TELNET(1) User's Reference Manual TELNET(1)
NAME
telnet — user interface to the TELNET protocol
SYNOPSIS
telnet [options ...] [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.
OPTIONS
-4, --ipv4
Use IPv4 to connect to hosts.
-6, --ipv6
Use IPv6 to connect to hosts.
-8, --binary
Specifies an 8-bit data path. This causes an attempt to nego-
tiate the TELNET BINARY option on both input and output.
-L, --binary-output
Specifies an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.
-e, --escape escape-char
Sets the initial telnet escape character to escape-char. If
escape-char is omitted, then there will be no escape character.
-E, --no-escape
Stops any character from being recognized as an escape charac-
ter.
-a, --login
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.
-K, --no-login
Specifies no automatic login to the remote system.
-l, --user user
When connecting to the remote system, if the remote system un-
derstands the ENVIRON option, then user will be sent to the re-
mote 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, --trace tracefile
Opens tracefile for recording trace information. See the set
tracefile command below.
-r, --rlogin
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.
-c, --no-rc
Disables the reading of the user's .telnetrc file. (See the
toggle skiprc command on this man page.)
-d, --debug
Sets the initial value of the debug toggle to TRUE
-x, --encrypt
Turns on encryption of the data stream if possible.
-X, --disable-auth atype
Disables the atype type of authentication.
-k, --realm realm
If Kerberos authentication is being used, the -k option re-
quests that telnet obtain tickets for the remote host in realm
realm instead of the remote host's realm, as determined by
krb_realmofhost(3).
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.
OPERATION
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, 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 ef-
fect 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 acknowl-
edges the TELNET sequence) and flush previous terminal input (in the
case of quit and intr).
While connected to a remote host, telnet command mode may be entered by
typing the telnet “escape character” (initially “^]”). When in command
mode, the normal terminal editing conventions are available.
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 be-
low).
encrypt argument ...
The encrypt command manipulates the information sent through
the TELNET ENCRYPT option.
Note: Because of export controls, the TELNET ENCRYPT option
is not supported outside of the United States and Canada.
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 out-
put are enabled. To obtain a list of avail-
able 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 available types,
use the encrypt enable ? command.
status Lists the current status of encryption.
stop [input|output]
Stops encryption. If you omit input and out-
put, 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 the variables
that my be sent through the TELNET ENVIRON option. The ini-
tial 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 en-
closed 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 automati-
cally, other variables will only be sent if ex-
plicitly requested.
? Prints out help information for the environ com-
mand.
logout Sends the TELNET LOGOUT option to the remote side. This
command is similar to a close command; however, if the re-
mote side does not support the LOGOUT option, nothing hap-
pens. 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 reattach-
ment, the logout argument indicates that you should termi-
nate the session immediately.
mode type Type is one of several options, depending on the state of
the TELNET session. The remote host is asked for permission
to go into the requested mode. If the remote host is capa-
ble of entering that mode, the requested mode will be en-
tered.
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
system via the ENVIRON option. When connecting to a non-
standard port, telnet omits any automatic initiation of
TELNET options. When the port number is preceded by a minus
sign, the initial option negotiation is done. After estab-
lishing a connection, the file .telnetrc in the users home
directory is opened. Lines beginning with a # are comment
lines. Blank lines are ignored. Lines that begin without
white space 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 white space are assumed to be telnet com-
mands and are processed as if they had been typed in manu-
ally 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 speci-
fied (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 re-
spond.
brk Sends the TELNET BRK (Break) sequence, which may
have significance to the remote system.
ec Sends the TELNET EC (Erase Character) sequence,
which should cause the remote system to erase the
last character entered.
el Sends the TELNET EL (Erase Line) sequence, which
should cause the remote system to erase the line
currently being entered.
eof Sends the TELNET EOF (End Of File) sequence.
eor Sends the TELNET EOR (End of Record) sequence.
escape Sends the current telnet escape character (initially
“^”).
ga Sends the TELNET GA (Go Ahead) sequence, which
likely has no significance to the remote system.
getstatus
If the remote side supports the TELNET STATUS com-
mand, getstatus will send the subnegotiation to re-
quest 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 re-
mote system is a 4.2BSD system -- if it doesn't
work, a lower case “r” may be echoed on the termi-
nal).
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, in-
cluding 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 en-
abled, 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
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 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 char-
acter.
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
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 ter-
minal'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 en-
abled, 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 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 ter-
minal'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 ex-
ported.
check Verify the current settings for the current spe-
cial characters. The remote side is requested
to send all the current special character set-
tings, 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 ex-
plicitly 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 rec-
ognized (and transformed into TELNET se-
quences; see set above for details), telnet
refuses to display any data on the user's ter-
minal 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.
Note: Because of export controls, the TELNET
ENCRYPT option is not supported outside the
United States and Canada.
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
command.
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 car-
riage 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 charac-
ters typed by the user, only those received
from the remote host. This mode is not very
useful unless the remote host only sends car-
riage return, but never line feed. The ini-
tial 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 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 al-
ways be TRUE. If LINEMODE has ever been en-
abled, 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 begin-
ning 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 users
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 encryp-
tion is enabled or disabled. The initial
value for this toggle is FALSE. Note: Because
of export controls, data encryption is not
supported outside of the United States and
Canada.
? 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 in-
voked.
? [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.
GNU Network Utilities February 9, 2019 TELNET(1)
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