fix: Accept single words as valid hostnames

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,14 +14,15 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed an issue where `UnityTransport` would not accept single words as valid hostnames (notably "localhost"). (#3591)
 - Fixed issue where viewing a `NetworkBehaviour` with one or more `NetworkVariable` fields could throw an exception if running a distributed authority network topology with a local (DAHost) host and viewed on the host when the host is not the authority of the associated `NetworkObject`. (#3578)
 - Fixed issue when using a distributed authority network topology and  viewing a `NetworkBehaviour` with one or more `NetworkVariable` fields in the inspector view would not show editable fields. (#3578)
 
 ### Changed
 
+- Marked `UnityTransport.ConnectionAddressData.ServerEndPoint` as obsolete. It can't work when using hostnames as the server address, and its functionality can easily be replicated using `NetworkEndpoint.Parse`. (#3591)
 - Optimized `NetworkList<T>` indexer setter to skip operations when the new value equals the existing value, improving performance by avoiding unnecessary list events and network synchronization. (#3587)
 
-
 ## [2.5.0] - 2025-08-01
 
 ### Added

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

@@ -7,7 +7,7 @@
 
 using System;
 using System.Collections.Generic;
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
+#if HOSTNAME_RESOLUTION_AVAILABLE
 using System.Text.RegularExpressions;
 #endif
 using Unity.Burst;
@@ -235,7 +235,7 @@ public struct ConnectionAddressData
             [SerializeField]
             public string ServerListenAddress;
 
-            private static NetworkEndpoint ParseNetworkEndpoint(string ip, ushort port)
+            internal static NetworkEndpoint ParseNetworkEndpoint(string ip, ushort port)
             {
                 NetworkEndpoint endpoint = default;
                 if (!NetworkEndpoint.TryParse(ip, port, out endpoint, NetworkFamily.Ipv4))
@@ -245,31 +245,16 @@ private static NetworkEndpoint ParseNetworkEndpoint(string ip, ushort port)
                 return endpoint;
             }
 
-            private void InvalidEndpointError()
-            {
-                Debug.LogError($"Invalid network endpoint: {Address}:{Port}.");
-            }
-
             /// <summary>
             /// Endpoint (IP address and port) clients will connect to.
             /// </summary>
-            public NetworkEndpoint ServerEndPoint
-            {
-                get
-                {
-                    var networkEndpoint = ParseNetworkEndpoint(Address, Port);
-                    if (networkEndpoint == default)
-                    {
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-                        if (!IsValidFqdn(Address))
-#endif
-                        {
-                            InvalidEndpointError();
-                        }
-                    }
-                    return networkEndpoint;
-                }
-            }
+            /// <remarks>
+            /// If a DNS hostname was set as the address, this will return an invalid endpoint. This
+            /// is still handled correctly by NGO, but for this reason usage of this property is
+            /// discouraged.
+            /// </remarks>
+            [Obsolete("Use NetworkEndpoint.Parse on the Address field instead.")]
+            public NetworkEndpoint ServerEndPoint => ParseNetworkEndpoint(Address, Port);
 
             /// <summary>
             /// Endpoint (IP address and port) server will listen/bind on.
@@ -281,32 +266,27 @@ public NetworkEndpoint ListenEndPoint
                     NetworkEndpoint endpoint = default;
                     if (string.IsNullOrEmpty(ServerListenAddress))
                     {
-                        endpoint = NetworkEndpoint.LoopbackIpv4;
-
-                        // If an address was entered and it's IPv6, switch to using ::1 as the
-                        // default listen address. (Otherwise we always assume IPv4.)
-                        if (!string.IsNullOrEmpty(Address) && ServerEndPoint.Family == NetworkFamily.Ipv6)
-                        {
-                            endpoint = NetworkEndpoint.LoopbackIpv6;
-                        }
+                        endpoint = IsIpv6 ? NetworkEndpoint.LoopbackIpv6 : NetworkEndpoint.LoopbackIpv4;
                         endpoint = endpoint.WithPort(Port);
                     }
                     else
                     {
                         endpoint = ParseNetworkEndpoint(ServerListenAddress, Port);
                         if (endpoint == default)
                         {
-                            InvalidEndpointError();
+                            Debug.LogError($"Invalid listen endpoint: {ServerListenAddress}:{Port}. Note that the listen endpoint MUST be an IP address (not a hostname).");
                         }
                     }
                     return endpoint;
                 }
             }
 
