Add possibility of binding clients to a specific port

Author: simon-lemay-unity

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -245,6 +245,12 @@ internal static NetworkEndpoint ParseNetworkEndpoint(string ip, ushort port)
                 return endpoint;
             }
 
+            /// <summary>
+            /// The port the client will bind to. If 0 (the default), an ephemeral port will be used.
+            /// </summary>
+            [SerializeField]
+            public ushort ClientBindPort;
+
             /// <summary>
             /// Endpoint (IP address and port) clients will connect to.
             /// </summary>
@@ -683,6 +689,20 @@ private bool ClientBindAndConnect()
             }
 
             InitDriver();
+
+            // Don't bind yet if connecting to a hostname, since we don't know if it will resolve to IPv4 or IPv6.
+            if (serverEndpoint.Family != NetworkFamily.Invalid && ConnectionData.ClientBindPort != 0)
+            {
+                var bindEndpoint = serverEndpoint.Family == NetworkFamily.Ipv6
+                    ? NetworkEndpoint.AnyIpv6.WithPort(ConnectionData.ClientBindPort)
+                    : NetworkEndpoint.AnyIpv4.WithPort(ConnectionData.ClientBindPort);
+                if (m_Driver.Bind(bindEndpoint) != 0)
+                {
+                    Debug.LogError($"Couldn't create socket. Possibly another process is using port {ConnectionData.ClientBindPort}.");
+                    return false;
+                }
+            }
+
             Connect(serverEndpoint);
 
             return true;

com.unity.netcode.gameobjects/Tests/Editor/Transports/UnityTransportTests.cs

@@ -184,6 +184,20 @@ public void UnityTransport_EmptySecurityStringsShouldThrow([Values("", null)] st
             }
         }
 
+        [Test]
+        public void UnityTransport_BindClientToSpecificPort()
+        {
+            UnityTransport transport = new GameObject().AddComponent<UnityTransport>();
+            transport.Initialize();
+            transport.SetConnectionData("127.0.0.1", 4242);
+            transport.ConnectionData.ClientBindPort = 14242;
+
+            Assert.True(transport.StartClient());
+            Assert.AreEqual(14242, transport.GetLocalEndpoint().Port);
+
+            transport.Shutdown();
+        }
+
 #if HOSTNAME_RESOLUTION_AVAILABLE
         private static readonly (string, bool)[] k_HostnameChecks =
         {