Programming

Here is a sample code that may help u!
Most kernel support syn-cookies to prevent synflood attack...

Wow could not attach the file, leave an email for delivery

Which compiler are you using, BTW? Sorry, I've been away. Did you mention the compiler? GCC?

Neo

Yes. I'm using gcc.

Btw, I'm getting a very strange error with sendto. Here's my code. I have compiled it with gcc and I get error :

sendto:invalid argument.

int main(int argc, char *argv[])
{
struct pseudohdr {
unsigned long saddr; /* source address */
unsigned long daddr; /* dest address */
char zer0; /* zero */
unsigned char protocol; /* protocol to use */
unsigned short length; /* length of packet */
};

int one = 1, sent;
int packSize=sizeof(struct tcphdr)+sizeof(struct iphdr)
+sizeof(struct pseudohdr);
char packet[packSize]; /* Packet. */
struct sockaddr_in sin; /* Our address info structures. */
struct iphdr *ip = (struct iphdr *) packet;
struct tcphdr *tcp = (struct tcphdr *) packet + sizeof(struct iphdr);
struct pseudohdr *pseudo;
pseudo = (struct pseudohdr *) (packet + sizeof(struct iphdr) -
sizeof(struct pseudohdr));
char *remote_ip_str="128.112.139.72";

/* create the socket. */
if((sock = socket (AF_INET, SOCK_RAW, IPPROTO_TCP)) == -1) {
perror("socket"); /* Error creating socket. */
exit(-1);
}

bzero((char *) &sin, sizeof(sin));
sin.sin_port = htons(MYPORT);

if ((bind(sock, (struct sockaddr *)& sin, sizeof(sin))) < 0) {
perror("bind");
exit(1);
}

/* Tell the kernel we'll fill in the IP headers
* outselves. */
if((setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one))) < 0 )
{
perror("setsockopt");
exit(1);
}

/* Fill in the destination info. */
bzero((char *)& sin, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons(MYPORT);
sin.sin_addr.s_addr = inet_addr(remote_ip_str);

bzero(packet, sizeof(packet));

/* fill in the pseudo header - this is needed to help calculate
* the checksum for the tcp segment of the packet. */
pseudo->saddr = 0;
pseudo->daddr = (u_long)inet_ntoa(sin.sin_addr);
pseudo->zer0 = 0;
pseudo->protocol = IPPROTO_TCP;
pseudo->length = htons(sizeof(struct tcphdr));

/* Fill in IP headers. */
ip->ihl = 5;
ip->version = 4;
ip->tot_len = sizeof(struct iphdr) + sizeof(struct tcphdr);
ip->id = htons(0);
ip->saddr = 0;
ip->daddr = (u_long)inet_ntoa(sin.sin_addr);
ip->ttl = 64;
ip->protocol = IPPROTO_TCP;
ip->check = 0; /* calculate checksum later, below. */
ip->tos = 0;
ip->frag_off = 0;
/* set the checksum. */
ip->check = in_cksum((unsigned short *)ip, sizeof(struct iphdr));

/* Fill in the TCP headers. */
tcp->source = htons(MYPORT);
tcp->dest = htons(MYPORT);

tcp->seq = htons(random()); /* "random" sequence number. */
tcp->ack = 0; /* ACKnowledgement */
tcp->syn = 1;
tcp->window = htons(65535);
tcp->doff = 5;
tcp->rst = 0;
tcp->check = 0; /* calculate checksum later, below. */
tcp->psh = 0;
tcp->fin = 0;
tcp->urg = 0;
tcp->ack_seq = htons(0);
/* set the checksum. */
tcp->check = (unsigned short)in_cksum((unsigned short *)pseudo,
sizeof(struct tcphdr)+sizeof(struct pseudohdr));

//if((sent=sendto(sock, packet, ip->tot_len, 0, This didn't work either.
if((sent=sendto(sock, packet, sizeof(packet), 0,
(struct sockaddr *)&sin, sizeof(sin))) < 0 ) {
printf("sent is %d\n", sent);
perror("sendto");
exit(1);
}
printf("sent is %d\n", sent);
return(0);
}

unsigned short in_cksum(unsigned short *addr, int len) {
register int sum = 0;
u_short answer = 0;
register u_short *w = addr;
register int nleft = len;

while (nleft > 1) {
sum += *w++;
nleft -= 2;
}

if (nleft == 1) {
*(u_char *)(&answer) = *(u_char *)w ;
sum += answer;
}

sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;
return(answer); /* return the checksum value. */
}