TELNET(1) User Commands TELNET(1)

NAME

telnet - user interface to a remote system using the TELNET protocol

SYNOPSIS

telnet [-8EFKLacdfrx] [-X atype] [-e escape_char]
[-k realm] [-l user] [-n file]
[ [ [!] @hop1 [@hop2...] @] host [port]]

DESCRIPTION

The telnet utility communicates with another host using the TELNET
protocol. If telnet is invoked without arguments, it enters command
mode, indicated by its prompt, telnet>. In this mode, it accepts and
executes its associated commands. See USAGE. If it is invoked with
arguments, it performs an open command with those arguments.


If, for example, a host is specified as @hop1@hop2@host, the
connection goes through hosts hop1 and hop2, using loose source
routing to end at host. If a leading ! is used, the connection
follows strict source routing. Notice that when telnet uses IPv6, it
can only use loose source routing, and the connection ignores the !.


Once a connection has been opened, telnet enters input mode. In this
mode, text typed is sent to the remote host. The input mode entered
will be either "line mode", "character at a time", or "old line by
line", depending upon what the remote system supports.


In "line mode", character processing is done on the local system,
under the control of the remote system. When input editing or
character 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. (Use this mostly to enter passwords without the
password being echoed.).


If the "line mode" option is enabled, or if the localchars toggle is
TRUE (the default in "old line by line" mode), the user's quit, intr,
and flush characters are trapped locally, and sent as TELNET protocol
sequences to the remote side. If "line mode" has ever been enabled,
then the user's susp and eof are also sent as TELNET protocol
sequences. quit is then sent as a TELNET ABORT instead of BREAK. The
options toggle autoflush and toggle autosynch cause this action to
flush subsequent output to the terminal (until the remote host
acknowledges the TELNET sequence); and to flush previous terminal
input, in the case of quit and intr.


While connected to a remote host, the user can enter telnet command
mode by typing the telnet escape character (initially ^]). When in
command mode, the normal terminal editing conventions are available.
Pressing RETURN at the telnet command prompt causes telnet to exit
command mode.

OPTIONS

The following options are supported:

-8

Specifies an 8-bit data path. Negotiating the TELNET BINARY
option is attempted for both input and output.


-a

Attempts automatic login. This sends the user name by means of
the USER variable of the ENVIRON option, if supported by the
remote system. The name used is that of the current user as
returned by getlogin(3C) if it agrees with the current user ID.
Otherwise, it is the name associated with the user ID.


-c

Disables the reading of the user's telnetrc file. (See the toggle
skiprc command on this reference page.)


-d

Sets the initial value of the debug toggle to TRUE.


-e escape_char

Sets the initial escape character to escape_char. escape_char may
also be a two character sequence consisting of ^ (Control key)
followed by one character. If the second character is ?, the DEL
character is selected. Otherwise, the second character is
converted to a control character and used as the escape
character. If escape_char is defined as the null string (that is,
-e ''), this is equivalent to -e '^@' (Control-@). To specify
that no character can be the escape character, use the -E option.


-E

Stops any character from being recognized as an escape character.


-f

Forwards a copy of the local credentials to the remote system.


-F

Forwards a forwardable copy of the local credentials to the
remote system.


-k realm

If Kerberos authentication is being used, requests that telnet
obtain tickets for the remote host in realm instead of the remote
host's default realm as determined in krb5.conf(5).


-K

Specifies no automatic login to the remote system.


-l user

When connecting to a remote system that understands the ENVIRON
option, then user will be sent to the remote system as the value
for the ENVIRON variable USER.


-L

Specifies an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.


-n tracefile

Opens tracefile for recording trace information. See the set
tracefile command below.


-r

Specifies a user interface similar to rlogin. In this mode, the
escape character is set to the tilde (~) character, unless
modified by the -e option. The rlogin escape character is only
recognized when it is preceded by a carriage return. In this
mode, the telnet escape character, normally '^]', must still
precede a telnet command. The rlogin escape character can also be
followed by '.\r' or '^Z', and, like rlogin(1), closes or
suspends the connection, respectively. This option is an
uncommitted interface and may change in the future.


-x

Turns on encryption of the data stream. When this option is
turned on, telnet will exit with an error if authentication
cannot be negotiated or if encryption cannot be turned on.


-X atype

Disables the atype type of authentication.

USAGE

telnet Commands
The commands described in this section are available with telnet. It
is necessary to type only enough of each command to uniquely identify
it. (This is also true for arguments to the mode, set, toggle, unset,
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.


open [-l user ] [ [!] @hop1 [@hop2 ...]@host [ 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(4P) or inet6(4P)). If the host is specified
as @hop1@hop2@host, the connection goes through hosts hop1 and
hop2, using loose source routing to end at host. The @ symbol is
required as a separator between the hosts specified. If a leading
! is used with IPv4, the connection follows strict source
routing.

