Add validation of hostnames when starting clients

Author: simon-lemay-unity

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

@@ -655,16 +655,24 @@ private bool ClientBindAndConnect()
             else
             {
                 // This will result in an invalid endpoint if the address is a hostname.
-                // This is handled later in the Connect method if hostname resolution is available,
+                // This is handled later in the Connect method if hostname resolution is available
+                // (although we still check for hostname validity to error out early if it's not),
                 // but if not then we need to error out here.
                 serverEndpoint = ConnectionAddressData.ParseNetworkEndpoint(ConnectionData.Address, ConnectionData.Port);
-#if !HOSTNAME_RESOLUTION_AVAILABLE
+
                 if (serverEndpoint.Family == NetworkFamily.Invalid)
                 {
+#if HOSTNAME_RESOLUTION_AVAILABLE
+                    if (Uri.CheckHostName(ConnectionData.Address) != UriHostNameType.Dns)
+                    {
+                        Debug.LogError($"Provided connection address \"{ConnectionData.Address}\" is not a valid hostname.");
+                        return false;
+                    }
+#else
                     Debug.LogError($"Invalid server address: {ConnectionData.Address}:{ConnectionData.Port}.");
                     return false;
-                }
 #endif
+                }
             }
 
             InitDriver();

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

@@ -183,5 +183,37 @@ public void UnityTransport_EmptySecurityStringsShouldThrow([Values("", null)] st
                 }
             }
         }
+
+#if HOSTNAME_RESOLUTION_AVAILABLE
+        private static readonly (string, bool)[] HostnameChecks =
+        {
+            ("localhost", true),
+            ("unity3d.com", true),
+            ("unity3d.com.", true),
+            (string.Empty, false),
+            ("unity3d.com/test", false),
+            ("test%123.com", false),
+        };
+
+        [Test]
+        [TestCaseSource(nameof(HostnameChecks))]
+        public void UnityTransport_HostnameValidation((string, bool) testCases)
+        {
+            var (hostname, isValid) = testCases;
+
+            UnityTransport transport = new GameObject().AddComponent<UnityTransport>();
+            transport.Initialize();
+
+            if (!isValid)
+            {
+                LogAssert.Expect(LogType.Error, $"Provided connection address \"{hostname}\" is not a valid hostname.");
+            }
+
+            transport.SetConnectionData(hostname, 4242);
+            Assert.AreEqual(isValid, transport.StartClient());
+
+            transport.Shutdown();
+        }
+#endif
     }
 }