Install Nagios Core on a dedicated Ubuntu or Debian monitoring host, validate its configuration, then add local checks before expanding to remote Linux servers. For new mixed operating-system deployments, Nagios describes NCPA as the modern cross-platform agent; keep NRPE for existing legacy fleets and use check_by_ssh when SSH access is preferable to an installed agent.
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Choose an installation path
Nagios has two practical deployment routes. The Debian-family package route is the quickest way to get a working service and web interface. A source build gives you tighter control over versions and paths and is the better fit when your distribution does not provide the Core release you need.
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| Route | Use it when | Version and path details |
|---|---|---|
| Ubuntu or Debian packages | You want distribution-managed updates and a fast initial deployment. | The service is normally named nagios4; common configuration files are under /etc/nagios4/. |
| Source installation | You need the documented upstream release, custom install paths, or a system without a suitable package. | The Nagios Core source guide documents Core 4.5.10 with Nagios Plugins 2.4.11. OpenSSL has been required since Core 4.4.7. |
Nagios says its Easy Setup guides cover source installation on Ubuntu, Debian, RHEL, CentOS, Fedora, SUSE, FreeBSD, Solaris and other operating systems. Package names, service names and file locations vary, so use the commands below as an Ubuntu/Debian baseline and verify the equivalent names on another distribution.
“These Easy Setup guides are intended to provide you with simple instructions on how to install Nagios from source (code) and have it monitoring your local machine inside of 20 minutes.” — Nagios Core Documentation quickstart
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Install Nagios 4 on Ubuntu or Debian
1. Prepare the monitoring host
Use a supported, patched Ubuntu or Debian server with a fixed hostname and address. The monitoring host needs outbound access to the systems and services it will check, and administrators need web access to the Nagios interface.
sudo apt update
sudo apt install nagios4 nagios-plugins-basic nagios-plugins-contrib nagios-nrpe-plugin apache2The basic and contributed plugin packages provide the standard local checks; nagios-nrpe-plugin supplies the monitoring-side client used later for NRPE. Your distribution may split these packages differently. If apt proposes a different package name, use the package that provides the Nagios Core daemon, official plugins and NRPE client.
2. Enable the web server and Nagios
sudo systemctl enable --now apache2
sudo systemctl enable --now nagios4On Debian-family packages the web interface is commonly exposed at http://monitoring-server/nagios4. The exact alias is defined by the installed Apache configuration; inspect the Nagios package’s site configuration if that path is different on your release.
3. Protect the web interface
Create an HTTP basic-auth account for the Nagios interface. The package commonly expects the file /etc/nagios4/htpasswd.users and the user nagiosadmin.
sudo htpasswd -c /etc/nagios4/htpasswd.users nagiosadminEnter a long, unique password when prompted. Do not expose the interface directly to the public internet; put it behind your normal administrative network controls or a VPN.
4. Verify before every start or reload
Never restart a production daemon with an unverified configuration. On a package installation, run:
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sudo systemctl restart nagios4
sudo systemctl status nagios4 --no-pagerSource-install Nagios Core 4.5.10
Use this path when you need the upstream Core 4.5.10 release and Plugins 2.4.11 rather than your distribution’s package version. Download those release archives from the official Nagios release pages, then build them on the monitoring host. The commands below show the important build sequence; distribution-specific development package names may differ.
Install build and web prerequisites
sudo apt update
sudo apt install autoconf gcc libc6 make wget unzip apache2 php libapache2-mod-php libgd-dev libssl-dev daemonOpenSSL is not optional for current Core: the upstream documentation has required it since Core 4.4.7. Add your distribution’s equivalent development headers if the configure step reports a missing library.
