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Nagios Tutorial – Continuous Monitoring With Nagios

Last updated on Dec 15,2021 64.9K Views

Saurabh is a technology enthusiast working as a Research Analyst at Edureka.... Saurabh is a technology enthusiast working as a Research Analyst at Edureka. His areas of interest are - DevOps, Artificial Intelligence, Big Data and...
7 / 7 Blog from DevOps Lifecycle

Nagios Tutorial:

I believe there are very few posts on Nagios, and not much relevant information available over the internet. So, I think this post will give you a good clarity on this Monitoring tool. Nagios monitors your entire IT infrastructure to ensure systems, applications, services, and business processes are functioning properly. It is integral to the DevOps lifecycle and is a must for DevOps Certification. In this Nagios tutorial, I will be covering the below topics:

  1. Why We Need Continuous Monitoring?
  2. What Is Continuous Monitoring?
  3. What Is Nagios?
  4. How To Install Nagios?
  5. How To Add A Remote Server Using NRPE (Nagios Remote Plugin Executor).

Let’s begin this Nagios tutorial, by understanding why we need Continuous Monitoring because everything exists for a reason. So, let’s try to find out that reason.

Why We Need Continuous Monitoring?

Continuous Monitoring Tools resolve any system errors ( low memory, unreachable server etc. ) before they have any negative impact on your business productivity. 

Important reasons to use a monitoring tool are:

  • It detects any network or server problems
  • It determines the root cause of any issues
  • It maintains the security and availability of the service
  • It monitors and troubleshoot server performance issues
  • It allows us to plan for infrastructure upgrades before outdated systems cause failures
  • It can respond to issues at the first sign of a problem
  • It can be used to automatically fix problems when they are detected
  • It ensures IT infrastructure outages have a minimal effect on your organization’s bottom line
  • It can monitor your entire infrastructure and business processes

Yeah, it does a lot of cool work, but what is it?

What is Continuous Monitoring?

Let me first tell you where Continuous Monitoring lies in the DevOps life-cycle, consider the diagram below:

DevOps Life-Cyce - Nagios Tutorial - Edureka

Looking at the diagram you must be thinking this is the last stage in the DevOps lifecycle, but this is not the case. There is no end to a DevOps life cycle, and that is the reason for the infinity symbol. Continuous Monitoring comes into the picture, once the application is deployed on the production servers.

Continuous Monitoring is all about the ability of an organization to detect, report, respond, contain and mitigate the attacks that occur, in its infrastructure.

Continuous Monitoring is actually not new, it’s been around for some time. For years our security professionals are performing static analysis from – system log, firewall logs, IDS logs, IPS logs etc. But, it did not provide proper analysis and response. Today’s Continuous Monitoring approache gives us the ability to aggregate all of the events that I discussed above, co-relate them, compare them and then estimate the organization’s risk posture.

If we take all these pieces and ensure the interlinking between them. This is the crux of Continuous Monitoring.

Let me explain this with a use-case. Consider the diagram below:


Continuous Monitoring - Nagios Tutorial - EdurekaNow, let me explain you the above diagram:

  1. We have various security tools, like Firewall, IDS, End Point Protection etc. they are connected with a ‘Security Information and Event Management system.
  2. In order to achieve Continuous Monitoring, we need to have all the parts talking to each other, let me explain that to you.
  3. So we have security tools and series of ‘End Points’,  this can include client and servers, routers, switches, mobile devices and so on.
  4. These two groups can then talk to a Security Information and Event Management system (SIEM), through a common language and in more automated fashion.
  5. Connected to this SIEM there are two important components, first one is a Data Warehouse. Now to this Data Warehouse, we will connect ‘Analytics’ and ‘Security Intelligence’.
  6. Security intelligence (SI) is the information relevant to protecting an organization from external and insider threats as well as the processes, policies and tools designed to gather and analyze that information.
  7. This SIEM is also connected to a ‘Governance Risk and Compliance System’ it basically provides dashboarding. 
  8. To this ‘Governance Risk and Compliance System’  we attach a risk database. This gives us ‘Actionable Intelligence’. 
  9. Actionable Intelligence is nothing but information that can be acted upon, with the further implication that actions should be taken.

