defang: Interpose upon HTTP connections and filter animated gifs that
        come in. Can either replace the Animated Gif with a simple 
	static gif, or can make the animated GIF do just one loop.
	It can also screen connections based on remote hostname (allow
	and deny) and URL pathname (deny only).

defang evidently stands for: DElete Friggin ANimated Gifs :-)


Configuration: 

  Edit the top portion of the script. (or see cmd line options below)

  Setup your browser (e.g. Netscape) to use the machine and port you run
  this script on to be your browsers HTTP PROXY. Best to leave the FTP
  and GOPHER PROXYs alone. (although they might work too...). 

Options:
	-h, -help	This help.
	-on		Turn on (default mode: replace)
	-r		Turn on  GIF replacement mode. (red A)
	-once, -1	Turn on  GIF One Loop mode.
	-off		Turn off GIF filtering.

	-all		Turn off ACCESS filtering.
	-noall		Turn on  ACCESS filtering.
	

  Using any of the above options will not cause the server to be
  started, unless run via, say: defang -once start

Yow! Here are cmd line options for everything else:

	-c <file>	Use <file> as config file instead of
			./defang.conf.

	-F <file>	Use <file> as the flag file prefix instead
			of /tmp/defang.runge.

	-f <N>		Set initial Gif read block size to "N"
			(only this portion is checked for animation
			signature) Default: 16384

	-b <N>		Set the overall transfer read block size
			to "N". Default: 8192

	-v <httpvers>	Set to, say "HTTP/1.0" to force overwrite
			of the Http version in the GET line. Could
			be dangerous, really should be per-site.

	-nfg		Never return a fake gif or jpg to client.
			Only return HTTP 202.

	-d		Turn on debugging mode.

	-l <file>	Use <file> as debugging logfile instead of "defang.log"
			Implies -d.

	-u		The machine is Unix (ahhh)  Default.

	-w		The machine is Windows (gulp!)

	-sysv		The machine is SVR4 wrt sockets. (e.g. Solaris)

	-p <port>	Listen on port <port>, Default: 8082

	-proxy_host <host>	Use <host> as proxy to refer requests to.
				Specify "NONE" for no proxy.
				Default: NONE

	-proxy_port <port>	The port of the proxy to connect to.
				Ignored if host is "NONE".
				Default: 8080

			Or use -proxy_host  host:port

	-ef		Do an extra fork() call on Unix for
			the two-way data flow. Not needed for most
			HTTP. (But may be for secure/CONNECT method).

	-sf		Do a single fork to not protect against 
			zombie creation.

	-ka		Use Connection: Keep-Alive protocol.

	-pf <n>		Prefork <n> children. Use 0 to skip prefork
			mechanism.

	-jpf <n>	Under prefork mode, each child handles this
			many connections.

	-allow <list>	Allow hosts matching names or ips in patterns
			<list> E.g. -allow *.nick.com,*.disney.com
			
	-deny  <list>   Deny hosts matching patterns <list>
			E.g. -deny *.xxx.com,*.lycos.com

	-url_deny <list> Deny urls with pathnames matching the patterns
			 in <list>.

			Spaces may be used instead of commas in pattern
			lists.  Use /<regex>/ to use a perl regex w/o
			commas or spaces.

Params to set in Config_File:
	Hosts_Allow		(space/comma sep and additive)
	Hosts_Deny		(space/comma sep and additive)
	Url_Deny		(space/comma sep and additive)
	Url_Redir		(additive)
	Url_Rewrite		(additive)
	Flag_File_Prefix	(filename)
	Proxy_host		(host or host:port)
	Proxy_port		(port #)
	My_Listen_Port		(port #)
	First_Block_Size	(number)
	Buf_Size		(number)
	Pre_Fork		(number)
	Jobs_Per_Fork		(number)
	Http_Version		(e.g. HTTP/1.1)
	Check_Multi_Gif89_Ext	(0 or 1)
	Check_Any_Gif89_Ext	(0 or 1)
	No_Fake_Gifs		(0 or 1)
	Do_Shutdown		(0 or 1)
	Do_Keepalive		(0 or 1)
	Unix			(0 or 1)
	Windows			(0 or 1)
	IS_SYSV			(0 or 1)
	Extra_Fork		(0 or 1)
	Single_Fork		(0 or 1)
	Debug			(0 or 1)
	Argv_Url		(0 or 1)

ENV hacks:
	$ENV{DENIED_FILE} for internal Denied_Log_File

Notes:
	The Config_File is reread if it is modified.

	Send signals to program named defang_master

		USR1		Restart_Children
		USR2, HUP	Restart_All 
		TERM, INT	Kill everything and exit.

Bugs:
	Loop once does not seem to work any more.
	Allow/Deny host lists take 2 connections to get back to server.
	Cannot turn off and on Host and Url access easily.
	Does not fork on Windows. 1 connection at a time.

Copyright (c) 1997-2001 by Karl J. Runge <runge@karlrunge.com>