feat: Improve error message on connection failure

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

@@ -1146,7 +1146,7 @@
                         // means a clean disconnect that doesn't require logging.
                         if (!m_Driver.Listening && m_ServerClientId == default)
                         {
-                            Debug.LogError("Failed to connect to server.");
+                            LogClientConnectionFailureMessage();
                         }
 
                         // Set the disconnect event error code
@@ -1174,6 +1174,40 @@
             return false;
         }
 
+        private void LogClientConnectionFailureMessage()
+        {
+            if (OnCurrentTopology() == NetworkTopologyTypes.ClientServer)
+            {
+                var reasons = new List<string> { "Server/host is not running." };
+
+                if (m_ProtocolType == ProtocolType.RelayUnityTransport && !m_UseWebSockets)
+                {
+                    reasons.Add("Relay port might be blocked on your network. Try using WebSockets.");
+                    if (m_RelayServerData.IsSecure != 0)
+                    {
+                        reasons.Add("DTLS might be blocked on your network. Try using UDP or WebSockets.");
+                    }
+                }
+                else if (m_ProtocolType != ProtocolType.RelayUnityTransport)
+                {
+                    reasons.Add($"Server/host might not be reachable. Try \"ping {ConnectionData.Address}\" in a terminal.");
+                    reasons.Add("The firewall on the server/host might be blocking the connection attempts.");
+                    if (m_UseEncryption && !m_UseWebSockets)
+                    {
+                        reasons.Add("DTLS might be blocked on your network. Try using WebSockets.");
+                    }
+                }
+
+                const string prefix = "\n\t- ";
+                Debug.LogError("Failed to connect to server. Possible reasons:" + prefix + string.Join(prefix, reasons));
+            }
+            else
+            {
+                // If using DA the SDK should have logged an actual useful error.
+                Debug.LogError("Failed to connect to server.");
+            }
+        }
+
         /// <summary>
         /// Handles accepting new connections and processing transport events.
         /// </summary>

com.unity.netcode.gameobjects/Tests/Runtime/Connection/ClientOnlyConnectionTests.cs

@@ -1,4 +1,5 @@
 using System.Collections;
+using System.Text.RegularExpressions;
 using NUnit.Framework;
 using Unity.Netcode.TestHelpers.Runtime;
 using Unity.Netcode.Transports.UTP;
@@ -50,7 +51,7 @@ public IEnumerator ClientFailsToConnect()
             m_ClientNetworkManager.StartClient();
 
             // Unity Transport throws an error when it times out
-            LogAssert.Expect(LogType.Error, "Failed to connect to server.");
+            LogAssert.Expect(LogType.Error, new Regex("^Failed to connect to server\\."));
             yield return NetcodeIntegrationTest.WaitForConditionOrTimeOut(() => m_WasDisconnected, m_TimeoutHelper);
             Assert.False(m_TimeoutHelper.TimedOut, "Timed out waiting for client to timeout waiting to connect!");