NAME
telnet — user interface to the TELNET protocol
SYNOPSIS
telnet [-468ELadr] [-S tos] [-b address] [-e escapechar] [-l user]
[-n tracefile] [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’’.
-L Specifies an 8-bit data path on output. This causes the TELNET BINARY option to be negotiated on just output.
-a Attempt automatic login. Currently, this sends the user name via the USER variable of the NEW-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 connec‐ tion to the value tos.
-e escapechar Sets the escape character to escapechar. If no character is sup‐ plied, 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 envi‐ ronment variable, so it requires that the remote system support the TELNET NEW-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.
host Specifies a host to contact over the network.
port Specifies a port number or service name to contact. If not speci‐ fied, 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 sys‐ tem, under the control of the remote system. When input editing or char‐ acter echoing is to be disabled, the remote system will relay that infor‐ mation. The remote system will also relay changes to any special charac‐ ters that happen on the remote system, so that they can take effect on 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 char‐ acter” (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).
Commands:
The following telnet commands are available. Unique prefixes are under‐ stood 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.
Note that the current version of telnet does not support
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).
encrypt argument … The encrypt command controls the TELNET ENCRYPT protocol option. If telnet was compiled without encryption, the encrypt command will not be supported.
Valid arguments are as follows:
disable type [input|output]
Disable the specified type of encryption. If
you do not specify input or output, encryption
of both is disabled. To obtain a list of avail‐
able types, use ``encrypt disable ?''.
enable type [input|output]
Enable the specified type of encryption. If you
do not specify input or output, encryption of
both is enabled. To obtain a list of available
types, use ``encrypt enable ?''.
input This is the same as ``encrypt start input''.
-input This is the same as ``encrypt stop input''.
output This is the same as ``encrypt start output''.
-output This is the same as ``encrypt stop output''.
start [input|output]
Attempt to begin encrypting. If you do not
specify input or output, encryption of both
input and output is started.
status Display the current status of the encryption
module.
stop [input|output]
Stop encrypting. If you do not specify input or
output, encryption of both is stopped.
type type Sets the default type of encryption to be used
with later ``encrypt start'' or ``encrypt stop''
commands.
Note that the current version of telnet does not support
encryption.
environ arguments… The environ command is used to propagate environment variables across the telnet link using the TELNET NEW-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 sin‐
gle 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 explic‐
itly for the rest.
? Prints out help information for the environ com‐
mand.
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 com‐
mand.
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 com‐ mand-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 begin‐
ning with a #, and blank lines, are ignored. The rest of the
file should consist of hostnames and sequences of telnet com‐
mands 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 includ‐
ing ‘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 condi‐ tion on input, when in command mode, will trigger this opera‐ tion 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 char‐
acter entered.
el Sends the TELNET EL (Erase Line) sequence, which
should cause the remote system to erase the line cur‐
rently being entered.
eof Sends the TELNET EOF (End Of File) sequence.
eor Sends the TELNET EOR (End of Record) sequence.
escape Sends the current telnet escape character.
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 cur‐
rently 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 variables. 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.
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 termi‐
nal'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 enter‐
ing, 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
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 char‐
acter is taken to be the terminal's erase character.
escape This is the telnet escape character (initially “^]”)
which causes entry into telnet command mode (when con‐
nected to a remote system).
flushoutput
If telnet is in localchars mode (see toggle localchars
below) and the flushoutput character is typed, a
TELNET AO sequence (see send ao above) is sent to the
remote host. The initial value for the flush charac‐
ter is taken to be the terminal's flush character.
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 termi‐
nal'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 charac‐
ter is taken to be the terminal's kill character.
lnext If telnet is operating in LINEMODE or “old line by
line“ mode, then this character is taken to be the
terminal's lnext character. The initial value for the
lnext character is taken to be the terminal's lnext
character.
quit If telnet is in localchars mode (see toggle localchars
below) and the quit character is typed, a TELNET BRK
sequence (see send brk above) is sent to the remote
host. The initial value for the quit character is
taken to be the terminal's quit character.
reprint
If telnet is operating in LINEMODE or “old line by
line“ mode, then this character is taken to be the
terminal's reprint character. The initial value for
the reprint character is taken to be the terminal's
reprint character.
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 termi‐
nal's start character. The initial value for the kill
character is taken to be the terminal's start charac‐
ter.
stop If the TELNET TOGGLE-FLOW-CONTROL option has been
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 charac‐
ter.
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 char‐
acter 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 spe‐
cial 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 char‐
acters. 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 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)).
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 that this flag exists only if encryption
support is enabled.
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 NEW-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 car‐
riage returns will be send as <CR><NUL>. The
initial value for this toggle is FALSE.
crmod Toggle carriage return mode. When this mode is
enabled, most carriage return characters
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 encryp‐
tion code. Note that this flag only exists if
encryption support is available.
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 pro‐
tocol 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.
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. This flag only exists if
encryption support is available.
? 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 informa‐ tion for just that command.
ENVIRONMENT
Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environment vari‐
ables. Other environment variables may be propagated to the other side
via the TELNET NEW-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.