The -l option passes the user as the value of the ENVIRON
variable USER to the remote system.


close

Close any open TELNET session and exit telnet. An EOF (in command
mode) will also close a session and exit.


encrypt

The encrypt command manipulates the information sent through the
TELNET ENCRYPT option.

Valid arguments for the encrypt command are as follows:

disable type [input|output]

Disables the specified type of encryption. If you omit the
input and output, both input and output are disabled. To
obtain a list of available types, use the encrypt disable ?
command.


enable type [input|output]

Enables the specified type of encryption. If you omit input
and output, both input and output are enabled. To obtain a
list of available types, use the encrypt enable ? command.


input

This is the same as the encrypt start input command.


-input

This is the same as the encrypt stop input command.


output

This is the same as the encrypt start output command.


-output

This is the same as the encrypt stop output command.


start [input|output]

Attempts to start encryption. If you omit input and output,
both input and output are enabled. To obtain a list of
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.


quit

Same as close.


z

Suspend telnet. This command only works when the user is using a
shell that supports job control, such as sh(1).


mode type

The remote host is asked for permission to go into the requested
mode. If the remote host is capable of entering that mode, the
requested mode will be entered. The argument type is one of the
following:

character

Disable the TELNET LINEMODE option, or, if the remote side
does not understand the LINEMODE option, then enter
"character at a time" mode.


line

Enable the TELNET LINEMODE option, or, if the remote side
does not understand the LINEMODE option, then attempt to
enter "old-line-by-line" mode.


isig (-isig)

Attempt to enable (disable) the TRAPSIG mode of the LINEMODE
option. This requires that the LINEMODE option be enabled.


edit (-edit)

Attempt to enable (disable) the EDIT mode of the LINEMODE
option. This requires that the LINEMODE option be enabled.


softtabs (-softtabs)

Attempt to enable (disable) the SOFT_TAB mode of the LINEMODE
option. This requires that the LINEMODE option be enabled.


litecho (-litecho)

Attempt to enable (disable) the LIT_ECHO mode of the LINEMODE
option. This requires that the LINEMODE option be enabled.


?

Prints out help information for the mode command.


status

Show the current status of telnet. This includes the peer one is
connected to, as well as the current mode.


display

[argument...] Display all, or some, of the set and toggle values
(see toggle argument...).


?

[command] Get help. With no arguments, telnet prints a help
summary. If a command is specified, telnet will print the help
information for just that command.


send argument...

Send one or more special character sequences to the remote host.
The following are the arguments that can be specified (more than
one argument may be specified at a time):

escape

Send the current telnet escape character (initially ^]).


synch

Send the TELNET SYNCH sequence. This sequence discards all
previously typed, but not yet read, input on the remote
system. This sequence is sent as TCP urgent data and may not
work if the remote system is a 4.2 BSD system. If it does not
work, a lowercase "r" may be echoed on the terminal.


brk or break

Send the TELNET BRK (Break) sequence, which may have
significance to the remote system.


ip

Send the TELNET IP (Interrupt Process) sequence, which aborts
the currently running process on the remote system.


abort

Send the TELNET ABORT (Abort Process) sequence.


ao

Send the TELNET AO (Abort Output) sequence, which flushes all
output from the remote system to the user's terminal.


ayt

Send the TELNET AYT (Are You There) sequence, to which the
remote system may or may not respond.


ec

Send the TELNET EC (Erase Character) sequence, which erases
the last character entered.


el

Send the TELNET EL (Erase Line) sequence, which should cause
the remote system to erase the line currently being entered.


eof

Send the TELNET EOF (End Of File) sequence.


eor

Send the TELNET EOR (End Of Record) sequence.


ga

Send the TELNET GA (Go Ahead) sequence, which probably has no
significance for the remote system.


getstatus

If the remote side supports the TELNET STATUS command,
getstatus will send the subnegotiation to request that the
server send its current option status.


nop

Send the TELNET NOP (No Operation) sequence.


susp

Send the TELNET SUSP (Suspend Process) sequence.


do option
dont option
will option
wont option

Send the TELNET protocol option negotiation indicated. Option
may be the text name of the protocol option, or the number
corresponding to the option. The command will be silently
ignored if the option negotiation indicated is not valid in
the current state. If the option is given as help or ?, the
list of option names known is listed. This command is mostly
useful for unusual debugging situations.


?

