IP首部校验和计算

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

 

unsigned short checksum(int count,unsigned short* addr)
{
    long sum = 0;

    while(count > 1)
    {
        sum   +=*addr++;
        count -= sizeof(unsigned short);
    }

    if(count > 0)
    {
        sum  +=*(unsigned char*)addr;
    }

    while(sum >> 16)
    {
        sum = (sum & 0xFFFF) + (sum >> 16);
    }
    
    return (unsigned short)(~sum);
}
int main(int argc,char* argv[])
{
    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0xe6e3

    printf("0x%0x\n",checksum(sizeof(IpHeader)-1,(unsigned short *)IpHeader));
  
    return 0;
}