Nagios
Nagios is a free and open-source monitoring system that enables organizations to monitor IT infrastructure, including servers, networks, applications, and services, in order to detect and resolve issues proactively before they impact critical business processes.[1] Originally developed by Ethan Galstad, Nagios traces its roots to 1996 when Galstad created a simple MS-DOS application to ping Novell NetWare servers, followed by a Linux-based monitoring tool in 1998.[2] In 1999, Galstad released the software as an open-source project under the name NetSaint, which was renamed Nagios in 2002 due to trademark concerns.[2] The project quickly gained traction through its extensible plugin architecture, with the Nagios Plugins project emerging as a separate initiative to support community-developed extensions.[2] At its core, Nagios provides comprehensive monitoring capabilities, such as tracking system metrics, network protocols, and service performance across Windows, Linux, and other environments, while sending alerts for failures and recoveries via email, SMS, or custom scripts.[1] It features intuitive dashboards, automated reporting on outages, events, notifications, and SLA compliance, as well as tools for trending analysis, capacity planning, and scheduled downtime management.[1] This flexibility has fostered a global community of users and developers, resulting in thousands of add-ons and over 8.5 million downloads of Nagios Core by 2024.[2] Nagios has evolved significantly since its inception, with key milestones including the founding of Nagios Enterprises, LLC in 2007 by Galstad to support commercial development, the release of Nagios XI as the first enterprise-grade product in 2009, and subsequent launches like Nagios Fusion in 2010 for centralized dashboards, Nagios Core 4 in 2013, and Nagios Log Server in 2014.[2] Today, it serves as the foundation for a suite of monitoring solutions trusted by more than 10,000 customers worldwide, emphasizing prevention of downtime, automation of issue resolution, and minimization of financial impacts from IT disruptions.[1][2]Introduction
Definition and Purpose
Nagios is an open-source system and network monitoring application designed to track the availability, performance, and uptime of various IT components, including hosts, services, and applications.[3] It operates by continuously checking specified targets for issues, such as service failures or resource thresholds, and alerting administrators when problems arise or recover.[4] The primary purpose of Nagios lies in enabling proactive IT infrastructure management, where it detects potential issues early to prevent disruptions, automates notifications and responses through mechanisms like event handlers, and supports overall business continuity by minimizing downtime.[3] This focus on real-time oversight allows organizations to maintain reliable operations across diverse environments, from on-premises servers to cloud-based systems.[5] Released under the GNU General Public License version 2, Nagios emphasizes flexibility, permitting users to customize its functionality through a simple plugin architecture that extends monitoring capabilities without altering the core system.[3] As of 2025, Nagios Core (version 4.5.10, released October 2025)—the free, community-supported version—remains actively maintained by both Nagios Enterprises and the open-source community, ensuring ongoing updates and compatibility with modern IT needs.[6]Core Principles
Nagios operates on the principle of plugin-based extensibility, which enables users to add new monitoring checks as modular plugins without modifying the core codebase. This design allows for the creation of custom plugins in various scripting languages or compiled binaries, each returning standardized output codes (OK, WARNING, CRITICAL, or UNKNOWN) to indicate service status. By separating check logic from the main application, this approach promotes flexibility and community-driven development, with thousands of third-party plugins available for diverse monitoring needs.[3][7] The system employs an event-driven architecture to handle status changes efficiently in real time. Nagios schedules periodic checks (polling) for services and hosts, processing the resulting events such as service checks, host states, and recoveries through an event broker interface, which triggers actions like notifications or event handlers. This model minimizes resource overhead by focusing on state transitions and supports proactive responses, such as executing scripts to mitigate issues automatically.[3][8] Alerting in Nagios is threshold-based, differentiating between warning conditions, critical failures, and subsequent recoveries to provide nuanced notifications. Plugins evaluate metrics against user-defined thresholds specified in configuration, escalating alerts only when states cross these boundaries, which helps reduce noise from transient issues. Notifications can be routed via email, SMS, or custom methods, ensuring timely awareness while allowing recovery confirmations to close alerts.[9][3] Scalability is a core tenet, achieved through distributed monitoring setups that offload checks to remote or polled hosts in large environments. This allows a central Nagios instance to aggregate data from multiple satellite systems, supporting redundancy and load balancing to monitor thousands of hosts without performance degradation. Tools like NRPE facilitate secure remote execution, enabling horizontal scaling across complex infrastructures.[10][11] Nagios emphasizes configuration-driven operation, relying on plain text files to define hosts, services, contacts, and rules in a declarative format. This approach provides maximum flexibility for administrators to version-control, automate, or script configurations, with inheritance and templates reducing redundancy. The daemon parses these files on startup or reload, enforcing a clear separation between setup and runtime logic.[12][13]History
Founding and Early Development
