RTNH_HDRLEN
#define RTNH_OK(_nh, _len) NL_ITEM_OK(_nh, _len, RTNH_HDRLEN, _RTNH_LEN)
#define RTNH_LENGTH(_len) (RTNH_HDRLEN + (_len))
#define RTNH_DATA(_nh) ((struct rtattr *)NL_ITEM_DATA(_nh, RTNH_HDRLEN))