Build and install Core
tar xzf nagios-4.5.10.tar.gz
cd nagios-4.5.10
./configure --with-httpd-conf=/etc/apache2/sites-enabled
make all
sudo make install-groups-users
sudo usermod -a -G nagios www-data
sudo make install
sudo make install-daemoninit
sudo make install-commandmode
sudo make install-config
sudo make install-webconfThese targets create the Nagios user and group, install the daemon and sample configuration, set command-file permissions, install the system-service definition and add the Apache configuration. If your web server uses a different configuration directory, pass that directory to --with-httpd-conf and install the resulting site file according to that distribution’s conventions.
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Build the official plugins
tar xzf nagios-plugins-2.4.11.tar.gz
cd nagios-plugins-2.4.11
./configure --with-nagios-user=nagios --with-nagios-group=nagios
make
sudo make installRestart Apache, create the web-auth file expected by the installed web configuration, verify the source configuration and enable the source service:
sudo systemctl restart apache2
sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
sudo systemctl enable --now nagiosSource installations normally use /usr/local/nagios/etc/, /usr/local/nagios/libexec/ and the service name nagios, unlike the /etc/nagios4/ and nagios4 conventions of Debian-family packages. Keep those paths consistent in every command definition.
Understand Nagios configuration before adding checks
Nagios separates daemon settings from monitored-object definitions.
nagios.cfg: global daemon directives such as the user and group, state retention, log locations, object-file paths and check intervals.- Object files: definitions for hosts, hostgroups, services, commands, contacts, contactgroups, time periods and templates.
On Ubuntu and Debian, begin with /etc/nagios4/nagios.cfg and inspect its cfg_file and cfg_dir entries to see which files are loaded. A common layout contains files under /etc/nagios4/conf.d/ and packaged examples under /etc/nagios4/objects/. Source installs use /usr/local/nagios/etc/nagios.cfg and commonly keep objects in /usr/local/nagios/etc/objects/.
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A minimal host and service
define host {
use linux-server
host_name server02
alias Server 02
address 172.10.100.2
}
define service {
use generic-service
host_name server02
service_description SSH
check_command check_ssh
}The template names in the example are supplied by the packaged sample objects. If your installation uses different templates, substitute the templates that define the check interval, retry behavior and notification defaults for your site.
Start with local health checks
The first monitored host should be the Nagios server itself. Nagios’s Linux/UNIX monitoring documentation describes private-resource checks for CPU load, memory, disk usage, logged-in users and running processes. Public service checks such as HTTP, FTP, SSH and SMTP use the same plugin model.
CPU load
The check_load plugin compares one-, five- and fifteen-minute load averages with warning and critical thresholds. A typical command definition is:
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define service {
use generic-service
host_name localhost
service_description CPU load
check_command check_load!5.0,4.0,3.0!10.0,8.0,6.0
}These numbers are examples, not universal limits. A warning means the measured value crossed the warning threshold; critical means it crossed the more severe threshold. Tune values to the number of CPUs and the workload, then deliberately observe both states.
Memory, disk, users and processes
Plugin names and arguments differ between the basic and contributed packages, so confirm the installed command definitions in commands.cfg before creating services. The usual checks are a memory-usage plugin, check_disk, check_users and check_procs. Keep their warning and critical limits explicit in the service’s check_command line and document why each limit was chosen.
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For disk capacity, the NRPE example below uses the official plugin command:
/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -eHere, warning begins when free space falls below 20 percent and critical begins below 10 percent. The -e option makes the check report each mounted filesystem separately. Exclude transient mounts if your operating system creates them and they are not operationally meaningful.
Public services
Use the standard network plugins from the monitoring host so that you test the path users actually rely on:
define service {
use generic-service
host_name localhost
service_description Web HTTP
check_command check_http
}
define service {
use generic-service
host_name localhost
service_description SSH
check_command check_ssh
}
define service {
use generic-service
host_name localhost
service_description FTP
check_command check_ftp
}
define service {
use generic-service
host_name localhost
service_description SMTP
check_command check_smtp
}Only define FTP or SMTP checks where those services are intentionally offered. Every service should have an owner, a contact group and a notification time period before you call the configuration production-ready.