So here, we are Monitoring the events on the on going basis and determining what level of risk we are experiencing. With this, we can correlate the events at the SIEM. We can perform Network Behavior and Anomaly Detection at the ‘Analytics Engine’. This is what Continuous Monitoring is all about:

The integration of an organization security tools, the aggregation, normalization and correlation of the data that is produced by the security tools. The analysis of that data, based on the organization’s risk goals and threat knowledge, and near real-time response to the risks identified.

‘If you can’t measure it you can’t manage it’. I hope you know what I am talking about.


Next up in this Nagios tutorial, I will be introducing you to one of the most famous monitoring tool ‘Nagios’.

What is Nagios?

Nagios is used for Continuous monitoring of systems, applications, services, and business processes etc in a DevOps culture. In the event of a failure, Nagios can alert technical staff of the problem, allowing them to begin remediation processes before outages affect business processes, end-users, or customers. With Nagios, you don’t have to explain why an unseen infrastructure outage affect your organization’s bottom line.

Let me explain to you how Nagios works. Consider the diagram below:

Nagios Working - Nagios Tutorial - EdurekaNagios runs on a server, usually as a daemon or a service.

It periodically runs plugins residing on the same server, they contact hosts or servers on your network or on the internet. One can view the status information using the web interface. You can also receive email or SMS notifications if something happens.
The Nagios daemon behaves like a scheduler that runs certain scripts at certain moments. It stores the results of those scripts and will run other scripts if these results change.

Plugins: These are compiled executables or scripts (Perl scripts, shell scripts, etc.) that can be run from a command line to check the status or a host or service. Nagios uses the results from the plugins to determine the current status of the hosts and services on your network.

Let’s now discuss it’s architecture.

Nagios Architecture:

  • Nagios is built on a server/agents architecture.
  • Usually, on a network, a Nagios server is running on a host, and Plugins interact with local and all the remote hosts that need to be monitored.
  • These plugins will send information to the Scheduler, which displays that in a GUI.

Nagios Architecture - Nagios Tutorial - Edureka

I also need to explain you NRPE (Nagios Remote Plugin Executor). 

The NRPE addon is designed to allow you to execute Nagios plugins on remote Linux/Unix machines. The main reason for doing this is to allow Nagios to monitor “local” resources (like CPU load, memory usage, etc.) on remote machines. Since these public resources are not usually exposed to external machines, an agent like NRPE must be installed on the remote Linux/Unix machines.

Consider the diagram below:


NRPE Architecture - Nagios Tutorial - Edureka

  • The check_nrpe plugin, resides on the local monitoring machine.
  • The NRPE daemon, runs on the remote Linux/Unix machine.
  • There is a SSL (Secure Socket Layer) connection between monitoring host and remote host as shown in the diagram above.

Now in this Nagios tutorial, it is time for some Hands-on.

Let’s start by installing Nagios Core

Install Nagios Core:

The complete process to install Nagios can be summarized in four steps:

  1. Install Required Packages In The Monitoring Server
  2. Install Nagios Core, Nagios Plugins And NRPE (Nagios Remote Plugin Executor)
  3. Set Nagios Password To Access The Web Interface
  4. Install NRPE In Client

Step – 1: Install Required Packages On The Monitoring Server:

Visit the website:

Epel Repository - Nagios Tutorial - Edureka

Click on i386, and then you will be redirected to a page.

Epel Repository - Nagios Tutorial - Edureka

Since I am using CentOS 6, so I will right click and copy the link location of ‘epel-release-6-8.noarch.rpm‘, as shown in the above screenshot.

Open the terminal and use rpm -Uvh command and paste the link.

Install EPEL Repository - Nagios Tutorial - Edureka

We need to download one more repository, for that visit the website ‘

Nagios Package - Nagios Tutorial - Edureka

Right-click and copy the link location for ‘remi-release-6.rpm

Again open the terminal and use rpm -Uvh command and paste the link.

