Today I wanted to use my Raspberry Pi to monitor my network with Nagios.
But I was lost trying to find out what was the best option to do this.
I saw many how-to guides about NEMS, Nagios3 but nothing perfect with the latest Nagios version.
I’ll explain here what I found and how to do the same step by step.

Nagios requires a few prerequisites: Raspberry Pi OS, Apache and PHP.
Then Nagios Core and its plugins can be installed from source, by downloading the latest version on the official website.
After that, the configuration can start to add all the hosts and services to monitor.

Yes, Nagios from source is the best way to monitor your network on Raspberry Pi.
Nagios is like a failure detector, it will check one thing regularly (like every 5 minutes) and alert you when this check doesn’t work as expected.

In this post I’ll explain how to install Nagios on your Raspberry Pi to check your network status.
But I’ll start by listing all the ways to get Nagios I found on Internet and to show you why most of them are useless or unsuitable.

What is the best way to get Nagios on Pi?

I’ll start this post by explaining to you what I found to get Nagios quickly on the Raspberry Pi, and why they are bad options for you.

NagiosPi?

My first search showed me “NagiosPi”.
I was excited to see that Nagios created a specific version for the Raspberry Pi.
This is the main reason that led me to install it on my Pi.
But I was quickly cooled, seeing this is a side project, with no update since 2013.

This was not suitable for me, I know that Nagios releases new updates at least once a year, so I want something recent.

APT?

Next step, I looked into apt repository what I could get.
Did the guy from NagiosPi stopped his project because Raspbian added the Nagios package in apt?
Maybe, but I only find Nagios 3 in the apt sources, the latest version when I wrote this line was Nagios 4.4.6.

It could be the easiest way to get Nagios, but I want the latest version or not too far from it.

NEMS?

Next search, I found a pre-configured SD card with Nagios and a lot of monitoring tools.
This is the NEMS Linux project, NEMS stands for Nagios Enterprise Monitoring Server.
Seems interesting no?

Yes, I tried it, but this was not suitable for Raspberry Pi.
Even when doing nothing, I have a lot of load, and couldn’t do anything without lag.
Maybe too many tools for the Raspberry Pi.

From sources?

The last option I was thinking about was to install Nagios manually from the sources.
But I thought this will be difficult and time-consuming to compile everything.

Hum … not so long finally, maybe 30min to get everything working with the latest version.
So for me, it’s really the best option I found, and I’ll now explain to you how to install it on your Raspberry Pi.

How to install Nagios on your Raspberry Pi

Install Raspberry Pi OS

First thing you need, is an operating system ?
Start with Raspbian, I think Raspbian Lite is perfect for this goal, but take the Desktop version if you prefer.

Here are the first steps you need to complete before going further:

That’s it, we are ready to start.

Downloads and prerequisites

Nagios is a web interface, so we need to install Apache and PHP:
sudo apt install apache2 libapache2-mod-php

Then there are two things we need to download and install:

• Nagios Core: It’s the main part of the Nagios system.
• Nagios Plugins:  This will add basic check commands to Nagios, so you’ll not have to create all the scripts yourself.

We need to download sources for them, check their websites for the latest version:
Nagios Core downloads
Nagios Plugins downloads

Finally, download archives with wget:
cd ~
mkdir Downloads
cd Downloads
wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.6.tar.gz
wget http://nagios-plugins.org/download/nagios-plugins-2.3.3.tar.gz

Now we are ready to move to the installation process.

