c - 无法验证 TCP 校验和

Question

我一直在使用原始套接字在 C 中创建自定义 TCP 数据包。为了验证，我将它们发送到环回接口(interface)，当我使用 TCPDUMP 检查接收到的数据包时，校验和与 TCP 数据包不匹配。以下是 TCP header 中的字段:

        tcp->th_sport = temp_port;      // The TCP structure. The source port, spoofed, we accept through the command line
        tcp->th_dport = atoi(argv[2]);  // The destination port, we accept through command line
        tcp->th_seq = htonl(random_id()%1000);
        tcp->th_ack = htonl(random_id()%1000);
        tcp->th_off = 5;
        tcp->th_flags = TH_SYN;
        tcp->th_win = 10000;
        tcp->th_sum = 0;
        tcp->th_urp = 0;
        
        //pseudo header for TCP checksum calculation
        p_hdr->source = t1->s_addr;
        p_hdr->dest = t2->s_addr;
        p_hdr->reserved = 0;
        p_hdr->protocol = IPPROTO_TCP;  //TCP
        p_hdr->tcp_size = sizeof(struct tcphdr);
        memcpy(buffer2 + sizeof(struct pseudo_tcp_header) , tcp , sizeof(struct tcphdr) );
        tcp->th_sum = htons(csum((unsigned short *) (buffer2 ), sizeof(struct tcphdr) + sizeof(struct pseudo_tcp_header)));

这是 random_id 函数:

int random_id()
{
    int lower = 1, upper = 65535,number;
    number = (rand() % (upper - lower + 1)) + lower;
    return number;
}

校验和由函数计算，

unsigned short csum(unsigned short *buf, int len)
{
    unsigned long sum;
    for(sum=0; len>0; len--)
        sum += *buf++;
    sum = (sum >> 16) + (sum &0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

是否有默认函数来计算 tcpchecksum在 C 中？

Answer 1

我了解有没有计算 ip 的默认函数/tcp校验和。可以通过以下代码找到:

unsigned short csum(unsigned short *buf, int len)
{
    unsigned long sum;
    for(sum=0; len>0; len--)
        sum += *buf++;
    while (sum>>16)
        sum = (sum >> 16) + (sum &0xffff);
    return (unsigned short)(~sum);
}

输入， buf如果数据带有伪 header 。
仅当数据包仅遍历 localhost 时才需要此计算。 .如果是把网卡传到外网，留下 sum字段空白。网卡会自动计算头部值。