telnet

NAME

telnet - user interface to the Telnet protocol

SYNOPSIS

telnet [-8acdELr] [-e escapechar] [-l user]
	 [-n tracefile] [-S tos] [-X authtype]
	 [host [port]]

DESCRIPTION

The telnet(1) command is used to communicate with another host using the Telnet protocol. If telnet(1) 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 in this topic. If it is invoked with arguments, it performs an open command with those arguments.

If Windows Services for UNIX Telnet Client is installed, a Windows-command version of the telnet command is available. For more information, see Telnet Client in Windows Services for UNIX Help.

The telnet(1) command uses the following options:

-8: Specifies an 8-bit data path. This causes an attempt to negotiate the TELNET BINARY option on both input and output.
-a: Attempt automatic login. Currently, this sends the user name through 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(3) if it agrees with the current user identifier (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 in this topic.)
-d: Sets the initial value of the debug toggle to TRUE.
-E: Stops any character from being recognized as an escape character.
-eescape_char: Sets the initial telnet(1) escape character to escape_char. If escape_char is omitted, there will be no escape character.
-L: Specifies an 8-bit data path on output. This causes the BINARY option to be negotiated on output.
-luser: When connecting to the remote system, if the remote system understands the ENVIRON option, user will be sent to the remote system as the value for the variable USER. This option implies the -a option. This option can also be used with the open command.
-ntracefile: Opens tracefile for recording trace information. See the set tracefile command later in this topic.
-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.
-Stos: Sets the Internet Protocol (IP) type-of-service (TOS) option for the Telnet connection to the value tos, which can be a numeric TOS value or, on systems that support it, a symbolic TOS name found in the /etc/iptos file.
-Xatype: Disables the atype type of authentication.
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(1) 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.

After a connection has been opened, telnet(1) will attempt to enable the TELNET LINEMODE option. If this fails, telnet(1) 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) can 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), 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, 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) that 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(1) command mode can be entered by typing the telnet(1) escape character (initially ^]). When in command mode, the normal terminal editing conventions are available.

The following telnet(1) commands are available. You need only to type 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).

auth argument ...

The auth command manipulates the information sent through the TELNET AUTHENTICATE option. Valid arguments for the auth command are as follows:

disable type: Disables the specified type of authentication. To obtain a list of available types, use the auth disable ? command.
enable type: Enables the specified type of authentication. To obtain a list of available types, use the auth enable ? command.
status: Lists the current status of the various types of authentication.

close

Close a Telnet session and return to command mode.

display argument ...

Displays all or some of the set and toggle values.

encrypt

The encrypt command manipulates the information sent through the TELNET ENCRYPT option.

Because of export controls, the TELNET ENCRYPT option is not supported outside of the United States and Canada.

Valid arguments for the encrypt command are as follows:

disable: Disables the specified type of encryption. If you omit the input and output, both input and output are disabled. To obtain a list of available types, use the encrypt disable ? command.
enable: Enables the specified type of encryption. If you omit input and output, both input and output are enabled. To obtain a list of available types, use the encrypt enable ? command.
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.
start [input|output]: Attempts to start encryption. If you omit input and output, both input and output are enabled. To obtain a list of available types, use the encrypt enable ? command.
status: Lists the current status of encryption.
stop [input|output]: Stops encryption. If you omit input and output, encryption is on both input and output.
type type: Sets the default type of encryption to be used with later encrypt start or encrypt stop commands.

environ arguments...

The environ command is used to manipulate the variables that can 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 if the -a or -l options are used.

