NAME
telnet
—
user interface to the TELNET
protocol
SYNOPSIS
telnet |
[-4678acDdEKLr ]
[-b hostalias]
[-e escapechar]
[-l user]
[-n tracefile]
[-V rtable]
[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 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:
-4
- Forces
telnet
to use IPv4 addresses only. -6
- Forces
telnet
to use IPv6 addresses only. -7
- Specifies a 7-bit data path. This attempts to disable the
TELNET BINARY
option on both input and output. -8
- Specifies an 8-bit data path. This attempts to negotiate the
TELNET BINARY
option on both input and output. This is the default. -a
- Attempt automatic login. Currently, this sends the user name via the
USER
variable of theENVIRON
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. -b
hostalias- Uses bind(2) on the local socket to bind it to an aliased address (see ifconfig(8) and the ``alias'' specifier) or to the address of another interface than the one naturally chosen by connect(2). This can be useful when connecting to services which use IP addresses for authentication and reconfiguration of the server is undesirable (or impossible).
-c
- Disables the reading of the user's .telnetrc file.
(See the
toggle skiprc
command on this man page.) -D
- Disables rewriting of the
DISPLAY
variable when it starts with ‘:’ or ‘unix:’. By default, these are replaced with the local hostname and a colon. -d
- Sets the initial value of the
debug
toggle toTRUE
. -E
- Stops any character from being recognized as an escape character.
-e
escapechar- Sets the initial
telnet
escape character to escapechar. If escapechar is omitted, then there will be no escape character. -K
- Specifies no automatic login to the remote system.
-L
- Specifies an 8-bit data path on output. This attempts to negotiate the
TELNET BINARY
option on output. -l
user- When connecting to the remote system, 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 implies the-a
option. This option may also be used with theopen
command. -n
tracefile- Opens tracefile for recording trace information. See
the
set tracefile
command below. -r
- Specifies a user interface similar to
rlogin
. In this mode, the escape character is set to the tilde (~) character, unless modified by the-e
option. -V
rtable- Set the routing table to be used.
- 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).
close
- Close a TELNET session and return to command mode.
display
argument ...- Displays all, or some, of the
set
andtoggle
values (see below). environ
argument ...- The
environ
command is used to manipulate the variables that may be sent through theTELNET ENVIRON
option. The initial set of variables is taken from the users environment, with only theDISPLAY
andPRINTER
variables being exported by default. TheUSER
variable is also exported if the-a
or-l
options are used.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 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 asked for by the remote side.
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 aclose
command; however, if the remote side does not support theLOGOUT
option, nothing happens. If, however, the remote side does support theLOGOUT
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. 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 theLINEMODE
option, then enter ``character at a time'' mode. line
- Enable the
TELNET LINEMODE
option, or, if the remote side does not understand theLINEMODE
option, then attempt to enter ``old-line-by-line'' mode. isig
(-isig
)- Attempt to enable (disable) the
TRAPSIG
mode of theLINEMODE
option. This requires that theLINEMODE
option be enabled. edit
(-edit
)- Attempt to enable (disable) the
EDIT
mode of theLINEMODE
option. This requires that theLINEMODE
option be enabled. softtabs
(-softtabs
)- Attempt to enable (disable) the
SOFT_TAB
mode of theLINEMODE
option. This requires that theLINEMODE
option be enabled. litecho
(-litecho
)- Attempt to enable (disable) the
LIT_ECHO
mode of theLINEMODE
option. This requires that theLINEMODE
option be enabled. ?
