path: root/usr.bin/telnet/telnet.1

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.\"	from: @(#)telnet.1	8.4 (Berkeley) 2/3/94
.\"
.Dd $Mdocdate: November 15 2014 $
.Dt TELNET 1
.Os
.Sh NAME
.Nm telnet
.Nd user interface to the TELNET protocol
.Sh SYNOPSIS
.Nm telnet
.Bk -words
.Op Fl 4678acDdEKLr
.Op Fl b Ar hostalias
.Op Fl e Ar escapechar
.Op Fl l Ar user
.Op Fl n Ar tracefile
.Op Fl V Ar rtable
.Oo
.Ar host
.Op Ar port
.Oc
.Ek
.Sh DESCRIPTION
The
.Nm
command
is used to communicate with another host using the TELNET protocol.
If
.Nm
is invoked without the
.Ar host
argument, it enters command mode,
indicated by its prompt
.Pq Nm telnet\&> .
In this mode, it accepts and executes the commands listed below.
If it is invoked with arguments, it performs an
.Ic open
command with those arguments.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.It Fl 7
Specifies a 7-bit data path.
This attempts to disable the
.Dv TELNET BINARY
option on both input and output.
.It Fl 8
Specifies an 8-bit data path.
This attempts to negotiate the
.Dv TELNET BINARY
option on both input and output.
This is the default.
.It Fl a
Attempt automatic login.
Currently, this sends the user name via the
.Ev USER
variable
of the
.Ev ENVIRON
option if supported by the remote system.
The name used is that of the current user as returned by
.Xr getlogin 2
if it agrees with the current user ID,
otherwise it is the name associated with the user ID.
.It Fl b Ar hostalias
Uses
.Xr bind 2
on the local socket to bind it to an aliased address (see
.Xr ifconfig 8
and the
.Cm alias
specifier) or to the address of
another interface than the one naturally chosen by
.Xr connect 2 .
This can be useful when connecting to services which use IP addresses
for authentication and reconfiguration of the server is undesirable (or
impossible).
.It Fl c
Disables the reading of the user's
.Pa \&.telnetrc
file.
(See the
.Ic toggle skiprc
command on this man page.)
.It Fl D
Disables rewriting of the
.Ev DISPLAY
variable when it starts with
.Sq :\&
or
.Sq unix: .
By default, these are replaced with the local hostname and a colon.
.It Fl d
Sets the initial value of the
.Ic debug
toggle to
.Dv TRUE .
.It Fl E
Stops any character from being recognized as an escape character.
.It Fl e Ar escapechar
Sets the initial
.Nm
escape character to
.Ar escapechar .
If
.Ar escapechar
is omitted, then
there will be no escape character.
.It Fl K
Specifies no automatic login to the remote system.
.It Fl L
Specifies an 8-bit data path on output.
This attempts to negotiate the
.Dv TELNET BINARY
option on output.
.It Fl l Ar user
When connecting to the remote system, if the remote system
understands the
.Ev ENVIRON
option, then
.Ar user
will be sent to the remote system as the value for the variable USER.
This option implies the
.Fl a
option.
This option may also be used with the
.Ic open
command.
.It Fl n Ar tracefile
Opens
.Ar tracefile
for recording trace information.
See the
.Ic set tracefile
command below.
.It Fl r
Specifies a user interface similar to
.Nm rlogin .
In this
mode, the escape character is set to the tilde (~) character,
unless modified by the
.Fl e
option.
.It Fl V Ar rtable
Set the routing table to be used.
.It Ar host
Indicates the official name, an alias, or the Internet address
of a remote host.
.It Ar port
Indicates a port number (address of an application).
If a number is not specified, the default
.Nm
port is used.
.El
.Pp
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.
.Pp
Once a connection has been opened,
.Nm
will attempt to enable the
.Dv TELNET LINEMODE
option.
If this fails,
.Nm
will revert to one of two input modes:
either
.Dq character at a time
or
.Dq old line by line
depending on what the remote system supports.
.Pp
When
.Dv 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.
.Pp
In
.Dq character at a time
mode, most
text typed is immediately sent to the remote host for processing.
