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+# Net Alerter — Device Presence Tripwire Daemon
+
+Persistent systemd daemon that passively monitors device presence on the network via DHCP sniffing and Netlink kernel events. Sends Matrix alerts on device arrival/departure.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `net_alerter.py` | Main daemon (stdlib-only, 15K) |
+| `net_alerter.service` | Systemd unit file |
+| `deploy-daemon.sh` | Universal deployment script |
+| `DAEMON_DEPLOYMENT.md` | General deployment guide |
+| `ORANGE_PI_DEPLOYMENT.md` | Orange Pi Zero 3 specific guide |
+| `deploy.sh` | Legacy cron-based deployer (deprecated) |
+
+## Quick Start (Orange Pi Zero 3 @ 10.0.0.0)
+
+```bash
+cd ~/tools/bigbrother/net_alerter
+./deploy-daemon.sh root@10.0.0.0
+```
+
+Then configure Matrix credentials:
+```bash
+ssh root@10.0.0.0 "cat > /opt/net_alerter/.env <<EOF
+MATRIX_HOMESERVER=https://m.example.org
+MATRIX_ACCESS_TOKEN=<your_token>
+MATRIX_ROOM_ID=!REDACTED:example.org
+EOF
+chmod 600 /opt/net_alerter/.env
+systemctl restart net_alerter"
+```
+
+Verify:
+```bash
+ssh root@10.0.0.0 "journalctl -u net_alerter -n 30"
+```
+
+## How It Works
+
+### Three Concurrent Threads
+
+1. **DHCP Sniffer** — AF_PACKET raw socket captures DHCP traffic
+   - Extracts MAC, IP, hostname from DHCP packets
+   - Triggers arrival on Discover/Request, departure on Release
+
+2. **Netlink RTM_NEWNEIGH Watcher** — Kernel ARP neighbor creation events
+   - Fires when device reaches REACHABLE state
+   - Handles static-IP devices
+
+3. **Netlink RTM_DELNEIGH Watcher** — Kernel ARP neighbor deletion events
+   - Fires when device times out from ARP cache (5-10 minutes)
+   - Triggers departure alert
+
+### Zero Active Probing
+- No nmap, nping, arp-scan, ping
+- Only passive listening on raw sockets
+- No traffic generated by this daemon
+
+### Dependencies
+- **Runtime**: Python 3.6+, root access
+- **Libraries**: stdlib only (socket, struct, threading, time, logging, os)
+- **Data**: OUI database at `/usr/share/hwdata/oui.txt` or `/usr/share/misc/oui.txt` (usually present on Debian)
+
+## Configuration
+
+`.env` file at `/opt/net_alerter/.env`:
+```bash
+MATRIX_HOMESERVER=https://m.example.org
+MATRIX_ACCESS_TOKEN=syt_...
+MATRIX_ROOM_ID=!REDACTED:example.org
+```
+
+### Optional Environment Variables
+- `NET_ALERTER_ENV` — Path to config file (default: `/opt/net_alerter/.env`)
+
+## Deployment Methods
+
+### Method 1: Automated Script (Recommended)
+```bash
+./deploy-daemon.sh root@10.0.0.0
+```
+
+### Method 2: Manual
+See `DAEMON_DEPLOYMENT.md` → Manual Deployment section
+
+### Method 3: Orange Pi Specific
+See `ORANGE_PI_DEPLOYMENT.md` for step-by-step guide with troubleshooting
+
+## Monitoring
+
+### Service Status
+```bash
+ssh root@10.0.0.0 "systemctl status net_alerter"
+```
+
+### Real-Time Logs
+```bash
+ssh root@10.0.0.0 "journalctl -u net_alerter -f"
+```
+
+### Check Device Count
+```bash
+ssh root@10.0.0.0 "journalctl -u net_alerter | grep 'Tracking.*devices'"
+```
+
+## Example Alerts
+
+### Arrival
+```
+[NET] ARRIVED: iphone.local (10.0.0.0) [Apple Inc.] MAC:a4:5e:60:ab:cd:ef
+```
+
+### Departure
+```
+[NET] DEPARTED: iphone.local (10.0.0.0) [Apple Inc.] MAC:a4:5e:60:ab:cd:ef — present 2h 34m
+```
+
+## Performance
+
+| Metric | Value |
+|--------|-------|
+| CPU | <1% (idle, thread sleeping on sockets) |
+| Memory | ~15-20 MB |
+| Network Traffic | 0 (passive only) |
+| Detection Latency | <100ms for Netlink events, 5-10min for ARP timeouts |
+| Startup Time | ~2 seconds |
+
+## Troubleshooting
+
+See `DAEMON_DEPLOYMENT.md` → Troubleshooting section for:
+- Service won't start
+- No alerts being sent
+- Not detecting devices
+- High CPU usage
+
+## Migration from Cron
+
+The legacy cron-based implementation (`deploy.sh`) ran every 5 minutes with active probing (nmap). The new daemon:
+- Runs continuously
+- Zero active probing
+- Real-time detection via kernel Netlink notifications
+- Lower CPU and network overhead
+
+To migrate, stop the old cron and deploy the daemon.
+
+## Development
+
+### Code Structure
+- `load_env()` — Parse `.env` configuration
+- `seed_from_arp_cache()` — Bootstrap known devices on startup
+- `lookup_oui()` — MAC vendor lookup
+- `on_arrival()/on_departure()` — Event handlers, trigger Matrix alerts
+- `dhcp_sniffer()` — DHCP packet capture thread
+- `parse_dhcp()` — Extract MAC/IP/hostname from DHCP payloads
+- `netlink_watcher()` — Netlink event listener thread
+- `parse_netlink()/parse_ndmsg()` — Kernel neighbor event parsing
+- `send_alert()` — POST to Matrix API
+
+### Testing
+```bash
+python3 -m py_compile net_alerter.py  # Syntax check
+python3 -c "import net_alerter"        # Import check
+```
+
+Note: Full functional tests require root, raw sockets, and live DHCP traffic.
+
+## References
+- Netlink: https://man7.org/linux/man-pages/man7/netlink.7.html
+- RTnetlink: https://man7.org/linux/man-pages/man7/rtnetlink.7.html
+- AF_PACKET: https://man7.org/linux/man-pages/man7/packet.7.html
+- DHCP (RFC 2131): https://tools.ietf.org/html/rfc2131
+
+## License
+Part of BigBrother project.