- Prints out help information for the
mode
command.
open
host [-a
] [-b
-hostalias
] [-l
user] [[-
] port]- Open a connection to the named host. 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 (see hosts(5)) or an Internet address specified in the ``dot notation'' (see inet_ntop(3)). The-a
,-b
, and-l
options are equivalent to the identical command line options, but only apply to the new telnet connection being opened. When connecting to a non-standard port,telnet
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 whitespace are the start of a machine entry. The first thing on the line is the name of the machine that is being connected to. The rest of the line, and successive lines that begin with whitespace are assumed to betelnet
commands and are processed as if they had been typed in manually to thetelnet
command prompt. quit
- Close any open TELNET session and exit
telnet
. An end-of-file (in command mode) will also close a session and exit. send
argument ...- 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 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 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. escape
- Sends the current
telnet
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 TCP urgent data (and may not work if the remote system is a 4.2BSD system -- if it doesn't work, a lower case ``r'' may be echoed on the terminal). do
cmd- 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 eitherhelp
or?
to print out help information, including a list of known symbolic names. dont
cmd- Sends the
TELNET DONT
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 eitherhelp
or?
to print out help information, including a list of known symbolic names. will
cmd- Sends the
TELNET WILL
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 eitherhelp
or?
to print out help information, including a list of known symbolic names. wont
cmd- Sends the
TELNET WONT
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 eitherhelp
or?
to print out help information, including a list of known symbolic names. ?
- Prints out help information for the
send
command.
set
argument valueunset
argument value- The
set
command will set any one of a number oftelnet
variables to a specific value or toTRUE
. The special valueoff
turns off the function associated with the variable; this is equivalent to using theunset
command. Theunset
command will disable or set toFALSE
any of the specified functions. The values of variables may be interrogated with thedisplay
command. The variables which may be set or unset, but not toggled, are listed here. In addition, any of the variables for thetoggle
command may be explicitly set or unset using theset
andunset
commands.ayt
- If TELNET is in
localchars
mode, orLINEMODE
is enabled, and the status character is typed, aTELNET AYT
sequence (seesend 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 inLINEMODE
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 theeof
character is taken to be the terminal'seof
character. erase
- If
telnet
is inlocalchars
mode (seetoggle
localchars
below), and iftelnet
is operating in ``character at a time'' mode, then when this character is typed, aTELNET EC
sequence (seesend
ec
above) is sent to the remote system. The initial value for theerase
character is taken to be the terminal'serase
character. escape
- This is the
telnet
escape character (initially ``^['') which causes entry intotelnet
command mode (when connected to a remote system). flushoutput
- If
telnet
is inlocalchars
mode (seetoggle
localchars
below) and theflushoutput
character is typed, aTELNET AO
sequence (seesend
ao
above) is sent to the remote host. The initial value for theflush
character is taken to be the terminal'sflush
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 inlocalchars
mode (seetoggle
localchars
below) and theinterrupt
character is typed, aTELNET IP
sequence (seesend
ip
above) is sent to the remote host. The initial value for theinterrupt
character is taken to be the terminal'sintr
character. kill
- If
telnet
is inlocalchars
mode (seetoggle
localchars
below), and iftelnet
is operating in ``character at a time'' mode, then when this character is typed, aTELNET EL
sequence (seesend
el
above) is sent to the remote system. The initial value for thekill
character is taken to be the terminal'skill
character. lnext
- If
telnet
is operating inLINEMODE
or ``old line by line'' mode, then this character is taken to be the terminal'slnext
character. The initial value for thelnext
character is taken to be the terminal'slnext
character. quit
- If
telnet
is inlocalchars
mode (seetoggle
localchars
below) and thequit
character is typed, aTELNET BRK
sequence (seesend
brk
above) is sent to the remote host. The initial value for thequit
character is taken to be the terminal'squit
character. reprint
- If
telnet
is operating inLINEMODE
or ``old line by line'' mode, then this character is taken to be the terminal'sreprint
character. The initial value for thereprint
character is taken to be the terminal'sreprint
character. rlogin
- This is the rlogin escape character. If set, the normal TELNET 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 therlogin
escape character. start
- If the
TELNET TOGGLE-FLOW-CONTROL
option has been enabled, then this character is taken to be the terminal'sstart
character. The initial value for thestart
character is taken to be the terminal'sstart
character. stop
- If the
TELNET TOGGLE-FLOW-CONTROL
option has been enabled, then this character is taken to be the terminal'sstop
character. The initial value for thestop
character is taken to be the terminal'sstop
character. susp
- If
telnet
is inlocalchars
mode, orLINEMODE
is enabled, and thesuspend
character is typed, aTELNET SUSP
sequence (seesend
susp
above) is sent to the remote host. The initial value for thesuspend
character is taken to be the terminal'ssuspend
character. tracefile
- This is the file to which the output, caused by
netdata
oroption
tracing beingTRUE
, will be written. If it is set to “-
”, then tracing information will be written to standard output (the default). worderase
- If
telnet
is operating inLINEMODE
or ``old line by line'' mode, then this character is taken to be the terminal'sworderase
character. The initial value for theworderase
character is taken to be the terminal'sworderase
character. ?
