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Install Nagios Core on a dedicated monitoring host, add the official plugins, validate the configuration, and then add local and remote checks one service at a time. For a new mixed Linux/UNIX or cross-platform deployment, use NCPA; keep NRPE for existing or older fleets, and use check_by_ssh when SSH-based execution is a better fit.
What you will build
This guide uses one server as the Nagios monitoring host and adds checks for that host before monitoring another machine. It covers the two practical installation routes:
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- Source installation: the version-controlled path documented for Nagios Core 4.5.10 and Nagios Plugins 2.4.11.
Nagios Core schedules checks and evaluates their results. Plugins do the actual tests: they read CPU load, inspect filesystems, connect to network services, or query an agent on another server. A service changes state when its plugin output crosses a warning or critical threshold, and the notification system then applies contacts, time periods, and escalation rules.
The Nagios Core documentation describes its quickstarts as instructions to install from source and have Nagios monitoring the local machine “inside of 20 minutes.” Treat that as a setup target rather than a guarantee: package availability, firewall policy, DNS, and web-server configuration can add work.
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Choose the installation route
| Route | Best fit | What to account for |
|---|---|---|
| Ubuntu or Debian packages | A first installation or a host managed with normal operating-system updates | Package names, configuration directories, web-server integration, and service units differ by release. |
| Source | A fixed Core/plugin version, a custom prefix, or a distribution without a suitable package | You must install build prerequisites, create the service integration, and record the installation paths yourself. |
Do not mix instructions from the two routes without checking paths. A package installation commonly uses an /etc/nagios4 configuration tree and a nagios4 service, while a source installation commonly uses a dedicated prefix such as /usr/local/nagios and a service named nagios.
Prepare the monitoring host
Set the host’s identity and time
Give the monitoring server a stable hostname and address. Use the same names in host definitions, DNS, certificates, firewall rules, and notification messages. Keep its clock synchronized; check scheduling and time-period rules become confusing when the system time is wrong.
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Plan the network before installing
- Allow the administrator’s network to reach the Nagios web interface.
- Allow the monitoring host to reach remote agents or services on the ports they require.
- If you use NRPE, allow the NRPE port only from the monitoring host’s address.
- Decide whether remote checks are polled by Nagios or submitted by the monitored host. The latter is the role of passive-result mechanisms such as NSCA or NRDP.
Record your layout
Before adding objects, write down the operating-system release, Nagios Core and plugin versions, configuration directories, service names, agent choice, firewall rules, and the account that runs the daemon. This record is valuable during upgrades and incident recovery.
Install Nagios 4 from Ubuntu or Debian packages
The Ubuntu package guide uses a two-server example: server01 runs Nagios and server02 runs NRPE. Exact package names and prompts vary by release, so confirm the names offered by your distribution rather than copying a package name that belongs to another release.
1. Install Core and the plugins
- Refresh the package index:
sudo apt update- Install the Nagios 4 package and the distribution’s Nagios plugin packages. On Ubuntu releases that provide the standard package set, this is commonly:
sudo apt install nagios4 nagios-plugins-contribIf your release splits the core, common files, or plugin packages differently, install the corresponding Nagios 4 Core and official-plugin packages shown by apt search nagios. The important requirement is that Core and the plugins are installed together; a missing plugin produces an “Unable to find command” or “plugin not found” result later.
2. Complete the web-interface setup
The package may ask for the web administrator account and may configure Apache automatically. Set a strong password for the initial Nagios web user, then confirm that the web server is enabled and running. If your distribution does not configure the web server automatically, follow that release’s Nagios package instructions rather than copying an Apache layout from a source installation.
3. Find the package configuration files
Typical package layouts place the main file at /etc/nagios4/nagios.cfg, object definitions below /etc/nagios4/objects, and plugins below /usr/lib/nagios/plugins. Verify the actual locations with the package manager and with the cfg_file and cfg_dir entries in nagios.cfg.
4. Verify before starting or restarting
sudo nagios4 -v /etc/nagios4/nagios.cfgUse the verifier that belongs to your package. It must finish without configuration errors before you start or reload the daemon. Warnings can be acceptable only when you understand them; an error means Nagios cannot reliably load the object database.
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5. Enable and start the service
sudo systemctl enable --now nagios4Open the Nagios web interface, sign in, and confirm that the local host is being checked. If your package uses a different service unit, use the name reported by systemctl list-unit-files | grep -i nagios.
Install Nagios Core and plugins from source
The documented source path covers Nagios Core 4.5.10 with Nagios Plugins 2.4.11. OpenSSL has been a required prerequisite since Core 4.4.7, so install the OpenSSL development files as well as your operating system’s compiler, make tools, web-server development files, and other prerequisites listed for your target operating system.
1. Keep Core and plugin versions together
Download the Core 4.5.10 source and Nagios Plugins 2.4.11 source from the official Nagios distribution channels. Keep the archives and checksums in your administrator’s software record. Build the plugins as well as Core; Core alone has no checks for CPU, disks, HTTP, or other services.
