Telnet Negotiations


        The telnet protocol is used to control textbased connections
        between a client (the 'telnet' program or a mud client) and a
        server (the game driver). Most of the options offered by the
        protocol are optional and need to be negotiated between the
        client and the server. Consequently, and due to their
        specialized nature, mud clients don't have to support the full
        telnet option feature set.

        For the server to find out if a client supports the telnet
        protocol at all, one good approach is to issue a simple,
        commonly used telnet command to the client. If the client reaction
        conforms to the protocol (or sends telnet commands itself), the
        mud can continue to negotiate further options. If the client
        does not react, the mud can safely refrain from further

        The following list is a more or less comprehensive overview of
        the telnet related RFCs (available for example on

         RFC Title                                              rel. Code

         495 TELNET Protocol Specification
         513 Comments on the new TELNET specifications
         559 Comments on the new TELNET Protocol and its Implem
         595 Some Thoughts in Defense of the TELNET Go-Ahead
         596 Second Thoughts on Telnet Go-Ahead
         652 Telnet Output Carriage-Return Disposition Option   NAOCRD     10
         653 Telnet Output Horizontal Tabstops Option           NAOHTS     11
         654 Telnet Output Horizontal Tab Disposition Option    NAOHTD     12
         655 Telnet Output Formfeed Disposition Option          NAOFFD     13
         656 Telnet Output Vertical Tabstops Option             NAOVTS     14
         657 Telnet Output Vertical Tab Disposition Option      NAOVTD     15
         658 Telnet Output Linefeed Disposition                 NAOLFD     16
         698 Telnet Extended Ascii Option                       X-ASCII    17
         727 Telnet Logout Option                               LOGOUT     18
         728 A Minor Pitfall in the Telnet Protocol
         735 Revised TELNET Byte Macro Option                   BM         19
         749 Telnet SUPDUP-OUTPUT Option                        SUPDUP     22
         764 Telnet Protocol Specification
         779 Telnet SEND-LOCATION Option                        SENDLOC    23
         818 The Remote User Telnet Service
         854 Telnet Protocol Specification
         855 Telnet Option Specifications
         856 Telnet Binary Transmission                         BINARY      0
         857 Telnet Echo Option                                 ECHO        1
         858 Telnet Suppress Go Ahead Option                    SGA         3
         859 Telnet Status Option                               STATUS      5
         860 Telnet Timing Mark Option                          TM          6
         861 Telnet Extended Options - List Option              EXOPL     255
         884 Telnet Terminal Type Option                        TTYPE      24
         885 Telnet End of Record Option                        EOR        25
         930 Telnet Terminal Type Option                        TTYPE      24
         933 Output Marking Telnet Option                       OUTMRK     27
         946 Telnet Terminal Location Number Option             TTYLOC     28
        1043 Telnet Data Entry Terminal Option DODIIS Implement DET        20
        1053 Telnet X.3 PAD Option                              X.3-PAD    30
        1073 Telnet Window Size Option                          NAWS       31
        1079 Telnet Terminal Speed Option                       TSPEED     32
        1080 Telnet Remote Flow Control Option                  FLOWCTRL   33
        1091 Telnet Terminal-Type Option                        TTYPE      24
        1096 Telnet X Display Location Option                   XDISPLOC   35
        1116 Telnet Linemode Option                             LINEMODE   34
        1143 The Q Method of Implementing TELNET Option Negotia
        1184 Telnet Linemode Option                             LINEMODE   34
        1372 Telnet Remote Flow Control Option                  FLOWCTRL   33
        1408 Telnet Environment Option                          ENVIRON    36
        1571 Telnet Environment Option Interoperability Issues
        1572 Telnet Environment Option                          NEWENV     39
        2066 Telnet Charset Option                              CHARSET    42
        2217 Telnet Com Port Control Option                     COMPORT    44
        2877 5250 Telnet Enhancements

        All negotiations start with the special character IAC which is
        defined in /usr/include/arpa/telnet.h (or in
        src/driver/telnet.h for 3.2(.1)) and has the decimal value of
        255. Negotiations are based on different telnetoptions (their
        values are defined in telnet.h too). Before a negotiation can
        start the client and the server have to agree that they
        support the option. This works in the following way:

        If a client wants to send something to the server it has to
        send 'IAC WILL option' (For terminaltype negotation this would
        be the 3 bytes 255,251,24; again, check telnet.h) to confirm
        that it is able to do that. If the server is supporting that
        option and wants to receive something it sends 'IAC DO option'

        If one side is receiving an 'IAC WILL option' and has not yet
        sent with DO or DONT it has to respond with either 'IAC DO
        option' if it will support this negotiation or 'IAC DONT
        option' if it won't.

