NAME
telnet - user interface to the TELNET protocol
SYNOPSIS
telnet [-468EKLadr] [-S tos] [-X authtype] [-b address] [-e escapechar]
[-l user] [-n tracefile] [-z option] [host [port]]
DESCRIPTION
The telnet command is used for interactive communication with another
host using the TELNET protocol. It begins in command mode, where it
prints a telnet prompt ("telnet> "). If telnet is invoked with a host
argument, it performs an open command implicitly; see the description
below.
Options:
-4 Force IPv4 address resolution.
-6 Force IPv6 address resolution.
-8 Request 8-bit operation. This causes an attempt to negotiate the
TELNET BINARY option for both input and output. By default telnet
is not 8-bit clean.
-E Disables the escape character functionality; that is, sets the
escape character to ‘‘no character’’.
-K Specifies no automatic login to the remote system.
-L Specifies an 8-bit data path on output. This causes the TELNET
BINARY option to be negotiated on just output.
-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 username is retrieved via getlogin(3).
-b address
Use bind(2) on the local socket to bind it to a specific local
address.
-d Sets the initial value of the debug toggle to TRUE.
-r Emulate rlogin(1). In this mode, the default escape character is
a tilde. Also, the interpretation of the escape character is
changed: an escape character followed by a dot causes telnet to
disconnect from the remote host. A ^Z instead of a dot suspends
telnet, and a ^] (the default telnet escape character) generates
a normal telnet prompt. These codes are accepted only at the
beginning of a line.
-S tos Sets the IP type-of-service (TOS) option for the telnet
connection to the value tos.
-e escapechar
Sets the escape character to escapechar. If no character is
supplied, no escape character will be used. Entering the escape
character while connected causes telnet to drop to command mode.
-l user
Specify user as the user to log in as on the remote system. This
is accomplished by sending the specified name as the USER
environment variable, so it requires that the remote system
support the TELNET ENVIRON option. This option implies the -a
option, and may also be used with the open command.
-n tracefile
Opens tracefile for recording trace information. See the set
tracefile command below.
-z option
Set SSL (Secure Socket Layer) parameters. The default is to
negotiate via telnet protocol if SSL is available at server side
and then to switch it on. In this mode you can connect to both
conventional and SSL enhanced telnetd’s. If the connection is
made to localhost and -z secure is not set, then SSL is not
enabled.
The SSL parameters are:
debug Send SSL related debugging information to stderr.
authdebug Enable authentication debugging.
ssl Negotiate SSL at first, then use telnet protocol. In
this mode you can connect to any server supporting
directly SSL like Apache-SSL. Use telnet -z ssl
ssl3.netscape.com https for example. telnet protocol
negotiation goes encrypted.
nossl, !ssl
switch off SSL negotiation
certrequired
server certificate is mandatory
secure Don’t switch back to unencrypted mode (no SSL) if SSL
is not available.
verbose Be verbose about certificates etc.
verify=int Set the SSL verify flags (SSL_VERIFY_* in ssl/ssl.h
).
cert=cert_file
Use the certificate(s) in cert_file.
key=key_file
Use the key(s) in key_file.
cipher=ciph_list
Set the preferred ciphers to ciph_list. (See
ssl/ssl.h ).
host Specifies a host to contact over the network.
port Specifies a port number or service name to contact. If not
specified, the telnet port (23) is used.
Protocol:
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).
Commands:
The following telnet commands are available. Unique prefixes are
understood as abbreviations.
auth argument ...
The auth command controls the TELNET AUTHENTICATE protocol
option. If telnet was compiled without authentication, the
auth command will not be supported. Valid arguments are as
follows:
disable type Disable the specified type of authentication.
To obtain a list of available types, use the
auth disable ? command.
enable type Enable the specified type of authentication. To
obtain a list of available types, use the auth
enable ? command.
status List the current status of the various types of
authentication.
close Close the connection to the remote host, if any, and return to
command mode.
display argument ...
Display all, or some, of the set and toggle values (see
below).
environ arguments...
