SLIP_HDRLEN
M_PREPEND(*m, SLIP_HDRLEN, M_DONTWAIT);
m_adj(*m, SLIP_HDRLEN);
m0.m_len = SLIP_HDRLEN;
#define BUFOFFSET (128+sizeof(struct ifnet **)+SLIP_HDRLEN)
bpf_attach(&sc->sc_if, DLT_SLIP, SLIP_HDRLEN);