2. Configure and build Core
./configure --with-nagios-user=nagios --with-nagios-group=nagios
make all
sudo make install
sudo make install-init
sudo make install-config
sudo make install-commandmodeThe exact targets and web-interface installation target can differ by release and operating system. Read the 4.5.10 installation output and make sure the service account, command socket, configuration prefix, and init/systemd integration all point to the same installation.
3. Build and install the plugins
./configure --with-nagios-user=nagios --with-nagios-group=nagios
make
sudo make installSet the plugin directory in nagios.cfg or in the relevant command definitions. A source installation often places plugins under /usr/local/nagios/libexec; do not assume that path if your configure prefix is different.
4. Install and protect the web interface
Install the web configuration supplied with your Core release, create an authenticated web user, and restrict access to trusted administrator networks. The web interface exposes host and service state; it should not be left open to the public internet.
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5. Verify and enable the daemon
sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
sudo systemctl enable --now nagiosReplace both paths with the prefix reported by your build. Run the verifier after every object-file change and before every reload or restart.
Understand the Nagios configuration model
nagios.cfg controls the daemon
The main configuration file contains daemon directives: the user and group, state and log locations, command and event settings, resource files, and the object files to load. It does not define a particular server’s address or a service’s check command.
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Object files define what is monitored
Object definitions describe:
- Hosts: servers, routers, virtual machines, or other endpoints with an address and availability check.
- Hostgroups: reusable groups such as web servers or database servers.
- Services: individual checks attached to hosts.
- Commands: plugin invocations and their argument format.
- Contacts, contact groups, time periods, and escalations: who receives alerts, when alerts are sent, and how they progress.
- Templates: shared defaults that prevent every host or service from repeating the same notification and check settings.
Keep a dedicated file per host or service group. A layout such as objects/hosts/server01.cfg, objects/services/linux.cfg, and objects/contacts/on-call.cfg makes reviews and rollbacks safer than editing one large file.
Use explicit thresholds
Most plugins return OK, WARNING, CRITICAL, or UNKNOWN. A warning threshold is an early indication; a critical threshold is the condition that needs immediate action. For example, -w 20% -c 10% for disk usage means the plugin should warn when the remaining-space condition reaches the warning boundary and report critical at the critical boundary. Always read the individual plugin’s threshold semantics because some checks measure used capacity, while others measure free capacity or response time.
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The official Linux/UNIX monitoring documentation identifies CPU load, memory, disk usage, logged-in users, and running processes as private-resource checks. Add one service at a time, verify its output in the web interface, and only then apply the pattern to more hosts.
Example local services
define service {
use generic-service
host_name server01
service_description CPU load
check_command check_local_load!5,4,3!10,8,6
}
define service {
use generic-service
host_name server01
service_description Root filesystem
check_command check_local_disk!20%!10%!/
}
define service {
use generic-service
host_name server01
service_description Logged-in users
check_command check_local_users!5!10
}
define service {
use generic-service
host_name server01
service_description Running processes
check_command check_local_procs!250!400
}The command names and argument order must match the command definitions shipped with your distribution. If a package uses different aliases, inspect its command object and adjust the service rather than creating a second command with ambiguous arguments.
Check public services
Public checks test what a user or another system can reach. Common examples include:
- HTTP: request a web endpoint and alert when it cannot be reached or returns an unacceptable response.
- FTP: test that the FTP service accepts a connection.
- SSH: test that the SSH port and protocol respond.
- SMTP: test that the mail service accepts a connection and responds correctly.
These checks do not prove that an application is healthy internally. Pair a public check with a private-resource or application-specific check when an outage could occur behind a reachable port.
Choose a remote Linux monitoring method
| Method | How it works | When to choose it | Main design concern |
|---|---|---|---|
| NCPA | A modern cross-platform agent with an API, web GUI, active and passive capabilities, and discovery features | New deployments, mixed operating systems, or fleets that benefit from a current agent model | Restrict the agent interface and use authentication and firewall rules appropriate to your environment. |
| NRPE | Nagios executes a narrowly defined plugin command on the remote host through the NRPE service | Legacy operating systems or an existing NRPE fleet | Use an explicit source-IP allowlist and avoid exposing the service to arbitrary clients. |
check_by_ssh |
Nagios runs a remote plugin over SSH without installing NRPE | Small deployments or environments where SSH is already approved | Design key storage, command restrictions, host-key verification, and remote permissions deliberately. |
| NSCA or NRDP | The monitored host submits passive check results to Nagios | Systems that initiate delivery or situations where polling is undesirable | Authenticate and restrict result submission so an untrusted host cannot forge state. |
For a new cross-platform deployment, NCPA is the preferred starting point. NRPE remains a practical compatibility choice, not a requirement for every new installation.
Monitor a second Linux server with NRPE
The following example follows the documented two-server arrangement: server01 is the Nagios host at 172.10.100.1, and server02 is the monitored host at 172.10.100.2.
