r2

/* IP CHECKSUM VERIFICATION */
	printf("Verifying IP checksum...\n");
	unsigned short ip_checksum = calculate_checksum((unsigned short*)ih, ih->header_length*4/2);

	if(ip_checksum == 0)
		printf("IP checksum -> OK (0x%.2x)\n", ip_checksum);
	else
		printf("IP checksum -> BAD (0x%.2x)\n", ip_checksum);


	/* UDP CHECKSUM CALCULATION */

	// Calculate size of (IP pseudo header + UDP datagram)
	int data_length = 12 + ntohs(uh->datagram_length);		// Length of pseudo header + length of datagram
	data_length = data_length + data_length % 2;			// Check sum have to be even number

	// Dynamic allocate memory of checksum data
	unsigned char* checksum_data = (unsigned char*)malloc(data_length);
	memset(checksum_data, 0, data_length);

	// Copy IP pseudo header
	memcpy(checksum_data, ih->src_addr, 4);					// IP:	Source Address		(4 bytes)
	memcpy(checksum_data + 4, ih->dst_addr, 4);				// IP:  Destination Address	(4 bytes)
															// IP:  Reserved - all 0's	(1 byte)
	checksum_data[9] = ih->next_protocol;					// IP:  Next Protocol		(1 byte)
	memcpy(checksum_data + 10, &(uh->datagram_length), 2);	// UDP:	Datagram Length		(2 bytes)
															//	--------------------- = 12 bytes
	// Copy UDP datagram (header + application data)
	memcpy(checksum_data + 12, uh, ntohs(uh->datagram_length));

	// Initialize UDP checksum
	memset(checksum_data + 18, 0, 2);	// 18 = 12 (pseudo header) + 6 (position in udp header)

	printf("Calculating UDP checksum...\n");
	unsigned short udp_checksum = calculate_checksum((unsigned short*)checksum_data, data_length/2);
	printf("UDP checksum: %.2x\n\n", udp_checksum);

	// Free dynamic alocated memory
	free(checksum_data);
}

// Calculates checksum for given data
unsigned short calculate_checksum(unsigned short * data, int data_length)
{
	unsigned int sum = 0;

	// Calculate sum of 16bits values
	for(int i = 0; i < data_length; i++)
	{
		sum += ntohs(*(data + i));
	}

	unsigned int carry;

	// Carry will be added to the rest of the value
	while (carry = sum >> 16)
	{
		sum = sum & 0x0000FFFF;
		sum = sum + carry;
	}

	// Flip every bit in that value, to obtain the checksum
	unsigned short checksum = ~(unsigned short)sum;

	return checksum;
}