Embedded Tailscale & HA Failover

The firmware embeds a full WireGuard and Tailscale coordination client via a custom-tuned build of microlink. This enables end-to-end encrypted remote access from anywhere in the world without port forwarding, dynamic DNS, or cloud subscription fees.

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Mesh Topology & Subnet Route Advertising

┌─────────────────────────────────────────────────────────────┐
│                   TAILSCALE MESH NETWORK                    │
│                (MagicDNS: http://esp32/main)                │
└───────────────┬─────────────────────────────┬───────────────┘
                │                             │
    Direct Peer-to-Peer            Relayed via Tailscale DERP
   (UDP WireGuard on LAN)           (Encrypted HTTPS Relay)
                │                             │
                ▼                             ▼
┌─────────────────────────────────────────────────────────────┐
│                 ESP32-SwitchBot Microcontroller             │
│            • Modified Microlink WireGuard Stack             │
│            • Advertises Subnet: 192.168.1.0/24              │
│            • Outbound CGNAT (100.64.0.0/10) Intercept       │
└─────────────────────────────────────────────────────────────┘

Key Networking Enhancements in components/microlink:

1. High-Availability Subnet Advertising: Advertises the local LAN subnet (192.168.1.0/24) directly to the Tailnet.
2. Linker-Wrapped Outbound Routing: Uses a GNU linker hook (__wrap_ip4_route_src_hook in ml_wg_mgr.c) to route return packets destined for CGNAT (100.64.0.0/10) through the WireGuard interface.
3. Inbound Packet Remapping: Interface hooks in wireguardif.c ensure packets addressed to the local IP (192.168.1.50) pass directly to the local lwIP stack rather than looping back.

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Cold Standby & Subnet Router Watchdog

To minimize power and thermal load, the ESP32 includes an automated failover engine:

1. Watchdog Monitoring: If a primary subnet router (such as an always-on phone or home server e.g. "moto-g32") is configured with a read-only TAILSCALE_API_KEY, the ESP32 periodically queries api.tailscale.com.
2. Cold Standby State: While the primary subnet router is alive, the ESP32 shuts down its WireGuard engine (ml_stop). In this state, the device remains fully reachable through the primary router's subnet bridge while keeping chip thermals low (~40°C).
3. Automated Failover: If the primary router goes offline or suffers an outage, the ESP32 instantly starts its embedded WireGuard client (ml_start), taking over subnet advertising and restoring remote connectivity within seconds.

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Multi-Network Wi-Fi Failover & ARP Keepalives

• 6 Network Profiles: The ESP32 can store up to 6 distinct Wi-Fi credentials (WIFI_SSID_1..6). If the primary network loses internet or connection, the firmware automatically cycles to fallback networks.
• Gratuitous ARP Keepalives: Consumer ISP routers frequently drop sleeping Wi-Fi clients from their ARP tables. The firmware emits periodic Gratuitous ARP packets (CONFIG_LWIP_ESP_GRATUITOUS_ARP=y), preventing router connection drops during modem sleep.