.\" Copyright (c) 1983, 1990 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"	from: @(#)telnet.1	6.16 (Berkeley) 7/27/91
.\"	$Id: telnet.1,v 1.5 2006-09-24 00:48:31 ianb Exp $
.\"
.Dd August 15, 1999
.Dt TELNET 1
.Os "Linux NetKit (0.17)"
.Sh NAME
.Nm telnet
.Nd user interface to the 
.Tn TELNET
protocol
.Sh SYNOPSIS
.Nm telnet
.Op Fl 468EKLadr
.Op Fl S Ar tos
.Op Fl X Ar authtype
.Op Fl b Ar address
.Op Fl e Ar escapechar
.Op Fl l Ar user
.Op Fl n Ar tracefile
.Op Fl z Ar option
.Oo
.Ar host
.Op Ar port
.Oc
.Sh DESCRIPTION
The
.Nm telnet
command
is used for interactive communication with another host using the 
.Tn TELNET
protocol. It begins in command mode, where it prints a telnet prompt 
("telnet\&> "). If
.Nm telnet
is invoked with a
.Ar host
argument, it performs an
.Ic open
command implicitly; see the description below.
.Pp
Options:
.Bl -tag -width indent
.It Fl 4
Force IPv4 address resolution.
.It Fl 6
Force IPv6 address resolution.
.It Fl 8
Request 8-bit operation. This causes an attempt to negotiate the
.Dv TELNET BINARY
option for both input and output. By default telnet is not 8-bit
clean. 
.It Fl E
Disables the escape character functionality; that is, sets the escape
character to ``no character''.
.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 
.Dv TELNET BINARY 
option to be negotiated on just output.
.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 username is retrieved
via
.Xr getlogin 3 .
.It Fl b Ar address
Use bind(2) on the local socket to bind it to a specific local address.
.It Fl d
Sets the initial value of the
.Ic debug
toggle to
.Dv TRUE.
.It Fl r
Emulate 
.Xr rlogin 1 .
In this mode, the default escape character is a tilde. Also, the
interpretation of the escape character is changed: an escape character
followed by a dot causes
.Nm telnet 
to disconnect from the remote host. A ^Z instead of a dot suspends
.Nm telnet ,
and a ^] (the default
.Nm telnet
escape character) generates a normal telnet prompt. These codes are
accepted only at the beginning of a line. 
.It Fl S Ar tos
Sets the IP type-of-service (TOS) option for the telnet
connection to the value
.Ar tos .
.It Fl e Ar escapechar
Sets the escape character to
.Ar escapechar.
If no character is supplied, no escape character will be used.
Entering the escape character while connected causes telnet to drop to
command mode.
.It Fl l Ar user
Specify 
.Ar user
as the user to log in as on the remote system. This is accomplished by
sending the specified name as the 
.Dv USER
environment variable, so it requires that the remote system support the
.Ev TELNET ENVIRON
option. This option implies the
.Fl a
option, and 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 z Ar option
Set SSL (Secure Socket Layer) parameters. The default is to negotiate
via telnet protocol if SSL is available at server side and then to
switch it on. In this mode you can connect to both conventional and
SSL enhanced telnetd's. If the connection is made to localhost and 
.Ic -z secure
is not set, then
SSL is not enabled.
.Pp
The SSL parameters are:
.Bl -tag -width Fl
.It Ic debug
Send SSL related debugging information to stderr.
.It Ic authdebug
Enable authentication debugging.
.It Ic ssl
Negotiate SSL at first, then use telnet protocol. In this mode you can
connect to any server supporting directly SSL like Apache-SSL. Use
.Ic telnet -z ssl ssl3.netscape.com https
for example. telnet protocol negotiation goes encrypted.
.It Ic nossl, Ic !ssl
switch off SSL negotiation
.It Ic certrequired
server certificate is mandatory
.It Ic secure
Don't switch back to unencrypted mode (no SSL) if SSL is not available.
.It Ic verbose
Be verbose about certificates etc.
.It Ic verify= Ns Ar int
.\" TODO
Set the SSL verify flags (SSL_VERIFY_* in 
.Ar ssl/ssl.h
).
.\" TODO
.It Ic cert= Ns Ar cert_file
.\" TODO
Use the certificate(s) in
.Ar cert_file .
.It Ic key= Ns Ar key_file
.\" TODO
Use the key(s) in
.Ar key_file .
