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TELNET(1) TELNET(1)
NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [-8] [-E] [-F] [-K] [-L] [-S tos] [-X authtype] [-a] [-c] [-d]
[-e escapechar] [-f] [-k realm] [-l user] [-n tracefile] [-r] [-x]
[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
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 with those argu-
ments.
OPTIONS
-8 Specify an 8-bit data path. This causes an attempt to negoti-
ate the TELNET BINARY option on both input and output.
-E Stop any character from being recognized as an escape charac-
ter.
-F forward a forwardable copy of the local credentials to the
remote system.
-K Specify no automatic login to the remote system.
-L Specify an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.
-S tos Set the IP type-of-service (TOS) option for the telnet connec-
tion to the value tos, which can be a numeric TOS value (in
decimal, or a hex value preceded by 0x, or an octal value pre-
ceded by a leading 0) or, on systems that support it, a sym-
bolic TOS name found in the /etc/iptos file.
-X atype
Disable the atype type of authentication.
-a Attempt automatic login. This sends the user name via the USER
variable of the ENVIRON option, if supported by the remote sys-
tem. The name used is that of the current user as returned by
getlogin(2) if it agrees with the current user ID; otherwise it
is the name associated with the user ID.
-c Disable the reading of the user's .telnetrc file. (See the
toggle skiprc command on this man page.)
-d Set the initial value of the debug flag to TRUE
-e escape char
Set the initial telnet escape character to escape char. If
escape char is omitted, then there will be no escape character.
-f forward a copy of the local credentials to the remote system.
-k realm
If Kerberos authentication is being used, request that telnet
obtain tickets for the remote host in realm realm instead of
the remote host's realm, as determined by krb_realmofhost(3).
-l user
If the remote system understands 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 command.
-n tracefile
Open tracefile for recording trace information. See the set
tracefile command below.
-r Specify a user interface similar to rlogin(1). In this mode,
the escape character is set to the tilde (~) character, unless
modified by the -e option.
-x Turn on encryption of the data stream. When this option is
turned on, telnet will exit with an error if authentication
cannot be negotiated or if encryption cannot be turned on.
host Indicates the name, alias, or Internet address of the remote
host.
port Indicates a port number (address of an application). If the
port is not specified, the default telnet port (23) is used.
When in rlogin mode, ~ is the telnet escape character; a line of the
form ~. disconnects from the remote host. Similarly, the line ~^Z
suspends the telnet session. The line ~^] escapes to the normal tel-
net escape prompt.
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
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.
In ''character at a time'' mode, most text typed is immediately sent
to the remote host for processing.
In ''old line by line'' mode, all text is echoed locally, and (nor-
mally) 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).
If the LINEMODE option is enabled, or if the localchars flag is TRUE
(the default for ''old line by line''; see below), the user's quit,
intr, and flush characters are trapped locally, and sent as TELNET
protocol sequences 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
command 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).
auth argument ...
The auth command manipulates the information sent through the
TELNET AUTHENTICATE option. Valid arguments for the auth com-
mand are as follows:
disable type
Disables the specified type of authentication. To
obtain a list of available types, use the auth disable ?
command.
enable type
Enables the specified type of authentication. To obtain
a list of available types, use the auth enable ? com-
mand.
status Lists the current status of the various types of authen-
tication.
close Close a TELNET session and return to command mode.
display argument ...
Displays some or all of the set and toggle values (see below).
encrypt argument ...
The encrypt command manipulates the information sent through
the TELNET ENCRYPT option.
Note: Because of export controls, the TELNET ENCRYPT option is not
supported outside of the United States and Canada.
Valid arguments for the encrypt command are as follows:
disable type [input|output]
Disables the specified type of encryption. If you omit
the input and output, both input and output are dis-
abled. To obtain a list of available types, use the
encrypt disable ? command.
enable type]fP [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 encrypt enable
? command.
input This is the same as the encrypt start input command.
-input This is the same as the encrypt stop input command.
output This is the same as the encrypt start output command.
-output
This is the same as the encrypt stop output command.
start [input|output]
Attempts to start encryption. If you omit input and
output, both input and output are enabled. To obtain a
list of available types, use the encrypt enable ? com-
mand.
status Lists the current status of encryption.
stop [input|output]
Stops encryption. If you omit input and output, encryp-
tion is on both input and output.
type type
Sets the default type of encryption to be used with
later encrypt start or encrypt stop commands.
environ arguments ...
