RFC 2221 (rfc2221) - Page 3 of 5
IMAP4 Login Referrals
Alternative Format: Original Text Document
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
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
; See [IMAP-URL] for definition of
; 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.
Gahrns Standards Track