NC-1883 - IPv6 peers (#281)

* Add the ipV6 flag to the datagram socket options, this creates a socket that handles both
ipv4 and ipv6 socket connections.
* Set the IPv6 flag state based on the presence or absence of an IPv6 address.

i.e. if your machine runs IPv6 it will accept IPv6 bootnodes and peers.  However link-local IPv6 is not fully supported (see issue #288).

ethereum/p2p/src/main/java/tech/pegasys/pantheon/ethereum/p2p/discovery/PeerDiscoveryAgent.java

@@ -150,7 +150,8 @@ public class PeerDiscoveryAgent implements DisconnectCallback {
       LOG.info("Starting peer discovery agent on host={}, port={}", host, port);
 
       vertx
-          .createDatagramSocket(new DatagramSocketOptions())
+          .createDatagramSocket(
+              new DatagramSocketOptions().setIpV6(NetworkUtility.isIPv6Available()))
           .listen(
               port,
               host,

ethereum/p2p/src/test/java/tech/pegasys/pantheon/ethereum/p2p/NetworkingServiceLifecycleTest.java

@@ -25,6 +25,7 @@ import tech.pegasys.pantheon.ethereum.p2p.config.NetworkingConfiguration;
 import tech.pegasys.pantheon.ethereum.p2p.discovery.PeerDiscoveryServiceException;
 import tech.pegasys.pantheon.ethereum.p2p.netty.NettyP2PNetwork;
 import tech.pegasys.pantheon.ethereum.p2p.peers.PeerBlacklist;
+import tech.pegasys.pantheon.util.NetworkUtility;
 
 import java.io.IOException;
 
@@ -56,7 +57,9 @@ public class NetworkingServiceLifecycleTest {
       service.run();
       final int port = service.getDiscoverySocketAddress().getPort();
 
-      assertEquals("/0.0.0.0:" + port, service.getDiscoverySocketAddress().toString());
+      assertEquals(
+          (NetworkUtility.isIPv6Available() ? "/0:0:0:0:0:0:0:0:" : "/0.0.0.0:") + port,
+          service.getDiscoverySocketAddress().toString());
       assertThat(service.getDiscoveryPeers()).hasSize(0);
     }
   }

util/src/main/java/tech/pegasys/pantheon/util/NetworkUtility.java

@@ -12,13 +12,56 @@
  */
 package tech.pegasys.pantheon.util;
 
+import java.net.Inet6Address;
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
+import java.net.NetworkInterface;
+import java.util.Enumeration;
+import java.util.function.Supplier;
+
+import com.google.common.base.Suppliers;
 
 public class NetworkUtility {
 
   private NetworkUtility() {}
 
+  private static final Supplier<Boolean> ipv6Available =
+      Suppliers.memoize(NetworkUtility::checkIpv6Availability);
+
+  /**
+   * Is IPv6 available?
+   *
+   * @return Returns true if the machine reports having any IPv6 addresses.
+   */
+  public static boolean isIPv6Available() {
+    return ipv6Available.get();
+  }
+
+  /**
+   * The standard for IPv6 availability is if the machine has any IPv6 addresses.
+   *
+   * @return Returns true if any IPv6 addresses are iterable via {@link NetworkInterface}.
+   */
+  private static Boolean checkIpv6Availability() {
+    try {
+      final Enumeration<NetworkInterface> networkInterfaces =
+          NetworkInterface.getNetworkInterfaces();
+      while (networkInterfaces.hasMoreElements()) {
+        final Enumeration<InetAddress> addresses =
+            networkInterfaces.nextElement().getInetAddresses();
+        while (addresses.hasMoreElements()) {
+          if (addresses.nextElement() instanceof Inet6Address) {
+            // Found an IPv6 address, hence the IPv6 stack is available.
+            return true;
+          }
+        }
+      }
+    } catch (final Exception ignore) {
+      // Any exception means we treat it as not available.
+    }
+    return false;
+  }
+
   /**
    * Checks the port is not null and is in the valid range port (1-65536).
    *