Address review feedback for RFC 6761 handling

- Fix IPv6 address order: use [IPv6Loopback, Loopback] to match Windows resolver behavior
- Add validation to reject malformed hostnames (starting with '.' or containing '..') in IsReservedName, letting them fall through to OS resolver
- Add tests for malformed hostname edge cases (.localhost, foo..localhost, .invalid, test..invalid)

Author: laveeshb

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

@@ -417,16 +417,24 @@ private static bool ValidateAddressFamily(ref AddressFamily addressFamily, strin
         }
 
         // Pre-allocated arrays for RFC 6761 localhost handling to avoid allocations on hot path.
+        // IPv6 is listed first to match Windows resolver behavior.
         private static readonly IPAddress[] s_localhostIPv4 = [IPAddress.Loopback];
         private static readonly IPAddress[] s_localhostIPv6 = [IPAddress.IPv6Loopback];
-        private static readonly IPAddress[] s_localhostBoth = [IPAddress.Loopback, IPAddress.IPv6Loopback];
+        private static readonly IPAddress[] s_localhostBoth = [IPAddress.IPv6Loopback, IPAddress.Loopback];
 
         /// <summary>
         /// Checks if the given host name matches a reserved name or is a subdomain of it.
         /// For example, IsReservedName("foo.localhost", "localhost") returns true.
+        /// Returns false for malformed hostnames (starting with dot or containing consecutive dots).
         /// </summary>
         private static bool IsReservedName(string hostName, string reservedName)
         {
+            // Reject malformed hostnames - let OS resolver handle them (and reject them)
+            if (hostName.StartsWith('.') || hostName.Contains(".."))
+            {
+                return false;
+            }
+
             // Matches "reservedName" exactly, or "*.reservedName" (subdomain)
             return hostName.EndsWith(reservedName, StringComparison.OrdinalIgnoreCase) &&
                    (hostName.Length == reservedName.Length ||

src/libraries/System.Net.NameResolution/tests/FunctionalTests/GetHostEntryTest.cs

@@ -374,6 +374,21 @@ public async Task DnsGetHostEntry_SimilarButNotReserved_ThrowsSocketException(st
             await Assert.ThrowsAnyAsync<SocketException>(() => Dns.GetHostEntryAsync(hostName));
         }
 
+        // Malformed hostnames should not be treated as RFC 6761 reserved names.
+        // They should fall through to the OS resolver which will reject them.
+        [Theory]
+        [InlineData(".localhost")]
+        [InlineData("foo..localhost")]
+        [InlineData(".invalid")]
+        [InlineData("test..invalid")]
+        public async Task DnsGetHostEntry_MalformedReservedName_NotTreatedAsReserved(string hostName)
+        {
+            // Malformed hostnames should go to OS resolver, not be special-cased.
+            // OS resolver will typically reject them with ArgumentException or SocketException.
+            Assert.ThrowsAny<Exception>(() => Dns.GetHostEntry(hostName));
+            await Assert.ThrowsAnyAsync<Exception>(() => Dns.GetHostEntryAsync(hostName));
+        }
+
         // RFC 6761: "*.localhost" subdomains should respect AddressFamily parameter.
         [Theory]
         [InlineData(AddressFamily.InterNetwork)]