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.\"	from: @(#)telnet.1	8.4 (Berkeley) 2/3/94
.\"
.Dd April 16, 2022
.Dt TELNET 1
.Os
.Sh NAME
.Nm telnet
.Nd user interface to the
.Tn TELNET
protocol
.Sh SYNOPSIS
.Nm
.Op Fl 468acdEFfKLNrx
.Op Fl e Ar escapechar
.Op Fl k Ar realm
.Op Fl l Ar user
.Op Fl n Ar tracefile
.Op Fl P Ar policy
.Op Fl S Ar tos
.Op Fl X Ar authtype
.Oo
.Ar host
.Op Ar port
.Oc
.Sh DESCRIPTION
The
.Nm
command
is used to communicate with another host using the
.Tn 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
Options:
.Bl -tag -width indent
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.It Fl 8
Specifies an 8-bit data path.
This causes an attempt to
negotiate the
.Dv TELNET BINARY
option on both input and output.
.It Fl E
Stops any character from being recognized as an escape character.
.It Fl F
If Kerberos V5 authentication is being used, the
.Fl 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.
.It Fl K
Specifies no automatic login to the remote system.
.It Fl L
Specifies an 8-bit data path on output.
This causes the
BINARY option to be negotiated on output.
.It Fl N
Numeric host address.
No attempt will be made to look up
symbolic names for host addresses.
.It Fl S Ar tos
Sets the IP type-of-service (TOS) option for the telnet
connection to the value
.Ar tos ,
which can be a numeric TOS value
or, on systems that support it, a symbolic
TOS name found in the /etc/iptos file.
.It Fl X Ar atype
Disables the
.Ar atype
type of authentication.
.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 c
Disables the reading of the user's
.Pa \&.telnetrc
file.
(See the
.Ic toggle skiprc
command on this man page.)
.It Fl d
Sets the initial value of the
.Ic debug
toggle to
.Dv TRUE .
.It Fl e Ar escape char
Sets the initial
.Nm
escape character to
.Ar escape char .
If
.Ar escape char
is omitted, then
there will be no escape character.
.It Fl f
If Kerberos V5 authentication is being used, the
.Fl f
option allows the local credentials to be forwarded to the remote system.
.It Fl k Ar realm
If Kerberos authentication is being used, the
.Fl k
option requests that telnet obtain tickets for the remote host in
realm
.Ar realm
instead of the remote host's realm.
.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 P Ar policy
Use IPsec policy specification string
.Ar policy ,
for the connections.
See
.Xr ipsec_set_policy 3
for details.
.It Fl r
Specifies a user interface similar to
.Xr rlogin 1 .
In this
mode, the escape character is set to the tilde (~) character,
unless modified by the
.Fl e
option.
.It Fl x
Turns on encryption of the data stream if possible.
.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, then
.Nm
will revert to one of two input modes:
either \*(Lqcharacter at a time\*(Rq
or \*(Lqold line by line\*(Rq
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 \*(Lqcharacter at a time\*(Rq mode, most
text typed is immediately sent to the remote host for processing.
.Pp
In \*(Lqold line by line\*(Rq mode, all text is echoed locally,
and (normally) only completed lines are sent to the remote host.
The \*(Lqlocal echo character\*(Rq (initially \*(Lq^E\*(Rq) 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 \*(Lqold line by line\*(Lq; see below),
the user's
.Ic quit  ,
.Ic intr ,
and
.Ic flush
characters are trapped locally, and sent as
.Tn 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
.Tn 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
.Tn 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
\*(Lqescape character\*(Rq (initially \*(Lq^]\*(Rq).
When in command mode, the normal terminal editing conventions are available.
.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 auth Ar argument ...
The auth command manipulates the information sent through the
.Dv TELNET AUTHENTICATE
option.
Valid arguments for the
auth command are as follows:
.Bl -tag -width "disable type"
.It Ic disable Ar type
Disables the specified type of authentication.
