The Hacker's Encyclopedia 1998

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Network Working Group T. Rinne Request for Comments: 1756 HUT Category: Experimental January 1995 REMOTE WRITE PROTOCOL - VERSION 1.0 Status of this Memo This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. 1. Background It is often convenient to use electronic communication somewhat lighter than electronic mail. Sometimes even the use of the talk(1) *) program seems like overkill. We like to offer to user something like UNIX **) command write(1) ***) except that it can also pass messages through the network instead of the single host. There have been few programs offering this kind of service, but they have either based on SUN-RPC protocol or used a strictly undocumented protocol. This document describes a simple Remote Write Protocol (RWP) that should have been documented at least 10 years ago. But late is better than never. Version number of the RWP protocol in this document is 1.0. 2. Overview RWP is a simple protocol that can be used to relay short messages through the network to other users. RWP looks pretty much like Simple Mail Transfer Protocol (SMTP) ****) though it is a bit more complicated due to the interactive nature of the RWP session. The idea behind the RWP session is that client program that is relaying message to the host in which the target user is logged in opens the tcp or udp connection to the server program running in the target machine Then the client gives the sender's and recipient's identification (usually login ids), actual message body and tells the server to deliver a message to the user. On tcp-connection server returns a status from each action taken. On udp-connection no responses are sent. RWP sessions through udp are implemented to support message broadcasting. Rinne [Page 1] RFC 1756 Remote Write Protocol January 1995 Message delivering methods are not defined within this document, but the basic method could be a simple write to users terminal. This is basically what UNIX command write(1) does. Depending on server implementation, the delivery method could be configurable personally by each user. 3. Description Server program answers to each command submitted by a response. All responses have two parts: three number unique response code and a short textual explanation of the response. Also whenever the server is ready to accept new commands a notification is submitted to the client. There are three kinds of commands in RWP. The first group is for querying a status of the server. The second group is actual message handling commands and the last set of commands are for RWP session control. When the server is ready to receive a command from the client, it sends a message code 100 to the client. This message is for example as follows: 100 Ready. Server commands are as follows: Status Query HELP Gives a short help message that contains legal RWP commands. Help lines have code 510. Example RWP implementation *****) gives a following response to HELP command: 510 Valid commands are: 510 BYE, DATA, HELP, HELO, 510 RSET, SEND, PROT, QUIT, 510 VRFY, VER 510 FROM senderlogin 510 FHST senderhost 510 TO recipentlogin [tty] 510 FWDS current_hop_count HELO Says hello to the server. Server response to HELO command has code 500. For example: 500 Hello remote.host. This is local.host speaking. Rinne [Page 2] RFC 1756 Remote Write Protocol January 1995 PROT Asks the RWP protocol version from the server. Response code to PROT command is 502. Protocol version described in this document is RWP 1.0 and the response is as follows: 502 RWP version 1.0. VRFY After the recipient of the message is set by to command described later, the possibility of message delivery can be queried by VRFY command. If message can be delivered the response code is 108. If message is about to be forwarded the response code is 110 and message is either form: 110 Recipient ok to forward. or if the server can tell the destination of the forwarding: 110 Recipient ok to forward <user@host.domain>. Other possible response codes are 669, 670, 671, 674 and 677 and they all indicate that message delivery is by one way or another currently impossible. Description of the codes is later in this document. After the SEND command the server may also give autoreply from the remote user before the actual response code. Autoreply lines are ones of code 300. VER Asks the version of the server program. Response code to VER command is 501 and the textual part of the response is the name and the version number of the RWP server, for example: 501 Rwrited version 1.0. Message Handling: FROM senderlogin Tells the server the identification information of the sender of the message. Usually this id information is user's login id. Response code to successful FROM command is 105, for example: 105 Sender ok. Rinne [Page 3] RFC 1756 Remote Write Protocol January 1995 TO recipentlogin [tty] Tells the server the identification information of the intended recipient of the message. Usually this id information is user's login id. If tty is submitted, the message is delivered to that tty. If tty is submitted between brackets '[]' the tty given is treated as a hint only. Response code to successful TO command is 106. FHST original.host [forwarder1.host forwarder2.host ...] Tells the server the host name that the message originates to and the path of the hosts that has forwarded the message. The host name of the machine that is currently submitting the message to the server should not be in the path list. This information is relevant if message is forwarded and it is not originally coming from the host that is forwarding it. Response code to successful FHST command is 111. DATA Tells the server to start receive the body of the message. Response code to DATA command is 200, for example: 200 Enter message. Single dot '.' on line terminates. After response 200 the message lines are submitted to the server one after another. Message is terminated by the line that contains a single dot '.'