我正在尝试使用RFC 1071的示例C代码来计算正确的IP头校验和,但是有一个问题最好用代码描述:
设置IP标头:
#include <linux/ip.h>
typedef struct iphdr tipheader;
int main(int argc, char **argv)
{
tipheader * iphead = (tipheader *) malloc(sizeof(tipheader));
iphead->ihl = 5;
iphead->version = 4;
iphead->tos = 0;
iphead->tot_len = 60;
....
unsigned short checksum = getChecksum((unsigned short *) iphead, 20);
}
校验和功能:
unsigned short getChecksum(unsigned short * iphead, int count)
{
unsigned long int sum = 0;
unsigned short checksum = 0;
printf("\nStarting adress: %p\n", iphead);
while(count > 1) {
sum += * (unsigned short *) (iphead);
count -=2;
printf("a: %p, content is: %d, new sum: %ld\n", iphead, (unsigned short) *(iphead), sum);
iphead++;
}
if(count > 0) {
sum += * (unsigned short *) (iphead);
}
while(sum >> 16) {
sum = (sum & 0xffff) + (sum >> 16);
}
checksum = ~sum;
return checksum;
}
使用无符号短指针迭代iphead指向的内存,在前两次迭代后显示以下输出:
Starting address: 0x603090
a: 0x603090, content is: 69, new sum: 69
a: 0x603092, content is: 60, new sum: 129
因此指针按预期工作'并且每次迭代增加2。 但是为什么前两个字节的内容被解释为69(0x45),它应该是0x4500
澄清的问题
答案 0 :(得分:0)
前两个字段只有4位长,所以, 在内存中是0x4500。
但是,由于Endian'ness,它被读为0x0045。
打印时,除非另外强制,否则会抑制前导0。
所以结果是0x45 = 69