CONTINUAL INTEGRATION

Problem scenario
You have Nagios core installed on a server in AWS.  You want a new AWS RedHat Enterprise Linux server to have the NRPE agent.  You want thorough directions for this process to potentially automate it.  How do you install and configure the NRPE agent on a RHEL server and configure a Nagios core server to monitor it?

Solution
Prerequisites

1. You need to know the internal IP address of 1) the Nagios core server; for RHEL 7.x, use ifconfig. 2) the server that will receive the NRPE agent. 
2. The AWS Security Group must have a certain rule configured for Nagios to perform ping tests.  We suggest you go to Security Groups.  Go to Inbound Rules and click "Edit."  Click "Add Rule."  For type use "ALL ICMP - IPv4." For "Source" keep it at Custom.  For the blank field use this (where x.x.x.x is the internal IP address of the Nagios core server):  x.x.x.x/32

Method
#1  (Done from Nagios client; the server destined to have the NRPE agent.)   Below is the content of a script.  You must change the x.x.x.x to the IP address of the Nagios core server.  Then run the script below on the server that you want to have the NRPE agent installed and configured on. (To run it you might run a command like this sudo bash /tmp/install.sh.)

# Script written by continualintegration.com
#!/bin/bash

yum -y update

yum -y install https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

yum -y install nrpe nagios-plugins-users nagios-plugins-load nagios-plugins-swap nagios-plugins-disk nagios-plugins-procs

cd /etc/nagios
cp nrpe.cfg nrpe.cfg.bak
cat nrpe.cfg.bak | sed -e 's/allowed_hosts=127.0.0.1,::1/allowed_hosts=x.x.x.x/g' > nrpe.cfg
# The old way of doing the above command required perl. Here it is:
# perl -i -pe 's/.*/allowed_hosts=x.x.x.x/ if $.==81' nrpe.cfg
​# /etc/nagios/nrpe.cfg has an allowed_hosts stanza
# This setting can support multiple Nagios core servers.  The IP addresses should be separated by a comma if you have more than one.

systemctl restart nrpe
systemctl enable nrpe

# Last line of script

#2 (Done from Nagios core server)  vi /usr/local/nagios/etc/servers/contint.cfg
The content below is what you'll need for this contint.cfg file.  You just have to change three parts:

i) Replace y.y.y.y with the IP address printed at the end of the script of #3 (the internal IP address of the RHEL server to get the NRPE agent).

ii) Replace the "contint" in every spot below with the hostname found in step #3.

# RedHat Linux Host configuration file.  (Largely taken from https://www.howtoforge.com/tutorial/ubuntu-nagios/)
# You can create other .cfg files with unique names in the same directory /usr/local/nagios/etc/servers/

define host {
        use                          linux-server
        host_name                    contint
        alias                        contint
        address                      y.y.y.y
        register                     1
}

define service {
      host_name                       contint
      service_description             PING
      check_command                   check_ping!100.0,20%!500.0,60%
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}

define service {
      host_name                       contint
      service_description             Check Users
      check_command           check_local_users!20!50
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}

define service {
      host_name                       contint
      service_description             Local Disk
      check_command                   check_local_disk!20%!10%!/
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}

define service {
      host_name                       contint
      service_description             Check SSH
      check_command                   check_ssh
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}

define service {
      host_name                       contint
      service_description             Total Process
      check_command                   check_local_procs!250!400!RSZDT
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}

#3 (Done from Nagios core server) 
Run this command:
sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

If an error is found in the output, correct the problem.  If zero errors are found, proceed to step #4.

#4  (Done from Nagios client; the server destined to have the NRPE agent.) 
Run these commands:

sudo systemctl restart nrpe
sudo systemctl enable nrpe

#5  (Done from Nagios core server) 
Run these commands:

sudo systemctl restart httpd
sudo systemctl restart nagios