telnet(1)

NAME

telnet — user interface to the TELNET protocol

SYNOPSIS

telnet [[options] host [port]]

DESCRIPTION

telnet is used to communicate with another host using the TELNET protocol. If telnet is invoked without arguments, it enters command mode, indicated by its prompt (telnet>). In this mode, it accepts and executes the commands listed below. If telnet is invoked with arguments, it performs an open command (see below) with those arguments.

Once a connection has been opened, telnet 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 character" (initially ^E) 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 localchars toggle is TRUE (the default in line mode; see below), the user's quit and intr characters are trapped locally, and sent as TELNET protocol sequences to the remote side. There are options (see toggle autoflush and toggle autosynch 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 quit and intr).

While connected to a remote host, telnet command mode can be entered by typing the telnet "escape character" (initially ^]). When in command mode, the normal terminal editing conventions are available.

telnet 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 binary toggle to enable an 8-bit data stream between telnet and the remote host.

Note: Some remote hosts may not provide the necessary support for eight-bit characters.

If, at any time, telnet is unable to read from or write to the server over the connection, the message Connection closed by foreign host. is printed on standard error. telnet then exits with a value of 1.

telnet 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 telnet into that host without being prompted for a second login sequence. The TAC User ID option uses the same security mechanism as rlogin 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 HP-UX System Administrator's Guide.

Options

The following telnet options are available:

-8: Enable cs8 (8 bit transfer) on local tty.
-e c: Set the telnet command mode escape character to be ^c instead of its default value of ^].
-l: 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 -l option executes the default setting.

Kerberos-Specific Options

By default, or by use of the -a or the -l option, the Kerberos version of telnet behaves as a client which supports authentication based on Kerberos V5. As a Kerberos client, telnet 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 krb5.conf configuration file. Refer to the appdefaults Section in the krb5.conf(4) manpage for more information. The options -a, -f, and -F described in the subsequent paragraphs, can be set in the krb5.conf file with the tag names autologin, forward, and forwardable respectively. Refer to the appdefaults Section of the krb5.conf(4) manpage for more information.

The fallback option can be set in the krb5.conf file within appdefaults Section. If fallback is set to true and the kerberos authentication fails, telnet will use the non-secure mode of authentication.

Note: Command line options override configuration file options.

The following options are Kerboros-specific:

-a

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 getpwnam() (see getpwent(3C)) if it agrees with the current user ID. Otherwise, it is the name associated with the user ID.

-P

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.

-f

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 -f or -F option is allowed. They cannot be used together.

-F

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 -f or -F option is allowed. They cannot be used together.

-l user

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 -l option executes the default setting. Only one -l 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 mode, set, toggle, and display commands).

open host [port]

Open a connection to the named host at the indicated port. If no port is specified, telnet 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 gethostbyname() (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 getnameinfo() (see getnameinfo(3N)) or an Internet address specified in the colon notation as described in hosts(4). If no hostname is given, telnet prompts for one.

close

Close a TELNET session. If the session was started from command mode, telnet returns to command mode; otherwise telnet exits.

quit

Close any open TELNET session and exit telnet. An end of file (in command mode) will also close a session and exit.

z

Suspend telnet. If telnet is run from a shell that supports job control, (such as csh(1) or ksh(1)), the z command suspends the TELNET session and returns the user to the shell that invoked telnet. Then the job can be resumed by using the fg command (see csh(1) or ksh(1)).

mode mode

Change telnet's user input mode to mode, which can be character (for "character at a time" mode) or line (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 character mode, telnet sends each character to the remote host as it is typed. In line mode, telnet gathers user input into lines and transmits each line to the remote host when the user types carriage return, linefeed, or EOF (normally ^D; see stty(1)).

Note: Setting line-mode also sets local echo. Applications that expect to interpret user input character by character (such as more, csh, ksh, and vi) do not work properly in line mode.

status

Show current status of telnet. telnet reports the current escape character. If telnet is connected, it reports the host to which it is connected and the current mode. If telnet is not connected to a remote host, it reports No connection. Once telnet has been connected, it reports the local flow control toggle value.

display [argument ...]

Displays all or some of the set and toggle values (see below).

? [command]

Get help. With no arguments, telnet prints a help summary. If a command is specified, telnet prints the help information available about that command only. Help information is limited to a one-line description of the command.

! [shell_command]

Shell escape. The SHELL environment variable is checked for the name of a shell to use to execute the command. If no shell_command is specified, a shell is started and connected to the user's terminal. If SHELL is undefined, /usr/bin/sh is used.

send arguments

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 send command):

escape: Sends the current telnet escape character (initially ^]).
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 to some systems -- if it doesn't work, a lower case ``r'' may be echoed on the terminal).
brk: Sends the TELNET BRK (Break) sequence, which may have significance to the remote system.
ip: Sends the TELNET IP (Interrupt Process) sequence, which should cause the remote system to abort the currently running process.
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.
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.
ga: Sends the TELNET GA (Go Ahead) sequence, which is likely to have no significance to the remote system.
nop: Sends the TELNET NOP (No OPeration) sequence.
?: Prints out help information for the send command.

set variable_name value

Set any of the telnet variables to a specific value. The special value off turns off the function associated with the variable. The values of variables can be shown by using the display command. The following variable_names can be specified:

echo: This is the value (initially ^E) 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.
escape: This is the telnet escape character (initially ^]) that causes entry into telnet command mode (when connected to a remote system).
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.
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.
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 ^O.
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.
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.
eof: If telnet 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 eof character is taken to be the terminal's eof character.

toggle arguments ...

Toggle (between TRUE and FALSE ) various flags that control how telnet responds to events. More than one argument can be specified. The state of these flags can be shown by using the display command. Valid arguments are:

localchars: If TRUE, the flush, interrupt, quit, erase, and kill characters (see set above) are recognized locally, and transformed into appropriate TELNET control sequences (respectively ao, ip, brk, ec, and el; see send above). The initial value for this toggle is TRUE in line-by-line mode, and FALSE in character-at-a-time mode.
autoflush: If autoflush and localchars are both TRUE, whenever the ao, intr, 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.
autologin: 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 -a option. The initial value for this toggle is TRUE.
autosynch: If autosynch and localchars are both TRUE, 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 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.
binary: 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.
crlf: 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.
crmod: 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.
echo: Toggle local echo mode or remote echo mode. In local echo mode, user input is echoed to the terminal by the local telnet 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 vi (see csh(1), ksh(1), and vi(1)), do not work correctly with local echo.
options: Toggle viewing of TELNET options processing. When options viewing is enabled, all TELNET option negotiations are displayed. Options sent by telnet are displayed as ``SENT'', while options received from the TELNET server are displayed as ``RCVD''. The initial value for this toggle is FALSE.
netdata: Toggles the display of all network data (in hexadecimal format). The initial value for this toggle is FALSE.
?: Displays the legal toggle commands.

RETURN VALUE

In the event of an error, or if the TELNET connection is closed by the remote host, telnet returns a value of 1. Otherwise, it returns 0.

DIAGNOSTICS

The following diagnostic messages are displayed by telnet:

Error! Could not retrieve authentication type.: 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 inetsvcs_sec. If the system file on either the local host or the remote host does not contain known authentication types, the above error is displayed.
telnet/tcp: Unknown service: telnet was unable to find the TELNET service entry in the services(4) database.
hostname: Unknown host: telnet 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 hosts database (see hosts(4)).
?Invalid command: An invalid command was typed in telnet command mode.
system call>: ...: An error occurred in the specified system call. See the appropriate manual entry for a description of the error.

AUTHOR

telnet was developed by the University of California, Berkeley.