IP校验和

  1 #include <stdio.h>
  2 #include <unistd.h>
  3 #include <linux/if_ether.h>
  4 #include <linux/ip.h>
  5 #include <linux/udp.h>
  6 #include <linux/types.h>
  7
  8 unsigned short check_sum(unsigned char *data, int len);
  9
 10 int main()
 11 {
 12         unsigned char data[1024] = {
 13         0x45, 0x00,
 14         0x00, 0x20, 0x00, 0x00, 0x40, 0x00, 0x40, 0x11,
 15         0x00, 0x00, 0xc0, 0xa8, 0x1f, 0x72, 0xc0, 0xa8,
 16         0x1f, 0x7a
 17         };
 18
 19         unsigned short ret = 0;
 20         ret = check_sum(data, 20);
 21         printf("check sum is %x\n", htons(ret));
 22 }
 23
 24 unsigned short check_sum(unsigned char *data, int len)
 25 {
 26         unsigned short ret = 0;
 27         int i = 0;
 28         int sum = 0;
 29         unsigned short *p = (short *)data;
 30         for(i=0; i<len/2; i++)
 31                 sum = sum + ntohs(p[i]);
 32         if(sum > 0xffff){
 33                 sum = (sum & 0x0ffff) + ((sum & 0xf0000) >> 16);
 34         }
 35
 36         ret = (short)sum;
 37         return ~ret;
 38 }

1. 0x00 0x00
2. 按两字节相加完回滚（如果结果是2856D，则2要按0002 继续加 856D）
3. ～
4. htons()