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Network Working Group                                             S. YehRequest for Comments: 1204                                        D. Lee                                              Netix Communications, Inc.                                                           February 1991Message Posting Protocol (MPP)Status of this Memo   This memo describes a protocol for posting messages from workstations   (e.g., PCs) to a mail service host.  This RFC specifies an   Experimental Protocol for the Internet community.  Discussion and   suggestions for improvement are requested.  Please refer to the   current edition of the "IAB Official Protocol Standards" for the   standardization state and status of this protocol.  Distribution of   this memo is unlimited.1. INTRODUCTION   Operating systems for personal computers do not provide a mechanism   for user authentication.  However, such a mechanism is crucial for   electronic mail system since authenticating message sender's identity   is important in preventing mail forgery.  Hence, adding personal   computers to an electronic mail network requires an agent (message   posting server) to authenticate sender's identity and then submit   mail to the message delivery system (e.g., Sendmail, MMDF) on behalf   of the sender at a PC.  The Netix Message Posting Protocol is   developed to be the interface between the message posting server and   the PC (client).  The protocol is designed to use TCP and is based on   the command and reply structures defined for Simple Mail Transfer   Protocol (RFC 821) and File Transfer Protocol (RFC 959).2. SPECIFICATIONS2.1.  Command List      USER <SP> <username> <CRLF>      PASS <SP> <password> <CRLF>      DATA <CRLF>      NOOP <CRLF>      QUIT <CRLF>2.2.  Reply List      220 Message Posting Service Ready.      221 Closing Connection.      250 Command OK.Yeh & Lee                                                       [Page 1]

RFC 1204                          MPP                      February 1991      354 Enter mail, end with <CRLF>.<CRLF>      451 Local error encountered.      500 Command unrecognized.      501 Argument syntax error.      503 Illegal command sequence.      530 Authentication Failure.      550 Error.2.3.  Command and Reply Descriptions      USER <SP> <username> <CRLF>         The USER command informs the message posting server about the         username of the user trying to submit mail to the network.  The         required argument for the USER command is a string specifying         the message sender's username.         The USER command can only be used under three conditions:         - when the session with the message posting server has just           started;         - right after a message text (terminated by the "<CRLF>.<CRLF>"           sequence) has been successfully submitted to the message           posting server;         - right after a USER command that gets the reply code 501.         List of possible reply codes for the USER command:         - 250   The username of the message sender has been accepted.         - 451   Internal error has occurred in the message posting                 server.         - 501   Syntax error detected in the username argument.         - 503   The USER command has been used under an inappropriate                 condition (i.e., one that is not specified above).         It is recommended that the message posting server should return         250 even if the username is not recognized by the message         posting server, as long as the username is syntactically         correct.  This is an attempt to prevent the message posting         server from releasing too much information about the user         database.  Client should not be able to test the existence of a         certain username.Yeh & Lee                                                       [Page 2]

RFC 1204                          MPP                      February 1991      PASS <SP> <password> <CRLF>         The PASS command is used to inform the message posting server         about the password associated with the username previously         specified.  The required argument for the PASS command is a         string specifying the message sender's password.         The PASS command can only be used under two conditions:         - right after a USER command that gets the reply code 250;         - right after a PASS command that gets the reply code 501.         List of possible reply codes for the PASS command:         - 250   The password has been accepted and verified to be                 correctly associated with the username previously                 specified.         - 451   Internal error has occurred in the message posting                 server.         - 501   Syntax error detected in the password argument.         - 503   The PASS command has been used under an inappropriate                 condition (i.e., one that is not specified above).         - 530   The password provided is not the one associated with the                 username previously specified.      DATA <CRLF>         The DATA command is used to inform the message posting server         to get ready to accept a mail message text.  No argument is         expected.  (This command has the same meaning as the DATA         command defined inRFC 821.)         The DATA command can only be used under two conditions:         - right after a PASS command that gets the reply code 250;         - right after a mail message text has been successfully           accepted from the client.         List of possible reply codes for the DATA command:         - 354   The message posting server is ready to accept the mail                 message text.Yeh & Lee                                                       [Page 3]

