Nagios installation 4.2.0 (Ubuntu Server 18.04.1 LTS)

Website: www.nagios.com/
DOWNLOAD Nagios Core: nag.is/coresys
DOWNLOAD Nagios Plugins: nag.is/plugsys
DOWNLOAD Nagios XI: nag.is/xisys
DOWNLOAD Nagios Log Server: nag.is/logsys
DOWNLOAD Nagios Network Analyzer: nag.is/netsys

Why Nagios:
Open-source
Customized Dashboards
Ease of Use
Infinite Scalability
Data in Real Time
Network Security
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Starting with the installation
Installing the prerequisites:
$ sudo apt install wget build-essential apache2 php libapache2-mod-php7.2 php-gd libgd-dev unzip postfix

User and group configuration
$ useradd nagios
$ groupadd nagcmd
$ usermod -a -G nagcmd nagios
$ usermod -a -G nagios,nagcmd www-data

Download and extract the Nagios core
$ wget assets.nagios.com/downloads/nagioscore/releases/na…
$ tar -xzf nagios*.tar.gz

You will have to configure it with the user and the group you have created earlier
$ ./configure --with-nagios-group=nagios --with-command-group=nagcmd
$ make all
$ make install
$ make install-commandmode
$ make install-init
$ make install-config
$ /usr/bin/install -c -m 644 sample-config/httpd.conf /etc/apache2/sites-available/nagios.conf

Copy evenhandler directory to the nagios directory
$ cp -R contrib/eventhandlers/ /usr/local/nagios/libexec/
$ chown -R nagios:nagios /usr/local/nagios/libexec/eventhandlers

Install the Nagios Plugins
$ wget nagios-plugins.org/download/nagios-plugins-2.1.2.t…
$ tar -xzf nagios-plugins*.tar.gz

Install the Nagios plugin's with the commands below
$ ./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
$ make
$ make install

You can find the default configuration of Nagios in /usr/local/nagios/. We will configure Nagios and Nagios contact. Edit default nagios configuration with nano

$ nano -c /usr/local/nagios/etc/nagios.cfg
uncomment line 51 for the host monitor configuration.
Save and exit.

Add a new folder named servers.
$ mkdir -p /usr/local/nagios/etc/servers

Change the user and group for the new folder to nagios:
$ chown nagios:nagios /usr/local/nagios/etc/servers

Enable Apache modules
$ sudo a2enmod rewrite
$ sudo a2enmod cgi

You can use the htpasswd command to configure a user nagiosadmin for the nagios web interface

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

Enable the Nagios virtualhost
$ sudo ln -s /etc/apache2/sites-available/nagios.conf /etc/apache2/sites-enabled/

Start Apache
$ service apache2 restart

When Nagios starts, you may see the following error
Starting nagios (via systemctl): nagios.serviceFailed
DON'T WORRY WE CAN FIX IT
FOLLOW THE COMMANDS
$ cd /etc/init.d/
If you could find the nagios file there in the folder
$ cp /etc/init.d/skeleton /etc/init.d/nagios
$ nano /etc/init.d/nagios
Paste this code at the end of the file
----------------------------------------
DESC="Nagios"
NAME=nagios
DAEMON=/usr/local/nagios/bin/$NAME
DAEMON_ARGS="-d /usr/local/nagios/etc/nagios.cfg"
PIDFILE=/usr/local/nagios/var/$NAME.lock
-------------------------------------------------------------------
Make it executable and start Nagios
$ chmod +x /etc/init.d/nagios
$ service apache2 restart
WE DIDN'T FINISH YET
First we are going to create/change the nagios.service
$ nano /etc/systemd/system/nagios.service
Paste the following code of the file
---------------------------------------------------------------
[Unit]
Description=Nagios
BindTo=network.target

[Install]
WantedBy=multi-user.target

[Service]
User=nagios
Group=nagios
Type=simple
ExecStart=/usr/local/nagios/bin/nagios /usr/local/nagios/etc/nagios.cfg
------------------------------------------------------------------
We need to enable created nagios.service config
$ systemctl enable /etc/systemd/system/nagios.service
$ service nagios start
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To check the service is working
$ service nagios status
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Don't forget to install htop to monitor your memory
$ apt install htop