TELNET(1C)

NAME
telnet - User interface to the TELNET protocol

SYNOPSIS
telnet [-d] [-n tracefile] [-l user] [-a] [-e[escapechar]] [host [port]]

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

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).

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

Options:

-d  Sets the initial value of the debug toggle to TRUE.

-n tracefile
    Opens tracefile for recording trace information. See the set tracefile command below.

[-l user]
    When connecting to the remote system, if the remote system understands the ENVIRON option, then user will be sent to the remote system as the value for the variable USER. This option may also be used with the open command.

[-a]
    When connecting to the remote system, if the remote system understands the ENVIRON option, then the user's login name will be sent to the remote system as the value for the variable USER. This option may also be used with the open command.

-e[escape char]
    Sets the initial telnet escape character to escape char. If escape char is ommitted, then there will be no escape character.

host
    Indicates the official name, an alias, or the Internet address of a remote host.

port
    Indicates a port number (address of an application). If a number is not specified, the default telnet port is used.

The following telnet commands are available. Only enough of each command to uniquely identify it need be typed (this is also true for arguments to the mode, set, toggle, unset, slc, environ, and display commands).

close
    Close a TELNET session and return to command mode.

display [ argument... ]
    Displays all, or some, of the set and toggle values (see below).

mode type
    Type is one of several options, depending on the state of the TELNET session. 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 will be entered.

    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 server at the default port. The host specification may be either a host name (see hosts(5)) or an Internet address specified in the “dot notation” (see inet(3N)). The -l option may be used to specify the user name to be passed to the remote system via the ENVIRON option. When connecting to a non-standard port, telnet omits any automatic initiation of TELNET options. When the port number is preceeded by a minus sign, the inital option negotiation is done. After establishing a connection, the .telnetrc in the users home directory is opened. Lines begining with a # are comment lines. Blank lines are ignored. Lines that begin without whitespace are the start of a machine entry. The first thing on the line is the name of the machine that is being connected to. The rest of the line, and successive lines that begin with whitespace are assumed to be telnet commands and are processed as if they had been typed in manually to the telnet command prompt.

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

send arguments
    Sends one or more special character sequences to the remote host. The following are the arguments which may be specified (more than one argument may be specified at a time):

    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 (initially “^]”).

    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 4.2 BSD system -- if it doesn't work, a lower case “r” may be echoed on the terminal).

    ?
        Prints out help information for the send command.

set argument value

unset arguments...
    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 functions. 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 using the set and unset commands.

    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.

    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.

    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.

    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.

        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.

        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 discrepencies with the local side, the local side will switch to the remote value.

        ?
            Prints out help information for the slc command.

    environ arguments...
        The environ command is used to manipulate the the variables that my be sent through the ENVIRON option. The initial set of variables is taken from the users environment; with only the USER and DISPLAY variables being exported.
        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 exported. The value may be enclosed in single or double quotes so that tabs and spaces may be included.

        undefine variable
            Remove variable from the list of environment variables.

        export variable
            Mark the variable variable to be exported to the remote side.

        unexport variable
            Mark the variable variable to not be exported unless explicitly asked for by the remote side.

        list
            List the current set of environment variables. Those marked with a * will be sent automatically, other variables will only be sent if explicitly requested.

        ?
            Prints out help information for the environ command.

    ?
        Displays the legal set (unset) commands.

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 listed above. More than one argument may be specified. The state of these flags may be interrogated with the display command. Valid arguments are:

    autoflush
        If autoflush and localchars are both TRUE, then when 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 if the terminal user had not done an "stty noflsh", otherwise FALSE (see stty(1)).

    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 superuser). 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 eofand suspend are sent as eofand 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 formated in a more user readable format. Spaces are put between each character in the output, and the begining of any TELNET escape sequence is preceeded by a '*' to aid in locating them.

    ?
        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 ommitted, then an interactive subshell is invoked.

status
    Show the current status of telnet. This includes the peer one is connected to, as well as the current mode.

? [ 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.

FILES
~/.telnetrc

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.