linux - telnet (1)
SYNOPSIS
telnet [-d] [-a] [-n tracefile] [-e escapechar] [[-l user] host [port]]
DESCRIPTION
The telnet command is used to communicate with another host using the
TELNET protocol. If telnet is invoked without the host argument, it en-
ters 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 with those arguments.
Options:
-d Sets the initial value of the debug toggle to TRUE
-a Attempt automatic login. Currently, this sends the user name via
the USER variable of the ENVIRON option if supported by the re-
mote system. The name used is that of the current user as re-
turned by getlogin(2) if it agrees with the current user ID, oth-
erwise it is the name associated with the user ID.
-n tracefile
Opens tracefile for recording trace information. See the set
tracefile command below.
-l user
When connecting to the remote system, if the remote system under-
stands the ENVIRON option, then user will be sent to the remote
system as the value for the variable USER. This option implies
the -a option. This option may also be used with the open com-
mand.
-e escape char
Sets the initial telnet telnet escape character to escape char.
If escape char is ommitted, then there will be no escape charac-
ter.
host Indicates the official name, an alias, or the Internet address of
a remote host.
port Indicates a port number (address of an application). If a number
is not specified, the default telnet port is used.
Once a connection has been opened, telnet will attempt to enable the
TELNET LINEMODE option. If this fails, then telnet will revert to one of
two input modes: either ``character at a time'' or ``old line by line''
depending on what the remote system supports.
When LINEMODE is enabled, character processing is done on the local sys-
tem, under the control of the remote system. When input editing or char-
acter echoing is to be disabled, the remote system will relay that infor-
mation. The remote system will also relay changes to any special charac-
ters that happen on the remote system, so that they can take effect on
quences to the remote side. If LINEMODE has ever been enabled, then the
user's susp and eof are also sent as TELNET protocol sequences, and quit
is sent as a TELNET ABORT instead of BREAK 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 com-
mand mode, the normal terminal editing conventions are available.
The following telnet 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, unset, slc, environ, and display commands).
close Close a TELNET session and return to command mode.
display argument ...
Displays all, or some, of the set and toggle values (see be-
low).
mode type Type is one of several options, depending on the state of the
TELNET session. The remote host is asked for permission to go
into the requested mode. If the remote host is capable of en-
tering that mode, the requested mode will be entered.
character Disable the TELNET LINEMODE option, or, if the
remote side does not understand the LINEMODE op-
tion, then enter ``character at a time`` mode.
line Enable the TELNET LINEMODE option, or, if the
remote side does not understand the LINEMODE op-
tion, then attempt to enter ``old-line-by-line``
mode.
isig (-isig) Attempt to enable (disable) the TRAPSIG mode of
the LINEMODE option. This requires that the
LINEMODE option be enabled.
edit (-edit) Attempt to enable (disable) the EDIT mode of the
LINEMODE option. This requires that the
LINEMODE option be enabled.
softtabs (-softtabs)
Attempt to enable (disable) the SOFT_TAB mode of
the LINEMODE option. This requires that the
LINEMODE option be enabled.
litecho (-litecho)
Attempt to enable (disable) the LIT_ECHO mode of
the LINEMODE option. This requires that the
the port number is preceeded by a minus sign, the inital op-
tion negotiation is done. After establishing a connection,
the file .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 telnet commands and are processed as if they had been typed
in manually to the telnet command prompt.
quit Close any open TELNET session and exit telnet. An end of file
(in command mode) will also close a session and exit.
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):
abort Sends the TELNET ABORT (Abort processes) sequence.
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 re-
spond.
brk Sends the TELNET BRK (Break) sequence, which may have
significance to the remote system.
ec Sends the TELNET EC (Erase Character) sequence, which
should cause the remote system to erase the last char-
acter entered.
el Sends the TELNET EL (Erase Line) sequence, which
should cause the remote system to erase the line cur-
rently being entered.
eof Sends the TELNET EOF (End Of File) sequence.
eor Sends the TELNET EOR (End of Record) sequence.
escape Sends the current telnet escape character (initially
``^'').
ga Sends the TELNET GA (Go Ahead) sequence, which likely
has no significance to the remote system.
getstatus
If the remote side supports the TELNET STATUS command,
the remote system to discard all previously typed (but
not yet read) input. This sequence is sent as TCP ur-
gent data (and may not work if the remote system is a
4.2BSD system -- if it doesn't work, a lower case
``r'' may be echoed on the terminal).
? Prints out help information for the send command.
set argument value
unset argument value
The set command will set any one of a number of telnet vari-
ables to a specific value or to TRUE. The special value off
turns off the function associated with the variable, this is
equivalent to using the unset command. The unset command will
disable or set to FALSE any of the specified functions. The
values of variables may be interrogated with the display com-
mand. The variables which may be set or unset, but not tog-
gled, are listed here. In addition, any of the variables for
the toggle command may be explicitly set or unset using the
set and unset commands.
