telnet(1)telnet(1)NAMEtelnet - user interface to the TELNET protocol
SYNOPSIS
[[options] host [port]]
DESCRIPTION
is used to communicate with another host using the TELNET protocol. If
is invoked without arguments, it enters command mode, indicated by its
prompt In this mode, it accepts and executes the commands listed below.
If is invoked with arguments, it performs an command (see below) with
those arguments.
Once a connection has been opened, enters an input mode. The input
mode will be either "character at a time" or "line by line" , depending
on what the remote system supports.
In "character at a time" mode, most text typed is immediately sent to
the remote host for processing.
In "line by line" mode, all text is echoed locally, and (normally) only
completed lines are sent to the remote host. The "local echo charac‐
ter" (initially can be used to turn-off and turn-on the local echo
(this would mostly be used to enter passwords without the password
being echoed).
In either mode, if the toggle is TRUE (the default in line mode; see
below), the user's and characters are trapped locally, and sent as TEL‐
NET protocol sequences to the remote side. There are options (see and
below) that cause this action to flush subsequent output to the termi‐
nal (until the remote host acknowledges the TELNET sequence) and flush
previous terminal input (in the case of and
While connected to a remote host, command mode can be entered by typing
the "escape character" (initially When in command mode, the normal ter‐
minal editing conventions are available.
supports 8-bit characters when communicating with the server on the
remote host. To use eight-bit characters you may need to reconfigure
your terminal or the remote host appropriately (see stty(1)). Further‐
more, you may have to use the toggle to enable an 8-bit data stream
between and the remote host.
Note: Some remote hosts may not provide the necessary support
for eight-bit characters.
If, at any time, is unable to read from or write to the server over the
connection, the message is printed on standard error. then exits with
a value of 1.
supports the TAC User ID (also known as the TAC Access Control System,
or TACACS User ID) option. Enabling the option on a host server allows
the user to into that host without being prompted for a second login
sequence. The TAC User ID option uses the same security mechanism as
for authorizing access by remote hosts and users. The system adminis‐
trator must enable the (telnetd) option only on systems, which are des‐
ignated as participating hosts. The system administrator must also
assign to each user of TAC User ID the very same UID on every system
for which he is allowed to use the feature. See the telnetd(1M) man‐
page and the
Options
The following options are available:
Enable cs8 (8 bit transfer) on local tty.
Set the command mode escape character to be instead of its default
value of
Disable the TAC User ID option if enabled on the client,
to cause the user to be prompted for login username and pass‐
word. Omitting the option executes the default setting.
Kerberos-Specific Options
By default, or by use of the or the option, the Kerberos version of
behaves as a client which supports authentication based on Kerberos V5.
As a Kerberos client, will authenticate and authorize the user to
access the remote system. See the sis(5) manpage for details on Ker‐
beros authentication and authorization. However, it will not support
integrity-checked or encrypted sessions.
The default Kerberos options for the applications are set in the con‐
figuration file. Refer to the in the krb5.conf(4) manpage for more
information. The options and described in the subsequent paragraphs,
can be set in the file with the tag names and respectively. Refer to
the of the krb5.conf(4) manpage for more information.
The option can be set in the file within If is set to true and the ker‐
beros authentication fails, will use the non-secure mode of authentica‐
tion.
Note: Command line options override configuration file options.
The following options are Kerboros-specific:
This option is applicable only in a secure environment based
on Kerberos V5. Attempt automatic login into the Kerberos
realm and disable the TAC User ID option.
Note: This is the default login mode.
Sends the user name via the NAME subnegotiation of the
Authentication option. The name used is that of the current
user as returned by the USER environment variable. If this
variable is not defined, the name used is that returned by
(see getpwent(3C)) if it agrees with the current user ID.
Otherwise, it is the name associated with the user ID.
This option is applicable only in a secure environment based
on Kerberos V5. Disable use of Kerberos authentication and
authorization. When this option is specified, a password is
required that is sent across the network in a readable form.
See the sis(5) manpage.
This option is applicable only in a secure environment based
on Kerberos V5. Allows local credentials to be forwarded to
the remote system. Only one or option is allowed. They can‐
not be used together.
