feat: subnet scan fallback when Bonjour fails (handles iOS Personal Hotspot)

Matthias Nott

2026-03-25 9e5953ced9c02f883203c8e49b2c7a78333ef34b

feat: subnet scan fallback when Bonjour fails (handles iOS Personal Hotspot)

 lib/services/mqtt_service.dart
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@@ -156,8 +156,10 @@
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   }
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   /// Discover AIBroker on local network via Bonjour/mDNS.
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+  /// Falls back to subnet scan if Bonjour fails (iOS blocks mDNS on Personal Hotspot).
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   /// Returns the IP address or null if not found within timeout.
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   Future<String?> _discoverViaMdns({Duration timeout = const Duration(seconds: 3)}) async {
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+    // Try Bonjour first
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     try {
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       final discovery = BonsoirDiscovery(type: '_mqtt._tcp');
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       await discovery.initialize();
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@@ -187,9 +189,62 @@
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       await sub?.cancel();
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       await discovery.stop();
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-      return ip;
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+      if (ip != null) return ip;
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     } catch (e) {
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       _mqttLog('MQTT: Bonjour discovery error: $e');
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+    }
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+
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+    // Fallback: scan local subnet for MQTT port (handles Personal Hotspot)
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+    _mqttLog('MQTT: Bonjour failed, trying subnet scan...');
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+    return _scanSubnetForMqtt();
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+  }
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+
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+  /// Scan the local subnet for an MQTT broker by probing the configured port.
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+  /// Useful when iOS Personal Hotspot blocks mDNS.
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+  Future<String?> _scanSubnetForMqtt() async {
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+    try {
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+      // Get device's own IP to determine the subnet
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+      final interfaces = await NetworkInterface.list(type: InternetAddressType.IPv4);
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+      for (final iface in interfaces) {
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+        for (final addr in iface.addresses) {
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+          final parts = addr.address.split('.');
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+          if (parts.length != 4) continue;
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+          // Skip loopback
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+          if (parts[0] == '127') continue;
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+          // Only scan small subnets (hotspot = /28, max 14 hosts)
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+          final subnet = '${parts[0]}.${parts[1]}.${parts[2]}';
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+          _mqttLog('MQTT: scanning $subnet.0/24 on ${iface.name}');
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+
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+          // Probe hosts 1-14 in parallel (covers /28 hotspot subnet)
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+          final futures = <Future<String?>>[];
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+          for (int i = 1; i <= 14; i++) {
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+            final probe = '$subnet.$i';
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+            if (probe == addr.address) continue; // skip self
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+            futures.add(_probeHost(probe, config.port));
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+          }
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+
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+          final results = await Future.wait(futures);
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+          final found = results.firstWhere((r) => r != null, orElse: () => null);
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+          if (found != null) {
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+            _mqttLog('MQTT: subnet scan found broker at $found');
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+            return found;
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+          }
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+        }
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+      }
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+    } catch (e) {
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+      _mqttLog('MQTT: subnet scan error: $e');
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+    }
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+    return null;
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+  }
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+
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+  /// Probe a single host:port with a TCP connection attempt (1s timeout).
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+  Future<String?> _probeHost(String host, int port) async {
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+    try {
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+      final socket = await Socket.connect(host, port,
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+          timeout: const Duration(seconds: 1));
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+      await socket.close();
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+      return host;
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+    } catch (_) {
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       return null;
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     }
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   }