Install Nagios on Raspberry Pi

Create Nagios user and group

We need to create a specific user for Nagios, and a group for Nagios and Apache users (nagcmd):
sudo useradd -m -s /bin/bash nagios
sudo groupadd nagcmd
sudo usermod -a -G nagcmd nagios
sudo usermod -a -G nagcmd www-data

Compile and install Nagios

Now we have to extract files from the archive, compile the sources and install Nagios on the Raspberry Pi:

• Move to the “Downloads” directory:
  cd /home/pi/Downloads
• Extract files from the archive downloaded before:
  tar zxvf nagios-4.4.6.tar.gz
• Don’t forget to change the version number if you download another one.
• Compile :
  cd nagios-4.4.6/
./configure --with-command-group=nagcmd
make all
  This could take a few minutes.
• Install:
  sudo make install
sudo make install-init
sudo make install-config
sudo make install-commandmode

This is the end of the Nagios core installation.
We now need to configure the Apache part to allow access to the Nagios page.

Configure Apache for Nagios

• Enable the CGI module for Apache:
  sudo a2enmod cgi
• Copy the Apache configuration to the Apache folder:
  sudo cp sample-config/httpd.conf /etc/apache2/sites-enabled/nagios.conf
• Create the first user for the web interface:
  sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
• Restart Apache:
  sudo service apache2 restart
• Check if you have access to Nagios:
  Go to http://<RASPBERRY IP>/nagios
  (check this tutorial if you don’t know how to find the current IP address).
  You should get something like this:
  
  It’s normal, we’ll fix that now.

The Nagios installation is almost done, we just need to run the Nagios service.

Start the Nagios service

Nagios has a service running in background to launch a check at a predefined frequency (every 5min for example).
We need to start it to finish the installation:
sudo service nagios start

And to start Nagios automatically on boot:
sudo ln -s /etc/init.d/nagios /etc/rcS.d/S99nagios

If you refresh the web interface, you should now see “Daemon running with PID XXX”.
Everything is fine with Nagios, we just want to add plugins now.

Install Nagios Plugins

Follow this procedure to install the Nagios plugins:

• Get back to the Downloads folder:
  cd /home/pi/Downloads
• Extract files from the archive:
  tar zxvf nagios-plugins-2.3.3.tar.gz
• Compile and install:
  cd nagios-plugins-2.3.3/
./configure --with-nagios-user=nagios --with-nagios-group=nagios
make
sudo make install

All plugins are now available in the /usr/local/nagios/libexec/ folder and ready to use in Nagios.

You may need to restart Nagios to apply changes:
sudo service nagios restart

How to configure Nagios to fit your needs?

Nagios vocabulary

Nagios uses a specific vocabulary to define each object in the configuration
So first, I have to be sure you know it:

• Command: a command is basically a script from the Nagios plugins folder, we define it in Nagios to use it later with parameters (ex: check_load is a command).
• Service: a service is a check that applies to one or more computers, and that uses a command to run a particular verification (ex: check if the load is above 10 on Raspberry Pi is a service).
• Host: a host is a computer, a server, anything with an IP address.
• Hostgroup: you can define a host group, like “Raspberry Pi”, and apply the same services to all the hosts inside this group.

We’ll now see how to configure each one in the Nagios configuration file.
For the example I’ll monitor my Raspberry Pi Zero from the Raspberry Pi where Nagios is installed.

Prepare the host

Nagios uses mainly SNMP to connect to other hosts and get all the information.
So we need to install SNMP on every new host before adding it in the configuration file.

This is not mandatory, it depends on which services you’ll create for this host.
Anyway, I’ll show you how to do it, so you have a complete tutorial.

In my case, I followed these steps on the Raspberry Pi Zero:

• Connect to the host via SSH.
• Install snmpd:
  sudo apt install snmpd
• Move to the configuration folder:
  cd /etc/snmp
• Remove (or backup) the snmpd.conf file:
  sudo rm snmpd.conf
• Create a new snmpd.conf file:
  sudo nano snmpd.conf
• Paste these lines into:
  com2sec readonly default RASPBERRYTIPS
group MyROGroup v1 readonly
group MyROGroup v2c readonly
view all included .1 80
access MyROGroup "" any noauth exact all none none
sysName Pi Zero
  Replace RASPBERRYTIPS by the community you want to use.
  Replace sysName by your host name or description.
  There are many options available in snmpd.conf, but for this test we don’t need more than that.