. The termination of the message is acknowledged by the server with the response code 107. Server does not notify client about receiving the single message lines. If empty message is submitted (i.e. single dot is on the first line) the response code is 672 and DATA command only cancels possible previous DATA command. Because of this all dots or at least dots that are standing alone in the line have to be quoted. SEND Sends the message. If commands FROM, TO and DATA are successfully given before SEND command, the message is delivered to the target user. If delivery is successful the response code is 103. If message is not delivered directly to the target user but instead forwarded to another host the response code is 104. Response codes 669, 670 and 671, 677 indicate an error on message delivery and codes 673, 674, 675 indicate that either command FROM, TO or DATA has not been Rinne [Page 4] RFC 1756 Remote Write Protocol January 1995 successfully given before SEND command. After the SEND command the server may also give autoreply from the remote user before the actual response code. Autoreply lines are ones of code 300. FWDS n Tells the server that message has been forwarded n times. If the server forwards the message to the another server, it increments the counter and tells the remote server the current count of forwards. Response code to the FWDS command is 110 if n is less than the server specific forward limit. If this limit is exceeded the response code is 676. If the response code is 676 the client can either quit the session and fail the message or it can give the message to the server despite the fact that the forward limit is exceeded. If the message is given when forward limit is exceeded, the server tries to deliver it, but does not forward it to another server. If forward count is given as -1, the message is considered as a autoreply and never forwarded. Session Control: RSET Resets the RWP session. FROM, TO and DATA -commands that are given before are canceled and they have to be given again before SEND command can be used. Also possible FWDS and FHST commands are canceled. BYE Terminates the RWP session. Server gives a response code 101 and closes the connection. QUIT Is the synonym to bye, but it's a lot more impolite. Response code is however 101 as in bye. Server specific command: QUOTE command Relay a command to the server. If the QUOTE command is successfully completed response code 112 is returned. If QUOTE command is failed the response code is 678. If RWP server doesn't recognize the given QUOTE command the response code is 679. Currently reserved QUOTE commands are AGENT, CHARSET, IDENT, KEY and KEYID. Rinne [Page 5] RFC 1756 Remote Write Protocol January 1995 4. Response Codes Here are all legal response codes of RWP server followed by short textual explanation. Only the numeral codes are important and texts can contain practically anything, however in response code 110 there is possibly useful information between '<' and '>' characters. No characters '<' or '>' should be present in other responses. Also response 502 has possibly interesting information about the RWP protocol version the server supports. 100 Ready. The RWP server is ready to accept next command. 101 Goodbye. The RWP server is closing connection. 103 Message delivered. The SEND command is successfully completed and the message is delivered directly to its destination. 104 Message forwarded. The SEND command is completed and message is forwarded to the user. 105 Sender ok. The FROM command successful. 106 Recipient ok. The TO command successful. 107 Message ok. The DATA command successful. 108 Recipient ok to send. The VRFY command successful and direct message delivery is possible. 109 RSET ok. The RWP server has received the RSET command and reset itself. Rinne [Page 6] RFC 1756 Remote Write Protocol January 1995 110 Ok to forward. or 110 Ok to forward <user@host.domain>. The VRFY command successful and direct message delivery by forwarding is possible. If response has also forwarding address the client can either forward the message itself or give it to server for forwarding. 111 Original sender host ok. The FHST command successful and original sender host is set as given by the client. 200 Enter message. Single dot '.' on line terminates. The RWP server is ready to receive the message. Single dot on message line terminates the message. 300 |I'm not in right now but I'll be back tomorrow 300 |at 8 o'clock a.m. Automatical response to the delivered message. Every line of this user defined reply message is delivered in its own 300 line. Response code 300 lines may appear only after SEND command before response code 103 (message delivered). Client receiving autoreply 300 should show the text of the autoreply to the user. Actual autoreply line begins after the '|' -character in the line. 500 Hello remote.host. This is local.host speaking. Response to the HELO command. This message can also occur in the beginning of the conversation without the VER command and it can be ignored. 501 Rwrited version X.X. Response to the VER command. This message can also occur in the beginning of the conversation without the VER command and it can be ignored. Rinne [Page 7] RFC 1756 Remote Write Protocol January 1995 502 RWP version 1.0. Response to the VER command. This message can also occur in the beginning of the conversation without the VER command and it can be ignored. 