telnet.netkit
TELNET(1) BSD General Commands Manual TELNET(1) telnet — user interface to the TELNET protocol
Synopsis
telnet
[-468ELadr] [-S tos]
[-b address]
[-e escapechar]
[-l user]
[-n tracefile] [
host [port] ]
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description
The telnet command is
used for interactive communication with another host using
the TELNET protocol. It begins in command mode, where it
prints a telnet prompt ("telnet> "). If
telnet is invoked with a host argument, it
performs an open command implicitly; see the
description below.
Options:
-4
Force IPv4
address resolution.
-6
Force IPv6
address resolution.
-8
Request 8-bit
operation. This causes an attempt to negotiate the TELNET
BINARY option for both input and output. By default telnet
is not 8-bit clean.
-E
Disables the
escape character functionality; that is, sets the escape
character to ’’no character’’.
-L
Specifies an
8-bit data path on output. This causes the TELNET BINARY
option to be negotiated on just output.
-a
Attempt
automatic login. Currently, this sends the user name via the
USER variable of the ENVIRON option if supported by the
remote system. The username is retrieved via
getlogin(3).
-b
address
Use bind(2) on the local socket
to bind it to a specific local address.
-d
Sets the
initial value of the debug toggle to TRUE.
-r
Emulate
rlogin(1). In this mode, the default escape character is a
tilde. Also, the interpretation of the escape character is
changed: an escape character followed by a dot causes
telnet to disconnect from the remote host. A ^Z
instead of a dot suspends telnet, and a ^] (the
default telnet escape character) generates a normal
telnet prompt. These codes are accepted only at the
beginning of a line.
-S tos
Sets the IP
type-of-service (TOS) option for the telnet connection to
the value tos.
-e
escapechar
Sets the escape character to
escapechar. If no character is supplied, no escape
character will be used. Entering the escape character while
connected causes telnet to drop to command mode.
-l user
Specify user as the user
to log in as on the remote system. This is accomplished by
sending the specified name as the USER environment variable,
so it requires that the remote system support the TELNET
ENVIRON option. This option implies the -a
option, and may also be used with the open
command.
-n
tracefile
Opens tracefile for
recording trace information. See the set tracefile
command below.
host
Specifies a
host to contact over the network.
port
Specifies a
port number or service name to contact. If not specified,
the telnet port (23) is used.
Protocol:
Once a
connection has been opened, telnet will attempt to
enable the TELNET LINEMODE option. If this fails, then
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).
Commands:
The following
telnet commands are available. Unique prefixes are
understood as abbreviations.
auth argument
...
The auth command
controls the TELNET AUTHENTICATE protocol option. If
telnet was compiled without authentication, the
auth command will not be supported. Valid arguments
are as follows:
disable type
Disable the
specified type of authentication. To obtain a list of
available types, use the auth disable ? command.
enable type
Enable the
specified type of authentication. To obtain a list of
available types, use the auth enable ? command.
status
List the
current status of the various types of authentication.
Note that the
current version of telnet does not support
authentication.
close
Close the
connection to the remote host, if any, and return to command
mode.
display argument
...
Display all, or some, of the
set and toggle values (see below).
encrypt argument
...
The encrypt command controls
the TELNET ENCRYPT protocol option. If telnet was
compiled without encryption, the encrypt command will
not be supported.
Valid arguments
are as follows:
disable type
[input|output]
Disable the specified type of
encryption. If you do not specify input or output,
encryption of both is disabled. To obtain a list of
available types, use ’’encrypt disable
?’’.
enable type
[input|output]
Enable the specified type of
encryption. If you do not specify input or output,
encryption of both is enabled. To obtain a list of available
types, use ’’encrypt enable ?’’.
input
This is the
same as ’’encrypt start input’’.
-input
This is the
same as ’’encrypt stop input’’.
output
This is the
same as ’’encrypt start
output’’.
-output
This is the
same as ’’encrypt stop output’’.
start [input|output]
Attempt to begin encrypting. If
you do not specify input or output, encryption of both input
and output is started.
status
Display the
current status of the encryption module.
stop [input|output]
Stop encrypting. If you do not
specify input or output, encryption of both is stopped.
type type
Sets the
default type of encryption to be used with later
’’encrypt start’’ or
’’encrypt stop’’ commands.
Note that the
current version of telnet does not support
encryption.
environ
arguments...
The environ command is
used to propagate environment variables across the
telnet link using the TELNET ENVIRON protocol option.
