`telnet`

user interface to the TELNET protocol

Syntax:

telnet [-8] [-a] [-D] [-d] [-e escape_char] 
       [-n tracefile] [[-l user] host [port]]

Options:

-8: Allow an eight-bit input data path at all times. Without this option, parity bits are stripped whenever the remote side's stop and start characters are ^S and ^Q.
-a: Attempt automatic login. Currently, this sends the username via the USER variable of the ENVIRON option if supported by the remote system. The name used is that of the current user (as returned by getlogin()) if that name agrees with the current user ID. Otherwise, the name used is the one associated with the user ID.
-D: Disable telnet escape character.
-d: Set the initial value of the debug toggle to TRUE.
-e escape_char: Set the initial telnet escape character to escape_char (default is Ctrl -] ). This character lets you switch to telnet's command mode.
-l user: When connecting to the remote system, if the remote system understands the ENVIRON option, send user to the remote system as the value for the variable USER. This option implies the -a option and may may also be used with the open command.
-n tracefile: Record trace information in the specified file. See the set tracefile command, below.
host: The official name, an alias, or the Internet address of a remote host.
port: A port number (address of an application). If a number isn't specified, the default telnet port is used.

Description:

You use the telnet command to communicate with another host via the TELNET protocol. If you invoke telnet without a host argument, it enters command mode and displays this prompt:

telnet>

In command mode, telnet accepts and executes the commands listed in the following section, ``Telnet commands.''

If you invoke telnet with a host argument, it opens a connection to the named host (i.e. it performs an open command).

If LINEMODE is enabled, character processing is done on the local system, 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 ``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 (in most cases) only completed lines are sent to the remote host. You can use the ``local echo character'' (initially Ctrl -E ) to turn off and on the local echo (you would use this mostly 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''), your quit, intr, and flush characters are trapped locally and sent as TELNET protocol sequences to the remote side.

If LINEMODE has ever been enabled, then your susp and eof are also sent as TELNET protocol sequences, and quit is sent as a TELNET ABORT instead of as a TELNET BREAK. There are options (see toggle autoflush and toggle autosynch below) that cause this action to flush subsequent terminal output (until the remote host acknowledges the TELNET sequence) and to flush previous terminal input (in the case of quit and intr).

While you're connected to a remote host, you can enter telnet's command mode by entering the telnet escape sequence (initially Ctrl -] ). The normal terminal editing conventions are available in command mode.

Telnet command

In the following commands, you need to type only enough of each command to uniquely identify it; 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...

Display all, or some, of the set and toggle values (see below).

mode type

The type argument can have one of several values, 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 can enter that mode, the mode will be entered.

?: Print out help information for the mode command.
character: Disable the TELNET LINEMODE option; if the remote side doesn't understand the LINEMODE option, enter character-at-a-time mode instead.
line: Enable the TELNET LINEMODE option; if the remote side doesn't understand the LINEMODE option, attempt to enter old-line-by-line mode instead.
[-]edit: Attempt to enable (disable) the EDIT mode of the LINEMODE option. LINEMODE must be enabled.
[-]isig: Attempt to enable (disable) the TRAPSIG mode of the LINEMODE option. LINEMODE must be enabled.
[-]litecho: Attempt to enable (disable) the LIT_ECHO mode of the LINEMODE option. LINEMODE must be enabled.
[-]softtabs: Attempt to enable (disable) the SOFT_TAB mode of the LINEMODE option. LINEMODE must be enabled.

open host [[-l] user] [-port]

Open a connection to the named host. If no port number is specified, telnet will try to contact a TELNET server at the default port. The host may be specified either as a hostname (see the hosts file) or as an Internet address specified in the ``dot notation'' (see the Internet address manipulation routines, inet*()). .

With the -l option, you can specify the username to be passed to the remote system via the ENVIRON option.

When connecting to a nonstandard port, telnet omits any automatic initiation of TELNET options. When port is preceded by a minus sign, the initial option negotiation is done. Once telnet establishes a connection, it opens the .telnetrc file that's in your home directory.

In this file, lines beginning with a pound sign (#) are comments. 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's being connected to. The rest of the line and the successive lines that begin with whitespace are assumed to be telnet commands; they're processed as if you had typed them manually at 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

Send one or more special character sequences to the remote host. You can specify the following arguments (more than one argument may be specified at a time):

?: Print out help information for the send command.
abort: Send the TELNET ABORT (Abort processes) sequence.
ao: Send the TELNET AO (Abort Output) sequence, which should cause the remote system to flush all its output to the user's terminal.
ayt: Send the TELNET AYT (Are You There) sequence, to which the remote system may or may not choose to respond.
brk: Send the TELNET BRK (Break) sequence, which may have significance to the remote system.
ec: Send the TELNET EC (Erase Character) sequence, which should cause the remote system to erase the last character entered.
el: Send the TELNET EL (Erase Line) sequence, which should cause the remote system to erase the line currently being entered.
eof: Send the TELNET EOF (end-of-file) sequence.
eor: Send the TELNET EOR (end-of-record) sequence.
escape: Send the current telnet escape character (initially Ctrl -] ).
ga: Send 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, send the subnegotiation to request that the server send its current option status.
ip: Send the TELNET IP (Interrupt Process) sequence, which should cause the remote system to abort the currently running process.
nop: Send the TELNET NOP (No OPeration) sequence.
susp: Send the TELNET SUSP (SUSPend process) sequence.
synch: Send 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 urgent data - it might not work if the remote system is a 4.2BSD system. If it doesn't work, a lowercase ``r'' might be echoed on the terminal.

