The reliable Nagios 4 setup is a sequence, not a single command: install Nagios Core and the official plugins, verify the configuration, start with local checks, then add remote Linux or UNIX hosts through NCPA, NRPE, or SSH. Validate the configuration before every reload or restart, restrict agent access by source address, and confirm each result in the web interface.
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This guide uses Ubuntu or Debian for the package example and also describes the source-install path documented for Nagios Core 4.5.10 with Nagios Plugins 2.4.11. Package names, service names, paths, and web aliases differ across distributions, so treat the examples as an explicitly labelled starting point and check the files installed by your release.
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Choose the installation path
Nagios Core can be installed from distribution packages or compiled from source. Packages are the fastest route on Ubuntu and Debian because the service unit, web configuration, and default object files are supplied by the distribution. Source installation gives you control over the Core and plugin versions and is the path covered by the official cross-platform quickstart documentation.
| Path | Best fit | Version and maintenance notes |
|---|---|---|
| Ubuntu or Debian packages | A first installation on a supported server where operating-system updates should manage dependencies | The Ubuntu guide demonstrates a two-server layout: server01 runs Nagios and server02 runs NRPE. Package names and paths vary by release. |
| Source | Administrators who need a controlled Core/plugin version or a distribution without a suitable package | The source guide documents Nagios Core 4.5.10 and Nagios Plugins 2.4.11. OpenSSL has been required since Core 4.4.7. |
The Nagios Core quickstart describes its Easy Setup guides as instructions to install Nagios from source and have it monitoring the local machine inside of 20 minutes. Actual time depends on dependencies, authentication, firewall rules, and configuration.
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Prepare the monitoring server
Define the roles and network flows
Use one host as the monitoring server and decide whether checks will be pulled by Nagios or pushed by an agent. In the Ubuntu example, server01 is the monitoring server at 172.10.100.1, while server02 is the monitored Linux host at 172.10.100.2. Replace those addresses with your real, routable addresses.
- Allow the monitoring server to reach the target’s agent or SSH port only when that protocol is required.
- Allow the web interface only to administrators and operators who need it.
- Record the operating-system release, Core version, plugin version, agent choice, firewall rules, and configuration-file locations before you begin.
Install the package route on Ubuntu or Debian
Package names change between Ubuntu and Debian releases. Search your release’s repository if a package name is unavailable. A typical Ubuntu or Debian starting point is:
sudo apt update
sudo apt install nagios4 nagios-plugins-contrib nagios-nrpe-plugin apache2 libapache2-mod-phpThe package route normally creates a Nagios configuration directory under /etc/nagios4, plugin files under a distribution-specific plugin directory, and a service commonly named nagios4. Confirm the actual locations with your package manager before editing files.
Enable the web server and Nagios service using the service names installed by your release. For the common Ubuntu naming:
sudo systemctl enable --now apache2
sudo systemctl enable --now nagios4Do not regard a successful service start as proof that the configuration is correct. Run the verifier first, and fix every error it reports.
Compile Nagios Core and plugins from source
Use the source route when you need the documented Core 4.5.10 and Plugins 2.4.11 versions, or when your operating system does not provide a suitable package. Install a compiler, development libraries, a web server, PHP support, and OpenSSL development files using your distribution’s package manager. The exact dependency package names differ by operating system.
The documented source workflow is:
- Download and unpack the Nagios Core source archive and the Nagios Plugins source archive for the versions you intend to maintain.
- Create the Nagios user and group, and add the web-server account to the Nagios group where the distribution’s instructions require it.
- From the Core source directory, run the configure step with the installation prefix and web settings required by your system, then run
make. - Install the Core binaries, init or systemd integration, sample configuration, command-mode directory, and web configuration with the
make install,make install-daemoninit,make install-config,make install-commandmode, andmake install-webconftargets documented for your release. - Build and install Nagios Plugins 2.4.11 after Core so the plugin paths in the object definitions point to the installed plugin directory.
- Create an authenticated web user with the web server’s password-file tool, enable the required web-server modules, and reload the web server.
A source installation commonly uses /usr/local/nagios/etc/nagios.cfg, /usr/local/nagios/bin/nagios, and a service named nagios, but verify those paths in the output from your configure and install steps. OpenSSL is not optional for Core versions from 4.4.7 onward.
Understand the Nagios configuration model
Daemon directives belong in nagios.cfg
The main configuration file controls the Core daemon: log locations, state retention, object-file directories, check intervals, notification behavior, and command-processing settings. It does not describe every server and service. Keep daemon-wide settings in this file and avoid mixing host-specific definitions into it.
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Object definitions describe what is monitored
Object files define hosts, hostgroups, services, commands, contacts, contactgroups, time periods, and templates. A host represents a machine or network endpoint. A service represents one check on that host. A command maps a service to a plugin and its arguments. Templates hold shared defaults such as notification intervals or check periods.
