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''.
-K
Specifies no automatic login to the remote system.
-L
Specifies an 8-bit data path on output. This causes the
TELNET BINARY
option to be negotiated on just 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 username is retrieved
via
getlogin(3).
-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.
-z option
Set SSL (Secure Socket Layer) parameters. The default is to negotiate
via telnet protocoll if SSL is availlable at server side and then to
switch it on. In this mode you can connect to both conventional and
SSL enhanced telnetd's.
The SSL parameters are:
debug
Send SSL related debugging information to stderr.
authdebug
Enable authentication debugging.
ssl
Negotiate SSL at first, then use telnet protocol. In this mode you can
connect to any server supporting directly SSL like Apache-SSL. Use
telnet -z ssl ssl3.netscape.com https
for example. telnet protocol negotiation goes encrypted.
nossl, !ssl
switch off SSL negotiation
certrequired
client certificate is mandatory
secure
Don't switch back to unencrypted mode (no SSL) if SSL is not available.
verbose
Be verbose about certificates etc.
verify=int
Set the SSL verify flags (SSL_VERIFY_* in
ssl/ssl.h
).
cert=cert_file
Use the certificate(s) in
cert_file
key=key_file
Use the key(s) in
key_file
cipher=ciph_list
Set the preferred ciphers to
ciph_list
(See
ssl/ssl.h
).
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
toggleautoflush
and
toggleautosynch
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. 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
BSD 4.2
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 DOcmd
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
togglelocalchars
below),
and
if
telnet
is operating in character at a time mode, then when this
character is typed, a
TELNET EC
sequence (see
sendec
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
togglelocalchars
below)
and the
flushoutput
character is typed, a
TELNET AO
sequence (see
sendao
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
togglelocalchars
below)
and the
interrupt
character is typed, a
TELNET IP
sequence (see
sendip
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
togglelocalchars
below),
and
if
telnet
is operating in character at a time mode, then when this
character is typed, a
TELNET EL
sequence (see
sendel
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
togglelocalchars
below)
and the
quit
character is typed, a
TELNET BRK
sequence (see
sendbrk
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
sendsusp
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 interruptquit erase
and
kill
characters (see
set
above) are recognized locally, and transformed into (hopefully) appropriate
TELNET
control sequences
(respectively
ao ipbrk 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 andsuspend
are sent as
eof andsusp
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
TRUEtelnet
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
TRUETELNET
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
HOMESHELLDISPLAY
and
TERM
environment variables.
Other environment variables may be propagated
to the other side via the
TELNET ENVIRON
option.
FILES
/etc/telnetrc
global telnet startup values
~/.telnetrc
user customized telnet startup values
HISTORY
The
Telnet
command appeared in
BSD 4.2
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.