C程序设计TCP校验 [英] C Programming TCP Checksum

问题描述

我一直有麻烦现在做的TCP校验了好几天。我已经看了看互联网上的许多资源，但没有的，我所看到的例子告诉你怎么做TCP校验和。我也看了看 RFC文档，仍然我有麻烦：

I have been having trouble doing the checksum for TCP for several days now. I have looked at many sources on the Internet but none of the examples that I have seen show you how to do the TCP checksum. I have also looked at the RFC document and still I am having trouble:

下面是code我使用生成校验：

Below is the code I am using to generate the checksum:

unsigned short checksum(unsigned short * buffer, int bytes)
{
    unsigned long sum = 0;
    unsigned short answer = 0;
    int i = bytes;
    while(i>0)
    {
            sum+=*buffer;
            buffer+=1;
            i-=2;
    }
    sum = (sum >> 16) + (sum & htonl(0x0000ffff));
    sum += (sum >> 16);
    return ~sum;
}

此功能适用于IP校验和。

This function works for the IP checksum.

下面是我为我的TCP报头所做的结构：

Below is the struct I have made for my TCP header:

struct tcp_header
{
    unsigned short tcp_sprt;
    unsigned short tcp_dprt;
    unsigned int tcp_seq;
    unsigned int tcp_ack;
    unsigned char tcp_res:4;
    unsigned char tcp_off:4;
    unsigned char tcp_flags;
    unsigned short tcp_win;
    unsigned short tcp_csum;
    unsigned short tcp_urp;
};

我一直在使用Wireshark来测试这些包，唯一错的是校验码。

I have been using Wireshark to test these packets and the only thing wrong is the checksum.

最后这里是伪报头结构，我加载了从IP头中的TCP报头和信息：

Finally here is the pseudo header struct that I load up with the TCP header and information from the IP header:

struct pseudoTcpHeader
{
    unsigned int ip_src;
    unsigned int ip_dst;
    unsigned char zero;//always zero
    unsigned char protocol;// = 6;//for tcp
    unsigned short tcp_len;
    struct tcp_header tcph;
};

在我这个结构加载正确的信息，然后我用了整个伪报头结构的校验功能和TCP校验分配给该值。你看到什么错我提供什么？如果问题不在这里可能是一个粗心的错误，我不能看到的。

Once I load up this struct with the correct information I then use the checksum function on the entire pseudo header struct and assign the TCP checksum to that value. Do you see anything wrong with what I have provided? If the problem isn't here it may be a careless error that I can't see.

推荐答案

我发现了的 WinPcap的用户邮件列表应该解决有关奇数长度的数据格雷格的评论，并给你的东西来比较你的code反对。

I found a fairly good example on the winpcap-users mailing list which should address Greg's comment about odd length data and give you something to compare your code against.

USHORT CheckSum(USHORT *buffer, int size)
{
    unsigned long cksum=0;
    while(size >1)
    {
        cksum+=*buffer++;
        size -=sizeof(USHORT);
    }
    if(size)
        cksum += *(UCHAR*)buffer;

    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >>16);
    return (USHORT)(~cksum);
}

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