TELNET(1)TELNET(1)NAMEtelnet - user interface to a remote system using the TELNET protocol
SYNOPSIStelnet [-8EFKLacdfrx] [-X atype] [-e escape_char]
[-k realm] [-l user] [-n file]
[ [ [!] @hop1 [@hop2...] @] host [port]]
DESCRIPTION
The telnet utility communicates with another host using the TELNET pro‐
tocol. If telnet is invoked without arguments, it enters command mode,
indicated by its prompt, telnet>. In this mode, it accepts and executes
its associated commands. See USAGE. If it is invoked with arguments, it
performs an open command with those arguments.
If, for example, a host is specified as @hop1@hop2@host, the connection
goes through hosts hop1 and hop2, using loose source routing to end at
host. If a leading ! is used, the connection follows strict source
routing. Notice that when telnet uses IPv6, it can only use loose
source routing, and the connection ignores the !.
Once a connection has been opened, telnet enters input mode. In this
mode, text typed is sent to the remote host. The input mode entered
will be either "line mode", "character at a time", or "old line by
line", depending upon what the remote system supports.
In "line mode", character processing is done on the local system, under
the control of the remote system. When input editing or character echo‐
ing 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 char‐
acter" (initially ^E) may be used to turn off and on the local echo.
(Use this mostly to enter passwords without the password being
echoed.).
If the "line mode" option is enabled, or if the localchars toggle is
TRUE (the default in "old line by line" mode), the user's quit, intr,
and flush characters are trapped locally, and sent as TELNET protocol
sequences to the remote side. If "line mode" has ever been enabled,
then the user's susp and eof are also sent as TELNET protocol
sequences. quit is then sent as a TELNET ABORT instead of BREAK. The
options toggle autoflush and toggle autosynch cause this action to
flush subsequent output to the terminal (until the remote host acknowl‐
edges the TELNET sequence); and to flush previous terminal input, in
the case of quit and intr.
While connected to a remote host, the user can enter telnet command
mode by typing the telnet escape character (initially ^]). When in com‐
mand mode, the normal terminal editing conventions are available.
Pressing RETURN at the telnet command prompt causes telnet to exit com‐
mand mode.
OPTIONS
The following options are supported:
-8
Specifies an 8-bit data path. Negotiating the TELNET BINARY option
is attempted for both input and output.
-a
Attempts automatic login. This sends the user name by means of 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(3C) if it agrees with the current user ID. Otherwise, it
is the name associated with the user ID.
-c
Disables the reading of the user's telnetrc file. (See the toggle
skiprc command on this reference page.)
-d
Sets the initial value of the debug toggle to TRUE.
-e escape_char
Sets the initial escape character to escape_char. escape_char may
also be a two character sequence consisting of ^ (Control key) fol‐
lowed by one character. If the second character is ?, the DEL char‐
acter is selected. Otherwise, the second character is converted to
a control character and used as the escape character. If
escape_char is defined as the null string (that is, -e ''), this is
equivalent to -e '^@' (Control-@). To specify that no character can
be the escape character, use the -E option.
-E
Stops any character from being recognized as an escape character.
-f
Forwards a copy of the local credentials to the remote system.
-F
Forwards a forwardable copy of the local credentials to the remote
system.
-k realm
If Kerberos authentication is being used, requests that telnet
obtain tickets for the remote host in realm instead of the remote
host's default realm as determined inkrb5.conf(4).
-K
Specifies no automatic login to the remote system.
-l user
When connecting to a remote system that understands the ENVIRON
option, then user will be sent to the remote system as the value
for the ENVIRON variable USER.
-L
Specifies an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.
-n tracefile
Opens tracefile for recording trace information. See the set trace‐
file 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. The rlogin escape character is only recognized
when it is preceded by a carriage return. In this mode, the telnet
escape character, normally '^]', must still precede a telnet com‐
mand. The rlogin escape character can also be followed by '.\r' or
'^Z', and, like rlogin(1), closes or suspends the connection,
respectively. This option is an uncommitted interface and may
change in the future.
