How To Install Nagios 4 and Monitor Your Servers on Ubuntu 18.04

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Introduction

Nagios is a popular open-source monitoring system. It keeps an inventory of your servers and monitors them so you know your critical services are up and running. Using a monitoring system like Nagios is an essential tool for any production environment, because by monitoring uptime, CPU usage, or disk space, you can head off problems before they occur, or before your users call you.

In this tutorial, you’ll install Nagios 4 and configure it so you can monitor host resources via Nagios’ web interface. You’ll also set up the Nagios Remote Plugin Executor (NRPE), which runs as an agent on remote hosts so you can monitor their resources.

Prerequisites

To follow this tutorial, you will need:

• Two Ubuntu 18.04 servers set up by following our Initial Server Setup Guide for Ubuntu 18.04, including a non-root user with sudo privileges and a firewall configured with ufw. On one server, you will install Nagios; this tutorial will refer to this as the Nagios server. It will monitor your second server; this second server will be referred to as the second Ubuntu server.
• The server that will run the Nagios server needs Apache and PHP installed. Follow this guide to configure those on one of your servers. You can skip the MySQL steps in that tutorial.

Typically, Nagios runs behind a hardware firewall or VPN. If your Nagios server is exposed to the public internet, you should secure the Nagios web interface by installing a TLS/SSL certificate. This is optional but strongly encouraged. You can follow the Let’s Encrypt on Ubuntu 18.04 guide to obtain the free TLS/SSL certificate.

This tutorial assumes that your servers have private networking enabled so that monitoring happens on the private network rather than the public network. If you don’t have private networking enabled, you can still follow this tutorial by replacing all the references to private IP addresses with public IP addresses.

Step 1 — Installing Nagios 4

There are multiple ways to install Nagios, but you’ll install Nagios and its components from source to ensure you get the latest features, security updates, and bug fixes.

Log in to your server that runs Apache. In this tutorial, we’ll call this the Nagios server:

ssh sammy@your_nagios_server_ip

Because you’re building Nagios and its components from source, you must install a few development libraries to complete the build, including compilers, development headers, and OpenSSL.

Update your package lists to ensure you can download the latest versions of the prerequisites:

sudo apt update

Then install the required packages:

sudo apt install autoconf gcc make unzip libgd-dev libmcrypt-dev libssl-dev dc snmp libnet-snmp-perl gettext

With the prerequisites installed, you can install Nagios itself. Download the source code for the latest stable release of Nagios Core. Go to the Nagios downloads page, and click the Skip to download link below the form. Copy the link address for the latest stable release so you can download it to your Nagios server.

Download the release to your home directory with the curl command:

cd ~
curl -L -O https://github.com/NagiosEnterprises/nagioscore/archive/nagios-4.4.4.tar.gz

Extract the Nagios archive:

tar zxf nagios-4.4.4.tar.gz

Then change to the extracted directory:

cd nagioscore-nagios-4.4.4

Before building Nagios, run the configure script and specify the Apache configs directory:

./configure --with-httpd-conf=/etc/apache2/sites-enabled

You’ll see the following output from the configure command:

Output
*** Configuration summary for nagios 4.4.4 2019-07-29 ***:

 General Options:
 -------------------------
        Nagios executable:  nagios
        Nagios user/group:  nagios,nagios
       Command user/group:  nagios,nagios
             Event Broker:  yes
        Install ${prefix}:  /usr/local/nagios
    Install ${includedir}:  /usr/local/nagios/include/nagios
                Lock file:  /run/nagios.lock
   Check result directory:  /usr/local/nagios/var/spool/checkresults
           Init directory:  /lib/systemd/system
  Apache conf.d directory:  /etc/apache2/sites-enabled
             Mail program:  /bin/mail
                  Host OS:  linux-gnu
          IOBroker Method:  epoll

 Web Interface Options:
 ------------------------
                 HTML URL:  http://localhost/nagios/
                  CGI URL:  http://localhost/nagios/cgi-bin/
 Traceroute (used by WAP):


Review the options above for accuracy.  If they look okay,
type 'make all' to compile the main program and CGIs.

