if_resolvemulti
int (*if_resolvemulti) /* validate/resolve multicast */
int if_resolvemulti(if_t ifp, struct sockaddr **, struct sockaddr *);
if (ifp->if_resolvemulti == NULL)
return (ifp->if_resolvemulti(ifp, srcs, dst));
ifp->if_resolvemulti = NULL; // done in the stack