.Pp
In
.Dq 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
.Pq initially Sq ^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).
.Pp
If the
.Dv LINEMODE
option is enabled, or if the
.Ic localchars
toggle is
.Dv TRUE
(the default for
.Dq old line by line ;
see below),
the user's
.Ic quit ,
.Ic intr ,
and
.Ic flush
characters are trapped locally, and sent as TELNET
protocol sequences to the remote side.
If
.Dv LINEMODE
has ever been enabled, then the user's
.Ic susp
and
.Ic eof
are also sent as TELNET protocol sequences, and
.Ic quit
is sent as a
.Dv TELNET ABORT
instead of
.Dv BREAK .
There are options (see
.Ic toggle
.Ic autoflush
and
.Ic toggle
.Ic 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
.Ic quit
and
.Ic intr ) .
.Pp
While connected to a remote host,
.Nm
command mode may be entered by typing the
.Nm
escape character
.Pq initially Sq ^] .
When in command mode, the normal terminal editing conventions are available.
Note that the escape character will return to the command mode of the initial
invocation of
.Nm
that has the controlling terminal.
Use the
.Cm send escape
command to switch to command mode in subsequent
.Nm
processes on remote hosts.
.Pp
The following
.Nm
commands are available.
Only enough of each command to uniquely identify it need be typed
(this is also true for arguments to the
.Ic mode ,
.Ic set ,
.Ic toggle ,
.Ic unset ,
.Ic slc ,
.Ic environ ,
and
.Ic display
commands).
.Bl -tag -width "mode type"
.It Ic close
Close a TELNET session and return to command mode.
.It Ic display Ar argument ...
Displays all, or some, of the
.Ic set
and
.Ic toggle
values (see below).
.It Ic environ Ar argument ...
The
.Ic environ
command is used to manipulate the
variables that may be sent through the
.Dv TELNET ENVIRON
option.
The initial set of variables is taken from the users
environment, with only the
.Ev DISPLAY
and
.Ev PRINTER
variables being exported by default.
The
.Ev USER
variable is also exported if the
.Fl a
or
.Fl l
options are used.
.Pp
Valid arguments for the
.Ic environ
command are:
.Bl -tag -width Fl
.It Ic define Ar variable value
Define the variable
.Ar variable
to have a value of
.Ar value .
Any variables defined by this command are automatically exported.
The
.Ar value
may be enclosed in single or double quotes so
that tabs and spaces may be included.
.It Ic undefine Ar variable
Remove
.Ar variable
from the list of environment variables.
.It Ic export Ar variable
Mark the variable
.Ar variable
to be exported to the remote side.
.It Ic unexport Ar variable
Mark the variable
.Ar variable
to not be exported unless
explicitly asked for by the remote side.
.It Ic list
List the current set of environment variables.
Those marked with a
.Cm *
will be sent automatically,
other variables will only be sent if explicitly requested.
.It Ic \&?
Prints out help information for the
.Ic environ
command.
.El
.It Ic logout
Sends the
.Dv TELNET LOGOUT
option to the remote side.
This command is similar to a
.Ic close
command; however, if the remote side does not support the
.Dv LOGOUT
option, nothing happens.
If, however, the remote side does support the
.Dv 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.
.It Ic mode Ar type
.Ar 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.
.Bl -tag -width Ar
.It Ic character
Disable the
.Dv TELNET LINEMODE
option, or, if the remote side does not understand the
.Dv LINEMODE
option, then enter
.Dq character at a time
mode.
.It Ic line
Enable the
.Dv TELNET LINEMODE
option, or, if the remote side does not understand the
.Dv LINEMODE
option, then attempt to enter
.Dq old line by line
mode.
.It Ic isig Pq Ic \-isig
Attempt to enable (disable) the
.Dv TRAPSIG
mode of the
.Dv LINEMODE
option.
This requires that the
.Dv LINEMODE
option be enabled.
.It Ic edit Pq Ic \-edit
Attempt to enable (disable) the
.Dv EDIT
mode of the
.Dv LINEMODE
option.
This requires that the
.Dv LINEMODE
option be enabled.
.It Ic softtabs Pq Ic \-softtabs
Attempt to enable (disable) the
.Dv SOFT_TAB
mode of the
.Dv LINEMODE
option.
