telnet(1)

Requires Optional ARPA Services Software

NAME

telnet − user interface to the TELNET protocol

SYNOPSIS

telnet [host [port]]

DESCRIPTION

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

Once a connection has been opened, telnet enters an input mode. The input mode entered will be either “character at a time” or “line by line”, depending on what the remote system supports.

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

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

In either mode, if the localchars toggle is TRUE (the default in line mode; see below), the user’s quit and intr characters are trapped locally, and sent as TELNET protocol sequences to the remote side. 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 can be entered by typing the telnet “escape character” (initially ^] ). When in command mode, the normal terminal editing conventions are available.

telnet supports eight-bit characters when communicating with the server on the remote host. To use eight-bit characters you may need to reconfigure your terminal or the remote host appropriately (see stty(1)). Furthermore, you may have to use the binary toggle to enable an 8-bit data stream between telnet and the remote host. Note that some remote hosts may not provide the necessary support for eight-bit characters.

If, at any time, telnet is unable to read from or write to the server over the connection, the message Connection closed by foreign host. is printed on stderr, and telnet exits with a value of one.

Commands

The following commands are available in command mode. You need only type enough of each command to uniquely identify it (this is also true for arguments to the mode, set, toggle, and display commands).

open host [ port ]
Open a connection to the named host at the indicated port. If no port is specified, telnet attempts to contact a TELNET server at the standard TELNET port. The hostname can be either the official name or an alias as understood by gethostbyname() (see gethostent(3n)), or an Internet address specified in the dot notation as described in hosts(4). If no hostname is given, telnet prompts for one.

close Close a TELNET session. If the session was started from command mode, then telnet returns to command mode; otherwise telnet exits.

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

z Suspend telnet. If telnet is run from a shell that supports job control, (csh(1) or ksh(1)), the z command suspends the TELNET session and returns the user to the shell that invoked telnet. The job can then be resumed with the fg command (see csh(1) or ksh(1)).

mode Mode
Change telnet’s user input mode to Mode. Mode can be character (for “character at a time” mode) or line (for “line by line” mode). The remote host is asked for permission to go into the requested mode. If the remote host is capable of entering that mode, the requested mode is entered. In character mode, telnet sends each character to the remote host as it is typed. In line mode, telnet gathers user input into lines and transmits each line to the remote host when the user types carriage return, linefeed, or EOF (normally ^D; see stty(1)). Note that setting line mode also sets local echo. Applications that expect to interpret user input character by character (such as more(1), csh(1), ksh(1), and vi(1), do not work correctly in line mode.

status
Show current status of telnet. telnet reports the current escape character. If telnet is connected, it reports the host to which it is connected and the current mode. If telnet is not connected to a remote host, it reports No connection.

display [ argument... ]
Displays all or some of the set and toggle values (see below).

? [ command ]
Get help. With no arguments, telnet prints a help summary. If a command is specified, telnet prints the help information available about that command only. Help information is limited to a one-line description of the command.

! [ shellcommand ]
Shell escape. The SHELL environment variable is checked for the name of a shell to use to execute the command. If no command is specified, a shell is started and connected to the user’s terminal. If SHELL is undefined, /bin/sh is used.

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

escape
Sends the current telnet escape character (initially “^]”).

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 urgent data (and may not work to some systems -- if it doesn’t work, a lower case “r” may be echoed on the terminal).

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

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

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 respond.

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

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

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

nop
Sends the TELNET NOP (No OPeration) sequence.

?
Prints out help information for the send command.

set argument value
Set any one of a number of telnet variables to a specific value. The special value “off” turns off the function associated with the variable. The values of variables may be interrogated with the display command. The variables which may be specified are:

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 entering, say, a password).

escape
This is the telnet escape character (initially “^[”) which causes entry into telnet command mode (when connected to a remote system).

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.

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.

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 “^O”.

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 character is taken to be the terminal’s erase 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 character is taken to be the terminal’s kill character.

eof
If telnet is operating in “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 character.

toggle arguments...
Toggle (between TRUE and FALSE) various flags that control how telnet responds to events. More than one argument may be specified. The state of these flags may be interrogated with the display command. Valid arguments are:

localchars
If this is TRUE, then the flush, interrupt, quit, erase, and kill characters (see set above) are recognized locally, and 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 “line by line” mode, and FALSE in “character at a time” mode.

autoflush
If autoflush and localchars are both TRUE, then when the ao, intr, 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 sequences. The initial value for this toggle is TRUE.

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. This option should be enabled in order to send and receive 8-bit characters to and from the TELNET server.

crlf
If this is TRUE, then carriage returns will be sent as <CR><LF>. If this is FALSE, then carriage 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, any carriage return characters received from the remote host are mapped into a carriage return and a line feed. This mode does not affect those characters typed by the user, only those received. This mode is only required for some hosts that require the client to do local echoing, but output “naked” carriage returns. The initial value for this toggle is FALSE.

echo Toggle local echo mode or remote echo mode. In local echo mode, user input is echoed to the terminal by the local telnet before being transmitted to the remote host. In remote echo, any echoing of user input is done by the remote host. Applications that handle echoing of user input themselves, such as csh(1), ksh(1), and vi(1), will not work correctly with local echo.

options
Toggle viewing of TELNET options processing. When options viewing is enabled, all TELNET option negotiations are displayed. Options sent by telnet are displayed as “SENT”, while options received from the TELNET server are displayed as “RCVD”. The initial value for this toggle is FALSE.

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

?
Displays the legal toggle commands.

RETURN VALUE

In the event of an error, or if the TELNET connection is closed by the remote host, telnet returns a value of one. Otherwise it returns a zero.

DIAGNOSTICS

The following diagnostic messages are displayed by telnet:

telnet/tcp: Unknown service
telnet was unable to find the TELNET service entry in the services(4) database .

<hostname>: Unknown host
telnet was unable to map the host name to an Internet address. Your next step should be to contact the system administrator to check whether there is an entry for the remote host in the hosts(4) database.

?Invalid command
You have typed an invalid command in telnet command mode.

< system call >: ...
An error occurred in the specified system call. See the appropriate manual page for a description of the error.

AUTHOR

telnet was developed by the University of California, Berkeley.

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