Keep a separate file for each host or logical service group, such as server02.cfg or web-services.cfg. The distribution’s nagios.cfg must include that directory or file pattern. This arrangement makes reviews, backups, and rollbacks practical.
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For a package installation, the verifier is commonly:
sudo nagios4 -v /etc/nagios4/nagios.cfgFor a source installation, use the binary and configuration path selected during installation, commonly:
sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfgProceed only when the output reports zero errors. Warnings may still represent a real operational problem, such as a service without a notification contact, so review them rather than ignoring them. After a successful verification, reload or restart the service used by your installation.
Add local checks first
The local host is the fastest way to prove that Core, the plugins, permissions, and notifications work together. The Linux and UNIX monitoring documentation groups the first private-resource checks as CPU load, memory, disk usage, logged-in users, and running processes. Add one service at a time and confirm the result in the web interface before adding more.
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CPU load
Use the load plugin to alert when the one-, five-, or fifteen-minute load averages cross thresholds appropriate to the number of CPU cores. A warning should indicate sustained pressure that deserves attention; critical should indicate a condition likely to affect service.
Memory
Monitor available memory using the plugin supplied with your distribution. Define warning and critical limits in the same unit the plugin reports, and account for the operating system’s use of filesystem cache before selecting thresholds.
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Disk usage
Check each filesystem that can fill and stop an application. Exclude temporary or virtual filesystems when they are not meaningful for capacity planning. A common disk command uses percentage thresholds, for example:
/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -eIn this example, warning begins when the available percentage reaches the warning boundary and critical begins at the critical boundary; the exact interpretation is shown by the plugin’s help output. The -e option makes the check evaluate each filesystem separately.
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Checks for logged-in users can reveal unexpected access or a broken session policy. Process checks can confirm that a required daemon exists and can alert when too many copies are running. Scope process checks narrowly: a generic count often produces noise during normal maintenance.
Public services
Public checks test the service from the monitoring server’s point of view. Add HTTP, FTP, SSH, and SMTP checks when those services are part of the system’s contract. The corresponding plugins usually are named check_http, check_ftp, check_ssh, and check_smtp. Test the same hostname, port, encryption mode, and authentication behavior that users rely on.
Every service should have an owner and a notification route before it is considered production-ready. A green status without a responsible contact is not an operational control.
Select a method for remote Linux and UNIX monitoring
| Method | What it does | When to choose it |
|---|---|---|
| NCPA | A modern cross-platform agent with an API, web GUI, active and passive capabilities, and discovery features | Preferred for new mixed Linux, UNIX, and cross-platform deployments where a current agent can be installed |
| NRPE | Runs narrowly defined Nagios plugins on the monitored host and returns the result to Core | Useful for legacy operating systems or fleets that already standardize on NRPE |
check_by_ssh |
Executes a plugin over SSH without installing NRPE | Useful for small numbers of hosts when SSH keys, command permissions, and account controls can be designed carefully |
| NSCA or NRDP | Accepts passive check-result submissions initiated by the monitored side or another sender | Useful when polling is undesirable or the monitored host must deliver results through an outbound path |
Do not choose an agent only because it is familiar. Consider operating-system support, firewall direction, credential rotation, command restrictions, and whether you need discovery or passive submission.
Configure NRPE with the two-server Ubuntu example
The following procedure follows the documented example in which server01 runs Nagios and server02 runs the NRPE service.
Install and configure the target host
- Install the NRPE service and the Nagios plugins on
server02using that host’s package manager. - Edit the NRPE configuration on
server02, commonly/etc/nagios/nrpe.cfgor a distribution-specific drop-in file. - Set the source allowlist to the monitoring server’s address:
allowed_hosts=172.10.100.1This setting is an access control, not a substitute for a firewall. Permit the NRPE port from 172.10.100.1 only, and keep the service account at the least privilege needed to run its plugins.
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Define the disk command on the target
Add the narrowly scoped command requested by the monitoring server:
command[check_all_disks]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -eDo not enable arbitrary remote command arguments unless your design explicitly requires them. Fixed command definitions reduce the impact of a compromised monitoring credential or an incorrectly written service object.
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In the host or service object file on server01, add a service using the NRPE one-argument command and the target address:
define service {
host_name server02
service_description All disks
check_command check_nrpe_1arg!check_all_disks!172.10.100.2
check_interval 5
retry_interval 1
max_check_attempts 3
check_period 24x7
notification_interval 60
notification_period 24x7
contacts nagiosadmin
}
Match the object syntax, contact names, and time-period names already present in your installation. If your package’s command definition uses a different argument format, keep the service’s intent but follow the command definition supplied by that package.
Restart both sides in the correct order
- Verify the NRPE configuration on
server02with the validation command supplied by its package, then restart the agent:
sudo systemctl restart nagios-nrpe-server- On
server01, run the Nagios configuration verifier and fix every error. - Restart Nagios on the monitoring server:
sudo systemctl restart nagios4Open the web interface, locate server02, and confirm that the All disks service returns plugin output. Deliberately fill a safe test filesystem or adjust a temporary test command if you need to observe OK, WARNING, and CRITICAL transitions; never manufacture an outage on a production filesystem.
