1、尝试了 gethostbyname(...) 和 Qt598中的qhostinfo::fromname(...)，都可以解析出来IP，但是有一个问题：域名指向的IP改变了，这两种方式需要等较长时间 才会解析到更新后的ip地址。而CMD中 nslookup却一下子就解析到了更新后的IP...

　1.1、查了一下资料，貌似原因如下（大概的说下原理，可能不严谨准确）：

　　（1）域名服务器在 域名的TTL时间段内 它不会去实时的查找域名对应的IP，而是要等到TTL超时 域名服务器才回去再次查询 然后更新新的地址。

　　（2）又∵ gethostbyname 和 qhostinfo::fromname 都没有参数可以输入 DNS服务器地址，猜测可能使用的是 OS中设置的DNS服务器。

　　ZC：综上2个原因 造成了这个现象

　　ZC：改 OS的DNS设置的话，如果 这个DNS服务器不是 常用上网的DNS服务器的话，到时候还要改回来，比较麻烦，暂不考虑使用

　1.2、于是猜测，指定 DNS解析服务器的话，就能解决这个现象

2、C++实现DNS域名解析 - Snser - 博客园.html（https://www.cnblogs.com/snser/p/4101729.html）

　　ZC：DNS解析的 数据包结构、原理、代码 都在这个文章中查看

　　ZC：只解析IPv4地址的精简版代码在 2.2

　2.1、这是文章中的代码（少许改动，运行OK）（项目->属性->C/C++->预处理器->"预处理器定义"中添加 "_ITERATOR_DEBUG_LEVEL=0"）

　　（1）CDNSLookup.h

#pragma once

// C++实现DNS域名解析 - Snser - 博客园.html (https://www.cnblogs.com/snser/p/4101729.html)

#define _CRT_SECURE_NO_WARNINGS

//这里需要导入库 Ws2_32.lib，在不同的IDE下可能不太一样

//#pragma comment(lib, "Ws2_32.lib")

#include <winsock2.h>

#pragma comment(lib, "Ws2_32.lib")

//#include <windows.h>

#include <string>

#include <vector>

#define MAX_DOMAINNAME_LEN  255

#define DNS_PORT            53

#define DNS_TYPE_SIZE       2

#define DNS_CLASS_SIZE      2

#define DNS_TTL_SIZE        4

#define DNS_DATALEN_SIZE    2

#define DNS_TYPE_A          0x0001 //1 a host address

#define DNS_TYPE_CNAME      0x0005 //5 the canonical name for an alias

#define DNS_PACKET_MAX_SIZE (sizeof(DNSHeader) + MAX_DOMAINNAME_LEN + DNS_TYPE_SIZE + DNS_CLASS_SIZE)

struct DNSHeader

{

    USHORT usTransID; //标识符

    USHORT usFlags; //各种标志位

    USHORT usQuestionCount; //Question字段个数

    USHORT usAnswerCount; //Answer字段个数

    USHORT usAuthorityCount; //Authority字段个数

    USHORT usAdditionalCount; //Additional字段个数

};

class CDNSLookup

{

public:

    CDNSLookup();

    ~CDNSLookup();

    BOOL DNSLookup(ULONG ulDNSServerIP, char *szDomainName,

        std::vector<ULONG> *pveculIPList = NULL, std::vector<std::string> *pvecstrCNameList = NULL,

        ULONG ulTimeout = , ULONG *pulTimeSpent = NULL);

    BOOL DNSLookup(ULONG ulDNSServerIP, char *szDomainName,

        std::vector<std::string> *pvecstrIPList = NULL, std::vector<std::string>

        *pvecstrCNameList = NULL, ULONG ulTimeout = , ULONG *pulTimeSpent = NULL);

private:

    BOOL Init();

    BOOL UnInit();

    BOOL DNSLookupCore(ULONG ulDNSServerIP, char *szDomainName, std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList, ULONG ulTimeout, ULONG *pulTimeSpent);

    BOOL SendDNSRequest(sockaddr_in sockAddrDNSServer, char *szDomainName);

    BOOL RecvDNSResponse(sockaddr_in sockAddrDNSServer, ULONG ulTimeout,

        std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList, ULONG *pulTimeSpent);

    BOOL EncodeDotStr(char *szDotStr, char *szEncodedStr, USHORT nEncodedStrSize);

    BOOL DecodeDotStr(char *szEncodedStr, USHORT *pusEncodedStrLen, char *szDotStr, USHORT nDotStrSize, char *szPacketStartPos = NULL);

private:

    BOOL m_bIsInitOK;

    SOCKET m_sock;

};

　　（2）CDNSLookup.cpp

#include "CDNSLookup.h"

#include <stdio.h>

#include <string.h>

CDNSLookup::CDNSLookup() :

    m_bIsInitOK(FALSE),

    m_sock(INVALID_SOCKET)

