Monitoring pfSense with Nagios XI Using SSH – part 1

Monitoring pfSense with Nagios XI Using SSH – part 1

Monitoring pfSense with Nagios XI Using SSH Series

This walkthrough will guide you through the process of monitoring your pfSense using Nagios XI and SSH. The scripts could also be used with NRPE without issue, although I discuss why SSH is my preferred route below. Similar configuration steps could be taken on Nagios Core, however, I don’t have a running copy of Core to verify. I also included a service config file at the end of part 3 so Nagios Core users can see what the underlying config looks like. This process is based on very sound practices and I can say that I used it to monitor numerous pfSense firewalls for several years with zero issues. Not to mention, once you understand the process it is trivial to replicate the configuration to additional firewalls. At some point, I might make this methodology a pfSense package to assist with the install and versioning if there is enough interest.

Nagios pfSense services detail

Why SSH When There’s NRPE and SNMP?

Quite honestly, I’ve never been a huge fan of NRPE or SNMP. By default, NRPE uses ADH (anonymous Diffie Hellman) and it’s a little messy dealing with certs if you want to make it secure IMO. I admittedly haven’t tried the pfSense NRPE package recently, but years ago the service would frequently require a restart, i.e. false positives. I also tried SNMP monitoring because I was quite familiar using it for network equipment monitoring, but 1) I found it difficult to get what I needed and 2) the OIDs ended up being a bit of a moving target. On top of that, SSH is built into pfSense!

As you see above, I like monitoring the holy heck out of my firewalls so I know exactly what’s happening whether in real-time or via historical data <- fantastic for data correlation. I’ll continue adding more checks over time including more security-focused ones, but if you have any you’d like to see just give me a holler and I’ll look at adding them. Better yet, write the script and I’ll acknowledge your work! 😉

I’ve broken this guide down to 3 parts. You can click on any one of the links to go to that section. Note: I suggest going in order and *not* skipping anything. If you receive any errors, please double-check your work and make sure you followed the steps outlined here.

Part 1: Setting up password-less SSH (below)

Part 2: Downloading and testing the checks

Part 3: Configuring the checks on Nagios XI

Monitoring pfSense with Nagios XI Using SSH – part 1 – Setting up password-less SSH

Enable SSH on pfSense

First and foremost, you need to enable SSH on your pfSense box if you haven’t already. From the web GUI, go to System -> Advanced and put a checkmark in the box to ‘Enable Secure Shell.’ Hit save!

Creating the SSH keys on Nagios

From the command line on the Nagios system, type in the following as the nagios user. Hint: If you are logged in as root, just run ‘su – nagios’ to make the switch.

$ ssh-keygen -t ed25519

Hit enter a few times to accept the defaults. If all goes well, the new keys will get created in the /home/nagios/.ssh directory. Now we are looking for the public key to copy/paste. Display that key using the cat command below.

$ cat /home/nagios/.ssh/
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBinKKG0cGsC9rtsD1Ty4Q9fCed1bjUGMazNCTKgJAjT nagios@localhost.localdomain

Don’t close that window just yet! We’ll need it here in a bit! It’s also worth noting that you can copy this same public key to multiple pfSense firewalls (or other Linux, Unix, or BSD systems) that you want to monitor and you don’t need a separate one for each of them.

Creating the Nagios user on pfSense

Now we need to go over to the pfSense web GUI and create the Nagios user. FWIW, the password doesn’t matter because we won’t use it to log in… Just make it something ridiculously long that you can copy/paste twice. Also, note the ‘Authorized SSH Keys’ is identical to the one from the previous step. Don’t forget to click save after filling in the highlighted information.

Creating a nagios user on pfSense

Install and configure the sudo package

Before you get out of the pfSense web GUI, you need to install one other package. As you would when installing any pfSense package, go to System -> Package Manager and click on Available Packages. Type in ‘sudo’ and click install on the only package that shows up. Click ‘Confirm’ and you should be ready to go.

After the sudo package is installed, go to System -> sudo. Click on add and change the user dropdown to Nagios, place a checkmark in the “no password” section and then type ‘ALL’ in the command list as shown below. Click Save.

Sudo Package configuration on pfSense

Testing it

Next, go back to your Nagios command line and test to ensure you can log in. Don’t forget that you need to be the ‘nagios’ user! The very first time you’ll get asked a question about continuing your connection as seen below. Type in ‘yes’ and hit enter. If all goes well, you will end up at a command line on the pfSense box (last line in it).

[nagios@localhost ~]$ ssh nagios@
The authenticity of host ' (' can't be established.
ED25519 key fingerprint is SHA256:QIWv1m3iAfsPu/mqP1XhonpElGxDPEy5ggjRc6fQeew.
ED25519 key fingerprint is MD5:5b:92:18:de:18:65:e9:71:06:12:d5:b1:cb:cb:fd:78.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '' (ED25519) to the list of known hosts.

If you exit out of the firewall shell with Control-D and then re-connect, you’ll see this login dialog isn’t quite as messy.

[nagios@localhost ~]$ ssh nagios@

So we’re now able to login from the Nagios box, but what good does that do otherwise? Now that we have a secure connection between the systems, we can download and eventually run any check command we want using the SSH proxy on Nagios XI (or the check_by_ssh on Nagios Core).

Go to Part 2: Downloading and testing the checks

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