The environ command is used to propagate environment variables
across the telnet link using the TELNET ENVIRON protocol
option. All variables exported from the shell are defined,
but only the DISPLAY and PRINTER variables are marked to be
sent by default. The USER variable is marked to be sent if
the -a or -l command-line options were 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 marked for propagation
(‘‘exported’’). The value may be enclosed in
single or double quotes so that tabs and spaces
may be included.
undefine variable
Remove any existing definition of variable.
export variable
Mark the specified variable for propagation to the
remote host.
unexport variable
Do not mark the specified variable for propagation
to the remote host. The remote host may still ask
explicitly for variables that are not exported.
list List the current set of environment variables.
Those marked with a * will be propagated to the
remote host. The remote host may still ask
explicitly for the rest.
? Prints out help information for the environ
command.
logout Send the TELNET LOGOUT protocol option to the remote host.
This command is similar to a close command. If the remote host
does not support the LOGOUT option, nothing happens. But if
it does, this command should cause it to close the connection.
If the remote side also supports the concept of suspending a
user’s session for later reattachment, the logout command
indicates that the session should be terminated immediately.
mode type Type is one of several options, depending on the state of the
session. Telnet asks the remote host to go into the requested
mode. If the remote host says it can, that mode takes effect.
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 daemon at
the standard port (23). The host specification may be a host
name or IP address. The -l option may be used to specify a
user name to be passed to the remote system, like the -l
command-line option.
When connecting to ports other than the telnet port, telnet
does not attempt telnet protocol negotiations. This makes it
possible to connect to services that do not support the telnet
protocol without making a mess. Protocol negotiation can be
forced by placing a dash before the port number.
After establishing a connection, any commands associated with
the remote host in /etc/telnetrc and the user’s .telnetrc file
are executed, in that order.
The format of the telnetrc files is as follows: Lines
beginning with a #, and blank lines, are ignored. The rest of
the file should consist of hostnames and sequences of telnet
commands to use with that host. Commands should be one per
line, indented by whitespace; lines beginning without
whitespace are interpreted as hostnames. Lines beginning with
the special hostname ‘DEFAULT’ will apply to all hosts.
Hostnames including ‘DEFAULT’ may be followed immediately by a
colon and a port number or string. If a port is specified it
must match exactly with what is specified on the command line.
If no port was specified on the command line, then the value
‘telnet’ is used. Upon connecting to a particular host, the
commands associated with that host are executed.
quit Close any open session and exit telnet. An end of file
condition on input, when in command mode, will trigger this
operation as well.
send arguments
Send one or more special telnet protocol character sequences
to the remote host. The following are the codes which may be
specified (more than one may be used in one command):
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.
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 variables.
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.
ayt If telnet is in localchars mode, or LINEMODE is
enabled, and the status character is typed, a TELNET
AYT sequence 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 mode escape character. Setting it
enables rlogin mode, as with the r command-line option
(q.v.)
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.
startssl Attempt to negotiate telnet-over-SSL (as with the -z ssl
option). This is useful when connecting to non-telnetds such
as imapd (with the STARTTLS command). To control SSL when
connecting to a SSL-enabled telnetd, use the auth command
instead.
status Show the current status of telnet. This includes the name of
the remote host, if any, 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. More than
one flag may be toggled at once. The state of these flags may
be examined with the display command. Valid flags are:
authdebug Turns on debugging for the authentication code.
This flag only exists if authentication support
is enabled.
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)).
autologin If the remote side supports the TELNET
AUTHENTICATION option, telnet attempts to use it
to perform automatic authentication. If the
TELNET AUTHENTICATION option is not supported,
the user’s login name is propagated using the
TELNET ENVIRON option. Setting this flag is the
same as specifying the a option to the open
command or on the command line.
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.
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 telnet escape sequences are preceded by a ’*’
to aid in locating them.
skiprc When the skiprc toggle is TRUE, telnet does not
read the telnetrc files. 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.
? 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
/etc/telnetrc global telnet startup values
~/.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.
BUGS
The source code is not comprehensible.