根据RFC1071文档的计算方法,编写代码实现IP首部校验和的计算

 

计算步骤:

1.首先将IP首部中校验和字段置0

2.将IP首部每16bit进行相加,如果有进位产生,则将进位加到最低位。

3.将计算的结果取反即可

 

1 unsigned short checksum(int count,unsigned short* addr) 2 { 3     long sum = 0; 4  5     while(count > 1) 6     { 7         sum   +=*addr++; 8         count -= sizeof(unsigned short); 9     }10 11     if(count > 0)12     {13         sum  +=*(unsigned char*)addr;14     }15 16     while(sum >> 16)17     {18         sum = (sum & 0xFFFF) + (sum >> 16);19     }20     21     return (unsigned short)(~sum);22 }23 int main(int argc,char* argv[])24 {25     unsigned char IpHeader[] = "\xa1\x00\x00\x45\x00\x00\x21\x2c\x00\x00\x11\x01\x2f\x0a\x1e\xac\xfa\xff\xff\xef"; //抓包实例  检验和是0xe6e326 27     printf("0x%0x\n",checksum(sizeof(IpHeader)-1,(unsigned short *)IpHeader));28   29     return 0;30 }