To obtain a list of available types, use the
.Ic auth disable \&?
command.
.It Ic enable Ar type
Enables the specified type of authentication.
To obtain a list of available types, use the
.Ic auth enable \&?
command.
.It Ic status
Lists the current status of the various types of
authentication.
.El
.It Ic close
Close a
.Tn 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 encrypt Ar argument ...
The encrypt command manipulates the information sent through the
.Dv TELNET ENCRYPT
option.
.Pp
.Bf -symbolic
Note:  By modern standards the encryption provided by
.Nm
is weak and insecure.
.Ef
This option is provided for compatibility.
.Pp
Valid arguments for the encrypt command are:
.Bl -tag -width Ar
.It Ic disable Ar type Ic [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
.Ic encrypt disable \&?
command.
.It Ic enable Ar type Ic [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
.Ic encrypt enable \&?
command.
.It Ic input
This is the same as the
.Ic encrypt start input
command.
.It Ic -input
This is the same as the
.Ic encrypt stop input
command.
.It Ic output
This is the same as the
.Ic encrypt start output
command.
.It Ic -output
This is the same as the
.Ic encrypt stop output
command.
.It Ic start Ic [input|output]
Attempts to start encryption.
If you omit
.Ic input
and
.Ic output ,
both input and output are enabled.
To obtain a list of available types, use the
.Ic encrypt enable \&?
command.
.It Ic status
Lists the current status of encryption.
.It Ic stop Ic [input|output]
Stops encryption.
If you omit input and output,
encryption is on both input and output.
.It Ic type Ar type
Sets the default type of encryption to be used
with later
.Ic encrypt start
or
.Ic encrypt stop
commands.
.El
.It Ic environ Ar arguments ...
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
.Tn 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
.Tn 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 \*(Lqcharacter at a time\*(Lq 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 \*(Lqold-line-by-line\*(Lq 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 Ic open Ar host Oo Fl l Ar user Oc Oo Fl a Oc Oo Oo \&- Oc Ns Ar port Oc
Open a connection to the named host.
If no port number
is specified,
.Nm
will attempt to contact a
.Tn 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 \*(Lqdot notation\*(Rq (see
.Xr inet 3 ) .
The
.Fl l
option may be used to specify the user name
to be passed to the remote system via the
.Ev ENVIRON
option.
If a port is specified
.Nm
omits any automatic initialisation of
.Tn TELNET
options.
When the port number is preceded by a minus sign,
the initial option negotiation is done.
.Pp
After establishing a connection, the file
.Pa \&.telnetrc
in the
user's home directory is read.
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 such a line is a string identifying the machine
that is being connected to.
It may be the hostname or numeric address specified as the argument
.Ar host ,
the canonical name of that string as determined by
.Xr getaddrinfo 3 ,
or the string
.Dq DEFAULT
indicating all hosts.
The rest of the line, and successive
lines that begin with white space 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
.Tn TELNET
session and exit
.Nm .
An end of file (in command mode) will also close a session and exit.
.It Ic send Ar 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):
.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 (initially \*(Lq^\*(Rq).
.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
.Tn TCP
urgent
data (and may not work if the remote system is a
.Bx 4.2
system \(em if
it doesn't work, a lower case \*(Lqr\*(Rq may be echoed on the terminal).
.It Ic do Ar cmd
.It Ic dont Ar cmd
.It Ic will Ar cmd
.It Ic wont 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
.Dv 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
.Tn TELNET
is in localchars mode, or
.Dv LINEMODE
is enabled, and the status character is typed, a
.Dv TELNET AYT
sequence (see
.Ic send ayt
above) 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 (initially \*(Lq^E\*(Rq) which, when in
\*(Lqline by line\*(Rq 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 \*(Lqold line by line\*(Rq 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
.Ic eof
character.
