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 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 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 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!\n";
           system("/sbin/iptables -A smtp_connect -j REJECT");
       }

       sub some_free_slaves {
           print STDERR "Some free slaves.\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_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)