-x
Turns on encryption of the data stream. When this option is turned
on, telnet will exit with an error if authentication cannot be
negotiated or if encryption cannot be turned on.
-X atype
Disables the atype type of authentication.
USAGEtelnet Commands
The commands described in this section are available with telnet. It is
necessary to type only enough of each command to uniquely identify it.
(This is also true for arguments to the mode, set, toggle, unset, envi‐
ron, and display commands.)
auth argument ...
The auth command manipulates the information sent through the TEL‐
NET 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 authentica‐
tion.
open [-l user ] [ [!] @hop1 [@hop2 ...]@host [ port ]
Open a connection to the named host. If no port number is speci‐
fied, telnet will attempt to contact a TELNET server at the default
port. The host specification may be either a host name (see
hosts(4)) or an Internet address specified in the "dot notation"
(see inet(7P) or inet6(7P)). If the host is specified as
@hop1@hop2@host, the connection goes through hosts hop1 and hop2,
using loose source routing to end at host. The @ symbol is required
as a separator between the hosts specified. If a leading ! is used
with IPv4, the connection follows strict source routing.
The -l option passes the user as the value of the ENVIRON variable
USER to the remote system.
close
Close any open TELNET session and exit telnet. An EOF (in command
mode) will also close a session and exit.
encrypt
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 the
input and output, both input and output are disabled. To obtain
a list of available 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 output 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 command.
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 avail‐
able 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 commands.
quit
Same as close.
z
Suspend telnet. This command only works when the user is using a
shell that supports job control, such as sh(1).
mode type
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. The argument type is one of the
following:
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.
status
Show the current status of telnet. This includes the peer one is
connected to, as well as the current mode.
display
[argument...] Display all, or some, of the set and toggle values
(see toggle argument...).
?
[command] Get help. With no arguments, telnet prints a help sum‐
mary. If a command is specified, telnet will print the help infor‐
mation for just that command.
send argument...
Send one or more special character sequences to the remote host.
The following are the arguments that can be specified (more than
one argument may be specified at a time):
escape
Send the current telnet escape character (initially ^]).
synch
Send the TELNET SYNCH sequence. This sequence discards all pre‐
viously typed, but not yet read, input on the remote system.
This sequence is sent as TCP urgent data and may not work if
the remote system is a 4.2 BSD system. If it does not work, a
lowercase "r" may be echoed on the terminal.
brk or break
Send the TELNET BRK (Break) sequence, which may have signifi‐
cance to the remote system.
ip
Send the TELNET IP (Interrupt Process) sequence, which aborts
the currently running process on the remote system.
abort
Send the TELNET ABORT (Abort Process) sequence.
ao
Send the TELNET AO (Abort Output) sequence, which flushes all
output from the remote system to the user's terminal.
ayt
Send the TELNET AYT (Are You There) sequence, to which the
remote system may or may not respond.
ec
Send the TELNET EC (Erase Character) sequence, which erases the
last character entered.
el
Send the TELNET EL (Erase Line) sequence, which should cause
the remote system to erase the line currently being entered.
eof
Send the TELNET EOF (End Of File) sequence.
eor
Send the TELNET EOR (End Of Record) sequence.
ga
Send the TELNET GA (Go Ahead) sequence, which probably has no
significance for the remote system.
getstatus
If the remote side supports the TELNET STATUS command, getsta‐
tus will send the subnegotiation to request that the server
send its current option status.
nop
Send the TELNET NOP (No Operation) sequence.
susp
Send the TELNET SUSP (Suspend Process) sequence.
do option
dont option
will option
wont option
Send the TELNET protocol option negotiation indicated. Option
may be the text name of the protocol option, or the number cor‐
responding to the option. The command will be silently ignored
if the option negotiation indicated is not valid in the current
state. If the option is given as help or ?, the list of option
names known is listed. This command is mostly useful for
unusual debugging situations.