Install Packages - Nagios Tutorial -Edureka

Fine, so we are done with the pre-requisites. Let’s proceed to the next step.

Step – 2: Install Nagios Core, Nagios Plugins And NRPE (Nagios Remote Plugin Executor):

Execute the below command in the terminal:

yum -y install nagios nagios-plugins-all nagios-plugins-nrpe nrpe httpd php

This will install Nagios, Nagios Plugins, Plugins for NRPE, NRPE, Apache and PHP

Apache web server is required to monitor the current web server status.

Php is used to process dynamic content of the site date.

Install Nagios Plugins NRPE Apache Php - Nagios Core - Edureka

Next, we need to enable Apache and Nagios service:

chkconfig httpd on && chkconfig nagios on

Start Nagios And Apache Service - Nagios Tutoria - Edureka

Our next step is to start Nagios and Apache:

service httpd start && service nagios start

Start Nagios And Apache - Nagios Tutoria - Edureka

Now, I will enable swap memory of at least 1GB. It’s time to create the swap file itself using the dd command:

dd if=/dev/zero of=/swap bs=1024 count=2097152

Enable Swap Memory - Nagios Tutorial - EdurekaSwap is basically used to free some, not so frequently accessed information from RAM, and move it to a specific partition on our hard drive.

Now that you have created the swap partition, use the command mkswap to setup the swap partition. This is going to prepare the swap file by creating a linux swap area.

mkswap /swap

To prevent the file from being world-readable, you should set up the correct permissions on the swap file:

chown root. /swap 
chmod 0600 /swap

If you see no errors, your swap space is ready to use. To activate it immediately, type:

swapon /swap

Make Swap - Nagios Tutorial - Edureka

This file will last on the virtual private server until the machine reboots. You can ensure that the swap is permanent by adding it to the fstab file.

echo /swap swap swap defaults 0 0 >> /etc/fstab

Edit Fstab File - Nagios Tutorial - Edureka

The operating system kernel can adjust how often it relies on swap through a configuration parameter known as swappiness.

To find the current swappiness settings, type:

cat /proc/sys/vm/swappiness

Swapiness can be a value from 0 to 100. Swappiness near 100 means that the operating system will swap often and usually, too soon. Although swap provides extra resources, RAM is much faster than swap space. Anytime something is moved from RAM to swap, it slows down.

A swappiness value of 0 means that the operating will only rely on swap when it absolutely needs to. We can adjust the swappiness with the sysctl command. To make your VPS automatically apply this setting every time it boots up, you can add the setting to the /etc/sysctl.conf file:

echo vm.swappiness = 0 >> /etc/sysctl.conf && sysctl -p

Edit Sysctl.conf - Nagios Tutorial - Edureka

Finally, we are done with the second step.

Let’s proceed further and set Nagios password to access the web interface.

Step – 3: Set Nagios Password To Access The Web Interface:

Set the password to access the web interface, use the below command:

htpasswd -c /etc/nagios/passwd nagiosadmin

Nagios Credentials -Nagios Tutorial - Edureka

Type the password and confirm it by retyping it.

Now, open the browser. Here, type your public IP or hostname/nagios. Consider the example below:

Nagios Password - Nagios Tutorial - Edureka

Here, give the user name and password. By default, the user name is nagiosadmin, and password is what you have set in the previous step. Finally, press OK.

After this, you will directed to Nagios Core dashboard.

Nagios Core Dashboard - Nagios Tutorial - Edureka

You can click on hosts and see the what all hosts your Nagios Core is currently monitoring.

Nagios Hosts - Nagios Tutorial - Edureka

You can notice it is only monitoring one host, i.e. localhost. If I want my Nagios Core to monitor a remote host, I need to install NRPE in that remote host. This brings us to the next step, install NRPE In client/machine that you want Nagios to monitor.

Step – 4: Install NRPE In Client:

Alrighty then, let’s install NRPE in the client machine.

