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NAME
telnet — user interface to the TELNET protocol
SYNOPSIS
telnet [-78EFKLacdfrx] [-S tos] [-X authtype] [-e escapechar] [-k realm] [-l user]
[-n tracefile] [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 Specifies an 8-bit data path. This causes an attempt to negotiate the TELNET BINARY
option on both input and output.
-7 Do not try to negotiate TELNET BINARY option.
-E Stops any character from being recognized as an escape character.
-F If Kerberos V5 authentication is being used, the -F option allows the local
credentials to be forwarded to the remote system, including any credentials that
have already been forwarded into the local environment.
-K Specifies no automatic login to the remote system.
-L Specifies an 8-bit data path on output. This causes the BINARY option to be
negotiated on output.
-S tos Sets the IP type-of-service (TOS) option for the telnet connection to the value tos,
which can be a numeric TOS value or, on systems that support it, a symbolic TOS name
found in the /etc/iptos file.
-X atype
Disables the atype type of authentication.
-a Attempt automatic login. Currently, this sends the user name via the USER variable
of the ENVIRON option if supported by the remote system. The 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 Disables the reading of the user's .telnetrc file. (See the toggle skiprc command
on this man page.)
-d Sets the initial value of the debug toggle to TRUE
-e escape char
Sets the initial telnet telnet escape character to escape char. If escape char is
omitted, then there will be no escape character.
-f If Kerberos V5 authentication is being used, the -f option allows the local
credentials to be forwarded to the remote system.
-k realm
If Kerberos authentication is being used, the -k option requests 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
When connecting to the remote system, 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
Opens tracefile for recording trace information. See the set tracefile command
below.
-r 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.
-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 official name, an alias, or the Internet address of a remote host.
port Indicates a port number (address of an application). If a number is not specified,
the default telnet port is used.
When in rlogin mode, a line of the form ~. disconnects from the remote host; ~ is the
telnet escape character. 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 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 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 all, or some, 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 [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. 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 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 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 [-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 establishing 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 commands and are processed as if they had been
typed in manually 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 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 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 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 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 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 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
toggle is FALSE.
prettydump When the netdata toggle is enabled, if prettydump is enabled the
output from the netdata command will be formatted in a more user
readable format. Spaces are put between each character in the
output, and the beginning of 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 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 is using the csh(1).
! [command]
Execute a single command in a subshell on the local system. If command is
omitted, then an interactive subshell is invoked.
? [command]
Get help. With no arguments, telnet prints a help summary. If a command is
specified, telnet will print the help information for just that command.
ENVIRONMENT
Telnet uses at least the 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.