Network Working Group C. Newman
Internet Draft: IMAP URL Scheme Innosoft
Document: draft-newman-url-imap-02.txt November 1996
IMAP URL Scheme
Status of this memo
This document is an Internet Draft. Internet Drafts are working
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Discussion and suggestions for improvement are requested. This
document will expire six months after publication. Distribution of
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1. Introduction
IMAP [IMAP4] is a rich protocol for accessing remote message
stores. It provides an ideal mechanism for accessing public
mailing list archives as well as private and shared message stores.
This document defines a URL scheme for referencing objects on an
IMAP server.
2. IMAP scheme
The IMAP URL scheme is used to designate mailboxes, messages,
files, and search programs on Internet hosts accessible using the
IMAP protocol.
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Internet Draft IMAP URL Scheme November 1996
The IMAP URL follows the common Internet scheme syntax as defined
in RFC 1738 [RFC1738]. If :<port> is omitted, the port defaults to
143.
An IMAP URL takes one of the following forms:
imap://<login>/<enc_list_mailbox>;TYPE=<list_type>[iauth]
imap://<login>/<enc_mailbox>?<enc_search_program>[uidvalidity][iauth]
imap://<login>/<enc_mailbox>[uidvalidity]<iuid>[isection][iauth]
The first form is used to refer to a list of mailboxes, the second
form refers to a list of messages, and the final form refers to a
specific message or message part.
3. IMAP User Name, Password and Authentication mechanism
A user name, password and/or authentication mechanism may be
supplied. They are used in the "LOGIN" or "AUTHENTICATE" commands
after making the connection to the IMAP server. If no user name,
password or authentication mechanism is supplied, the user name
"anonymous" is used with the "LOGIN" command and the password is
supplied as the Internet e-mail address of the end user accessing
the resource. If the URL supplies a user name but no password, the
program interpreting the IMAP URL should request one from the user
if necessary.
An authentication mechanism can be expressed by adding
";AUTH=<enc_auth_type>" to the end of the URL. When such an
<enc_auth_type> is indicated, the client should request appropriate
credentials from that mechanism and use the "AUTHENTICATE" command
instead of the "LOGIN" command. If no user name is specified, one
should be obtained from the mechanism or requested from the user as
appropriate.
The string ";AUTH=*" indicates that the client should select an
appropriate authentication mechanism. It MAY use any mechanism
listed in the CAPABILITY command or use an out of band security
service resulting in a PREAUTH connection. If no user name is
specified and no appropriate authentication mechanisms are
available, the client should fall back to anonymous login as
described above. This allows a URL which grants read-write access
to authorized users, and read-only anonymous access to other users.
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4. Lists of mailboxes
An IMAP URL referring to a list of mailboxes has the following
form:
imap://<login>/<enc_list_mailbox>;TYPE=<list_type>[iauth]
The <list_type> may be either "LIST" or "LSUB", and is case
insensitive. The field ";TYPE=<list_type>" MUST be included.
The <enc_list_mailbox> is any argument suitable for the
list_mailbox field of the IMAP [IMAP4] LIST or LSUB commands. The
field <enc_list_mailbox> may be omitted, in which case the program
interpreting the IMAP URL may use "*" or "%" as the
<enc_list_mailbox>. The program should use "%" if it supports a
hierarchical view, otherwise it should use "*".
Note that if unsafe or reserved characters such as " " or "%" are
present in <enc_list_mailbox> they must be encoded as defined in
RFC 1738 [RFC1738].
5. Lists of messages
An IMAP URL referring to a list of messages has the following form:
imap://<login>/<enc_mailbox>?<enc_search_program>[uidvalidity][iauth]
The <enc_mailbox> field is used as the argument to the IMAP4
"SELECT" command. Note that if unsafe or reserved characters such
as " ", ";", or "?" are present in <enc_mailbox> they must be
encoded as defined in RFC 1738 [RFC1738].
The [uidvalidity] field is optional. If it is present, it must be
the argument to the IMAP4 UIDVALIDITY status response at the time
the URL was created. This MAY be used by the program interpreting
the IMAP URL to determine if the URL is stale.
The "?<enc_search_program>" field is optional. If it is not
present, a list of all messages in the mailbox should be presented
by the program interpreting the URL. If it is present, it should
be used as the arguments following an IMAP4 SEARCH command with
unsafe characters such as " " (which are likely to be present in
the <enc_search_program>) encoded as defined in RFC 1738 [RFC1738].
6. A specific message or message part
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An IMAP URL referring to a specific message or message part has the
following form:
imap://<login>/<enc_mailbox>[uidvalidity]<iuid>[isection][iauth]
The <enc_mailbox> and [uidvalidity] are as defined above.
If [uidvalidity] is present in this form, it SHOULD be used by the
program interpreting the URL to determine if the URL is stale.
The <uid> refers to an IMAP4 message UID, and should be used as the
<set> argument to the IMAP4 "UID FETCH" command.
The [isection] field is optional. If not present, the URL refers
to the entire RFC 822 message as returned by the IMAP command "UID
FETCH <uid> RFC822.PEEK". If present, the URL refers to the object
returned by a "UID FETCH <uid> BODY.PEEK[<section>]" command. The
type of the object may be determined with a "UID FETCH <uid>
BODYSTRUCTURE" command and locating the appropriate part in the
resulting BODYSTRUCTURE. Note that unsafe characters in
[isection], such as " " must be encoded as defined in RFC 1738
[RFC1738].
