telnet(TC) telnet(TC) Name telnet - User interface to a remote system using the TELNET protocol Syntax telnet [ -8 ] [ -E ] [ -L ] [ -a ] [ -d ] [ -e escapechar ] [ -l user ] [ -n tracefile ] [ -r ] [ host [ port ] Description The telnet command communicates with another host using the TELNET proto- col. If telnet is invoked without the host argument, it will enter com- mand mode as indicated by its prompt telnet>. In this mode, telnet will accept and execute the commands listed below; if telnet is invoked with arguments, it will perform an open command (see ``telnet commands'' below) with those arguments. The following options are available: -8 Use an eight bit data path. This will cause an attempt to nego- tiate the BINARY option on both input and output. -E Option stops any character from being recognized as an escape character. -L Use an eight bit data path on output. This causes the BINARY option to be negotiated on output. -a Automatic login into the remote system. If the remote system understands the ENVIRON option, then the variable USER will be sent to the remote system. This option may also be used with the open command. -d Toggles socket level debugging (useful only to the super user). Sets the initial value of the debug toggle to ``TRUE''. -e [ escapechar ] Sets the initial TELNET escape character to escapechar. If escapechar is omitted, then there will be no pre-defined escape character. -l user When connecting to the remote system and if the remote system understands the ENVIRON option, then user will be sent to the remote system as the value for the variable USER. This option may also be used with the open com- mand. -n tracefile Opens tracefile for recording the trace information. (See the settracefile command below.) -r Use a user interface similar to rlogin(TC). In this mode, the escape character is set to the tilde (~) character, unless modi- fied by the -e flag. host Indicates the host's official name: an alias or the Internet address of a remote host. port Indicates a port number (that is, the address of an application). If a number is not specified, the default TELNET port will be used. Once a connection has been opened, TELNET will enter the ``input mode''. TELNET will attempt to enable the TELNET LINEMODE option. If this fails, then TELNET will revert to one of two input modes: either the ``charac- ter at a time'' mode or the old ``line by line'' mode, depending on what the remote system supports. When LINEMODE is enabled, character processing will be done on the local system while under the control of the remote system. When input editing or character echoing is to be disabled, the remote system will relay that information. The remote system will also relay changes to any special characters that happen on the remote system, so that they can take effect on the local system. In the character at a time mode, most entered text will be sent immedi- ately to the remote host for processing. In the old line by line mode, all text will be echoed locally, but (nor- mally) only completed lines will be sent to the remote host. The ``local echo character'' (initially ``^E'') may be used to enable and disable the local echo mode; normally, this would be used only for entering passwords so that the password will not be echoed. If the LINEMODE option is enabled or if the localchars toggle is ``TRUE'' (the default value for the old line by line mode; see below), the user's quit, intr, and flush characters will be trapped locally and sent as TEL- NET protocol sequences to the remote machine. If LINEMODE had been enabled at any earlier time, then the user's susp and eof characters will also be sent as TELNET protocol sequences; quit will be sent as a TELNET ABORT instead of BREAK. There are options (see toggle autoflush and tog- gle autosynch below) which cause this action to flush any subsequent out- put to the terminal (until the remote host acknowledges the TELNET sequence) and to flush previous terminal input (in the case of quit and intr). While connected to a remote host, the telnet command mode may be entered by typing the TELNET <Esc> (initially ``^]''). When in command mode, the normal terminal editing conventions will be available. The following TELNET commands are available, but only enough of each com- mand need be typed to uniquely identify it (this is also true for argu- ments pertaining to the mode, toggle, set, unset, slc, environ, and dis- play commands). close Close a TELNET session and return to command mode. display argument ... Displays all, or some, of the set and toggle values (see descrip- tion below). mode [ type ] Depending on the state of the TELNET session, the type argument is one of several available options. The remote host will be asked for permission to go into the requested mode. If the remote host is capable of entering that mode, the requested mode will be entered. character Disable the TELNET LINEMODE option; or, if the remote side does not understand the TELNET LINEMODE option, then enter the character at a time mode. line Enable the TELNET LINEMODE option; or, if the remote side does not understand the TELNET LINEMODE option, then attempt to enter the old line by line mode. isig -isig Attempt to enable (disable) the TRAPSIG mode of the TEL- NET 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 SOFTTAB mode of the LINEMODE option. This requires that the LINEMODE option be enabled. litecho -litecho Attempt to enable (disable) the LITECHO mode of the LINEMODE option. This requires that the LINEMODE option be enabled. ? Prints out help information for the mode command. open host [ user ] [ [ - ] port ] [ -a ] [ -l user ] Open a connection to the named host. If no port number is speci- fied, telnet will attempt to contact a TELNET server at the default port. The host specification may be either a host name (see hosts(SFF)) or an