sample dnet code

int
send_ack(u_char *data) {
        struct ip_hdr *ip;
        struct tcp_hdr *tcp;
        u_char buf[40];
        ip_t *ipdev;

        ip = (struct ip_hdr *)data;
        tcp = (struct tcp_hdr *)(data + ip->ip_off + IP_HDR_LEN);
        tcp_pack_hdr (buf + IP_HDR_LEN, tcp->th_dport, tcp->th_sport, rand(),
                      tcp->th_seq + 1, 1024, TH_SYN + TH_ACK, NULL);
        ip_pack_hdr (buf, 0x10, IP_HDR_LEN, arc4random() ^ 0xFFFFFFFF, 0, 64,
                     IP_PROTO_TCP, ip->ip_dst, ip->ip_src);
        ipdev = ip_open();
        (void)ip_send (ipdev, buf, sizeof(buf));
        ip_close (ipdev);

        return (1);
}

int
main(int argc, char **argv) {
        ...

        send_ack("foobar");
}