NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [-8EFKLacdfrx] [-S tos] [-X authtype] [-e escapechar] [-k realm]
[-l user] [-n tracefile] [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 arguments.
Options:
-8 Specifies an 8-bit data path. This causes an attempt to
negotiate the TELNET BINARY option on both input and output.
-E Stops any character from being recognized as an escape character.
-F If Kerberos V5 authentication is being used, the -F option allows
the local credentials to be forwarded to the remote system,
including any credentials that have already been forwarded into
the local environment.
-K Specifies no automatic login to the remote system.
-L Specifies an 8-bit data path on output. This causes the BINARY
option to be negotiated on output.
-S tos Sets the IP type-of-service (TOS) option for the telnet
connection to the value tos, which can be a numeric TOS value or,
on systems that support it, a symbolic TOS name found in the
/etc/iptos file.
-X atype
Disables the atype type of authentication.
-a Attempt automatic login. Currently, 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.
-c Disables the reading of the user’s .telnetrc file. (See the
toggle skiprc command on this man page.)
-d Sets the initial value of the debug toggle to TRUE
-e escape char
Sets the initial telnet telnet escape character to escape char.
If escape char is omitted, then there will be no escape
character.
-f If Kerberos V5 authentication is being used, the -f option allows
the local credentials to be forwarded to the remote system.
-k realm
If Kerberos authentication is being used, the -k option requests
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
When connecting to the remote system, 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
Opens tracefile for recording trace information. See the set
tracefile command below.
-r Specifies 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 Turns on encryption of the data stream if possible. This option
is not available outside of the United States and Canada.
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.
When in rlogin mode, a line of the form ~. disconnects from the remote
host; ~ is the telnet escape character. Similarly, the line ~^Z suspends
the telnet session. The line ~^] escapes to the normal telnet 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 (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).
If the LINEMODE option is enabled, or if the localchars toggle 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
command 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 ? command.
status Lists the current status of the various types of
authentication.
close Close a TELNET session and return to command mode.
display argument ...
Displays all, or some, 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 disabled. To obtain a list of
available types, use the encrypt disable ?
command.
enable type [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 ? command.
status Lists the current status of encryption.
stop [input|output]
Stops encryption. If you omit input and output,
encryption 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. 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
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,
other variables will only be sent if explicitly
requested.
? Prints out help information for the environ
command.
logout Sends the TELNET LOGOUT option to the remote side. This
command is similar to a close command; however, if the remote
side does not support the LOGOUT option, nothing happens. If,
however, the remote side does support the LOGOUT option, this
command should cause the remote side to close the TELNET
connection. If the remote side also supports the concept of
suspending a user’s session for later reattachment, the logout
argument indicates that you should terminate the session
immediately.
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 [[-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)). The [-l] option may be used to
specify the user name to be passed to the remote system via
the ENVIRON option. 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. After establishing a connection,
the file .telnetrc in the users 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.
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
character 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 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,
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
variables 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
command. The variables which may be set or unset, but not
toggled, 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
echoing 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
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.
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.
escape This is the telnet escape character (initially “^[”)
which causes entry into telnet command mode (when
connected 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
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.
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
terminal’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
terminal’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
terminal’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
terminal’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
characters 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 terminal at the time when
telnet was started.
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.
? 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 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)).
autodecrypt When the 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.
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 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 <CR><LF>. If this is FALSE, then
carriage 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
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 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 “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 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 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 formatted 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 toggle is TRUE, TELNET skips the
reading of the .telnetrc file in the users home
directory when connections are opened. The
initial value for this toggle is FALSE.
termdata Toggles the display of all terminal data (in
hexadecimal format). The initial value for this
toggle is FALSE.
verbose_encrypt
When the verbose_encrypt toggle 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 supported
outside of the United States and Canada.
? 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 omitted, then an interactive subshell is
invoked.
? [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.
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
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.