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 complete this tutorial, you will need the following:

Two Ubuntu 16.04 servers with private networking configured, set up by following the Ubuntu 16.04 initial server setup guide, including a sudo non-root user and a firewall. You'll use the first server to run Nagios, and the second server will be configured as a remote host that Nagios can monitor.

The server that will run Nagios also needs Apache and PHP installed, which you can do by following How To Install Linux, Apache, MySQL, PHP (LAMP stack) on Ubuntu 16.04 . 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 with TLS. To do this, you should do one of the following:

Configure a domain name to point to your server. You can learn how to point domains to cloud servers by following the How To Set Up a Host Name with the cloud provider tutorial, and then follow How To Secure Apache with Let's Encrypt on Ubuntu 16.04 to set up HTTPS support

Secure Apache with a self-signed certificate by following How To Create a Self-Signed SSL Certificate for Apache in Ubuntu 16.04

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 we'll install Nagios and its components from source to ensure we get the latest features, security updates, and bug fixes.

Log into your server that runs Apache. We'll call this the Nagios server.

				
					
[environment local]

ssh &lt;^&gt;sammy&lt;^&gt;@&lt;^&gt;your_nagios_server_ip&lt;^&gt;

Create a nagios user and nagcmd group. You'll use these to run the Nagios process.

				
					
sudo useradd &lt;^&gt;nagios&lt;^&gt;

sudo groupadd &lt;^&gt;nagcmd&lt;^&gt;

Then add the user to the group:

				
					
sudo usermod -a -G &lt;^&gt;nagcmd nagios&lt;^&gt;

Because we are building Nagios and its components from source, we 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-get update

Then install the required packages:

				
					
sudo apt-get install build-essential libgd2-xpm-dev openssl libssl-dev unzip

With the prerequisites installed, we 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://assets.nagios.com/downloads/nagioscore/releases/nagios-&lt;^&gt;4.3.4&lt;^&gt;.tar.gz

Extract the Nagios archive:

				
					
tar zxf nagios-*.tar.gz

Then change to the extracted directory:

				
					
cd nagios-*

Before building Nagios, run the configure script to specify the user and group you want Nagios to use. Use the nagios user and nagcmd group you created:

				
					
./configure --with-nagios-group=&lt;^&gt;nagios&lt;^&gt; --with-command-group=&lt;^&gt;nagcmd&lt;^&gt;

If you want Nagios to send emails using Postfix, you must install Postfix and configure Nagios to use it by adding --with-mail=/usr/sbin/sendmail to the configure command. We won't cover Postfix in this tutorial, but if you choose to use Postfix and Nagios later, you'll need to reconfigure and reinstall Nagios to use Postfix support.

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

				
					
[secondary_label Output]

*** Configuration summary for nagios &lt;^&gt;4.3.4 2017-08-24&lt;^&gt; ***:



 General Options:

 -------------------------

 Nagios executable: nagios

 Nagios user/group: nagios,nagios

 Command user/group: nagios,nagcmd

 Event Broker: yes

 Install ${prefix}: /usr/local/nagios

 Install ${includedir}: /usr/local/nagios/include/nagios

 Lock file: /run/nagios.lock

 Check result directory: ${prefix}/var/spool/checkresults

 Init directory: /etc/init.d

 Apache conf.d directory: /etc/apache2/sites-available

 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

Now run these make commands to install Nagios, its init scripts, and its default configuration files:

				
					
sudo make install

sudo make install-commandmode

sudo make install-init

sudo make install-config

You'll use Apache to serve Nagios' web interface, so copy the sample Apache configuration file to the /etc/apache2/sites-available folder:

				
					
sudo /usr/bin/install -c -m 644 sample-config/httpd.conf /etc/apache2/sites-available/nagios.conf

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

				
					
sudo usermod -G nagcmd &lt;^&gt;www-data&lt;^&gt;

Nagios is now installed. Let's install a plugin which will allow Nagios to collect data from various hosts.

Step 2 — 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 which is used by Nagios server.

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 Nagios Exchange site.

Download it to your home directory with curl:

				
					
cd ~

curl -L -O https://github.com/NagiosEnterprises/nrpe/releases/download/&lt;^&gt;nrpe-3.2.1/nrpe-3.2.1&lt;^&gt;.tar.gz

Extract the NRPE archive:

				
					
tar zxf nrpe-*.tar.gz

Then change to the extracted directory:

				
					
cd nrpe-*

Configure the check_nrpe plugin:

				
					
./configure

Now build and install check_nrpe:

				
					
make check_nrpe

sudo make install-plugin

Let's configure the Nagios server next.

Step 3 — Configuring Nagios

Now let's perform the initial Nagios configuration, which involves editing some configuration files and configuring Apache to serve the Nagios web interface. You only need to perform this section once on your Nagios server.

Open the main Nagios configuration file in your text editor:

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

Find this line in the file:

				
					
[label /usr/local/nagios/etc/nagios.cfg]

...

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

...

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

				
					
[label /usr/local/nagios/etc/nagios.cfg]

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

Save the file and exit the editor.