Select a remote Linux monitoring method
| Method | Best fit | Important characteristics |
|---|---|---|
| NCPA | New cross-platform deployments. | Nagios describes it as the modern agent, with an API, web GUI, active and passive capabilities, and discovery features. |
| NRPE | Legacy operating systems or fleets already using NRPE. | The monitoring server requests a named command from the agent. Restrict the agent with an explicit allowed_hosts list. |
check_by_ssh |
Hosts where installing an agent is undesirable. | Runs a plugin over SSH; key management and remote-command permissions must be designed deliberately. |
| NSCA or NRDP | Passive monitoring, where the monitored host initiates result delivery. | Nagios addon documentation describes both for submitting check results when polling is undesirable. |
Choose one method per host initially. Mixing agents without a clear ownership model makes it harder to know which process owns a service result and which firewall rule is required.
Add a remote Linux host with NRPE
The following two-server example uses server01 as the Nagios host at 172.10.100.1 and server02 as the monitored host at 172.10.100.2.
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1. Install the NRPE service on server02
Install the distribution’s NRPE server package and the official plugins on server02. On Ubuntu/Debian this is commonly:
sudo apt update
sudo apt install nagios-nrpe-server nagios-plugins-basicThe service configuration is commonly /etc/nagios/nrpe.cfg, with additional snippets under /etc/nagios/nrpe.d/. Confirm the active path with the package’s systemd unit if your release differs.
2. Allow only the monitoring server
Set the NRPE allowlist on server02:
allowed_hosts=172.10.100.1Do not replace this with a broad network range unless every address in that range is trusted to request plugin execution. Also restrict the NRPE listening port with the host firewall so that only 172.10.100.1 can reach it.
3. Define a narrow disk command
Add this command to the NRPE command configuration on server02:
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command[check_all_disks]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -eThe command accepts no user-supplied arguments, which avoids turning the agent into a general remote shell. Add additional named commands only when the monitoring requirement is clear.
4. Restart the agent and verify locally
sudo systemctl restart nagios-nrpe-server
sudo systemctl enable nagios-nrpe-server
sudo systemctl status nagios-nrpe-server --no-pagerCheck the system journal and the NRPE configuration if the service does not start. The plugin path must exist and be executable by the NRPE service account.
5. Add the NRPE command on server01
Define a command that passes the second argument as the target address and the first as the permitted NRPE command name:
define command {
command_name check_nrpe_1arg
command_line /usr/lib/nagios/plugins/check_nrpe -H $ARG2$ -c $ARG1$
}Then add the host and disk service. The service expression follows the Ubuntu example:
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use linux-server
host_name server02
alias Server 02
address 172.10.100.2
}
define service {
use generic-service
host_name server02
service_description All disks
check_command check_nrpe_1arg!check_all_disks!172.10.100.2
}Run the verifier on server01 and restart Nagios only after it reports no errors:
sudo nagios4 -v /etc/nagios4/nagios.cfg
sudo systemctl restart nagios4If the check is still UNKNOWN, confirm that the NRPE service is listening, the firewall allows the monitoring host, allowed_hosts contains the exact source address, and the command name matches character-for-character on both sides.
Use NCPA for new cross-platform deployments
NCPA is the preferred starting point when you are designing a new fleet that includes more than one operating system. Its API and web GUI expose the agent’s data, while active and passive modes support both polling and host-initiated delivery. Discovery features can help identify available measurements before you create service definitions.
- Install the NCPA agent package appropriate for each operating system.
- Set a strong authentication token and bind the agent only to interfaces that need to serve monitoring traffic.
- Permit the monitoring host through the operating system firewall; deny other sources.
- From the Nagios host, install the NCPA monitoring plugin supplied for your distribution and test one endpoint manually.
- Create one host definition and one service in Nagios, run the configuration verifier, then restart Nagios.
- Use discovery to select stable measurements, and document whether each service is polled actively or receives passive results.
NCPA’s web interface is useful for troubleshooting, but it is not a substitute for Nagios notification policy. Keep ownership, escalation and maintenance periods in the Nagios object definitions.
