Installing Nagios

Update the system:

sudo apt-get update && sudo apt-get upgrade

Install the dependencies:

sudo apt-get install autoconf gcc libc6 make wget unzip apache2 php libapache2-mod-php libgd-dev libsasl2-modules mailutils -y

Download the nagios 4 source codes:

wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.6.tar.gz

Extract the downloaded source codes:
```
tar xzf nagios-4.4.6.tar.gz
```
Change to the directory of the newly extracted source codes:
```
cd nagios-4.4.6
```

Run the configure script with integration to apache2:

./configure --with-httpd-conf=/etc/apache2/sites-enabled

Compile the source:
```
sudo make all
```
Install the nagios user and group:
```
sudo make install-groups-users
```
Make nagios user a member of the www-data group:
```
sudo usermod -aG nagios www-data
```
Install nagios.
```
sudo make install
```
Install the init script:
```
sudo make install-init
```
Initialize the init script:
```
sudo make install-daemoninit
```
Install and configures permissions on the directory for holding the external command:
```
sudo make install-commandmode
```
Install the sample config files on the /usr/local/nagios/etc directory:
```
sudo make install-config
```
Install the nagios config file to apache2:
```
sudo make install-webconf
```
Update the SUID of the ping command:
```
sudo chmod 4755 /bin/ping
```

Installing the Nagios Plugins

Run the following command:
```
sudo apt install nagios-plugins
```
Update the $USER1$ property in the /usr/local/nagios/etc/resource.cfg file to have /usr/lib/nagios/plugins like as follows:
```
$USER1$=/usr/lib/nagios/plugins
```

Create nagiosadmin as a nagios web user:

sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

Assign a strong password