telnet

TELNET(1)                 BSD General Commands Manual                TELNET(1)

NAME
     telnet -- user interface to the TELNET protocol

SYNOPSIS
     telnet [-468ELadr] [-S tos] [-b address] [-e escapechar] [-l user]
            [-n tracefile] [host [port]]

DESCRIPTION
     The telnet command is used for interactive communication with another
     host using the TELNET protocol. It begins in command mode, where it
     prints a telnet prompt ("telnet> "). If telnet is invoked with a host ar-
     gument, it performs an open command implicitly; see the description be-
     low.

     Options:

     -4      Force IPv4 address resolution.

     -6      Force IPv6 address resolution.

     -8      Request 8-bit operation. This causes an attempt to negotiate the
             TELNET BINARY option for both input and output. By default telnet
             is not 8-bit clean.

     -E      Disables the escape character functionality; that is, sets the
             escape character to ``no character''.

     -L      Specifies an 8-bit data path on output.  This causes the TELNET
             BINARY option to be negotiated on just output.

     -a      Attempt automatic login.  Currently, this sends the user name via
             the USER variable of the NEW-ENVIRON option if supported by the
             remote system. The username is retrieved via getlogin(3).

     -b address
             Use bind(2) on the local socket to bind it to a specific local
             address.

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

     -r      Emulate rlogin(1).  In this mode, the default escape character is
             a tilde. Also, the interpretation of the escape character is
             changed: an escape character followed by a dot causes telnet to
             disconnect from the remote host. A ^Z instead of a dot suspends
             telnet, and a ^] (the default telnet escape character) generates
             a normal telnet prompt. These codes are accepted only at the be-
             ginning of a line.

     -S tos  Sets the IP type-of-service (TOS) option for the telnet connec-
             tion to the value tos.

     -e escapechar
             Sets the escape character to escapechar. If no character is sup-
             plied, no escape character will be used.  Entering the escape
             character while connected causes telnet to drop to command mode.

     -l user
             Specify user as the user to log in as on the remote system. This
             is accomplished by sending the specified name as the USER envi-
             ronment variable, so it requires that the remote system support
             the TELNET NEW-ENVIRON option. This option implies the -a option,
             and may also be used with the open command.

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

     host    Specifies a host to contact over the network.

     port    Specifies a port number or service name to contact. If not speci-
             fied, the telnet port (23) is used.

     Protocol:

     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" de-
     pending 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
     the local system.

     In "character at a time" mode, most text typed is immediately sent to the
     remote host for processing.

     In "old line by line" mode, all text is echoed locally, and (normally)
     only completed lines are sent to the remote host.  The "local echo char-
     acter" (initially "^E") may be used to turn off and on the local echo
     (this would mostly be used to enter passwords without the password being
     echoed).

     If the LINEMODE option is enabled, or if the localchars toggle is TRUE
     (the default for "old line by line"; see below), the user's quit, intr,
     and flush characters are trapped locally, and sent as TELNET protocol se-
     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).

     Commands:

     The following telnet commands are available. Unique prefixes are under-
     stood as abbreviations.

     auth argument ...
                The auth command controls the TELNET AUTHENTICATE protocol op-
                tion.  If telnet was compiled without authentication, the auth
                command will not be supported.  Valid arguments are as fol-
                lows:

                disable type  Disable the specified type of authentication.
                              To obtain a list of available types, use the
                              auth disable ? command.

                enable type   Enable the specified type of authentication.  To
                              obtain a list of available types, use the auth
                              enable ? command.

                status        List the current status of the various types of
                              authentication.

                Note that the current version of telnet does not support au-
                thentication.

     close      Close the connection to the remote host, if any, and return to
                command mode.

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

     encrypt argument ...
                The encrypt command controls the TELNET ENCRYPT protocol op-
                tion. If telnet was compiled without encryption, the encrypt
                command will not be supported.

                Valid arguments are as follows:

                disable type [input|output]
                              Disable the specified type of encryption.  If
                              you do not specify input or output, encryption
                              of both is disabled.  To obtain a list of avail-
                              able types, use ``encrypt disable ?''.

                enable type [input|output]
                              Enable the specified type of encryption.  If you
                              do not specify input or output, encryption of
                              both is enabled.  To obtain a list of available
                              types, use ``encrypt enable ?''.

                input         This is the same as ``encrypt start input''.

                -input        This is the same as ``encrypt stop input''.

                output        This is the same as ``encrypt start output''.

