Network Working Group M. Gahrns
Request for Comments: 2221 Microsoft
Category: Standards Track October 1997
IMAP4 Login Referrals
Status 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.
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.
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 SERVER2
5. 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_code
6. 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.
7. References
[RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC2060, December 1996.
[IMAP-URL], Newman, C., "IMAP URL Scheme", RFC2192, Innosoft,
September 1997.
[RFC-1738], Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
Resource Locators (URL)", RFC1738, December 1994.
[RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, 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.com
10. Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights Reserved.
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