pf_check_out
static int pf_check_out(void *arg, struct mbuf **m, struct ifnet *ifp,
pfil_add_hook(pf_check_out, NULL, PFIL_OUT, pfh_inet);
pfil_remove_hook(pf_check_out, NULL, PFIL_OUT, pfh_inet);
pfil_remove_hook(pf_check_out, NULL, PFIL_OUT, pfh_inet);