For the monitoring of our network infrastructure we use Check_MK. Check_MK supports a variety of services and agents. Data can be monitored with the Check_MK agent, SNMP or simply by ping. In the past, we used to monitor our firewall via SNMP, which is already integrated in pfSense (You can enable SNMP via Services → SNMP). This already covers most of the services, such as all network interfaces. But some important information is missing. This includes the current CPU load or the hard disk(s). To get this information, we need the Check_MK agent on the firewall.

Installing the Check_Mk Agent in pfSense

I’ll describe the installation via a SSH connection to firewall. Alternatively, you can also enter all commands under Diagnostics → Command Prompt.

The Check_MK-Agent is a bash script. Therefore bash must be installed first:

Then we create two directories:

Next, we need to download the Check_MK-Agent to our newly created directory and then make it executable:

The data of the Check_MK agent is retrieved via telnet. Therefore we need a configuration file. To do this, copy the following content (preferably via Diagnostics → Edit file) to the file /opt/etc/xinetd.d/check_mk:

Now we have to configure some filter settings. The following script makes the necessary changes in the file /etc/inc/ automatically. To do this, create the file /opt/filter_check_mk_cron (again with Diagnostics → Edit file).

We have to set the executable bit again and run it.

Finally, reload the filters (via Status → Filter Reload) and now we can check if the Check_MK-Agent works (adjust IP of the firewall accordingly):

If everything is configured correctly, you should see the output of the Check_MK agent.

Setting up pfSense on Check_MK Server

Now we have to add the firewall as a host in Check_MK or edit the existing host in WATO. In addition to the IP, SNMP community and hostname, we select Dual Check: Check_MK Agent + SNMP as the agent type. With a click on Save & go to Services we can activate the desired services.

Check Settings

Now all important information is always available at a glance. Here is an excerpt:

Check Services


Check_MK is a very flexible and great monitoring software. We have been using it for some time now in our school and the possibilities for customization are almost infinite as far as the notifications and devices to be monitored are concerned.

What software do you use for monitoring?

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Categories: HowTo



I'm a teacher and IT system administrator in an international school. I love open source software and I used it over a decade in my private and work life. My passion is to solve problems with open source software!