Now compile Nagios with this command:

make all

Next create a nagios user and nagios group. They will be used to run the Nagios process:

sudo make install-groups-users

Now run these make commands to install Nagios binary files, service files, and its sample configuration files:

sudo make install
sudo make install-daemoninit
sudo make install-commandmode
sudo make install-config

You’ll use Apache to serve Nagios’ web interface, so run the following to install the Apache configuration files and configure its settings:

sudo make install-webconf

Enable the Apache rewrite and cgi modules with the a2enmod command:

sudo a2enmod rewrite
sudo a2enmod cgi

In order to issue external commands via the web interface to Nagios, add the web server user, www-data, to the nagios group:

sudo usermod -a -G nagios www-data

Use the htpasswd command to create an admin user called nagiosadmin that can access the Nagios web interface:

sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

Enter a password at the prompt. Remember this password, as you will need it to access the Nagios web interface.

Restart Apache to load the new Apache configuration:

sudo systemctl restart apache2

You’ve now installed Nagios. But for this to work, it is necessary to install the Nagios Plugins, which you’ll cover in the next step.

Step 2 — Installing the Nagios Plugins

Nagios needs plugins to operate properly. The official Nagios Plugins package contains over 50 plugins that allow you to monitor basic services such as uptime, disk usage, swap usage, NTP, and others.

Let’s install the the plugins bundle.

You can find the latest version of the Nagios Plugins on the official site.

Download it to your home directory with curl:

cd ~
curl -L -O https://nagios-plugins.org/download/nagios-plugins-2.2.1.tar.gz

Extract the NRPE archive and navigate into the extracted directory:

tar zxf nagios-plugins-2.2.1.tar.gz
cd nagios-plugins-2.2.1

Next configure their installation:

./configure

Now build and install the plugins:

make
sudo make install

Now the plugins are installed, but you need one more plugin for monitoring remote servers. Let’s install it next.

Step 3 — Installing the check_nrpe Plugin

Nagios monitors remote hosts using the Nagios Remote Plugin Executor, or NRPE. It consists of two pieces:

• The check_nrpe plugin that the Nagios server uses.
• The NRPE daemon, which runs on the remote hosts and sends data to the Nagios server.

Let’s install the check_nrpe plugin on our Nagios server.

Find the download URL for the latest stable release of NRPE at the GitHub page.

Download it to your home directory with curl:

cd ~
curl -L -O https://github.com/NagiosEnterprises/nrpe/releases/download/nrpe-3.2.1/nrpe-3.2.1.tar.gz

Extract the NRPE archive:

tar zxf nrpe-3.2.1.tar.gz

Then change to the extracted directory:

cd nrpe-3.2.1

Configure the check_nrpe plugin:

./configure

Now build and install check_nrpe plugin:

make check_nrpe
sudo make install-plugin

Let’s configure the Nagios server next.

Step 4 — Configuring Nagios

Now let’s perform the initial Nagios configuration, which involves editing some configuration files. You only need to perform this section once on your Nagios server.

Open the main Nagios configuration file in your preferred text editor. Here, you’ll use nano:

sudo nano /usr/local/nagios/etc/nagios.cfg

Find this line in the file:

...
#cfg_dir=/usr/local/nagios/etc/servers
...

Uncomment this line by deleting the # character from the front of the line:

cfg_dir=/usr/local/nagios/etc/servers

Save and close nagios.cfg by pressing CTRL+X, followed by Y, and then ENTER (if you’re using nano).

Now create the directory that will store the configuration file for each server that you will monitor:

sudo mkdir /usr/local/nagios/etc/servers

Open the Nagios contacts configuration in your text editor:

sudo nano /usr/local/nagios/etc/objects/contacts.cfg

Find the email directive and replace its value with your own email address:

...
define contact{
        contact_name                    nagiosadmin             ; Short name of user
        use                             generic-contact         ; Inherit default values from generic-contact template (defined above)
        alias                           Nagios Admin            ; Full name of user
        email                           your_email@your_domain.com        ; <<***** CHANGE THIS TO YOUR EMAIL ADDRESS ******
...

Save and exit the editor.