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 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.

You can query the values of variables with the display command.

The telnet variables that can be set or unset, but not toggled, are listed here. Note that you also explicitly set or unset any of the toggle command's variables.

?: Display the legal set (unset) commands.
echo: In old-line-by-line mode, this value (initially Ctrl -E ) toggles between doing local echoing of entered characters (for normal processing) and suppressing echoing of entered characters (e.g. for entering a password).
eof: If telnet is operating in LINEMODE or old-line-by-line mode, this character will be sent to the remote system if entered as the first character on a line. The initial value of the eof character is the terminal's eof character.
erase: If telnet is in localchars mode (see toggle localchars below) and 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 character is the terminal's erase character.
escape: This is the telnet escape character; entering it switches you to telnet's command mode when telnet is connected to a remote system. (Initially, the character is Ctrl -] to switch).
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 character is the terminal's flush character.
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 the terminal's intr character.
kill: If telnet is in localchars mode (see toggle localchars below) and 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 character is the terminal's kill character.
lnext: If telnet is in LINEMODE or old-line-by-line mode, then this character is the terminal's lnext> character. The initial value for the lnext character is 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 the terminal's quit character.
reprint: If telnet is in LINEMODE or old-line-by-line mode, then this character is the terminal's reprint character. The initial value for the reprint character is the terminal's reprint character.
start: If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is the terminal's start character. The initial value for the kill character is the terminal's start character.
stop: If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is the terminal's stop character. The initial value for the kill character is the terminal's stop character.
susp: If telnet is in localchars mode, or if LINEMODE is enabled, and the suspend character is typed, a TELNET SUSP sequence (see send susp above) is sent to the remote host. The initial value for the suspend character is the terminal's suspend character.
tracefile: The file where the output (caused by netdata or options tracing being TRUE) will be written. If it's 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 the terminal's worderase character. The initial value for the worderase character is the terminal's worderase character.

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. Special characters are characters that get mapped to TELNET command sequences (like ip or quit) or line-editing characters (like erase and kill).

By default, the local special characters are exported.

?: Print out help information for the slc command.
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 remote value.
export: Switch to the local defaults for the special characters. 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.

environ argument...

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 DISPLAY and PRINTER variables being exported by default. The USER variable is also exported when you invoke telnet with -a or -l. Valid arguments are:

?: Print out help information for the environ command.
define variable value: Define this variable to have this value. Any variables defined by this command aren't automatically exported. To include Tabs and Spaces in value, enclose it in single or double quotes.
export variable: Mark variable to be exported to the remote side.
list: List the current set of environment variables. Those marked with a * will be sent automatically; other variables will be sent only if explicitly requested.
send variable: Send variable to the remote site.
undefine variable: Remove variable from the list of environment variables.
unexport variable: Mark variable not to be exported unless explicitly asked for by the remote side.

toggle flag...

Toggle between TRUE and FALSE various flags that control how telnet responds to events. You can explicitly set these flags to TRUE or FALSE with the set and unset commands listed above. To query the state of these flags, use the display command. Note that you can specify more than one flag.

Valid arguments are:

?: Display the legal toggle commands.
autoflush: If autoflush and localchars are both TRUE, and the ao or quit characters are recognized (and transformed into TELNET sequences; see set above for details), 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. If the terminal user hasn't done an stty noflsh, the initial value for this toggle is TRUE; otherwise it is FALSE. See stty.
autosynch: If autosynch and localchars are both TRUE, and either the intr or quit character is typed (see set above), send the resulting TELNET sequence and follow it with 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.
crlf: If this is TRUE, send carriage returns as CR LF. If this is FALSE, send carriage returns 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 received from the remote host will be mapped into a carriage return followed by a line feed. This mode doesn't affect characters you type, only those received from the remote host. This mode is useful only if the remote host sends only carriage returns and never sends line feeds. The initial value for this toggle is FALSE.
debug: Toggle socket-level debugging (useful only to the superuser). The initial value for this toggle is FALSE.
inbinary: Enable or disable the TELNET BINARY option on input only.
localchars: If this is TRUE, then the flush, interrupt, quit, erase, and kill characters (see set above) are recognized locally, and should be transformed into 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 be always TRUE. If LINEMODE has ever been enabled, then quit is sent as abort, and eof and susp are sent as eof and susp (see send above).
netdata: Toggle the display of all network data (in hex format). The initial value for this toggle is FALSE.
options: Toggle the display of some internal telnet protocol processing (having to do with TELNET options). The initial value for this toggle is FALSE.
outbinary: Enable or disable the TELNET BINARY option on output only.
prettydump: If the netdata and prettydump toggles are both 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 any TELNET escape sequence is preceded by a * to aid in locating it.

z: Suspend telnet. This command works only if you're using csh.
! [command]: Execute a single command in a subshell on the local system. If command is omitted, an interactive subshell is invoked.
status: Show the current status of telnet. This includes the peer you're 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 for that command.

Files:

$HOME/.telnetrc: User-customized startup values for telnet.

Environment variables:

The telnet utility uses at least the HOME, SHELL, DISPLAY, and TERM environment variables. The environment variables may be propagated to the other side via the TELNET ENVIRON option.

Caveats:

On some remote systems, you have to turn echo off manually when you're in old-line-by-line mode.

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