Choose ipv4 or ipv6 google dns servers as default fallback based on the settings for this system / jvm

Motivation:

We should not use ipv4 google dns servers if the app is configured to run ipv6.

Modifications:

Use either ipv4 or ipv6 dns servers depending on the system config.

Result:

More correct behaviour

Author: normanmaurer

resolver-dns/src/main/java/io/netty/resolver/dns/DefaultDnsServerAddressStreamProvider.java

@@ -15,6 +15,7 @@
  */
 package io.netty.resolver.dns;
 
+import io.netty.util.NetUtil;
 import io.netty.util.internal.SocketUtils;
 import io.netty.util.internal.UnstableApi;
 import io.netty.util.internal.logging.InternalLogger;
@@ -25,6 +26,7 @@
 import javax.naming.directory.DirContext;
 import javax.naming.directory.InitialDirContext;
 import java.lang.reflect.Method;
+import java.net.Inet6Address;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.net.URISyntaxException;
@@ -117,10 +119,21 @@ public final class DefaultDnsServerAddressStreamProvider implements DnsServerAdd
                         "Default DNS servers: {} (sun.net.dns.ResolverConfiguration)", defaultNameServers);
             }
         } else {
-            Collections.addAll(
-                    defaultNameServers,
-                    SocketUtils.socketAddress("8.8.8.8", DNS_PORT),
-                    SocketUtils.socketAddress("8.8.4.4", DNS_PORT));
+            // Depending if IPv6 or IPv4 is used choose the correct DNS servers provided by google:
+            // https://developers.google.com/speed/public-dns/docs/using
+            // https://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html
+            if (NetUtil.isIpV6AddressesPreferred() ||
+                    (NetUtil.LOCALHOST instanceof Inet6Address && !NetUtil.isIpV4StackPreferred())) {
+                Collections.addAll(
+                        defaultNameServers,
+                        SocketUtils.socketAddress("2001:4860:4860::8888", DNS_PORT),
+                        SocketUtils.socketAddress("2001:4860:4860::8844", DNS_PORT));
+            } else {
+                Collections.addAll(
+                        defaultNameServers,
+                        SocketUtils.socketAddress("8.8.8.8", DNS_PORT),
+                        SocketUtils.socketAddress("8.8.4.4", DNS_PORT));
+            }
 
             if (logger.isWarnEnabled()) {
                 logger.warn(