SYN Packet

raw_packet.c

uint16_t csum (uint16_t *addr, int len) {   
//RFC 1071
	register long sum = 0;
	int count = len;
	uint16_t temp;

	while (count > 1)  {
		temp = htons(*addr++);
		sum += temp;
		count -= 2;
	}

	/*  Add left-over byte, if any */
	if(count > 0)
		sum += *(unsigned char *)addr;

	/*  Fold 32-bit sum to 16 bits */
	while (sum >> 16)
		sum = (sum & 0xffff) + (sum >> 16);

	uint16_t checksum = ~sum;
	return checksum;
}


void syn()
{
	char src_ip[17];
	char dst_ip[17];
	
	short dst_port = 80;
	short th_sport = 1234;
	
	short tcp_flags = TH_SYN;
	
	//Headers
	struct ip *iph = (struct ip *) datagram;
	struct tcpheader *tcph = (struct tcpheader *) (datagram + sizeof (struct ip));
	
	struct sockaddr_in servaddr;
	
	snprintf(src_ip,16,"%s", getLocalIP());  //src ip
	snprintf(dst_ip,16,"%s","209.85.175.104"); //google's ip
	
	printf("Source IP %s\nDestination IP %s\n", src_ip, dst_ip);
		
	memset(datagram, 0, 4096); //clearing the buffer
	
	int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);
	servaddr.sin_family = AF_INET;
	inet_pton(AF_INET, dst_ip, &servaddr.sin_addr);
	
	int tcpheader_size = sizeof(struct tcpheader);
	
	printf("TCP Header Size %d\n", tcpheader_size);
	
	iph->ip_hl = 5;					//header length 5
	iph->ip_v = 4;					//version 4
	iph->ip_tos = 0;				//type of service
	iph->ip_len = sizeof(struct ip) + sizeof(struct tcpheader);  //no data
	iph->ip_id = htons(31337);		//id
	iph->ip_off = 0;				//no fragmentation
	iph->ip_ttl = 250;				//time to live
	iph->ip_p = IPPROTO_TCP;		//6
	iph->ip_sum = 0;				//let kernel fill the checksum
	
	printf("IP len %d\n", iph->ip_len);
	
	inet_pton(AF_INET, src_ip, &(iph->ip_src));	//local device ip
	iph->ip_dst.s_addr = servaddr.sin_addr.s_addr;	//destination address
	
	tcph->th_sport = htons(th_sport);	//any port
	tcph->th_dport = htons(dst_port);	//destination port
	tcph->th_seq = htonl(31337);		//random
	tcph->th_ack = htonl(0);		//ACK not needed
	tcph->th_x2 = 0;			//
	tcph->th_off = 0x50;	 		//data offset
	tcph->th_flags = tcp_flags;	        //SYN flag
	tcph->th_win = htons(65535);		//window size
	tcph->th_sum = 0;			//later
	tcph->th_urp = 0;		        //no urgent pointer
	
	struct pseudo_hdr *phdr = (struct pseudo_hdr *) (datagram + sizeof(struct ipheader) + sizeof(struct tcpheader));

	memset(phdr, 0, sizeof(phdr));
	
	phdr->src = iph->ip_src.s_addr;
	phdr->dst = iph->ip_dst.s_addr;
	phdr->mbz = 0;
	phdr->proto = IPPROTO_TCP;
	phdr->len = ntohs(0x14);

	tcph->th_sum = htons(csum((unsigned short *)tcph, sizeof(struct pseudo_hdr)+ sizeof(struct tcpheader)));
			
	int one = 1;
	const int *val = &one;
	if(setsockopt(s, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
		printf("Cannot set HDRINCL for port %d", th_sport);	
	
	if (sendto(s, datagram, iph->ip_len, 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0)
	{
		printf("Error in sending");
	}
	else printf("Sent\n");
}