.It Ic cipher= Ns Ar ciph_list
.\" TODO
Set the preferred ciphers to
.Ar ciph_list .
.\" TODO: possible values; comma-separated list?
(See 
.Ar ssl/ssl.h
).
.El
.It Ar host
Specifies a host to contact over the network.
.It Ar port
Specifies a port number or service name to contact. If not specified,
the 
.Nm telnet
port (23) is used.
.El
.Pp
Protocol:
.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
Commands:
.Pp
The following
.Nm telnet
commands are available. Unique prefixes are understood as abbreviations.
.Pp
.Bl -tag -width "mode type"
.It Ic auth Ar argument ... 
The
.Ic auth
command controls the
.Dv TELNET AUTHENTICATE
protocol option.  If 
.Nm telnet
was compiled without authentication, the 
.Ic auth
command will not be supported. 
Valid arguments are as follows:
.Bl -tag -width "disable type"
.It Ic disable Ar type
Disable the specified type of authentication.  To
obtain a list of available types, use the
.Ic auth disable \&?
command.
.It Ic enable Ar type
Enable the specified type of authentication.  To
obtain a list of available types, use the
.Ic auth enable \&?
command.
.It Ic status
List the current status of the various types of
authentication.
.El
.It Ic close
Close the connection to the remote host, if any, and return to command
mode.
.It Ic display Ar argument ... 
Display all, or some, of the
.Ic set
and
.Ic toggle
values (see below).
.\" .It Ic encrypt Ar argument ...
.\" The encrypt command controls the
.\" .Dv TELNET ENCRYPT
.\" protocol option. If 
.\" .Nm telnet
.\" was compiled without encryption, the
.\" .Ic encrypt
.\" command will not be supported. 
.\" .Pp
.\" Valid arguments are as follows:
.\" .Bl -tag -width Ar
.\" .It Ic disable Ar type Ic [input|output]
.\" Disable the specified type of encryption.  If you do not specify input
.\" or output, encryption of both is disabled.  To obtain a list of
.\" available types, use ``encrypt disable \&?''.
.\" .It Ic enable Ar type Ic [input|output]
.\" Enable the specified type of encryption.  If you do not specify input
.\" or output, encryption of both is enabled.  To obtain a list of
.\" available types, use ``encrypt enable \&?''.
.\" .It Ic input
.\" This is the same as ``encrypt start input''.
.\" .It Ic -input
.\" This is the same as ``encrypt stop input''.
.\" .It Ic output
.\" This is the same as ``encrypt start output''.
.\" .It Ic -output
.\" This is the same as ``encrypt stop output''.
.\" .It Ic start Ic [input|output]
.\" Attempt to begin encrypting.  If you do not specify input or output, 
.\" encryption of both input and output is started. 
.\" .It Ic status
.\" Display the current status of the encryption module.
.\" .It Ic stop Ic [input|output]
.\" Stop encrypting.  If you do not specify input or output, encryption of
.\" both is stopped.
.\" .It Ic type Ar type
.\" Sets the default type of encryption to be used with later ``encrypt start''
.\" or ``encrypt stop'' commands.
.\" .El
.\" .Pp
.\" Note that the current version of 
.\" .Nm telnet
.\" does not support encryption.
.It Ic environ Ar arguments... 
The
.Ic environ
command is used to propagate environment variables across the 
.Nm telnet
link using the
.Dv TELNET ENVIRON
protocol option.
All variables exported from the shell are defined, but only the 
.Ev DISPLAY
and
.Ev PRINTER
variables are marked to be sent by default.  The
.Ev USER
variable is marked to be sent if the
.Fl a
or 
.Fl l
command-line options were 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 marked for
propagation (``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 any existing definition of
.Ar variable .
.It Ic export Ar variable 
Mark the specified variable for propagation to the remote host.
.It Ic unexport Ar variable 
Do not mark the specified variable for propagation to the remote
host. The remote host may still ask explicitly for variables that are
not exported.
.It Ic list
List the current set of environment variables.
Those marked with a
.Cm *
will be propagated to the remote host. The remote host may still ask
explicitly for the rest.
.It Ic \&?
Prints out help information for the
.Ic environ
command.
.El
.It Ic logout
Send the
.Dv TELNET LOGOUT
protocol option to the remote host.
