Understanding Nagios and its features

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What is Nagios?

Nagios Core, open-source software tool used for continuous monitoring of the applications, system metrics, server resources, network elements and custom services. The important feature of Nagios Core is that apart from working as a monitoring tool, it works as an altering service at the times when a critical error occurs and gets resolved. Originally designed to support Linux, it now works well on other Unix alternatives too. Noted for its flexible and scalability nature, it is the widely preferred monitoring tool among the DevOps culture. 
Need to know about how to install and configure Nagios on your system, visit the link, Installing and Configuring Nagios on Ubuntu 18.04

What it’s used for?

Nagios is used for several monitoring reasons, they include;

• For monitoring host resources such as disk usage, memory usage, processor load, system logs, etc.
• Continuous monitoring  of the infrastructure and processes.
• Does service checks in a parallel manner. 
• Used to identify network issues such as HTTP, POP3, SMTP, SSH, etc.
• Automatically resolves errors and other issues in critical situations.
• Helps you to maintain continuous security and system performance.

Types of Nagios

Nagios core

It is the free and open-source platform of Nagios Core (which is originally hailed as ‘Nagios’). The primary functions of the Nagios core are to monitor scheduling, monitor execution, monitor processing, event handling and as an alert manager. Further, it comes with several integrated APIs which facilitates in extending the performance of this tool with add-ons. 

Features:

Basic web interface for basic monitoring of,

1. Processor load
2. Disk usage
3. Currently active processes
4. Log files
5. SMTP protocol

Also, comes with extendable architecture and professional support options. 

Nagios XI

Being the enterprise-class version, it is the most powerful and reliable monitoring tool in trend. Besides offering all features as similar to the Nagios Core, it also provides extensive GUI, intuitive dashboard, monitoring wizards, scalability-rich back-end and front-end, performance graphing, mobile application, advanced reporting and large-scale configuration. 
In simple words, Nagios XI is an extended interface of Nagios Core with feature-rich capabilities.

Features:

1. Powerful Monitoring Engine
2. Updated Web Interface
3. Configuration Wizards
4. Advanced User Management
5. Intuitive Graphs and Dashboard
6. Capacity Planning

Nagios Addons:

Addons are additional software packages which are used to extend the functionality of software on which the addons are integrated. Nagios addons also work for the same purpose and use to extend the functionalities of Nagios to a larger level. It can be also used to integrate Nagios with other applications. There are numerous add-ons available for Nagios, they are,

• NRPE
• NSCA
• NDOUtils

To know about the other Nagios addons, visit https://exchange.nagios.org/
One of the widely used addons is the NRPE (Nagios Remote Plugin Executor). This NRPE add-on enables you to remotely execute Nagios plug-ins on Linux/Unix systems (remote hosts) and can also be used to communicate with some of the Windows’ add-ons too. 

NRPE (Nagios Remote Plugin Executor)

The Nagios Remote Plugin Executor, NRPE acts as an intermediate agent in allowing the remote monitoring of system metrics using scripts. The resources which NRPE allows to monitor includes; system load, memory usage and disk usage.The NRPE (Nagios Remote Plugin Executor) a.k.a. the agent is generally installed on a remote host (be it a Linux/Unix/Windows system) for which you need to monitor from the Nagios server. Also, it is what responsible for processing requests for the Nagios server. 
It does this work by using a plugin which is located on your remote host. At the time of request processing, NRPE assigns a plugin to execute the request and sends back the result to Nagios. The plugins are binary scripts and responsible for performing actual checks like disk space usage, load or logged on users. 
The NRPE is comprised of two fragments, namely,

check_nrpe plug-in

This is a plugin which will be located on the local monitoring server and responsible for contacting the NRPE daemon plug-in located on the remote host for processing requests. As soon as it receives check results from the NRPE daemon, it sends back to Nagios. 

NRPE daemon plug-in

In contrast to the check_nrpe plugin, the NRPE daemon plug-in will be running on the remote hosts (Linux/Unix and Windows too). Once it gets a request from the check_nrpe plugin or Nagios server, it executes the Nagios plugins to perform the check services and sends back the check results back to check_nrpe.

Comparing Nagios Core with Nagios XI

S.No:	Nagios Core	Nagios XI
1.	It is free and open-source software.	It is a freemium and extended interface of Nagios Core. (As free license limited to only 7 hosts)
2.	Offers a basic web interface for monitoring basic features.	Comprehensive Infrastructure monitoring with advanced features.
3.	Alerting services limited to mail, phone and custom methods.	Atop the same features as Nagios Core, adds RSS Feed and Per-user notifications.
4.	Designed to offer only basic reporting.	Advanced reporting with performance graphs, capacity planning and custom report creation.
5.	Basic level of offering User Interface and Distributed Monitoring.	Intuitive UI and advanced capabilities in offering Distributed Monitoring.
6.	Offers no more configuration editor and modification tools.	Comes with extensive configuration wizard such as  Bulk-Host Cloning Auto-discovery & auto-decommissioning Automated-backup scheduler
7.	A basic map network and no database back-end.	Available with database back-end and  Google Maps integration Custom Maps with Nagvis Network Replay

Note: Further, Nagios XI is available in two different editions called Standard Edition and Enterprise Edition. 
Both the Nagios Core and Nagios XI comes up with 4000+ plug-ins, extendable architecture, multi-tenant capabilities, proven OSS core, professional support options and other customizable options.

Nagios Installation Steps

1. Downloading Nagios 
2. Installing Nagios with prerequisites
3. Installation and configuration of Nagios
4. Installation and configuration of NRPE
5. Configuration of the web interface
6. Testing the Nagios web interface

Nagios GUI

The Wrap-Up:

Basically, both Nagios Core and Nagios XI is developed to offer continuous monitoring of system metrics, applications, networks and server resources. While Nagios Core is sufficient to carry out basic-level monitoring such as to monitor processing, execution, event handling and so forth, Nagios XI is designed to facilitate large-scale configurations and scheduling reporting. Additionally, the Nagios addons like NRPE, NSCA and NDOUtils are used to extend the functionalities of Nagios. The Nagios Plugins like check_nrpe and NRPE daemon are used to execute the check results.