All variables exported from the shell are defined, but only
the DISPLAY and PRINTER variables are marked to be sent by
default. The USER variable is marked to be sent if the
-a or -l command-line options were
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 marked
for propagation (’’exported’’). The
value may be enclosed in single or double quotes so
that tabs and spaces may be included.
undefine
variable
Remove any existing definition
of variable.
export
variable
Mark the specified variable for
propagation to the remote host.
unexport
variable
Do not mark the specified
variable for propagation to the remote host. The remote host
may still ask explicitly for variables that are not
exported.
list
List the
current set of environment variables. Those marked with a
* will be propagated to the remote host. The remote
host may still ask explicitly for the rest.
?
Prints out help
information for the environ command.
logout
Send the TELNET
LOGOUT protocol option to the remote host. This command is
similar to a close command. If the remote host does
not support the LOGOUT option, nothing happens. But if it
does, this command should cause it to close the connection.
If the remote side also supports the concept of suspending a
user’s session for later reattachment, the logout
command indicates that the session should be terminated
immediately.
mode type
Type is
one of several options, depending on the state of the
session. Telnet asks the remote host to go into the
requested mode. If the remote host says it can, that mode
takes effect.
character
Disable the
TELNET LINEMODE option, or, if the remote side does not
understand the LINEMODE option, then enter
’’character at a time’’ mode.
line
Enable the
TELNET LINEMODE option, or, if the remote side does not
understand the LINEMODE option, then attempt to enter
’’old-line-by-line’’ mode.
isig
(-isig)
Attempt to
enable (disable) the TRAPSIG mode of the 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 [
[-l] user][
- port]
Open a connection to the named
host. If no port number is specified, telnet will
attempt to contact a telnet daemon at the standard port
(23). The host specification may be a host name or IP
address. The -l option may be used to specify a
user name to be passed to the remote system, like the
-l command-line option.
When connecting
to ports other than the telnet port, telnet
does not attempt telnet protocol negotiations. This makes it
possible to connect to services that do not support the
telnet protocol without making a mess. Protocol negotiation
can be forced by placing a dash before the port number.
After
establishing a connection, any commands associated with the
remote host in /etc/telnetrc and the user’s
.telnetrc file are executed, in that order.
The format of
the telnetrc files is as follows: Lines beginning with a #,
and blank lines, are ignored. The rest of the file should
consist of hostnames and sequences of telnet commands
to use with that host. Commands should be one per line,
indented by whitespace; lines beginning without whitespace
are interpreted as hostnames. Lines beginning with the
special hostname ’DEFAULT’ will apply to all
hosts. Hostnames including ’DEFAULT’ may be
followed immediately by a colon and a port number or string.
If a port is specified it must match exactly with what is
specified on the command line. If no port was specified on
the command line, then the value ’telnet’ is
used. Upon connecting to a particular host, the commands
associated with that host are executed.
quit
Close any open
session and exit telnet. An end of file condition on
input, when in command mode, will trigger this operation as
well.
send arguments
Send one or more special telnet
protocol character sequences to the remote host. The
following are the codes which may be specified (more than
one may be used in one command):
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.
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
dont cmd
will cmd
wont 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 either help or ? to
print out help information, including a list of known
symbolic names.
?
Prints out help
information for the send command.
set argument
value
unset argument
value
The set command will set
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 variables.
The values of variables may be interrogated with 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 set or unset.
ayt
If telnet is in
localchars mode, or LINEMODE is enabled, and the status
character is typed, a TELNET AYT sequence is sent to the
remote host. The initial value for the "Are You
There" character is the terminal’s status
character.
echo
This is the
value (initially ’’^E’’) which, when
in ’’line by line’’ mode, toggles
between doing local echoing of entered characters (for
normal processing), and suppressing echoing of entered
characters (for entering, say, a password).
eof
If
telnet is operating in LINEMODE or ’’old
line by line’’ mode, entering this character as
the first character on a line will cause this character to
be sent to the remote system. The initial value of the eof
character is taken to be the terminal’s eof
character.
erase
If
telnet is in localchars mode (see toggle
localchars below), and if telnet is
operating in ’’character at a time’’
mode, then when this character is typed, a TELNET EC
sequence (see send ec above) 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 escape character (initially
’’^[’’) which causes entry into
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 typed, a TELNET AO
sequence (see send ao above) 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 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 typed, a TELNET IP
sequence (see send ip above) 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 is in localchars mode (see toggle
localchars below), and if telnet is
operating in ’’character at a time’’
mode, then when this character is typed, a TELNET EL
sequence (see send el above) is sent to the remote
system. The initial value for the kill character is taken to
be the terminal’s kill character.
lnext
If
telnet is operating in LINEMODE or ’’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
typed, a TELNET BRK sequence (see send brk above) is
sent to the remote host. The initial value for the quit
character is taken to be the terminal’s quit
character.
reprint
If telnet is operating
in LINEMODE or ’’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 mode escape character. Setting it enables rlogin
mode, as with the r command-line option (q.v.)
