Know Every Device on Your Network

You cannot protect what you cannot see. A Pi can inventory your home network, remember known devices and notify you when something new appears.

Video tutorial

▶ Shannon MorseWatch on YouTube ↗

Video by Shannon Morse, embedded from YouTube. The written steps below are our own and match the parts list.

Mission steps

1. Install nmap

   sudo apt install nmap. Only scan networks you own or have written permission to test.

2. Run a discovery scan

   nmap -sn 192.168.1.0/24 lists live hosts and, run with sudo, their MAC addresses.

3. Save known devices

   Run the script once to record your current devices, then label them.

4. Alert on new ones

   Schedule the script every 15 minutes with cron; it sends a notification for any unknown MAC.

5. Act

   Unknown device? Check your router, change your Wi-Fi password and move smart gadgets to a guest network.

Code

# netwatch.py — run with sudo for MAC addresses
import subprocess, re, json, os, requests
NET, DB = "192.168.1.0/24", "/home/maker/known.json"
TOPIC = "https://ntfy.sh/mc-net-k3j9d8"      # your own random topic

out = subprocess.run(["nmap", "-sn", NET], capture_output=True, text=True).stdout
seen = set(m.upper() for m in re.findall(r"MAC Address: ([0-9A-Fa-f:]{17})", out))
known = set(json.load(open(DB))) if os.path.exists(DB) else set()

if not known:
    json.dump(sorted(seen), open(DB, "w"), indent=1)
    print("Saved", len(seen), "known devices")
else:
    for mac in seen - known:
        requests.post(TOPIC, data=f"New device on network: {mac}")

Tip: test every script on the bench before installing it in a moving machine.

Questions