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

NAME
     telnetd — DARPA TELNET protocol server

SYNOPSIS
     /usr/libexec/telnetd [-46BUhlkn] [-D debugmode] [-S tos] [-X authtype]
			  [-a authmode] [-edebug] [-p loginprog] [-u len]
			  [-debug [port]]

DESCRIPTION
     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:

     -4	     Forces telnetd to use IPv4 addresses only.

     -6	     Forces telnetd to use IPv6 addresses only.

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

	     debug  Turn on authentication debugging code.

	     user   Only allow connections when the remote user can provide
		    valid authentication information to identify the remote
		    user, and is allowed access to the specified account with‐
		    out providing a password.

	     valid  Only allow connections when the remote user can provide
		    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 account.

	     other  Only allow connections that supply some authentication
		    information.  This option is currently not supported by
		    any of the existing authentication mechanisms, and is thus
		    the same as specifying -a valid.

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

	     off    Disable the authentication code.  All user verification
		    will happen through the login(1) program.

     -B	     Specify bftp server mode.	In this mode, telnetd causes login to
	     start a bftp(1) session rather than the user's normal shell.  In
	     bftp daemon mode normal logins are not supported, 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 connection,
	     allowing the user to see what telnetd is doing.  There are sev‐
	     eral possible values for debugmode:

	     options   Print information about the negotiation of TELNET
		       options.

	     report    Print the options information, plus some additional
		       information about what processing is going on.

	     netdata   Display the data stream received by telnetd.

	     ptydata   Display data written to the pty.

	     exercise  Has not been implemented yet.

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

     -edebug
	     If telnetd has been compiled with support for data encryption,
	     then the -edebug option may be used to enable encryption debug‐
	     ging code.

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

     -k	     This option is only useful if telnetd has been compiled with both
	     linemode and kludge linemode support.  If the -k option is speci‐
	     fied, 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 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	     Specify line mode.	 Try 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.

     -p loginprog
	     Specify an alternate login(1) command to run to complete the
	     login.  The alternate command must understand the same command
	     arguments as the standard login.

     -S tos  Sets the IP type-of-service (TOS) option for the telnet connec‐
	     tion to the value tos, which can be a numeric TOS value or, on
	     systems that support it, a symbolic TOS name found in the
	     /etc/iptos file.

     -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 over‐
	     flow 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.  Avail‐
	     able authtype values include KERBEROS_V4, KERBEROS_V5, SPX, MINK,
	     and SRA.  These options are completely independent of the -a
	     option.

     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:

	   DO AUTHENTICATION
	   WILL ENCRYPT
	   DO TERMINAL TYPE
	   DO TSPEED
	   DO XDISPLOC
	   DO NEW-ENVIRON
	   DO ENVIRON
	   WILL SUPPRESS GO AHEAD
	   DO ECHO
	   DO LINEMODE
	   DO NAWS
	   WILL STATUS
	   DO LFLOW
	   DO TIMING-MARK

     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	Indicate 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		Indicate that it will not be sending IAC GA, go ahead,
			commands.

     WILL STATUS	Indicate 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	Indicate 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		Indicate 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	Indicate a desire to be able to request information
			about the speed of the serial line to which the client
			is attached.

     DO XDISPLOC	Indicate a desire to be able to request the name of
			the X Window System display that is associated with
			the telnet client.

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

     DO ENVIRON		Indicate 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
			processing.

     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.

NOTES
     By default telnetd will read the he, hn, and im capabilities from
     /etc/gettytab and use that information (if present) to determine what to
     display before the login: prompt.	You can also use a System V style
     /etc/issue file by using the if capability, which will override im.  The
     information specified in either im or if will be displayed to both con‐
     sole and remote logins.

FILES
     /etc/services
     /etc/gettytab
     /etc/iptos	    (if supported)
     /usr/ucb/bftp  (if supported)

SEE ALSO
     bftp(1), login(1), telnet(1) (if supported), gettytab(5)

STANDARDS
     RFC-854   TELNET PROTOCOL SPECIFICATION
     RFC-855   TELNET OPTION SPECIFICATIONS
     RFC-856   TELNET BINARY TRANSMISSION
     RFC-857   TELNET ECHO OPTION
     RFC-858   TELNET SUPPRESS GO AHEAD OPTION
     RFC-859   TELNET STATUS OPTION
     RFC-860   TELNET TIMING MARK OPTION
     RFC-861   TELNET EXTENDED OPTIONS - LIST OPTION
     RFC-885   TELNET END OF RECORD OPTION
     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

HISTORY
     IPv6 support was added by WIDE/KAME project.

BUGS
     Some TELNET commands are only partially implemented.

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

BSD				August 28, 2008				   BSD
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