linux - telnet (1)



SYNOPSIS
     telnet [-d] [-a] [-n tracefile] [-e escapechar] [[-l user] host [port]]


DESCRIPTION
     The telnet command is used to communicate with another host using the
     TELNET protocol.  If telnet is invoked without the host argument, it en-
     ters command mode, indicated by its prompt (telnet>). In this mode, it
     accepts and executes the commands listed below.  If it is invoked with
     arguments, it performs an open command with those arguments.

     Options:

     -d      Sets the initial value of the debug toggle to TRUE

     -a      Attempt automatic login.  Currently, this sends the user name via
             the USER variable of the ENVIRON option if supported by the re-
             mote system.  The name used is that of the current user as re-
             turned by getlogin(2) if it agrees with the current user ID, oth-
             erwise it is the name associated with the user ID.

     -n tracefile
             Opens tracefile for recording trace information.  See the set
             tracefile command below.

     -l user
             When connecting to the remote system, if the remote system under-
             stands the ENVIRON option, then user will be sent to the remote
             system as the value for the variable USER.  This option implies
             the -a option.  This option may also be used with the open com-
             mand.

     -e escape char
             Sets the initial telnet telnet escape character to escape char.
             If escape char is ommitted, then there will be no escape charac-
             ter.

     host    Indicates the official name, an alias, or the Internet address of
             a remote host.

     port    Indicates a port number (address of an application).  If a number
             is not specified, the default telnet port is used.

     Once a connection has been opened, telnet will attempt to enable the
     TELNET LINEMODE option.  If this fails, then telnet will revert to one of
     two input modes: either ``character at a time'' or ``old line by line''
     depending on what the remote system supports.

     When LINEMODE is enabled, character processing is done on the local sys-
     tem, under the control of the remote system.  When input editing or char-
     acter echoing is to be disabled, the remote system will relay that infor-
     mation.  The remote system will also relay changes to any special charac-
     ters that happen on the remote system, so that they can take effect on
     quences to the remote side.  If LINEMODE has ever been enabled, then the
     user's susp and eof are also sent as TELNET protocol sequences, and quit
     is sent as a TELNET ABORT instead of BREAK There are options (see toggle
     autoflush and toggle autosynch below) which cause this action to flush
     subsequent output to the terminal (until the remote host acknowledges the
     TELNET sequence) and flush previous terminal input (in the case of quit
     and intr).

     While connected to a remote host, telnet command mode may be entered by
     typing the telnet ``escape character'' (initially ``^]'').  When in com-
     mand mode, the normal terminal editing conventions are available.

     The following telnet commands are available.  Only enough of each command
     to uniquely identify it need be typed (this is also true for arguments to
     the mode, set, toggle, unset, slc, environ, and display commands).

     close      Close a TELNET session and return to command mode.

     display argument ...
                Displays all, or some, of the set and toggle values (see be-
                low).

     mode type  Type is one of several options, depending on the state of the
                TELNET session.  The remote host is asked for permission to go
                into the requested mode.  If the remote host is capable of en-
                tering that mode, the requested mode will be entered.

                character     Disable the TELNET LINEMODE option, or, if the
                              remote side does not understand the LINEMODE op-
                              tion, then enter ``character at a time`` mode.

                line          Enable the TELNET LINEMODE option, or, if the
                              remote side does not understand the LINEMODE op-
                              tion, then attempt to enter ``old-line-by-line``
                              mode.

                isig (-isig)  Attempt to enable (disable) the TRAPSIG mode of
                              the LINEMODE option.  This requires that the
                              LINEMODE option be enabled.

                edit (-edit)  Attempt to enable (disable) the EDIT mode of the
                              LINEMODE option.  This requires that the
                              LINEMODE option be enabled.

                softtabs (-softtabs)
                              Attempt to enable (disable) the SOFT_TAB mode of
                              the LINEMODE option.  This requires that the
                              LINEMODE option be enabled.

                litecho (-litecho)
                              Attempt to enable (disable) the LIT_ECHO mode of
                              the LINEMODE option.  This requires that the

                the port number is preceeded by a minus sign, the inital op-
                tion negotiation is done.  After establishing a connection,
                the file .telnetrc in the users home directory is opened.
                Lines begining with a # are comment lines.  Blank lines are
                ignored.  Lines that begin without whitespace are the start of
                a machine entry.  The first thing on the line is the name of
                the machine that is being connected to.  The rest of the line,
                and successive lines that begin with whitespace are assumed to
                be telnet commands and are processed as if they had been typed
                in manually to the telnet command prompt.

     quit       Close any open TELNET session and exit telnet. An end of file
                (in command mode) will also close a session and exit.

     send arguments
                Sends one or more special character sequences to the remote
                host.  The following are the arguments which may be specified
                (more than one argument may be specified at a time):

                abort   Sends the TELNET ABORT (Abort processes) sequence.