        If one side is receiving an 'IAC DO option' and has not yet
        sent a WILL or WONT it has to reply with either 'IAC WILL
        option' if it supports the option or 'IAC WONT option' if not.

        A small example: Lets assume we want to negotiate
        terminaltype. (TELOPT_TTYPE with value 24). client is the
        telnet executable on the playerside, the server is the

                client                        server
            IAC WILL TTYPE
                                           IAC DO TTYPE

                                           IAC DO TTYPE
            IAC WILL TTYPE

        After this we are ready to transfer the terminaltype from the
        client to the server as explained below.

        Now we are ready to start the real negotiations. I explain the
        3 options I have currently implemented.

        First TerminalType aka TTYPE aka 24 aka TELOPT_TTYPE assuming
        the client and the server have exchanged WILL/DO.

        The server is now free to send 'IAC SB TELOPT_TTYPE
        TELQUAL_SEND IAC SE' which will be replied with 'IAC SB
        TELOPT_TTYPE TELQUAL_IS terminaltype IAC SE' where
        terminaltype is a non-zero terminated string (it's terminated
        by the IAC) (For values look up telnet.h) AND switch the
        client's terminalemulation to 'terminaltype'. terminaltype is
        case-insensitive. terminal-type may be UNKNOWN. The server may
        repeat the SEND request and the client will respond with the
        next preferred terminaltype. If this is the same as the
        previous received, it marks the end of the list of
        terminaltypes. The next SEND request will start the
        terminaltypes from the beginning.

        Example: (we have exchanged WILL/DO already)
                  client                                server
                                              IAC SB TTYPE SEND IAC SE
                                              IAC SB TTYPE SEND IAC SE
                                              IAC SB TTYPE SEND IAC SE
                                              IAC SB TTYPE SEND IAC SE
        /* this marks that we have all terminaltypes. We decide to use the
         * vt200 mode so we have to skip to VT200
                                              IAC SB TTYPE SEND IAC SE

        Next important option is NAWS (31) or WindowSizeNegotiation.

        This one is a bit easier than terminaltype. After having
        received a IAC DO NAWS from the server, the client will reply
        with IAC WILL NAWS and immediately after that send IAC SB NAWS
        columns_high columns_low lines_high lines_low IAC SE where
        xx_low refers to the lowbyte of xx and xx_high refers to the
        highbyte of xx. This will be automagically resent at every
        windowresize (when the client gets a SIGWINCH for example) or
        at your request with 'IAC SB NAWS SEND IAC SE'.

        Example: (WILL/DO exchanged)
                client                                server
        IAC SB NAWS 0 80 0 24 IAC SE         /* the standard vt100 windowsize */
                                             /* no reply */

        And, a bit less important but most complex, the LINEMODE (34)
        option. It was implemented it due to the fact, that
        some weird DOS telnets would not work otherwise. Implemented
        are only the absolute basic feature, which is the actual
        switching the telnet to linemode. After exchanging WILL/DO the
        server sends a modechange request to the client using IAC SB
        LINEMODE LM_MODE MODE_EDIT IAC SE, which should turn on local
        commandline-editing for the client. If a client supports
        LINEMODE it HAS to support this modechange. The client will
        (x|y is bitwise or). That's it for linemode. (You will perhaps
        receive other IAC SB LINEMODEs with other LM_xxx ... you may
        ignore them. (At least IRIX 5.x sends IAC SB LINEMODE LM_SLC
        .... IAC SE which declares the local characterset.)).

        Example: (WILL/DO negotiated)

                client                                        server
                                                IAC SB LINEMODE LM_MODE
                                                       MODE_EDIT IAC SE

        Note: The option is more interesting than it looks here. For
          example it supports a mixed mode between linemode and
          charactermode, flushing the input at certain characters (at
          ESC or TAB for shell-like commandline completition). We suggest
          reading RFC 1184.

        You might be interested in TELOPT_XDISPLAYLOC and TELOPT_ENVIRON too.

        Now, how to implement this using LDMud?

