telnetd man page on 4.4BSD

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TELNETD(8)		  BSD System Manager's Manual		    TELNETD(8)

     telnetd — DARPA TELNET protocol server

     /usr/libexec/telnetd [-BUhlkns] [-D debugmode] [-Iinitid] [-S tos]
			  [-X authtype] [-a authmode] [-edebug]
			  [-rlowpty-highpty] [-u len] [-debug [port]]

     The telnetd command is a server which supports the DARPA standard TELNET
     virtual terminal protocol.	 Telnetd is normally invoked by the internet
     server (see inetd(8)) for requests to connect to the TELNET port as indi‐
     cated by the /etc/services file (see services(5)).	 The -debug option may
     be used to start up telnetd manually, instead of through inetd(8).	 If
     started up this way, port may be specified to run telnetd on an alternate
     TCP port number.

     The telnetd command accepts the following options:

     -a authmode  This option may be used for specifying what mode should be
		  used for authentication.  Note that this option is only use‐
		  ful if telnetd has been compiled with support for the
		  AUTHENTICATION option.  There are several valid values for

		  debug	 Turns on authentication debugging code.

		  user	 Only allow connections when the remote user can pro‐
			 vide valid authentication information to identify the
			 remote user, and is allowed access to the specified
			 account without providing a password.

		  valid	 Only allow connections when the remote user can pro‐
			 vide valid authentication information to identify the
			 remote user.  The login(1) command will provide any
			 additional user verification needed if the remote
			 user is not allowed automatic access to the specified

		  other	 Only allow connections that supply some authentica‐
			 tion information.  This option is currently not sup‐
			 ported by any of the existing authentication mecha‐
			 nisms, and is thus the same as specifying -a valid.

		  none	 This is the default state.  Authentication informa‐
			 tion is not required.	If no or insufficient authen‐
			 tication information is provided, then the login(1)
			 program will provide the necessary user verification.

		  off	 This disables the authentication code.	 All user ver‐
			 ification will happen through the login(1) program.

     -B		  Specifies bftp server mode.  In this mode, telnetd causes
		  login to start a bftp(1) session rather than the user's nor‐
		  mal shell.  In bftp daemon mode normal logins are not sup‐
		  ported, and it must be used on a port other than the normal
		  TELNET port.

     -D debugmode
		  This option may be used for debugging purposes.  This allows
		  telnetd to print out debugging information to the connec‐
		  tion, allowing the user to see what telnetd is doing.	 There
		  are several possible values for debugmode:

		  options   Prints information about the negotiation of TELNET

		  report    Prints the options information, plus some addi‐
			    tional information about what processing is going

		  netdata   Displays the data stream received by telnetd.

		  ptydata   Displays data written to the pty.

		  exercise  Has not been implemented yet.

     -debug	  Enables debugging on each socket created by telnetd (see
		  SO_DEBUG in socket(2)).

     -edebug	  If telnetd has been compiled with support for data encryp‐
		  tion, then the -edebug option may be used to enable encryp‐
		  tion debugging code.

     -h		  Disables the printing of host-specific information before
		  login has been completed.

     -I initid	  This option is only applicable to UNICOS systems prior to
		  7.0.	It specifies the ID from /etc/inittab to use when init
		  starts login sessions.  The default ID is fe.

     -k		  This option is only useful if telnetd has been compiled with
		  both linemode and kludge linemode support.  If the -k option
		  is specified, then if the remote client does not support the
		  LINEMODE option, then telnetd will operate in character at a
		  time mode.  It will still support kludge linemode, but will
		  only go into kludge linemode if the remote client requests
		  it.  (This is done by by the client sending DONT
		  SUPPRESS-GO-AHEAD and DONT ECHO.)  The -k option is most
		  useful when there are remote clients that do not support
		  kludge linemode, but pass the heuristic (if they respond
		  with WILL TIMING-MARK in response to a DO TIMING-MARK) for
		  kludge linemode support.

     -l		  Specifies line mode.	Tries to force clients to use line-
		  at-a-time mode.  If the LINEMODE option is not supported, it
		  will go into kludge linemode.

     -n		  Disable TCP keep-alives.  Normally telnetd enables the TCP
		  keep-alive mechanism to probe connections that have been
		  idle for some period of time to determine if the client is
		  still there, so that idle connections from machines that
		  have crashed or can no longer be reached may be cleaned up.

     -r lowpty-highpty
		  This option is only enabled when telnetd is compiled for
		  UNICOS. It specifies an inclusive range of pseudo-terminal
		  devices to use.  If the system has sysconf variable
		  _SC_CRAY_NPTY configured, the default pty search range is 0
		  to _SC_CRAY_NPTY; otherwise, the default range is 0 to 128.
		  Either lowpty or highpty may be omitted to allow changing
		  either end of the search range.  If lowpty is omitted, the -
		  character is still required so that telnetd can differenti‐
		  ate highpty from lowpty.

     -s		  This option is only enabled if telnetd is compiled with sup‐
		  port for SecurID cards.  It causes the -s option to be
		  passed on to login(1), and thus is only useful if login(1)
		  supports the -s flag to indicate that only SecurID validated
		  logins are allowed, and is usually useful for controlling
		  remote logins from outside of a firewall.

     -S tos

     -u len	  This option is used to specify the size of the field in the
		  utmp structure that holds the remote host name.  If the
		  resolved host name is longer than len, the dotted decimal
		  value will be used instead.  This allows hosts with very
		  long host names that overflow this field to still be
		  uniquely identified.	Specifying -u0 indicates that only
		  dotted decimal addresses should be put into the utmp file.

