How to Install Nagios on Rocky Linux 9

Nagios is a monitoring system that tracks the availability and performance of network services, servers, and applications. Installing Nagios on Rocky Linux 9 enables users to run scheduled service checks for HTTP, SSH, disk usage, and system processes using plugins. Nagios Core is the central component of the Nagios system, responsible for executing monitoring checks, managing configurations, and handling notifications. Nagios Core triggers alerts if a service or host experiences an issue, such as downtime or a performance threshold breach.

This article explains how to install Nagios on Rocky Linux. You'll also configure Nagios to monitor services on a remote host.

Prerequisites

Before you begin, you need to:

• Have access to a Rocky Linux server to use as the Nagios server as a non-root user with sudo privileges.
• Have access to an Ubuntu server instance to use as the Nagios client.

Compile and Install Nagios Core From Source

Nagios is not available in the default package repositories on Rocky Linux 9. Follow the steps below to compile and install the Nagios Core and its plugins.

Compile Nagios Core

1. Update the existing server packages.

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   $ sudo dnf update -y

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2. On your Nagios server, install all required dependency packages.

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   $ sudo dnf install gcc glibc glibc-common wget perl net-snmp openssl-devel make unzip gd gd-devel epel-release httpd php php-cli php-common php-gd -y

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   This command installs compilers, libraries, and tools needed to compile Nagios Core and its plugins.

3. Navigate to the user's home directory.

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   $ cd ~

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4. Create a new nagios project directory to store Nagios Core files.

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   $ mkdir nagios

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5. Navigate to the directory.

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   $ cd nagios

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6. Visit the Nagios Core page and download the latest Nagios Core source code.

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   $ wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.5.9.tar.gz

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7. Install tar to extract the Nagios core.

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   $ sudo dnf install tar -y

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8. Extract files from the downloaded archive.

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   $ tar -xvf nagios-4.5.9.tar.gz

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9. Navigate to the extracted plugins directory.

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   $ cd nagios-4.5.9

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10. Run the Nagios Core configuration script.

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    $ ./configure

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11. Compile Nagios Core.

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    $ make all

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12. Compile and create a Nagios system user and group.

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    $ sudo make install-groups-users

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    Your output should be similar to the one below:

    groupadd -r nagios
    useradd -g nagios nagios

13. Add the apache user to the Nagios group to allow the Apache web server to interact with Nagios files and directories.

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    $ sudo usermod -a -G nagios apache

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Install and Configure Nagios on Rocky Linux 9

Nagios Core consists of essential binaries, configuration files, and system services that enable monitoring capabilities. Follow these steps to install Nagios on Rocky Linux 9.

1. Install the core Nagios binaries.

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   $ sudo make install

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2. Install the init scripts required to start, stop, and manage Nagios as a service on your server.

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   $ sudo make install-init

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3. Install the Nagios command mode package to allow access to external web-based commands.

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   $ sudo make install-commandmode

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4. Install the default Nagios configuration files.

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   $ sudo make install-config

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5. Install the Apache configuration file to enable Nagios's web interface.

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   $ sudo make install-webconf

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6. Verify the Nagios configuration to ensure there are no errors.

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   $ sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

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   Your output should be similar to the one below:

   ...
   Total Warnings: 0
   Total Errors: 0
   
   Things look okay - No serious problems were detected during the pre-flight check

7. Create a new vultr-admin user to access the web interface and set a strong password when prompted.

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   $ sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users vultr-admin

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   Your output should be similar to the one below:

    Adding password for user vultr-admin

8. Grant Apache access to the Nagios authentication file.

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   $ sudo chown apache:nagios /usr/local/nagios/etc/htpasswd.users

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   This allows the Apache web server to read the authentication credentials for the Nagios web interface.

9. Start the Nagios service.

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   $ sudo systemctl start nagios

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   Your output should be similar to the one below:

   Created symlink /etc/systemd/system/multi-user.target.wants/nagios.service → /usr/lib/systemd/system/nagios.service.