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 kill 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 kill character is taken
to be the terminal’s stop character.
susp
If
telnet is in localchars mode, or LINEMODE is
enabled, and the suspend character is typed, a TELNET
SUSP sequence (see send susp above) is sent to the
remote host. The initial value for the suspend character is
taken to be the terminal’s suspend
character.
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
’’-’’, then tracing
information will be written to standard output (the
default).
worderase
If telnet is operating
in LINEMODE or ’’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 (unset) commands.
slc state
The slc
command (Set Local Characters) is used to set or change the
state of the 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, 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 name of
the remote host, if any, as well as the current mode.
toggle arguments
...
Toggle (between TRUE and FALSE)
various flags that control how telnet responds to
events. These flags may be set explicitly to TRUE or FALSE
using the set and unset commands. More than
one flag may be toggled at once. The state of these flags
may be examined with the display command. Valid flags
are:
authdebug
Turns on
debugging for the authentication code. This flag only exists
if authentication support is enabled.
autoflush
If
autoflush and localchars are both TRUE, then
when the ao, or quit characters are recognized
(and transformed into TELNET sequences; see set above
for details), telnet refuses to display any data on
the user’s terminal until the remote system
acknowledges (via a TELNET TIMING MARK option) that it has
processed those TELNET sequences. The initial value for this
toggle is TRUE if the terminal user had not done an
"stty noflsh", otherwise FALSE (see stty(1)).
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.
Note that this
flag exists only if encryption support is enabled.
autologin
If the remote
side supports the TELNET AUTHENTICATION option, telnet
attempts to use it to perform automatic authentication. If
the TELNET AUTHENTICATION option is not supported, the
user’s login name is propagated using the TELNET
ENVIRON option. Setting this flag is the same as specifying
the a option to the open command or on the
command line.
autosynch
If
autosynch and localchars are both TRUE, then
when either the intr or quit characters is
typed (see set above for descriptions of the
intr and quit characters), the resulting
telnet sequence sent is followed by the TELNET SYNCH
sequence. This procedure should cause the remote
system to begin throwing away all previously typed input
until both of the telnet sequences have been read and acted
upon. The initial value of this toggle is FALSE.
binary
Enable or
disable the TELNET BINARY option on both input and
output.
inbinary
Enable or
disable the TELNET BINARY option on input.
outbinary
Enable or
disable the TELNET BINARY option on output.
crlf
If this is
TRUE, then carriage returns will be sent as
<CR><LF>. If this is FALSE, then carriage
returns will be send as <CR><NUL>. The initial
value for this toggle is FALSE.
crmod
Toggle carriage
return mode. When this mode is enabled, most carriage return
characters received from the remote host will be mapped into
a carriage return followed by a line feed. This mode does
not affect those characters typed by the user, only those
received from the remote host. This mode is not very useful
unless the remote host only sends carriage return, but never
line feed. The initial value for this toggle is FALSE.
debug
Toggles socket
level debugging (useful only to the super user). The
initial value for this toggle is FALSE.
encdebug
Turns on
debugging information for the encryption code. Note that
this flag only exists if encryption support is
available.
localchars
If this is
TRUE, then the flush, interrupt, quit,
erase, and kill characters (see set
above) are recognized locally, and transformed into
(hopefully) appropriate TELNET control sequences
(respectively ao, ip, brk, ec,
and el; see send above). The initial value for
this toggle is TRUE in ’’old line by
line’’ mode, and FALSE in
’’character at a time’’ mode. When
the LINEMODE option is enabled, the value of
localchars is ignored, and assumed to always be TRUE.
If LINEMODE has ever been enabled, then quit is sent
as abort, and eof and are sent as eof and
susp, see send above).
netdata
Toggles the
display of all network data (in hexadecimal format). The
initial value for this toggle is FALSE.
options
Toggles the
display of some internal telnet protocol processing
(having to do with telnet options). The initial value for
this toggle is FALSE.
prettydump
When the
netdata toggle is enabled, if prettydump is
enabled the output from the netdata command will be
formatted in a more user-readable format. Spaces are put
between each character in the output, and the beginning of
telnet escape sequences are preceded by a ’*’ to
aid in locating them.
skiprc
When the skiprc
toggle is TRUE, telnet does not read the telnetrc files. The
initial value for this toggle is FALSE.
termdata
Toggles the
display of all terminal data (in hexadecimal format). The
initial value for this toggle is FALSE.
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. This flag only exists if encryption
support is available.
?
Displays the
legal toggle commands.
z
Suspend
telnet. This command only works when the user is
using the csh(1).
! [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.
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.
bugs
The source code is not
comprehensible.
Linux NetKit
(0.17) August 15, 1999 Linux NetKit (0.17)
history
/etc/telnetrc
global telnet
startup values
~/.telnetrc
user customized
telnet startup values
The Telnet command
appeared in 4.2BSD.