7. Examples
The following examples demonstrate how an IMAP4 client program
might translate various IMAP4 URL into a series of IMAP4 commands.
Commands sent from the client to the server are prefixed with "C:",
and responses sent from the server to the client are prefixed with
"S:".
The URL:
<imap://minbari.org/grey-council;UIDVALIDITY=3857529045;UID=20>
Results in the following client commands:
<connect to minbari.org, port 143>
C: A001 LOGIN ANONYMOUS sheridan@babylon5.org
C: A002 SELECT grey-council
<client verifies the UIDVALIDITY matches>
C: A003 UID FETCH 20 RFC822.PEEK
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The URL:
<imap://michael@minbari.org/users.*;type=list>
Results in the following client commands:
<client requests password from user>
<connect to minbari.org, port 143>
C: A001 LOGIN MICHAEL zipper
C: A002 LIST "" users.*
The URL:
<imap://minbari.org/grey-council;uid=20;section=1.2;auth=KERBEROS_V4>
Results in the following client commands:
<connect to minbari.org, port 143>
C: A001 AUTHENTICATE KERBEROS_V4
<authentication exchange>
C: A002 SELECT grey-council
C: A003 UID FETCH 20 BODY.PEEK[1.2]
The URL:
<imap://minbari.org/grey%20council?SUBJECT%20shadows;AUTH=*>
Could result in the following:
<connect to minbari.org, port 143>
C: A001 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI
S: A001 OK
C: A002 AUTHENTICATE GSSAPI
<authentication exchange>
S: A002 OK user lennier authenticated
C: A003 SELECT "grey council"
...
C: A004 SEARCH SUBJECT shadows
S: * SEARCH 8 10 13 14 15 16
S: A004 OK SEARCH completed
C: A005 FETCH 8,10,13:16 ALL
...
NOTE: In this final example, the client has implementation dependant
choices. The authentication mechanism could be anything, including
PREAUTH. And the final FETCH command could fetch more or less
information about the messages, depending on what it wishes to display
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to the user.
8. ABNF for IMAP URL scheme
This uses ABNF as used in the IMAP specification [IMAP4].
Terminals from the BNF for URLs [RFC1738] are also used. Strings
are not case sensitive.
encoded_char ::= uchar / ":" / "@" / "&" / "=" / "/"
; see RFC 1738 for "uchar" definition
enc_auth_type ::= 1*encoded_char
; further defined in RFC 1730 "auth_type"
enc_list_mailbox ::= *encoded_char
; further defined in RFC 1730 "list_mailbox"
enc_mailbox ::= 1*encoded_char
; further defined in RFC 1730 "mailbox"
enc_search_program
::= 1*encoded_char
; encoded version of search_program below
imapurl ::= "imap://" login "/" [ icommand ] [ iauth ]
; see RFC 1738 for "login" terminal
iauth ::= ";AUTH=" ( "*" / enc_auth_type )
icommand ::= imailboxlist / ipath / isearch
imailboxlist ::= enc_list_mailbox [ ";TYPE=" list_type ]
ipath ::= enc_mailbox [uidvalidity] iuid [isection]
isearch ::= enc_mailbox [ "?" enc_search_program ] [uidvalidity]
isection ::= ";SECTION=" section
; See RFC 1730 for "section" definition
iuid ::= ";UID=" nz_number
; See RFC 1730 for "nz_number" definition
list_type ::= "LIST" / "LSUB"
search_program ::= ["CHARSET" SPACE astring SPACE] search_criteria
; IMAP4 literals may not be used
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; See RFC 1730 for "astring" and "search_criteria"
uidvalidity ::= ";UIDVALIDITY=" nz_number
; See RFC 1730 for "nz_number" definition
9. References
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4", RFC 1730, University of Washington, December 1994.
<ftp://ds.internic.net/rfc/rfc1730.txt>
[RFC1738] Berners-Lee, Masinter, McCahill, "Uniform Resource
Locators (URL)", RFC 1738, CERN, Xerox Coproration, University of
Minnesota, December 1994.
<ftp://ds.internic.net/rfc/rfc1738.txt>
10. Security Considerations
IMAP URLs have the same security considerations as general Internet
URLs [RFC1738]. Specifically, including passwords in the URL makes
the password vulnerable to network evesdroppers both when the URL
is transmitted and when the "LOGIN" command is sent to the IMAP
server. For this reason, including passwords in the URL is
discouraged.
Security considerations discussed in the IMAP specification [IMAP4]
are also relevant.
Client authors should be careful when selecting an authentication
mechanism if ";AUTH=*" is specified. Clients SHOULD NOT fall back
to the "LOGIN" command with a user other than "anonymous" if more
secure mechanisms are available to the client. A client which
violates this rule is vulnerable to an active attacker which spoofs
the server and does not declare support for any AUTHENTICATE
mechanisms.
11. Author's Address
Chris Newman
Innosoft International, Inc.
1050 East Garvey Ave. South
West Covina, CA 91790 USA
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Email: chris.newman@innosoft.com
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