unpack_header
int unpack_header(struct packed*, struct header*);
if (unpack_header(&p, &h) == -1) {
if (unpack_header(&p, &h) == -1) {
unpack_header(&pack, &h);
unpack_header(&pack, &h);
unpack_header(&pack, &h);
int unpack_header(struct unpack *, struct dns_header *);