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rfc:rfc2384

Network Working Group R. Gellens Request for Comments: 2384 QUALCOMM, Incorporated Category: Standards Track August 1998

                           POP 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.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

1. Introduction

 [POP3] is a widely-deployed mail access protocol.  Many programs
 access POP3 message stores, and thus need POP3 configuration
 information.  Since there are multiple configuration elements which
 are required in order to access a mailbox, a single string
 representation is convenient.
 A POP3 mailbox (like an [IMAP4] mailbox) is a network resource, and
 URLs are a widely-supported generalized representation of network
 resources.
 A means of specifying a POP3 mailbox as a URL will likely be useful
 in many programs and protocols. [ACAP] is one case where a string
 encapsulation of elements required to access network services is
 needed.  For example, an [IMAP4] message store is usually specified
 in ACAP datasets as an [IMAP-URL].
 This memo defines a URL scheme for referencing a POP mailbox.

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 "Key words for
 use in RFCs to Indicate Requirement Levels" [KEYWORDS].

Gellens Standards Track [Page 1] RFC 2384 POP URL Scheme August 1998

3. POP Scheme

 The POP URL scheme designates a POP server, and optionally a port
 number, authentication mechanism, authentication ID, and/or
 authorization ID.
 The POP 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 110.
 The POP URL is described using [ABNF] in Section 8.
 A POP URL is of the general form:
      pop://<user>;auth=<auth>@<host>:<port>
 Where <user>, <host>, and <port> are as defined in RFC 1738, and some
 or all of the elements, except "pop://" and <host>, may be omitted.

4. POP User Name and Authentication Mechanism

 An authorization (which mailbox to access) and authentication (whose
 password to check against) identity (referred to as "user name" for
 simplicity) and/or authentication mechanism name may be supplied.
 These are used in a "USER", "APOP", "AUTH" [POP-AUTH], or extension
 command after making the connection to the POP server.  If the URL
 doesn't supply an authentication identifier, the program interpreting
 the POP URL SHOULD request one from the user.
 An authentication mechanism can be expressed by adding ";AUTH=<enc-
 auth-type>" to the end of the user name.  If the authentication
 mechanism name is not preceded by a "+", it is a SASL POP [SASL]
 mechanism.  If it is preceded by a "+", it is either "APOP" or an
 extension mechanism.
 When an <enc-auth-type> is specified, the client SHOULD request
 appropriate credentials from that mechanism and use the "AUTH",
 "APOP", or extension command instead of the "USER" 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
 supported by the POP server.
 If an <enc-auth-type> other than ";AUTH=*" is specified, the client
 SHOULD NOT use a different mechanism without explicit user
 permission.

Gellens Standards Track [Page 2] RFC 2384 POP URL Scheme August 1998

 If a user name is included with no authentication mechanism, then
 ";AUTH=*" is assumed.
 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.
 A URL containing ";AUTH=*" should 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).
 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].

5. Relative POP URLs

 Relative POP URLs are not permitted.

Gellens Standards Track [Page 3] RFC 2384 POP URL Scheme August 1998

6. Multinational Considerations

 Since 8-bit characters are not permitted in URLs, [UTF8] characters
 are encoded as required by the URL specification [BASIC-URL].

7. Examples

 The following examples demonstrate how a POP client program might
 translate various POP URLs into a series of POP 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:
      <pop://rg@mailsrv.qualcomm.com>
 Results in the following client commands:
      <request password from user>
      <connect to mailsrv.qualcomm.com, port 110>
      S: +OK POP3 server ready <1896.697170952@mailsrv.qualcomm.com>
      C: USER rg
      S: +OK
      C: PASS secret
      S: +OK rg's mailbox has 2 messages (320 octets)
 The URL:
      <pop://rg;AUTH=+APOP@mail.eudora.com:8110>
 Results in the following client commands:
      <client requests password from user>
      <connect to mail.eudora.com, port 8110>
      S: +OK POP3 server ready <1896.697170952@mail.eudora.com>
      C: APOP rg c4c9334bac560ecc979e58001b3e22fb
      S: +OK mailbox has 1 message (369 octets)
 The URL:
      <pop://baz;AUTH=SCRAM-MD5@foo.bar>
 Results in the following client commands:
      <connect to foo.bar, port 110>
      S: +OK POP3 server ready <1896.697170952@foo.bar>
      C: AUTH SCRAM-MD5 AGNocmlzADx0NG40UGFiOUhCMEFtL1FMWEI3MmVnQGVsZW