             /// <summary>
-            /// Returns true if the end point address is of type <see cref="NetworkFamily.Ipv6"/>.
+            /// Returns true if the end point address is of type <see cref="NetworkFamily.Ipv6"/> or
+            /// if it is a hostname (because in current versions of the engine, hostname resolution
+            /// prioritizes IPv6 addresses).
             /// </summary>
-            public bool IsIpv6 => !string.IsNullOrEmpty(Address) && NetworkEndpoint.TryParse(Address, Port, out NetworkEndpoint _, NetworkFamily.Ipv6);
+            public bool IsIpv6 => !string.IsNullOrEmpty(Address) && !NetworkEndpoint.TryParse(Address, Port, out NetworkEndpoint _, NetworkFamily.Ipv4);
         }
 
 
@@ -667,16 +647,6 @@ private NetworkPipeline SelectSendPipeline(NetworkDelivery delivery)
             }
         }
 
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-        private static bool IsValidFqdn(string fqdn)
-        {
-            // Regular expression to validate FQDN
-            string pattern = @"^(?=.{1,255}$)(?!-)[A-Za-z0-9-]{1,63}(?<!-)\.(?!-)(?:[A-Za-z0-9-]{1,63}\.?)+[A-Za-z]{2,6}$";
-            var regex = new Regex(pattern);
-            return regex.IsMatch(fqdn);
-        }
-#endif
-
         private bool ClientBindAndConnect()
         {
             var serverEndpoint = default(NetworkEndpoint);
@@ -687,44 +657,20 @@ private bool ClientBindAndConnect()
             }
             else
             {
-                serverEndpoint = ConnectionData.ServerEndPoint;
-            }
-
-            // Verify the endpoint is valid before proceeding
-            if (serverEndpoint.Family == NetworkFamily.Invalid)
-            {
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-
-                // If it's not valid, assure it meets FQDN standards
-                if (IsValidFqdn(ConnectionData.Address))
-                {
-                    // If so, then proceed with driver initialization and attempt to connect
-                    InitDriver();
-                    m_Driver.Connect(ConnectionData.Address, ConnectionData.Port);
-                    return true;
-                }
-                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,
+                // 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 not then log an error and return false
-                    Debug.LogError($"Target server network address ({ConnectionData.Address}) is not a valid Fully Qualified Domain Name!");
+                    Debug.LogError($"Invalid server address: {ConnectionData.Address}:{ConnectionData.Port}.");
                     return false;
                 }
-#else
-                Debug.LogError($"Target server network address ({ConnectionData.Address}) is {nameof(NetworkFamily.Invalid)}!");
-                return false;
 #endif
             }
 
             InitDriver();
-
-            var bindEndpoint = serverEndpoint.Family == NetworkFamily.Ipv6 ? NetworkEndpoint.AnyIpv6 : NetworkEndpoint.AnyIpv4;
-            int result = m_Driver.Bind(bindEndpoint);
-            if (result != 0)
-            {
-                Debug.LogError("Client failed to bind");
-                return false;
-            }
-
             Connect(serverEndpoint);
 
             return true;
@@ -737,30 +683,22 @@ private bool ClientBindAndConnect()
         /// <returns>A <see cref="NetworkConnection"/> representing the connection to the server, or an invalid connection if the connection attempt fails.</returns>
         protected virtual NetworkConnection Connect(NetworkEndpoint serverEndpoint)
         {
+#if HOSTNAME_RESOLUTION_AVAILABLE
+            // If the server endpoint is invalid, it means whatever the user entered in the address
+            // field was not an IP address, and must be presumed to be a hostname.
+            if (serverEndpoint.Family == NetworkFamily.Invalid)
+            {
+                return m_Driver.Connect(ConnectionData.Address, ConnectionData.Port);
+            }
+#endif
             return m_Driver.Connect(serverEndpoint);
         }
 
         private bool ServerBindAndListen(NetworkEndpoint endPoint)
         {
-            // Verify the endpoint is valid before proceeding
             if (endPoint.Family == NetworkFamily.Invalid)
             {
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-                // If it's not valid, assure it meets FQDN standards
-                if (!IsValidFqdn(ConnectionData.Address))
-                {
-                    // If not then log an error and return false
-                    Debug.LogError($"Listen network address ({ConnectionData.Address}) is not a valid {NetworkFamily.Ipv4} or {NetworkFamily.Ipv6} address!");
-                }
-                else
-                {
-                    Debug.LogError($"While ({ConnectionData.Address}) is a valid Fully Qualified Domain Name, you must use a valid {NetworkFamily.Ipv4} or {NetworkFamily.Ipv6} address when binding and listening for connections!");
-                }
                 return false;
-#else
-                Debug.LogError($"Network listen address ({ConnectionData.Address}) is {nameof(NetworkFamily.Invalid)}!");
-                return false;
-#endif
             }
 