This command is similar to a
.Ic close
command. If the remote host does not support the
.Dv LOGOUT
option, nothing happens.  But if it does, this command should cause it
to close the connection.  If the remote side also supports the concept
of suspending a user's session for later reattachment, the logout
command indicates that the session should be terminated immediately.
.It Ic mode Ar type 
.Ar Type
is one of several options, depending on the state of the session.
.Tn Telnet
asks the remote host to go into the requested mode. If the remote host
says it can, that mode takes effect.
.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
daemon at the standard port (23).
The host specification may be a host name or IP address.
The
.Fl l
option may be used to specify a user name to be passed to the remote
system, like the
.Fl l
command-line option.
.Pp
When connecting to ports other than the 
.Nm telnet
port,
.Nm telnet
does not attempt 
.Tn telnet
protocol negotiations. This makes it possible to connect to services
that do not support the
.Tn telnet
protocol without making a mess. Protocol negotiation can be forced by
placing a dash before the port number.
.Pp
After establishing a connection, any commands associated with the
remote host in
.Pa /etc/telnetrc
and the user's
.Pa .telnetrc
file are executed, in that order.
.Pp
The format of the telnetrc files is as follows: Lines beginning with a
#, and blank lines, are ignored.  The rest of the file should consist
of hostnames and sequences of
.Nm telnet
commands to use with that host. Commands should be one per line,
indented by whitespace; lines beginning without whitespace are
interpreted as hostnames.  Lines beginning with the special hostname
.Ql DEFAULT
will apply to all hosts.  Hostnames including
.Ql DEFAULT
may be followed immediately by a colon and a port number or string.
If a port is specified it must match exactly with what is specified
on the command line.  If no port was specified on the command line,
then the value
.Ql telnet
is used.
Upon connecting to a particular host, the
commands associated with that host are executed.
.It Ic quit
Close any open session and exit
.Nm telnet .
An end of file condition on input, when in command mode, will trigger
this operation as well.
.It Ic send Ar arguments 
Send one or more special 
.Tn telnet
protocol character sequences to the remote host.  The following are
the codes which may be specified (more than one may be used in one
command):
.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.
.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 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 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 variables.
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.
.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 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 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 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 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 rlogin
This is the rlogin mode escape character. Setting it enables rlogin
mode, as with the
.Ar r
command-line option (q.v.)
.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
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 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 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 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 \&?
Prints out help information for the
.Ic slc
command.
.El
.It Ic startssl
Attempt to negotiate telnet-over-SSL (as with the
.Ic -z ssl
option). This is useful when connecting to non-telnetds such
as imapd (with the
.Ic STARTTLS
command). To control SSL when connecting to a SSL-enabled
telnetd, use the
.Ic auth
command instead.
.It Ic status
Show the current status of
.Nm telnet .
This includes the name of the remote host, if any, as well as the
current mode.
.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.
More than one flag may be toggled at once.
The state of these flags may be examined with the
.Ic display
command.
Valid flags are:
.Bl -tag -width Ar
.It Ic authdebug
Turns on debugging for the authentication code. This flag only exists
if authentication support is enabled.
.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 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
.\" Note that this flag exists only if encryption support is enabled.
.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 TELNET AUTHENTICATION
option is not supported, the user's login name is propagated using the
.Dv TELNET ENVIRON
option.
Setting this flag is the same as specifying the
.Ar a
option to the
.Ic open
command or on the command line.
.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.
.\" Note that this flag only exists if encryption support is available.
.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 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 formatted in a more user-readable format.
Spaces are put between each character in the output, and the
beginning of
.Tn telnet
escape sequences are preceded by a '*' to aid in locating them.
.It Ic skiprc
When the skiprc toggle is
.Dv TRUE ,
.Tn telnet
does not read the telnetrc files.  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 ,
.\" .Tn TELNET
.\" prints out a message each time encryption is enabled or
.\" disabled.  The initial value for this toggle is
.\" .Dv FALSE.
.\" This flag only exists if encryption support is available.
.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 omitted, then an interactive subshell is invoked.
.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
environment variables.
Other environment variables may be propagated
to the other side via the
.Dv TELNET ENVIRON
option.
.Sh FILES
.Bl -tag -width /etc/telnetrc -compact
.It Pa /etc/telnetrc
global telnet startup values
.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.
.Sh BUGS
The source code is not comprehensible.