func (container *Container) allocatePort(eng *engine.Engine, port nat.Port, bindings nat.PortMap) error {
	binding := bindings[port]
	if container.hostConfig.PublishAllPorts && len(binding) == 0 {
		binding = append(binding, nat.PortBinding{})
	}

	for i := 0; i < len(binding); i++ {
		b := binding[i]

		job := eng.Job("allocate_port", container.ID)
		job.Setenv("HostIP", b.HostIp)
		job.Setenv("HostPort", b.HostPort)
		job.Setenv("Proto", port.Proto())
		job.Setenv("ContainerPort", port.Port())

		portEnv, err := job.Stdout.AddEnv()
		if err != nil {
			return err
		}
		if err := job.Run(); err != nil {
			eng.Job("release_interface", container.ID).Run()
			return err
		}
		b.HostIp = portEnv.Get("HostIP")
		b.HostPort = portEnv.Get("HostPort")

		binding[i] = b
	}
	bindings[port] = binding
	return nil
}

// Allocate an external port and map it to the interface
func AllocatePort(id string, port nat.Port, binding nat.PortBinding) (nat.PortBinding, error) {
	var (
		ip            = defaultBindingIP
		proto         = port.Proto()
		containerPort = port.Int()
		network       = currentInterfaces.Get(id)
	)

	if binding.HostIp != "" {
		ip = net.ParseIP(binding.HostIp)
		if ip == nil {
			return nat.PortBinding{}, fmt.Errorf("Bad parameter: invalid host ip %s", binding.HostIp)
		}
	}

	// host ip, proto, and host port
	var container net.Addr
	switch proto {
	case "tcp":
		container = &net.TCPAddr{IP: network.IP, Port: containerPort}
	case "udp":
		container = &net.UDPAddr{IP: network.IP, Port: containerPort}
	default:
		return nat.PortBinding{}, fmt.Errorf("unsupported address type %s", proto)
	}

	//
	// Try up to 10 times to get a port that's not already allocated.
	//
	// In the event of failure to bind, return the error that portmapper.Map
	// yields.
	//

	var (
		host net.Addr
		err  error
	)
	hostPort, err := nat.ParsePort(binding.HostPort)
	if err != nil {
		return nat.PortBinding{}, err
	}
	for i := 0; i < MaxAllocatedPortAttempts; i++ {
		if host, err = portMapper.Map(container, ip, hostPort); err == nil {
			break
		}
		// There is no point in immediately retrying to map an explicitly
		// chosen port.
		if hostPort != 0 {
			logrus.Warnf("Failed to allocate and map port %d: %s", hostPort, err)
			break
		}
		logrus.Warnf("Failed to allocate and map port: %s, retry: %d", err, i+1)
	}

	if err != nil {
		return nat.PortBinding{}, err
	}

	network.PortMappings = append(network.PortMappings, host)

	switch netAddr := host.(type) {
	case *net.TCPAddr:
		return nat.PortBinding{HostIp: netAddr.IP.String(), HostPort: strconv.Itoa(netAddr.Port)}, nil
	case *net.UDPAddr:
		return nat.PortBinding{HostIp: netAddr.IP.String(), HostPort: strconv.Itoa(netAddr.Port)}, nil
	default:
		return nat.PortBinding{}, fmt.Errorf("unsupported address type %T", netAddr)
	}
}