10. Enable Nagios to start automatically on system boot.

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    $ sudo systemctl enable nagios

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11. Verify the Nagios service is up and running.

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    $ sudo systemctl status nagios

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    Your output should be similar to the one below:

    ● nagios.service - Nagios Core 4.5.9
       Loaded: loaded (/usr/lib/systemd/system/nagios.service; enabled; preset: disabled)
       Active: active (running) since Sun 2025-02-16 06:05:12 UTC; 14s ago
         Docs: https://www.nagios.org/documentation
     Main PID: 74321 (nagios)
        Tasks: 6 (limit: 23116)
       Memory: 5.9M
          CPU: 321ms

12. Allow HTTP traffic on port 80 through the firewall.

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    $ sudo firewall-cmd --add-service=http --permanent

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    Output:

    success

13. Reload the firewall to apply the new rules.

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    $ sudo firewall-cmd --reload

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14. Enable Apache to start automatically when the system boots.

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    $ sudo systemctl enable httpd

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15. Restart both Nagios and Apache to apply the changes.

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    $ sudo systemctl restart nagios httpd

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Install Nagios Plugins

Nagios plugins allow Nagios to monitor various services, hosts, and resources. They acts as an executable script or binary that checks the status of a specific service or system component and returns the results to the Nagios core. Follow the steps below to install Nagios plugins.

1. Navigate to the Nagios directory you created.

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   $ cd ~/nagios/

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2. Visit the the Nagios plugins page and download the latest Nagios plugins source code.

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   $ wget https://nagios-plugins.org/download/nagios-plugins-2.4.11.tar.gz

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3. Extract files from the downloaded archive.

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   $ tar -xvf nagios-plugins-2.4.11.tar.gz

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4. Navigate to the extracted plugins directory.

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   $ cd nagios-plugins-2.4.11

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5. Run the Nagios plugins configuration script.

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   $ ./configure

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6. Compile the plugins.

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   $ make

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7. Install the compiled plugins.

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   $ sudo make install

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8. Restart both Nagios and Apache to apply the changes.

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   $ sudo systemctl restart nagios httpd

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Access the Nagios Web Monitoring Dashboard

Follow the steps below to access the Nagios web monitoring dashboard using your Nagios server's public IP address.

1. Open a new tab in web browser and enter the following URL to access the Nagios web interface.

   http://<your-server-ip>/nagios

   Replace <your-server-ip> with the actual IP address of your Rocky Linux server.

2. Enter the username vultr-admin and the password you configured. After logging in, the Nagios dashboard appears, as shown in the image below.

   

3. On the dashboard, navigate to side menu and click Tactical Overview to view a summary of monitored hosts, services, monitoring performance, and active alerts.

   

4. Click Event Log to check logs for events such as alerts, outages, and recovery logs.

   

Monitor Remote Hosts

Nagios enables you to monitor servers, routers, and other networked devices by defining them as hosts. The Nagios Remote Plugin Executor (NRPE) extends Nagios monitoring capabilities to remote hosts. It allows Nagios to execute plugins on remote machines, providing system metrics and resource monitoring when direct access is restricted.

Install Nagios Plugins on a Remote Host

Before Nagios can monitor a remote host, NRPE must be installed on it. NRPE listens for connection requests from a Nagios server, allowing remote monitoring of system metrics and resources. Follow the steps below to configure NRPE on your remote host.

1. Update the APT package index.

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   $ sudo apt update

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2. Install Nagios Remote Plugin Executor (NRPE) and Nagios plugins.

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   $ sudo apt install -y nagios-nrpe-server nagios-plugins nagios-plugins-basic nagios-nrpe-plugin

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3. Open the NRPE configuration file using a text editor, such as vi.

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   $ sudo vi /etc/nagios/nrpe.cfg

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4. Locate the allowed_hosts directive and add your Nagios server’s IP address.

