TCP校验和的问题

饭团呀饭团 2014-07-15 03:02:23

代码部分：



u_short CheckSum(u_short *pBuffer, int nSize)

{

        u_long dwChecksum = 0;

        int cnt = nSize;



        while (cnt > 1)

        {

                dwChecksum += *pBuffer++;

                cnt -= sizeof(u_short);

        }



        if (cnt)

        {

                dwChecksum += *(u_char*)pBuffer;

        }

        while (dwChecksum >> 16)

        {

                dwChecksum = (dwChecksum >> 16) + (dwChecksum & 0xFFFF);

        }

        return (u_short)~dwChecksum;

}



void TcpCheckSum(tcp_packet *packet)

{

        unsigned short attachsize = 0;                                //传输层协议头以及附加数据的总长度

        _tcp_hdr *ptcp_header = NULL;                //TCP头指针

        FakeHead_t *ptcp_psd_header = NULL;



        ptcp_header = (_tcp_hdr *)(packet + 14 + ((packet->ip.ip.ucVersionAndHeadLength) & 15) * 4);

        attachsize = ntohs(packet->ip.ip.usTotalLength) - ((packet->ip.ip.ucVersionAndHeadLength) & 15) * 4;

        ptcp_psd_header = (FakeHead_t *)malloc(attachsize + sizeof(FakeHead_t));



        if (!ptcp_psd_header)

        {

                return;

        }



        memset(ptcp_psd_header, 0, attachsize + sizeof(FakeHead_t));



        //填充伪TCP头

        ptcp_psd_header->dest_address = packet->ip.ip.dest_ip;

        ptcp_psd_header->source_address = packet->ip.ip.sour_ip;

        ptcp_psd_header->placeholder = 0;

        ptcp_psd_header->protocol = 0x06;

        ptcp_psd_header->tcp_length = htons(attachsize);



        //计算TCP校验和

        ptcp_header->usCheckSum = 0;

        memcpy((unsigned char *)ptcp_psd_header + sizeof(FakeHead_t), (unsigned char *)ptcp_header, attachsize);

        ptcp_header->usCheckSum = CheckSum((unsigned short *)ptcp_psd_header, attachsize + sizeof(FakeHead_t));





        free(ptcp_psd_header);

}

数据类型部分：





struct FakeHead_t

{

        u_int source_address;

        u_int dest_address;

        u_char placeholder;

        u_char protocol;

        u_short tcp_length;

};

struct tcp_packet

{

        ip_packet ip;

        _tcp_hdr tcp;

};

struct ip_head

{

        u_char ucVersionAndHeadLength;

        u_char ucTos;

        u_short usTotalLength;

        u_short usIdentification;

        u_short usFlagsAndFragmentOffset;

        u_char ucTtl;

        u_char ucProtocol;

        u_short usCheckSum;

        unsigned long sour_ip;

        unsigned long dest_ip;

};

struct ip_packet

{

        ethernet_head eth;

        ip_head ip;

};



struct _tcp_hdr

{

 u_short usSourcePort;

        u_short usDestPort;

        DWORD dwSeq;

        DWORD dwAck;

        u_char ucLength;

        u_char ucFlag;

        u_short usWindow;

        u_short usCheckSum;

        u_short usUrgent;

        u_int unMssOpt;

        u_short usNopOpt;

        u_short usSackOpt;

};

struct ethernet_head

{

        unsigned char dest_mac[6];  //目标主机MAC地址

unsigned char source_mac[6]; //源端MAC地址

unsigned short eh_type;   //以太网类型

};