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check_by_ssh is appropriate when SSH is already the approved management channel and adding NRPE or NCPA is not acceptable. Create a dedicated, least-privilege SSH key for the Nagios service account. On the target, restrict the key in authorized_keys and permit only the required plugin commands where your SSH policy supports command restrictions.
/usr/lib/nagios/plugins/check_by_ssh -H 172.10.100.2 -C '/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -e'Do not allow arbitrary command text from a Nagios service definition unless the deployment explicitly requires it. Rotate the key under your normal credential process, monitor failed logins and account for the additional SSH connection overhead when setting check intervals.
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Notifications, ownership and thresholds
A green service result is useful only when a human knows what to do with a warning or critical event. For each production service, define:
- A contact or contact group responsible for remediation.
- A notification time period matching the team’s on-call schedule.
- Escalation rules for unacknowledged failures.
- Warning and critical thresholds with a short explanation of the operational consequence.
- Maintenance periods for planned work, so expected outages do not page the on-call engineer.
Test notification routing with a non-production contact before enabling broad paging. Keep the service description specific enough that an alert identifies the resource and failure type without requiring a web-interface search.
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Secure the deployment
- Use explicit source-IP allowlists for NRPE and host-firewall rules that match them.
- Keep agent commands narrowly defined; avoid accepting arbitrary command arguments.
- Run agents and plugin processes with least-privilege service accounts.
- Protect the Nagios web interface with strong authentication and network controls.
- Use dedicated SSH keys and restricted remote commands for
check_by_ssh. - Record every exposed agent endpoint, firewall rule and configuration-file location.
These controls are operational safeguards, not a single universal hardening profile. Apply your organization’s patching, certificate, logging and access-review requirements as well.
Validate a new host and troubleshoot failures
Configuration errors
Run the appropriate verifier after every object-file change:
sudo nagios4 -v /etc/nagios4/nagios.cfg
sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfgUse the command matching your installation path. Fix unknown templates, duplicate object names, missing commands and invalid contact groups before restarting.
Plugin or permission errors
Run the plugin manually as the Nagios service account when possible. Confirm that the executable exists, its shared libraries are present and the account can read the paths it checks. A command that works as root can still fail under Nagios because of permissions or environment differences.
Remote timeout or connection refusal
- Check the target agent’s service status and logs.
- Confirm the target firewall allows the monitoring host’s address.
- Confirm the monitoring host is using the target’s current address.
- For NRPE, compare
allowed_hosts, the command name and the plugin path. - For SSH, test key authentication and the exact remote command as the Nagios account.
Unexpected WARNING or CRITICAL
Read the plugin’s complete output in the Nagios interface, then compare the measured value with the threshold in the command definition. Check whether a filesystem, process count or load average is transient. Adjust a threshold only after deciding whether the underlying condition is acceptable; do not hide a real capacity problem by making the limit less sensitive.
Prove state transitions
Add one remote host and one service first. Confirm an OK result, then deliberately create a safe warning condition and a safe critical condition, and finally restore the service. Verify that Nagios records each state, sends the intended notification and recovers cleanly. Remove the test condition immediately after validation.
Operational checklist
- Confirm the Nagios Core service is enabled and running.
- Run the configuration verifier and fix every error before a reload or restart.
- Verify that the local host reports the expected plugin output.
- Add one remote host and one service before onboarding the rest of the fleet.
- Test OK, WARNING and CRITICAL transitions deliberately.
- Check contacts, escalation rules and time periods against the on-call schedule.
- After changing an agent, restart the agent and the Nagios service, as in the Ubuntu NRPE example.
- Record plugin versions, OS release, agent choice, firewall rules and configuration-file locations for future upgrades.
With that record in place, upgrading Core or an agent becomes a controlled change: install the new component, run the verifier against the intended configuration file, test one host and one service, and then expand the change only after the web interface and notifications show the expected results.
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