LOGIN-REFERRALS capability, it is potentially easier to   write a rogue 'password catching' server that collects login data and   then refers the client to their actual IMAP4 server.  Although   referrals reduce the effort to write such a server, the referral   response makes detection of the intrusion easier.Gahrns                      Standards Track                     [Page 3]

RFC 2221                 IMAP4 Login Referrals              October 19977. References   [RFC-2060], Crispin, M., "Internet Message Access Protocol - Version   4rev1",RFC 2060, December 1996.   [IMAP-URL], Newman, C., "IMAP URL Scheme",RFC 2192, Innosoft,   September 1997.   [RFC-1738], Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform   Resource Locators  (URL)",RFC 1738, December 1994.   [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate   Requirement Levels",RFC 2119, March 1997.   [ABNF], DRUMS working group, Dave Crocker Editor, "Augmented BNF for   Syntax Specifications: ABNF", Work in Progress.8.  Acknowledgments   Many valuable suggestions were received from private discussions and   the IMAP4 mailing list.  In particular, Raymond Cheng, Mark Crispin,   Mark Keasling Chris Newman and Larry Osterman made significant   contributions to this document.9. Author's Address   Mike Gahrns   Microsoft   One Microsoft Way   Redmond, WA, 98072   Phone: (206) 936-9833   EMail: mikega@microsoft.comGahrns                      Standards Track                     [Page 4]

RFC 2221                 IMAP4 Login Referrals              October 199710.  Full Copyright Statement   Copyright (C) The Internet Society (1997).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implmentation may be prepared, copied, published   andand distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."Gahrns                      Standards Track                     [Page 5]