             InitDriver();

com.unity.netcode.gameobjects/Runtime/Unity.Netcode.Runtime.asmdef

@@ -68,11 +68,6 @@
             "expression": "6000.0.11f1",
             "define": "COM_UNITY_MODULES_PHYSICS2D_LINEAR"
         },
-        {
-            "name": "com.unity.transport",
-            "expression": "2.4.0",
-            "define": "UTP_TRANSPORT_2_4_ABOVE"
-        },
         {
             "name": "Unity",
             "expression": "6000.1.0a1",

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

@@ -127,16 +127,11 @@ public void UnityTransport_RestartSucceedsAfterFailure()
             UnityTransport transport = new GameObject().AddComponent<UnityTransport>();
             transport.Initialize();
 
-            transport.SetConnectionData("127.0.0.", 4242, "127.0.0.");
+            transport.SetConnectionData("127.0.0.1", 4242, "foobar");
 
             Assert.False(transport.StartServer());
-            LogAssert.Expect(LogType.Error, "Invalid network endpoint: 127.0.0.:4242.");
+            LogAssert.Expect(LogType.Error, "Invalid listen endpoint: foobar:4242. Note that the listen endpoint MUST be an IP address (not a hostname).");
 
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-            LogAssert.Expect(LogType.Error, "Listen network address (127.0.0.) is not a valid Ipv4 or Ipv6 address!");
-#else
-            LogAssert.Expect(LogType.Error, "Network listen address (127.0.0.) is Invalid!");
-#endif
             transport.SetConnectionData("127.0.0.1", 4242, "127.0.0.1");
             Assert.True(transport.StartServer());
 
@@ -156,24 +151,6 @@ public void UnityTransport_StartServerWithoutAddresses()
             transport.Shutdown();
         }
 
-        // Check that StartClient returns false with bad connection data.
-        [Test]
-        public void UnityTransport_StartClientFailsWithBadAddress()
-        {
-            UnityTransport transport = new GameObject().AddComponent<UnityTransport>();
-            transport.Initialize();
-
-            transport.SetConnectionData("foobar", 4242);
-            Assert.False(transport.StartClient());
-            LogAssert.Expect(LogType.Error, "Invalid network endpoint: foobar:4242.");
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-            LogAssert.Expect(LogType.Error, "Target server network address (foobar) is not a valid Fully Qualified Domain Name!");
-#else
-            LogAssert.Expect(LogType.Error, "Target server network address (foobar) is Invalid!");
-#endif
-            transport.Shutdown();
-        }
-
         [Test]
         public void UnityTransport_EmptySecurityStringsShouldThrow([Values("", null)] string cert, [Values("", null)] string secret)
         {

com.unity.netcode.gameobjects/Tests/Editor/Unity.Netcode.Editor.Tests.asmdef

@@ -38,11 +38,6 @@
             "expression": "(0,2022.2.0a5)",
             "define": "UNITY_UNET_PRESENT"
         },
-        {
-            "name": "com.unity.transport",
-            "expression": "2.4.0",
-            "define": "UTP_TRANSPORT_2_4_ABOVE"
-        },
         {
             "name": "Unity",
             "expression": "6000.1.0a1",

com.unity.netcode.gameobjects/Tests/Runtime/Transports/UnityTransportConnectionTests.cs

@@ -5,6 +5,7 @@
 using NUnit.Framework;
 using Unity.Netcode.TestHelpers.Runtime;
 using Unity.Netcode.Transports.UTP;
+using Unity.Networking.Transport;
 using UnityEngine;
 using UnityEngine.TestTools;
 using static Unity.Netcode.RuntimeTests.UnityTransportTestHelpers;
@@ -55,45 +56,52 @@ public IEnumerator Cleanup()
             yield return null;
         }
 
