NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [-8] [-E] [-F] [-K] [-L] [-S tos] [-X authtype] [-a]
[-c] [-d] [-e escapechar] [-f] [-k realm] [-l user] [-n tra-
cefile] [-r] [-x] [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
-8 Specify an 8-bit data path. This causes an attempt to
negotiate the TELNET BINARY option on both input and
output.
-E Stop any character from being recognized as an escape
character.
-F forward a forwardable copy of the local credentials to
the remote system.
-K Specify no automatic login to the remote system.
-L Specify an 8-bit data path on output. This causes the
BINARY option to be negotiated on output.
-S tos
Set the IP type-of-service (TOS) option for the telnet
connection to the value tos, which can be a numeric TOS
value (in decimal, or a hex value preceded by 0x, or an
octal value preceded by a leading 0) or, on systems
that support it, a symbolic TOS name found in the
/etc/iptos file.
-X atype
Disable the atype type of authentication.
-a Attempt automatic login. 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.
-c Disable the reading of the user's .telnetrc file. (See
the toggle skiprc command on this man page.)
-d Set the initial value of the debug flag to TRUE
-e escape char
Set the initial telnet escape character to escape char.
If escape char is omitted, then there will be no escape
character.
-f forward a copy of the local credentials to the remote
system.
-k realm
If Kerberos authentication is being used, request 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
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
Open tracefile for recording trace information. See
the set tracefile command below.
-r Specify 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 Turn 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.
host Indicates the name, alias, or Internet address of the
remote host.
port Indicates a port number (address of an application).
If the port is not specified, the default telnet port
(23) is used.
When in rlogin mode, ~ is the telnet escape character; a
line of the form ~. disconnects from the remote host. Simi-
larly, the line ~^Z suspends 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 tel-
net will revert to one of two input modes: either ``charac-
ter 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 immedi-
ately 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 flag
is TRUE (the default for ``old line by line''; see below),
the user's quit, intr, and flush characters are trapped
locally, and sent as TELNET protocol sequences to the remote
side. If LINEMODE has ever been enabled, then the user's
susp and eof are also sent as TELNET protocol sequences, and
quit is sent as a TELNET ABORT instead of BREAK. There are
options (see toggle autoflush and toggle autosynch below)
which cause this action to flush subsequent output to the
terminal (until the remote host acknowledges the TELNET
sequence) and flush previous terminal input (in the case of
quit and intr).
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 command to uniquely identify it need be typed (this is
also true for arguments to the mode, set, toggle, unset,
slc, environ, and display commands).
auth argument ...
The auth command manipulates the information sent
through the TELNET AUTHENTICATE option. Valid argu-
ments 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 some or all of the set and toggle values (see
below).
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]fP [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 com-
mand.
-input
This is the same as the encrypt stop input com-
mand.
output
This is the same as the encrypt start output com-
mand.
-output
This is the same as the encrypt stop output com-
mand.
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 commands.
environ arguments ...
The environ command is used to manipulate the the vari-
ables that my 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 vari-
ables.
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
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 reattachment, the logout argu-
ment indicates that you should terminate 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 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 command.
open host [-a] [[-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).
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 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 commands and are pro-
cessed as if they had been typed in manually to the
telnet command prompt.
-a Attempt automatic login. 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 getlo-
gin(2) if it agrees with the current user ID; oth-
erwise it is the name associated with the user ID.
[-l] user
may be used to specify the user name to be passed
to the remote system via the ENVIRON option.
-port
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.
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
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 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
status.
ip Sends the TELNET IP (Interrupt Process) sequence,
which should cause the remote system to abort the
currently running process.
nop Sends the TELNET NOP (No OPeration) sequence.
susp Sends the TELNET SUSP (SUSPend process) sequence.
synch
Sends the TELNET SYNCH sequence. This sequence
causes the remote system to discard all previously
typed (but not yet read) input. This sequence is
sent as TCP urgent data (and may not work if the
remote system is a 4.2BSD system -- if it doesn't
work, a lower case ``r'' may be echoed on the ter-
minal).
do cmd
dont cmd
will cmd
wont cmd
Sends the TELNET DO cmd sequence. Cmd can be
either a decimal number between 0 and 255, or a
symbolic name for a specific TELNET command. Cmd
can also be either help or ? to print out help
information, including a list of known symbolic
names.
