计算 ip udp tcp 效验和

下面叙述的是ipv4的 ip udp tcp 效验和的计算。
ip效验和计算相对简单，只需对ip协议头进行计算；
ip协议头长20字节；
udp协议头长8字节；
tcp协议头长20～60字节；
udp和tcp的校验和不仅要对整个ip协议负载(包括
udp/tcp协议头和udp/tcp协议负载)进行计算，
还要先对一个伪协议头进行计算。
在计算效验和前，协议头效验和字段要清0。

伪协议头结构如下：
  0     7 8      15 16     23 24  31
  +--------+--------+--------+--------+
  |           source address            |
  +--------+--------+--------+--------+
  |        destination address        |
  +--------+--------+--------+--------+
  | zero   |protocol| udp/tcp length |
  +--------+--------+--------+--------+

伪协议头中的字段同样都是网络字节序；

下面代码片段展现的是在gnu/linux中具体的计算方式：

//=====================================================
#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;
}

摘自 leeshuheng的专栏