This requires that the
.Dv LINEMODE
option be enabled.
.It Ic litecho Pq Ic \-litecho
Attempt to enable (disable) the
.Dv LIT_ECHO
mode of the
.Dv LINEMODE
option.
This requires that the
.Dv LINEMODE
option be enabled.
.It Ic \&?
Prints out help information for the
.Ic mode
command.
.El
.It Xo
.Ic open Ar host
.Op Fl a
.Op Fl b hostalias
.Op Fl l Ar user
.Oo Op Fl
.Ar port Oc
.Xc
Open a connection to the named host.
If no port number
is specified,
.Nm
will attempt to contact a TELNET server at the default port.
The host specification may be either a host name (see
.Xr hosts 5 )
or an Internet address specified in the
.Dq dot notation
(see
.Xr inet_ntop 3 ) .
The
.Fl a ,
.Fl b ,
and
.Fl l
options are equivalent to the identical command line options,
but only apply to the new telnet connection being opened.
When connecting to a non-standard port,
.Nm
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
.Pa \&.telnetrc
in the
user's home directory is opened.
Lines beginning with a
.Sq #
are comment lines.
Blank lines are ignored.
Lines that begin
without whitespace 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 whitespace are assumed to be
.Nm
commands and are processed as if they had been typed
in manually to the
.Nm
command prompt.
.It Ic quit
Close any open TELNET session and exit
.Nm telnet .
An end-of-file (in command mode) will also close a session and exit.
.It Ic send Ar argument ...
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):
.Bl -tag -width escape
.It Ic abort
Sends the
.Dv TELNET ABORT
(Abort
processes)
sequence.
.It Ic ao
Sends the
.Dv TELNET AO
(Abort Output) sequence, which should cause the remote system to flush
all output
.Em from
the remote system
.Em to
the user's terminal.
.It Ic ayt
Sends the
.Dv TELNET AYT
(Are You There)
sequence, to which the remote system may or may not choose to respond.
.It Ic brk
Sends the
.Dv TELNET BRK
(Break) sequence, which may have significance to the remote
system.
.It Ic ec
Sends the
.Dv TELNET EC
(Erase Character)
sequence, which should cause the remote system to erase the last character
entered.
.It Ic el
Sends the
.Dv TELNET EL
(Erase Line)
sequence, which should cause the remote system to erase the line currently
being entered.
.It Ic eof
Sends the
.Dv TELNET EOF
(End Of File)
sequence.
.It Ic eor
Sends the
.Dv TELNET EOR
(End of Record)
sequence.
.It Ic escape
Sends the current
.Nm
escape character
.Pq initially Sq ^] .
.It Ic ga
Sends the
.Dv TELNET GA
(Go Ahead)
sequence, which likely has no significance to the remote system.
.It Ic getstatus
If the remote side supports the
.Dv TELNET STATUS
command,
.Ic getstatus
will send the subnegotiation to request that the server send
its current option status.
.It Ic ip
Sends the
.Dv TELNET IP
(Interrupt Process) sequence, which should cause the remote
system to abort the currently running process.
.It Ic nop
Sends the
.Dv TELNET NOP
(No OPeration)
sequence.
.It Ic susp
Sends the
.Dv TELNET SUSP
(SUSPend process)
sequence.
.It Ic synch
Sends the
.Dv 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
.Bx 4.2
system -- if
it doesn't work, a lower case
.Sq r
may be echoed on the terminal).
.It Ic do Ar cmd
Sends the
.Dv TELNET DO
.Ar cmd
sequence.
.Ar cmd
can be either a decimal number between 0 and 255,
or a symbolic name for a specific TELNET command.
.Ar cmd
can also be either
.Ic help
or
.Ic ?\&
to print out help information, including
a list of known symbolic names.
.It Ic dont Ar cmd
Sends the
.Dv TELNET DONT
.Ar cmd
sequence.
.Ar cmd
can be either a decimal number between 0 and 255,
or a symbolic name for a specific TELNET command.
.Ar cmd
can also be either
.Ic help
or
.Ic ?\&
to print out help information, including
a list of known symbolic names.
