Install Nagios3 in Ubuntu

Nagios:
http://www.nagios.org/
Nagios can be used to monitor your entire IT infrastructure, such as network, switch, database, CPU and security in any of your computers.
In Ubuntu,
1) sudo apt-get install nagios3
2) To avoid error in service nagios3 start
sudo /etc/init.d/nagios3 stop
sudo /etc/init.d/nagios3 start
3) Remember nagiosadmin and its password
4) In you hostname/nagios3, for example
http://192.168.56.101/nagios3/
to start
nagios monitoring
Note: the following may need to be changed before nagios start
edit /etc/nagios3/nagios.cfg
change
 check_external_commands=0
to
check_external_commands=1
edit /etc/groups
add www-data at the end of nagios group
chmod g+x /var/lib/nagios3/rw/
chmod g+x /var/lib/nagios3

Video:  Install Nagios3 in Ubuntu