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Network Working Group C. Newman Internet Draft: IMAP URL Scheme Innosoft Document: draft-newman-url-imap-05.txt January 1997 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. 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 this section. Newman [Page 1] Internet Draft IMAP URL Scheme January 1997 1. MUST This word means that the definition is an absolute requirement of the specification. 2. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. 3. SHOULD This word means that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. 4. SHOULD NOT This phrase means that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. 5. MAY This word means that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.) 3. 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. 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 Newman [Page 2] Internet Draft IMAP URL Scheme January 1997 specific message or message part. 4. 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]. 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 Newman [Page 3] Internet Draft IMAP URL Scheme January 1997 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. 6. 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 SHOULD 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]. 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 Newman [Page 4] Internet Draft IMAP URL Scheme January 1997 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]. 8. 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. 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 Newman [Page 5] Internet Draft IMAP URL Scheme January 1997 enclosing message itself SHOULD be checked in that order for "Content-Base" or "Content-Location" headers. 9. 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.* 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] Newman [Page 6] Internet Draft IMAP URL Scheme January 1997 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 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. Newman [Page 7] Internet Draft IMAP URL Scheme January 1997 10. 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] isearch ::= enc_mailbox [ "?" enc_search_program ] [uidvalidity] isection ::= ";SECTION=" enc_section Newman [Page 8] Internet Draft IMAP URL Scheme January 1997 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 11. 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> [RFC1808] Fielding, "Relative Uniform Resource Locators", RFC 1808, UC Irvine, June 1995. <ftp://ds.internic.net/rfc/rfc1808.txt> Newman [Page 9] Internet Draft IMAP URL Scheme January 1997 [HTTP] Fielding, Gettys, Mogul, Frystyk, Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, UC Irvine, DEC, MIT/LCS, January 1997. <ftp://ds.internic.net/rfc/rfc2068.txt> 12. 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. Some web browsers store URLs in history files which may expose them further. For these reasons, passwords SHOULD NOT be in the URL. 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. 13. 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 10]