.It Ic will Ar cmd
Sends the
.Dv TELNET WILL
.Ar cmd
sequence.
.Ar cmd
can be either a decimal number between 0 and 255,
or a symbolic name for a specific TELNET command.
.Ar cmd
can also be either
.Ic help
or
.Ic ?\&
to print out help information, including
a list of known symbolic names.
.It Ic wont Ar cmd
Sends the
.Dv TELNET WONT
.Ar cmd
sequence.
.Ar cmd
can be either a decimal number between 0 and 255,
or a symbolic name for a specific TELNET command.
.Ar cmd
can also be either
.Ic help
or
.Ic ?\&
to print out help information, including
a list of known symbolic names.
.It Ic \&?
Prints out help information for the
.Ic send
command.
.El
.It Ic set Ar argument value
.It Ic unset Ar argument value
The
.Ic set
command will set any one of a number of
.Nm
variables to a specific value or to
.Dv TRUE .
The special value
.Ic off
turns off the function associated with
the variable; this is equivalent to using the
.Ic unset
command.
The
.Ic unset
command will disable or set to
.Dv FALSE
any of the specified functions.
The values of variables may be interrogated with the
.Ic display
command.
The variables which may be set or unset, but not toggled, are
listed here.
In addition, any of the variables for the
.Ic toggle
command may be explicitly set or unset using
the
.Ic set
and
.Ic unset
commands.
.Bl -tag -width escape
.It Ic ayt
If TELNET is in
.Ic localchars
mode, or
.Dv LINEMODE
is enabled, and the status character is typed, a
.Dv TELNET AYT
sequence (see
.Ic send ayt
preceding) is sent to the
remote host.
The initial value for the "Are You There"
character is the terminal's status character.
.It Ic echo
This is the value
.Pq initially Sq ^E
which, when in
.Dq 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).
.It Ic eof
If
.Nm
is operating in
.Dv LINEMODE
or
.Dq 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
.Ic eof
character is taken to be the terminal's
.Ic eof
character.
.It Ic erase
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below),
and if
.Nm
is operating in
.Dq character at a time
mode, then when this character is typed, a
.Dv TELNET EC
sequence (see
.Ic send
.Ic ec
above)
is sent to the remote system.
The initial value for the
.Ic erase
character is taken to be
the terminal's
.Ic erase
character.
.It Ic escape
This is the
.Nm
escape character
.Pq initially Sq ^[
which causes entry into
.Nm
command mode (when connected to a remote system).
.It Ic flushoutput
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below)
and the
.Ic flushoutput
character is typed, a
.Dv TELNET AO
sequence (see
.Ic send
.Ic ao
above)
is sent to the remote host.
The initial value for the
.Ic flush
character is taken to be
the terminal's
.Ic flush
character.
.It Ic forw1
.It Ic forw2
If TELNET is operating in
.Dv 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.
.It Ic interrupt
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below)
and the
.Ic interrupt
character is typed, a
.Dv TELNET IP
sequence (see
.Ic send
.Ic ip
above)
is sent to the remote host.
The initial value for the
.Ic interrupt
character is taken to be
the terminal's
.Ic intr
character.
.It Ic kill
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below),
and if
.Nm
is operating in
.Dq character at a time
mode, then when this character is typed, a
.Dv TELNET EL
sequence (see
.Ic send
.Ic el
above)
is sent to the remote system.
The initial value for the
.Ic kill
character is taken to be
the terminal's
.Ic kill
character.
.It Ic lnext
If
.Nm
is operating in
.Dv LINEMODE
or
.Dq old line by line
mode, then this character is taken to be the terminal's
.Ic lnext
character.
The initial value for the
.Ic lnext
character is taken to be
the terminal's
.Ic lnext
character.
.It Ic quit
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below)
and the
.Ic quit
character is typed, a
.Dv TELNET BRK
sequence (see
.Ic send
.Ic brk
above)
is sent to the remote host.
The initial value for the
.Ic quit
character is taken to be
the terminal's
.Ic quit
character.
.It Ic reprint
If
.Nm
is operating in
.Dv LINEMODE
or
.Dq old line by line
mode, then this character is taken to be the terminal's
.Ic reprint
character.
