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telnet(1) General Commands Manual telnet(1) NAME telnet - Provides the TELNET interface for logging in to a remote host SYNOPSIS telnet [-d] [-l user] [-n trace_file] [host] [port] The telnet command implements the TELNET protocol, which allows remote login to other hosts. OPTIONS Turns debugging mode on. Sends user to the remote system as the value for the variable USER when connecting to the remote system, if the re- mote system understands the ENVIRON option. This option can also be used with the open command. Records network trace information in the file specified by trace_file. DESCRIPTION The telnet command uses the Transmission Control Protocol/Internet Pro- tocol (TCP/IP) to communicate with other hosts in the network. The tel- net command operates in two different modes: command mode and input mode. When issued without arguments, telnet enters command mode, as indicated by the telnet> prompt. Command mode can also be entered by typing the telnet Escape character (initially ^]). When in command mode, the normal terminal editing conventions are available. In command mode, the subcommands listed in the SUBCOMMANDS section can be entered. Some of these subcommands return you to the remote session upon completion. For those that do not, pressing <Return> returns you to the remote session. If telnet is issued with arguments, it performs an open subcommand with those arguments, then enters input mode. Once a connection is opened, telnet attempts to enable the TELNET LINEMODE option. If this fails, telnet reverts to one of two input modes: either character-at-a-time or old line-by-line mode, 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 relays that in- formation. The remote system also relays changes to any special char- acters 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 (in most cases) only completed lines are sent to the remote host. The local echo character (initially ^E) can be used to turn off and on the local echo (this would mostly be used to enter passwords without the password be- ing 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 BRK. 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 acknowl- edges the TELNET sequence) and flush previous terminal input (in the case of quit and intr). The telnet command uses the default Type-of-Service value recommended by RFC1060, which is as follows: telnet Low delay You can configure this value by specifying it in the /etc/iptos file. For more information, see iptos(4). SUBCOMMANDS For each of the subcommands in the following list, you only need to type enough letters to uniquely identify the command. (For example, q is sufficient for the quit subcommand.) This is also true for the argu- ments to the mode, set, toggle, unset, slc, and display commands. The subcommands for telnet are as follows: Requests help on telnet. Without arguments, telnet prints a help summary. If a subcommand is specified, telnet prints help information for just that subcommand. Closes the telnet connection and returns to command mode. Displays all of the set and toggle values if no argument is specified; otherwise, lists only those values that match argument. Manipulates the variables that can be sent through the telnet ENVIRON option. The initial set of variables is taken from the user's environment, with only the USER and DISPLAY variables being exported. Valid arguments for the environ subcommand are as follows: De- fines variable to have the specified value. Any variables de- fined by this command are automatically exported. value can be enclosed in single or double quotes so that tabs and spaces can be included. Removes variable from the list of environment variables. Marks variable to be exported to the remote side. Marks variable to not be exported unless explicitly asked for by the remote side. Lists the current set of environment vari- ables. Those marked with an * (asterisk) are sent automati- cally; other variables are sent only if explicitly requested. The type option is one of several options, depending on the state of the TELNET session. The remote host is asked for per- mission to go into the requested mode. If the remote host is ca- pable of entering that mode, the requested mode is entered. Prints out help information for the mode command. Disables the LINEMODE option, or, if the remote side does not understand the LINEMODE option, enters character-at-a-time mode. Enables the LINEMODE option, or, if the remote side does not understand the LINEMODE option, then attempts to enter old line-by-line mode. Attempts to enable (disable) the TRAPSIG mode of the LINEMODE option. This requires that the LINEMODE option be enabled. At- tempts to enable (disable) the EDIT mode of the LINEMODE option. This requires that the LINEMODE option be enabled. Attempts to enable (disable) the SOFT_TAB mode of the LINEMODE option. This requires that the LINEMODE option be enabled. Attempts to en- able (disable) the LIT_ECHO mode of the LINEMODE option. This requires that the LINEMODE option be enabled. Opens a connec- tion to the specified host. The host specification can be ei- ther a hostname or an Internet address in dotted decimal form. If no port is given, telnet attempts to contact a TELNET server at the default port. When connecting to a nonstandard port, telnet omits any auto- matic initiation of TELNET options. When the port number is preceded by a - (dash), the initial option negotiation is done. After establishing a connection, the file in the user's home di- rectory is opened. Lines beginning with a # (number sign) are comment lines. Blank lines are ignored. Lines that begin without empty spaces are the start of a machine entry. The first thing on the line is the name of the machine to which the user is connected. The rest of the line and successive