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MIMEDEFANG-NOTIFY(7)   Miscellaneous Information Manual	  MIMEDEFANG-NOTIFY(7)

NAME
       mimedefang-notify  -  Conventions  used by mimedefang-multiplexor(8) to
       notify an external program of state changes.

DESCRIPTION
       If you supply the -O option to mimedefang-multiplexor, then  it	allows
       external	 programs  to  connect	to a socket and	be notified of certain
       state changes in	the multiplexor.  The external programs	can  react  in
       whatever	 way they choose to these state	changes.  The external program
       that listens for	state changes is referred to as	a listener.

NOTIFICATION OVERVIEW
       From the	point of view of a listener, notification works	like this:

       1) The listener connects	to a TCP or UNIX-domain	socket.

       2) The listener informs mimedefang-multiplexor of the message types  it
       is interested in.

       3) The listener loops, reading messages from the	socket and reacting to
       them.

MESSAGES
       Each  message from the multiplexor normally consists of a single	upper-
       case letter, possibly followed by a space and some arguments, and  then
       followed	by a newline.

       Two  special  messages are "*OK"	followed by a newline, which is	issued
       when a listener first connects, and "*ERR" followed by some text	and  a
       newline,	which is issued	when an	error occurs.

       The normal messages are:

       B      This  message is issued whenever a worker	is killed because of a
	      busy timeout.

       F n    This message is issued  whenever	the  number  of	 free  workers
	      changes.	The parameter n	is the number of free workers.

       R      This  message  is	 issued	 whenever  someone has forced a	filter
	      reread.

       S n nmsg
	      This message is issued whenever worker n's status	 tag  changes.
	      The  status  tag	is a string indicating what the	worker is cur-
	      rently doing; the	-Z option to the multiplexor allows  the  Perl
	      code  to update the status tag so	you have a good	idea what each
	      worker is	doing.

       U      This message is issued whenever a	worker has died	unexpectedly.

       Y      This message is issued  whenever	the  number  of	 free  workers
	      changes from zero	to non-zero.

       Z      This message is issued whenever the number of free workers falls
	      to zero.

EXPRESSING INTEREST
       A  listener does	not receive any	messages until it has expressed	inter-
       est in various message types.  To express interest, the listener	should
       send a question mark ("?") followed by the types	of messages it is  in-
       terested	 in,  followed	by  a newline over the socket.	For example, a
       listener	interested in the R and	F messages would send this line:

	    ?RF

       A listener interested in	every possible message type should send:

	    ?*

       Once a listener has expressed interest, it may receive messages at  any
       time, and should	monitor	the socket for messages.

       Note  that  a  listener	always receives	the special messages "*OK" and
       "*ERR", even if it has not expressed interest in	them.

EXAMPLE
       The following Perl script implements a listener that, on	Linux, rejects
       new SMTP	connections if all workers are busy, and  accepts  them	 again
       once  a	worker	is free.  Existing SMTP	connections are	not shut down;
       the system merely refuses new connections if all	the workers are	busy.

       This script assumes that	you have  used	the  -O	 inet:4567  option  to
       mimedefang-multiplexor.

       #!/usr/bin/perl -w
       #
       # On Linux, prepare to use this script like this:
       #     /sbin/iptables -N smtp_connect
       #     /sbin/iptables -A INPUT --proto tcp --dport 25 --syn -j smtp_connect
       # Then run the script as	root.

       use IO::Socket::INET;

       sub no_free_workers {
	   print STDERR	"No free workers!\n";
	   system("/sbin/iptables -A smtp_connect -j REJECT");
       }

       sub some_free_workers {
	   print STDERR	"Some free workers.\n";
	   system("/sbin/iptables -F smtp_connect");
       }

       sub main	{
	   my $sock;

	   $sock = IO::Socket::INET->new(PeerAddr => '127.0.0.1',
					 PeerPort => '4567',
					 Proto => 'tcp');
	   # We	are only interested in Y and Z messages
	   print $sock "?YZ\n";
	   $sock->flush();
	   while(<$sock>) {
	       if (/^Z/) {
		   no_free_workers();
	       }
	       if (/^Y/) {
		   some_free_workers();
	       }
	   }

	   # EOF from multiplexor?? Better undo	firewalling
	   system("/sbin/iptables -F smtp_connect");
       }

       main();

SEE ALSO
       mimedefang.pl(8), mimedefang(8),	mimedefang-multiplexor(8), mimedefang-
       filter(5)

4th Berkeley Distribution	8 February 2005		  MIMEDEFANG-NOTIFY(7)

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