Nagios provides enterprise-class Open Source IT monitoring, network monitoring, server and applications monitoring.

We'll cover Linux (Ubuntu) Server Monitoring.

Update your system.
apt update and optionally upgrade using apt upgrade

Install the required dependencies using:

apt-get install -y build-essential dos2unix gcc git libmcrypt4 libpcre3-dev ntp unzip make python2.7-dev python-pip re2c supervisor unattended-upgrades whois vim libnotify-bin pv cifs-utils 

Run this command to Add these missing software sources to your distro:

apt-get install -y software-properties-common curl
apt-add-repository ppa:ondrej/php -y
apt-add-repository ppa:nginx/development -y 

install PHP Specific Dependencies using :

apt-get install php7.1-cli php7.1-dev \
php7.1-pgsql php7.1-sqlite3 php7.1-gd \
php7.1-curl php7.1-memcached \
php7.1-imap php7.1-mysql php7.1-mbstring \
php7.1-xml php7.1-zip php7.1-bcmath php7.1-soap \
php7.1-intl php7.1-readline php-xdebug 

Set these PHP Module variables:

sudo sed -i "s/error_reporting = .*/error_reporting = E_ALL/" /etc/php/7.1/cli/php.ini
sudo sed -i "s/display_errors = .*/display_errors = On/" /etc/php/7.1/cli/php.ini
sudo sed -i "s/memory_limit = .*/memory_limit = 512M/" /etc/php/7.1/cli/php.ini
sudo sed -i "s/;date.timezone.*/date.timezone = UTC/" /etc/php/7.1/cli/php.ini

Install PHP-FPM and NGINX
apt-get install nginx php7.1-fpm

Setup Some PHP-FPM Options

echo "xdebug.remote_enable = 1" >> /etc/php/7.1/mods-available/xdebug.ini
echo "xdebug.remote_connect_back = 1" >> /etc/php/7.1/mods-available/xdebug.ini
echo "xdebug.remote_port = 9000" >> /etc/php/7.1/mods-available/xdebug.ini
echo "xdebug.max_nesting_level = 512" >> /etc/php/7.1/mods-available/xdebug.ini
echo "opcache.revalidate_freq = 0" >> /etc/php/7.1/mods-available/opcache.ini
sed -i "s/error_reporting = .*/error_reporting = E_ALL/" /etc/php/7.2/fpm/php.ini
sed -i "s/display_errors = .*/display_errors = On/" /etc/php/7.1/fpm/php.ini
sed -i "s/;cgi.fix_pathinfo=1/cgi.fix_pathinfo=0/" /etc/php/7.1/fpm/php.ini
sed -i "s/memory_limit = .*/memory_limit = 512M/" /etc/php/7.1/fpm/php.ini
sed -i "s/upload_max_filesize = .*/upload_max_filesize = 100M/" /etc/php/7.1/fpm/php.ini
sed -i "s/post_max_size = .*/post_max_size = 100M/" /etc/php/7.1/fpm/php.ini
sed -i "s/;date.timezone.*/date.timezone = UTC/" /etc/php/7.1/fpm/php.ini

Disable XDebug On The CLI

sudo phpdismod -s cli xdebug

Copy fastcgi_params to Nginx because they broke it on the PPA

cat > /etc/nginx/fastcgi_params << EOF
fastcgi_param	QUERY_STRING		\$query_string;
fastcgi_param	REQUEST_METHOD		\$request_method;
fastcgi_param	CONTENT_TYPE		\$content_type;
fastcgi_param	CONTENT_LENGTH		\$content_length;
fastcgi_param	SCRIPT_FILENAME		\$request_filename;
fastcgi_param	SCRIPT_NAME		\$fastcgi_script_name;
fastcgi_param	REQUEST_URI		\$request_uri;
fastcgi_param	DOCUMENT_URI		\$document_uri;
fastcgi_param	DOCUMENT_ROOT		\$document_root;
fastcgi_param	SERVER_PROTOCOL		\$server_protocol;
fastcgi_param	GATEWAY_INTERFACE	CGI/1.1;
fastcgi_param	SERVER_SOFTWARE		nginx/\$nginx_version;
fastcgi_param	REMOTE_ADDR		\$remote_addr;
fastcgi_param	REMOTE_PORT		\$remote_port;
fastcgi_param	SERVER_ADDR		\$server_addr;
fastcgi_param	SERVER_PORT		\$server_port;
fastcgi_param	SERVER_NAME		\$server_name;
fastcgi_param	HTTPS			\$https if_not_empty;
fastcgi_param	REDIRECT_STATUS		200;
EOF