510 Valid commands are: 510 BYE, DATA, HELP, HELO, 510 RSET, SEND, PROT, QUIT, 510 VRFY, VER 510 FROM senderlogin 510 FHST senderhost 510 TO recipentlogin 510 FWDS current_hop_count Response to the HELP command. 511 Information to the user. Server specific informational response. These responses may occur anytime during the conversation. The client can ignore them. 512 Debug information to the user. Server specific informational response. Reserved for server debugging. These messages may occur anytime during the conversation. The client can ignore them. 666 FATAL ERROR! The RWP server got into the fatal error situation and is about to exit immediately. Client programs are strongly encouraged to close the connection. 668 Syntax error. The RWP server has received an invalid command. 669 Permission denied. The RWP server is unable to deliver the message because the target user has denied the send permission. 670 User not logged in. The RWP server is unable to deliver the message because the target user is not logged in. Rinne [Page 8] RFC 1756 Remote Write Protocol January 1995 671 No such user. The RWP server is unable to deliver the message because the target user does not exist. Error code 670 can be used to replace this message. 672 No message. The DATA command is terminated with empty message body. No SEND command can be executed before a new DATA command is given. 673 FROM command required. Tried to give the SEND command before FROM. 674 TO command required. Tried to give the SEND command before TO. 675 DATA command required. Tried to give the SEND command before DATA. 676 Forward limit exceeded. Response to the FWDS command that had an argument that exceeded the server specific limit of message forwarding steps. 677 Unable to forward message. or 677 Unable to forward message to <user@host.domain>. Response to the SEND or VRFY command if message forwarding is attempted and the server specific limit of message forwarding steps has been exceeded or if message forwarding has otherwise failed. If message forwarding fails with message 669, 670 or 671, server will not use response 667 but gives response but instead it gives the response analogous with the error occured. If message 677 includes address the message was to be forwarded, the client may try to deliver it itself. 698 Unknown error. RWP server has faced an internal error that is not fatal. Rinne [Page 9] RFC 1756 Remote Write Protocol January 1995 699 Unknown error. RWP server has faced an unknown error that is not fatal. 5. RWP Compliant Software Simple RWP 1.0 compliant server and client software RWrite-1.1 will be available during the fall 1994. 6. Security of RWP RWP version 1.0 does not offer any mean to verify the identity of the user connecting the RWP server program. It's possible to identify the sender using ident-service, but not all hosts currently support that. This vulnerability is analogous with the weakness of the SMTP protocol. Cryptographic user verification and message hiding method is under development and is to be defined in RWP version 2.0 during the year 1995. RWP server also may offer a way to the intruder to get to know user ids within the target host by trying the TO and VRFY commands. This vulnerability is also present in SMTP. It is however possible to build servers so that they never give message 671 (no such user) but use response 670 (user not logged in) instead. Another way to increase security even within RWP-1.0 described in the document is to design RWP servers so that they do not deliver messages directly to user but instead connect to some kind of RWP agent process that is executed by each user willing to receive RWP messages. This user configurable message agent could then decide whether to deliver the message to the user and which way of delivery to use. Message agent is the best way to prevent hostile user from sending uncontrolled message flood to the user's terminal. Sample implementation (RWrite-1.0) of the RWP server includes the support for user configuration files in which each user can either allow or deny messages from some user(s), host(s) or network domains(s). Support for message agents is currently under development. The user that is receiving the message should be able to define characters to be stripped from the incoming messages to prevent terminal mess-up. Rinne [Page 10] RFC 1756 Remote Write Protocol January 1995 7. RWP Connection Type It is suggested that tcp (and udp) port 18 should be allocated for rwp in future versions of RFCs listing the reserved tcp/udp/rpc ports. Currently port 18 is assigned to the service called Message Send Protocol (msp) that is not known to be implemented. Actually port 18 is not currently defined at all in the /etc/services -file of the any common UNIX-like system. Entry for /etc/services -file is as follows rwrite 18/udp # RWP rwrite rwrite 18/tcp # RWP rwrite Given that RWP compliant daemon program is /usr/sbin/rwrited the entry for /etc/inetd.conf -file would be: rwrite stream tcp nowait nobody /usr/sbin/rwrited rwrited 8. Character quotation To offer a safe method to transfer various character sets RWP defines a method to quote characters in both message and autoreply. RWP uses quotation similar to MIME `quoted-printable' encoding. Quoted character is presented as a '=' -sign followed by a two character hex code. This means also that all '='-signs have to be quoted. Quotation is also needed when message contains a line with only a single dot '.' in it. For example: '.' -> =2E '=' -> =3D '\a' -> =07 '\t' -> =09 9. Security Considerations Security issues are not discussed in this memo. 10. Author's Address Timo J. Rinne Helsinki University of Technology. Cirion oy PO-BOX 250 FIN-00121 Helsinki, Finland EMail: Timo.Rinne@hut.fi Rinne [Page 11]