tcp/ip通信中tcp头部结构tcphdrp->check校验计算

通过raw socket修改通信数据后，可通过函数

set_tcp_checksum1(iph);

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 > 1 ) { 
        /* This is the inner loop */
        sum += ntohs(* (u_int16_t*) addr);
        addr += 2;
        count -= 2;
    } /* Add left-over byte, if any */
    if( count > 0 )
        sum += ntohs(* (u_int8_t*) addr); /* Fold 32-bit sum to 16 bits */ 
    while (sum>>16)
    sum = (sum & 0xffff) + (sum >> 16); 
    return (u_int16_t)~sum;
} 
static u_int16_t tcp_checksum2(struct iphdr* iphdrp, struct tcphdr* tcphdrp){ 
    size_t tcplen = ntohs(iphdrp->tot_len) - (iphdrp->ihl<<2); 
    u_int32_t cksum = 0;

    cksum += ntohs((iphdrp->saddr >> 16) & 0x0000ffff);
    cksum += ntohs(iphdrp->saddr & 0x0000ffff);
    cksum += ntohs((iphdrp->daddr >> 16) & 0x0000ffff);
    cksum += ntohs(iphdrp->daddr & 0x0000ffff);
    cksum += iphdrp->protocol & 0x00ff;
    cksum += tcplen; 
    return checksum(cksum, (u_int8_t*)tcphdrp, tcplen);
} 

static u_int16_t tcp_checksum1(struct iphdr* iphdrp){ 
    struct tcphdr *tcphdrp = (struct tcphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<2)); 
    return tcp_checksum2(iphdrp, tcphdrp);
} 
static void set_tcp_checksum2(struct iphdr* iphdrp, struct tcphdr* tcphdrp){
    tcphdrp->check = 0;
    tcphdrp->check = htons(tcp_checksum2(iphdrp, tcphdrp));
} 
static void set_tcp_checksum1(struct iphdr* iphdrp){ 
    struct tcphdr *tcphdrp = (struct tcphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<2));
    set_tcp_checksum2(iphdrp, tcphdrp);
}