Next, add a new command to your Nagios configuration that lets you use the check_nrpe command in Nagios service definitions. Open the file /usr/local/nagios/etc/objects/commands.cfg in your editor:

sudo nano /usr/local/nagios/etc/objects/commands.cfg

Add the following to the end of the file to define a new command called check_nrpe:

...
define command{
        command_name check_nrpe
        command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

This defines the name and specifies the command-line options to execute the plugin.

Save and exit the editor.

Then start Nagios and enable it to start when the server boots:

sudo systemctl start nagios

Nagios is now running, so let’s log in to its web interface.

Step 5 — Accessing the Nagios Web Interface

Open your favorite web browser, and go to your Nagios server by visiting http://nagios_server_public_ip/nagios.

Enter the login credentials for the web interface in the popup that appears. Use nagiosadmin for the username, and the password you created for that user.

After authenticating, you will see the default Nagios home page. Click on the Hosts link in the left navigation bar to see which hosts Nagios is monitoring:

As you can see, Nagios is monitoring only “localhost”, or itself.

Let’s monitor our other server with Nagios,

Step 6 — Installing Nagios Plugins and NRPE Daemon on a Host

Let’s add a new host so Nagios can monitor it. You’ll install the Nagios Remote Plugin Executor (NRPE) on the remote host, install some plugins, and then configure the Nagios server to monitor this host.

Log in to the second server, which we’ll call the second Ubuntu server:

ssh sammy@your_monitored_server_ip

First create a nagios user which will run the NRPE agent:

sudo useradd nagios

You’ll install NRPE from source, which means you’ll need the same development libraries you installed on the Nagios server in Step 1. Update your package sources and install the NRPE prerequisites:

sudo apt update
sudo apt install autoconf gcc libmcrypt-dev make libssl-dev wget dc build-essential gettext

NRPE requires that Nagios Plugins is installed on the remote host. Let’s install this package from source.

Find the latest release of Nagios Plugins from the downloads page.

Download Nagios Plugins to your home directory with curl:

cd ~
curl -L -O https://nagios-plugins.org/download/nagios-plugins-2.2.1.tar.gz

Extract the Nagios Plugins archive and change to the extracted directory:

tar zxf nagios-plugins-2.2.1.tar.gz
cd nagios-plugins-2.2.1

Before building Nagios Plugins, configure them with the following command:

./configure

Now compile the plugins:

make

Then install them by running:

sudo make install

Next, install NRPE daemon. Find the download URL for the latest stable release of NRPE at the GitHub page just like you did in Step 3. Download the latest stable release of NRPE to your monitored server’s home directory with curl:

cd ~
curl -L -O https://github.com/NagiosEnterprises/nrpe/releases/download/nrpe-3.2.1/nrpe-3.2.1.tar.gz

Extract the NRPE archive with this command:

tar zxf nrpe-3.2.1.tar.gz

Then change to the extracted directory:

cd nrpe-3.2.1

Configure NRPE:

./configure

Now build and install NRPE and its startup script with these commands:

make nrpe
sudo make install-daemon
sudo make install-config
sudo make install-init

Now, let’s update the NRPE configuration file and add some basic checks that Nagios can monitor.

First, let’s monitor the disk usage of this server. Use the df -h command to look for the root filesystem. You’ll use this filesystem name in the NRPE configuration:

df -h /

You’ll see output similar to this:

Output
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        25G  1.4G   23G   6% /

Now open /usr/local/nagios/etc/nrpe.cfg file in your editor:

sudo nano /usr/local/nagios/etc/nrpe.cfg

The NRPE configuration file is very long and full of comments. There are a few lines that you will need to find and modify:

• server_address: Set to the private IP address of the monitored server.
• allowed_hosts: Add the private IP address of your Nagios server to the comma-delimited list.
• command[check_hda1]: Change /dev/hda1 to whatever your root filesystem is called.

Locate these settings and alter them appropriately:

...
server_address=second_ubuntu_server_private_ip
...
allowed_hosts=127.0.0.1,::1,your_nagios_server_private_ip
...
command[check_vda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/vda1
...

