NAGIOS SERVER 4.0.8

sudo su
apt-get update
apt-get upgrade -y
apt-get dist-upgrade -y
apt-get autoremove -y
apt-get install build-essential libgd2-xpm-dev openssl libssl-dev apache2-utils apache2 php5 -y
dd if=/dev/zero of=/swap bs=1024 count=2097152
mkswap /swap && sudo chown root. /swap && sudo chmod 0600 /swap && sudo swapon /swap
sh -c "echo /swap swap swap defaults 0 0 >> /etc/fstab"
sh -c "echo vm.swappiness = 0 >> /etc/sysctl.conf && sysctl -p"
useradd nagios
groupadd nagcmd
usermod -a -G nagcmd nagios
cd /root
curl -L -O http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-4.0.8.tar.gz
tar xvf nagios-*.tar.gz
cd nagios-4.0.8
./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
cd ../
curl -L -O http://nagios-plugins.org/download/nagios-plugins-2.0.3.tar.gz
tar xvf nagios-plugins-*.tar.gz
cd nagios-plugins-2.0.3
./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
make
make install
nano /usr/local/nagios/etc/nagios.cfg

Remove the hashtag in front of this line. 
----------------------------------------
Before --
#cfg_dir=/usr/local/nagios/etc/servers

After --
cfg_dir=/usr/local/nagios/etc/servers

mkdir /usr/local/nagios/etc/servers

Add Your Email
--------------
nano /usr/local/nagios/etc/objects/contacts.cfg

a2enmod rewrite cgi
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
ln -s /etc/apache2/sites-available/nagios.conf /etc/apache2/sites-enabled/
service nagios start
service apache2 restart
ln -s /etc/init.d/nagios /etc/rcS.d/S99nagios

nano /usr/local/nagios/etc/servers/monitor1.cfg

define host {
        use                             linux-server
        host_name                       monitor1 
        alias                           My first monitoring server.
        address                         PRIVATE_IP_NAGIOS_SERVER
        max_check_attempts              5
        check_period                    24x7
        notification_interval           30
        notification_period             24x7
}

define service {
        use                             generic-service
        host_name                       monitor1
        service_description             PING
        check_command                   check_ping!100.0,20%!500.0,60%
}

service nagios reload

NAGIOS CLIENT MACHINE

sudo su
apt-get update
apt-get upgrade -y
apt-get dist-upgrade -y
apt-get autoremove -y
apt-get install nagios-plugins nagios-nrpe-server -y

nano /etc/nagios/nrpe.cfg

Edit
----
allowed_hosts=127.0.0.1 PRIVATE_IP_NAGIOS_SERVER

service nagios-nrpe-server restart