TELNET(1) BSD General Commands Manual TELNET(1)
telnet − user interface to the TELNET protocol
telnet [−8EFKLacdfrx] [−X authtype] [−b hostalias] [−e escapechar] [−k realm] [−l user] [−n tracefile] [
The telnet command is used to communicate with another host using the TELNET protocol. If telnet is invoked without the host argument, it enters 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.
The options are as follows:
−8’ Specifies an 8-bit data path. This causes an attempt tonegotiate the TELNET BINARY option on both input and output.
−E’ Stops any character from being recognized as an escape character.
−F’ If Kerberos V5 authentication is being used, the −F option allows the local credentials to be forwarded to the remote system, including any credentials that have already been forwarded into the local environment.
−K’ Specifies no automatic login to the remote system.
−L’ Specifies an 8-bit data path on output. This causes the BINARY option to be negotiated on output.
−a’ Attempt automatic login. Currently, this sends the user name 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(2) if it agrees with the current user ID, otherwise it is the name associated with the user ID.
−c’ Disables the reading of the user’s .telnetrc file. (See the toggle skiprc command on this man page.)
−d’ Sets the initial value of the debug toggle to TRUE.
−f’ If Kerberos V5 authentication is being used, the −f option allows the local credentials to be forwarded to the remote system.
−r’ Specifies a user interface similar to rlogin(1). In this mode, the escape character is set to the tilde (~) character, unless modified by the −e option.
−x’ Turns on encryption of the data stream if possible.
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.
When in rlogin mode, a line of the form ~. disconnects from the remote host; ~ is the telnet escape character. Similarly, the line ~^Z suspends the telnet session. The line ~^] escapes to the normal telnet escape prompt.
Once a connection has been opened, telnet will attempt to enable the TELNET LINEMODE option. If this fails, 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 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 (normally) only completed lines are sent to the remote host. The ‘‘local echo character’’ (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 sequences 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 command mode, the normal terminal editing conventions are available. Note that the escape character will return to the command mode of the initial invocation of telnet that has the controlling terminal. Use the send escape command to switch to command mode in subsequent telnet processes on remote hosts.
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).
auth argument [...]
status’ Lists the current status of the various types of authentication.
close’ Close a TELNET session and return to command mode.
display argument [...]
encrypt argument [...]
Valid arguments for the encrypt command are as follows:
disable type [input|output]
enable type [input|output]
input’ This is the same as the encrypt start input command.
-input’ This is the same as the encrypt stop input command.
output’ This is the same as the encrypt start output command.
-output’ This is the same as the encrypt stop output command.
status’ Lists the current status of encryption.
type type’ Sets the default type of encryption to be used with later encrypt start or encrypt stop commands.
environ arguments [...]
define variable value
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 command.
logout’ Sends the TELNET LOGOUT option to the remote side. This command is similar to a close command; however, if the remote side does not support the LOGOUT option, nothing happens. If, however, the remote side does support the LOGOUT option, this command should cause the remote side to close the TELNET connection. If the remote side also supports the concept of suspending a user’s session for later reattachment, the logout argument indicates that you should terminate the session immediately.
character’ Disable the TELNET LINEMODE option, or, if the remote side does not understand the LINEMODE option, then enter ‘‘character at a time’’ mode.
line’ Enable the TELNET LINEMODE option, or, if the remote side does not understand the LINEMODE option, then attempt to enter ‘‘old-line-by-line’’ mode.
?’ Prints out help information for the mode command.
open host [−l user]
quit’ Close any open TELNET session and exit telnet. An end-of-file (in command mode) will also close a session and exit.
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.
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 character 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.
ga’ Sends the TELNET GA (Go Ahead) sequence, which likely has no 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.
nop’ Sends the TELNET NOP (No OPeration) sequence.
susp’ Sends the TELNET SUSP (SUSPend process) sequence.
?’ Prints out help information for the send command.
set argument value
unset argument value
ayt’ If TELNET is in localchars mode, or LINEMODE is enabled, and the status character is typed, a TELNET AYT sequence (see send ayt preceding) is sent to the remote host. The initial value for the "Are You There" character is the terminal’s status character.
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).
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 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.
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.
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 character.
susp’ If telnet is in localchars mode, or 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 taken to be the terminal’s suspend character.
?’ Displays the legal set (unset) commands.
skey sequence challenge
check’ Verify the current settings for the current special 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 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.
?’ Prints out help information for the slc command.
status’ Show the current status of telnet. This includes the peer one is connected to, as well as the current mode.
toggle arguments [...]
authdebug’ Turns on debugging information for the authentication code.
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 sequences. The initial value for this toggle is TRUE if the terminal user had not done an "stty noflsh", otherwise FALSE (see stty(1)).
autologin’ If the remote side supports the TELNET AUTHENTICATION option TELNET attempts to use it to perform automatic authentication. If the AUTHENTICATION option is not supported, the user’s login name are propagated through the TELNET ENVIRON option. This command is the same as specifying a option on the open command.
autosynch’ If autosynch and localchars are both TRUE, then when either the intr or quit character 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 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, 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 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 feeds. The initial value for this toggle is FALSE.
debug’ Toggles socket level debugging (useful only to the superuser). The initial value for this toggle is FALSE.
encdebug’ Turns on debugging information for the encryption code.
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 protocol processing (having to do with TELNET options). 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 any TELNET escape sequence is preceded by a ’*’ to aid in locating them.
skiprc’ When the skiprc toggle is TRUE, TELNET skips the reading of the .telnetrc file in the user’s home directory when connections are opened. 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.
?’ Displays the legal toggle commands.
z’ Suspend telnet. This command only works when the user is using the csh(1).
telnet uses at least the HOME, SHELL, DISPLAY, and TERM environment variables. Other environment variables may be propagated to the other side via the TELNET ENVIRON option.
user customized telnet startup values
The telnet command appeared in 4.2BSD.
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.
Source routing is not supported yet for IPv6.
BSD February 3, 1994 BSD