Valid arguments for the environ command are:

define: Define the variable variable to have a value of value. Any variables defined by this command are automatically exported. The value can be enclosed in single or double quotes so that tabs and spaces will be included.
undefine: Remove variable from the list of environment variables.
export: Mark the variable variable to be exported to the remote side.
unexport: Mark the variable variable so that it will not be exported unless explicitly asked for by the remote side.
list: List the current set of environment variables. Those marked with an asterisk (*) 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 suspending a user's session for later reattachment, the logout argument indicates that you should terminate the session immediately.

mode type

Type is one of several options, 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 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 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, attempt to enter old line-by-line mode mode.
isig
-isig: Attempt to enable (-isig disables) the TRAPSIG mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
edit
-edit: Attempt to enable(-edit disables) the EDIT mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
softtabs
-softtabs: Attempt to enable (-softtabs disables ) the SOFT_TAB mode of the LINEMODE option. This requires that the LINEMODE option be enabled.
litecho
-litecho: Attempt to enable(-litecho disables) 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 host [[-l] user ][-port]

Open a connection to the named host. If no port number is specified, telnet(1) will attempt to contact a Telnet server at the default port. The host specification can be either a host name (see hosts(1)) or an Internet address specified in the dot notation (see inet(3)). The -l option can be used to specify the user name to be passed to the remote system through the ENVIRON option. When connecting to a non-standard port, telnet(1) omits any automatic initiation of TELNET options. When the port number is preceded by a minus sign, the initial option negotiation is done. After establishing a connection, the file .telnetrc in the user's home directory is opened. Lines beginning with a # are comment lines. Blank lines are ignored. Lines that begin without white space are the start of a computer entry. The first thing on the line is the name of the computer to which the connection is being made. The rest of the line, and successive lines that begin with white space, are assumed to be telnet(1) commands and are processed as if they had been typed in manually to the telnet(1) command prompt.

quit

Close any open Telnet session and exit telnet(1). An end of file (in command mode) will also close a session and exit.

send

Sends one or more special character sequences to the remote host. The following are the arguments that can be specified (more than one argument can be specified at a 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, to which the remote system might or might not choose to respond.
brk: Sends the TELNET BRK (Break) sequence, which can 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.
escape: Sends the current telnet(1) escape character (initially ^).
ga: Sends 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, getstatus will send the subnegotiation to 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 is sent as Transmission Control Protocol (TCP) urgent data (and might not work if the remote system is a Berkeley Software Distribution (BSD) 4.2 system. If it does not work, a lowercase "r" might be echoed on the terminal).
do
dont
will
wont: Sends the TELNET DO cmd sequence. Cmd can be either a decimal number between 0 and 255, or a symbolic name for a specific TELNET command. Cmd can also be either help or ? to print out help information, including a list of known symbolic names.
?: Prints out help information for the send command.

set

unset

The set command will set any one of a number of telnet(1) 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 can be interrogated with the display command. The variables that can be set or unset, but not toggled, are listed here. In addition, any of the variables for the toggle command can be explicitly set or unset 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 preceding) is sent to the remote host. The initial value for the "Are You There" character is the terminal's status character.
echo: In line-by-line mode, this value toggles between several modes: local line-by-line mode, local echoing of entered characters (for normal processing), and echo suppression of entered characters (for entering, a password, for example, ). The initial value is ^E.
eof: If telnet(1) 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.
erase: If telnet(1) is in localchars mode (see toggle localchars later in this topic), and if telnet(1) is operating in character-at-a-time mode, when this character is typed, a TELNET EC sequence (see send ec discussed previously in this topic) is 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(1) escape character that causes entry into telnet(1) command mode (when connected to a remote system). The initial value is ^[.
flushoutput: If telnet(1) is in localchars mode (see toggle localchars later in this topic) and the flushoutput character is typed, a TELNET AO sequence (see send ao discussed previously in this topic) is sent to the remote host. The initial value for the flush character is taken to be the terminal's flush character.
forw1
forw2: If 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(1) is in localchars mode (see toggle localchars later in this topic) and the interrupt character is typed, a TELNET IP sequence (see send ip) is sent to the remote host. The initial value for the interrupt character is taken to be the terminal's intr character.
kill: If telnet(1) is in localchars mode (see toggle localchars), and if telnet(1) is operating in character at a time mode, when this character is typed, a TELNET EL sequence (see send el discussed previously in this topic) is sent to the remote system. The initial value for the kill character is taken to be the terminal's kill character.
lnext: If telnet(1) is operating in LINEMODE or old line-by-line mode, 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(1) is in localchars mode (see toggle localchars) and the quit character is typed, a TELNET BRK sequence (see send brk) is sent to the remote host. The initial value for the quit character is taken to be the terminal's quit character.
reprint: If telnet(1) is operating in LINEMODE or old line-by-line mode, 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 escape character is ignored unless it is preceded by this character at the beginning of a line. This character, at the beginning of a line followed by a "." closes the connection; when followed by a ^Z it suspends the telnet command. The initial state is to disable the rlogin escape character.
start: If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, this character is taken to be the terminal's start character. The initial value for the kill character is taken to be the terminal's start character.
stop: If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, this character is taken to be the terminal's stop character. The initial value for the kill character is taken to be the terminal's stop character.
susp: If telnet(1) is in localchars mode, or LINEMODE is enabled, and the suspend character is typed, a TELNET SUSP sequence (see send susp) is 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. If it is set to '-', tracing information will be written to standard output (the default).
worderase: If telnet(1) is operating in LINEMODE or old line-by-line mode, 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 (unset) commands.