                -output       This is the same as ``encrypt stop output''.

                start [input|output]
                              Attempt to begin encrypting.  If you do not
                              specify input or output, encryption of both in-
                              put and output is started.

                status        Display the current status of the encryption
                              module.

                stop [input|output]
                              Stop encrypting.  If you do not specify input or
                              output, encryption of both is stopped.

                type type     Sets the default type of encryption to be used
                              with later ``encrypt start'' or ``encrypt stop''
                              commands.

                Note that the current version of telnet does not support en-
                cryption.

     environ arguments...
                The environ command is used to propagate environment variables
                across the telnet link using the TELNET NEW-ENVIRON protocol
                option.  All variables exported from the shell are defined,
                but only the DISPLAY and PRINTER variables are marked to be
                sent by default.  The USER variable is marked to be sent if
                the -a or -l command-line options were 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 marked for propagation (``ex-
                            ported'').  The value may be enclosed in single or
                            double quotes so that tabs and spaces may be in-
                            cluded.

                undefine variable
                            Remove any existing definition of variable.

                export variable
                            Mark the specified variable for propagation to the
                            remote host.

                unexport variable
                            Do not mark the specified variable for propagation
                            to the remote host. The remote host may still ask
                            explicitly for variables that are not exported.

                list        List the current set of environment variables.
                            Those marked with a * will be propagated to the
                            remote host. The remote host may still ask explic-
                            itly for the rest.

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

     logout     Send the TELNET LOGOUT protocol option to the remote host.
                This command is similar to a close command. If the remote host
                does not support the LOGOUT option, nothing happens.  But if
                it does, this command should cause it to close the connection.
                If the remote side also supports the concept of suspending a
                user's session for later reattachment, the logout command in-
                dicates that the session should be terminated immediately.

     mode type  Type is one of several options, depending on the state of the
                session.  Telnet asks the remote host to go into the requested
                mode. If the remote host says it can, that mode takes effect.

                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
                              LINEMODE option be enabled.

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

     open host [[-l] user][- port]
                Open a connection to the named host.  If no port number is
                specified, telnet will attempt to contact a telnet daemon at
                the standard port (23).  The host specification may be a host
                name or IP address.  The -l option may be used to specify a
                user name to be passed to the remote system, like the -l com-
                mand-line option.

                When connecting to ports other than the telnet port, telnet
                does not attempt telnet protocol negotiations. This makes it
                possible to connect to services that do not support the telnet
                protocol without making a mess. Protocol negotiation can be
                forced by placing a dash before the port number.

                After establishing a connection, any commands associated with
                the remote host in /etc/telnetrc and the user's .telnetrc file
                are executed, in that order.

                The format of the telnetrc files is as follows: Lines begin-
                ning with a #, and blank lines, are ignored.  The rest of the
                file should consist of hostnames and sequences of telnet com-
                mands to use with that host. Commands should be one per line,
                indented by whitespace; lines beginning without whitespace are
                interpreted as hostnames.  Lines beginning with the special
                hostname 'DEFAULT' will apply to all hosts.  Hostnames includ-
                ing 'DEFAULT' may be followed immediately by a colon and a
                port number or string.  If a port is specified it must match
                exactly with what is specified on the command line.  If no
                port was specified on the command line, then the value
                'telnet' is used.  Upon connecting to a particular host, the
                commands associated with that host are executed.

     quit       Close any open session and exit telnet.  An end of file condi-
                tion on input, when in command mode, will trigger this opera-
                tion as well.

     send arguments
                Send one or more special telnet protocol character sequences
                to the remote host.  The following are the codes which may be
                specified (more than one may be used in one command):

                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.

                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,
                        getstatus will send the subnegotiation to request that
                        the server send its current option status.

                ip      Sends the TELNET IP (Interrupt Process) sequence,
                        which should cause the remote system to abort the cur-
                        rently running process.

                nop     Sends the TELNET NOP (No Operation) sequence.

                susp    Sends the TELNET SUSP (Suspend Process) sequence.

                synch   Sends the TELNET SYNCH sequence.  This sequence causes
                        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).

                do cmd

                dont cmd

                will cmd

                wont cmd
                        Sends the TELNET DO cmd sequence.  cmd can be either a
                        decimal number between 0 and 255, or a symbolic name
                        for a specific TELNET command.  cmd can also be either
                        help or ? to print out help information, including a
                        list of known symbolic names.

                ?       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 variables.  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.

                ayt     If telnet is in localchars mode, or LINEMODE is en-
                        abled, and the status character is typed, a TELNET AYT
                        sequence is sent to the remote host.  The initial
                        value for the "Are You There" character is the termi-
                        nal's status character.