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1. Install the NRPE service on the target
On server02, install the NRPE service and the plugins required by the commands you plan to expose. The package name varies by distribution; on Ubuntu it is commonly the nagios-nrpe-server package plus the relevant plugin package.
2. Allow only the monitoring host
In the NRPE configuration on server02, set:
allowed_hosts=172.10.100.1Use the real monitoring address in your environment. If the target has multiple interfaces, do not replace an explicit allowlist with a broad subnet unless that is an intentional policy.
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Add this command to the NRPE command configuration on server02:
command[check_all_disks]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -eThe command checks disks with warning and critical thresholds and excludes the filesystem types selected by the plugin’s -e behavior. Confirm the plugin path on the target; source installations often use a different directory.
4. Restart the agent
sudo systemctl restart nagios-nrpe-serverRestart the agent after changing its allowlist or command definitions. Check the service status and local logs on server02 if the port does not accept connections.
5. Add the service on the Nagios host
On server01, define the remote service using the NRPE command wrapper:
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define service {
use generic-service
host_name server02
service_description All disks via NRPE
check_command check_nrpe_1arg!check_all_disks!172.10.100.2
}The command definition must exist on server01 and must pass the NRPE command name and target address in the order expected by your package’s command object.
6. Validate and restart Nagios
sudo nagios4 -v /etc/nagios4/nagios.cfg
sudo systemctl restart nagios4Again, substitute the verifier, configuration path, and service name used by your installation. The Ubuntu example restarts both sides: nagios-nrpe-server on the target and nagios4 on the monitoring server.
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Secure agents and remote execution
- Use an explicit NRPE
allowed_hostslist and enforce the same source restriction with the host firewall. - Expose agent ports only on the monitoring network; do not publish them on an internet-facing interface.
- Define only the plugin commands Nagios needs. Avoid arbitrary command arguments unless your design genuinely requires them.
- Run agents with the least privilege that still permits their checks. A disk check may need access to filesystem statistics, but it should not become a general-purpose root command runner.
- For SSH checks, protect private keys, verify host keys, and restrict the remote account to the intended plugin commands.
- For passive submissions, authenticate senders and reject results from untrusted hosts.
These controls follow from the documented allowlist and remote-execution designs; there is no single hardening profile that fits every network, operating system, or compliance regime.
Validate results in the web interface
Confirm the normal state
After the first local and remote checks run, open the host and service views. Confirm that plugin output is readable, the expected address is being checked, and the service is associated with the correct host and contact group.
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- Start with a known-good condition and record the OK output.
- Temporarily cross a warning threshold, such as lowering available disk space in a safe test environment, and confirm a WARNING state.
- Cross the critical threshold or stop a disposable test service and confirm CRITICAL.
- Restore the service or threshold condition and verify recovery.
Do not test alerts by stopping a production service. Use a maintenance window or an isolated host, and document the expected notification, acknowledgement, recovery, and escalation behavior.
Notifications and production readiness
A green dashboard is not a complete monitoring system. Before calling the deployment production-ready, check:
- Every important host and service has an owner and a contact route.
- Notification contacts, escalation rules, and time periods match the team’s on-call schedule.
- Host-down and service-failure notifications are distinct where the response differs.
- Checks have thresholds that represent an actionable condition rather than normal variation.
- Dependencies prevent a single failed network path from generating a misleading flood of alerts.
- Configuration changes are reviewed and the verifier is run before each reload or restart.
- The Core service is enabled and running after a reboot.
Troubleshoot the common failures
“Configuration error” during verification
Read the first error, not only the final summary. Common causes are a misspelled object attribute, a duplicate host or service name, an object file that was not included by nagios.cfg, or a command that does not exist in the configured plugin directory. Fix the first error, run the verifier again, and continue until errors are gone.
“Plugin not found” or “Unable to find command”
Compare the command definition’s path with the actual plugin directory and executable permissions. Package and source layouts differ, and a command copied from one host can point to a path that does not exist on another.
NRPE reports a timeout or connection refused
Check that the NRPE service is running on the target, that its firewall permits the monitoring host’s address, and that allowed_hosts contains the correct source address. Then confirm that the Nagios host is using the target’s reachable address and the expected NRPE port.
NRPE rejects the command
Ensure the command name in check_nrpe_1arg exactly matches the name inside command[...]. Restart NRPE after editing its command file, and test the plugin directly on the target to separate a plugin problem from an NRPE transport problem.
The web page is empty or stale
Check that the daemon is running, that the web server points to the same status files configured in nagios.cfg, and that you are viewing the intended monitoring instance. A successful configuration verification does not prove that the service restarted or that the web server can read the current status data.
Maintain the installation
Record Core and plugin versions, operating-system releases, agent versions, configuration-file locations, firewall rules, and custom commands. When upgrading, validate the configuration with the new binaries before restarting, review changes to plugin threshold semantics, and re-test one local service and one remote service. Keep source archives or package versions available for a controlled rollback, and update both ends of an agent-based check when compatibility requires it.
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