? Prints out help information for the send command.
set argument value
unset argument value
The set command will set any one of a number of telnet
variables to a specific value or to TRUE. The special
value off turns off the function associated with the
variable; this is equivalent to using the unset com-
mand. The unset command will disable or set to FALSE
any of the specified functions. The values of vari-
ables may be interrogated with the display command.
The variables which may be set or unset, but not tog-
gled, are listed here. In addition, any of the vari-
ables 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 TEL-
NET 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
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 charac-
ter 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 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 local-
chars 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 local-
chars 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 local-
chars below), and if telnet is operating in
``character at a time'' mode, then when this char-
acter 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 local-
chars below) and the quit character is typed, a
TELNET BRK sequence (see send brk above) is sent
to the remote host. The initial value for the
quit character is taken to be the terminal's quit
character.
reprint
If telnet is operating in LINEMODE or ``old line
by line'' mode, then this character is taken to be
the terminal's reprint character. The initial
value for the reprint character is taken to be the
terminal's reprint character.
rlogin
This is the rlogin 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 followed by a ^Z it suspends the telnet com-
mand. The initial state is to disable the rlogin
escape character.
start
If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, then this character is taken to be the
terminal's start character. The initial value for
the kill character is taken to be the terminal's
start character.
stop If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, then this character is taken to be the
terminal's stop character. The initial value for
the kill character is taken to be the terminal's
stop character.
susp If telnet is in localchars mode, or LINEMODE is
enabled, and the suspend character is typed, a
TELNET SUSP sequence (see send susp above) is sent
to the remote host. The initial value for the
suspend character is taken to be the terminal's
suspend character.
tracefile
This is the file to which the output, caused by
netdata or option tracing being TRUE, will be
written. If it is set to ``-'', then tracing
information will be written to standard output
(the default).
worderase
If telnet is operating in LINEMODE or ``old line
by line'' mode, then this character is taken to be
the terminal's worderase character. The initial
value for the worderase character is taken to be
the terminal's worderase character.
? Displays the legal set (unset) commands.
slc state
The slc command (Set Local Characters) is used to set
or change the state of the the special characters when
the TELNET LINEMODE option has been enabled. Special
characters are characters that get mapped to telnet
commands sequences (like ip or quit ) or line editing
characters (like erase and kill). By default, the
local special characters are exported.
check
Verify the current settings for the current 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 char-
acters. 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 con-
trol 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 interro-
gated with the display command. Valid arguments are:
authdebug
Turns on debugging information for the authentica-
tion 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 sys-
tem acknowledges (via a TELNET TIMING MARK option)
that it has processed those TELNET sequences. The
initial value for this toggle is TRUE if the ter-
minal user had not done an "stty noflsh", other-
wise 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: 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 AUTHENTICA-
TION 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 the -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 out-
put.
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 car-
riage 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.
localchars
If this is TRUE , then the flush, interrupt, quit,
erase, and kill characters (see set above) are
recognized locally, and transformed into (hope-
fully) 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 ``char-
acter 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 hexa-
decimal format). The initial value for this tog-
gle is FALSE.
options
Toggles the display of some internal telnet
protocol processing (having to do with TELNET
options). The initial value for this flag is
FALSE .
prettydump
When the netdata flag 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 out-
put, and the beginning of any TELNET escape
sequence is preceded by a '*' to aid in locating
them.
skiprc
When the skiprc flag is TRUE, TELNET skips the
reading of the .telnetrc file in the user's home
directory when connections are opened. The ini-
tial value for this flag is FALSE.
termdata
Toggles the display of all terminal data (in hexa-
decimal format). The initial value for this flag
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. Note: Because of export con-
trols, 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's shell is 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, prints a help summary.
If a command is specified, will print the help informa-
tion 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
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.
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