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   allowed_hosts=127.0.0.1,::1,<nagios-server-ip>

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   Replace <nagios-server-ip> with the actual IP address of your Nagios server.

5. Start NRPE server.

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   $ sudo systemctl start nagios-nrpe-server

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6. Enable NRPE to start at boot.

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   $ sudo systemctl enable nagios-nrpe-server

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7. Check NRPE service status.

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   $ sudo systemctl status nagios-nrpe-server

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   Your output should be similar to the one below:

   ● nagios-nrpe-server.service - Nagios Remote Plugin Executor
        Loaded: loaded (/usr/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
        Active: active (running) since Sun 2025-02-16 06:46:33 UTC; 4min 23s ago
          Docs: http://www.nagios.org/documentation
      Main PID: 2726 (nrpe)
         Tasks: 1 (limit: 4607)
        Memory: 1.0M (peak: 1.2M)
           CPU: 7ms

8. View the UFW status and verify that the firewall is active.

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   $ sudo ufw status

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   If the status is inactive, allow SSH connections to the server and start UFW.

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   $ sudo ufw allow ssh && sudo ufw enable

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9. The NRPE service listens on port 5666. Allow inbound connections on this port.

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   $ sudo ufw allow 5666/tcp

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   Output:

   Rule added
   Rule added (v6)

10. Reload UFW to apply the changes.

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    $ sudo ufw reload

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    Output:

    Firewall reloaded

Configure the Nagios Server To Monitor Remote Hosts

Follow the steps below to configure your Nagios server to monitor the remote host.

1. Install NRPE plugin.

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   $ sudo dnf install -y nagios-plugins-nrpe

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2. Open the Nagios command configuration file.

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   $ sudo vi /usr/local/nagios/etc/objects/commands.cfg

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3. Add the following custom command definition to the file.

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   define command {
       command_name check_nrpe
       command_line /usr/lib64/nagios/plugins/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
   }

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   This command definition allows the Nagios server to connect to the NRPE agent on a remote host, request the execution of specific monitoring plugins, and retrieve the results.

   Save and close the file.

   In the above configuration:

   • command_name: The name (check_nrpe) that Nagios uses to reference this command.

   • command_line: The actual command executed by Nagios, where:

     • /usr/lib64/nagios/plugins/check_nrpe runs the check_nrpe plugin.

     • -H $HOSTADDRESS$ specifies the target host's address.

     • -c $ARG1$ specifies the command for the remote host's NRPE agent to execute.

4. The NRPE service listens on port 5666. Allow inbound connections on this port.

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   $ sudo firewall-cmd --add-port=5666/tcp --permanent

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   Output:

   success

5. Reload the firewall to apply the changes.

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   $ sudo firewall-cmd --reload

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   Output:

   success

6. Create a new hosts configuration file for the remote host.

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   $ sudo vi /usr/local/nagios/etc/objects/hosts.cfg

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7. Add the following configuration to define the contact responsible for receiving notifications about host or service issues.

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   define contact {
       use                     generic-contact
       contact_name            <vultr-admin>
       alias                   <Vultr Admin>
       email                   <vultradmin@example.com>
   }

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   In the above configuration:

   • use generic-contact: Inherits properties from a pre-defined generic-contact template.
   • contact_name vultr-admin: Assigns the name vultr-admin to the contact.
   • alias Vultr Admin: Provides a descriptive label Vultr Admin, for the contact.
   • email vultradmin@example.com: Specifies the email address for sending notifications.

   Feel free to replace the contact_name, alias, and email values with your information.