                ao      Sends the TELNET AO (Abort Output) sequence, which
                        should cause the remote system to flush all output
                        from the remote system to the user's terminal.

                ayt     Sends the TELNET AYT (Are You There) sequence, to
                        which the remote system may or may not choose to re-
                        spond.

                brk     Sends the TELNET BRK (Break) sequence, which may have
                        significance to the remote system.

                ec      Sends the TELNET EC (Erase Character) sequence, which
                        should cause the remote system to erase the last char-
                        acter entered.

                el      Sends the TELNET EL (Erase Line) sequence, which
                        should cause the remote system to erase the line cur-
                        rently being entered.

                eof     Sends the TELNET EOF (End Of File) sequence.

                eor     Sends the TELNET EOR (End of Record) sequence.

                escape  Sends the current telnet escape character (initially
                        ``^'').

                ga      Sends the TELNET GA (Go Ahead) sequence, which likely
                        has no significance to the remote system.

                getstatus
                        If the remote side supports the TELNET STATUS command,
                        the remote system to discard all previously typed (but
                        not yet read) input.  This sequence is sent as TCP ur-
                        gent data (and may not work if the remote system is a
                        4.2BSD system -- if it doesn't work, a lower case
                        ``r'' may be echoed on the terminal).

                ?       Prints out help information for the send command.

     set argument value

     unset argument value
                The set command will set any one of a number of telnet vari-
                ables to a specific value or to TRUE. The special value off
                turns off the function associated with the variable, this is
                equivalent to using the unset command.  The unset command will
                disable or set to FALSE any of the specified functions.  The
                values of variables may be interrogated with the display com-
                mand.  The variables which may be set or unset, but not tog-
                gled, are listed here.  In addition, any of the variables for
                the toggle command may be explicitly set or unset using the
                set and unset commands.

                echo    This is the value (initially ``^E'') which, when in
                        ``line by line'' mode, toggles between doing local
                        echoing of entered characters (for normal processing),
                        and suppressing echoing of entered characters (for en-
                        tering, say, a password).

                eof     If telnet is operating in LINEMODE or ``old line by
                        line'' mode, entering this character as the first
                        character on a line will cause this character to be
                        sent to the remote system.  The initial value of the
                        eof character is taken to be the terminal's eof char-
                        acter.

                erase   If telnet is in localchars mode (see toggle localchars
                        below), and if telnet is operating in ``character at a
                        time'' mode, then when this character is typed, a
                        TELNET EC sequence (see send ec above) is sent to the
                        remote system.  The initial value for the erase char-
                        acter is taken to be the terminal's erase character.

                escape  This is the telnet escape character (initially ``^['')
                        which causes entry into telnet command mode (when con-
                        nected to a remote system).

                flushoutput
                        If telnet is in localchars mode (see toggle localchars
                        below) and the flushoutput character is typed, a
                        TELNET AO sequence (see send ao above) is sent to the
                        remote host.  The initial value for the flush charac-
                        ter is taken to be the terminal's flush character.
                        ter is taken to be the terminal's kill character.

                lnext   If telnet is operating in LINEMODE or ``old line by
                        line`` mode, then this character is taken to be the
                        terminal's lnext character.  The initial value for the
                        lnext character is taken to be the terminal's lnext
                        character.

                quit    If telnet is in localchars mode (see toggle localchars
                        below) and the quit character is typed, a TELNET BRK
                        sequence (see send brk above) is sent to the remote
                        host.  The initial value for the quit character is
                        taken to be the terminal's quit character.

                reprint
                        If telnet is operating in LINEMODE or ``old line by
                        line`` mode, then this character is taken to be the
                        terminal's reprint character.  The initial value for
                        the reprint character is taken to be the terminal's
                        reprint character.

                start   If the TELNET TOGGLE-FLOW-CONTROL option has been en-
                        abled, then this character is taken to be the termi-
                        nal's start character.  The initial value for the kill
                        character is taken to be the terminal's start charac-
                        ter.

                stop    If the TELNET TOGGLE-FLOW-CONTROL option has been en-
                        abled, then this character is taken to be the termi-
                        nal's stop character.  The initial value for the kill
                        character is taken to be the terminal's stop charac-
                        ter.

                susp    If telnet is in localchars mode, or LINEMODE is en-
                        abled, and the suspend character is typed, a TELNET
                        SUSP sequence (see send susp above) is sent to the re-
                        mote host.  The initial value for the suspend charac-
                        ter is taken to be the terminal's suspend character.

                tracefile
                        Thi is the file to which the output, caused by netdata
                        or option tracing being TRUE, will be written.  If it
                        is set to ``-'', then tracing information will be
                        written to standard output (the default).

                worderase
                        If telnet is operating in LINEMODE or ``old line by
                        line`` mode, then this character is taken to be the
                        terminal's worderase character.  The initial value for
                        the worderase character is taken to be the terminal's
                        worderase character.


                import      Switch to the remote defaults for the special
                            characters.  The remote default characters are
                            those of the remote system at the time when the
                            TELNET connection was established.

                check       Verify the current settings for the current spe-
                            cial characters.  The remote side is requested to
                            send all the current special character settings,
                            and if there are any discrepencies with the local
                            side, the local side will switch to the remote
                            value.

