LAN Fence

LAN Fence is an open-source defensive network device monitor for Linux (developed and tested on Raspberry Pi OS / Debian, and reasonably portable to other Debian/Ubuntu systems).

• Source & downloads: https://github.com/rosscooney/lanfence (releases)
• Package: lanfence on PyPI

LAN Fence runs on a small Linux box (a Raspberry Pi is the common case) sitting on your network. It continuously scans for connected devices via ARP, maintains an allowlist of devices you already trust, and alerts in plain language when something unknown joins - a rogue device, unauthorized hardware, or a supply-chain implant on your LAN.

LAN Fence only observes. It sends nothing beyond a standard ARP "who-has" request (the same thing every device on your LAN does routinely) and never touches, blocks, deauthenticates or spoofs anything.

⚠️ LAN Fence cannot prove a device is malicious, or that a MAC address is genuine. MAC vendor prefixes and hostnames are trivially spoofed by anyone deliberately trying to blend in. A finding is a lead worth checking by hand, not a verdict - use it as one input to your own judgement.

How it works

1. Active scanning - LAN Fence periodically ARP-sweeps your subnet (lanfence scan for one sweep, or on an interval inside lanfence monitor), collecting every MAC/IP that answers.
2. Passive monitoring - between active sweeps, lanfence monitor also listens for ARP traffic on the wire, so a device that joins mid-interval is caught sooner rather than waiting for the next sweep.
3. Every sighting is folded into a persistent SQLite database keyed by MAC address, which tracks each device's lifecycle: new_device the first time it's ever seen, reappeared if it had gone offline and came back, and disconnected when an active sweep no longer sees it.
4. Each device is fingerprinted: an offline OUI → vendor lookup, a set of built-in rogue-device signatures (see below), and a check of whether its MAC is locally administered (randomized/spoofed rather than vendor-assigned).
5. Each device is checked against your allowlist (lanfence allow <mac>). A brand-new or reappearing device not on the allowlist produces a plain-language finding with a severity (high/medium/info), a rationale, and a recommendation; an allowlisted device is downgraded to info so your own hardware stops shouting every time it reconnects.
6. Findings can be dispatched to syslog, email, or a webhook, and everything is available as a CLI table or JSON for automation.

Built-in rogue-device signatures

Heuristics, not proof - a match is a lead to check by hand:

Extend or override these with your own rogue_signatures_file: (same YAML shape as lanfence/data/rogue_signatures.yaml) and vendor_file: (same tab-separated shape as lanfence/data/oui_vendors.txt) in config.

Install

pipx install "lanfence[scan]"     # isolated, recommended - includes scapy for scanning

scan/monitor need the scan extra (scapy) to actually send/receive ARP packets; allow, report and check work without it. Already installed without the extra? Add it in place:

pipx inject lanfence scapy

or, without pipx:

python3 -m venv ~/.venvs/lanfence
~/.venvs/lanfence/bin/pip install 'lanfence[scan]'

Scanning needs raw-socket access, so scan/monitor typically need sudo (or CAP_NET_RAW on the interpreter). allow, report and check do not.

Commands

lanfence scan                  # one-time active ARP scan; table + findings
lanfence scan --format json    # same, machine-readable
lanfence monitor                # continuous: active sweeps + passive sniffing
lanfence allow <MAC> --name X   # trust a device; its findings become info
lanfence allow --list           # show the allowlist
lanfence allow --remove <MAC>   # untrust a device
lanfence report --since 24h     # summarize events/findings from the database
lanfence check                  # verify permissions, scapy, interface, storage
lanfence upgrade                # check PyPI and install a newer release, if any
lanfence upgrade --check        # only report whether an update is available