The initial value for the
.Ic reprint
character is taken to be
the terminal's
.Ic reprint
character.
.It Ic 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
.Nm
command.
The initial state is to
disable the
.Ic rlogin
escape character.
.It Ic start
If the
.Dv TELNET TOGGLE-FLOW-CONTROL
option has been enabled,
then this character is taken to
be the terminal's
.Ic start
character.
The initial value for the
.Ic start
character is taken to be
the terminal's
.Ic start
character.
.It Ic stop
If the
.Dv TELNET TOGGLE-FLOW-CONTROL
option has been enabled,
then this character is taken to
be the terminal's
.Ic stop
character.
The initial value for the
.Ic stop
character is taken to be
the terminal's
.Ic stop
character.
.It Ic susp
If
.Nm
is in
.Ic localchars
mode, or
.Dv LINEMODE
is enabled, and the
.Ic suspend
character is typed, a
.Dv TELNET SUSP
sequence (see
.Ic send
.Ic susp
above)
is sent to the remote host.
The initial value for the
.Ic suspend
character is taken to be
the terminal's
.Ic suspend
character.
.It Ic tracefile
This is the file to which the output, caused by
.Ic netdata
or
.Ic option
tracing being
.Dv TRUE ,
will be written.
If it is set to
.Dq Fl ,
then tracing information will be written to standard output (the default).
.It Ic worderase
If
.Nm
is operating in
.Dv LINEMODE
or
.Dq old line by line
mode, then this character is taken to be the terminal's
.Ic worderase
character.
The initial value for the
.Ic worderase
character is taken to be
the terminal's
.Ic worderase
character.
.It Ic \&?
Displays the legal
.Ic set
.Pq Ic unset
commands.
.El
.It Ic skey Ar sequence challenge
The
.Ic skey
command computes a response to the S/Key challenge.
See
.Xr skey 1
for more information on the S/Key system.
.It Ic slc Ar state
The
.Ic slc
command (Set Local Characters) is used to set
or change the state of the special
characters when the
.Dv TELNET LINEMODE
option has
been enabled.
Special characters are characters that get mapped to TELNET
commands sequences (like
.Ic ip
or
.Ic quit )
or line editing characters (like
.Ic erase
and
.Ic kill ) .
By default, the local special characters are exported.
.Bl -tag -width Fl
.It Ic 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.
.It Ic export
Switch to the local defaults for the special characters.
The local default characters are those of the local terminal at
the time when
.Nm
was started.
.It Ic 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.
.It Ic \&?
Prints out help information for the
.Ic slc
command.
.El
.It Ic status
Show the current status of
.Nm telnet .
This includes the peer one is connected to, as well
as the current mode.
.It Ic toggle Ar argument ...
Toggle (between
.Dv TRUE
and
.Dv FALSE )
various flags that control how
.Nm
responds to events.
These flags may be set explicitly to
.Dv TRUE
or
.Dv FALSE
using the
.Ic set
and
.Ic unset
commands listed above.
More than one argument may be specified.
The state of these flags may be interrogated with the
.Ic display
command.
Valid arguments are:
.Bl -tag -width Ar
.It Ic autoflush
If
.Ic autoflush
and
.Ic localchars
are both
.Dv TRUE ,
then when the
.Ic ao
or
.Ic quit
characters are recognized (and transformed into TELNET sequences; see
.Ic set
above for details),
.Nm
refuses to display any data on the user's terminal
until the remote system acknowledges (via a
.Dv TELNET TIMING MARK
option)
that it has processed those TELNET sequences.
The initial value for this toggle is
.Dv TRUE
if the terminal user had not
done an "stty noflsh", otherwise
.Dv FALSE
(see
.Xr stty 1 ) .
.It Ic autologin
The user's login
name is propagated through the
.Dv TELNET ENVIRON
option.
This command is the same as specifying
.Ar a
option on the
.Ic open
command.
.It Ic autosynch
If
.Ic autosynch
and
.Ic localchars
are both
.Dv TRUE ,
then when either the
.Ic intr
or
.Ic quit
character is typed (see
.Ic set
above for descriptions of the
.Ic intr
and
.Ic quit
characters), the resulting TELNET sequence sent is followed by the
.Dv TELNET SYNCH
sequence.
