1MIMEDEFANG-NOTIFY(7) Miscellaneous Information Manual MIMEDEFANG-NOTIFY(7)
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6 mimedefang-notify - Conventions used by mimedefang-multiplexor(8) to
7 notify an external program of state changes.
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11 If you supply the -O option to mimedefang-multiplexor, then it allows
12 external programs to connect to a socket and be notified of certain
13 state changes in the multiplexor. The external programs can react in
14 whatever way they choose to these state changes. The external program
15 that listens for state changes is referred to as a listener.
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19 From the point of view of a listener, notification works like this:
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21 1) The listener connects to a TCP or UNIX-domain socket.
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23 2) The listener informs mimedefang-multiplexor of the message types it
24 is interested in.
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26 3) The listener loops, reading messages from the socket and reacting to
27 them.
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31 Each message from the multiplexor normally consists of a single upper-
32 case letter, possibly followed by a space and some arguments, and then
33 followed by a newline.
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35 Two special messages are "*OK" followed by a newline, which is issued
36 when a listener first connects, and "*ERR" followed by some text and a
37 newline, which is issued when an error occurs.
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39 The normal messages are:
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42 B This message is issued whenever a slave is killed because of a
43 busy timeout.
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46 F n This message is issued whenever the number of free slaves
47 changes. The parameter n is the number of free slaves.
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50 R This message is issued whenever someone has forced a filter
51 reread.
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54 S n nmsg
55 This message is issued whenever slave n's status tag changes.
56 The status tag is a string indicating what the slave is cur‐
57 rently doing; the -Z option to the multiplexor allows the Perl
58 code to update the status tag so you have a good idea what each
59 slave is doing.
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62 U This message is issued whenever a slave has died unexpectedly.
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65 Y This message is issued whenever the number of free slaves
66 changes from zero to non-zero.
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69 Z This message is issued whenever the number of free slaves falls
70 to zero.
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74 A listener does not receive any messages until it has expressed inter‐
75 est in various message types. To express interest, the listener should
76 send a question mark ("?") followed by the types of messages it is
77 interested in, followed by a newline over the socket. For example, a
78 listener interested in the R and F messages would send this line:
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80 ?RF
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82 A listener interested in every possible message type should send:
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84 ?*
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86 Once a listener has expressed interest, it may receive messages at any
87 time, and should monitor the socket for messages.
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89 Note that a listener always receives the special messages "*OK" and
90 "*ERR", even if it has not expressed interest in them.
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94 The following Perl script implements a listener that, on Linux, rejects
95 new SMTP connections if all slaves are busy, and accepts them again
96 once a slave is free. Existing SMTP connections are not shut down; the
97 system merely refuses new connections if all the slaves are busy.
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99 This script assumes that you have used the -O inet:4567 option to
100 mimedefang-multiplexor.
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102 #!/usr/bin/perl -w
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104 # On Linux, prepare to use this script like this:
105 # /sbin/iptables -N smtp_connect
106 # /sbin/iptables -A INPUT --proto tcp --dport 25 --syn -j smtp_connect
107 # Then run the script as root.
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109 use IO::Socket::INET;
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111 sub no_free_slaves {
112 print STDERR "No free slaves!\n";
113 system("/sbin/iptables -A smtp_connect -j REJECT");
114 }
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116 sub some_free_slaves {
117 print STDERR "Some free slaves.\n";
118 system("/sbin/iptables -F smtp_connect");
119 }
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121 sub main {
122 my $sock;
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124 $sock = IO::Socket::INET->new(PeerAddr => '127.0.0.1',
125 PeerPort => '4567',
126 Proto => 'tcp');
127 # We are only interested in Y and Z messages
128 print $sock "?YZ\n";
129 $sock->flush();
130 while(<$sock>) {
131 if (/^Z/) {
132 no_free_slaves();
133 }
134 if (/^Y/) {
135 some_free_slaves();
136 }
137 }
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139 # EOF from multiplexor?? Better undo firewalling
140 system("/sbin/iptables -F smtp_connect");
141 }
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143 main();
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147 mimedefang.pl(8), mimedefang(8), mimedefang-multiplexor(8), mimedefang-
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1554th Berkeley Distribution 8 February 2005 MIMEDEFANG-NOTIFY(7)