-        // Check that invalid endpoint addresses are detected and return false if detected
-        [Test]
-        public void DetectInvalidEndpoint()
+        // Check connection with a single client (IP address).
+        [UnityTest]
+        public IEnumerator ConnectSingleClient_IPAddress()
         {
-            using var netcodeLogAssert = new NetcodeLogAssert(true);
             InitializeTransport(out m_Server, out m_ServerEvents);
             InitializeTransport(out m_Clients[0], out m_ClientsEvents[0]);
-            m_Server.ConnectionData.Address = "Fubar";
-            m_Server.ConnectionData.ServerListenAddress = "Fubar";
-            m_Clients[0].ConnectionData.Address = "MoreFubar";
-            Assert.False(m_Server.StartServer(), "Server failed to detect invalid endpoint!");
-            Assert.False(m_Clients[0].StartClient(), "Client failed to detect invalid endpoint!");
-#if HOSTNAME_RESOLUTION_AVAILABLE && UTP_TRANSPORT_2_4_ABOVE
-            LogAssert.Expect(LogType.Error, $"Listen network address ({m_Server.ConnectionData.Address}) is not a valid {Networking.Transport.NetworkFamily.Ipv4} or {Networking.Transport.NetworkFamily.Ipv6} address!");
-            LogAssert.Expect(LogType.Error, $"Target server network address ({m_Clients[0].ConnectionData.Address}) is not a valid Fully Qualified Domain Name!");
-
-            m_Server.ConnectionData.Address = "my.fubar.com";
-            m_Server.ConnectionData.ServerListenAddress = "my.fubar.com";
-            Assert.False(m_Server.StartServer(), "Server failed to detect invalid endpoint!");
-            LogAssert.Expect(LogType.Error, $"While ({m_Server.ConnectionData.Address}) is a valid Fully Qualified Domain Name, you must use a " +
-                $"valid {Networking.Transport.NetworkFamily.Ipv4} or {Networking.Transport.NetworkFamily.Ipv6} address when binding and listening for connections!");
-#else
-            netcodeLogAssert.LogWasReceived(LogType.Error, $"Network listen address ({m_Server.ConnectionData.Address}) is Invalid!");
-            netcodeLogAssert.LogWasReceived(LogType.Error, $"Target server network address ({m_Clients[0].ConnectionData.Address}) is Invalid!");
-#endif
 
-            UnityTransportTestComponent.CleanUp();
+            m_Clients[0].SetConnectionData("127.0.0.1", 7777);
+
+            m_Server.StartServer();
+            m_Clients[0].StartClient();
+
+            yield return WaitForNetworkEvent(NetworkEvent.Connect, m_ClientsEvents[0]);
+
+            // Check we've received Connect event on server too.
+            Assert.AreEqual(1, m_ServerEvents.Count);
+            Assert.AreEqual(NetworkEvent.Connect, m_ServerEvents[0].Type);
+
+            yield return null;
         }
 
-        // Check connection with a single client.
+#if HOSTNAME_RESOLUTION_AVAILABLE
+        // Check connection with a single client (hostname).
         [UnityTest]
-        public IEnumerator ConnectSingleClient()
+        public IEnumerator ConnectSingleClient_Hostname()
         {
             InitializeTransport(out m_Server, out m_ServerEvents);
             InitializeTransport(out m_Clients[0], out m_ClientsEvents[0]);
 
-            m_Server.StartServer();
+            // We don't know if localhost will resolve to 127.0.0.1 or ::1, so we wait until we know
+            // before starting the server. Because localhost is pretty much always defined locally
+            // it should resolve immediatly and thus waiting one frame should be enough.
+
+            // We'll need to retry connection requests most likely so make this fast.
+            m_Clients[0].ConnectTimeoutMS = 50;
+
+            m_Clients[0].SetConnectionData("localhost", 7777);
             m_Clients[0].StartClient();
 
+            yield return null;
+
+            var endpoint = m_Clients[0].GetLocalEndpoint();
+            var ip = endpoint.Family == NetworkFamily.Ipv4 ? "127.0.0.1" : "::1";
+            m_Server.SetConnectionData(ip, 7777, ip);
+            m_Server.StartServer();
+
             yield return WaitForNetworkEvent(NetworkEvent.Connect, m_ClientsEvents[0]);
 
             // Check we've received Connect event on server too.
@@ -102,6 +110,7 @@ public IEnumerator ConnectSingleClient()
 
             yield return null;
         }
+#endif
 
         // Check connection with multiple clients.
         [UnityTest]

com.unity.netcode.gameobjects/Tests/Runtime/Unity.Netcode.Runtime.Tests.asmdef

@@ -48,11 +48,6 @@
             "expression": "",
             "define": "COM_UNITY_MODULES_PHYSICS"
         },
-        {
-            "name": "com.unity.transport",
-            "expression": "2.4.0",
-            "define": "UTP_TRANSPORT_2_4_ABOVE"
-        },
         {
             "name": "Unity",
             "expression": "6000.1.0a1",