8. Add the following configuration to create a host block for a specific device to be monitored by Nagios.

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   define host {
       use                     linux-server
       host_name               vultr-example
       address                 <ip-address>
       max_check_attempts      5
       check_period            24x7
       notification_interval   30
       notification_period     24x7
   }

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   In the above configuration:

   • use linux-server: Applies the default settings from the linux-server template for standard Linux configurations.
   • host_name vultr-example: Sets a unique identifier for the host. Replace vultr-example with your hostname.
   • address <ip-address>: Specifies the IP address of the host to be monitored. Replace <ip-address> with the actual IP.
   • max_check_attempts 5: Retries failed checks up to 5 times before marking the host as down.
   • check_period 24x7: Specifies that checks are performed 24 hours a day, 7 days a week.
   • notification_interval 30: Sends notifications every 30 minutes during a problem state.
   • notification_period 24x7: Defines when Nagios will send notifications, 24 hours a day, 7 days a week.

Define Service Monitoring in Nagios

The following configurations define service checks for the vultr-example host to monitor SSH connectivity, system load, and total processes.

1. Add the following configuration to monitor for SSH connectivity for the vultr-example host.

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   define service {
       use                     generic-service
       host_name               vultr-example
       service_description     SSH
       check_command           check_ssh
       max_check_attempts      3
       check_interval          2
       retry_interval          1
   }

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   In the above configuration:

   • Monitors SSH using the check_ssh plugin.
   • Retries failed checks 3 times before marking the service as down.
   • Runs checks every 2 minutes, with a 1-minute retry interval.

2. Add the following configuration to define a System Load check for the vultr-example host to monitor its load levels.

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   define service {
       use                    generic-service
       host_name              vultr-example
       service_description    System Load
       check_command          check_nrpe!check_load
       max_check_attempts     5
       check_interval         2
       retry_interval         1
   }

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   In the above configuration:

   • Uses the NRPE plugin to monitor system load.
   • Retries failed checks 5 times before marking the service as down.
   • Runs checks every 2 minutes, with a 1-minute retry interval.

3. Add the configuration below to define a Total Processes check for the vultr-example host to monitor the total number of processes running on the system.

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   define service {
       use                    generic-service
       host_name              vultr-example
       service_description    Total Processes
       check_command          check_nrpe!check_total_procs
       max_check_attempts     5
       check_interval         2
       retry_interval         1
   }

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   In the above configuration:

   • Uses the NRPE plugin to monitor total running processes.
   • Retries failed checks 5 times before marking the service as down.
   • Runs checks every 2 minutes, with a 1-minute retry interval.

   Save and close the file.

4. Open Nagios configuration file with a text editor such as vi.

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   $ sudo vi /usr/local/nagios/etc/nagios.cfg

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5. Add the following line to include the hosts.cfg file during startup.

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   # Definitions for monitoring remote host
   cfg_file=/usr/local/nagios/etc/objects/hosts.cfg

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   Save and close the file.

6. Verify the Nagios configuration to ensure there are no errors.

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   $ sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

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   Your output should be similar to the one below:

   ...
   Total Warnings: 0
   Total Errors: 0
   
   Things look okay - No serious problems were detected during the pre-flight check

7. Restart Nagios to apply the changes.

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   $ sudo systemctl restart nagios

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Verify Remote Host and Service Monitoring

1. In the Nagios Dashboard, navigate to the side menu and click Hosts. The newly configured remote host should appear in the list with a status of UP, indicating that the host is operational and accessible by the Nagios server.

   

2. Click the vultr-example host to view the Host State Information page.

   

   This section provides detailed status information about the remote host.

3. Click View Status Detail For This Host in the Host State Information section to see the monitored services for vultr-example. Services with an OK status indicate successful checks.

   

   Note

   Initially, statuses may show as PENDING due to a 2-minute check interval. Nagios updates statuses after the next scheduled check.

4. To view all monitored services, navigate to the Services option in the side menu. The displayed information includes localhost, representing your Rocky Linux server and its monitored services, as well as the remote host vultr-example and its monitored services.

   

Conclusion

You have successfully installed the Nagios on Rocky Linux, set up Nagios plugins, and configured remote host monitoring. With Nagios running, you can extend its capabilities by adding more hosts and services as needed. For enhanced monitoring, use the Nagios NRPE package on remote hosts and configure settings accordingly. For more information on Nagios, visit the official documentation.