telnet(1) General Commands Manual telnet(1)
NAME
telnet - 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 char-
acter" (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
TELNET protocol sequences to the remote side. There are options (see and below) that 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 and
While connected to a remote host, command mode can be entered by typing the "escape character" (initially When in command mode, the normal
terminal 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)). Furthermore, 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 mecha-
nism as for authorizing access by remote hosts and users. The system administrator must enable the (telnetd) option only on systems, which
are designated 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) manpage 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 password. 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 Kerberos
authentication and authorization. However, it will not support integrity-checked or encrypted sessions.
The default Kerberos options for the applications are set in the configuration 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 kerberos authentication fails, will use the non-secure mode of authenti-
cation.
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 cannot 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 Kerberos realm as the specified user and disable the TAC User ID option.
The user name specified is sent via the NAME subnegotiation 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 environment, hostname
can be either the official name or an alias as understood by (see gethostent(3N)) or an Internet 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. Applications 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 control 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 com-
mand 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_command is speci-
fied, 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 (multiple 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 currently 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 values of vari-
ables 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 character is typed, a TELNET IP sequence (see above) is sent to the
remote host. The initial 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 terminal'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 character is typed, a TELNET
EC sequence (see above) is sent to the remote system. The initial 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 character is typed, a TELNET
EL sequence (see above) is sent to the remote system. The initial 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 character 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 command. Valid arguments are:
If TRUE, the and characters (see above) are recognized locally, and transformed into appropriate 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 TEL-
NET 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 character 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 initial 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 transmitted 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 viewing 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 administrator 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 ALSO
csh(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)