Internet address specified in the ``dot notation'' (see inet(SLIB)). The -l or the -a option may be used to specify the user name to be passed to the remote system via the ENVIRON option. When connecting to a non-standard port, telnet will omit the automatic initiation of any TELNET options. When the port number is preceded by a minus sign, the initial option negotiation will be done as follows: After establishing a connection, the file .telnetrc in the user's home directory will be opened. Lines beginning with a ``#'' will be treated as comment lines; blank lines will be ignored. Lines that begin without whitespace will be the start of a machine entry. The first thing on the line will be the name of the machine to which this host is being connected. The rest of the line -- and successive lines which begin with whitespace -- will be assumed to be telnet commands and will be processed as if they had been entered manually in response to the telnet command prompt. quit Close any open TELNET session and exit telnet. When in command mode, an End-of-File (EOF) 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 given 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; the remote system may or may not choose to respond to this transmission. brk Sends the TELNET BRK (Break) sequence which may have sig- nificance to the remote system. ec Sends the TELNET EC (Erase Character) sequence which should cause the remote system to erase the last charac- ter entered. el Sends the TELNET EL (Erase Line) sequence which should cause the remote system to erase the line currently 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 probably has no significance to the remote system. getstatus If the remote side supports the TELNET STATUS command, getstatus will send the subnegotiation 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 currently 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 will be sent as TCP urgent data (and may not work if the remote system is a 4.2 BSD system; if it does not work, a lowercase ``|'' 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 variables to a specific value or to ``TRUE''. The special value off will turn off the function associated with this 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 aid of the display command. The variables which may be set or unset -- but not toggled -- are listed here. In addition, any of the variables for the toggle command may be explicitly enabled or disabled using the set and unset commands. echo This is the value (initially ``^['') which, when in the line by line mode, will toggle between doing local echo- ing of entered characters (for normal processing) and suppressing echoing of entered characters (for example, for entering a password). eof If telnet is operating in LINEMODE or in the 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'' charac- ter is taken to be the terminal's eof character. erase If telnet is in localchars mode (see ``toggle'' local- chars below), and if telnet is operating in the character at a time mode, then when this character is entered, a TELNET EC sequence (see send ec above) will be 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 the 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 entered, a TELNET AO sequence (see send ao above) will be sent to the remote host. The initial value for the flush character is taken to be the terminal's flush character. interrupt If TELNET is in localchars mode (see toggle localchars below) and the interrupt character is entered, a TELNET IP sequence (see send ip above) will be sent to the remote host. The initial value for the interrupt charac- ter 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 the character at a time mode, then when this character is entered, a TELNET EL sequence (see send el above) will be sent to the remote system. The initial value for the kill character is taken to be the terminal's kill character. lnext If TELNET is operating in LINEMODE or in the 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 char- acter. quit If TELNET is in localchars mode (see toggle localchars below) and the quit character is entered, a TELNET BRK sequence (see send brk above) will be 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 in the 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 enabled, then this character is taken to be the terminal's start character. The initial value for the start character is taken to be the terminal's start char- acter. stop If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is taken to be the terminal's stop character. The initial value for the stop character is taken to be the terminal's stop charac- ter. forw1 If TELNET is in the localchars mode, then this character is taken to be an alternate end of line character. forw2 If TELNET is in the localchars mode, then this character is taken to be an alternate end of line character. ayt If TELNET is in the localchars mode, then this character is taken to be the alternate AYT character. susp If TELNET is in the localchars mode or if the LINEMODE is enabled and the suspend character is entered, a TELNET SUSP sequence (see send susp above) will be sent to the remote host. The initial value for the suspend character is taken to be the terminal's suspend character. tracefile This is the file to which the output generated by the netdata command will be written. worderase If TELNET is operating in LINEMODE or in the 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 and unset commands. slc [ state ] The slc command (Set Local Characters) sets (or changes) the state of the special characters when the TELNET LINEMODE option has been enabled. The ``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. export Switch to the local defaults for the ``special charac- ters''. 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 charac- ters''. 