This option is applicable only in a secure environment based
on Kerberos V5. Allows local credentials to be forwarded to
the remote system including any credentials that have already
been forwarded into the local environment. Only one or
option is allowed. They cannot be used together.
This option is applicable only in a secure environment
based on Kerberos V5. Attempt automatic login into the Ker‐
beros realm as the specified user and disable the TAC User ID
option. The user name specified is sent via the NAME subne‐
gotiation of the Authentication option. Omitting the option
executes the default setting. Only one option is allowed.
Commands
The following commands are available in command mode. You need to type
only sufficient prefix of each command to uniquely identify it (this is
also true for arguments to the and commands).
Open a connection to the named host at the indicated port.
If no port is specified, attempts to contact a TELNET
server at the standard TELNET port. In the IPv4 envi‐
ronment, hostname can be either the official name or an
alias as understood by (see gethostent(3N)) or an Inter‐
net address specified in the dot notation as described
in hosts(4). In the IPv6 environment, hostname can be
either the official name or an alias as understood by
(see getnameinfo(3N)) or an Internet address specified
in the colon notation as described in hosts(4). If no
hostname is given, prompts for one.
Close a TELNET session. If the session was started from command
mode, returns to command mode; otherwise exits.
Close any open TELNET session and exit An end of file (in command mode)
will also close a session and exit.
Suspend If is run from a shell that supports job control, (such
as csh(1) or ksh(1)), the command suspends the TELNET
session and returns the user to the shell that invoked
Then the job can be resumed by using the command (see
csh(1) or ksh(1)).
Change user input mode to mode, which can be (for "character at
a time" mode) or (for "line by line" mode). 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 is entered. In mode, sends
each character to the remote host as it is typed. In
mode, gathers user input into lines and transmits each
line to the remote host when the user types carriage
return, linefeed, or EOF (normally see stty(1)).
Note: Setting line-mode also sets local echo. Applica‐
tions that expect to interpret user input character by
character (such as and do not work properly in line
mode.
Show current status of
reports the current escape character. If is connected,
it reports the host to which it is connected and the
current If is not connected to a remote host, it reports
Once has been connected, it reports the local flow con‐
trol toggle value.
Displays all or some of the
and values (see below).
Get help. With no arguments, prints a help summary. If a command
is specified, prints the help information available
about that command only. Help information is limited to
a one-line description of the command.
Shell escape. The environment variable is checked for the name of a
shell to use to execute the command. If no shell_com‐
mand is specified, a shell is started and connected to
the user's terminal. If is undefined, is used.
Sends one or more special character sequences to the remote host.
Each argument can have any of the following values (mul‐
tiple arguments can be specified with each command):
Sends the current
escape character (initially
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
to some systems -- if it doesn't work, a lower
case ``r'' may be echoed on the terminal).
Sends the TELNET BRK (Break) sequence, which may have
significance to the remote system.
Sends the TELNET IP (Interrupt Process) sequence, which
should cause the remote system to abort the
currently running process.
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.
Sends the TELNET AYT (Are You There) sequence, to which
the remote system may or may not choose to
respond.
Sends the TELNET EC (Erase Character) sequence, which
should cause the remote system to erase the
last character entered.
Sends the TELNET EL (Erase Line) sequence, which should
cause the remote system to erase the line cur‐
rently being entered.
Sends the TELNET GA (Go Ahead) sequence, which is likely
to have no significance to the remote system.
Sends the TELNET NOP (No OPeration) sequence.
Prints out help information for the
command.
Set any of the variables to a specific value. The special value turns
off the function associated with the variable. The val‐
ues of variables can be shown by using the command. The
following variable_names can be specified:
This is the value (initially
which, toggles between doing local echoing of
entered characters (for normal processing),
and suppressing echoing of entered characters
(for entering, for example, a password) in
line-by-line mode.
This is the
escape character (initially that causes entry
into command mode (when connected to a remote
system).
If is in mode (see below) and the interrupt char‐
acter is typed, a TELNET IP sequence (see
above) is sent to the remote host. The ini‐
tial value for the interrupt character is
taken to be the terminal's character.
If is in mode (see below) and the character is
typed, a TELNET BRK sequence (see above) is
sent to the remote host. The initial value
for the quit character is taken to be the ter‐
minal's character.
