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.\"	@(#)telnet.1	6.16 (Berkeley) 7/27/91
.\"
.Dd July 27, 1991
.Dt TELNET 1
.Os BSD 4.2
.Sh NAME
.Nm telnet
.Nd User interface to the 
.Tn TELNET
protocol
.Sh SYNOPSIS
.Nm telnet
.Op Fl d
.Op Fl a
.Op Fl n Ar tracefile
.Op Fl e Ar escapechar
.Oo
.Op Fl l Ar user
.Ar host
.Op port
.Oc
.Sh DESCRIPTION
The
.Nm telnet
command
is used to communicate with another host using the 
.Tn TELNET
protocol.
If
.Nm telnet
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 d
Sets the initial value of the
.Ic debug
toggle to
.Dv TRUE
.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 n Ar tracefile 
Opens
.Ar tracefile
for recording trace information.
See the
.Ic set tracefile
command below.
.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 e Ar escape char 
Sets the initial
.Nm
.Nm telnet
escape character to
.Ar escape char.
If
.Ar escape char
is ommitted, then
there will be no escape character.
.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 telnet
port is used.
.El
.Pp
Once a connection has been opened,
.Nm telnet
will attempt to enable the
.Dv TELNET LINEMODE
option.
If this fails, then
.Nm telnet
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 telnet
command mode may be entered by typing the
.Nm telnet
\*(Lqescape character\*(Rq (initially \*(Lq^]\*(Rq).
When in command mode, the normal terminal editing conventions are available.
.Pp
The following
.Nm telnet
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).
.Pp
.Bl -tag -width "mode type"
.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 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 Xo
.Ic open Ar host
.Oo Op Fl l
.Ar user
.Oc Ns Oo Fl
.Ar port Oc
.Xc
Open a connection to the named host.
If no port number
is specified,
.Nm telnet
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
.Op Fl l
option may be used to specify the user name
to be passed to the remote system via the
.Ev ENVIRON
option.
When connecting to a non-standard port,
.Nm telnet
omits any automatic initiation of
.Tn TELNET
options.  When the port number is preceeded by a minus sign,
the inital option negotiation is done.
After establishing a connection, the file
.Pa \&.telnetrc
in the
users home directory is opened.  Lines begining with a # 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 telnet
commands and are processed as if they had been typed
in manually to the
.Nm telnet
command prompt.
.It Ic quit
Close any open
.Tn TELNET
session and exit
.Nm telnet  .
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):
.Pp
.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 telnet
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 -- if
it doesn't work, a lower case \*(Lqr\*(Rq may be echoed on the terminal).
.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 telnet
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 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 telnet
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 telnet
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below),
.Sy and
if
.Nm telnet
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 telnet
escape character (initially \*(Lq^[\*(Rq) which causes entry
into
.Nm telnet
command mode (when connected to a remote system).
.It Ic flushoutput
If
.Nm telnet
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 interrupt
If
.Nm telnet
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 telnet
is in
.Ic localchars
mode (see
.Ic toggle
.Ic localchars
below),
.Ic and
if
.Nm telnet
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 telnet
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 telnet
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 telnet
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 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 kill 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 kill character is taken to be
the terminal's
.Ic stop
character.
.It Ic susp
If
.Nm telnet
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
Thi 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 telnet
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 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 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 telnet
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 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 discrepencies with
the local side, the local side will switch to the remote value.
.It Ic \&?
Prints out help information for the
.Ic slc
command.
.El
.It Ic environ Ar arguments... 
The
.Ic environ
command is used to manipulate the
the variables that my 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.
.br
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 toggle Ar arguments ... 
Toggle (between
.Dv TRUE
and
.Dv FALSE )
various flags that control how
.Nm telnet
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
.Tn TELNET
sequences; see
.Ic set
above for details),
.Nm telnet
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 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 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
.B 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 telnet
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 formated in a more user readable format.
Spaces are put between each character in the output, and the
begining of any
.Tn TELNET
escape sequence is preceeded by a '*' to aid in locating them.
.It Ic \&?
Displays the legal
.Ic toggle
commands.
.El
.It Ic z
Suspend
.Nm telnet  .
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
.Ic command
is ommitted, then an interactive
subshell is invoked.
.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 \&? Op Ar command 
Get help.  With no arguments,
.Nm telnet
prints a help summary.
If a command is specified,
.Nm telnet
will print the help information for just that command.
.El
.Sh ENVIRONMENT
.Nm Telnet
uses at least the
.Ev HOME ,
.Ev SHELL ,
.Ev DISPLAY ,
and
.Ev TERM
environent variables.
Other envirnoment variables may be propogated
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 Telnet
command appeared in
.Bx 4.2 .
.Sh NOTES
.Pp
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.