Set The Nginx & PHP-FPM User

sed -i "s/user www-data;/user www-data;/" /etc/nginx/nginx.conf
sed -i "s/# server_names_hash_bucket_size.*/server_names_hash_bucket_size 64;/" /etc/nginx/nginx.conf

sed -i "s/user = www-data/user = www-data/" /etc/php/7.1/fpm/pool.d/www.conf
sed -i "s/group = www-data/group = www-data/" /etc/php/7.1/fpm/pool.d/www.conf

sed -i "s/listen\.owner.*/listen.owner = www-data/" /etc/php/7.1/fpm/pool.d/www.conf
sed -i "s/listen\.group.*/listen.group = www-data/" /etc/php/7.1/fpm/pool.d/www.conf
sed -i "s/;listen\.mode.*/listen.mode = 0666/" /etc/php/7.1/fpm/pool.d/www.conf

Create Nagios User

useradd nagios
usermod -a -G nagios www-data

Download Nagios Using:

wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.3.4.tar.gz

Extract the download using:

tar -zxvf nagios-4.3.4.tar.gz

Go into the extracted folder
cd nagios-4.3.4

configure the installer using:
./configure --with-nagios-group=nagios --with-command-group=www-data --with-mail=/usr/sbin/sendmail

If all runs fine. Execute: ```make all``

If that runs successuflly without any issues/errors and the configuration console output appears OK with you, then proceed to run the commands below:

make install 
make install-commandmode
make install-init
make install-config

Enable the Nagios service using:
systemctl enable nagios.service

Nagios Plugins

Download using:
wget https://nagios-plugins.org/download/nagios-plugins-2.2.1.tar.gz

Extract Using:
tar -zxvf nagios-plugins-2.2.1.tar.gz && cd nagios-plugins-2.2.1

Configure Using:
./configure --with-nagios-user=nagios --with-nagios-group=www-data --with-openssl

Compile Using:
make

Install Using:
make install

NRPE

Download to your local machine from:
http://downloads.sourceforge.net/project/nagios/nrpe-2.x/nrpe-3.1.1/nrpe-3.2.1.tar.gz

Push to remote machine using:
scp -P port_number Downloads/nrpe-3.2.1.tar.gz manu@eopio.com:/var/www/
where port_number is the ssh port number of your server. Default is 22

In your remote server, Extract the newly uploaded file using:
tar -zxvf nrpe-3.2.1.tar.gz && cd nrpe-3.2.1

Configure it Using:
./configure --enable-command-args --with-nagios-user=nagios --with-nagios-group=www-data --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/x86_64-linux-gnu --with-need-dh=no

dh sometimes causes config issues/errors so i set the glag --with-need-dh=no

Compile and Install Using:

make install
make install-init
make install-config

Enalbe NRPE Service:
systemctl enable nrpe.service

Configure Nagios

Open Config in your favourite terminal editor. I use vim:
vim /usr/local/nagios/etc/nagios.cfg

Uncomment #cfg_dir=/usr/local/nagios/etc/servers by Removing the # symbol

Create the direcory where we'll put all our server config files.
mkdir -p /usr/local/nagios/etc/servers

Configure Nagios Admin User

Install apache2-utils using:
apt get install apache2-utils

Create nagios admin user account using:
sudo htpasswd -b -c /usr/local/nagios/etc/.htpasswd nagiosadmin nagiospassword
-b enables the use of password from commandline rather than prompting for it.
-c creates a new file. Overwrites any existing in the same path.

