The Nagios is a very helpful  monitoring tool to the remote machines, we can Install and Configure NRPE in CentOS and Red Hat OS. Once you have installing  NRPE daemon on the machine that needs monitoring like,

Host Status
Disk Space
Port Monitoring
Website content Monitoring
SSH
Email queues status
Mysql Status
System Load average and more...

The following steps how to configure NRPE in the client system

Before start, supporting packages and nagios plugins must be installed on the remote machine,

# yum -y update

# yum -y install gcc

# yum -y install perl

# yum -y install libtool

# yum -y install openssl-devel

Create a nagios user
 
#useradd nagios

downlaod the nagios plugin and nrpe
 
# cd /root/download

# wget http://assets.nagios.com/downloads/nagiosplugins/nagios-plugins-1.5.tar.gz

# wget http://sourceforge.net/projects/nagios/files/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz

# tar -xf nagios-plugins-1.5.tar.gz

# cd nagios-plugins-1.5

# ./configure
....
            --with-apt-get-command:
              --with-ping6-command: /bin/ping6 -n -U -w %d -c %d %s
               --with-ping-command: /bin/ping -n -U -w %d -c %d %s
                       --with-ipv6: yes
                      --with-mysql: no
                    --with-openssl: no
                     --with-gnutls: no
               --enable-extra-opts: no
                       --with-perl: /usr/bin/perl
...

# make

# make install

# chown nagios.nagios /usr/local/nagios

# chown -R nagios.nagios /usr/local/nagios/libexec

Next, install xinetd using yum command.
 
# yum install xinetd

Configure nrpe :
 
# tar -xf nrpe-2.15.tar.gz

# cd nrpe-2.15

# ./configure

*** Configuration summary for nrpe 2.15 09-06-2013 ***:
 General Options:
 ----------------
 NRPE port:    5666
 NRPE user:    nagios
 NRPE group:   nagios
 Nagios user:  nagios
 Nagios group: nagios
Review the options above for accuracy.  If they look okay,
type 'make all' to compile the NRPE daemon and client.

# make all

# make install-plugin

# make install-daemon

# make install-daemon-config

# make install-xinetd   

Note : Assume that the Nagios IP address is 192.168.0.100

open nrpe file and add nagios IP Address
 
# vi /etc/xinetd.d/nrpe
        only_from  = 127.0.0.1  192.168.0.100

# vi  /etc/services
    5666    /tcp      # NRPE

Next, you want to configure nrpe.cfg files
 
# vi  /usr/local/nagios/etc/nrpe.cfg

allowed_hosts=127.0.0.1 192.168.0.100

command[check_hosts]=/usr/local/nagios/libexec/check_ping -H 192.168.0.200 -w 3000.0,80% -c 5000.0,100% -p 5
command[check_load]=/usr/local/nagios/libexec/check_load -w 15,10,5 -c 30,25,20
command[check_xvda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/sda2
command[check_http]=/usr/local/nagios/libexec/check_http -I 192.168.0.200 -p 80
command[check_mysql]=/usr/local/nagios/libexec/check_mysql -u nagios -p nagios -H 192.168.0.200 -P 3306

Restart xinetd daemon with  service xinetd restart
 
# /etc/init.d/xinetd start
Starting xinetd:                                           [  OK  ]

Test nrpe is functional ?
 
# netstat -at | grep nrpe

tcp        0      0 *:nrpe                      *:*                         LISTEN

Port number allow in IP tables, If are you using Fedora/CentOS/RedHat
 
iptables -I RH-Firewall-1-INPUT -p tcp -m tcp -dport 5666 -j ACCEPT

May be have you configured CSF (ConfigServer Firewall) allow the Port

Restart IP Tables or CSF

Finally, check the NRPE Version using below command,
 
# /usr/local/nagios/libexec/check_nrpe -H localhost

NRPE v2.15

If you want to check services  manually
 
/usr/local/nagios/libexec/check_users -w 5 -c 10

USERS OK - 4 users currently logged in |users=4;5;10;0

I have faced  errors while configuring   nagios,

Error 1 :   "checking for SSL headers... configure: error: Cannot find ssl headers"

Error 2 :  connection refused by host in nagios server

The xinetd services does not run
# /etc/init.d/xinetd start

Next, How to Configure Client Machine in Nagios Server.