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 DigitalOcean Droplets by following the How To Set Up a Host Name with DigitalOcean 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.

ssh sammy@your_nagios_server_ip

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

sudo useradd nagios
sudo groupadd nagcmd

Then add the user to the group:

sudo usermod -a -G nagcmd nagios

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-4.3.4.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=nagios --with-command-group=nagcmd

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

Output
*** Configuration summary for nagios 4.3.4 2017-08-24 ***:

 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 www-data

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/nrpe-3.2.1/nrpe-3.2.1.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:

...
#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 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:

...
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. 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 nagiosadmin

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

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:

#  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:

Order allow,deny
Allow from all

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

# Order allow,deny
# Allow from all

Then find the following lines:

#  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:

Order deny,allow
Deny from all
Allow from 127.0.0.1 your_ip_address

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

[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.

Log in to the second server, which we’ll call the monitored server.

ssh sammy@your_monitored_server_ip

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

sudo useradd nagios

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:

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:

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

Extract the Nagios Plugins archive:

tar zxf nagios-plugins-*.tar.gz

Change to the extracted directory:

cd nagios-plugins-*

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

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

Now compile the plugins:

make

Then install them:

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:

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-*.tar.gz

Then change to the extracted directory:

cd nrpe-*

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

./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:

make all
sudo make install
sudo make install-config
sudo make install-init

Next, let’s update the NRPE configuration file:

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:

allowed_hosts=127.0.0.1,::1,your_nagios_server_private_ip

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:

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
...
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:

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 remote_host_ip

You’ll see the following output:

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:

df -h /

You’ll see output similar to this:

Output
Filesystem      Size  Used Avail Use% Mounted on
udev            490M     0  490M   0% /dev
tmpfs           100M  3.1M   97M   4% /run
/dev/vda1        29G  1.4G   28G   5% /
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:

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:

...
server_address=monitored_server_private_ip
...
command[check_vda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/vda1
...

Save and exit the editor.

Restart the NRPE service to put the change into effect:

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/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 CPU usage:

define service {
        use                             generic-service
        host_name                       your_monitored_server_host_name
        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:

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 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.

ubuntu@ip-172-31-1-238:/usr/local/nagios/libexec$ sudo systemctl status nrpe
● nrpe.service - Nagios Remote Plugin Executor
   Loaded: loaded (/lib/systemd/system/nrpe.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2019-08-05 07:26:13 UTC; 36min ago
     Docs: http://www.nagios.org/documentation
  Process: 15332 ExecStopPost=/bin/rm -f /usr/local/nagios/var/nrpe.pid (code=exited, status=0/SUCCESS)
 Main PID: 15336 (nrpe)
    Tasks: 1
   Memory: 808.0K
      CPU: 11ms
   CGroup: /system.slice/nrpe.service
           └─15336 /usr/local/nagios/bin/nrpe -c /usr/local/nagios/etc/nrpe.cfg -f

Aug 05 07:26:13 ip-172-31-1-238 nrpe[15336]: Server listening on 0.0.0.0 port 5666.
Aug 05 07:26:13 ip-172-31-1-238 nrpe[15336]: Server listening on :: port 5666.
Aug 05 07:26:13 ip-172-31-1-238 nrpe[15336]: Listening for connections on port 5666
Aug 05 07:26:13 ip-172-31-1-238 nrpe[15336]: Allowing connections from: 172.31.8.319,127.0.0.1,::1
Aug 05 07:26:36 ip-172-31-1-238 nrpe[15508]: Host 172.31.8.139 is not allowed to talk to us!
Aug 05 07:26:42 ip-172-31-1-238 nrpe[15562]: Host 172.31.8.139 is not allowed to talk to us!
Aug 05 07:45:23 ip-172-31-1-238 systemd[1]: Started Nagios Remote Plugin Executor.
Aug 05 07:45:37 ip-172-31-1-238 nrpe[22799]: Host 172.31.8.139 is not allowed to talk to us!
Aug 05 07:46:22 ip-172-31-1-238 nrpe[23111]: Host 172.31.8.139 is not allowed to talk to us!
Aug 05 08:02:11 ip-172-31-1-238 nrpe[29244]: Host 172.31.8.139 is not allowed to talk to us!

     AuthName "Nagios Access"
     AuthType Basic
     AuthUserFile /usr/local/nagios/etc/htpasswd.users
     Require valid-user
  </RequireAll>

 Order deny,allow
 Deny from all
 Allow from 127.0.0.1 192.168.1.6

  AuthName "Nagios Access"
  AuthType Basic
  AuthUserFile /usr/local/nagios/etc/htpasswd.users
  Require valid-user

     AuthName "Nagios Access"
     AuthType Basic
     AuthUserFile /usr/local/nagios/etc/htpasswd.users
     Require valid-user
  </RequireAll>

 Order deny,allow
 Deny from all
 Allow from 127.0.0.1 192.168.1.6

  AuthName "Nagios Access"
  AuthType Basic
  AuthUserFile /usr/local/nagios/etc/htpasswd.users
  Require valid-user