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 ``special characters''. The remote side is requested to send all the current special character settings; if there are any discrepancies with the local side, the local side will switch to the set of remote values. ? Prints out help information for the slc command. environ[arguments[...] The environ command manipulates the variables that may be sent through the TELNET ENVIRON option. The initial set of variables is taken from the user's environment; with only the USER and DISPLAY variables being exported. The valid arguments for the environ command are: define variable value Define the variable to have a value of value. Any vari- ables defined by this command are automatically exported. The value may be enclosed in single or double quotes so that tabs and embedded spaces may be included. undefine variable Remove variable from the list of environment variables. export variable Mark the variable to be exported to the remote side. unexport variable Mark the variable to not be exported unless explicitly requested by the remote side. list List the current set of environment variables. Those marked with a ``*'' will be sent automatically; any other variables will be sent only if requested explicitly. send variable Send environment variable. ? Prints out help information for the environ command. toggle arguments [ ... ] Toggle various flags (between ``TRUE'' and ``FALSE'') that con- trol how TELNET responds to events. These flags may be set ex- plicitly to ``TRUE'' or ``FALSE'' using the set and unset com- mands listed above. More than one argument may be specified. The state of these flags may be interrogated with the aid of the display command. The valid arguments are: autoflush If autoflush and localchars are both ``TRUE'', then when the ao or the quit characters are recognized (and transformed into TELNET sequences; see set above for details), TELNET will refuse 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 sequences. The initial value for this toggle is ``TRUE'' if the terminal user had not executed an stty noflsh; otherwise ``FALSE'' (see stty(C)). autosynch If autosynch and localchars are both ``TRUE'', then when either the intr or quit character is entered (see set above for descriptions of the intr and quit characters), the resulting TELNET sequence sent will be followed by the TELNET SYNCH sequence. This procedure should cause the remote system to begin throwing away all previously entered input until both of the TELNET sequences have been read and acted upon. The initial value of this tog- gle is ``FALSE''. binary Enable or disable the TELNET BINARYs+1 option on both the 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 toggle value is ``TRUE'', then Carriage Returns will be sent as ``<CR><LF>''. If this is ``FALSE'', then Carriage Returns will be sent as ``<CR><NUL>''. The ini- tial value for this toggle is ``FALSE''. crmod Toggle the Carriage Return mode. When this mode is enabled, most Carriage Return characters received from the remote host will be mapped into a Carriage Return followed by a Line Feed. This mode does not affect those characters entered by the user, but only those received from the remote host. This mode is not very useful unless the remote host only sends Carriage Return, but never any Line Feeds. The initial value for this toggle is ``FALSE''. debug Toggles the socket level debugging mode (useful only to the super-user). The initial value for this toggle is ``FALSE''. localchars If this is ``TRUE'', then the flush, interrupt, quit, erase, and kill characters (see set above) are recognized locally and then transformed into appropriate TELNET con- trol 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 char- acter at a time mode. When the LINEMODE option is enabled, the value of local- chars is ignored and assumed to always be ``TRUE''. If ``LINEMODE'' has ever been enabled, then quit will be sent as abort; eof and suspend will be 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 protocol pro- cessing which pertain to TELNET options. The initial value for this toggle is ``FALSE''. prettydump When the netdata toggle is enabled and if prettydump is enabled, the output from the netdata command will be reorganized into a more user-friendly format. Spaces will be put between each character in the output and the beginning of any TELNET escape sequence will be preceded by a ``*'' to aid in locating them. skiprc Toggle does not process ~/.telnetrc file. The initial value for this toggle is ``FALSE''. termdata Toggles printing of hexadecimal terminal data. The ini- tial value for this toggle is ``FALSE''. ? Displays the legal toggle commands. <Ctrl>Z Suspend telnet. This command will work only when the user is using csh(C) or ksh(C). ! [ command ] Execute a single command in a subshell on the local system. If command is omitted, then an interactive subshell will be invoked. status Show the current status of telnet. This includes the peer to which one is connected, as well as the current mode. ? [ command ] Get help. When no command is specified, telnet will print a sum- mary for the help command. If a command is specified, telnet will print the help information for just that command. Files $HOME/.telnetrc user-customized telnet startup values Environments The telnet command uses at least the following environment variables: HOME, SHELL, USER, DISPLAY, and TERM. Other environment variables may be propagated to the other side via the TELNET ENVIRON option. History The telnet command appeared in 4.2 BSD. User considerations On some remote systems, the echo command has to be turned off manually when in the old line by line mode. When in the old line by line mode or in LINEMODE, the terminal's eof character is only recognized (and sent to the remote system) when it is the first character in a line.