.It Ic erase
If
.Nm
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below),
.Sy and
if
.Nm
is operating in \*(Lqcharacter at a time\*(Rq 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 erase character is taken to be
the terminal's
.Ic erase
character.
.It Ic escape
This is the
.Nm
escape character (initially \*(Lq^[\*(Rq) 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 flush character is taken to be
the terminal's
.Ic flush
character.
.It Ic forw1
.It Ic forw2
If
.Tn 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 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),
.Ic and
if
.Nm
is operating in \*(Lqcharacter at a time\*(Rq 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 kill character is taken to be
the terminal's
.Ic kill
character.
.It Ic lnext
If
.Nm
is operating in
.Dv LINEMODE
or \*(Lqold line by line\*(Lq mode, then this character is taken to
be the terminal's
.Ic lnext
character.
The initial value for the 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 quit character is taken to be
the terminal's
.Ic quit
character.
.It Ic reprint
If
.Nm
is operating in
.Dv LINEMODE
or \*(Lqold line by line\*(Lq mode, then this character is taken to
be the terminal's
.Ic reprint
character.
The initial value for the 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
.Tn 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 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 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 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 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 \*(Lqold line by line\*(Lq mode, then this character is taken to
be the terminal's
.Ic worderase
character.
The initial value for the 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 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
.Tn 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
.Tn 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 .
This includes the peer one is connected to, as well
as the current mode.
.It Ic toggle Ar arguments ...
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 authdebug
Turns on debugging information for the authentication code.
.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
.Tn 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
.Tn 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 autodecrypt
When the
.Dv 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.
.Pp
.Bf -symbolic
Note:  By modern standards the encryption provided by
.Nm
is weak and insecure.
.Ef
This option is provided for compatibility.
.It Ic autologin
If the remote side supports the
.Dv TELNET AUTHENTICATION
option
.Tn TELNET
attempts to use it to perform automatic authentication.
If the
.Dv AUTHENTICATION
option is not supported, the user's login
name are propagated through the
.Dv TELNET ENVIRON
option.
This command is the same as specifying the
.Fl 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
characters is typed (see
.Ic set
above for descriptions of the
.Ic intr
and
.Ic quit
characters), the resulting
.Tn TELNET
sequence sent is followed by the
.Dv TELNET SYNCH
sequence.
This procedure
.Ic should
cause the remote system to begin throwing away all previously
typed input until both of the
.Tn 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 send 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 feed.
The initial value for this toggle is
.Dv FALSE .
.It Ic debug
Toggles socket level debugging (useful only to the
.Ic super user  ) .
The initial value for this toggle is
.Dv FALSE .
.It Ic encdebug
Turns on debugging information for the encryption code.
.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
.Tn 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 \*(Lqold line by line\*(Rq mode,
and
.Dv FALSE
in \*(Lqcharacter at a time\*(Rq 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
.Tn 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
.Tn TELNET
escape sequence is preceded by a '*' to aid in locating them.
.It Ic skiprc
When the skiprc toggle is
.Dv TRUE ,
.Tn TELNET
skips the reading of the
.Pa \&.telnetrc
file in the users 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 verbose_encrypt
When the
.Ic verbose_encrypt
toggle is
.Dv TRUE ,
.Nm
prints out a message each time encryption is enabled or
disabled.
The initial value for this toggle is
.Dv FALSE .
Note:  Because of export controls, data encryption
is not supported outside of the United States and Canada.
.It Ic \&?
Displays the legal
.Ic toggle
commands.
.El
.It Ic z
Suspend
.Nm  .
This command only works when the user is using the
.Xr csh 1 .
.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 HISTORY
The
.Nm
command appeared in
.Bx 4.2 .
IPsec support was added by WIDE/KAME project, in 1999.
.Sh NOTES
On some remote systems, echo has to be turned off manually when in
\*(Lqold line by line\*(Rq mode.
.Pp
In \*(Lqold line by line\*(Rq 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.