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:

				
					
[label /usr/local/nagios/etc/objects/contacts.cfg]

...

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

 &lt;^&gt;email&lt;^&gt; &lt;^&gt;your_email@your_domain.com&lt;^&gt; ; &lt;&lt;***** 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:

				
					
[label /usr/local/nagios/etc/objects/commands.cfg]

...

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. You'll use this command in Step 5.

Save and exit the editor.

Now configure Apache to serve the Nagios user interface. Enable the Apache rewrite and cgi modules with the a2enmod command:

				
					
sudo a2enmod rewrite

sudo a2enmod cgi

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 &lt;^&gt;nagiosadmin&lt;^&gt;

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

[warning]

Note: If you create a user with a name other than nagiosadmin, you will need to edit /usr/local/nagios/etc/cgi.cfg and change all the nagiosadmin references to the user you created.

Now create a symbolic link for nagios.conf to the sites-enabled directory. This enables the Nagios virtual host.

				
					
sudo ln -s /etc/apache2/sites-available/nagios.conf /etc/apache2/sites-enabled/

Next, open the Apache configuration file for Nagios.

				
					
sudo nano /etc/apache2/sites-available/nagios.conf

If you've configured Apache to serve pages over HTTPS, locate both occurrences of this line:

				
					
[label /etc/apache2/sites-available/nagios.conf]

# SSLRequireSSL

Uncomment both occurrances by removing the # symbol.

If you want to restrict the IP addresses that can access the Nagios web interface so that only certain IP addresses can access the interface, find the following two lines:

				
					
[label /etc/apache2/sites-available/nagios.conf]

Order allow,deny

Allow from all

Comment them out by adding # symbols in front of them:

				
					
[label /etc/apache2/sites-available/nagios.conf]

&lt;^&gt;#&lt;^&gt; Order allow,deny

&lt;^&gt;#&lt;^&gt; Allow from all

Then find the following lines:

				
					
[label /etc/apache2/sites-available/nagios.conf]

# Order deny,allow

# Deny from all

# Allow from 127.0.0.1

Uncomment them by deleting the # symbols, and add the IP addresses or ranges (space delimited) that you want to allow to in the Allow from line:

				
					
[label /etc/apache2/sites-available/nagios.conf]

Order deny,allow

Deny from all

Allow from 127.0.0.1 &lt;^&gt;your_ip_address&lt;^&gt;

These lines appear twice in the configuration file, so ensure you change both occurrences. Then save and exit the editor.

Restart Apache to load the new Apache configuration:

				
					
sudo systemctl restart apache2

With the Apache configuration in place, you can set up the service for Nagios. Nagios does not provide a Systemd unit file to manage the service, so let's create one. Create the nagios.service file and open it in your editor:

				
					
sudo nano /etc/systemd/system/nagios.service

Enter the following definition into the file. This definition specifies when Nagios should start and where Systemd can find the Nagios application. Learn more about Systemd unit files in the tutorial Understanding Systemd Units and Unit Files

				
					
[label /etc/systemd/system/nagios.service]

[Unit]

Description=Nagios

BindTo=network.target



[Install]

WantedBy=multi-user.target



[Service]

Type=simple

User=nagios

Group=nagios

ExecStart=/usr/local/nagios/bin/nagios /usr/local/nagios/etc/nagios.cfg

Save the file and exit your editor.

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

				
					
sudo systemctl enable /etc/systemd/system/nagios.service

sudo systemctl start nagios

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

Step 4 — 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 5 — Installing NPRE on a Host

Let's add a new host so Nagios can monitor it. We'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.

				
					
[environment local]

ssh sammy@&lt;^&gt;your_monitored_server_ip&lt;^&gt;

First create create a "nagios" user which will run the NRPE agent.

				
					
[environment second]

sudo useradd &lt;^&gt;nagios&lt;^&gt;

We'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:

				
					
[environment second]

sudo apt-get update

sudo apt-get install build-essential libgd2-xpm-dev openssl libssl-dev unzip

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 Nagios Plugins Download page. Copy the link address for the latest version, and copy the link address so you can download it to your Nagios server.

Download Nagios Plugins to your home directory with curl:

				
					
[environment second]

cd ~

curl -L -O http://nagios-plugins.org/download/nagios-plugins-&lt;^&gt;2.2.1&lt;^&gt;.tar.gz

Extract the Nagios Plugins archive:

				
					
[environment second]

tar zxf nagios-plugins-*.tar.gz

Change to the extracted directory:

				
					
[environment second]

cd nagios-plugins-*

Before building Nagios Plugins, configure it to use the nagios user and group, and configure OpenSSL support:

				
					
[environment second]

./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl

Now compile the plugins:

				
					
[environment second]

make

Then install them:

				
					
[environment second]

sudo make install

Next, install NRPE. Find the download URL for the latest stable release of NRPE at the Nagios Exchange site just like you did in Step 1. Download the latest stable release of NRPE to your monitored server's home directory with curl:

				
					
