Preamble

Every Munin Plugin should have a preamble by default:

#!/usr/bin/env perl
# -*- perl -*-

=head1 NAME

dar_vpnd a Plugin for displaying VPN Stats for the Darwin (MacOS) vpnd Service.

=head1 INTERPRETATION

The Plugin displays the number of active VPN connections.

=head1 CONFIGURATION

No Configuration necessary!

=head1 AUTHOR

Philipp Haussleiter <philipp@haussleiter.de> (email)

=head1 LICENSE

GPLv2

=cut

# MAIN
use warnings;
use strict;

As you can see, this Plugin will use Perl as the Plugin language.

After that you have some information about the Plugin Usage:

• Name of the Plugin + some description
• Interpretation of the delivered Data
• Information about the Plugins Configuration (not necessary here, we will see that in the other Plugins)
• Author Name + Contact Email
• License

# MAIN marks the beginngin of the (main) code.

Next you see some Perl Setup, using strict Statements and also show warnings.

Gathering Data

First you should always have a basic idea how you want collect your Data (e.g. which user will use what command to get what kind of data).

For Example we can get all VPN Connections in Mac OS (Server) searching the process List for pppd processes.

ps -ef | grep ppp

    0   144     1   0  5Mär14 ??        10:35.34 vpnd -x -i com.apple.ppp.l2tp
    0 29881   144   0  4:12pm ??         0:00.04 pppd serverid com.apple.ppp.l2tp nodetach proxyarp plugin L2TP.ppp ms-dns 10.XXX.YYY.1 debug logfile /var/log/ppp/vpnd.log idle 7200 noidlesend lcp-echo-interval 60 lcp-echo-failure 5 mru 1500 mtu 1280 receive-all ip-src-address-filter 1 novj noccp intercept-dhcp require-mschap-v2 plugin DSAuth.ppp plugin2 DSACL.ppp l2tpmode answer :10.XXX.YYY.233
    0 22567   144   0  4:12pm ??         0:00.04 pppd serverid com.apple.ppp.l2tp nodetach proxyarp plugin L2TP.ppp ms-dns 10.XXX.YYY.1 debug logfile /var/log/ppp/vpnd.log idle 7200 noidlesend lcp-echo-interval 60 lcp-echo-failure 5 mru 1500 mtu 1080 receive-all ip-src-address-filter 1 novj noccp intercept-dhcp require-mschap-v2 plugin DSAuth.ppp plugin2 DSACL.ppp l2tpmode answer :10.XXX.YYY.234    

Collecting only the IP you need some more RegExp using awk:

ps -ef | awk '/[p]ppd/ {print substr($NF,2);}'

10.XXX.YYY.233
10.XXX.YYY.234

We are only interested in the total Connection Count. So we use wc for counting all IPs:

ps -ef | awk '/[p]ppd/ {print substr($NF,2);}' | wc -l

       2

So we now have a basic command that gives us the Count of currentyl connected users.

Configuration

The next thing is how your Data should be handled by the Munin System.
Your Plugin needs to provide Information about the Field Setup.

The most basic (Perl) Code looks like this:

if ( exists $ARGV[0] and $ARGV[0] eq "config" ) {
    # Config Output
    print "...";    
} else {
    # Data Output
    print "...";
}

For a more Information about fieldnames, please follow the above Link.

Our Plugin Source looks like this:

# MAIN
use warnings;
use strict;


my $cmd = "ps -ef | awk '/[p]ppd/ {print substr(\$NF,2);}' | wc -l";

if ( exists $ARGV[0] and $ARGV[0] eq "config" ) {
    print "graph_category VPN\n";
    print "graph_args --base 1024 -r --lower-limit 0\n";    
    print "graph_title Number of VPN Connections\n";
    print "graph_vlabel VPN Connections\n";
    print "graph_info The Graph shows the Number of VPN Connections\n"; 
    print "connections.label Number of VPN Connections\n";
    print "connections.type GAUGE\n";   
} else {
    my $output = `$cmd`;
    print "connections.value $output";
}

Implementation

To test the Plugin you can use munin-run:

> /opt/local/sbin/munin-run dar_vpnd config

graph_category VPN
graph_args --base 1024 -r --lower-limit 0
graph_title Number of VPN Connections
graph_vlabel VPN Connections
graph_info The Graph shows the Number of VPN Connections
connections.label Number of VPN Connections
connections.type GAUGE

> /opt/local/sbin/munin-run dar_vpnd

connections.value        1

Example Graphs

Some basic (long time) Graphs look like this: