To setup a minimal CentOS 6 64-bit Nagios server:
1] Setup CentOS 6 64-bit as in guide (http://blog.malaya-digital.org/setup-a-minimal-centos-6-0-64-bit-setup-with-networking/).
2] "yum update"
3] "vi /etc/sysconfig/selinux" and set "SELINUX=disabled".
4] "setenforce 0"
5] "shutdown -r now"
6] "rpm -ivh http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-7.noarch.rpm"
7] "yum install nagios nagios-plugins nagios-plugins-all nagios-plugins-nrpe php php-pear mod_ssl net-snmp net-snmp-utils sendmail"
8] To create the default Nagios user and assign a password to it:

htpasswd /etc/nagios/passwd nagiosadmin

9] To check Nagios configuration file for errors:

nagios -v /etc/nagios/nagios.cfg

10] Open port 443 for secure web access.
11] "usermod -a -G nagios apache"
12] "chkconfig nagios on"
13] "chkconfig httpd on"
14] "chkconfig sendmail on"
15] "service sendmail restart"
16] Edit /etc/httpd/conf/httpd.conf, and appropriately configure "ServerName". Also, edit /etc/httpd/conf.d/nagios.conf , and uncomment all "SSLRequireSSL" lines.
17] "service httpd restart"
18] Edit /etc/nagios/nagios.cfg, and uncomment "cfg_dir=/etc/nagios/servers".
19] Append to /etc/nagios/objects/commands.cfg:

define command{
command_name check_nrpe
command_line /usr/lib64/nagios/plugins/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

20] "mkdir /etc/nagios/servers"
21] "chown root:nagios /etc/nagios/servers"
22] "service nagios restart"
23] Open https://(IP or hostname of Nagios server)/nagios . Enter the credentials for the nagiosadmin user.

To configure a CentOS 6 64-bit Nagios NRPE client:
1] "yum update"
2] "vi /etc/sysconfig/selinux" and set "SELINUX=disabled".
3] "setenforce 0"
4] "shutdown -r now"
5] "rpm -ivh http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-7.noarch.rpm"
6] "yum install nagios-plugins-nrpe nagios-plugins-all nagios-nrpe openssl"
7] Edit /etc/yum.conf and append this to the "[main]" section:

exclude=nagios*

8] Edit /etc/nagios/nrpe.cfg . Look for:

allowed_hosts=127.0.0.1

Add the IP of your Nagios server. And add the IP of the Nagios client. Eg:

allowed_hosts=127.0.0.1,10.1.34.53,10.1.34.32

Also look for:

server_address=127.0.0.1

Assign the IP of your Nagios client. Eg:

server_address=10.1.34.32

9] "chown nrpe:nrpe /etc/nagios/nrpe.cfg"
10] Add to /etc/services :

nrpe 5666/tcp # NRPE

11] "chkconfig nrpe on"
12] "service nrpe restart"
13] open port 5666/tcp
14] To check if NRPE is working on the Nagios client(substitute the IP of the Nagios client; this command is to be executed on the client):

/usr/lib64/nagios/plugins/check_nrpe -H 10.1.34.32

15] You can also check NRPE daemon from the Nagios server(substitute the IP of the Nagios client; this command is to be executed on the server):

/usr/lib64/nagios/plugins/check_nrpe -H 10.1.34.32

16] Make sure the "COMMAND DEFINITIONS" in the /etc/nagios/nrpe.cfg file in the client are in order. They can be found near the end of the file.
17] "service nrpe restart"
18] Edit /etc/nagios/objects/templates.cfg in the Nagios server, and add the following(you may need to make adjustments in order to reconcile the settings described in this step to the /etc/nagios/nrpe.cfg settings in the client):

define host{
name linux-box ; Name of this template
use generic-host ; Inherit default values
check_period 24x7
check_interval 5
retry_interval 1
max_check_attempts 10
check_command check-host-alive
notification_period 24x7
notification_interval 30
notification_options d,r
contact_groups admins
register 0 ; DONT REGISTER THIS - ITS A TEMPLATE
}

19] Add a file to /etc/nagios/servers in the Nagios server(eg: "/etc/nagios/servers/linux-nagios-client.cfg" ; you can use any filename you like, just make sure it has a ".cfg" extension). It should look like this (substitute appropriate values for host_name, alias, and address)

define host{
use linux-box
host_name linux-nagios-client
alias CentOS 6
address 10.1.34.32
}

define service{
use generic-service
host_name linux-nagios-client
service_description Current Load
check_command check_nrpe!check_load
}

define service{
use generic-service
host_name linux-nagios-client
service_description Current users
check_command check_nrpe!check_users
}

define service{
use generic-service
host_name linux-nagios-client
service_description Root Partition
check_command check_nrpe!check_hda1
}

define service{
use generic-service
host_name linux-nagios-client
service_description Total Processes
check_command check_nrpe!check_total_procs
}

define service{
use generic-service
host_name linux-nagios-client
service_description Zombie Processes
check_command check_nrpe!check_zombie_procs
}

define service{
use generic-service
host_name linux-nagios-client
service_description SSH
check_command check_nrpe!check_ssh
}

define service{
use generic-service
host_name linux-nagios-client
service_description Swap Usage
check_command check_nrpe!check_swap
}

define service{
use generic-service
host_name linux-nagios-client
service_description Ping
check_command check_ping!100.0,20%!500.0,60%
}

20] Restart Nagios on the Nagios server:

service nagios restart

21] Open http://(IP or hostname of Nagios server)/nagios . Enter the credentials for the nagiosadmin user.