rfc2192.txt

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Network Working Group                                          C. Newman
Request for Comments: 2192                                      Innosoft
Category: Standards Track                                 September 1997


                            IMAP URL Scheme


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.


Abstract

     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.


1. Conventions used in this document

     The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in "Key words for
     use in RFCs to Indicate Requirement Levels" [KEYWORDS].


2. IMAP scheme

     The IMAP URL scheme is used to designate IMAP servers, mailboxes,
     messages, MIME bodies [MIME], and search programs on Internet hosts
     accessible using the IMAP protocol.

     The IMAP URL follows the common Internet scheme syntax as defined
     in RFC 1738 [BASIC-URL] except that clear text passwords are not
     permitted.  If :<port> is omitted, the port defaults to 143.








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RFC 2192                    IMAP URL Scheme               September 1997


     An IMAP URL takes one of the following forms:

         imap://<iserver>/
         imap://<iserver>/<enc_list_mailbox>;TYPE=<list_type>
         imap://<iserver>/<enc_mailbox>[uidvalidity][?<enc_search>]
         imap://<iserver>/<enc_mailbox>[uidvalidity]<iuid>[isection]

     The first form is used to refer to an IMAP server, the second form
     refers to a list of mailboxes, the third form refers to the
     contents of a mailbox or a set of messages resulting from a search,
     and the final form refers to a specific message or message part.
     Note that the syntax here is informal.  The authoritative formal
     syntax for IMAP URLs is defined in section 11.


3. IMAP User Name and Authentication Mechanism

     A user name 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 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 doesn't
     supply a user name, 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.

     If a user name is included with no authentication mechanism, then
     ";AUTH=*" is assumed.






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RFC 2192                    IMAP URL Scheme               September 1997


     Since URLs can easily come from untrusted sources, care must be
     taken when resolving a URL which requires or requests any sort of
     authentication.  If authentication credentials are supplied to the
     wrong server, it may compromise the security of the user's account.
     The program resolving the URL should make sure it meets at least
     one of the following criteria in this case:

     (1) The URL comes from a trusted source, such as a referral server
     which the client has validated and trusts according to site policy.
     Note that user entry of the URL may or may not count as a trusted
     source, depending on the experience level of the user and site
     policy.
     (2) Explicit local site policy permits the client to connect to the
     server in the URL.  For example, if the client knows the site
     domain name, site policy may dictate that any hostname ending in
     that domain is trusted.
     (3) The user confirms that connecting to that domain name with the
     specified credentials and/or mechanism is permitted.
     (4) A mechanism is used which validates the server before passing
     potentially compromising client credentials.
     (5) An authentication mechanism is used which will not reveal
     information to the server which could be used to compromise future
     connections.

     URLs which do not include a user name must be treated with extra
     care, since they are more likely to compromise the user's primary
     account.  A URL containing ";AUTH=*" must also be treated with
     extra care since it might fall back on a weaker security mechanism.
     Finally, clients are discouraged from using a plain text password
     as a fallback with ";AUTH=*" unless the connection has strong
     encryption (e.g. a key length of greater than 56 bits).

     A program interpreting IMAP URLs MAY cache open connections to an
     IMAP server for later re-use.  If a URL contains a user name, only
     connections authenticated as that user may be re-used.  If a URL
     does not contain a user name or authentication mechanism, then only
     an anonymous connection may be re-used.  If a URL contains an
     authentication mechanism without a user name, then any non-
     anonymous connection may be re-used.

     Note that if unsafe or reserved characters such as " " or ";" are
     present in the user name or authentication mechanism, they MUST be
     encoded as described in RFC 1738 [BASIC-URL].








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RFC 2192                    IMAP URL Scheme               September 1997


4. IMAP server

     An IMAP URL referring to an IMAP server has the following form:

         imap://<iserver>/

     A program interpreting this URL would issue the standard set of
     commands it uses to present a view of the contents of an IMAP
     server.  This is likely to be semanticly equivalent to one of the
     following URLs:

         imap://<iserver>/;TYPE=LIST
         imap://<iserver>/;TYPE=LSUB

     The program interpreting this URL SHOULD use the LSUB form if it
     supports mailbox subscriptions.


5. 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 [BASIC-URL].  If the character "/" is present in
     enc_list_mailbox, it SHOULD NOT be encoded.


6. Lists of messages

     An IMAP URL referring to a list of messages has the following form:

         imap://<iserver>/<enc_mailbox>[uidvalidity][?<enc_search>]





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     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 [BASIC-URL].  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 SHOULD be used by the program
     interpreting the IMAP URL to determine if the URL is stale.

     The [?<enc_search>] field is optional.  If it is not present, the
     contents of 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>)
     encoded as described in RFC 1738 [BASIC-URL].


7. 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 <iuid> 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 Internet message as returned by the IMAP command "UID
     FETCH <uid> BODY.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]
     MUST be encoded as described in [BASIC-URL].









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RFC 2192                    IMAP URL Scheme               September 1997


8. Relative IMAP URLs

     Relative IMAP URLs are permitted and are resolved according to the
     rules defined in RFC 1808 [REL-URL] with one exception.  In IMAP
     URLs, parameters are treated as part of the normal path with
     respect to relative URL resolution.  This is believed to be the
     behavior of the installed base and is likely to be documented in a
     future revision of the relative URL specification.

     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