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

                eof     If telnet is operating in LINEMODE or "old line by
                        line" mode, entering this character as the first char-
                        acter 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 character.

                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.

                forw1

                forw2   If TELNET is operating in LINEMODE, these are the
                        characters that, when typed, cause partial lines to be
                        forwarded to the remote system.  The initial value for
                        the forwarding characters are taken from the termi-
                        nal's eol and eol2 characters.

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

                kill    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 EL sequence (see send el above) is sent to the
                        remote system.  The initial value for the kill charac-
                        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.

                rlogin  This is the rlogin mode escape character. Setting it
                        enables rlogin mode, as with the r command-line option
                        (q.v.)

                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
                        This 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.

                ?       Displays the legal set (unset) commands.

     slc state  The slc command (Set Local Characters) is used to set or
                change the state of the the special characters when the TELNET
                LINEMODE option has been enabled.  Special characters are
                characters that get mapped to TELNET commands sequences (like
                ip or quit) or line editing characters (like erase and kill).
                By default, the local special characters are exported.

                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 discrepancies with the local
                            side, the local side will switch to the remote
                            value.

                export      Switch to the local defaults for the special char-
                            acters.  The local default characters are those of
                            the local terminal at the time when telnet was
                            started.

                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.

                ?           Prints out help information for the slc command.

     status     Show the current status of telnet.  This includes the name of
                the remote host, if any, as well as the current mode.

     toggle arguments ...
                Toggle (between TRUE and FALSE) various flags that control how
                telnet responds to events.  These flags may be set explicitly
                to TRUE or FALSE using the set and unset commands.  More than
                one flag may be toggled at once.  The state of these flags may
                be examined with the display command.  Valid flags are:

                authdebug     Turns on debugging for the authentication code.
                              This flag only exists if authentication support
                              is enabled.

                autoflush     If autoflush and localchars are both TRUE, then
                              when the ao, or quit characters are recognized
                              (and transformed into TELNET sequences; see set
                              above for details), telnet refuses to display
                              any data on the user's terminal until the remote
                              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)).

                autodecrypt   When the TELNET ENCRYPT option is negotiated, by
                              default the actual encryption (decryption) of
                              the data stream does not start automatically.
                              The autoencrypt (autodecrypt) command states
                              that encryption of the output (input) stream
                              should be enabled as soon as possible.

                              Note that this flag exists only if encryption
                              support is enabled.

                autologin     If the remote side supports the TELNET
                              AUTHENTICATION option, telnet attempts to use it
                              to perform automatic authentication.  If the
                              TELNET AUTHENTICATION option is not supported,
                              the user's login name is propagated using the
                              TELNET NEW-ENVIRON option.  Setting this flag is
                              the same as specifying the a option to the open
                              command or on the command line.

                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
                              never 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
                              toggle is FALSE.

                encdebug      Turns on debugging information for the encryp-
                              tion code.  Note that this flag only exists if
                              encryption support is available.

                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
                              TRUE in "old line by line" mode, and FALSE in
                              "character at a time" mode.  When the LINEMODE
                              option is enabled, the value of localchars is
                              ignored, and assumed to always be TRUE.  If
                              LINEMODE has ever been enabled, then quit is
                              sent as abort, and eof and suspend are sent as
                              eof and susp, see send above).

                netdata       Toggles the display of all network data (in
                              hexadecimal format).  The initial value for this
                              toggle is FALSE.

                options       Toggles the display of some internal telnet pro-
                              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 formatted in a more
                              user-readable format.  Spaces are put between
                              each character in the output, and the beginning
                              of telnet escape sequences are preceded by a '*'
                              to aid in locating them.

                skiprc        When the skiprc toggle is TRUE, telnet does not
                              read the telnetrc files.  The initial value for
                              this toggle is FALSE.

                termdata      Toggles the display of all terminal data (in
                              hexadecimal format).  The initial value for this
                              toggle is FALSE.

                verbose_encrypt
                              When the verbose_encrypt toggle is TRUE, TELNET
                              prints out a message each time encryption is en-
                              abled or disabled.  The initial value for this
                              toggle is FALSE. This flag only exists if en-
                              cryption support is available.

                ?             Displays the legal toggle commands.

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

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

     ? [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 environment vari-
     ables.  Other environment variables may be propagated to the other side
     via the TELNET NEW-ENVIRON option.

FILES
     /etc/telnetrc  global telnet startup values
     ~/.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.

BUGS
     The source code is not comprehensible.

Linux NetKit (0.17)             August 15, 1999            Linux NetKit (0.17)
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