tcp/ip通信中udp头部结构udphdrp->check校验计算

通过raw socket修改通信数据后，可通过函数

set_udp_checksum1

重新校验计算iph->check值

在http://www.cnblogs.com/dpf-10/p/7899237.html查看实际应用

static u_int16_t checksum(u_int32_t init, u_int8_t *addr, size_t count){
    /* Compute Internet Checksum for "count" bytes * beginning at location "addr". */
    u_int32_t sum = init; 

    while( count >  ) {
        /* This is the inner loop */
        sum += ntohs(* (u_int16_t*) addr);
        addr += ;
        count -= ;
    } /* Add left-over byte, if any */
    if( count >  )
        sum += ntohs(* (u_int8_t*) addr); /* Fold 32-bit sum to 16 bits */
    while (sum>>)
    sum = (sum & 0xffff) + (sum >> );
    return (u_int16_t)~sum;
} 
static u_int16_t udp_checksum2(struct iphdr* iphdrp, struct udphdr* udphdrp){
    size_t udplen = ntohs(iphdrp->tot_len) - (iphdrp->ihl<<);
    u_int32_t cksum = ;

    cksum += ntohs((iphdrp->saddr >> ) & 0x0000ffff);
    cksum += ntohs(iphdrp->saddr & 0x0000ffff);
    cksum += ntohs((iphdrp->daddr >> ) & 0x0000ffff);
    cksum += ntohs(iphdrp->daddr & 0x0000ffff);
    cksum += iphdrp->protocol & 0x00ff;
    cksum += udplen;
    return checksum(cksum, (u_int8_t*)udphdrp, udplen);
} 

static u_int16_t udp_checksum1(struct iphdr* iphdrp){
    struct udphdr *udphdrp = (struct udphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<));
    return udp_checksum2(iphdrp, udphdrp);
}
static void set_udp_checksum2(struct iphdr* iphdrp, struct udphdr* udphdrp){
    udphdrp->check = ;
    udphdrp->check = htons(udp_checksum2(iphdrp, udphdrp));
}
static void set_udp_checksum1(struct iphdr* iphdrp){
    struct udphdr *udphdrp = (struct udphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<));
    set_udp_checksum2(iphdrp, udphdrp);
}