DNS lookup of IPv6-only hosts fails

[TekkiWuff]

Steps to Reproduce

1. Ensure you have propper IPv6 connectivity e. g. by opening ipv6.google.com in a browser
2. Write a simple oneliner C# program to do a DNS lookup

var addresse = Dns.GetHostAddresses("ipv6.google.com");

3. An exception is thrown

Current Behavior

The DNS lookup does not return any IPv6 addresses and fails with an exception for IPv6-only hosts.

Expected Behavior

The DNS lookup returns all IPv6 addresses associated with the hostname.

On which platforms did you notice this

[ ] macOS
[x] Linux
[ ] Windows

Distributor ID: Ubuntu
Description:    Ubuntu 17.10
Release:        17.10
Codename:       artful

Version Used:

Mono JIT compiler version 5.8.0.108 (tarball Fri Jan 19 18:15:21 UTC 2018)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       altstack
        Notifications: epoll
        Architecture:  amd64
        Disabled:      none
        Misc:          softdebug
        LLVM:          supported, not enabled.
        GC:            sgen (concurrent by default)

Installed from repository as described here http://www.mono-project.com/download/#download-lin

Stacktrace

Unhandled Exception:
System.Net.Sockets.SocketException (0x80004005): Could not resolve host 'ipv6.google.com'
  at System.Net.Dns.Error_11001 (System.String hostName) [0x00015] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.hostent_to_IPHostEntry (System.String originalHostName, System.String h_name, System.String[] h_aliases, System.String[] h_addrlist) [0x00068] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostByName (System.String hostName) [0x00027] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostEntry (System.String hostNameOrAddress) [0x00052] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostAddresses (System.String hostNameOrAddress) [0x00056] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at IPV6Test.MainClass.Main (System.String[] args) [0x00053] in <7972d22e6aeb495781af83f5a15dede8>:0
[ERROR] FATAL UNHANDLED EXCEPTION: System.Net.Sockets.SocketException (0x80004005): Could not resolve host 'ipv6.google.com'
  at System.Net.Dns.Error_11001 (System.String hostName) [0x00015] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.hostent_to_IPHostEntry (System.String originalHostName, System.String h_name, System.String[] h_aliases, System.String[] h_addrlist) [0x00068] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostByName (System.String hostName) [0x00027] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostEntry (System.String hostNameOrAddress) [0x00052] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at System.Net.Dns.GetHostAddresses (System.String hostNameOrAddress) [0x00056] in <a63b52a3a7644f60a95b388cb72441c2>:0
  at IPV6Test.MainClass.Main (System.String[] args) [0x00053] in <7972d22e6aeb495781af83f5a15dede8>:0

Further details

The (obsolote) Socket.SupportsIPv6 gets always initialised with false here for the PAL (FEATURE_PAL)

mono/mcs/class/referencesource/System/net/System/Net/Sockets/Socket.cs

Lines 6072 to 6073 in 0bcbe39

	s_SupportsIPv4 = true;
	s_SupportsIPv6 = false;

As visible in the Stacktrace, System.Net.Dns.hostent_to_IPHostEntry is called at some point during the execution. It iterates over all addresses and removes IPv6 addresses based on the value of Socket.SupportsIPv6.

mono/mcs/class/System/System.Net/Dns.cs

Lines 323 to 327 in 4fa0463

	#pragma warning disable 618
	if( (Socket.SupportsIPv6 && newAddress.AddressFamily == AddressFamily.InterNetworkV6) ||
	(Socket.SupportsIPv4 && newAddress.AddressFamily == AddressFamily.InterNetwork) )
	addrlist.Add(newAddress);
	#pragma warning restore 618

For IPv6-only hosts this results in an empty list which then throws an exception

mono/mcs/class/System/System.Net/Dns.cs

Lines 337 to 338 in 4fa0463

	if(addrlist.Count == 0)
	Error_11001 (originalHostName);

Interestingly the not obsolete Socket.OSSupportsIPv6 return true

Setting Socket.SupportsIPv6 to true via reflection fixes the issue. Tested DNS lookups and WebRequests (see example code below)

I'm not sure about the correct solution though, since I have no deeper knowledge of internals of Mono. Maybe it's enough to use Socket.OSSupportsIPv6 instead of Socket.SupportsIPv6 in System.Net.Dns.hostent_to_IPHostEntry?

Complete test code

using System;
using System.Net;
using System.Net.Sockets;
using System.Reflection;
using System.IO;

namespace IPV6Test
{
        class MainClass
        {
                public static void Main(string[] args)
                {
                        // Check IPv6 support
                        Console.WriteLine("Socket.OSSupportsIPv6: " + Socket.OSSupportsIPv6);
                        Console.WriteLine("Socket.SupportsIPv6: " + Socket.SupportsIPv6);

                        // Use reflection to change the IPv6 support
                        // remove coments to temporarily fix the issues
//                      var field = typeof(System.Net.Sockets.Socket).GetField("s_SupportsIPv6", BindingFlags.Static|BindingFlags.NonPublic);
//                      field.SetValue(null, true);
//                      Console.WriteLine("Socket.SupportsIPv6: " + Socket.SupportsIPv6);

                        // Test DNS lookup
                        var addresses = Dns.GetHostAddresses("ipv6.google.com");
                        foreach(var address in addresses)
                        {
                                Console.WriteLine(address.ToString());
                        }

                        // Test WebRequest to IPv6-only host
                        var webRequest = WebRequest.Create(@"http://ipv6.google.com");
                        using(var response = webRequest.GetResponse())
                        using(var content = response.GetResponseStream())
                        using(var reader = new StreamReader(content))
                        {
                                Console.Write(reader.ReadToEnd());
                        }
                }
        }
}

[dominiqueroux]

👍

[marek-safar]

@baulig this looks quite serious/ugly I think we should try to get it into 2017-12

[marek-safar]

@baulig any update on this?

[alexischr]

Thank you for figuring out the issue @TekkiWuff !