下面敘述的是IPv4的 ip udp tcp 效驗和的計算。
ip效驗和計算相對簡單,只需對ip協議頭進行計算;
ip協議頭長20字節;
udp協議頭長8字節;
tcp協議頭長20~60字節;
udp和tcp的校驗和不僅要對整個ip協議負載(包括
udp/tcp協議頭和udp/tcp協議負載)進行計算,
還要先對一個偽協議頭進行計算。
在計算效驗和前,協議頭效驗和字段要清0。
偽協議頭結構如下:
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| source address |
+--------+--------+--------+--------+
| destination address |
+--------+--------+--------+--------+
| zero |protocol| udp/tcp length |
+--------+--------+--------+--------+
偽協議頭中的字段同樣都是網絡字節序;
下面代碼片段展現的是在GNU/Linux中具體的計算方式:
//=====================================================
#include <stdint.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ether.h>
#include <netinet/udp.h>
#include <netinet/tcp.h>
struct pseudo_head {
uint32_t saddr;
uint32_t daddr;
char zero;
char proto;
unsigned short len;
};
int cal_cksum(char *p)
{
struct iphdr *ip = (struct iphdr *)
(p + sizeof(struct ether_header));
cal_ip_cksum(ip);
struct tcphdr *tcp;
struct udphdr *udp;
char *f = (char *)ip + 20;
switch(ip->protocol) {
case IPPROTO_TCP:
tcp = (struct tcphdr *)f;
tcp->check = 0;
tcp->check = cal_udptcp_cksum(
(unsigned short *)f, ip);
break;
case IPPROTO_UDP:
udp = (struct udphdr *)f;
udp->check = 0;
udp->check = cal_udptcp_cksum(
(unsigned short *)f, ip);
break;
case IPPROTO_IP:
break;
default:
return 1;
}
return 0;
}
inline static void cal_ip_cksum(struct iphdr *ip)
{
ip->check = 0;
ip->check = in_cksum((unsigned short *)ip,
20, 0);
}
inline static unsigned short cal_udptcp_cksum(
unsigned short *p, struct iphdr *ip)
{
int len = ntohs(ip->tot_len) - 20;
struct pseudo_head ph;
ph.len = htons(len);
ph.saddr = ip->saddr;
ph.daddr = ip->daddr;
ph.proto = ip->protocol;
ph.zero = 0;
int sum = cal_sum(
(unsigned short *)&ph, sizeof(ph));
return in_cksum(p, len, sum);
}
unsigned short in_cksum(unsigned short *p,
int len, int sum)
{
int n = len;
while(n > 1) {
sum += *p++;
n -= 2;
}
if(n == 1)
sum += *(unsigned char *)p;
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
unsigned short ret = ~sum;
return ret;
}
inline static int cal_sum(unsigned short *p, int len)
{
int sum = 0;
while(len > 0) {
sum += *p++;
len -= 2;
}
return sum;
}