Save and exit the editor. Now you can start NRPE:

sudo systemctl start nrpe.service

Ensure that the service is running by checking its status:

sudo systemctl status nrpe.service

You’ll see the following output:

Output
...
Aug 01 06:28:31 client systemd[1]: Started Nagios Remote Plugin Executor.
Aug 01 06:28:31 client nrpe[8021]: Starting up daemon
Aug 01 06:28:31 client nrpe[8021]: Server listening on 0.0.0.0 port 5666.
Aug 01 06:28:31 client nrpe[8021]: Server listening on :: port 5666.
Aug 01 06:28:31 client nrpe[8021]: Listening for connections on port 5666
Aug 01 06:28:31 client nrpe[8021]: Allowing connections from: 127.0.0.1,::1,165.22.212.38

Next, allow access to port 5666 through the firewall. If you are using UFW, configure it to allow TCP connections to port 5666 with the following command:

sudo ufw allow 5666/tcp

You can learn more about UFW in How To Set Up a Firewall with UFW on Ubuntu 18.04.

Now you can check the communication with the remote NRPE server. Run the following command on the Nagios server:

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

You’ll see the following output:

Output
NRPE v3.2.1

Repeat the steps in this section for each additional server you want to monitor.

Once you are done installing and configuring NRPE on the hosts that you want to monitor, you will have to add these hosts to your Nagios server configuration before it will start monitoring them. Let’s do that next.

Step 7 — Monitoring Hosts with Nagios

To monitor your hosts with Nagios, you’ll add configuration files for each host specifying what you want to monitor. You can then view those hosts in the Nagios web interface.

On your Nagios server, create a new configuration file for each of the remote hosts that you want to monitor in /usr/local/nagios/etc/servers/. Replace the highlighted word, monitored_server_host_name with the name of your host:

sudo nano /usr/local/nagios/etc/servers/your_monitored_server_host_name.cfg

Add the following host definition, replacing the host_name value with your remote hostname, the alias value with a description of the host, and the address value with the private IP address of the remote host:

define host {
        use                             linux-server
        host_name                       your_monitored_server_host_name
        alias                           My client server
        address                         your_monitored_server_private_ip
        max_check_attempts              5
        check_period                    24x7
        notification_interval           30
        notification_period             24x7
}

With this configuration, Nagios will only tell you if the host is up or down. Let’s add some services to monitor.

First, add this block to monitor load average:

define service {
        use                             generic-service
        host_name                       your_monitored_server_host_name
        service_description             Load average
        check_command                   check_nrpe!check_load
}

The use generic-service directive tells Nagios to inherit the values of a service template called generic-service, which is predefined by Nagios.

Next, add this block to monitor disk usage:

define service {
        use                             generic-service
        host_name                       your_monitored_server_host_name
        service_description             /dev/vda1 free space
        check_command                   check_nrpe!check_vda1
}

Now save and quit. Restart the Nagios service to put any changes into effect:

sudo systemctl restart nagios

After several minutes, Nagios will check the new hosts and you’ll see them in the Nagios web interface. Click on the Services link in the left navigation bar to see all of your monitored hosts and services.

Conclusion

You’ve installed Nagios on a server and configured it to monitor load average and disk usage of at least one remote machine.

Now that you’re monitoring a host and some of its services, you can start using Nagios to monitor your mission-critical services. You can use Nagios to set up notifications for critical events. For example, you can receive an email when your disk utilization reaches a warning or critical threshold, or a notification when your main website is down. This way you can resolve the situation promptly, or even before a problem occurs.

$ /usr/local/nagios/libexec/check_nrpe -H <monitored_IP> -c check_hda1
$ NRPE: Unable to read output

// The default window url may have been overridden with a permalink...
// Parse the URL and remove permalink option from base.
$a = parse_url($_GET['corewindow']);

// Build the base url.
$url = htmlentities($a['path']).'?';
$url = (isset($a['host'])) ? $a['scheme'].'://'.$a['host'].$url : '/'.$url;

$query = isset($a['query']) ? $a['query'] : '';
$pairs = explode('&', $query);
foreach ($pairs as $pair) {
	$v = explode('=', $pair);
	if (is_array($v)) {
		$key = urlencode($v[0]);
		$val = urlencode(isset($v[1]) ? $v[1] : '');
		$url .= "&$key=$val";
	}
}
if (preg_match("/^http:\/\/|^https:\/\/|^\//", $url) != 1)