[dotnet] Fix find port for IPv4 only environments

dotnet/src/webdriver/Internal/PortUtilities.cs

@@ -30,13 +30,23 @@ public static class PortUtilities
     /// <summary>
     /// Finds a random, free port to be listened on.
     /// </summary>
+    /// <remarks>
+    /// Prefers IPv4, but falls back to IPv6 if necessary.
+    /// </remarks>
     /// <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;
-
+        try
+        {
+            using var ipV4socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            ipV4socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+            return ((IPEndPoint)ipV4socket.LocalEndPoint!).Port;
+        }
+        catch (SocketException)
+        {
+            using var ipV6socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
+            ipV6socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
+            return ((IPEndPoint)ipV6socket.LocalEndPoint!).Port;
+        }
     }
 }