ifa_ifwithdstaddr
struct ifaddr *ifa_ifwithdstaddr(struct sockaddr *);
ifa = ifa_ifwithdstaddr(dst);
ifa = ifa_ifwithdstaddr(gateway);
ia = ifatoia(ifa_ifwithdstaddr(sintosa(sin)));
if ((ia = (INA)ifa_ifwithdstaddr((SA)&ipaddr))
if ((ia = ifatoia(ifa_ifwithdstaddr(sintosa(dst)))) == NULL &&