trunk_gethdr
trunk_gethdr(m, off, EVL_ENCAPLEN, &vlanbuf)) == NULL)
trunk_gethdr(m, off, sizeof(*ip), &ipbuf)) == NULL)
trunk_gethdr(m, off, sizeof(*ip6), &ip6buf)) == NULL)
const void *trunk_gethdr(struct mbuf *, u_int, u_int, void *);