If is in mode (see below) and the character is
typed, a TELNET AO sequence (see above) is
sent to the remote host. The initial value
for the flush character is
If is in mode (see below), if is operating in
character-at-a-time mode, then when this char‐
acter is typed, a TELNET EC sequence (see
above) is sent to the remote system. The ini‐
tial value for the erase character is taken to
be the terminal's character.
If is in mode (see below), if is operating in
character-at-a-time mode, then when this char‐
acter is typed, a TELNET EL sequence (see
above) is sent to the remote system. The ini‐
tial value for the kill character is taken to
be the terminal's character.
If is operating in line-by-line mode, entering
this character as the first character on a
line causes this character to be sent to the
remote system. The initial value of the char‐
acter is taken to be the terminal's character.
Toggle (between
TRUE and FALSE ) various flags that control how responds
to events. More than one argument can be specified.
The state of these flags can be shown by using the com‐
mand. Valid arguments are:
If TRUE, the and characters (see above) are rec‐
ognized locally, and transformed into appro‐
priate TELNET control sequences (respectively
and see above). The initial value for this
toggle is in line-by-line mode, and in charac‐
ter-at-a-time mode.
If and are both TRUE, whenever the or characters
are recognized (and transformed into TELNET
sequences − see above for details), 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.
This flag is available only in a secure environment
based on Kerberos V5.
Enable or disable automatic login into the
Kerberos realm. Using this option yields the
same results as using the option. The initial
value for this toggle is TRUE.
If and are both TRUE, when either the or charac‐
ter is typed (see above for descriptions of
the and characters), the resulting TELNET
sequence sent is followed by the TELNET SYNCH
sequence. This procedure causes the remote
system to begin discarding all previously
typed input until both of the TELNET sequences
have been read and acted upon. The initial
value of this toggle is FALSE.
Enable or disable the
TELNET BINARY option on both input and output.
This option should be enabled in order to send
and receive 8-bit characters to and from the
TELNET server.
If TRUE, end-of-line sequences are sent as an
ASCII carriage-return and line-feed pair. If
FALSE, end-of-line sequences are sent as an
ASCII carriage-return and NUL character pair.
The initial value for this toggle is FALSE.
Toggle carriage return mode.
When this mode is enabled, any carriage return
characters received from the remote host are
mapped into a carriage return and a line feed.
This mode does not affect those characters
typed by the user; only those received. This
mode is only required for some hosts that
require the client to do local echoing, but
output ``naked'' carriage returns. The ini‐
tial value for this toggle is FALSE.
Toggle local echo mode or remote echo mode.
In local echo mode, user input is echoed to
the terminal by the local before being trans‐
mitted to the remote host. In remote echo,
any echoing of user input is done by the
remote host. Applications that handle echoing
of user input themselves, such as C shell,
Korn shell, and (see csh(1), ksh(1), and
vi(1)), do not work correctly with local echo.
Toggle viewing of
TELNET options processing. When options view‐
ing is enabled, all TELNET option negotiations
are displayed. Options sent by are displayed
as while options received from the TELNET
server are displayed as The initial value for
this toggle is FALSE.
Toggles the display of all network data (in hexadecimal
format).
The initial value for this toggle is FALSE.
Displays the legal
commands.
RETURN VALUE
In the event of an error, or if the TELNET connection is closed by the
remote host, returns a value of Otherwise, it returns
DIAGNOSTICS
The following diagnostic messages are displayed by
There are two authentication mechanisms used by TELNET.
One authentication mechanism is based on Kerberos and the
other is not. The type of authentication mechanism is
obtained from a system file, which is updated by If the
system file on either the local host or the remote host
does not contain known authentication types, the above
error is displayed.
was unable to find the
TELNET service entry in the services(4) database.
was unable to map the host name to an Internet address.
Your next step should be to contact the system adminis‐
trator to check whether there is an entry for the remote
host in the database (see hosts(4)).
An invalid command was typed in
command mode.
An error occurred in the specified system call.
See the appropriate manual entry for a description of the
error.
AUTHOR
was developed by the University of California, Berkeley.
SEE ALSOcsh(1), ksh(1), login(1), rlogin(1), stty(1), telnetd(1M),
inetsvcs_sec(1M), hosts(4), krb5.conf(4), services(4), sis(5),
termio(7).
telnet(1)
