It’s pretty straightforward to install Nagios on a Debian system but if you want to be able to use the web interface to control the nagios process a little more work is required.

Starting with a blank slate (apt/dpkg will ensure any required prerequisites will be installed):

# apt-get install nagios3 apache2-suexec

You’ll be asked to set a password for the nagiosadmin user for the web interface.

Enable check_external_commands in Nagios to enable the ability to mute alarms, make comments, restart the nagios process etc from the web interface (pretty much invaluable, but be aware of the inherent risks in enabling the ability to influence the process from “outside”)

# sed -i -e 's/check_external_commands=0/check_external_commands=1/' /etc/nagios3/nagios.cfg
# /etc/init.d/nagios3 restart

Edit the nagios3 apache2 config include to make the web interface scripts run as the nagios user so that the web interface can write to the nagios command pipe; inserting the following at the top of /etc/nagios3/apache2.conf:

User nagios
Group nagios

Restart apache..

# /etc/init.d/apache2 restart

And you’re pretty much done! You can go to http://YOUR_HOST_NAME/nagios3/ and log in with your nagiosadmin password you set up when prompted at the start of this process.

Now, you can get started with creating host and service configuration files in /etc/nagios3/conf.d/ to monitor your servers/network/etc