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 (C) The Internet Society (1997). All Rights Reserved.
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
2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
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
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.