telnet(1tcp)

telnet -- user interface to a remote system using the TELNET protocol

Synopsis

telnet [ -8ELacdr ] [ -e escape_char ] [ -l user ] [ -n tracefile ] [ host [ -port | port ]

Description

The telnet command communicates with another host using the TELNET protocol. If telnet is invoked without the host argument, it will enter command 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 with those arguments.

The following options are available:

-8

Use an eight bit data path. This will cause an attempt to negotiate the BINARY option on both input and output. Binary mode is enabled by default if the LANG environment variable is set to one of the following Asian locales:

ja, ja_JP, or ja_JP.EUC (Japanese)
ko (Korean)
zh_cn (Simplified Chinese)
zh_tw (Traditional Chinese)

-E

Stop 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 USER variable is exported to the remote system. This option may also be used with the open command.

-c

Do not process the user's ~/.telnetrc file (see the toggle skiprc command below).

-d

Toggles socket level debugging (useful only to root). Sets the initial value of the debug toggle to TRUE.

-e escape_char

Sets the initial TELNET escape character to escape_char. If escape_char is the null string (""), then there is no pre-defined escape character.

-l user

When connecting to the remote system and if the remote system understands the ENVIRON option, then user is exported to the remote system as the value for the USER variable. This option may also be used with the open command.

-n tracefile

Opens tracefile for recording the trace information. (See the set tracefile command below.)

-r

Use a user interface similar to rlogin(1tcp). In this mode, the escape character is set to the tilde (~) character, unless modified with the -e option.

host

Indicates the host's official name, an alias, or the Internet address of a remote host.

host may be specified as an IPv4 or IPv6 loose source route (where each element does not need to be a neighbor) or strict source route (where each element must be a neighbor). An IPv4 loose source route takes the form:

@addr[@addr...]:dest

An IPv4 strict source route takes the form:

!@addr[@addr...]:dest

An IPv6 loose source route takes the form:

@addr[@addr...]@dest

An IPv6 strict source route takes the form:

!@addr[@addr...]@dest

Additionally, IPv6 allows the specification of a mixed strict and loose source route:

@addr[!][@addr[!]...]@dest[!]

In this case, strict elements specify a trailing ``!''; loose elements do not specify a trailing ``!''.

IPv4 allows up to 9 intermediate addresses to be specified in the route to the destination. IPv6 allows up to 23 intermediate addresses to be specified in the route to the destination. See RFC 791 and RFC 1883 for more information about source routing in IPv4 and IPv6.

NOTE: This exists primarily for debugging or for use when network connectivity is problematic. Use of source routing is not usually recommended.

port

Indicates a port number (that is, the address of an application). If a number is not specified, the default TELNET port is used.

When a port number is specified, telnet will omit the automatic initiation of any TELNET options, unless the port number is preceded by a minus sign.

After establishing a connection, file .telnetrc file in the user's home directory is opened. Lines beginning with a ``#'' are treated as comment lines; 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 the host is connecting to. The rest of the line (and successive lines which begin with whitespace) are assumed to be telnet commands, and are processed as if they had been entered manually in response to the telnet command prompt.

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 character-at-a-time mode or the old line-by-line mode, depending on what the remote system supports.

When LINEMODE is enabled, character processing is 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 immediately to the remote host for processing.

In the old line-by-line mode, all text is echoed locally, but usually only completed lines are 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 are trapped locally and sent as TELNET 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 are sent as a TELNET ABORT instead of <Break>. There are options (see toggle autoflush and toggle autosynch below) which cause this action to flush any subsequent output 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> sequence (initially ``^]'').

When in command mode, the normal terminal editing conventions are available.

The following TELNET commands are available, but only enough of each command need be typed to uniquely identify it (this is also true for arguments pertaining to the mode, toggle, set, 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 description below).

environ [ argument [...] ]

Manipulate the variables that may be sent through the TELNET ENVIRON option. The initial set of variables is taken from the user's environment; only the DISPLAY, LANG, and PRINTER variables are automatically exported, although USER is also exported if the -a or -l options are used.

The valid arguments for the environ command are:

define variable value: Define the variable to have a value of value. Any variables 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 not to be exported unless explicitly requested by the remote side.
list: List the current set of environment variables. Those marked with a ``'' are sent automatically; any other variables are sent only if explicitly requested.
send variable: Send environment variable.
?: Prints out help information for the environ command.

logout

Send the TELNET LOGOUT option to the remote site. This command is similar to the close command. However, nothing will happen if the remote site does not support the LOGOUT option. If the remote site does support this option, then it should cause the remote site to close the TELNET connection. Additionally, if a user's session can be suspended for future attachment, the logout option indicates that you should terminate the session immediately.

mode [ type ]

Depending on the state of the TELNET session, the type argument is one of several available options. 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.