Print out help information for the send command.


set argument [value]
unset argument

Set any one of a number of telnet variables to a specific value.
The special value off turns off the function associated with the
variable. The values of variables may be interrogated with the
display command. If value is omitted, the value is taken to be
true, or "on". If the unset form is used, the value is taken to
be false, or off. The variables that may be specified are:

echo

This is the value (initially ^E) that, when in "line by line"
mode, toggles between local echoing of entered characters for
normal processing, and suppressing echoing of entered
characters, for example, entering a password.


escape

This is the telnet escape character (initially ^]) that
enters telnet command mode when connected to a remote system.


interrupt

If telnet is in localchars mode (see toggle, localchars) and
the interrupt character is typed, a TELNET IP sequence (see
send and ip) is sent to the remote host. The initial value
for the interrupt character is taken to be the terminal's
intr character.


quit

If telnet is in localchars mode and the quit character is
typed, a TELNET BRK sequence (see send, brk) is sent to the
remote host. The initial value for the quit character is
taken to be the terminal's quit character.


flushoutput

If telnet is in localchars mode and the flushoutput character
is typed, a TELNET AO sequence (see send, ao) is sent to the
remote host. The initial value for the flush character is
taken to be the terminal's flush character.


erase

If telnet is in localchars mode and operating in "character
at a time" mode, then when the erase character is typed, a
TELNET EC sequence (see send, ec) is sent to the remote
system. The initial value for the erase character is taken to
be the terminal's erase character.


kill

If telnet is in localchars mode and operating in "character
at a time" mode, then when the kill character is typed, a
TELNET EL sequence (see send, el) is sent to the remote
system. The initial value for the kill character is taken to
be the terminal's kill character.


eof

If telnet is operating in "line by line"/ mode, entering the
eof character as the first character on a line sends this
character to the remote system. The initial value of eof is
taken to be the terminal's eof character.


ayt

If telnet is in localchars mode, or LINEMODE is enabled, and
the status character is typed, a TELNET AYT ("Are You There")
sequence is sent to the remote host. (See send, ayt above.)
The initial value for ayt is the terminal's status character.


forw1
forw2

If telnet is operating in LINEMODE, and the forw1 or forw2
characters are typed, this causes the forwarding of partial
lines to the remote system. The initial values for the
forwarding characters come from the terminal's eol and eol2
characters.


lnext

If telnet is operating in LINEMODE or "old line by line"
mode, then the lnext character is assumed to be the
terminal's lnext character. The initial value for the lnext
character is taken to be the terminal's lnext character.


reprint

If telnet is operating in LINEMODE or "old line by line"
mode, then the reprint character is assumed to be the
terminal's reprint character. The initial value for reprint
is taken to be the terminal's reprint character.


rlogin

This is the rlogin escape character. If set, the normal
telnet escape character is ignored, unless it is preceded by
this character at the beginning of a line. The rlogin
character, at the beginning of a line followed by a "."
closes the connection. When followed by a ^Z, the rlogin
command suspends the telnet command. The initial state is to
disable the rlogin escape character.


start

If the TELNET TOGGLE-FLOW-CONTROL option has been enabled,
then the start character is taken to be the terminal's start
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 the stop 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, generated when the
netdata or the debug option is TRUE, will be written. If
tracefile is set to "-", then tracing information will be
written to standard output (the default).


worderase

If telnet is operating in LINEMODE or "old line by line"
mode, then this character is taken to be the terminal's
worderase character. The initial value for the worderase
character is taken to be the terminal's worderase character.


?

Displays the legal set and unset commands.


slc state

The slc (Set Local Characters) command is used to set or change
the state of special characters when the TELNET LINEMODE option
has been enabled. Special characters are characters that get
mapped to TELNET commands sequences (like ip or quit) or line
editing characters (like erase and kill). By default, the local
special characters are exported. The following values for state
are valid:

check

Verifies the settings for the current special characters. The
remote side is requested to send all the current special
character settings. If there are any discrepancies with the
local side, the local settings will switch to the remote
values.


export

Switches to the local defaults for the special characters.
The local default characters are those of the local terminal
at the time when telnet was started.


import

Switches to the remote defaults for the special characters.
The remote default characters are those of the remote system
at the time when the TELNET connection was established.


?

Prints out help information for the slc command.


toggle argument...

Toggle between TRUE and FALSE the various flags that control how
telnet responds to events. More than one argument may be
specified. The state of these flags may be interrogated with the
display command. Valid arguments are:

authdebug
Turns on debugging information for the
authentication code.


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.


