To install the latest version of Nagios, we need to build it from the source:

1. First, update the system packages:

   sudo apt update
2. Then, install the necessary tools and libraries:

   sudo apt install autoconf \
gcc \
libc6 \
make \
wget \
unzip \
apache2 \
apache2-utils \
php \
libapache2-mod-php \
libgd-dev \
libssl-dev -y

3. Next, head to the Nagios downloads page and download the latest source tarball. We need to replace `VER` with the latest version number:

   VER=4.5.2
wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-${VER}.tar.gz

4. Then, extract the downloaded archive and go to the source directory:

   tar xzf nagios-${VER}.tar.gz

   Now we should have the Nagios source directory in the current working directory:

   
ls
nagios-4.5.2 nagios-4.5.2.tar.gz

5. Then, go to the Nagios source directory and run the configure script to adapt Nagios to our system while checking for any missing required dependency before the installation:

   cd nagios-${VER}
6. Now, it is time to configure Nagios to use Apache as the web server:

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

   We can use ./configure –help for more options.
7. Then compile and install Nagios:

   make all
8. After that, create a Nagios user and group. Then, add the user to the Apache web server group.

   sudo make install-groups-users
sudo usermod -aG nagios www-data
9. Then, install the Nagios Server main program, CGIs, and HTML files.

   sudo make install
10. After that, install Nagios Server startup scripts:

    sudo make install-init

    We can install and configure permissions on the directory for holding the external command file:

    sudo make install-commandmode

    Also, initialize the init scripts:

    sudo make install-daemoninit

    We can install Nagios Server sample configuration files in /usr/local/nagios/etc with this command:

    sudo make install-config
11. Furthermore, we can install the Apache configuration file for the Nagios web interface:

    sudo make install-webconf
12. Then, enable Apache modules and configure basic authentication:

    sudo a2enmod rewrite cgi
13. Next, enable Apache rewrite and CGI modules:

    sudo a2dissite 000-default.conf
14. Similarly, we can disable the default Apache site as seen here:

    sudo a2dissite 000-default.conf
15. Now, configure Nagios Web Interface basic authentication:

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

    Here, nagiosadmin is the default admin user for accessing Nagios web interface. We can create or use any other user name and be sure to specify it in the /usr/local/nagios/etc/cgi.cfg configuration file.

    Furthermore, we can add more users by omitting option -c in the command above.
16. Now, we can start and enable the Nagios service:

    sudo systemctl enable --now nagios

    If it was already running, we can restart it:

    sudo systemctl restart nagios

    We can check the status with this command:

    systemctl status nagios