BIOCGDLT
if (ioctl(BpfFd, BIOCGDLT, (caddr_t)&n) < 0) {
case BIOCGDLT:
if (ioctl(fd, BIOCGDLT, (caddr_t)&dlt) == -1) {
if (ioctl(bpf, BIOCGDLT, &dlt) == -1)