?
Print out help information for the send command.
set argument [value]
unset argument
Set any one of a number of telnet variables to a specific value.
The special value off turns off the function associated with the
variable. The values of variables may be interrogated with the
display command. If value is omitted, the value is taken to be
true, or "on". If the unset form is used, the value is taken to be
false, or off. The variables that may be specified are:
echo
This is the value (initially ^E) that, when in "line by line"
mode, toggles between local echoing of entered characters for
normal processing, and suppressing echoing of entered charac‐
ters, for example, entering a password.
escape
This is the telnet escape character (initially ^]) that enters
telnet command mode when connected to a remote system.
interrupt
If telnet is in localchars mode (see toggle, localchars) and
the interrupt character is typed, a TELNET IP sequence (see
send and ip) is sent to the remote host. The initial value for
the interrupt character is taken to be the terminal's intr
character.
quit
If telnet is in localchars mode and the quit character is
typed, a TELNET BRK sequence (see send, brk) is sent to the
remote host. The initial value for the quit character is taken
to be the terminal's quit character.
flushoutput
If telnet is in localchars mode and the flushoutput character
is typed, a TELNET AO sequence (see send, ao) is sent to the
remote host. The initial value for the flush character is taken
to be the terminal's flush character.
erase
If telnet is in localchars mode and operating in "character at
a time" mode, then when the erase character is typed, a TELNET
EC sequence (see send, ec) is sent to the remote system. The
initial value for the erase character is taken to be the termi‐
nal's erase character.
kill
If telnet is in localchars mode and operating in "character at
a time" mode, then when the kill character is typed, a TELNET
EL sequence (see send, el) is sent to the remote system. The
initial value for the kill character is taken to be the termi‐
nal's kill character.
eof
If telnet is operating in "line by line"/ mode, entering the
eof character as the first character on a line sends this char‐
acter to the remote system. The initial value of eof is taken
to be the terminal's eof character.
ayt
If telnet is in localchars mode, or LINEMODE is enabled, and
the status character is typed, a TELNET AYT ("Are You There")
sequence is sent to the remote host. (See send, ayt above.) The
initial value for ayt is the terminal's status character.
forw1
forw2
If telnet is operating in LINEMODE, and the forw1 or forw2
characters are typed, this causes the forwarding of partial
lines to the remote system. The initial values for the forward‐
ing characters come from the terminal's eol and eol2 charac‐
ters.
lnext
If telnet is operating in LINEMODE or "old line by line" mode,
then the lnext character is assumed to be the terminal's lnext
character. The initial value for the lnext character is taken
to be the terminal's lnext character.
reprint
If telnet is operating in LINEMODE or "old line by line" mode,
then the reprint character is assumed to be the terminal's re‐
print character. The initial value for reprint 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. The rlogin character, at
the beginning of a line followed by a "." closes the connec‐
tion. When followed by a ^Z, the rlogin command suspends the
telnet command. The initial state is to disable the rlogin
escape character.
start
If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then
the start character is taken to be the terminal's start charac‐
ter. 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
the stop character is taken to be the terminal's stop charac‐
ter. 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, generated when the net‐
data or the debug option is TRUE, will be written. If tracefile
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 and unset commands.
slc state
The slc (Set Local Characters) command is used to set or change the
state of 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 charac‐
ters (like erase and kill). By default, the local special charac‐
ters are exported. The following values for state are valid:
check
Verifies the settings for the current special characters. The
remote side is requested to send all the current special char‐
acter settings. If there are any discrepancies with the local
side, the local settings will switch to the remote values.
export
Switches 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
Switches 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.
toggle argument...
Toggle between TRUE and FALSE the various flags that control how
telnet responds to events. More than one argument may be specified.
The state of these flags may be interrogated with the display com‐
mand. Valid arguments are:
authdebug
Turns on debugging information for the authen‐
tication code.
