Draft: RFC 6761 handling for invalid and *.localhost

src/libraries/System.Net.NameResolution/src/System/Net/Dns.cs

@@ -432,6 +432,18 @@ private static object GetHostEntryOrAddressesCore(string hostName, bool justAddr
             object result;
             try
             {
+                if (MatchesReservedName(hostName, "invalid"))
+                {
+                    LogSpecialUse(hostName);
+                    throw new SocketException((int)SocketError.HostNotFound);
+                }
+
+                if (MatchesReservedName(hostName, "localhost"))
+                {
+                    LogSpecialUse(hostName);
+                    return GetLoopbacksForAddressFamily(addressFamily);
+                }
+
                 SocketError errorCode = NameResolutionPal.TryGetAddrInfo(hostName, justAddresses, addressFamily, out string? newHostName, out string[] aliases, out IPAddress[] addresses, out int nativeErrorCode);
 
                 if (errorCode != SocketError.Success)
@@ -702,6 +714,33 @@ private static void ValidateHostName(string hostName)
             }
         }
 
+        private static IPAddress[] GetLoopbacksForAddressFamily(AddressFamily addressFamily)
+        {
+            bool ipv4Enabled = SocketProtocolSupportPal.OSSupportsIPv4;
+            bool ipv6Enabled = SocketProtocolSupportPal.OSSupportsIPv6;
+
+            return addressFamily switch
+            {
+                AddressFamily.InterNetwork when ipv4Enabled => new[] { IPAddress.Loopback },
+                AddressFamily.InterNetworkV6 when ipv6Enabled => new[] { IPAddress.IPv6Loopback },
+                AddressFamily.Unspecified when ipv4Enabled && ipv6Enabled => new[] { IPAddress.Loopback, IPAddress.IPv6Loopback },
+                AddressFamily.Unspecified when ipv4Enabled => new[] { IPAddress.Loopback },
+                AddressFamily.Unspecified when ipv6Enabled => new[] { IPAddress.IPv6Loopback },
+                _ => Array.Empty<IPAddress>(),
+            };
+        }
+
+        private static bool MatchesReservedName(string name, string reservedName)
+        {
+            return name.Equals(reservedName, StringComparison.OrdinalIgnoreCase) ||
+                name.EndsWith("." + reservedName, StringComparison.OrdinalIgnoreCase);
+        }
+
+        private static void LogSpecialUse(string hostName)
+        {
+            if (NetEventSource.Log.IsEnabled()) NetEventSource.Info(hostName, $"Special-use domain intercepted before calling OS resolver: {hostName}");
+        }
+
         private static bool LogFailure(object hostNameOrAddress, in NameResolutionActivity activity, Exception exception)
         {
             NameResolutionTelemetry.Log.AfterResolution(hostNameOrAddress, activity, answer: null, exception: exception);