Gellens Standards Track [Page 4] RFC 2384 POP URL Scheme August 1998

         Fub3IuaW5ub3NvZnQuY29tPg==
      S: + dGVzdHNhbHQBAAAAaW1hcEBlbGVhbm9yLmlubm9zb2Z0LmNvbQBq
         aGNOWmxSdVBiemlGcCt2TFYrTkN3
      C: AQAAAMg9jU8CeB4KOfk7sUhSQPs=
      S: + U0odqYw3B7XIIW0oSz65OQ==
      C:
      S: +OK mailbox has 1 message (369 octets)

8. ABNF for POP URL scheme

 The POP URL scheme is described using [ABNF]:
      achar            = uchar / "&" / "=" / "~"
                              ; see [BASIC-URL] for "uchar" definition
      auth             = ";AUTH=" ( "*" / enc-auth-type )
      enc-auth-type    = enc-sasl / enc-ext
      enc-ext          = "+" ("APOP" / 1*achar)
                            ;APOP or encoded extension mechanism name
      enc-sasl         = 1*achar
                            ;encoded version of [SASL] "auth_type"
      enc-user         = 1*achar
                            ;encoded version of [POP3] mailbox
      pop-url          = "pop://" server
      server           = [user-auth "@"] hostport
                            ;See [BASIC-URL] for "hostport" definition
      user-auth        = enc-user [auth]

9. Security Considerations

 Security considerations discussed in the [POP3] specification and the
 [BASIC-URL] specification are relevant.  Security considerations
 related to authenticated URLs are discussed in section 4 of this
 document.
 Many email clients store the plain text password for later use after
 logging into a POP server.  Such clients MUST NOT use a stored
 password in response to a POP URL without explicit permission from
 the user to supply that password to the specified host name.

Gellens Standards Track [Page 5] RFC 2384 POP URL Scheme August 1998

10. Acknowledgements

 This document borrows heavily from Chris Newman's [IMAP-URL]
 specification, and has attempted to follow the advice in [URL-
 GUIDELINES].

11. References

 [ABNF]           Crocker, D., and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", RFC 2234, November
                  1997.
 [ACAP]           Newman, C., and J. Myers, "ACAP -- Application
                  Configuration Access Protocol", RFC 2244, November
                  1997.
 [BASIC-URL]      Berners-Lee, T., Masinter, L., and M. McCahill,
                  "Uniform Resource Locators (URL)", RFC 1738,
                  December 1994.
 [IMAP-URL]       Newman, C., "IMAP URL Scheme", RFC 2192, September
                  1997.
 [IMAP4]          Crispin, M., "Internet Message Access Protocol -
                  Version 4rev1", RFC 2060, December 1996.
 [KEYWORDS]       Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
 [POP-AUTH]       Myers, J., "POP3 AUTHentication command", RFC 1734,
                  December 1994.
 [POP3]           Myers, J., and M. Rose, "Post Office Protocol --
                  Version 3", STD 53, RFC 1939, May 1996.
 [SASL]           Myers, J., "Simple Authentication and Security Layer
                  (SASL)", RFC 2222, October 1997.
 [URL-GUIDELINES] Masinter, Alvestrand, Zigmond, "Guidelines for new
                  URL Schemes", Work in Progress.
 [UTF8]           Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", RFC 2279, January 1998.

Gellens Standards Track [Page 6] RFC 2384 POP URL Scheme August 1998

12. Author's Address

 Randall Gellens
 QUALCOMM, Incorporated
 6455 Lusk Blvd.
 San Diego, CA  92121-2779
 U.S.A.
 Phone: +1 619 651 5115
 Fax:   +1 619 651 5334
 EMail: Randy@Qualcomm.Com

Gellens Standards Track [Page 7] RFC 2384 POP URL Scheme August 1998

13. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Gellens Standards Track [Page 8]

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