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 AUTHEN‐
TICATION option, telnet attempts to use it to
perform automatic authentication. If the
AUTHENTICATION option is not supported, the
user's login name is propagated through the
TELNET ENVIRON option. This command is the same
as specifying the -a option on the open com‐
mand.
autoflush
If autoflush and localchars are both TRUE, then
when the ao, intr, or quit characters are rec‐
ognized (and transformed into TELNET sequences;
see set for details), telnet refuses to display
any data on the user's terminal until the
remote system acknowledges (using a TELNET Tim‐
ing Mark option) that it has processed those
TELNET sequences. The initial value for this
toggle is TRUE if the terminal user has not
done an "stty noflsh". Otherwise, the value is
FALSE (see stty(1)).
autosynch
If autosynch and localchars are both TRUE, then
when either the interrupt or quit characters
are typed (see set for descriptions of inter‐
rupt and quit), 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
Determines how carriage returns are sent. If
the value is TRUE, then carriage returns will
be sent as <CR><LF>. If the value is FALSE,
then carriage returns will be send as
<CR><NUL>. The initial value for this toggle is
FALSE.
crmod
Toggle RETURN mode. When this mode is enabled,
most RETURN characters received from the remote
host will be mapped into a 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
useful only for remote hosts that send RETURN
but never send LINEFEED. The initial value for
this toggle is FALSE.
debug
Toggle socket level debugging (only available
to the super-user). The initial value for this
toggle is FALSE.
encdebug
Turns on debugging information for the encryp‐
tion code.
localchars
If this toggle is TRUE, then the flush, inter‐
rupt, quit, erase, and kill characters (see
set) are recognized locally, and transformed
into appropriate TELNET control sequences,
respectively ao, ip, brk, ec, and el (see
send). The initial value for this toggle is
TRUE in "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 always to 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
Toggle the display of all network data (in
hexadecimal format). The initial value for this
toggle is FALSE.
options
Toggle the display of some internal TELNET pro‐
tocol processing (having to do with telnet
options). The initial value for this toggle is
FALSE.
prettydump
When the netdata toggle is enabled, if pretty‐
dump is enabled, the output from the netdata
command will be formatted in a more user read‐
able format. Spaces are put between each char‐
acter in the output. The beginning of any TEL‐
NET escape sequence is preceded by an asterisk
(*) 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 flag is TRUE, TELNET
prints out a message each time encryption is
enabled or disabled. The initial value for this
toggle is FALSE.
?
Display the legal toggle commands.
environ argument...
The environ command is used to manipulate variables that may be
sent through the TELNET ENVIRON option. The initial set of vari‐
ables is taken from the users environment. Only the DISPLAY and
PRINTER variables are exported by default. Valid arguments for the
environ command are:
define variable value
Define 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 to be exported to the remote side.
unexport variable
Mark the variable to not be exported unless explicitly
requested by the remote side.
list
List the current set of environment variables. Those marked
with an asterisk (*) will be sent automatically. Other vari‐
ables will be sent only 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 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 reat‐
tachment, the logout argument indicates that the remote side should
terminate the session immediately.
FILES
$HOME/.telnetrc
file that contains commands to be executed before
initiating a telnet session
SEE ALSOrlogin(1), sh(1), stty(1), getlogin(3C), hosts(4), krb5.conf(4), nolo‐
gin(4), telnetrc(4), attributes(5), inet(7P), inet6(7P)DIAGNOSTICS
NO LOGINS: System going down in N minutes
The machine is in the process of being shut down and logins have
been disabled.
NOTES
On some remote systems, echo has to be turned off manually when in
"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.
The telnet protocol only uses single DES for session protection—clients
request service tickets with single DES session keys. The KDC must know
that host service principals that offer the telnet service support sin‐
gle DES, which, in practice, means that such principals must have sin‐
gle DES keys in the KDC database.
Aug 17, 2006 TELNET(1)