Example: an unknown device joins

$ sudo lanfence scan

Devices seen (4)
┌───────────────────┬──────────────┬──────────────┬────────────────────┬────────┬─────────┐
│ MAC                │ IP           │ Hostname     │ Vendor              │ Status │ Trusted │
├───────────────────┼──────────────┼──────────────┼────────────────────┼────────┼─────────┤
│ b8:27:eb:12:34:56  │ 192.168.1.10 │ nas.local    │ Raspberry Pi        │ online │ yes (NAS)│
│ 52:8a:1c:99:f4:2d  │ 192.168.1.47 │ [unknown]    │ [unknown]           │ online │ no      │
└───────────────────┴──────────────┴──────────────┴────────────────────┴────────┴─────────┘

Findings (1)

  MEDIUM Unknown device connected
    MAC: 52:8a:1c:99:f4:2d
    The vendor bit pattern indicates a locally administered address rather
    than one assigned by a hardware vendor. Common causes: MAC-randomization
    privacy features on modern phones/laptops, virtual machines/containers,
    or a device deliberately spoofing its address.
    Recommendation: Verify this device belongs on your network. If it's
    yours, run `lanfence allow 52:8a:1c:99:f4:2d` to stop future alerts.
      • MAC: 52:8a:1c:99:f4:2d
      • IP: 192.168.1.47
      • Hostname: [unknown]
      • Vendor: [unknown]
      • mac = 52:8a:1c:99:f4:2d (U/L bit set, no vendor OUI match)

Overall: 1 finding(s), highest severity: medium

Running unattended

LAN Fence does not ship its own scheduler; use systemd (recommended on a Pi) or cron.

Continuous monitoring - /etc/systemd/system/lanfence.service:

[Unit]
Description=LAN Fence continuous monitoring
After=network-online.target
Wants=network-online.target

[Service]
ExecStart=/usr/local/bin/lanfence monitor --config /etc/lanfence/config.yaml
Restart=on-failure
User=root

[Install]
WantedBy=multi-user.target

sudo systemctl enable --now lanfence

Daily report - a cron entry (sudo crontab -e):

0 7 * * * /usr/local/bin/lanfence report --since 24h --format json > /var/log/lanfence/daily.json

Configuration

All settings are optional; everything has a sensible default. Pass --config path/to/config.yaml to any command.

scan:
  interface: null              # null = auto-detect
  subnet: null                 # null = derive from the interface's own address
  scan_interval_seconds: 60    # how often `monitor` repeats an active sweep
  active_scan_timeout_seconds: 3
  passive: true                # also sniff ARP traffic between sweeps
  resolve_hostnames: true      # try reverse DNS for each device
  dns_timeout_seconds: 1

alerts:
  min_severity: medium         # info | medium | high - dispatch threshold
  syslog:
    enabled: false
    address: /dev/log
    facility: user
  email:
    enabled: false
    smtp_host: localhost
    smtp_port: 587
    use_tls: true
    username: null
    password: null
    from_addr: null
    to_addrs: []
  webhook:
    enabled: false
    url: null
    timeout_seconds: 5

db_path: ~/.local/share/lanfence/lanfence.db
allowlist_file: ~/.config/lanfence/allowlist.yaml
vendor_file: null             # extra OUI table, merged with the packaged one
rogue_signatures_file: null   # extra signatures, merged with the packaged ones

Exit codes (--fail-on-findings)

scan and report accept --fail-on-findings for CI/scripting use:

Privacy and security

• No telemetry, no external calls. LAN Fence never phones home. The only network destinations it ever contacts are ones you configure: your own syslog daemon, your own SMTP relay, or your own webhook URL.
• The vendor and signature databases are bundled, offline, and static - nothing is fetched to "keep them fresh".
• The device database and allowlist are written atomically and are owner-readable only where the platform supports it.

Development

python3 -m venv .venv && . .venv/bin/activate
pip install -e ".[dev,scan]"
pytest

See CONTRIBUTING.md for scope and pull-request guidelines, and DISTRIBUTING.md for licensing notes on the optional scapy (GPL-2.0) dependency.

License

MIT - see LICENSE.