The following commands will demonstrate how to compile Nagios Core and Nagios plugins from the source tarballs downloaded from the previous step, and how to install all the necessary files for Nagios Core to run and begin monitoring:

cd /tmp/nagios
./configure -–with-command-group=nagcmd
make all
make install
make install-init
make install-config
make install-commandmode
make install-webconf

As mentioned earlier, Nagios monitors hosts and services through the execution of check commands, so let's go ahead and install the Nagios plugins bundle as well using the commands:

cd /tmp/nagios-plugins-1.4.16
./configure
make
make install

The following commands will verify the Nagios object configuration and start the monitoring engine:

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
/etc/init.d/nagios start

From here, the monitoring engine should be up and running.

NRPE allows you to remotely execute Nagios plugins as active checks on remote machines by means of an agent. NRPE can communicate with agents on both Windows and Linux/Unix machines. NRPE contacts the agent on the remote machine, asks it to run a check command, and returns the results back to Nagios, as illustrated in the following diagram:

Getting started with NRPE and active checks will be covered in the Top features you need to know about section.

NRDP functions as a form of web API to submit both check results and commands using POST and GET over standard HTTP and HTTPS ports. Using NRDP, Nagios acts as a listener for results for a host or service, while cron or some other processes run the check on their own schedule. This is known as a passive check. NRDP allows for batch submissions of passive checks, and uses an XML format for data transport. NRDP is considered to be the most lightweight way to submit passive checks to Nagios.

The following steps will guide you through the installation process for NRDP:

The following commands will verify that the apache web server is running, and will create a default account for accessing the Nagios web interface. The htpasswd command will prompt for an initial password for the nagiosadmin user.

/etc/init.d/httpd start
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

For RHEL/CentOS systems, the Nagios and httpd processes should be added to the chkconfig list so that they're started automatically each time the system boots. This can be accomplished by the following commands:

chkconfig –-add nagios
chkconfig –-level 35 nagios on
chkconfig –-add httpd
chkconfig –-level 35 httpd on

SELinux functions as a secondary layer of system security in addition to firewall rules and user account privileges. However, if you are unfamiliar with black box of SELinux, it just tends to make a lot of system events fail with little to no trail on what prevented it from working, and acts more like an unnecessary middle manager. For simplicity's sake, we'll be disabling SELinux to prevent complications. Disable SELinux by updating /etc/selinux/config to match the following settings:

SELINUX=disabled
SETLOCALDEFS=0

Then run the following command to immediately disable SELinux:

setenforce 0

Firewall settings are ultimately up to the administrator with regards to which ports should be allowed, but the following settings are some basic guidelines as to which ports need to be opened, and their corresponding services. On a CentOS/RHEL-based system, the following lines can be added to the /etc/sysconfig/iptables file somewhere before the COMMIT statement:

# HTTP/HTTPS
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT
# NRPE
-A INPUT -m state --state NEW -m tcp -p tcp --dport 5666 -j ACCEPT
# NSCA
-A INPUT -m state --state NEW -m tcp -p tcp --dport 5667 -j ACCEPT

Once the firewall rules are in place, the iptables service will need to be restarted to apply the settings using the following command:

/etc/init.d/iptables restart