The environ command is used to manipulate the the variables
that my be sent through the TELNET ENVIRON option. The initial
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.
Variables defined by this command are not 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 variables.
export variable
Mark the variable variable to be exported to the remote
side.
unexport variable
Mark the variable variable to not be exported unless
explicitly asked for by the remote side.
list List the current set of environment variables. Those
marked with a * will be sent automatically; those marked
with a + will be sent if the other end requests their
values, and other variables will not be sent.
? Prints out help information for the environ command.
logout Sends the TELNET LOGOUT option to the remote side. This com-
mand is similar to a close command; however, if the remote side
does not support the LOGOUT option, nothing happens. If, how-
ever, the remote side does support the LOGOUT option, this com-
mand should cause the remote side to close the TELNET connec-
tion. If the remote side also supports the concept of suspend-
ing a user's session for later reattachment, the logout argu-
ment indicates that you should terminate the session immedi-
ately.
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
entering that mode, the requested mode will be entered.
character
Disable the TELNET LINEMODE option, or, if the remote
side does not understand the LINEMODE option, then enter
''character at a time'' mode.
line Enable the TELNET LINEMODE option, or, if the remote
side does not understand the LINEMODE option, 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 LINEMODE option
be enabled.
? Prints out help information for the mode command.
open host [-a] [[-l] user] [-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(3). After establishing a connec-
tion, the file .telnetrc in the user's home directory is
opened. 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 the line is
the name of the machine that is being connected to. The rest
of the line, and successive lines that begin with white space
are assumed to be telnet commands and are processed as if they
had been typed in manually to the telnet command prompt.
-a Attempt automatic login. This sends the user name via
the USER variable of the ENVIRON option, if supported by
the remote system. The name used is that of the current
user as returned by getlogin(2) if it agrees with the
current user ID; otherwise it is the name associated
with the user ID.
[-l] user
may be used to specify the user name to be passed to the
remote system via the ENVIRON option.
-port When connecting to a non-standard port, telnet omits any
automatic initiation of TELNET options. When the port
number is preceded by a minus sign, the initial option
negotiation is done.
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 respond.
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 charac-
ter entered.
el Sends the TELNET EL (Erase Line) sequence, which should
cause the remote system to erase the line currently
being entered.
eof Sends the TELNET EOF (End Of File) sequence.
eor Sends the TELNET EOR (End of Record) sequence.
escape Sends the current 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,
getstatus will send the subnegotiation to request that
the server send its current option status.
ip Sends the TELNET IP (Interrupt Process) sequence, which
should cause the remote system to abort the currently
running process.
nop Sends the TELNET NOP (No OPeration) sequence.
susp Sends the TELNET SUSP (SUSPend process) sequence.
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
4.2BSD system -- if it doesn't work, a lower case ''r''
may be echoed on the terminal).
do cmd
dont cmd
will cmd
wont cmd
Sends the TELNET DO cmd sequence. Cmd can be either a
decimal number between 0 and 255, or a symbolic name for
a specific TELNET command. Cmd can also be either help
or ? to print out help information, including a list of
known symbolic names.
? 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.
ayt If telnet is in localchars mode, or LINEMODE is enabled,
and the status character is typed, a TELNET AYT sequence
(see send ayt preceding) is sent to the remote host.
The initial value for the "Are You There" character is
the terminal's status character.
echo This is the value (initially ''^E'') which, when in
''line by line'' mode, toggles between doing local echo-
ing of entered characters (for normal processing), and
suppressing echoing of entered characters (for entering,
say, a password).
eof If telnet is operating in LINEMODE or ''old line by
line'' mode, entering this character as the first char-
acter on a line will cause this character to be sent to
the remote system. The initial value of the eof charac-
ter is taken to be the terminal's eof character.
erase If telnet is in 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.
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 character is
taken to be the terminal's flush character.
forw1
forw2 If telnet is operating in LINEMODE, these are the char-
acters that, when typed, cause partial lines to be for-
warded to the remote system. The initial value for the
forwarding characters are taken from the terminal's eol
and eol2 characters.
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.
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.
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 ter-
minal's reprint character. The initial value for the
reprint character is taken to be the terminal's reprint
character.
rlogin This is the rlogin escape character. If set, the normal
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 telnet command. The initial state is to disable the
rlogin escape character.
start If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, 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 character.
stop If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, 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 character.
susp If telnet is in localchars mode, or LINEMODE is enabled,
and the suspend character is typed, a TELNET SUSP
sequence (see send susp above) is sent to the remote
host. The initial value for the suspend character is
taken to be the terminal's suspend character.
tracefile
This 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 ter-
minal's worderase character. The initial value for the
worderase character is taken to be the terminal's
worderase character.
