fix: cache discovered host to prevent repeated subnet scan floods

Matthias Nott

2026-03-25 a7b094a1284fd1fa9c8aaf84598e5ee12ed6041d

fix: cache discovered host to prevent repeated subnet scan floods

 lib/services/mqtt_service.dart
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@@ -44,6 +44,7 @@
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   ConnectionStatus _status = ConnectionStatus.disconnected;
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   bool _intentionalClose = false;
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   String? _clientId;
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+  String? _lastDiscoveredHost;
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   StreamSubscription? _updatesSub;
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   // Message deduplication
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@@ -104,12 +105,15 @@
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     final clientId = await _getClientId();
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-    // Connection order: local → Bonjour → VPN → remote
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+    // Connection order: local → cached discovery → Bonjour/scan → VPN → remote
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     final attempts = <MapEntry<String, int>>[];  // host → timeout ms
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     if (config.localHost != null && config.localHost!.isNotEmpty) {
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       attempts.add(MapEntry(config.localHost!, 2500));
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     }
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-    // Bonjour placeholder — inserted dynamically below
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+    // Try cached discovered host before scanning again
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+    if (_lastDiscoveredHost != null) {
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+      attempts.add(MapEntry(_lastDiscoveredHost!, 3000));
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+    }
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     if (config.vpnHost != null && config.vpnHost!.isNotEmpty) {
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       attempts.add(MapEntry(config.vpnHost!, 3000));
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     }
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@@ -118,7 +122,6 @@
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     }
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     _mqttLog('MQTT: attempts=${attempts.map((e) => e.key).join(", ")} port=${config.port}');
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-    // Try configured local host first
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     for (final attempt in attempts) {
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       if (_intentionalClose) return;
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       _mqttLog('MQTT: trying ${attempt.key}:${config.port}');
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@@ -127,21 +130,22 @@
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       } catch (e) {
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         _mqttLog('MQTT: ${attempt.key} error=$e');
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       }
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+    }
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-      // After local host fails, try Bonjour discovery before VPN/remote
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-      if (attempt.key == config.localHost && !_intentionalClose) {
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-        _mqttLog('MQTT: trying Bonjour discovery...');
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-        final bonjourHost = await _discoverViaMdns();
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-        if (bonjourHost != null && !_intentionalClose) {
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-          _mqttLog('MQTT: Bonjour found $bonjourHost');
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-          try {
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-            if (await _tryConnect(bonjourHost, clientId, timeout: 3000)) return;
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-          } catch (e) {
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-            _mqttLog('MQTT: Bonjour host $bonjourHost error=$e');
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-          }
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-        } else {
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-          _mqttLog('MQTT: Bonjour discovery returned nothing');
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+    // All configured hosts failed — try Bonjour/subnet scan (only once, not on retry)
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+    if (_lastDiscoveredHost == null && !_intentionalClose) {
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+      _mqttLog('MQTT: trying Bonjour/subnet discovery...');
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+      final discovered = await _discoverViaMdns();
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+      if (discovered != null && !_intentionalClose) {
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+        _lastDiscoveredHost = discovered;
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+        _mqttLog('MQTT: discovered $discovered, connecting...');
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+        try {
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+          if (await _tryConnect(discovered, clientId, timeout: 3000)) return;
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+        } catch (e) {
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+          _mqttLog('MQTT: discovered host $discovered error=$e');
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         }
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+      } else {
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+        _mqttLog('MQTT: discovery returned nothing');
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       }
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     }
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