Use NCPA for new cross-platform deployments
NCPA is described by Nagios as its modern cross-platform agent. Its API and web GUI provide more than a single NRPE-style command endpoint, while active and passive modes support both polling and agent-initiated delivery. Discovery features can help identify available metrics before you create service objects.
- Install the NCPA agent package appropriate to the monitored operating system.
- Set an authentication token or equivalent credential and restrict which interfaces and source addresses can reach the agent.
- Choose the NCPA check command or API metric for CPU, memory, disk, process, or service state.
- Create one host and one service object in Nagios, using a dedicated configuration file.
- Verify the Core configuration, restart the agent after agent-side changes, restart or reload Nagios after object changes, and confirm the returned value in the web interface.
Keep the agent’s exposed surface small. Disable features you do not use, use firewall rules to limit reachability, and rotate credentials under the same process used for other monitoring secrets.
Use check_by_ssh when SSH is the deliberate choice
check_by_ssh can run a plugin on a remote host without installing NRPE. Create a dedicated, restricted account, use a key that is limited to the monitoring purpose, and control which commands that account may execute. Host-key verification, key rotation, sudo policy, and timeout behavior must be designed before deployment.
SSH-based checks are easy to prototype but can become difficult to operate when every host has a different key or command policy. If a fleet needs discovery, active/passive operation, or a consistent cross-platform agent, NCPA is usually the cleaner starting point. If the fleet already has NRPE-compatible command definitions, retaining NRPE may reduce migration risk.
Secure the deployment
- Use explicit source-IP allowlists such as NRPE’s
allowed_hosts. - Apply host-firewall rules so agent ports are reachable only from the monitoring server or approved relay.
- Define fixed plugin commands and avoid accepting arbitrary command arguments.
- Run agents and plugins with the least privilege that still permits the required checks.
- Protect web-interface credentials and restrict administrative access to the management network.
- Separate monitoring secrets from ordinary user accounts and document their rotation procedure.
- Review notification contacts and escalation rules so a check failure reaches the team that owns the service.
These controls are operational safeguards derived from the documented allowlist and remote-execution designs; no single hardening profile fits every operating system or network.
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Validate the result in the web interface
Use the interface as an end-to-end test, not merely as a dashboard. A host should show the expected address and availability. A service should show the exact plugin output, its current state, the last check time, and the next scheduled check.
- Confirm that the Core service is enabled and running.
- Run the configuration verifier and correct all errors before every reload or restart.
- Verify that the local host reports expected CPU, memory, disk, user, process, and public-service output.
- Add one remote host and one remote service before onboarding a large fleet.
- Confirm OK, WARNING, and CRITICAL transitions deliberately with a safe test.
- Check that contacts, escalations, and time periods match the on-call schedule.
- After changing an agent, restart the agent and then restart or reload Nagios as appropriate.
- Record versions, operating-system releases, agent choice, firewall rules, and configuration-file locations for future upgrades.
Troubleshoot the failures you are most likely to see
The service will not start after an object change
Run the verifier directly and read the first error, not just the final systemd failure. Common causes include a misspelled template, a service referring to a nonexistent host, a command name that is not defined, an object file that is not included by nagios.cfg, or a duplicate object name. Correct the object, run the verifier again, and restart only after it reports no errors.
A remote check shows timeout or connection refused
Check routing and the host firewall first. Then confirm that the agent is listening, that its service is running, and that the monitoring server’s address appears in the NRPE allowlist or equivalent NCPA policy. A refusal usually indicates a closed port or stopped service; a timeout commonly indicates filtering or a path problem.
NRPE reports an unknown command
Compare the service’s command argument with the target’s exact command[...] name. Confirm that the plugin exists at the path in the command definition, is executable by the NRPE account, and has the required permissions for the filesystem or process being checked. Restart NRPE after changing its command file.
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UNKNOWN generally means the plugin could not interpret its arguments or could not perform the check. Run the plugin locally on the target as the agent account, inspect its help output, and verify paths, permissions, environment variables, and timeout values. Threshold mistakes should be corrected in the command definition rather than hidden with a larger timeout.
The web page is green but nobody is notified
Check that the service has a contact or contactgroup, that notifications are enabled, that the current time falls inside the assigned notification period, and that the contact’s delivery method is configured. A healthy status page does not prove that an alert path was exercised.
When the setup is ready for production
Production readiness means more than a running daemon. The configuration has been verified, local and remote checks return meaningful output, agent access is restricted, and every alert has an owner and an escalation route. Keep the configuration in version control, review changes before deployment, and preserve the version and path record needed to repeat the installation after an operating-system or Core upgrade.
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