slc state

The slc command (Set Local Characters) is used to set or change the state of the special characters when the TELNET LINEMODE option has been enabled. 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(1) 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(1). This includes the peer one is connected to, as well as the current mode.

toggle toggle

Toggle (between TRUE and FALSE) various flags that control how telnet(1) responds to events. These flags can be set explicitly to TRUE or FALSE using the set and unset commands discussed previously in this topic. More than one argument can be specified. The state of these flags can be interrogated with the display command. Valid arguments are provided in the following list:

authdebug: Turn on debugging information for the authentication code.
autoflush: If autoflush and localchars are both TRUE, when the ao, or quit characters are recognized (and transformed into TELNET sequences; see set for details), telnet(1) refuses to display any data on the user's terminal until the remote system acknowledges (through a TELNET TIMING MARK option) that it has processed those TELNET sequences. The initial value for this toggle is TRUE if the terminal user has not done an stty noflsh; otherwise, the value is FALSE (see stty(1)).
autodecrypt: When the TELNET ENCRYPT option is negotiated, by default the actual encryption (decryption) of the data stream does not start automatically. The autoencrypt (autodecrypt) command states that encryption of the output (input) stream should be enabled as soon as possible.
Because of export controls, the TELNET ENCRYPT option is not supported outside the United States and Canada.
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 is 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, when either the intr or quit character is typed (see set 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 discarding 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, carriage returns will be sent as <CR><LF>. If this is FALSE, carriage returns will be sent 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, it only affects those received from the remote host. This mode is not very useful unless the remote host only sends carriage return, but never line feed. 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.
encdebug: Turn on debugging information for the encryption code.
localchars: If this is TRUE, the flush, interrupt, quit, erase, and kill characters (see set) are recognized locally, and should be transformed into appropriate TELNET control sequences (respectively ao, ip, brk, ec, and el; see send discussed previously in this topic). 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, quit is sent as abort, and eof and suspend are sent as eof and susp (see send).
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(1) 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 readable format. Spaces are put between each character in the output, and the beginning of any TELNET escape sequence is preceded by a '*' to make them easier to locate.
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: Toggle the display of all terminal data (in hexadecimal format). The initial value for this toggle is FALSE.
verbose_encrypt: When the verbose_encrypt toggle is TRUE, TELNET prints out a message each time encryption is enabled or disabled. The initial value for this toggle is FALSE.
Because of export controls, data encryption is not supported outside of the United States and Canada.
?: Displays the legal toggle commands.

z

Suspend telnet(1). This command works only when the user is using csh(1).

! [command]

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

? [command]

Get help. With no arguments, telnet(1) prints a help summary. If a command is specified, telnet(1) will print the help information for that command only.

ENVIRONMENT VARIABLES

The telnet(1) command uses at least the HOME, SHELL, DISPLAY, and TERM environment variables. Other environment variables can be propagated to the other side through the TELNET ENVIRON option.

FILES

~/.telnetrc: User customized telnet startup values.

NOTES

On some remote systems, echo must 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.