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Screenshot of the Nagios web interface
|Original author(s)||Ethan Galstad and others|
|Initial release||March 14, 1999|
|Stable release||3.5.0 / March 15, 2013|
Nagios (pron.: //) is an open source computer system monitor, network monitoring and infrastructure monitoring software application. Nagios offers monitoring and alerting for servers, switches, applications, and services. It alerts users when things go wrong and alerts them again when the problem has been resolved.
Nagios, originally created under the name NetSaint, was written and is currently maintained by Ethan Galstad, along with a group of developers actively maintaining both official and unofficial plugins. Nagios is a recursive acronym: "Nagios Ain't Gonna Insist On Sainthood", "Sainthood" being a reference to the original name NetSaint, which was changed in response to a legal challenge by owners of a similar trademark. "Agios" is also a transliteration of the Greek word άγιος which means "saint".
Nagios was originally designed to run under Linux, but also runs well on other Unix variants. It is a free software licensed under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.
In 2006, a survey among the nmap-hackers mailing list asked people what their favorite network security tools was. In the survey 3243 people voted; Nagios came in 67th overall and 5th among traffic monitoring tools. Nmap itself was excluded from the list. Another survey, in 2011, placed Nagios at 69th place. 
Nagios is open source software licensed under the GNU GPL V2. It provides:
- Monitoring of network services (SMTP, POP3, HTTP, NNTP, ICMP, SNMP, FTP, SSH)
- Monitoring of host resources (processor load, disk usage, system logs) on a majority of network operating systems, including Microsoft Windows with the NSClient++ plugin or Check MK.
- Monitoring of anything else like probes (temperature, alarms...) which have the ability to send collected data via a network to specifically written plugins
- Monitoring via remotely-run scripts via Nagios Remote Plugin Executor
- Remote monitoring supported through SSH or SSL encrypted tunnels.
- A simple plugin design that allows users to easily develop their own service checks depending on needs, by using their tools of choice (shell scripts, C++, Perl, Ruby, Python, PHP, C#, etc.)
- Available data graphing plugins
- Parallelized service checks
- The ability to define network host hierarchies using "parent" hosts, allowing detection of and distinction between hosts that are down or unreachable
- Contact notifications when service or host problems occur and get resolved (via e-mail, pager, SMS, or any user-defined method through plugin system)
- The ability to define event handlers to be run during service or host events for proactive problem resolution
- Automatic log file rotation
- Support for implementing redundant monitoring hosts
- An optional web-interface for viewing current network status, notifications, problem history, log files, etc.
- Data storage via text files rather than database
Nagios Agents 
Nagios Remote Plugin Executor (NRPE) is a Nagios agent that allows remote system monitoring using scripts that are hosted on the remote systems. It allows for monitoring resources such as disk usage, system load or number of users currently logged in. Nagios periodically polls the agent on the remote system using the
Nagios Remote Data Processor (NRDP) is a Nagios agent with a flexible data transport mechanism and processor. It is designed with an architecture that allows it to be easily extended and customized. NRDP uses standard ports and protocols (HTTP(S) and XML) and can be implemented as a replacement for NSCA.
This program is mainly used to monitor Windows machines. Being installed on a remote system NSClient++ listens to port TCP 1248. Nagios plugin that is used to collect information from this addon is called
check_nt. As NRPE, NSClient++ allows to monitor the so-called "private services" (memory usage, CPU load, disk usage, running processes, etc.)
See also 
- Comparison of network monitoring systems
- Icinga -- fork of Nagios
- Shinken—a Nagios compatible replacement
- N2rrd—a Nagios add-on to record data in a Round Robin Database
- NConf—a tool for configuring Nagios
- Opsview—an integrated bundle of tools that includes Nagios core
- Check MK an extension to Nagios that offloads work from the Nagios core, and allows distributed monitoring from several Nagios servers
- first release of NetSaint from the changelog at http://web.archive.org/web/20060501150621/http://www.netsaint.org/changelog.php
- Nagios 3.x Version History
- Galstad, Ethan (2003-05-03). offi "Nagios: FAQs : What does Nagios mean?". Nagios: Frequently Asked Questions. Nagios Enterprises, LLC. Retrieved 2009-03-06. "The official meaning is that N.A.G.I.O.S. is a recursive acronym which stands for "Nagios Ain't Gonna Insist On Sainthood"."
- "2005-02-22 - Ethan Galstad". Fosdem 2005. 2005-02-22. Retrieved 2009-03-06. "Although we were able to eventually reach an amicable agreement on my future use of the name "NetSaint", I felt it was prudent to change the name in order to prevent any future mishaps."
- Top 6 Traffic Monitoring Tools http://sectools.org/traffic-monitors.html
Further reading 
- Barth, Wolfgang; (2006) Nagios: System And Network Monitoring - No Starch Press ISBN 1-59327-070-4
- Barth, Wolfgang; (2008) "Nagios: System And Network Monitoring, 2nd edition - No Starch Press ISBN 1-59327-179-4
- Turnbull, James; (2006) Pro Nagios 2.0 - San Francisco: Apress ISBN 1-59059-609-9
- Josephsen, David; (2007) Building a Monitoring Infrastructure with Nagios - Prentice Hall ISBN 0-13-223693-1
- Dondich, Taylor; (2006) Network Monitoring with Nagios - O'Reilly ISBN 0-596-52819-1