     -U		  This option causes telnetd to refuse connections from
		  addresses that cannot be mapped back into a symbolic name
		  via the gethostbyaddr(3) routine.

     -X authtype  This option is only valid if telnetd has been built with
		  support for the authentication option.  It disables the use
		  of authtype authentication, and can be used to temporarily
		  disable a specific authentication type without having to
		  recompile telnetd.

     Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for
     a client, then creating a login process which has the slave side of the
     pseudo-terminal as stdin, stdout and stderr.  Telnetd manipulates the
     master side of the pseudo-terminal, implementing the TELNET protocol and
     passing characters between the remote client and the login process.

     When a TELNET session is started up, telnetd sends TELNET options to the
     client side indicating a willingness to do the following TELNET options,
     which are described in more detail below:


     The pseudo-terminal allocated to the client is configured to operate in
     cooked mode, and with XTABS and CRMOD enabled (see tty(4)).

     Telnetd has support for enabling locally the following TELNET options:

     WILL ECHO		When the LINEMODE option is enabled, a WILL ECHO or
			WONT ECHO will be sent to the client to indicate the
			current state of terminal echoing.  When terminal echo
			is not desired, a WILL ECHO is sent to indicate that
			telnetd will take care of echoing any data that needs
			to be echoed to the terminal, and then nothing is
			echoed.	 When terminal echo is desired, a WONT ECHO is
			sent to indicate that telnetd will not be doing any
			terminal echoing, so the client should do any terminal
			echoing that is needed.

     WILL BINARY	Indicates that the client is willing to send a 8 bits
			of data, rather than the normal 7 bits of the Network
			Virtual Terminal.

     WILL SGA		Indicates that it will not be sending IAC GA, go
			ahead, commands.

     WILL STATUS	Indicates a willingness to send the client, upon
			request, of the current status of all TELNET options.

     WILL TIMING-MARK	Whenever a DO TIMING-MARK command is received, it is
			always responded to with a WILL TIMING-MARK

     WILL LOGOUT	When a DO LOGOUT is received, a WILL LOGOUT is sent in
			response, and the TELNET session is shut down.

     WILL ENCRYPT	Only sent if telnetd is compiled with support for data
			encryption, and indicates a willingness to decrypt the
			data stream.

     Telnetd has support for enabling remotely the following TELNET options:

     DO BINARY		Sent to indicate that telnetd is willing to receive an
			8 bit data stream.

     DO LFLOW		Requests that the client handle flow control charac‐
			ters remotely.

     DO ECHO		This is not really supported, but is sent to identify
			a 4.2BSD telnet(1) client, which will improperly
			respond with WILL ECHO. If a WILL ECHO is received, a
			DONT ECHO will be sent in response.

     DO TERMINAL-TYPE	Indicates a desire to be able to request the name of
			the type of terminal that is attached to the client
			side of the connection.

     DO SGA		Indicates that it does not need to receive IAC GA, the
			go ahead command.

     DO NAWS		Requests that the client inform the server when the
			window (display) size changes.

     DO TERMINAL-SPEED	Indicates a desire to be able to request information
			about the speed of the serial line to which the client
			is attached.

     DO XDISPLOC	Indicates a desire to be able to request the name of
			the X windows display that is associated with the tel‐
			net client.

     DO NEW-ENVIRON	Indicates a desire to be able to request environment
			variable information, as described in RFC 1572.

     DO ENVIRON		Indicates a desire to be able to request environment
			variable information, as described in RFC 1408.

     DO LINEMODE	Only sent if telnetd is compiled with support for
			linemode, and requests that the client do line by line

     DO TIMING-MARK	Only sent if telnetd is compiled with support for both
			linemode and kludge linemode, and the client responded
			with WONT LINEMODE. If the client responds with WILL
			TM, the it is assumed that the client supports kludge
			linemode.  Note that the [-k] option can be used to
			disable this.

     DO AUTHENTICATION	Only sent if telnetd is compiled with support for
			authentication, and indicates a willingness to receive
			authentication information for automatic login.

     DO ENCRYPT		Only sent if telnetd is compiled with support for data
			encryption, and indicates a willingness to decrypt the
			data stream.

     /etc/inittab (UNICOS systems only)
     /etc/iptos (if supported)
     /usr/ucb/bftp (if supported)

     telnet(1), login(1), bftp(1) (if supported)

     RFC-1073  Telnet Window Size Option
     RFC-1079  Telnet Terminal Speed Option
     RFC-1091  Telnet Terminal-Type Option
     RFC-1096  Telnet X Display Location Option
     RFC-1123  Requirements for Internet Hosts -- Application and Support
     RFC-1184  Telnet Linemode Option
     RFC-1372  Telnet Remote Flow Control Option
     RFC-1416  Telnet Authentication Option
     RFC-1411  Telnet Authentication: Kerberos Version 4
     RFC-1412  Telnet Authentication: SPX
     RFC-1571  Telnet Environment Option Interoperability Issues
     RFC-1572  Telnet Environment Option

     Some TELNET commands are only partially implemented.

     Because of bugs in the original 4.2 BSD telnet(1), telnetd performs some
     dubious protocol exchanges to try to discover if the remote client is, in
     fact, a 4.2 BSD telnet(1).

     Binary mode has no common interpretation except between similar operating
     systems (Unix in this case).

     The terminal type name received from the remote client is converted to
     lower case.

     Telnetd never sends TELNET IAC GA (go ahead) commands.

4.2 Berkeley Distribution	 June 1, 1994	     4.2 Berkeley Distribution

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