[environment second]

cd ~

curl -L -O https://github.com/NagiosEnterprises/nrpe/releases/download/&lt;^&gt;nrpe-3.2.1/nrpe-3.2.1&lt;^&gt;.tar.gz

Extract the NRPE archive with this command:

				
					
[environment second]

tar zxf nrpe-*.tar.gz

Then change to the extracted directory:

				
					
[environment second]

cd nrpe-*

Configure NRPE by specifying the Nagios user and group, and tell it you want SSL support:

				
					
[environment second]

./configure --enable-command-args --with-nagios-user=nagios --with-nagios-group=nagios --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/x86_64-linux-gnu

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

				
					
[environment second]

make all

sudo make install

sudo make install-config

sudo make install-init

Next, let's update the NRPE configuration file:

				
					
[environment second]

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

Find the allowed_hosts directive, and add the private IP address of your Nagios server to the comma-delimited list:

				
					
[environment second]

[label /usr/local/nagios/etc/nrpe.cfg]

allowed_hosts=127.0.0.1,::1&lt;^&gt;,your_nagios_server_private_ip&lt;^&gt;

This configures NRPE to accept requests from your Nagios server via its private IP address.

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

				
					
[environment second]

sudo systemctl start nrpe.service

Ensure that the service is running by checking its status:

				
					
[environment second]

sudo systemctl status nrpe.service

You'll see the following output:

				
					
[environment second]

[secondary_label Output]

...

Oct 16 07:10:00 nagios systemd[1]: Started Nagios Remote Plugin Executor.

Oct 16 07:10:00 nagios nrpe[14653]: Starting up daemon

Oct 16 07:10:00 nagios nrpe[14653]: Server listening on 0.0.0.0 port 5666.

Oct 16 07:10:00 nagios nrpe[14653]: Server listening on :: port 5666.

Oct 16 07:10:00 nagios nrpe[14653]: Listening for connections on port 5666

Oct 16 07:10:00 nagios nrpe[14653]: Allowing connections from: 127.0.0.1,::1,207.154.249.232

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

				
					
[environment second]

sudo ufw allow 5666/tcp

You can learn more about UFW in How To Set Up a Firewall with UFW on Ubuntu 16.04.

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 &lt;^&gt;remote_host_ip&lt;^&gt;

You'll see the following output:

				
					
[secondary_label Output]

NRPE v3.2.1

Now let's configure 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:

				
					
[environment second]

df -h /

You'll see output similar to this:

				
					
[secondary_label Output]

Filesystem Size Used Avail Use% Mounted on

udev 490M 0 490M 0% /dev

tmpfs 100M 3.1M 97M 4% /run

&lt;^&gt;/dev/vda1 29G 1.4G 28G 5% /&lt;^&gt;

tmpfs 497M 0 497M 0% /dev/shm

tmpfs 5.0M 0 5.0M 0% /run/lock

tmpfs 497M 0 497M 0% /sys/fs/cgroup

/dev/vda15 105M 3.4M 102M 4% /boot/efi

tmpfs 100M 0 100M 0% /run/user/0

Locate the filesystem associated with /. On a Droplet, the filesystem you want is probably /dev/vda1.

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

				
					
[environment second]

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

command[check_hda1]: Change /dev/hda1 to whatever your root filesystem is called

Locate these settings and alter them appropriately:

				
					
[environment second]

[label /usr/local/nagios/etc/nrpe.cfg]

...

server_address=&lt;^&gt;monitored_server_private_ip&lt;^&gt;

...

command[check_vda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p &lt;^&gt;/dev/vda1&lt;^&gt;

...

Save and exit the editor.

Restart the NRPE service to put the change into effect:

				
					
[environment second]

sudo systemctl restart nrpe.service

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 6 – 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/&lt;^&gt;your_monitored_server_host_name&lt;^&gt;.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:

				
					
[label /usr/local/nagios/etc/servers/your_monitored_server_host_name.cfg]

define host {

 use linux-server

 host_name &lt;^&gt;your_monitored_server_host_name&lt;^&gt;

 alias &lt;^&gt;My client server&lt;^&gt;

 address &lt;^&gt;your_monitored_server_private_ip&lt;^&gt;

 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 CPU usage:

				
					
[label /usr/local/nagios/etc/servers/your_monitored_server_host_name.cfg]

define service {

 use generic-service

 host_name &lt;^&gt;your_monitored_server_host_name&lt;^&gt;

 service_description CPU load

 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:

				
					
[label /usr/local/nagios/etc/servers/your_monitored_server_host_name.cfg]

define service {

 use generic-service

 host_name &lt;^&gt;your_monitored_server_host_name&lt;^&gt;

 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 CPU 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 even occurs.

How To Install Nagios 4 and Monitor Your Servers on Ubuntu 16.04

Table of Contents

Introduction

Prerequisites

Step 1 — Installing Nagios 4

Step 2 — Installing the check_nrpe Plugin

Step 3 — Configuring Nagios

Step 4 — Accessing the Nagios Web Interface

Step 5 — Installing NPRE on a Host

Step 6 – Monitoring Hosts with Nagios

Conclusion

Links

Newsletter

Contact