? Displays the legal set (unset) commands.
slc state
The slc command (Set Local Characters) is used to set or change
the state of the the special characters when the TELNET
LINEMODE option has been enabled. Special characters are char-
acters that get mapped to telnet commands sequences (like ip or
quit ) or line editing characters (like erase and kill). By
default, the local special characters are exported.
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.
export Switch to the local defaults for the special characters.
The local default characters are those of the local ter-
minal at the time when telnet was started.
import Switch to the remote defaults for the special charac-
ters. The remote default characters are those of the
remote system at the time when the TELNET connection was
established.
? Prints out help information for the slc command.
status Show the current status of telnet. This includes the peer one
is connected to, as well as the current mode.
toggle arguments ...
Toggle (between TRUE and FALSE) various flags that control how
telnet responds to events. These flags may be set explicitly
to TRUE or FALSE using the set and unset commands listed above.
More than one argument may be specified. The state of these
flags may be interrogated with the display command. Valid
arguments are:
authdebug
Turns on debugging information for the authentication
code.
autoflush
If autoflush and localchars are both TRUE , then when
the ao, or quit characters are recognized (and trans-
formed 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 tog-
gle is TRUE if the terminal user had not done an "stty
noflsh", otherwise FALSE (see stty(1).
autodecrypt
When the TELNET ENCRYPT option is negotiated, by default
the actual encryption (decryption) of the data stream
does not start automatically. The autoencrypt (autode-
crypt) command states that encryption of the output
(input) stream should be enabled as soon as possible.
Note: Because of export controls, the TELNET ENCRYPT option is
not supported outside the United States and Canada.
autologin
If the remote side supports the TELNET AUTHENTICATION
option telnet attempts to use it to perform automatic
authentication. If the AUTHENTICATION option is not
supported, the user's login name are propagated through
the TELNET ENVIRON option. This command is the same as
specifying the -a option on the open command.
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
. If this is FALSE, then carriage returns will
be send as . The initial value for 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 super
user). The initial value for this toggle is FALSE .
encdebug
Turns on debugging information for the encryption code.
localchars
If this is TRUE , then the flush, interrupt, quit,
erase, and kill characters (see set above) are recog-
nized locally, and transformed into (hopefully) appro-
priate TELNET control sequences (respectively ao, ip,
brk, ec, and el; see send above). The initial value for
this toggle is TRUE in ''old line by line'' mode, and
FALSE in ''character at a time'' mode. When the
LINEMODE option is enabled, the value of localchars is
ignored, and assumed to always be TRUE. If LINEMODE has
ever been enabled, then quit is sent as abort, and eof
and suspend are sent as eof and susp, see send above).
netdata
Toggles the display of all network data (in hexadecimal
format). 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 ini-
tial value for this flag is FALSE .
prettydump
When the netdata flag is enabled, if prettydump is
enabled the output from the netdata command will be for-
matted in a more user-readable format. Spaces are put
between each character in the output, and the beginning
of any TELNET escape sequence is preceded by a '*' to
aid in locating them.
skiprc When the skiprc flag is TRUE, TELNET skips the reading
of the .telnetrc file in the user's home directory when
connections are opened. The initial value for this flag
is FALSE.
termdata
Toggles the display of all terminal data (in hexadecimal
format). The initial value for this flag is FALSE.
verbose_encrypt
When the verbose_encrypt flag is TRUE, TELNET prints out
a message each time encryption is enabled or disabled.
The initial value for this toggle is FALSE. Note:
Because of export controls, data encryption is not sup-
ported outside of the United States and Canada.
? Displays the legal toggle commands.
z Suspend telnet. This command only works when the user's shell
is csh(1).
! [command]
Execute a single command in a subshell on the local system. If
command is omitted, then an interactive subshell is invoked.
? command
Get help. With no arguments, prints a help summary. If a com-
mand is specified, will print the help information for just
that command.
ENVIRONMENT
Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environment
variables. Other environment variables may be propagated to the other
side via the TELNET ENVIRON option.
FILES
~/.telnetrc user-customized telnet startup values
~/.k5login (on remote host) - file containing Kerberos prin-
cipals that are allowed access.
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.
TELNET(1)
UNIX/Linux commands referenced on this page: