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 tracefile] [-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. Similarly, 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 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 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 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 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 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 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 variables 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. Variables defined by this
command are not automatically exported. The value may be enclosed in single or
double quotes so that tabs and spaces may be included.
undefine variable
Remove variable from the list of environment variables.
export variable
Mark the variable variable to be exported to the remote side.
unexport variable
Mark the variable variable to not be exported unless explicitly asked for by the
remote side.
list
List the current set of environment variables. Those marked with a * will be
sent automatically; those marked with a + will be sent if the other end requests
their values, and other variables will not be sent.
?
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 argument 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 processed
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 getlogin(2) if it agrees with the current user ID;
otherwise 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 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 4.2BSD 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 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 command. The unset command
will disable or set to FALSE any of the specified functions. The values of
variables may be interrogated with the display command. The variables which may
be set or unset, but not toggled, are listed here. In addition, any of the
variables for the toggle command may be explicitly set or unset using the set
and unset commands.
ayt
If telnet is in localchars mode, or LINEMODE is enabled, and the status
character is typed, a TELNET AYT sequence (see send ayt preceding) is sent to
the remote host. The initial value for the "Are You There" character is the
terminal's status character.
echo
This is the value (initially ``^E'') which, when in ``line by line'' mode,
toggles between doing local echoing of entered characters (for normal
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 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 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 command. 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 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 peer one is connected to,
as well as the current mode.
toggle arguments ...
Toggle (between TRUE and FALSE) various flags that control how telnet responds
to events. These flags may be set explicitly to TRUE or FALSE using the set and
unset commands listed above. More than one argument may be specified. The state
of these flags may be interrogated with the display command. Valid arguments
are:
authdebug
Turns on debugging information for the authentication code.
autoflush
If autoflush and localchars are both TRUE , then when the ao, or quit characters
are recognized (and transformed into TELNET sequences; see set above for
details), telnet refuses to display any data on the user's terminal until the
remote system acknowledges (via a TELNET TIMING MARK option) that it has
processed those TELNET sequences. The initial value for this toggle is TRUE if
the terminal user had not done an "stty noflsh", otherwise FALSE (see stty(1).
autodecrypt
When the TELNET ENCRYPT option is negotiated, by default the actual encryption
(decryption) of the data stream does not start 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 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 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 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.
localchars
If this is TRUE , then the flush, interrupt, quit, erase, and kill characters
(see set above) are recognized locally, and transformed into (hopefully)
appropriate TELNET control sequences (respectively ao, ip, brk, ec, and el; see
send above). The initial value for this toggle is TRUE in ``old line by line''
mode, and FALSE in ``character at a time'' mode. When the LINEMODE option is
enabled, the value of localchars is ignored, and assumed to always be TRUE. If
LINEMODE has ever been enabled, then quit is sent as abort, and eof and suspend
are sent as eof and susp, see send above).
netdata
Toggles the display of all network data (in hexadecimal format). The initial
value for this toggle is FALSE.
options
Toggles the display of some internal telnet protocol processing (having to do
with TELNET options). The initial value for this 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 output, 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 initial value for
this flag is FALSE.
termdata
Toggles the display of all terminal data (in hexadecimal 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 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'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 information 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
~/.k5login
(on remote host) - file containing Kerberos principals that are allowed access.
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.