Add the following setting to your Nginx Web server Config in /etc/nginx/sites-available

  auth_basic "Private";
  auth_basic_user_file /usr/local/nagios/etc/.htpasswd;

Configure Nagios Contact

Open the contact config file in your editor:
vim /usr/local/nagios/etc/objects/contacts.cfg

Change the default contact address to your email address

email   nagios@localhost   ; <<***** CHANGE THIS TO YOUR EMAIL ADDRESS ******

Configure NRPE Command

Open the commands config file in your editor:
vim /usr/local/nagios/etc/objects/commands.cfg

Append the following to the end of the file.

define command {
    command_name check_nrpe
    command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

This allows you to see check_nrpe command in your Nagios service definition.

NGINX Config

Ensure that your Nginx site config file in /etc/nginx/sites-available/ looks like this:

    server {
    

        listen 443;
        listen [::]:443;

        # force https-redirect

        ssl on;
        ssl_certificate /etc/letsencrypt/live/eopio.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/eopio.com/privkey.pem;

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;

        add_header Strict-Transport-Security "max-age=31536000";


        server_name localhost;

        root /var/www/ghost/system/nginx-root;

        index index.php index.html;

        location / {
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header Host $http_host;
            proxy_pass http://127.0.0.1:2368;

        }

        location ~ /.well-known {
            allow all;
    #	root /var/www/ghost/system/nginx-root;
        default_type "text/plain";	
        }

        location ~ /js {
        allow all;
        root /var/www/ghost/system/nginx-root;
        default_type "text/javascript";
        }

        location /apple-app-site-association {
        default_type application/json;
       }

        location /nagios {
            alias /usr/local/nagios/share;
            allow all;
            
            auth_basic "Private";
            auth_basic_user_file /usr/local/nagios/etc/.htpasswd;
    
            location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_param SCRIPT_FILENAME $request_filename;
                fastcgi_param AUTH_USER $remote_user;
                fastcgi_param REMOTE_USER $remote_user;
                fastcgi_pass unix:/run/php/php7.1-fpm.sock;
            }
            location ~ \.cgi$ {
                root /usr/local/nagios/sbin;
                rewrite ^/nagios/cgi-bin/(.*)\.cgi /$1.cgi break;
                include /etc/nginx/fastcgi_params;
                fastcgi_param SCRIPT_FILENAME $request_filename;
                fastcgi_param AUTH_USER $remote_user;
                fastcgi_param REMOTE_USER $remote_user;
                fastcgi_pass unix:/var/run/fcgiwrap.socket;
            }
        }

        location ~ ^/nagiosgraph/cgi-bin/(.*\.cgi)$ {
            alias /usr/local/nagiosgraph/cgi/$1;
            include /etc/nginx/fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $request_filename;
            fastcgi_param AUTH_USER $remote_user;
            fastcgi_param REMOTE_USER $remote_user;
            fastcgi_pass unix:/var/run/fcgiwrap.socket;
        }


        location /nagiosgraph {
            alias /usr/local/nagiosgraph/share;
        }

        location ~ \.php$ {
            include snippets/fastcgi-php.conf;
            fastcgi_pass unix:/run/php/php7.1-fpm.sock; 
        }


        client_max_body_size 50m;
    }


    server {
        listen 80;
        listen [::]:80;
    #
        server_name eopio.com;
    #	rewrite ^ https://$server_name$request_uri? permanent;
    #
        root /var/www/ghost/system/nginx-root;
        index index.html;
    #
        location / {
            rewrite ^ https://$server_name$request_uri? permanent;
    #		try_files $uri $uri/ =404;
        }
    }

Start Nagios service

systemctl start nagios

and restart nginx

systemctl restart nginx

If the console has no output, then all is good.
Otherwise, Keep Calm and read the error message.

If no errors, add nagios to startup commands:
sudo ln -s /etc/init.d/nagios /etc/rcS.d/S99nagios

Goto your_domain/nagios e.g eopio nagios
Enter your nagios admin username and password.

Get Monitoring!

Thank you and Feel free to reach out!