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  pro-
       gram 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 slave is killed because of a busy timeout.

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

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

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

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

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

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

EXPRESSING INTEREST

       A  listener  does  not  receive	any  messages until it has expressed interest in various message types.  To express interest, the listener
       should send a question mark ("?") followed by the types of messages it is interested in, followed by a newline over the socket.	For  exam-
       ple, 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 slaves are busy, and accepts them again
       once a slave is free.  Existing SMTP connections are not shut down; the system merely refuses new connections if all the slaves 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_slaves {
	   print STDERR "No free slaves!
";
	   system("/sbin/iptables -A smtp_connect -j REJECT");
       }

       sub some_free_slaves {
	   print STDERR "Some free slaves.
";
	   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
";
	   $sock->flush();
	   while(<$sock>) {
	       if (/^Z/) {
		   no_free_slaves();
	       }
	       if (/^Y/) {
		   some_free_slaves();
	       }
	   }

	   # 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)