1:telnet
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telnet - user interface to the TELNET protocol
TELNET(1) BSD General Commands Manual TELNET(1)
Contents |
SYNOPSIS
telnet [-8] [-E] [-F] [-K] [-L] [-S tos] [-X authtype] [-a] [-b hostalias] [-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 with-
out 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.
The options are as follows:
-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.
-b hostalias
Uses bind(2) on the local socket to bind it to an aliased address (see ifconfig(8) and the ``alias speci-
fier) or to the address of another interface than the one naturally chosen by connect(2). This can be use-
ful when connecting to services which use IP addresses for authentication and reconfiguration of the server
is undesirable (or impossible).
-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 escapechar
Sets the initial telnet escape character to escapechar. If escapechar 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.
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, 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 acknowl-
edges 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. Note that the
escape character will return to the command mode of the initial invocation of telnet that has the controlling ter-
minal. Use the send escape command to switch to command mode in subsequent telnet processes on remote hosts.
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 argu-
ments 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.
Valid arguments for the encrypt command are as follows:
disable type [input|output]
Disables the specified type of encryption. If you omit input and output, both input and
output are disabled. To obtain a list of available types, use the encrypt disable ? com-
mand.
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 ? com-
mand.
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 variables that may be sent through the TELNET ENVIRON
option. The initial set of variables is taken from the user's environment, with only the DISPLAY,
PRINTER, and XAUTHORITY variables being exported by default. The USER variable is also exported if the
-a or -l options are used. Additionally, the value of the TERM variable is by default available to be
queried by the server.
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 com-
mand 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.
list List the current set of environment variables. Those marked with a * will be sent automati-
cally, those marked with a + will only be sent if explicitly requested by the server, and
others won't be revealed to the server even if 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 connec-
tion. If the remote side also supports the concept of suspending a user's session for later reattach-
ment, 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 spec-
ify 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 pre-
ceded by a minus sign, the initial option negotiation is done. After establishing a connection, 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.
quit Close any open TELNET session and exit telnet. An end-of-file (in command mode) will also close a ses-
sion 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 previ-
ously 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 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.
dont cmd
Sends the TELNET DONT 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.
will cmd
Sends the TELNET WILL 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.
wont cmd
Sends the TELNET WONT 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 val-
ues 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 ini-
tial 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 start 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 stop character is taken to be the ter-
minal'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.
skey sequence challenge
The skey command computes a response to the S/Key challenge. See skey(1) for more information on the
S/Key system.
slc state The slc command (Set Local Characters) is used to set or change the state of 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 discrep-
ancies 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 recog-
nized (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", other-
wise FALSE (see stty(1)).
autodecrypt When the TELNET ENCRYPT option is negotiated, by default the actual encryption (decryp-
tion) of the data stream does not start automatically. The autoencrypt (autodecrypt) com-
mand states that encryption of the output (input) stream should be enabled as soon as pos-
sible.
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 character 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 car-
riage return, but never line feeds. 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 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 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
user's 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.
? 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.
Source routing is not supported yet for IPv6.
BSD February 3, 1994 BSD