I did bump on this software few year back.. but never got chance to play with it.
just saw someone else using it at that moment..

let’s give a try.. with rpmforge yum repo enabled ..

yum search nagios

wow.. someone else already made it before..

yum install nagios nagios-plugins nagios-plugins-nrpe nagios-devel -y

Hmmm.. re-configure and set password file.

vi /etc/nagios/cgi.cfg
htpasswd -c /etc/nagios/htpasswd.users nagiosadmin

start the daemon & restart HTTPd

service nagios restart
service httpd restart

then use http://localhost/nagios
enter the password..

should be able to see localhost service being monitor and all..
to add few other thing..

I love to define the folder into /etc/nagios/nagios.cfg
add this line

cfg_dir=/etc/nagios/namran

then create a directory and set permission accordingly.

mkdir /etc/nagios/namran
chown nagios:nagios /etc/nagios/namran

then create the config file to be added.

vi /etc/nagios/namran/wwwnamran.cfg

add the content such as below

define host{
use linux-server
host_name www.namran.net
alias www.namran.net
address 210.48.152.20
}
define service{
use generic-service
host_name www.namran.net
service_description PING
check_command check_ping!100.0,20%!500.0,60%
}
define service{
use generic-service
host_name www.namran.net
service_description HTTP
check_command check_http
notifications_enabled 0
}
define service{
use generic-service
host_name www.namran.net
service_description Cpanel
check_command check_tcp!2082!
notifications_enabled 0
}

chown nagios:nagios /etc/nagios/namran/*.cfg

restart the nagios service

service nagios restart

repeat that with next few server and service that you might want to monitor..

and the result is such as below.. and few other..

Tactical overview