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Network Working Group C. Newman
Internet Draft: IMAP URL Scheme Innosoft
Document: draft-newman-url-imap-04.txt December 1996
IMAP URL Scheme
Status of this memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress``.
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
munnari.oz.au.
A revised version of this draft document will be submitted to the
RFC editor as a Proposed Standard for the Internet Community.
Discussion and suggestions for improvement are requested. This
document will expire six months after publication. Distribution of
this draft is unlimited.
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, MIME
bodies [MIME], and search programs on Internet hosts accessible
using the IMAP protocol.
Newman [Page 1]
Internet Draft IMAP URL Scheme December 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://<iserver>/<enc_list_mailbox>;TYPE=<list_type>
imap://<iserver>/<enc_mailbox>[uidvalidity]?<enc_search_program>
imap://<iserver>/<enc_mailbox>[uidvalidity]<iuid>[isection]
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, Authentication Mechanism and Password
A user name, authentication mechanism and/or password may be
supplied. They are used in the "LOGIN" or "AUTHENTICATE" commands
after making the connection to the IMAP server. If no user name,
authentication mechanism or password 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 user name. 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.
Note that if unsafe or reserved characters such as " " or ";" are
present in the user name, authentication mechanism or password,
they MUST be encoded as described in RFC 1738 [RFC1738].
Newman [Page 2]
Internet Draft IMAP URL Scheme December 1996
4. Lists of mailboxes
An IMAP URL referring to a list of mailboxes has the following
form:
imap://<iserver>/<enc_list_mailbox>;TYPE=<list_type>
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 described in
RFC 1738 [RFC1738]. If the character "/" is present in
enc_list_mailbox, it SHOULD NOT be encoded.
5. Lists of messages
An IMAP URL referring to a list of messages has the following form:
imap://<iserver>/<enc_mailbox>[uidvalidity]?<enc_search_program>
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 described in RFC 1738 [RFC1738]. If the character "/"
is present in enc_mailbox, it SHOULD NOT be encoded.
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 described in RFC 1738
[RFC1738].
Newman [Page 3]
Internet Draft IMAP URL Scheme December 1996
6. A specific message or message part
An IMAP URL referring to a specific message or message part has the
following form:
imap://<iserver>/<enc_mailbox>[uidvalidity]<iuid>[isection]
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 described in RFC 1738
[RFC1738].
7. Relative IMAP URLs
Relative IMAP URLs are permitted and are resolved according to the
rules defined in RFC 1808 [RFC1808] with one exception. When the
relative URL includes a "SECTION=" parameter and does not include a
"UID=" parameter, then the "UID=" parameter is inherited from the
base URL. The following observations are also important:
The <iauth> grammar element is considered part of the user name for
purposes of resolving relative IMAP URLs. This means that unless a
new login/server specification is included in the relative URL, the
authentication mechanism is inherited from a base IMAP URL.
URLs always use "/" as the hierarchy delimiter for the purpose of
resolving paths in relative URLs. IMAP4 permits the use of any
hierarchy delimiter in mailbox names. For this reason, relative
mailbox paths will only work if the mailbox uses "/" as the
hierarchy delimiter. Relative URLs may be used on mailboxes which
use other delimiters, but in that case, the entire mailbox name
MUST be specified in the relative URL or inherited as a whole from
the base URL.
Newman [Page 4]
Internet Draft IMAP URL Scheme December 1996
The base URL for a list of mailboxes or messages which was referred
to by an IMAP URL is always the referring IMAP URL itself. The
base URL for a message or message part which was referred to by an
IMAP URL may be more complicated to determine. The program
interpreting the relative URL will have to check the headers of the
MIME entity and any enclosing MIME entities in order to locate the
"Content-Base" and "Content-Location" headers. These headers are
used to determine the base URL as defined in [HTTP]. For example,
if the referring IMAP URL contains a ";SECTION=1.2" parameter, then
the MIME headers for section 1.2, for section 1, and for the
enclosing message itself SHOULD be checked in that order for
"Content-Base" or "Content-Location" headers.
