Over the past few months, I’ve had a feeling that strangers (or actually people I know but never gave my Wi-Fi password to) have been using my Wi-Fi behind my back. Sure, my internet is unlimited, and I don’t have a problem sharing it. But having a whole bunch of people using it simultaneously can sometimes affect its performance. I’ve experienced that, and it’s what gave me the hunch.

Also, having strangers access your internet is a security risk. And trust me, I’ve changed my password multiple times, yet I still get the same feeling. Unfortunately, my router can only show me the devices that are currently connected. It doesn’t let me see which devices have connected in the past.

So, I started looking for a solution to my hunch, and that’s when I came across a free, open-source tool called NetAlertX.

NetAlertX runs on your own hardware

I installed it with Docker, and here’s what worked for me

NetAlertX is a self-hosted network monitoring and asset intelligence tool that runs on your own hardware. I deployed mine through Docker, but you can also install it as an Unraid app, through a supervised Home Assistant instance, or directly on bare metal.

If you’re comfortable with self-hosting, the developers’ Docker installation documentation should be enough to get you going. But I don’t really self-host stuff, so I kind of struggled a bit, mostly because my laptop was missing some prerequisites. Here’s the process that eventually worked for me:

• Install Docker and Docker Desktop on my Windows 11 laptop
• Download NetAlertX project files from GitHub
• Open the NetAlertX Docker installation folder in Windows Terminal
• Run docker compose up -d to start the NetAlertX Docker container
• Open http://localhost:20211 in your browser to reach the NetAlertX interface

I hit a couple of errors along the way (when starting the container with Docker Compose), so I asked Docker Desktop’s built-in AI assistant, Gordon, to help. It identified the problem and fixed the container issues. Then I ran the command again, and the container started.

NetAlertX helped me fish out unknown guests

Turns out my hunch was right

NetAlertX scans and tracks devices on your Wi-Fi or LAN, using discovery methods that can include ARP, DHCP, and ICMP. Unlike my router’s admin page, which only shows what’s connected at the moment, NetAlertX keeps a history of every device that has ever connected to my router.

It also shows timestamps, so I can see when devices were last detected or when their connection status changed. So, after finishing the installation and leaving it running for a few days, I checked the dashboard. And there they were: not one, not two, but four unknown devices that had connected to my router. I could see their IP and MAC addresses, which gave me the information I needed to block them from my router’s admin page.

It can alert you the moment a new device joins

Dozens of notification services, from Telegram to email

I don’t want to keep opening the NetAlertX dashboard just to check whether something new has connected to my Wi-Fi. Thankfully, the tool supports notifications through Apprise integration, which opens up more than 80 notification services.

That includes Telegram, Pushover, Pushsafer, NTFY, Slack, Discord, email, webhooks, and more. Once you configure a notification service, NetAlertX can send alerts when it detects events such as new devices or connection-state changes.

That means I can get notified when a new device joins my network instead of finding out days later when I happen to check the dashboard. You can also fine-tune the notification triggers to avoid notification fatigue. For example, you can choose which device changes should trigger an alert, so you’re not constantly getting notifications every time a familiar device disconnects and reconnects.

Plugins let you go further

Add new discovery sources and integrations as your setup grows

Plugins are NetAlertX’s biggest strengths. The tool isn’t limited to its core features, including the network scanning method. It has a plugin system that lets third-party integrations and custom plugins feed information into the interface, trigger notifications, and add data to its database.

Plugins can do things like add a different scanning method, import information from other services, and monitor websites. For example, I was able to use a plugin to pull device information from Pi-hole and UniFi.

You can also use these plugins to create UI elements, filter values, and import objects into NetAlertX’s database.

Keep an eye on who’s using your Wi-Fi

I can’t say for sure that every unknown device you’ll find is a stranger using your internet. Devices can show up with unfamiliar names, and MAC randomization can make identification tricky. But NetAlertX gave me a way to investigate network activity that my router’s admin page simply didn’t provide. And its results depend on your scanning method, network configuration, and how often scans run.