Install Nagios 4 which is an enterprise open source monitoring system.
Before proceed with Nagios installation make sure that the Apache and PHP are installed and working properly ,
1.) Install Nagios.Also Install basic plugins to monitor nagios server itself.
# install from EPEL
[root@ ~]# yum --enablerepo=epel -y install nagios nagios-plugins-{ping,disk,users,procs,load,swap,ssh,http}
2.) Configure Nagios. [root@~]# vi /etc/httpd/conf.d/nagios.conf
# line 24-26, change settings to set access permissionlike follows ( set for line 54-56, too )
#
Require all granted
#
Require local
Require ip 127.0.0.1 10.0.0.0/24
# add nagios admin user[root@~]# htpasswd /etc/nagios/passwd nagiosadmin
New password: # set any password
Re-type new password:Adding password for user nagiosadmin[root@~]# systemctl start nagios
[root@~]# systemctl enable nagios
[root@~]# systemctl restart httpd
3.) Allow HTTP service in the server firewall.[root@dlp ~]# firewall-cmd --add-service={http,https} --permanent
success
[root@dlp ~]# firewall-cmd --reload
success
4.) Access to the "http://(Nagios server's hostname or IP address)/nagios/" from a client which is in the network allowed by Nagios server and authenticate with the Nagios admin user "nagiosadmin" to login.
5.) After successing authentication, the Nagios admin site is displayed
6.) It's possible to view system status to click "Tactical Overview" and so on. 