        0. Patch src/driver/comm1.c, function init_telopts() to include
            telopts_do[TELOPT_XXX] = reply_h_telnet_neg;
            telopts_dont[TELOPT_XXX] = reply_h_telnet_neg;
            telopts_will[TELOPT_XXX] = reply_h_telnet_neg;
            telopts_wont[TELOPT_XXX] = reply_h_telnet_neg;
           for every telnet negotiation you want to use.
           Do not overwrite the TELOPT_ECHO and TELOPT_SGA hooks.

           Alternatively, set the driver hook H_NOECHO in master.c:
           this diverts _all_ telnet data into the mudlib.

        1. Add a new driver hook to master.c just below the others.
        2. Make a telnet.h for your mudlib... just change the arrays in
        3. define a function

                void telnet_neg(int cmd, int option, int * optargs)

           in your interactive objects (login.c , shells, player.c or
           whereever). And note, in ALL objects, through which a
           player is handed through (in TAPPMud these are login.c and
           player.c). [Ok, master.c is interactive for a very short
           time too, but it won't accept input, will it?]
           'cmd' will be TELCMD_xxxx (see telnet.h), 'option' one of
           TELOPT_xxxx and 'optargs' will be an array of ints (bytes in
           fact) when 'cmd' is SB.
           Parse 'cmd'/'option' and reply with appropiate answers
           using binary_message() (appropiate meaning sending the
           right DO/DONT/WILL/WONT if not sent before and using the SB
           return values).
           first time you can do it (before cat()ing /WELCOME perhaps).
        3.2. Note all sent and received WILL/WONT/DO/DONT options for
           conforming to the standard, avoiding endless loops and for
           easy debugging :)
        3.3. Pass those recevied/sent data and other data when the
           interactive object is changed (from login.c to player.c or
           at other bodychanges). Clear the data when the player goes
           linkdead or quits. You won't need to save this data.
        3.4. Lower_case() terminaltypes... ;)
        3.5. Use reasonable defaultvalues if the client does not
           support one of the options. (columns 80, lines 24 if not
           NAWS, unknown or vt100 for no terminaltype)

        The WILL/WONT/DO/DONT data is best saved in a mapping looking
        like this:
          ([ "received": ([ option1: DO_DONT_OR_0;WILL_WONT_OR_0, ... ])
           , "sent"    : ([ option1: DO_DONT_OR_0;WILL_WONT_OR_0, ... ])

        (Ok, it can be done better. But not without confusing *me*

        Before sending anything check
        so you don't enter endless loops, save network traffic and the

        The windowsize is best saved in the players environment
        variables so that he can modify them later on. (Or in two
        integers in the player object...). Use for these values is
        clear I think.

        The terminaltypes received using above mentioned method are
        best stored in an array. The actual set terminaltype is best
        stored in an environment variable where the player can modify
        it. Upon modifying it the IAC SB TTYPE SEND IAC SE cycle
        should be started to match the emulation to the entered new
        terminaltype. You then may use data retrieved from
        /etc/termcap (man 5 termcap) or /usr/lib/terminfo/*/* (SysVID,
        man 5 terminfo) to implement terminalcontrol codes dependend
        on the terminaltype. /etc/termcap may prove to be the easiest
        way tough /usr/lib/terminfo/*/* is the newer (and better) SysV
        way of doing it.

        [Anyone got a description of the internal terminfo format for
        me? -Marcus]

        LINEMODE replies may be left alone if only using the mode
        change to MODE_EDIT

        Some statistics about what clients support telnet negotiations:

        Tinyfugue and some other mudclients usually do not support

        Except for TF, which supports the Telnet End-Of-Record option
        as marker for the end of the prompt. So if you send IAC EOR
        after every prompt, it will print the prompt always in the
        input window. (Do not forget to negotiate that. First IAC WILL
        TELOPT_EOR/wait for IAC DO TELOPT_EOR). Newer versions of
        TF will support NAWS and there will be a patch for TTYPE
        negotiation available soon.

        All telnets able to do negotiations I've encountered support
        the TTYPE option.
        HP9.x,Irix5.x,Linux,EP/IX,CUTELNET/NCSATELNET (Novell) and
        perhaps more support NAWS.
        At least Irix5.x,Linux,CU/NCSATELNET support LINEMODE.
        SUN does not support NAWS and LINEMODE neither in SunOS 4.1.3
        nor in Solaris 2.3.

        For getting RFCs you can for example use


        Not all aspects of the options are mentioned to keep this doc
        at a reasonable size. Refer to the RFCs to get more confused.


        Provided by Marcus@TAPPMud (Marcus Meissner,

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