[dotnet] Fix find port for IPv4 only environments

Author: nvborisenko

dotnet/src/webdriver/Internal/PortUtilities.cs

@@ -33,10 +33,30 @@ public static class PortUtilities
     /// <returns>A random, free port to be listened on.</returns>
     public static int FindFreePort()
     {
-        using var socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
-        socket.DualMode = true;
-        socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
-        return (socket.LocalEndPoint as IPEndPoint)!.Port;
+        // Prefer IPv6 dual-mode when available, but fall back robustly to IPv4
+        try
+        {
+            using var ipV6socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
 
+            // Some platforms may not support DualMode or IPv6 at all; ignore failures and let bind decide
+            try
+            {
+                ipV6socket.DualMode = true;
+            }
+            catch
+            {
+                // Ignored; we'll still attempt to bind to IPv6 loopback
+            }
+
+            ipV6socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
+            return ((IPEndPoint)ipV6socket.LocalEndPoint!).Port;
+        }
+        catch(SocketException)
+        {
+            // If creating/binding the IPv6 socket fails for any reason, fall back to IPv4
+            using var ipV4socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            ipV4socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+            return ((IPEndPoint)ipV4socket.LocalEndPoint!).Port;
+        }
     }
 }