Monitor Remote Linux Host Using Nagios: A Beginner’s Guide

If you manage a network with more than a few devices, you need monitoring software to keep tabs on all of them. Nagios is a leading open-source monitoring solution that alerts you when something goes wrong and gives detailed information when something needs your attention. This article covers how to monitor remote Linux hosts with Nagios. If you don’t have much experience with network monitoring or systems administration, you may find some of the jargon in this article difficult to understand. But don’t worry—we’ll explain everything that you need to know about monitoring software, remote hosts, and Linux servers. By the end of this article, you’ll be ready to begin monitoring your own network using Nagios.

This article will help you to install NRPE service on your Linux system and add a host in the Nagios server for monitoring.

What is NRPE?

Nagios Remote Plugin Execution (NRPE) is an open-source monitoring plugin that allows you to monitor remote Linux hosts from a Nagios server. It uses TCP port 5666, which is normally not open on a firewall. If a firewall is blocking this port, you can install the Nagios NRPE proxy on the Nagios server and configure the firewall to allow TCP port 5666 from the IP address of the proxy server. NRPE is much more efficient than authentication methods like SSH and Telnet, which are more resource-intensive and require the Nagios server to have a user account on each remote host. With NRPE, Nagios authenticates with the user accounts on the remote host and uses the account’s permissions to access the remote host’s system information.

Step 1 – Configure NRPE on Linux Host

Follow the below steps to install and configure NRPE on the client machine and check connectivity with the Nagios server.

A. Installing NRPE Client

First, you need to install the remote plugin on each remote host that you want to monitor with Nagios. Open a terminal window on each remote host, and type the following to install the Nagios remote plugin on the host.

• On CentOS/RHEL/Fedora

  sudo dnf install nrpe nagios-plugins* 
  

• On Debian/Ubuntu/LinuxMint

  sudo apt update && sudo apt install nagios-nrpe-server nagios-plugins 
  

B. Configure NRPE

After successfully installing the NRPE service, Edit the NRPE configuration file /etc/nagios/nrpe.cfg in your favorite editor and add your Nagios service IP in allowed hosts.

vim /etc/nagios/nrpe.cfg 

allowed_hosts=127.0.0.1, 192.168.1.100

Where 192.168.1.100 is your Nagios server ip address.

After making the above changes in nrpe configuration file, Lets restart NRPE service as per your system

sudo service nrpe restart       # On CentOS/RHEL/Fedora 
sudo /etc/init.d/nagios-nrpe-server restart   # On Debian/Ubuntu/LinuxMint

C. Verify Connectivity from Nagios

Once you’ve finished configuring Nagios to monitor remote Linux hosts, test the connection. Open a new terminal window, and type the following commands on the Nagios server. Here 192.168.1.11 is the remote host system IP address.

/usr/local/nagios/libexec/check_nrpe -H 192.168.1.11 

NRPE v2.15

Step 2 – Add Linux Host in Nagios

We recommend using the NagiosQL3 web interface for managing the configuration of the Nagios server. The below steps is for CLI lovers. To add a host to your Nagios server from the command line.

First, create a configuration file /usr/local/nagios/etc/servers/MyLinuxHost001.cfg using the below values. for example you Linux hosts IP is 192.168.1.11. We also need to define a service with the host. So add a ping check service, which will continuously check whether the host is up or not.

sudo vim /usr/local/nagios/etc/servers/MyLinuxHost001.cfg 

#########################################################
# Linux Host 001 configuration file
#########################################################

define host {
        use                          linux-server
        host_name                    Linux_Host_001
        alias                        Linux Host 001
        address                      192.168.1.11
        register                     1
}
define service{
      host_name                       Linux_Host_001
      service_description             PING
      check_command                   check_ping!100.0,20%!500.0,60%
      max_check_attempts              2
      check_interval                  2
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           2
      notification_period             24x7
      notifications_enabled           1
      register                        1
}


#########################################################
# END OF FILE
#########################################################

Now verify configuration files using the following command. If there are no errors found in the configuration, restart the Nagios service.

nagios -v /usr/local/nagios/etc/nagios.cfg 
service nagios restart 

Step 3 – Check Host in Nagios Web Interface

Open your Nagios web interface and check for new Linux hosts added to Nagios core service. In my case, it looks like the below.

Conclusion

If you manage a network with more than a few devices, you need monitoring software to keep tabs on all of them. Nagios is a leading open-source monitoring solution that alerts you when something goes wrong and gives detailed information when something needs your attention. If you don’t have much experience with network monitoring or systems administration, you may find some of the jargon in this article difficult to understand. But don’t worry – we’ll explain everything that you need to know about monitoring software, remote hosts, and Linux servers. By the end of this article, you’ll be ready to begin monitoring your own network using Nagios. Ready to get started? Now that you know what NRPE is and how it works, you can install the corresponding software on your remote hosts and Nagios server. Then, you can start monitoring your network with Nagios to keep tabs on all of your devices.