• Save and exit (CTRL+O, CTRL+X)

• Restart snmpd

  sudo service snmpd restart

We can now add this host in Nagios.

Add a host

• Go back to the Raspberry Pi hosting Nagios.
• Move to the objects configuration folder of Nagios:
  cd /usr/local/nagios/etc/objects
• Create a new file for your host, like pizero.cfg:
  sudo nano pizero.cfg
• Paste these lines into:
  define host {
use linux-server  ; Host group to use
host_name Pi Zero  ; Name of this host
alias pizero  ; Alias
address 192.168.1.18  ; IP Address
}

  This is really a basic host definition, with the host name, alias and IP address.
  We also add it to the Linux servers host group to inherit group-defined services if you add some later

• Save and exit (CTRL+O, CTRL+X).
• Now we need to tell Nagios that we added a new file.
  Add this line in /usr/local/nagios/etc/nagios.cfg (just under localhost.cfg will be perfect) .
  cfg_file=/usr/local/nagios/etc/objects/pizero.cfg
• Restart Nagios:
  sudo service nagios restart
• Go back to the Nagios web interface, in the Hosts menu.
  You should have a new line like this:
  

Nagios already monitors the ping response for our new host, but we want more.
So we need to add a service inside, but first we need to define the command to use.

Add a command

Previously in this tutorial you downloaded and installed Nagios plugins
Nagios plugins provide many commands to check the local host, but not so much for remote hosts.
You can also download many other commands on this website (specifically look for SNMP plugins).

For this example, I’ll monitor the time synchronization between my two Raspberry Pi.

To add a command in Nagios, follow these steps:

The command is now ready to be used for your host, but we need to create a new service to apply it.

Add a service

To create a new service, follow these steps:

• Open your host configuration file:
  sudo nano /usr/local/nagios/etc/objects/pizero.cfg
  Adapt the filename to what you have created before
• Paste these lines:
  define service {
use local-service  ; Name of service template to use
host_name Pi Zero
service_description Time synchronization
check_command check-ntp!1!5
}

  Don’t forget to adapt these lines to your previous configuration.
  On the check_command line, we define all arguments, separated by exclamation points

• Save and exit.
• Check your configuration:
  sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
  This could be useful to know if you have configuration errors before restarting Nagios.
  In my case, I wrote check_ntp instead of check-ntp ?
• Restart Nagios:
  sudo service nagios restart

Now in the status detail from your host you should see the new service.

If you understood how to create a host, a command and a service, you are now ready to do anything in Nagios.
Feel free to download new plugins on Internet, or even code your plugins (it’s nothing more than a basic script to check something).

Video

Related questions

Is it possible to create the configuration from the web interface?  Unfortunately no, Nagios doesn’t provide a configuration tool in the Core version. But there are several configuration tools you could install. The one I often use is Thruk, I didn’t try it yet on Raspberry Pi, but you can download it here if you want.

Is it possible to monitor Windows or other hosts? Yes. You could monitor anything on the network. SNMP is also available for Windows and most of the network equipments (router, switch, home automation device, …).  As you can create your check commands, everything is possible.

Conclusion

That’s it, you know how to install and configure a Nagios server on your Raspberry Pi to monitor your network or anything you want.
With this tutorial, you get the latest Nagios version, with no excessive load average (currently 0.00 on my Pi ^^).

Once Nagios is installed, it’s not really different from any Nagios server on Linux.
So feel free to check Linux tutorials for advanced configuration.

And if you want to create easy-to-read graphs with the data generated in Nagios, take a look at how to install Grafana on your Raspberry Pi.

Additional Resources

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sudo apt-get install nagios3

define host {
    host_name    raspberrypi
    address      192.168.1.1
    check_command check-host-alive
}

define service {
    host_name    raspberrypi
    service_description    PING
    check_command check_ping!100.0,20%!500.0,60%
}

sudo service nagios3 start