This procedure
.Em 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
.Dv FALSE .
.It Ic binary
Enable or disable the
.Dv TELNET BINARY
option on both input and output.
.It Ic inbinary
Enable or disable the
.Dv TELNET BINARY
option on input.
.It Ic outbinary
Enable or disable the
.Dv TELNET BINARY
option on output.
.It Ic crlf
If this is
.Dv TRUE ,
then carriage returns will be sent as
.Li <CR><LF> .
If this is
.Dv FALSE ,
then carriage returns will be sent as
.Li <CR><NUL> .
The initial value for this toggle is
.Dv FALSE .
.It Ic 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 feeds.
The initial value for this toggle is
.Dv FALSE .
.It Ic debug
Toggles socket level debugging (useful only to the superuser).
The initial value for this toggle is
.Dv FALSE .
.It Ic localchars
If this is
.Dv TRUE ,
then the
.Ic flush ,
.Ic interrupt ,
.Ic quit ,
.Ic erase ,
and
.Ic kill
characters (see
.Ic set
above) are recognized locally, and transformed into (hopefully) appropriate
TELNET control sequences
(respectively
.Ic ao ,
.Ic ip ,
.Ic brk ,
.Ic ec ,
and
.Ic el ;
see
.Ic send
above).
The initial value for this toggle is
.Dv TRUE
in
.Dq old line by line
mode, and
.Dv FALSE
in
.Dq character at a time
mode.
When the
.Dv LINEMODE
option is enabled, the value of
.Ic localchars
is ignored, and assumed to always be
.Dv TRUE .
If
.Dv LINEMODE
has ever been enabled, then
.Ic quit
is sent as
.Ic abort ,
and
.Ic eof
and
.Ic suspend
are sent as
.Ic eof
and
.Ic susp
(see
.Ic send
above).
.It Ic netdata
Toggles the display of all network data (in hexadecimal format).
The initial value for this toggle is
.Dv FALSE .
.It Ic options
Toggles the display of some internal
.Nm
protocol processing (having to do with TELNET options).
The initial value for this toggle is
.Dv FALSE .
.It Ic prettydump
When the
.Ic netdata
toggle is enabled, if
.Ic prettydump
is enabled the output from the
.Ic 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.
.It Ic skiprc
When the skiprc toggle is
.Dv TRUE ,
TELNET skips the reading of the
.Pa \&.telnetrc
file in the user's home
directory when connections are opened.
The initial value for this toggle is
.Dv FALSE .
.It Ic termdata
Toggles the display of all terminal data (in hexadecimal format).
The initial value for this toggle is
.Dv FALSE .
.It Ic \&?
Displays the legal
.Ic toggle
commands.
.El
.It Ic z
Suspend
.Nm telnet .
This command only works when the user is using a shell with
job control enabled.
.It Ic \&! Op Ar command
Execute a single command in a subshell on the local
system.
If
.Ar command
is omitted, then an interactive
subshell is invoked.
.It Ic \&? Op Ar command
Get help.
With no arguments,
.Nm
prints a help summary.
If a command is specified,
.Nm
will print the help information for just that command.
.El
.Sh ENVIRONMENT
.Nm
uses at least the
.Ev HOME ,
.Ev SHELL ,
.Ev DISPLAY ,
and
.Ev TERM
environment variables.
Other environment variables may be propagated
to the other side via the
.Dv TELNET ENVIRON
option.
.Sh FILES
.Bl -tag -width ~/.telnetrc -compact
.It Pa ~/.telnetrc
user customized telnet startup values
.El
.Sh SEE ALSO
.Xr skey 1 ,
.Xr ssh 1 ,
.Xr stty 1
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.2 .
.Sh NOTES
On some remote systems, echo has to be turned off manually when in
.Dq old line by line
mode.
.Pp
In
.Dq old line by line
mode or
.Dv LINEMODE
the terminal's
.Ic eof
character is only recognized (and sent to the remote system)
when it is the first character on a line.
.Pp
Source routing is not supported yet for IPv6.