tcp_bridge:	0.4 Copyright (c) 1995-1998 by Karl J. Runge. <runge@karlrunge.com>

	Listen on a port (4001) and redirect connections to tcp
	service (23) on another machine ().
Usage:

tcp_bridge
	-p <port> 		Local port to listen to.
	-r <remote_machine> 	Remote machine to connect to.
	-s <service>		Remote port to connect to.
	-l <logfile>		Place info in logfile
	-limit xxx.xxx.xxx 	Limiting connecting IP number to pattern(s).
				(use : as separator)
	-d			Debug reporting
	-q			Quiet about logging
	-h -help		Print this help

	-bytes <n>		Try to read/write n bytes at a time, n=1
				implies select or getc(). (default mode is one
				line at a time, e.g. http, smtp, nntp)
	-binary			Use select + sysread + syswrite to transfer
				data (default blksize: 4096)
	-proxy			Run as a proxy server
	-connect_only		Do not be a server, just connect once, using
				STDIN and STDOUT as "client".
	-firewall		Make connections using default firewall command.
	-firewall_cmd <cmd>	Make connections using alternative firewall
				command "cmd".
	-sysv <n>		use <n> = 0 or 1 depending if machine is sysv.


Notes:
	If "service" or "port" is a standard service, the name may be used
	instead of the number (e.g. smtp = 25).

	Limit xxx.xxx.xxx limits access of incoming IP's number to those
	matching that pattern (e.g. xxx.xxx.xxx.yyy is OK).
	Separate multiple ones with `:'

	Default logging to (/tmp/tcp_bridge_18894_log).
	Default IP addr limiting to ().
	File to look for default machine (/home/runge/.machine).

	Firewall command is:
	    %Firewall_Perl script/tcp_bridge -connect_only -q -r %HOST -s %PORT

	When supplying your own firewall command, the STDIO is hooked to it
	and %HOST %PORT are expanded to the desired remote machine.
	However if Firewall_Command matches:

	TCP:fwhost:fwport:send_string:skip_lines:trim_string

	you can use a TCP connection to a firewall machine (instead of
	running a separate command). In this case a TCP socket is opened
	to port "fwport" on machine "fwhost". The number of bytes in
	"trim_string" is then read from the firewall host and discarded.
	Then "send_string" plus a newline is sent to the firewall host,
	with %HOST and %PORT expanded accordingly.  If "send_string" is
	empty, then "%HOST %PORT" is used. Then, the next "skip_lines" lines
	from the connection are discarded. The TCP connection is then
	handed over to the client.

	E.g.: -firewall_cmd "TCP:myproxy.foo.com:3666::2:(to) "
	This works for itelnet/telnet-passthru service.

	The "trim_string" is basically a prompt, to which "%HOST %PORT"
	is sent back. Then the number "skip_lines" are skipped, and
	finally the connection proceeds back and forth as normal.

Environment used:
	TCP_B_CHAR_AT_A_TIME	use getc()
	TCP_B_VERBOSE		verbose
	TCP_B_BINARY		run in binary mode (?)
	TCP_B_ALLSTR		in binary xfer mode, print out all data to log
	HOME			used to fine ~/.machine
	PAGER			used for -help

Bugs:
	Telnet login very slow [due to getc()]. (much better now with
		select/sysread/syswrite)
	Ftp will not work (multiple ports 20+21, 2nd cannot reach
	original caller).

Tested on:
	echo(7), daytime(13), chargen(19), telnet(23), mail(25), finger(79),
	pop(110), news(119). ssh(22). Proxy: http.