NRPE configuration

Configuration in remote machine:
Create a new nagios user account and give it a password.
/usr/sbin/useradd nagios
passwd nagios

Download and extract nagios plugins
wget https://www.nagios-plugins.org/download/nagios-plugins-1.5.tar.gz

apt-get install libssl-dev
tar zxvf nagios-plugins-1.5.tar.gz
cd nagios-plugins-1.5

Compile and install the plugins.
./configure --with-nagios-user=nagios --with-nagios-group=nagios
make
make install

The permissions on the plugin directory and the plugins will need to be fixed at this point, so run the following commands.
chown nagios.nagios /usr/local/nagios
chown -R nagios.nagios /usr/local/nagios/libexec

Install the NRPE daemon as a service under xinetd.
apt-get install xinetd

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

./configure --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/i386-linux-gnu/
make all

for 64bit users:
ln -s /usr/lib/x86_64-linux-gnu/libssl.so /usr/lib/libssl.so
./configure
make all

Install the NRPE plugin (for testing), daemon, and sample daemon config file.
make install-plugin
make install-daemon
make install-daemon-config

Install the NRPE daemon as a service under xinetd.
make install-xinetd

Edit the /etc/xinetd.d/nrpe file and add the IP address of the monitoring server to the only_from directive.
only_from = 127.0.0.1 <nagios_ip_address>

save and exit
:wq

Add the following entry for the NRPE daemon to the /etc/services file.
nrpe 5666/tcp # NRPE

save and exit
:wq

Restart the xinetd service.
service xinetd restart

Test NRPE locally
Make sure the nrpe daemon is running under xinetd.
netstat -at | grep nrpe

The output out this command should show something like this:
tcp 0 0 *:nrpe *:* LISTEN

Configuration in Nagios Machine:
Install nrpe plugin in the machine that runs Nagios
wget http://sourceforge.net/projects/nagios/files/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz
tar zxvf nrpe-2.15.tar.gz
cd nrpe-2.15

./configure --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/i386-linux-gnu/
make all
make install-plugin

for 64bit users:
ln -s /usr/lib/x86_64-linux-gnu/libssl.so /usr/lib/libssl.so
./configure
make all
make install-plugin

Test the communication between host and remote machine.
/usr/local/nagios/libexec/check_nrpe -H <ip_address_remote_machine>

You should get a string like this:
NRPE v2.15

Configure Nconf with nrpe commands (some of them)
Inside the file /usr/local/nagios/etc/nrpe.cfg it is possible to see the following lines:

#Check current users
command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10      

#Current Load
command[check_load]=/usr/local/nagios/libexec/check_load -w 15,10,5 -c 30,25,20

#Root Partition
command[check_sda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/sda1

#Zombie Processes
command[check_zombie_procs]=/usr/local/nagios/libexec/check_procs -w 5 -c 10 -s Z

#Total Processes
command[check_total_procs]=/usr/local/nagios/libexec/check_procs -w 150 -c 200

#Swap Usage
command[check_remote_swap]=/usr/local/nagios/libexec/check_swap -w 70% -c 90%

Before use the string check_disk make sure that /dev/xxx is same as you have for example:

df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       109G   18G   86G  17% /

Configure nconf to use nrpe:
In nconf menu click on add in checkcommands option and write:

check command name: check_users
default service name: Check Users
check command line: $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_users

Default service template: generic_service

then click on submit.

Now the command is ready and can be add as service in the configuration.

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