- Displays the legal
set
(unset
) commands.
skey
sequence challenge- The
skey
command computes a response to the S/Key challenge. See skey(1) for more information on the S/Key system. slc
state- The
slc
command (Set Local Characters) is used to set or change the state of the special characters when theTELNET LINEMODE
option has been enabled. Special characters are characters that get mapped to TELNET commands sequences (likeip
orquit
) or line editing characters (likeerase
andkill
). 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, 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
argument ...- Toggle (between
TRUE
andFALSE
) various flags that control howtelnet
responds to events. These flags may be set explicitly toTRUE
orFALSE
using theset
andunset
commands listed above. More than one argument may be specified. The state of these flags may be interrogated with thedisplay
command. Valid arguments are:autoflush
- If
autoflush
andlocalchars
are bothTRUE
, then when theao
orquit
characters are recognized (and transformed into TELNET sequences; seeset
above for details),telnet
refuses to display any data on the user's terminal until the remote system acknowledges (via aTELNET TIMING MARK
option) that it has processed those TELNET sequences. The initial value for this toggle isTRUE
if the terminal user had not done an "stty noflsh", otherwiseFALSE
(see stty(1)). autologin
- The user's login name is propagated through the
TELNET ENVIRON
option. This command is the same as specifying a option on theopen
command. autosynch
- If
autosynch
andlocalchars
are bothTRUE
, then when either theintr
orquit
character is typed (seeset
above for descriptions of theintr
andquit
characters), the resulting TELNET sequence sent is followed by theTELNET 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 isFALSE
. 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 isFALSE
, then carriage returns will be sent as<CR><NUL>
. The initial value for this toggle isFALSE
. 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
. localchars
- If this is
TRUE
, then theflush
,interrupt
,quit
,erase
, andkill
characters (seeset
above) are recognized locally, and transformed into (hopefully) appropriate TELNET control sequences (respectivelyao
,ip
,brk
,ec
, andel
; seesend
above). The initial value for this toggle isTRUE
in ``old line by line'' mode, andFALSE
in ``character at a time'' mode. When theLINEMODE
option is enabled, the value oflocalchars
is ignored, and assumed to always beTRUE
. IfLINEMODE
has ever been enabled, thenquit
is sent asabort
, andeof
andsuspend
are sent aseof
andsusp
(seesend
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 isFALSE
. prettydump
- When the
netdata
toggle is enabled, ifprettydump
is enabled the output from thenetdata
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 isFALSE
. 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 a shell with job control enabled. !
[command]- Execute a single command in a subshell on the local system. If command is omitted, then an interactive subshell is invoked.
?
[command]- Get help. With no arguments,
telnet
prints a help summary. If a command is specified,telnet
will print the help information for just that command.
ENVIRONMENT
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.
FILES
- ~/.telnetrc
- user customized telnet startup values
SEE ALSO
HISTORY
The telnet
command appeared in
4.2BSD.
NOTES
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.