RFC 1204                          MPP                      February 1991         - 451   Internal error has occurred in the message posting                 server.         - 503   The DATA command has been used under an inappropriate                 condition (i.e., one that is not specified above).         Upon receiving the reply code 354 for the DATA command, the         client should submit the mail message text to message posting         server and terminate the text by the sequence "<CRLF>.<CRLF>"         as defined inRFC 821.  If the message text includes the         "<CRLF>.<CRLF>" sequence, then the sequence is replaced by the         "<CRLF>..<CRLF>" sequence as defined inRFC 821.  The extra "."         token will not be included in the final copy of the submitted         message.         Upon receiving the mail message text terminated by the         "<CRLF>.<CRLF>" sequence, list of possible reply codes is:         - 250   The mail message text has been successfully queued for                 delivery.         - 451   Internal error has occurred in the message posting                 server and the mail message text has not been queued.      NOOP <CRLF>         The NOOP command does not cause any action to be performed by         the message posting server.  Instead, it tests the status of         the message posting server.  No argument is expected.         The NOOP command cannot be used under one condition:         - right after a DATA command that gets the reply code 354           (i.e., when the message posting server is expecting the client           to submit the mail message text).         List of possible reply codes for the NOOP command:         - 250   The message posting server has not encountered any                 internal error.         - 451   Internal error has occurred in the message posting                 server during the current session.      QUIT <CRLF>         The QUIT command is used to terminate the session with the         message posting server.  No argument is expected.Yeh & Lee                                                       [Page 4]

RFC 1204                          MPP                      February 1991         The QUIT command can be used under any condition.  The message         posting server should always return the reply code 221 for the         QUIT command.3. IMPLEMENTATION OF THE MESSAGE POSTING SERVER   There are several issues to be considered when implementing the   message posting server:   - secured environment   - port number assignment   - handling of idle client   - local/remote password database   - message queuing   - handling of message delivery failure3.1  Secured Environment   The message posting server is responsible for authenticating message   senders and submitting mail to the message delivery system.  Hence,   it should be running in a secured environment, such as running on a   system (UNIX, VMS, MS-DOS) with well restricted physical and network   access.3.2  Port Number Assignment   Port 218 is assigned for the Netix Message Posting Protocol.3.3  Handling of Idle Client   The message posting server should terminate a session if the client   has been idle for too long, to release the resource allocated for the   session.3.4  Local/Remote Password Database   To take advantage of existing password databases, such as the passwd   file in UNIX, the message posting server can use FTP and POP3 to   perform the username and password checking with the appropriate   server.   For network that does not have any password database, the message   posting server should let the system administrator specify a local   password file on the host that the message posting server is running.Yeh & Lee                                                       [Page 5]

RFC 1204                          MPP                      February 19913.5  Message Queuing   The message posting server should attempt to submit accepted messages   to the message delivery system as soon as possible.3.6  Handling of Message Delivery Failure   Failure in delivering messages should be handled by the message   delivery system and the message posting server should not interfere.4. REFERENCES   [1] Postel, J., "Simple Mail Transfer Protocol",RFC 821,       USC/Information Sciences Institute, August 1982.   [2] Postel, J., and J. Reynolds, "File Transfer Protocol",RFC 959,       USC/Information Sciences Institute, October 1985.Security Considerations   Security issues are discussed insection 3.1.Authors' Addresses   Shannon Yeh   Netix Communications, Inc.   15375 Barranca Parkway, Suite A-215   Irvine, CA 92718   Phone: (714) 727-9335   Email: yeh@netix.com   David Lee   Netix Communications, Inc.   15375 Barranca Parkway, Suite A-215   Irvine, CA 92718   Phone: (714) 727-9335   EMail: dlee@netix.comYeh & Lee                                                       [Page 6]