TUN_HDR
CLR(sc->sc_flags, TUN_ASYNC|TUN_HDR);
SET(sc->sc_flags, TUN_HDR);
if (ISSET(sc->sc_flags, TUN_HDR)) {
CLR(sc->sc_flags, TUN_HDR);
if (len > 0 && ISSET(sc->sc_flags, TUN_HDR))
if (ISSET(sc->sc_flags, TUN_HDR)) {
if (ISSET(sc->sc_flags, TUN_HDR))
if (ISSET(sc->sc_flags, TUN_HDR)) {