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 is propagated through
the TELNET ENVIRON option. This command is
the same as specifying the -a option on the
open command.


autoflush
If autoflush and localchars are both TRUE,
then when the ao, intr, or quit characters
are recognized (and transformed into TELNET
sequences; see set for details), telnet
refuses to display any data on the user's
terminal until the remote system acknowledges
(using a TELNET Timing Mark option) that it
has processed those TELNET sequences. The
initial value for this toggle is TRUE if the
terminal user has not done an "stty noflsh".
Otherwise, the value is FALSE (see stty(1)).


autosynch
If autosynch and localchars are both TRUE,
then when either the interrupt or quit
characters are typed (see set for
descriptions of interrupt and quit), the
resulting TELNET sequence sent is followed by
the TELNET SYNCH sequence. This procedure
should cause the remote system to begin
throwing away all previously typed input
until both of the TELNET sequences have been
read and acted upon. The initial value of
this toggle is FALSE.


binary
Enable or disable the TELNET BINARY option on
both input and output.


inbinary
Enable or disable the TELNET BINARY option on
input.


outbinary
Enable or disable the TELNET BINARY option on
output.


crlf
Determines how carriage returns are sent. If
the value is TRUE, then carriage returns will
be sent as <CR><LF>. If the value is FALSE,
then carriage returns will be send as
<CR><NUL>. The initial value for this toggle
is FALSE.


crmod
Toggle RETURN mode. When this mode is
enabled, most RETURN characters received from
the remote host will be mapped into a RETURN
followed by a line feed. This mode does not
affect those characters typed by the user,
only those received from the remote host.
This mode is useful only for remote hosts
that send RETURN but never send LINEFEED. The
initial value for this toggle is FALSE.


debug
Toggle socket level debugging (only available
to the super-user). The initial value for
this toggle is FALSE.


encdebug
Turns on debugging information for the
encryption code.


localchars
If this toggle is TRUE, then the flush,
interrupt, quit, erase, and kill characters
(see set) are recognized locally, and
transformed into appropriate TELNET control
sequences, respectively ao, ip, brk, ec, and
el (see send). The initial value for this
toggle is TRUE in "line by line" mode, and
FALSE in "character at a time" mode. When the
LINEMODE option is enabled, the value of
localchars is ignored, and assumed always to
be TRUE. If LINEMODE has ever been enabled,
then quit is sent as abort, and eof and
suspend are sent as eof and susp (see send
above).


netdata
Toggle the display of all network data (in
hexadecimal format). The initial value for
this toggle is FALSE.


options
Toggle the display of some internal TELNET
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. The beginning
of any TELNET escape sequence is preceded by
an asterisk (*) to aid in locating them.


skiprc
When the skiprc toggle is TRUE, TELNET skips
the reading of the .telnetrc file in the
user's home directory when connections are
opened. The initial value for this toggle is
FALSE.


termdata
Toggles the display of all terminal data (in
hexadecimal format). The initial value for
this toggle is FALSE.


verbose_encrypt
When the verbose_encrypt flag is TRUE, TELNET
prints out a message each time encryption is
enabled or disabled. The initial value for
this toggle is FALSE.


?
Display the legal toggle commands.


environ argument...

The environ command is used to manipulate variables that may be
sent through the TELNET ENVIRON option. The initial set of
variables is taken from the users environment. Only the DISPLAY
and PRINTER variables are exported by default. Valid arguments
for the environ command are:

define variable value

Define variable to have a value of value. Any variables
defined by this command are automatically exported. The value
may be enclosed in single or double quotes, so that tabs and
spaces may be included.


undefine variable

Remove variable from the list of environment variables.


export variable

Mark the variable to be exported to the remote side.


unexport variable

Mark the variable to not be exported unless explicitly
requested by the remote side.


list

List the current set of environment variables. Those marked
with an asterisk (*) will be sent automatically. Other
variables will be sent only if explicitly requested.


?

Prints out help information for the environ command.


logout

Sends the telnet logout option to the remote side. This command
is similar to a close command. However, if the remote side does
not support the logout option, nothing happens. If, however, the
remote side does support the logout option, this command should
cause the remote side to close the TELNET connection. If the
remote side also supports the concept of suspending a user's
session for later reattachment, the logout argument indicates
that the remote side should terminate the session immediately.

FILES

$HOME/.telnetrc
file that contains commands to be executed before
initiating a telnet session

SEE ALSO

rlogin(1), sh(1), stty(1), getlogin(3C), inet(4P), inet6(4P),
hosts(5), krb5.conf(5), nologin(5), telnetrc(5), attributes(7)

DIAGNOSTICS

NO LOGINS: System going down in N minutes

The machine is in the process of being shut down and logins have
been disabled.

NOTES

On some remote systems, echo has to be turned off manually when in
"line by line" mode.


In "old line by line" mode, or LINEMODE, the terminal's EOF character
is only recognized (and sent to the remote system) when it is the
first character on a line.


The telnet protocol only uses single DES for session
protection--clients request service tickets with single DES session
keys. The KDC must know that host service principals that offer the
telnet service support single DES, which, in practice, means that
such principals must have single DES keys in the KDC database.

August 17, 2006 TELNET(1)