What is Nagios?

In 1996 the American, Ethan Galstad developed an ap­pli­ca­tion for MS-DOS, which made it possible to check the ac­ces­si­bil­i­ty of Novell NetWare servers using third party programs. Based on this simple con­struc­tion, he released the open source project, NetSaint, three years later, which is the pre­de­ces­sor of what’s known today as the mon­i­tor­ing software Nagios. Galstad founded the company Nagios En­ter­pris­es in 2007, re­spon­si­ble for the de­vel­op­ment and dis­tri­b­u­tion of the open source variant Nagios Core as well as different fee-based editions.

Nagios is licensed under the GNU-GPL license and runs natively on various Unix-like systems. But with the help of virtual machine software it can also be used on Windows operating systems. To install and operate the mon­i­tor­ing tool, you require a web server with a PHP module: Apache is rec­om­mend­ed by the man­u­fac­tur­er. In addition to the program core, you also need to download the official Nagios plugin. These modules are re­spon­si­ble for the im­ple­men­ta­tion and analysis of various mon­i­tor­ing requests from networks, hosts, and services, and are therefore in­dis­pens­able for the IT software’s func­tion­al­i­ty.

It doesn’t matter if you want to monitor an in­di­vid­ual computer system, a database, or a server: you don’t install the software on the re­spec­tive devices, but rather on a separate Nagios server. From here, you configure (in an object-ori­en­tat­ed way) which systems and processed are to be observed. Every­thing revolves around these four com­po­nents or object types:

• Hosts: as a host, you define all devices, servers, databases, etc. belonging to the network that you want to monitor. The most important indicator of a host is the re­spec­tive IP address.
  
  
• Services: with services, you define which host features Nagios should check. These features may be services that run on the host (i.e. HTTP, FTP, etc.), internal at­trib­ut­es such as available disk space, or physical char­ac­ter­is­tics such as the tem­per­a­ture value of the hardware.
  
  
• Commands: with commands, you control the sequence of mon­i­tor­ing. You can configure how the host and service mon­i­tor­ing is designed and when Nagios should notify you if something happens.
  
  
• Contacts: due to the defined contacts, Nagios knows who to notify should an event occur. The software sends an e-mail, a text message, or a voice message depending on what you’ve specified.

Although Nagios itself isn’t installed on different hosts, plugins (which check internal data) run directly on the hosts. Remote access can be used to solve this, but it’s rarely used as it’s with more effort and dis­pro­por­tion­ate system use (with a secure con­nec­tion). The easier al­ter­na­tive is to install specific programs on the hosts. These perform only from the pre-con­fig­ured system queries and send the results via network ports to the Nagios server. The Nagios Remote Plugin Executor (NRPE) is used by default here and with its support, Nagios can also monitor Windows systems.

An ad­di­tion­al pos­si­bil­i­ty for using Nagios for checking re­spec­tive data is by carrying out a passive check. In this case, no in­for­ma­tion is requested from the mon­i­tor­ing software, but rather from another program such as Nagios Service Check Acceptor (NSCA). The mon­i­tor­ing server is only notified of the result after it’s been processed. Re­gard­less of the type of mon­i­tor­ing, the Nagios server always transmits one of the following status values:

• 0 (OK): no problems were found during the review.
• 1 (WARNING): the review resulted in a warning message.
• 2 (CRITICAL): a critical problem arose during the review.
• 3 (UNKNOWN): the status of the host or service could not be de­ter­mined.

In addition, the mon­i­tor­ing program also receives a time stamp, an internal command as well as an optional text that can be issued during the review.

Nagios is in demand as a mon­i­tor­ing solution due to its huge range of functions. The free source code means that the software can be cus­tomized and extended exactly how you want it. More than 1,000 plugins have been developed as well as various front ends, which are mostly provided for free. Even in­ex­pe­ri­enced ad­min­is­tra­tors can build a feature set that meets the needs of their network without having to program their own in­ter­faces them­selves. The only major drawback of Nagios is the complex con­fig­u­ra­tion of different types of servers and objects. For this reason, you’re advised to read the tool’s in­struc­tions and watch tutorials on the official Nagios website.

Nagios has become the norm of IT mon­i­tor­ing, but other al­ter­na­tives also exist when it comes to network mon­i­tor­ing. There are several similar, but less complex tools which are also rec­om­mend­ed. Also belonging to the open source sector is the en­ter­prise solution, Zabbix. Pro­pri­etary solutions such as CA deal with more than just mon­i­tor­ing; they also manage the complete network man­age­ment and are – as long as the budget covers it – good al­ter­na­tives to Nagios for large busi­ness­es.