Account dcoker0 IPv6 address on daemon start

- In case --fixed-cidr-v6 is specified and docker0 bridge already
  has a global scope IPv6 address belonging to that v6 network
  (likely from a previous daemon instance), to maintain consistency
  with what done for the docker0 IPv4 address, daemon has to pass it
  down to libnetwork in the IPAMConfig as network gateway to make
  sure that the address is not given to some container.

Signed-off-by: Alessandro Boch <aboch@docker.com>

daemon/daemon_unix.go

@@ -528,12 +528,16 @@ func initBridgeDriver(controller libnetwork.NetworkController, config *Config) e
 		netOption[bridge.DefaultBindingIP] = config.Bridge.DefaultIP.String()
 	}
 
-	ipamV4Conf := libnetwork.IpamConf{}
+	var (
+		ipamV4Conf *libnetwork.IpamConf
+		ipamV6Conf *libnetwork.IpamConf
+	)
 
-	ipamV4Conf.AuxAddresses = make(map[string]string)
+	ipamV4Conf = &libnetwork.IpamConf{AuxAddresses: make(map[string]string)}
 
-	if nw, _, err := ipamutils.ElectInterfaceAddresses(bridgeName); err == nil {
-		ipamV4Conf.PreferredPool = nw.String()
+	nw, nw6List, err := ipamutils.ElectInterfaceAddresses(bridgeName)
+	if err == nil {
+		ipamV4Conf.PreferredPool = types.GetIPNetCanonical(nw).String()
 		hip, _ := types.GetHostPartIP(nw.IP, nw.Mask)
 		if hip.IsGlobalUnicast() {
 			ipamV4Conf.Gateway = nw.IP.String()
@@ -564,10 +568,7 @@ func initBridgeDriver(controller libnetwork.NetworkController, config *Config) e
 		ipamV4Conf.AuxAddresses["DefaultGatewayIPv4"] = config.Bridge.DefaultGatewayIPv4.String()
 	}
 
-	var (
-		ipamV6Conf     *libnetwork.IpamConf
-		deferIPv6Alloc bool
-	)
+	var deferIPv6Alloc bool
 	if config.Bridge.FixedCIDRv6 != "" {
 		_, fCIDRv6, err := net.ParseCIDR(config.Bridge.FixedCIDRv6)
 		if err != nil {
@@ -587,6 +588,16 @@ func initBridgeDriver(controller libnetwork.NetworkController, config *Config) e
 			ipamV6Conf = &libnetwork.IpamConf{AuxAddresses: make(map[string]string)}
 		}
 		ipamV6Conf.PreferredPool = fCIDRv6.String()
+
+		// In case the --fixed-cidr-v6 is specified and the current docker0 bridge IPv6
+		// address belongs to the same network, we need to inform libnetwork about it, so
+		// that it can be reserved with IPAM and it will not be given away to somebody else
+		for _, nw6 := range nw6List {
+			if fCIDRv6.Contains(nw6.IP) {
+				ipamV6Conf.Gateway = nw6.IP.String()
+				break
+			}
+		}
 	}
 
 	if config.Bridge.DefaultGatewayIPv6 != nil {
@@ -596,13 +607,13 @@ func initBridgeDriver(controller libnetwork.NetworkController, config *Config) e
 		ipamV6Conf.AuxAddresses["DefaultGatewayIPv6"] = config.Bridge.DefaultGatewayIPv6.String()
 	}
 
-	v4Conf := []*libnetwork.IpamConf{&ipamV4Conf}
+	v4Conf := []*libnetwork.IpamConf{ipamV4Conf}
 	v6Conf := []*libnetwork.IpamConf{}
 	if ipamV6Conf != nil {
 		v6Conf = append(v6Conf, ipamV6Conf)
 	}
 	// Initialize default network on "bridge" with the same name
-	_, err := controller.NewNetwork("bridge", "bridge",
+	_, err = controller.NewNetwork("bridge", "bridge",
 		libnetwork.NetworkOptionGeneric(options.Generic{
 			netlabel.GenericData: netOption,
 			netlabel.EnableIPv6:  config.Bridge.EnableIPv6,