TELNET linux command manual
TELNET(1) BSD General Commands Manual TELNET(1)
NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [-8EFKLacdfrx] [-X authtype] [-b hostalias] [-e escapechar]
[-k realm] [-l user] [-n tracefile] [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:
-7 Strip 8th bit on input and output. Telnet is 8-bit clean by
default but doesn't send the TELNET BINARY option unless forced.
-8 Specifies an 8-bit data path. This causes an attempt to negoti-
ate the TELNET BINARY option on both input and output.
-E Stops any character from being recognized as an escape charac-
ter.
-F If Kerberos V5 authentication is being used, the -F option
allows the local credentials to be forwarded to the remote sys-
tem, including any credentials that have already been forwarded
into the local environment.
-K Specifies no automatic login to the remote system.
-L Specifies an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.
-X atype
Disables the atype type of authentication.
-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 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 Sets the initial value of the debug toggle to TRUE.
-e escapechar
Sets the initial telnet escape character to escapechar. If
escapechar is omitted, then there will be no escape character.
-f If Kerberos V5 authentication is being used, the -f option
allows the local credentials to be forwarded to the remote sys-
tem.
-k realm
If Kerberos authentication is being used, the -k option requests
that telnet obtain tickets for the remote host in realm realm
instead of the remote host's realm, as determined by
krb_realmofhost(3).
-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 the
open command.
-n tracefile
Opens tracefile for recording trace information. See the set
tracefile command below.
-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.
-x Turns on encryption of the data stream if possible.
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 num-
ber 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 sus-
pends 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 sys-
tem, 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 pro-
tocol 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 com-
mand 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 com-
mand to uniquely identify it need be typed (this is also true for argu-
ments to the mode, set, toggle, unset, slc, environ, and display com-
mands).
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 (see
below).
encrypt argument [...]
The encrypt command manipulates the information sent through
the TELNET ENCRYPT option.
Valid arguments for the encrypt command are as follows:
disable type [input|output]
Disables the specified type of encryption. If
you omit input and output, both input and out-
put are disabled. To obtain a list of avail-
able types, use the encrypt disable ? command.
enable type [input|output]
Enables the specified type of encryption. If
you omit input and output, both input and out-
put 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 com-
mand.
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 com-
mands.
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 users 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 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 com-
mand.
logout Sends the TELNET LOGOUT option to the remote side. This com-
mand 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 the concept of
suspending a user's session for later reattachment, the
logout argument indicates that you should terminate the ses-
sion 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, 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 com-
mand.
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 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(3)). The -l option may be
used to specify the user name to be passed to the remote sys-
tem via the ENVIRON option. When connecting to a non-stan-
dard 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 be telnet commands and are pro-
cessed as if they had been typed in manually to the telnet
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 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 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 cur-
rently 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 com-
mand, getstatus will send the subnegotiation to
request that the server send its current option sta-
tus.
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 termi-
nal).
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
either help 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 sym-
bolic name for a specific TELNET command. cmd can
also be either help or ? to print out help informa-
tion, 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 sym-
bolic name for a specific TELNET command. cmd can
also be either help or ? to print out help informa-
tion, 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 sym-
bolic name for a specific TELNET command. cmd can
also be either help or ? to print out help informa-
tion, 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 vari-
ables to a specific value or to TRUE. The special value off
turns off the function associated with the variable; this is
equivalent to using the unset command. The unset command
will disable or set to FALSE any of the specified functions.
The values of variables may be interrogated with the 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 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 This is the value (initially ''^E'') which, when in
''line by line'' mode, toggles between doing local
echoing of entered characters (for normal process-
ing), 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 char-
acter.
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 charac-
ter 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 ter-
minal'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 ter-
minal'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 charac-
ter 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 ter-
minal'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 charac-
ter.
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 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 fol-
lowed by a ^Z it suspends the telnet command. The
initial state is to disable the rlogin escape charac-
ter.
start If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, then this character is taken to be the ter-
minal'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 ter-
minal's stop character. The initial value for the
stop 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 char-
acter is taken to be the terminal's suspend charac-
ter.
tracefile
This is the file to which the output, caused by
netdata or option tracing being TRUE, will be writ-
ten. 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 termi-
nal's worderase 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 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 spe-
cial 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 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
listed above. More than one argument may be specified. The
state of these flags may be interrogated with the display
command. Valid arguments are:
authdebug Turns on debugging information for the authen-
tication code.
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 automati-
cally. The autoencrypt (autodecrypt) command
states that encryption of the output (input)
stream should be enabled as soon as possible.
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 are propagated through the
TELNET ENVIRON option. This command is the
same as specifying a option on the open com-
mand.
autosynch If autosynch and localchars are both TRUE, then
when either the intr or quit character 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 . If this is FALSE, then car-
riage returns will be send as . 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 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.
encdebug Turns on debugging information for the encryp-
tion code.
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 suspend 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 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 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.
? 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 infor-
mation 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
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.
BSD February 3, 1994 BSD