Nagios originated as the NetSaint project, initiated by Ethan Galstad in 1999 as an open-source monitoring tool. Galstad, then working as a systems administrator, developed NetSaint to address his personal need for a straightforward, customizable system to monitor hosts and services on his home network, building on earlier concepts from a 1996 MS-DOS ping application he created for Novell NetWare servers. In 1998, Galstad began developing a Linux-based monitoring tool based on these earlier concepts.[2][14] The early development of NetSaint was a solo endeavor by Galstad, who coded the core daemon in C to perform periodic checks on network hosts and services, generating alerts for issues such as downtime or performance thresholds. Released under the GNU General Public License version 2, the project was designed to foster community involvement by allowing users to extend functionality through plugins and configurations. The initial versions of NetSaint, starting around 1999, introduced basic monitoring via a web-based CGI interface for viewing status and logs, emphasizing simplicity and extensibility over complex enterprise features.[2][15][16] In 2002, due to a trademark dispute with another company using the name NetSaint, Galstad renamed the project to Nagios, an acronym for "Nagios Ain't Gonna Insist On Sainthood." This rebranding coincided with the first public release of Nagios version 1.0, which retained NetSaint's core architecture while continuing the focus on basic host and service monitoring through the CGI interface. The transition marked the project's evolution from a personal tool to a widely adopted open-source solution, with Galstad maintaining primary development responsibilities in its formative years.[2][17]Key Milestones and Releases
Nagios 2.0 was released in February 2006, marking a significant milestone with a stable plugin architecture that allowed for extensible monitoring capabilities and enhancements to the web interface for better usability and configuration management.[18] In 2007, Nagios Enterprises, LLC was formed by founder Ethan Galstad to provide consulting, support, and commercial development services while maintaining the open-source core of the project.[2] Nagios Core 4.0, released in September 2013, represented a major overhaul with substantial performance improvements, including optimized event handling and reduced resource usage, along with support for passive checks and enhanced scalability for large-scale deployments.[19] The open-source version was officially rebranded as Nagios Core in 2009 to distinguish it from emerging commercial offerings, a naming convention that has persisted to clarify its role as the free monitoring engine.[2] As of 2025, the Nagios Core 4.5.x series, with the latest release 4.5.10 in October 2025, includes enhanced security features such as vulnerability patches and improved authentication handling, alongside better API integrations for external command processing; ongoing community contributions via GitHub have driven numerous bug fixes since 2020, ensuring stability and compatibility with modern environments.[6][20]Architecture
Core Components
The Nagios daemon, known as thenagios process, serves as the central engine of the monitoring system, responsible for scheduling and executing checks on hosts and services, processing results, managing notifications, and maintaining overall system state. It operates continuously, reading configuration data to determine monitoring parameters and utilizing plugins to perform the actual checks while handling event handlers for automated responses. This daemon ensures real-time monitoring by updating status information and integrating with external modules for extended functionality.[21]
Configuration files form the foundational structure for defining the monitoring environment in Nagios. The primary file, nagios.cfg, specifies global settings such as log file locations, command timeouts, and feature toggles like notifications or external command processing, typically located at /usr/local/nagios/etc/nagios.cfg. Object definition files, often organized in directories like /usr/local/nagios/etc/objects/, detail hosts, services, contacts, commands, time periods, and groups using directive-based syntax, with support for templates to enable inheritance and reduce redundancy in setups. These files are parsed by the daemon at startup or reload to instantiate the monitoring logic.[21][22]
Retention and status files provide mechanisms for persisting monitoring data across daemon restarts and enabling historical analysis. The status file, usually /usr/local/nagios/var/status.dat, captures current host and service states, updated periodically (default every 10-15 seconds), to support real-time queries and prevent data loss during interruptions. The retention file, such as /usr/local/nagios/var/retention.dat, stores longer-term information including check results, comments, scheduled downtime, and notification history, with configurable masks to control what attributes are retained for efficiency. These files are written by the daemon and read by the web interface for dashboards and reports.[21]
The web interface in Nagios Core primarily relies on CGI scripts to deliver a browser-based view of monitoring status, accessible via a web server like Apache at paths such as /nagios/. Core CGIs like status.cgi for real-time host/service overviews, cmd.cgi for submitting external commands (e.g., acknowledgments or downtime scheduling), and extinfo.cgi for detailed views provide essential visualization, with authentication enforced through files like htpasswd.users. Modern frontends, such as add-on tools, can extend this CGI foundation for enhanced dashboards, but the core setup emphasizes lightweight, script-driven access to status and historical data from the retention files.[21]
The Event Broker module, also known as the Nagios Event Broker (NEB), acts as an interface for exporting and processing internal events in real-time to external systems, enhancing extensibility without altering the core daemon. It employs an API to callback modules (e.g., shared object files like ndomod.o) during events such as check executions or state changes, configurable via options in nagios.cfg like event_broker_options to control data flow (e.g., logging all events with -1). This module facilitates integrations like data export to databases via add-ons such as NDOUtils, allowing third-party applications to react to Nagios events seamlessly.[21]
Plugin System