lines that begin with empty spaces are assumed to be telnet commands and are processed as if they were typed in manually to the telnet> command prompt. Closes a TELNET connection and exits telnet. An End-of-File in command mode also closes the connection and exits. Sends one or more arguments (special character sequences) to the remote host. (Not all hosts will respond to all of these sequences.) Multi- ple arguments are separated by spaces. The following arguments can be used: Prints help information for the send subcommand. Sends the TELNET ABORT (Abort Processes) sequence. Sends the TELNET AO (Abort Output) sequence, which causes the remote host to flush all output from the remote system to the local termi- nal. Sends the TELNET AYT (Are You There) sequence, to which the remote system can respond. Sends the TELNET BRK (Break) se- quence, which might have significance to the remote system. Sends the TELNET EC (Erase Character) sequence, which causes the remote host to erase the last character entered. Sends the TEL- NET EL (Erase Line) sequence, which causes the remote system to erase the line currently being entered. Sends the TELNET EOF (End-of-File) sequence. Sends the TELNET EOR (End-of-Record) sequence. Sends the current TELNET Escape character (^] by de- fault). Sends the TELNET GA (Go Ahead) sequence, which provides the remote system with a mechanism to signal the local system to return control to the user. If the remote side supports the TELNET STATUS command, getstatus sends the subnegotiation to re- quest that the server send its current option status. Sends the TELNET IP (Interrupt Process) sequence, which causes the remote system to terminate the currently running process. Sends the TELNET NOP (No Operation) sequence. Sends the TELNET SUSP (Sus- pend Process) sequence. Sends the TELNET SYNC sequence, which causes the remote system to discard all previously typed input that has not yet been read. This sequence is sent as TCP urgent data. Sets a telnet variable to the specified value or to true. The off special value turns off the function associated with the variable name entered; this is equivalent to using the unset command. The unset command disables or sets to false any of the specified functions. The values of variables can be queried with the display subcommand. The variables that can be set or unset, but not toggled, are listed here. In addition, any of the variables for the toggle subcommand can be explicitly set or unset by using the set and unset commands. Displays the legal set (unset) commands. Toggles between enabling and suppressing local echo of entered characters. Local echo is used for normal processing, while suppressing the echo is used for entering text that should not appear on the display, such as passwords. This variable is initially ^E, and can only be used in line-by-line mode. Defines the End-of-File character for telnet. When tel- net is in line-by-line mode, entering the eof character as the first character on a line sends the character to the remote host. The initial value for the eof character is the local ter- minal's End-of-File character. Defines the erase character for telnet. When telnet is in character-at-a-time mode and lo- calchars is true, typing the erase character sends the TELNET EC sequence to the remote host. The initial value for the erase character is the local terminal's erase character. Specifies the telnet escape character (initially ^]), which puts telnet into command mode when connected to a remote host. Defines the flush character for telnet. When localchars is true, typing the flushoutput character sends the TELNET AO sequence to the remote host. The initial value for the flush character is the termi- nal's flush character. Defines alternate end-of-line character. Defines the interrupt character for telnet. When localchars is true, typing the interrupt character sends the TELNET IP se- quence to the remote host. The initial value for the interrupt character is the local terminal's interrupt character. Defines the kill character for telnet. When telnet is in character-at- a-time mode and localchars is true, typing the kill character sends the TELNET EL sequence to the remote host. The initial value for the kill character is the local terminal's kill char- acter. Defines the lnext (literal next) character for telnet. If telnet is operating in old line-by-line mode and localchars is true, this character is taken to be the terminal's lnext character. The initial value for the lnext character is the lo- cal terminal's lnext character. Defines the quit character for telnet. When localchars is true, typing the quit character sends the TELNET BRK sequence to the remote host. The initial value for the quit character is the local terminal's quit char- acter. Defines the terminal's reprint character, if telnet is operating in LINEMODE or old line-by-line mode. The initial value for the reprint character is the terminal's reprint char- acter. Defines the terminal's start character, if the TELNET TOGGLE-FLOW-CONTROL option was enabled. The initial value for the start character is the terminal's start character. Defines the terminal's stop character, if the TELNET TOGGLE-FLOW-CONTROL option was enabled. The initial value for the stop character is the terminal's stop character. Sends a TELNET SUSP sequence (see send susp) to the remote host, if telnet is in localchars mode, or LINEMODE is enabled, and the suspend character is typed. The initial value for the suspend character is the termi- nal's suspend character. Specifies the file to which the out- put, caused by netdata or option tracing being true, is written. If it is set to a - (dash), then tracing information is written to standard output (the default). Defines the terminal's worderase character, if telnet is operating in LINEMODE or old