I am trying to receive the RST packets sent to my program after I send several SYN packet to a closed port, the case is I am seeing any RST segments being sent or received, as I confirmed this through Wireshark.
Here is the SYN segment configuration:
` char packet[BUF_SIZE];
struct iphdr* ip_h = (struct iphdr*) packet;
struct tcphdr* tcp_h = (struct tcphdr*) (packet + sizeof(struct iphdr));

ip_h->ihl = 5;
ip_h->version = 4;
ip_h->tos = 0;
ip_h->tot_len = sizeof (struct iphdr) + sizeof (struct tcphdr);
ip_h->id = htons(54321);
ip_h->frag_off = 0;
ip_h->ttl = 255;
ip_h->protocol = IPPROTO_TCP;
ip_h->check = 0;
ip_h->saddr = inet_addr(cf->src_ip);
ip_h->daddr = inet_addr(cf->server_ip);

tcp_h->source = htons(12345);
tcp_h->dest = htons(dest_port);
tcp_h->seq = 0;
tcp_h->ack_seq = 0;
tcp_h->doff = 5;
tcp_h->fin = 0;
tcp_h->syn = 1;
tcp_h->rst = 0;
tcp_h->psh = 0;
tcp_h->ack = 0;
tcp_h->urg = 0;
tcp_h->check = 0;
tcp_h->window = htons (5840);
tcp_h->urg_ptr = 0;`

Screenshot For SYN Segment

In addition, when I check the SYN segment through Wireshark, this kinda looks off to me, as ECN and CWR aren’t supposed to be set, including Reserved as well, but I don’t think I will be able to manually set them off in the program.

For now, I could only think of two issues, it is either my SYN segment is not correctly setup, but it looks good to me. Or the server simply does not send RST, for this, I tried with nc -l -p 9999(destination port where SYN is sent to) to open up this port on server end, hoping it to send SYN-ACK, but still no response.