Nagios plugins serve as modular, standalone executables or scripts that perform specific monitoring checks on hosts and services, acting as an abstraction layer between the Nagios Core daemon and the resources being monitored. These plugins, which can be written in various languages such as shell scripts, Perl, Python, or compiled binaries, are executed to gather status information and return standardized results without the core system needing to comprehend the underlying check logic.[23][24] The execution model relies on the Nagios Core daemon to schedule and invoke plugins based on commands defined in configuration files, such as host and service definitions that specify check intervals and plugin paths. Active checks are initiated proactively by the daemon at defined intervals (e.g., every 5 minutes via thecheck_interval directive), with support for parallel processing through configurable worker processes (defaulting to the number of CPU cores, minimum 4) and no inherent limit on concurrent checks unless specified. To prevent hangs, each plugin execution has a default timeout of 60 seconds for service and host checks, after which the daemon terminates the process and logs the event as a critical failure for services or down state for hosts.[25][26] In contrast, passive checks are not scheduled by the daemon but instead receive results submitted externally, such as via the external command file in a format like PROCESS_SERVICE_CHECK_RESULT;<host>;<service>;<code>;<output>, allowing integration with third-party systems.[27]
Upon execution, plugins must adhere to strict output guidelines to ensure compatibility: they return an exit code indicating status, followed by text output to stdout, optionally including performance data separated by a pipe (|) character. The standard exit codes are defined as follows:
| Exit Code | Service State | Host State |
|---|---|---|
| 0 | OK | UP |
| 1 | WARNING | UP or DOWN/UNREACHABLE* |
| 2 | CRITICAL | DOWN/UNREACHABLE |
| 3 | UNKNOWN | DOWN/UNREACHABLE |
use_aggressive_host_checking option.[24]
Performance data, when included, follows the text output in a key-value format (e.g., disk_usage=80%;90%;95;0;100), enabling graphing and further analysis via macros like $SERVICEPERFDATA$. Output is limited to 4 KB total, with the first line as short output and subsequent lines as optional long output for detailed diagnostics.[24][28]
The official Nagios Plugins repository provides over 50 standardized plugins for common monitoring tasks, including check_ping for network reachability, check_http for web server status, and check_disk for storage usage, all maintained under the Nagios Plugins project for cross-platform compatibility. Developers are encouraged to follow these guidelines, such as using short, descriptive names (prefixed with check_), supporting standard options like -w for warning thresholds and -c for critical thresholds, and incorporating timeouts via DEFAULT_SOCKET_TIMEOUT for network-based plugins to align with Nagios expectations. Source code for these plugins is available on GitHub, promoting easy extension while maintaining the core's lightweight design.[7][9][29]
Features
Monitoring Capabilities
Nagios Core supports host monitoring to track the availability and status of network devices, servers, and endpoints through various protocols and methods. For basic availability checks, it employs ICMP echo requests (ping) to detect if a host is up or down.[11] SNMP is utilized for querying device-specific information, such as interface status or hardware health on routers and switches, enabling passive data collection without disrupting operations.[30] For more detailed, internal monitoring on remote systems, agent-based approaches like the Nagios Remote Plugin Executor (NRPE) allow execution of plugins on Linux/Unix hosts to gather metrics such as disk usage or memory consumption, which are then reported back to the central Nagios server.[31] Service monitoring in Nagios extends to applications, databases, and system processes by defining checks against predefined thresholds to ensure operational health. For instance, it can monitor CPU load on servers by comparing current usage against warning and critical levels, alerting if thresholds are exceeded.[11] Web server response times are checked via HTTP plugins that measure latency and status codes, with configurable thresholds for acceptable performance.[32] Database services, such as MySQL or PostgreSQL, are probed for connection availability and query performance using specialized plugins that return status based on response times or error rates.[33] Performance data collection enhances monitoring by capturing quantitative metrics from check plugins for historical analysis and graphing. Plugins output data in a standardized format following the status pipe, such asrta=0.80 ms for response latency or percent_packet_loss=0 for bandwidth-related metrics, which Nagios stores in variables like `SERVICEPERFDATA.[28] This data can be processed via commands or written to files for external tools to generate graphs tracking trends over time, providing insights into bandwidth usage or latency patterns without overwhelming the core system.[28]
In large-scale environments, distributed monitoring distributes the workload using secondary pollers or remote agents to handle checks efficiently. Secondary pollers act as additional Nagios instances that execute a subset of checks and forward results to a central server, reducing load on the primary instance.[34] Open-source modules like mod-gearman enable remote workers to poll check queues and perform executions locally, supporting scalability for thousands of hosts by balancing distribution across multiple machines.[34]
Dependency mapping defines relationships between hosts and services to accurately interpret monitoring results in interconnected environments. Host dependencies link the status of one host to another, such as making a web server host dependent on its upstream router, suppressing checks if the parent is down.[35] Service dependencies similarly connect services across hosts, for example, flagging a database service as critical only if its underlying host and network service are operational, using criteria like OK, WARNING, or CRITICAL states to control execution and propagation.[35] These mappings ensure that monitoring reflects real-world dependencies without generating false positives.[35]