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 TELNET 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 command.

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

Open a connection to the named host. If host is ``?'' then the help information for the open command is displayed. If no port number is specified, telnet will attempt to contact a TELNET server at the default port. The host specification may be either a host name or an Internet address specified in the dot notation (see inet(3N)).

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

Send 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: Send the TELNET ABORT (ABORT processes) sequence.
ao: Send 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: Send the TELNET AYT (Are You There?) sequence; the remote system may or may not respond to this transmission.
brk: Send the TELNET BRK (Break) sequence which may have significance for 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 ``^]'').
ga: Send 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, send the subnegotiation request to the server for 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 (and may not work if the remote system is a 4.2 BSD system; if it does not work, a lowercase ``l'' may be echoed on the terminal).
do cmd
dont cmd
will cmd
wont cmd: Send the TELNET DO, DONT, WILL or WONT command sequence. cmd can be either a decimal number between 0 and 255, or a symbolic name for a specific TELNET command. cmd may also be help or ? which prints help information, including the list of symbolic names.
?: Print out help information for the send command.

set argument value
unset argument value

The set command sets 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. 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.

ayt: If telnet is in localchars mode, or LINEMODE is enabled, and the status character is typed, a TELNET AYT sequence (see send ayt above) 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 the line-by-line mode, will toggle between doing local echoing 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 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 the character at a time mode, then when this character is entered, a TELNET EC sequence (see send ec above) are sent to the remote system. The initial value for the erase character 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 connected 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) are sent to the remote host. The initial value for the flush character 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 terminal's eol and eol2 characters.
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) are 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 the character at a time mode, then when this character is entered, a TELNET EL sequence (see send el above) are 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 character.
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) are 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.
rlogin: This is the rlogin escape character. If set, the normal TELNET escape character is ignored, unless preceded by this character at the beginning of a line. If the character is followed by a ``.'', then the connection is closed. If the character is followed by a ``^Z'', then the telnet command is suspended. The initial state is to disable the rlogin escape 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 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 the localchars mode or if the LINEMODE is enabled and the suspend character is entered, a TELNET SUSP sequence (see send susp above) are 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, caused by netdata or option tracing being TRUE, will be written. The default value is ``-'', which sends tracing information to the standard output.
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.
?: Display 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.

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.
?: Print out help information for the slc command.

status

Show the current status of telnet. This includes the peer to which one is connected, as well as the current mode.

toggle arguments [...]

Toggle various flags (between TRUE and FALSE) that control how TELNET responds to events. These flags may be set explicitly to TRUE or FALSE using the set and unset commands listed above. More than one argument may be specified. The state of these flags may be interrogated using display.

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(1)).
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 are 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 toggle is FALSE.
binary: Enable or disable the TELNET BINARY 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 are sent as <CR><LF>. If this is FALSE, then carriage returns are sent as <CR><NUL>. The initial 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 are 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: Toggle the socket level debugging mode (useful only to root). 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 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; eof and suspend are sent as eof and susp; (see send above).
netdata: Toggle the display of all network data (in hexadecimal format). The initial value for this toggle is FALSE.
options: Toggle the display of some internal telnet protocol processing 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 are reorganized into a more user-friendly format. Spaces are put between each character in the output and the beginning of any TELNET escape sequence are preceded by a ``'' to aid in locating them.
skiprc: If this is TRUE, then telnet does not process the ~/.telnetrc file. The initial value is set to FALSE.
termdata: Toggle printing of hexadecimal terminal data. The initial value for this toggle is FALSE.
?: Display the legal toggle commands.

<Ctrl>Z

Suspend telnet. This command will work only when the user is using csh(1) or ksh(1).

! [ command ]

Execute a single command in a subshell on the local system. If command is omitted, then an interactive subshell are invoked.

? [ command ]

Get help. When no command is specified, telnet will print a summary for the help command. If a command is specified, telnet will print the help information for just that command.

Environment variables

If set, the telnet command exports at least the following environment variables to the remote machine: DISPLAY, LANG, and PRINTER. Note that DISPLAY are set to local_hostname:0.0 if DISPLAY is set to :0.0 or to unix:0.0 locally.

Other environment variables may be propagated to the remote side via the TELNET ENVIRON option.

USER is set to LOGNAME if LOGNAME is set locally but USER is not. By default, USER is not set.

Files

$HOME/.telnetrc: user-customized telnet startup values

References

telnetd(1Mtcp)

RFC 727, RFC 854, RFC 856, RFC 857, RFC 858, RFC 859, RFC 860, RFC 1073, RFC 1079, RFC 1091, RFC 1096, RFC 1123, RFC 1184, RFC 1372, RFC 1572

Notices

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.