line-by-line mode. The initial value for the worderase charac- ter is taken to be the terminal's worderase character. Sets or changes the state of the special characters when the TELNET LINEMODE option is enabled (Set Local Characters). Special characters are characters that get mapped to TELNET commands se- quences (like ip or quit) or line editing characters (like erase and kill). By default, the local special characters are ex- ported. Switches to the local defaults for the special charac- ters. The local default characters are those of the local ter- minal at the time when telnet was started. Switches to the re- mote defaults for the special characters. The remote default characters are those of the remote system at the time when the TELNET connection was established. Verifies the current set- tings for the current special 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 lo- cal side switches to the remote value. Prints out help informa- tion for the slc command. Shows the current status of telnet. This includes the host to which you are connected, as well as the current mode. Toggles one or more arguments that control how telnet responds to events. Possible values are true and false. These options can be explicitly set to true or false with the set and unset subcommands. Multiple arguments are separated by spaces. The display subcommand can be used to query the cur- rent setting of each argument. The following arguments can be used: Displays valid arguments to toggle. If autoflush and localchars are both true and the AO, interrupt, and quit characters are recognized and transformed into TELNET sequences, telnet does not display any data on the user's terminal until the remote system acknowledges (with a TELNET timing mark option) that it has processed those TELNET sequences. The initial value of autoflush is true if the termi- nal has not done an stty noflsh, and false if it has. If auto- synch and localchars are both true, then typing the interrupt or quit character sends that character's TELNET sequence, followed by the TELNET SYNC sequence. This procedure causes the remote host to discard all previously typed input until both of the telnet sequences are read and acted upon. The initial value of this toggle is false. Enables or disables the TELNET BINARY op- tion on both input and output. Enables or disables the TELNET BINARY option on input. Enables or disables the TELNET BINARY option on output. Toggles carriage-return feature. When true, carriage-returns are sent as carriage-return/linefeed. When false, carriage-returns are sent as carriage-return/NULL. The initial value for this toggle is false. Toggles carriage-return mode. When set to true, most carriage-return characters re- ceived from the remote host are mapped into a carriage-return followed by a linefeed. This mode does not affect the charac- ters typed by the user, only those received from the remote host. This mode is useful when the remote host sends only a carriage-return and not a linefeed. The initial value of this toggle is false. Toggles debugging at the socket level. This argument can only be entered by a user with superuser privi- leges. The initial value of this toggle is false. Determines the handling of telnet special characters. When this value is true, the erase, flush, interrupt, kill, and quit characters are recognized locally and transformed into the appropriate TELNET control sequences (EC, AO, IP, BRK, and EL, respectively). When this value is false, these special characters are sent to the remote host as literal characters. The initial value of lo- calchars is true in 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, quit is sent as abort, and eof and suspend are sent as eof and susp (see the send subcommand). Toggles the display of all network data (in hexadecimal format). The data is written to standard output unless a trace_file is specified with the -n option on the telnet command line. The initial value of this toggle is false. Toggles the display of internal TELNET protocol processing options, such as terminal negotiation and local or remote echo of characters. The initial value of this toggle is false, indicating that the current options are not displayed. When the netdata toggle is enabled and prettydump is also enabled, the output from the netdata toggle appears in a more readable format. Spaces are placed between the characters in the output, and the beginning of any TELNET escape sequence is preceded by an * (asterisk) to aid in locating it. Toggles printing of hexadecimal terminal data (used for debugging). Suspends TELNET execution and returns you to your original login shell. This subcommand works only when you are using csh or ksh. 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 termi- nal's End-of-File character is only recognized (and sent to the remote system) when it is the first character on a line. EXAMPLES To log in to host1 and do terminal type negotiation, enter: telnet host1 Information similar to the following is displayed on your screen: Trying ... Connected to host1 Escape character is ^] login: _ To log in to remote host host3 and then check the sta- tus of the telnet program, enter: $ telnet host3 Trying ... Connected to host3 Escape character is ^] login: _ Enter your login ID and password when you are prompted to do so. Press ^] to receive the telnet> prompt. Enter the status subcom- mand at the prompt. Information similar to the following is then displayed on your screen: Connected to host3. Operating in single character mode. Escape character is '^]'. _ Press <Re- turn> $_ Upon completion of the status subcommand, you must press <Re- turn> to return to the remote prompt. FILES User-customized telnet startup values. SEE ALSO Commands: env(1), printenv(1), rexecd(8), rlogin(1), rsh(1), tel- netd(8) Files: iptos(4) telnet(1)