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 en-
tering, say, a password).
eof If telnet is operating in LINEMODE or ``old line by
line'' mode, entering this character as the first
character on a line will cause this character to be
sent to the remote system. The initial value of the
eof character is taken to be the terminal's eof char-
acter.
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 char-
acter is taken to be the terminal's erase character.
escape This is the telnet escape character (initially ``^['')
which causes entry into telnet command mode (when con-
nected to a remote system).
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 charac-
ter is taken to be the terminal's flush character.
ter is taken to be the terminal's kill character.
lnext If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's lnext character. The initial value for the
lnext character is taken to be the terminal's lnext
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.
reprint
If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's reprint character. The initial value for
the reprint character is taken to be the terminal's
reprint character.
start If the TELNET TOGGLE-FLOW-CONTROL option has been en-
abled, then this character is taken to be the termi-
nal's start character. The initial value for the kill
character is taken to be the terminal's start charac-
ter.
stop If the TELNET TOGGLE-FLOW-CONTROL option has been en-
abled, then this character is taken to be the termi-
nal's stop character. The initial value for the kill
character is taken to be the terminal's stop charac-
ter.
susp If telnet is in localchars mode, or LINEMODE is en-
abled, and the suspend character is typed, a TELNET
SUSP sequence (see send susp above) is sent to the re-
mote host. The initial value for the suspend charac-
ter is taken to be the terminal's suspend character.
tracefile
Thi is the file to which the output, caused by netdata
or option tracing being TRUE, will be written. If it
is set to ``-'', then tracing information will be
written to standard output (the default).
worderase
If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's worderase character. The initial value for
the worderase character is taken to be the terminal's
worderase character.
import Switch to the remote defaults for the special
characters. The remote default characters are
those of the remote system at the time when the
TELNET connection was established.
check Verify the current settings for the current spe-
cial 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.
? Prints out help information for the slc command.
environ arguments...
The environ command is used to manipulate the the variables
that my be sent through the TELNET ENVIRON option. The ini-
tial set of variables is taken from the users environment,
with only the DISPLAY and PRINTER variables being exported by
default. The USER variable is also exported if the -a or -l
options are used.
Valid arguments for the environ command are:
define variable value
Define the variable variable to have a value of
value. Any variables defined by this command are
automatically exported. The value may be enclosed
in single or double quotes so that tabs and spaces
may be included.
undefine variable
Remove variable from the list of environment vari-
ables.
export variable
Mark the variable variable to be exported to the
remote side.
unexport variable
Mark the variable variable to not be exported un-
less explicitly asked for by the remote side.
list List the current set of environment variables.
Those marked with a * will be sent automatically,
other variables will only be sent if explicitly
requested.
? Prints out help information for the environ com-
mand.
toggle arguments ...
system acknowledges (via a TELNET TIMING MARK
option) that it has processed those TELNET se-
quences. 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.
binary Enable or disable the TELNET BINARY option on
both input and output.
inbinary Enable or disable the TELNET BINARY option on
input.
outbinary Enable or disable the TELNET BINARY option on
output.
crlf If this is TRUE, then carriage returns will be
sent as <CR><LF>. If this is FALSE, then car-
riage returns will be send as <CR><NUL>. The
initial value for this toggle is FALSE.
crmod Toggle carriage return mode. When this mode is
enabled, most carriage return characters re-
ceived 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 nev-
er line feed. The initial value for this toggle
is FALSE.
debug Toggles socket level debugging (useful only to
the super user). The initial value for this tog-
gle is FALSE.
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
tocol processing (having to do with TELNET op-
tions). The initial value for this toggle is
FALSE.
prettydump When the netdata toggle is enabled, if
prettydump is enabled the output from the
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
TELNET escape sequence is preceeded by a '*' to
aid in locating them.
? Displays the legal toggle commands.
z Suspend telnet. This command only works when the user is using
the csh(1).
! [command]
Execute a single command in a subshell on the local system.
If command is ommitted, then an interactive subshell is in-
voked.
status Show the current status of telnet. This includes the peer one
is connected to, as well as the current mode.
? [command]
Get help. With no arguments, telnet prints a help summary.
If a command is specified, telnet will print the help informa-
tion for just that command.
ENVIRONMENT
Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environent vari-
ables. Other envirnoment variables may be propogated to the other side
via the TELNET ENVIRON option.
FILES
~/.telnetrc user customized telnet startup values
HISTORY
The Telnet command appeared in 4.2BSD.
NOTES
On some remote systems, echo has to be turned off manually when in ``old
line by line'' mode.
In ``old line by line'' mode or LINEMODE the terminal's eof character is
only recognized (and sent to the remote system) when it is the first
character on a line.
4.2 Berkeley Distribution July 27, 1991 8