NAME

telnet − User interface to a remote system using the TELNET protocol

SYNOPSIS

telnet [−8] [−E] [−K] [−L] [−a] [−d] [−e escape-character] [−l user] [−n tracefile] [−r] [host] [port]

DESCRIPTION

The telnet command is used to communicate 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 (see “TELNET COMMANDS” below) 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. 

−E option stops 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 variable USER will be sent to the remote system.  This option may also be used with the open command. 

−d Toggles socket level debugging (useful only to the super-user) .  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 omitted, then there will be no pre-defined escape character. 

−l user When connecting to the remote system and if the remote system understands the ENVIRON option, then user will be sent to the remote system as the value for the variable USER .  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(1) .  In this mode, the escape character is set to the tilde (“~”) character, unless modified by the −e flag. 

host Indicates the host’s official name: an alias or the Internet address of a remote host. 

port Indicates a port number or service name (i.e., the address or name of an application).  If a number or name is not specified, the default TELNET port will be used. 

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 will be 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 will be echoed locally, but (normally) only completed lines will be sent to the remote host.  The “local echo character” (initially “^]”) 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 will be 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 will be 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 “escape character” (initially “^]”). 

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

TELNET COMMANDS

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, set, toggle, unset, slc, environ, and display commands). 

close Close a TELNET session and return to command mode. 

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

mode [type]
Depending on the state of the TELNET session, the type argument is one of several available options.  The remote host will be 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 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 host [user] [[−]port ] [ −a ] [ −l user ]
Open a connection to the named host.  If no port number or service name is specified, telnet will attempt to contact a TELNET server at the default port.  The host specification may be either a host name [see hosts(4N)] or an Internet address specified in the “dot notation” [see inet(3)]. 

The −l or the −a option may be used to specify the user name to be passed to the remote system via the ENVIRON option. 

When connecting to a non-standard port, telnet will omit the automatic initiation of any TELNET options.  When the port number is preceded by a minus sign, the initial option negotiation will be done as follows: After establishing a connection, the file .telnetrc in the user’s home directory will be opened.  Lines beginning with a # will be treated as comment lines; blank lines will be ignored.  Lines that begin without whitespace will be the start of a machine entry.  The first thing on the line will the name of the machine to which this host is being connected.  The rest of the line − and successive lines which begin with whitespace − will be assumed to be telnet commands and will be processed as if they had been entered manually in response to the telnet command prompt. 

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
Sends 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 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; the remote system may or may not choose to respond to this transmission. 

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. 

escape
Sends the current TELNET escape character (initially “^E”). 

ga Sends 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, getstatus will send the subnegotiation 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 will be sent as TCP urgent data (and may not work if the remote system is a 4.2 BSD system; if it doesn’t work, a lower case “|” may be echoed on the terminal). 

? Prints out help information for the send command. 

set argument value

unset argument value
The set command will set anyone of a number of TELNET variables to a specific value or to TRUE.  The special value off will turn off the function associated with this 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. 

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) will be 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) will be sent to the remote host.  The initial value for the flush character is taken to be the terminal’s flush character. 

interrupt
If telnet is in localchars mode (see “toggle” localchars below) and the interrupt character is entered, a TELNET.SM IP sequence (see send ip above) will be 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) will be 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) will be 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. 

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) will be 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 generated by the netdata command will be written. 

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. 

? Displays the legal set and 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.  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. 

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.

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 set of remote values.

? Prints out help information for the slc command. 

environ[arguments[...] ]
The environ command is used to manipulate 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 USER and DISPLAY variables being exported. 

The valid arguments for the environ command are:

define variable value
Define the variable 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 variable to be exported to the remote side. 

unexport
variable
Mark the variable variable to not be exported unless explicitly requested by the remote side. 

list List the current set of environment variables.  Those marked with a ∗ will be sent automatically; any other variables will be sent only if requested explicitly. 

? Prints out help information for the environ command. 

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 with the aid of the display command.  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 will be 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 will be sent as <CR> <LF>.  If this is FALSE, then Carriage Returns will be 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 will be 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
Toggles the socket level debugging mode (useful only to the super-user). 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 (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 will be sent as abort; eof and suspend will be 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 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 will be reorganized into a more user-friendly format.  Spaces will be put between each character in the output and the beginning of any TELNET escape sequence will be preceded by a ‘∗’ to aid in locating them. 

? Displays 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 will be invoked. 

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

?[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

The telnet command uses at least the following environent variables: HOME, SHELL, USER, DISPLAY, and TERM.  Other environment variables may be propagated to the other side via the TELNET  ENVIRON option. 

FILES

$HOME/.telnetrc user-customized telnet startup values

HISTORY

The telnet command appeared in 4.2 BSD. 

USER CONSIDERATIONS

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. 

  —  Internet Utilities