Firstly, you need to install the required packages like I did on my Nagios server machine. So, just execute the same commands, consider the below screenshots:

Nagios Packages - Nagios Tutorial - Edureka

Nagios Packages - Nagios Tutorial - Edureka

Now install Nagios, Nagios Plugins and NRPE in client:

yum -y install nagios nagios-plugins-all nrpe

Install Nagios, Nagios Plugins, NRPE - Nagios Tutorial - Edureka

Once it is installed, enable the NRPE service:

chkconfig nrpe on

Enable The NRPE Service - Nagios Tutorial - Edureka
Our next step is to edit the, nrpe.cfg file. I will be using the vi editor, you can choose any other editor also:

Edit nrpe.cfg File - Nagios Tutorial - Edureka

You need to add the IP address of your monitoring server, in the allowed host line, consider the below screenshot:

Allowed Host - Nagios Tutorial - Edureka

Here, the IP address of my monitoring server is

Now, we need to setup firewall rules to allow connection between monitoring server and client. 

iptables -N NRPE

-A option is used to append the new rule to the end of a chain. If you want to put it somewhere else in the chain, you can use the -I option which allows you to specify the position of the new rule.

The below command accepts tcp requests on ports 5666.

iptables -I INPUT -s 0/0 -p tcp --dport 5666 -j NRPE
iptables -I NRPE -s -j ACCEPT
iptables -A NRPE -s 0/0 -j DROP

This will basically configure iptables to accept packets from a particular host, in my case –, and drops the packets from other hosts.

Now, I will save these configurations:

/etc/init.d/iptables save

Save iptables Configurations - Nagios Tutorial - Edureka

Start NRPE service now.

service nrpe start

Start Nrpe - Nagios Tutorial - Edureka

Now go back to the Monitoring server.

Here, I need to edit nagios.cfg file.

vi /etc/nagios/nagios.cfg

Edit Nagios.cfg File - Nagios Tutorial - Edureka

Uncomment the the line – cfg_dir = etc/nagios/servers

Uncomment cfg_dir - Nagios Tutorial - Edureka

Make ‘server’ directory, for that use mkdir command.

mkdir /etc/nagios/servers/

Make Directory Servers - Nagios Tutorial - Edureka

Change your working directory to servers.

cd /etc/nagios/servers

Enter Servers Directory - Nagios Tutorial - Edureka

Create a new file in this directory with .cfg extension and edit it. I will name it as client.cfg, and I will be using vi editor.

vi /etc/nagios/servers/client.cfg

Edit Clients.cfg - Nagios Tutorial - Edureka

Here add the below lines:

Monitoring Services In Hosts - Nagios Tutorial - Edureka This basically includes the kind of services I want to monitor. Give the hostname of the machine and its ip address which you want Nagios to monitor.

Similarly, you can add number of services that you want to monitor. The same configurations can be used to add ‘n’ number of clients.

Last step guys, set the folder permissions correctly and restart Nagios.

chown -R nagios. /etc/nagios/

Set Folder Permissions - Nagios Tutorial - Edureka

Now, restart Nagios

service nagios restart

Restart Nagios - Nagios Tutorial - Edureka

Open the browser and again type the host name or public ip/nagios/. In my case it is localhost/nagios/.

Nagios Dashboard - Nagios Tutorial - Edureka

Click on hosts to see all the machines Nagios is currently monitoring.

Added Remote Hosts - Nagios Tutorial - Edureka

Here you can notice, it is currently monitoring the client machine (hostname of the machine that I want Nagios to monitor). Basically, we have added a remote host using NRPE.

I hope you have enjoyed reading this Nagios tutorial, I will be coming up with more blogs on Nagios soon.


If you found this Nagios tutorial relevant, check out the DevOps training by Edureka, a trusted online learning company with a network of more than 250,000 satisfied learners spread across the globe. The Edureka DevOps Certification Training course helps learners gain expertise in various DevOps processes and tools such as Puppet, Jenkins, Nagios, Ansible, Chef, Saltstack and GIT for automating multiple steps in SDLC.

Got a question for me? Please mention it in the comments section and I will get back to you.

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Nagios Tutorial – Continuous Monitoring With Nagios