host address retrieving through assigned subnet information and broadcast message to learn host address

app/android/app/src/main/kotlin/com/dmc/dmc_app/MlrsNetworkPlugin.kt

@@ -19,6 +19,7 @@ import io.flutter.plugin.common.MethodChannel
 import java.net.BindException
 import java.net.DatagramPacket
 import java.net.DatagramSocket
+import java.net.Inet4Address
 import java.net.InetAddress
 import java.net.InetSocketAddress
 import java.util.concurrent.Executors
@@ -167,11 +168,18 @@ class MlrsNetworkPlugin(
         val callback = object : ConnectivityManager.NetworkCallback() {
             override fun onAvailable(network: Network) {
                 currentNetwork = network
-                val gateway = gatewayAddressOf(connectivityManager.getLinkProperties(network))
+                val linkProperties = connectivityManager.getLinkProperties(network)
+                val gateway = gatewayAddressOf(linkProperties)
+                val broadcast = broadcastAddressOf(linkProperties)
                 val ssid = ssidOf(connectivityManager.getNetworkCapabilities(network))
                 if (ssid != null) reportedSsid = ssid
                 eventSink?.success(
-                    mapOf("type" to "available", "gatewayAddress" to gateway, "ssid" to ssid),
+                    mapOf(
+                        "type" to "available",
+                        "gatewayAddress" to gateway,
+                        "broadcastAddress" to broadcast,
+                        "ssid" to ssid,
+                    ),
                 )
             }
 
@@ -225,6 +233,32 @@ class MlrsNetworkPlugin(
         return route?.gateway?.hostAddress
     }
 
+    /**
+     * Berechnet die Subnetz-Broadcast-Adresse (z.B. 192.168.4.255 fuer
+     * 192.168.4.42/24) aus der dem Geraet auf diesem Netz zugewiesenen
+     * IPv4-Adresse + Praefixlaenge. Dient als Bootstrap-Ziel fuer die erste
+     * MSP-Anfrage, wenn die tatsaechliche IP der mLRS-Bruecke (noch) nicht
+     * bekannt ist (Doku Kommunikationsschicht v2, Abschnitt 4/9:
+     * "Peer-Learning" versagt, wenn die Bruecke kein initiales Paket
+     * schickt und die geratene Standard-IP falsch liegt) - eine an die
+     * Broadcast-Adresse gesendete Anfrage erreicht die Bruecke unabhaengig
+     * von ihrer konkreten IP, ihre (unicast) Antwort verrät dann die echte
+     * Adresse.
+     */
+    private fun broadcastAddressOf(linkProperties: LinkProperties?): String? {
+        val linkAddress = linkProperties?.linkAddresses
+            ?.firstOrNull { it.address is Inet4Address } ?: return null
+        val ipBytes = linkAddress.address.address
+        val prefixLength = linkAddress.prefixLength
+        val broadcastBytes = ByteArray(4)
+        for (i in 0 until 4) {
+            val hostBitsInByte = (prefixLength - i * 8).coerceIn(0, 8)
+            val hostMaskByte = 0xFF shr hostBitsInByte
+            broadcastBytes[i] = (ipBytes[i].toInt() or hostMaskByte).toByte()
+        }
+        return InetAddress.getByAddress(broadcastBytes).hostAddress
+    }
+
     /**
      * Ermittelt die tatsaechliche SSID des verbundenen Netzes. Erfordert
      * normalerweise ACCESS_FINE_LOCATION, ausser fuer die anfragende App
@@ -260,6 +294,10 @@ class MlrsNetworkPlugin(
                 // hier erkennbar mit BindException scheitert (Doku 3/9: "ist
                 // 14550 belegt, ausweichen, aber protokollieren").
                 newSocket.reuseAddress = false
+                // Ohne dies wirft send() an eine Broadcast-Adresse
+                // (broadcastAddressOf()) eine SocketException - noetig fuer
+                // den Bootstrap-Fallback in UdpTransport.connect().
+                newSocket.broadcast = true
                 var fallbackFromPort: Int? = null
                 try {
                     newSocket.bind(InetSocketAddress(requestedPort))