8. 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/gray-council;UIDVALIDITY=385759045;UID=20>
Results in the following client commands:
<connect to minbari.org, port 143>
C: A001 LOGIN ANONYMOUS sheridan@babylon5.org
C: A002 SELECT gray-council
<client verifies the UIDVALIDITY matches>
C: A003 UID FETCH 20 RFC822.PEEK
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.*
Newman [Page 5]
Internet Draft IMAP URL Scheme December 1996
The URL:
<imap://;AUTH=KERBEROS_V4@minbari.org/gray-council;uid=20;section=1.2>
Results in the following client commands:
<connect to minbari.org, port 143>
C: A001 AUTHENTICATE KERBEROS_V4
<authentication exchange>
C: A002 SELECT gray-council
C: A003 UID FETCH 20 BODY.PEEK[1.2]
If the following relative URL is located in that body part:
<;section=1.4>
This could result in the following client commands:
C: A004 UID FETCH 20 (BODY.PEEK[1.2.MIME]
BODY.PEEK[1.MIME]
BODY.PEEK[HEADER.FIELDS (Content-Base Content-Location)])
<Client looks for Content-Base or Content-Location headers in
result. If no such headers, then it does the following>
C: A005 UID FETCH 20 BODY.PEEK[1.4]
The URL:
<imap://;AUTH=*@minbari.org/gray%20council?SUBJECT%20shadows>
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 "gray 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 dependent
choices. The authentication mechanism could be anything, including
Newman [Page 6]
Internet Draft IMAP URL Scheme December 1996
PREAUTH. And the final FETCH command could fetch more or less
information about the messages, depending on what it wishes to display
to the user.
9. 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.
achar ::= uchar / "&" / "=" / "~"
; see RFC 1738 for "uchar" definition
bchar ::= achar / ":" / "@" / "/"
enc_auth_type ::= 1*achar
; encoded version of [IMAP-AUTH] "auth_type"
enc_list_mailbox ::= 1*bchar
; encoded version of [IMAP4] "list_mailbox"
enc_mailbox ::= 1*bchar
; encoded version of [IMAP4] "mailbox"
enc_pass ::= *achar
; encoded version of [IMAP4] "password"
enc_search_program
::= 1*bchar
; encoded version of search_program below
enc_section ::= 1*bchar
; encoded version of section below
enc_user ::= *achar
; encoded version of [IMAP4] "userid"
imapurl ::= "imap://" iserver "/" [ icommand ] [ iauth ]
iauth ::= ";AUTH=" ( "*" / enc_auth_type )
icommand ::= imailboxlist / ipath / isearch
imailboxlist ::= [enc_list_mailbox] [ ";TYPE=" list_type ]
ipath ::= enc_mailbox [uidvalidity] iuid [isection]
Newman [Page 7]
Internet Draft IMAP URL Scheme December 1996
isearch ::= enc_mailbox [ "?" enc_search_program ] [uidvalidity]
isection ::= ";SECTION=" enc_section
iserver ::= [enc_user [ iauth ] [ ":" enc_pass ] "@"] hostport
; See RFC 1738 for "hostport" definition
iuid ::= ";UID=" nz_number
; See [IMAP4] for "nz_number" definition
list_type ::= "LIST" / "LSUB"
search_program ::= ["CHARSET" SPACE astring SPACE] 1#search_key
; IMAP4 literals may not be used
; See [IMAP4] for "astring" and "search_key"
section ::= section_text / (nz_number *["." nz_number]
["." (section_text / "MIME")])
; See [IMAP4] for "section_text" and "nz_number"
uidvalidity ::= ";UIDVALIDITY=" nz_number
; See [IMAP4] for "nz_number" definition
10. References
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, University of Washington, December 1996.
<ftp://ds.internic.net/rfc/rfc2060.txt>
[IMAP-AUTH] Myers, J., "IMAP4 Authentication Mechanism", RFC 1731,
Carnegie-Mellon University, December 1994.
<ftp://ds.internic.net/rfc/rfc1731.txt>
[MIME] Freed, N., Borenstein, N., "Multipurpose Internet Mail
Extensions", RFC 2045, Innosoft, First Virtual, November 1996.
<ftp://ds.internic.net/rfc/rfc2045.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>
Newman [Page 8]
Internet Draft IMAP URL Scheme December 1996
[RFC1808] Fielding, "Relative Uniform Resource Locators", RFC 1808,
UC Irvine, June 1995.
<ftp://ds.internic.net/rfc/rfc1808.txt>
[HTTP] Fielding, Gettys, Mogul, Frystyk, Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", Work in Progress, UC Irvine, Dec,
MIT/LCS, August 1996.
11. 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 eavesdroppers 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". 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.
Many email clients store the plain text password for later use
after logging into an IMAP server. Such clients MUST NOT use a
stored password in response to an IMAP URL without explicit
permission from the user to supply that password to the specified
host name.
12. Author's Address
Chris Newman
Innosoft International, Inc.
1050 East Garvey Ave. South
West Covina, CA 91790 USA
Email: chris.newman@innosoft.com
Newman [Page 9]