telnet(1)
NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [ host [ port ] ]
DESCRIPTION
Telnet is used to communicate with another host using the TELNET
protocol. If telnet is invoked without arguments, it enters command
mode, indicated by its prompt ("telnet>"). In this mode, it accepts and
executes the commands listed below. If it is invoked with arguments, it
performs an open command (see below) with those arguments.
Once a connection has been opened, telnet enters an input mode. The
input mode entered will be either "character at a time" or "line by line"
depending on what the remote system supports.
In "character at a time" mode, most text typed is immediately sent to the
remote host for processing.
In "line by line" mode, all text is echoed locally, and (normally) only
completed lines are sent to the remote host. The "local echo character"
(initially "^E") may be used to turn off and on the local echo (this
would mostly be used to enter passwords without the password being
echoed).
In either mode, if the localchars toggle is TRUE (the default in line
mode; see below), the user's quit, intr, and flush characters are trapped
locally, and sent as TELNET protocol sequences to the remote side. There
are options (see toggle autoflush and toggle 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 quit and intr).
While connected to a remote host, telnet command mode may be entered by
typing the telnet "escape character" (initially "^]"). When in command
mode, the normal terminal editing conventions are available.
COMMANDS
The following commands are available. Only enough of each command to
uniquely identify it need be typed (this is also true for arguments to
the mode, set, toggle, and display commands).
open host [ port ]
Open a connection to the named host. If no port number is
specified, telnet will attempt to contact a TELNET server at the
default port. The host specification may be either a host name (see
hosts(5)) or an Internet address specified in the "dot notation"
(see inet(3N)).
close
Close a TELNET session and return to command mode.
quit
Close any open TELNET session and exit telnet. An end of file (in
command mode) will also close a session and exit.
z
Suspend telnet. This command only works when the user is using the
csh(1).
mode type
Type is either line (for "line by line" mode) or character (for
"character at a time" mode). 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.
status
Show the current status of telnet. This includes the peer one is
connected to, as well as the current mode.
display [ argument... ]
Displays all, or some, of the set and toggle values (see below).
? [ command ]
Get help. With no arguments, telnet prints a help summary. If a
command is specified, telnet will print the help information for
just that command.
send 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):
escape
Sends the current telnet escape character (initially "^]").
synch
Sends the 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 4.2 BSD system -- if it doesn't work, a lower
case "r" may be echoed on the terminal).
brk
Sends the TELNET BRK (Break) sequence, which may have significance
to the remote system.
ip
Sends the TELNET IP (Interrupt Process) sequence, which should cause
the remote system to abort the currently running process.
ao
Sends the TELNET AO (Abort Output) sequence, which should cause the
remote system to flush all output from the remote system to the
user's terminal.
ayt
Sends the TELNET AYT (Are You There) sequence, to which the remote
system may or may not choose to respond.
ec
Sends the TELNET EC (Erase Character) sequence, which should cause
the remote system to erase the last character entered.
el
Sends the TELNET EL (Erase Line) sequence, which should cause the
remote system to erase the line currently being entered.
ga
Sends the TELNET GA (Go Ahead) sequence, which likely has no
significance to the remote system.
nop
Sends the TELNET NOP (No OPeration) sequence.
?
Prints out help information for the send command.
set argument value
Set any one of a number of telnet variables to a specific value.
The special value "off" turns off the function associated with the
variable. The values of variables may be interrogated with the
display command. The variables which may be specified are:
echo
This is the value (initially "^E") which, when in "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).
escape
This is the telnet escape character (initially "^[") which causes
entry into telnet command mode (when connected to a remote system).
interrupt
If telnet is in localchars mode (see toggle localchars below) and
the interrupt character is typed, a TELNET IP sequence (see send ip
above) is sent to the remote host. The initial value for the
interrupt character is taken to be the terminal's intr character.
quit
If telnet is in localchars mode (see toggle localchars below) and
the quit character is typed, a TELNET BRK sequence (see send brk
above) is sent to the remote host. The initial value for the quit
character is taken to be the terminal's quit character.
flushoutput
If telnet is in localchars mode (see toggle localchars below) and
the flushoutput character is typed, a TELNET AO sequence (see send
ao above) is sent to the remote host. The initial value for the
flush character is taken to be the terminal's flush character.
erase
If telnet is in localchars mode (see toggle localchars below), and
if telnet is operating in "character at a time" mode, then when this
character is typed, a TELNET EC sequence (see send ec above) is sent
to the remote system. The initial value for the erase character is
taken to be the terminal's erase character.
kill
If telnet is in localchars mode (see toggle localchars below), and
if telnet is operating in "character at a time" mode, then when this
character is typed, a TELNET EL sequence (see send el above) is sent
to the remote system. The initial value for the kill character is
taken to be the terminal's kill character.
eof
If telnet is operating in "line by line" mode, entering 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 eof character.
toggle arguments...
Toggle (between TRUE and FALSE) various flags that control how
telnet responds to events. More than one argument may be specified.
The state of these flags may be interrogated with the display
command. Valid arguments are:
localchars
If this is TRUE, then the flush, interrupt, quit, erase, and kill
characters (see set above) are recognized locally, and transformed
into (hopefully) appropriate TELNET control sequences (respectively
ao, ip, brk, ec, and el; see send above). The initial value for
this toggle is TRUE in "line by line" mode, and FALSE in "character
at a time" mode.
autoflush
If autoflush and localchars are both TRUE, then when the ao, intr,
or quit characters are recognized (and transformed into TELNET
sequences; see set above for details), telnet refuses to display any
data on the user's terminal until the remote system acknowledges
(via a TELNET Timing Mark option) that it has processed those TELNET
sequences. The initial value for this toggle is TRUE if the
terminal user had not done an "stty noflsh", otherwise FALSE (see
stty(1)).
autosynch
If autosynch and localchars are both TRUE, then when either the intr
or quit characters is typed (see set above for descriptions of the
intr and quit characters), the resulting TELNET sequence sent is
followed by the TELNET SYNCH sequence. This procedure should cause
the remote system to begin throwing away all previously typed input
until both of the TELNET sequences have been read and acted upon.
The initial value of this toggle is FALSE.
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 FALSE.
debug
Toggles socket level debugging (useful only to the superuser). The
initial value for this toggle is FALSE.
options
Toggles the display of some internal telnet protocol processing
(having to do with TELNET options). The initial value for this
toggle is FALSE.
netdata
Toggles the display of all network data (in hexadecimal format).
The initial value for this toggle is FALSE.
?
Displays the legal toggle commands.
BUGS
There is no adequate way for dealing with flow control.
On some remote systems, echo has to be turned off manually when in "line
by line" mode.
There is enough settable state to justify a .telnetrc file.
No capability for a .telnetrc file is provided.
In "line by line" mode, the terminal's eof character is only recognized
(and sent to the remote system) when it is the first character on a line.