OPT_HDR
edns->opt_reserved_space += OPT_HDR + 24;
edns->opt_reserved_space += OPT_HDR + nsd->nsid_len;
unsigned char nsid[OPT_HDR];
unsigned char cookie[OPT_HDR];
buffer_write(q->packet, edns->nsid, OPT_HDR);
buffer_write(q->packet, edns->cookie, OPT_HDR);