EE402-Lab4_Q7

Q7

#include <stdio.h>
typedef unsigned short u16;

#define BSWAP_16(x) \
(u16) ( ((((u16)(x) & 0x00ff)) << 8) \
	| (((u16)(x) & 0xff00) >> 8) )
u16 bswap_16(u16 x)
{
	return ((x & 0x00ff) << 8) | ((x & 0xff00) >> 8);
} 

unsigned short check_sum(unsigned short *a, int len)
{
	unsigned int sum = 0;
	
	while (len > 1) {
		sum += *a++;
		len -= 2;
	}
	
	if (len) {
		sum += *(unsigned char *)a;
	}
	
	while (sum >> 16) {
		sum = (sum >> 16) + (sum & 0xffff);
	}
	
	return (unsigned short)(~sum);
}

int main()
{
	unsigned short sum;
	unsigned char
	buffer[] = {0xc0,0xa8,0x2b,0xea,0xc0,0xa8,0x2b,0x01,0x00,0x11,0x00,0x2d,\ // message pseudo header
		0xc2,0x00,0x00,0x35,0x00,0x2d,0x00,0x00,\  // UDP header
		// data segment
		0xd3,0x04,0x01,0x00,0x00,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x06,0x61,\
		0x70,0x70,0x68,0x75,0x62,0x05,0x71,0x7a,0x6f,0x6e,0x65,0x02,0x71,0x71,0x03,0x63,0x6f,0x6d,0x00,0x00,0x01,0x00,0x01};	

	sum = check_sum((unsigned short*)buffer, sizeof(buffer));

	u16 num_16 = sum;
	printf("IP CheckSum:%x\n", bswap_16(num_16));
	printf("IP CheckSum:%x\n", BSWAP_16(num_16));
	return 0;
}

输出的就是检验后理论上的checksum，和你抓取的checksum不符就说明数据有误。

蓝色部分是UDP header，最后的d87a是待验证的checksum，代码中统一为0x00,0x00。蓝色部分后的全是data segment。蓝色部分前8对，是source IP和destination IP。

• PS：借鉴的童鞋改改变量名以防查重。