                ?           Prints out help information for the slc command.

     environ arguments...
                The environ command is used to manipulate the the variables
                that my be sent through the TELNET ENVIRON option.  The ini-
                tial set of variables is taken from the users environment,
                with only the DISPLAY and PRINTER variables being exported by
                default.  The USER variable is also exported if the -a or -l
                options are used.
                Valid arguments for the environ command are:

                define variable value
                            Define the variable variable to have a value of
                            value. Any variables defined by this command are
                            automatically exported.  The value may be enclosed
                            in single or double quotes so that tabs and spaces
                            may be included.

                undefine variable
                            Remove variable from the list of environment vari-
                            ables.

                export variable
                            Mark the variable variable to be exported to the
                            remote side.

                unexport variable
                            Mark the variable variable to not be exported un-
                            less explicitly asked for by the remote side.

                list        List the current set of environment variables.
                            Those marked with a * will be sent automatically,
                            other variables will only be sent if explicitly
                            requested.

                ?           Prints out help information for the environ com-
                            mand.

     toggle arguments ...
                              system acknowledges (via a TELNET TIMING MARK
                              option) that it has processed those TELNET se-
                              quences.  The initial value for this toggle is
                              TRUE if the terminal user had not done an "stty
                              noflsh", otherwise FALSE (see stty(1)).

                autosynch     If autosynch and localchars are both TRUE, then
                              when either the intr or quit characters is typed
                              (see set above for descriptions of the intr and
                              quit characters), the resulting TELNET sequence
                              sent is followed by the TELNET SYNCH sequence.
                              This procedure should cause the remote system to
                              begin throwing away all previously typed input
                              until both of the TELNET sequences have been
                              read and acted upon.  The initial value of this
                              toggle is FALSE.

                binary        Enable or disable the TELNET BINARY option on
                              both input and output.

                inbinary      Enable or disable the TELNET BINARY option on
                              input.

                outbinary     Enable or disable the TELNET BINARY option on
                              output.

                crlf          If this is TRUE, then carriage returns will be
                              sent as <CR><LF>. If this is FALSE, then car-
                              riage returns will be send as <CR><NUL>. The
                              initial value for this toggle is FALSE.

                crmod         Toggle carriage return mode.  When this mode is
                              enabled, most carriage return characters re-
                              ceived from the remote host will be mapped into
                              a carriage return followed by a line feed.  This
                              mode does not affect those characters typed by
                              the user, only those received from the remote
                              host.  This mode is not very useful unless the
                              remote host only sends carriage return, but nev-
                              er line feed.  The initial value for this toggle
                              is FALSE.

                debug         Toggles socket level debugging (useful only to
                              the super user). The initial value for this tog-
                              gle is FALSE.

                localchars    If this is TRUE, then the flush, interrupt,
                              quit, erase, and kill characters (see set above)
                              are recognized locally, and transformed into
                              (hopefully) appropriate TELNET control sequences
                              (respectively ao, ip, brk, ec, and el; see send
                              above).  The initial value for this toggle is
                              tocol processing (having to do with TELNET op-
                              tions).  The initial value for this toggle is
                              FALSE.

                prettydump    When the netdata toggle is enabled, if
                              prettydump is enabled the output from the
                              netdata command will be formated in a more user
                              readable format.  Spaces are put between each
                              character in the output, and the begining of any
                              TELNET escape sequence is preceeded by a '*' to
                              aid in locating them.

                ?             Displays the legal toggle commands.

     z          Suspend telnet. This command only works when the user is using
                the csh(1).

     ! [command]
                Execute a single command in a subshell on the local system.
                If command is ommitted, then an interactive subshell is in-
                voked.

     status     Show the current status of telnet. This includes the peer one
                is connected to, as well as the current mode.

     ? [command]
                Get help.  With no arguments, telnet prints a help summary.
                If a command is specified, telnet will print the help informa-
                tion for just that command.


ENVIRONMENT
     Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environent vari-
     ables.  Other envirnoment variables may be propogated to the other side
     via the TELNET ENVIRON option.


FILES
     ~/.telnetrc  user customized telnet startup values


HISTORY
     The Telnet command appeared in 4.2BSD.


NOTES
     On some remote systems, echo has to be turned off manually when in ``old
     line by line'' mode.

     In ``old line by line'' mode or LINEMODE the terminal's eof character is
     only recognized (and sent to the remote system) when it is the first
     character on a line.


4.2 Berkeley Distribution July 27, 1991 8