{

    m_bIsInitOK = Init();

}

CDNSLookup::~CDNSLookup()

{

    UnInit();

}

BOOL CDNSLookup::DNSLookup(ULONG ulDNSServerIP, char *szDomainName,

    std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList,

    ULONG ulTimeout, ULONG *pulTimeSpent)

{

    return DNSLookupCore(ulDNSServerIP, szDomainName, pveculIPList, pvecstrCNameList, ulTimeout, pulTimeSpent);

}

BOOL CDNSLookup::DNSLookup(ULONG ulDNSServerIP, char *szDomainName,

    std::vector<std::string> *pvecstrIPList, std::vector<std::string> *pvecstrCNameList,

    ULONG ulTimeout, ULONG *pulTimeSpent)

{

    std::vector<ULONG> *pveculIPList = NULL;

    if (pvecstrIPList != NULL)

    {

        std::vector<ULONG> veculIPList;

        pveculIPList = &veculIPList;

    }

    BOOL bRet = DNSLookupCore(ulDNSServerIP, szDomainName, pveculIPList, pvecstrCNameList, ulTimeout, pulTimeSpent);

    if (bRet)

    {

        pvecstrIPList->clear();

        char szIP[] = { '\0' };

        for (std::vector<ULONG>::iterator iter = pveculIPList->begin(); iter != pveculIPList->end(); ++iter)

        {

            ULONG ul = (*iter);

            BYTE *pbyIPSegment = (BYTE*)(&ul);

            //BYTE *pbyIPSegment = (BYTE*)(&(*iter));

            //sprintf_s(szIP, 16, "%d.%d.%d.%d", pbyIPSegment[0], pbyIPSegment[1], pbyIPSegment[2], pbyIPSegment[3]);

            sprintf(szIP, "%d.%d.%d.%d", pbyIPSegment[], pbyIPSegment[], pbyIPSegment[], pbyIPSegment[]);

            pvecstrIPList->push_back(szIP);

        }

    }

    return bRet;

}

BOOL CDNSLookup::Init()

{

    WSADATA wsaData;

    if (WSAStartup(MAKEWORD(, ), &wsaData) == SOCKET_ERROR)

    {

        printf("WSAStartup(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    if ((m_sock = socket(AF_INET, SOCK_DGRAM, )) == INVALID_SOCKET)

    {

        printf("socket(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    DWORD dwTimeOut =  * ;

    if (setsockopt(m_sock, SOL_SOCKET, SO_RCVTIMEO, (char*)&dwTimeOut, sizeof(dwTimeOut)) == SOCKET_ERROR)

    {

        printf("setsockopt(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    return TRUE;

}

BOOL CDNSLookup::UnInit()

{

    WSACleanup();

    return TRUE;

}

BOOL CDNSLookup::DNSLookupCore(ULONG ulDNSServerIP, char *szDomainName,

    std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList, ULONG ulTimeout, ULONG *pulTimeSpent)

{

    if (m_bIsInitOK == FALSE || szDomainName == NULL)

    {

        return FALSE;

    }

    //配置SOCKET

    sockaddr_in addrDnsServer;

    addrDnsServer.sin_family = AF_INET;

    addrDnsServer.sin_addr.s_addr = ulDNSServerIP;

    addrDnsServer.sin_port = htons(DNS_PORT);

    //DNS查询与解析

    if (!SendDNSRequest(addrDnsServer, szDomainName))

    {

        return FALSE;

    }

    if (!RecvDNSResponse(addrDnsServer, ulTimeout, pveculIPList, pvecstrCNameList, pulTimeSpent))

    {

        return FALSE;

    }

    return TRUE;

}

BOOL CDNSLookup::SendDNSRequest(sockaddr_in _addrDnsServer, char *_szDomainName)

{

    char bufSend[DNS_PACKET_MAX_SIZE] = {};

    char* pcSend = &bufSend[];

    //填充DNS查询报文头部

    DNSHeader *pDnsHdr = (DNSHeader*)pcSend;

    pDnsHdr->usTransID = (USHORT)GetCurrentProcessId();

    pDnsHdr->usFlags = htons(0x0100);

    pDnsHdr->usQuestionCount = htons(0x0001);

    pDnsHdr->usAnswerCount = 0x0000;

    pDnsHdr->usAuthorityCount = 0x0000;

    pDnsHdr->usAdditionalCount = 0x0000;

    //设置DNS查询报文内容

    USHORT usQType = htons(0x0001);

    USHORT usQClass = htons(0x0001);

    USHORT nDomainNameLen = strlen(_szDomainName);

    char *szEncodedDomainName = (char *)malloc(nDomainNameLen + );

    if (szEncodedDomainName == NULL)

    {

        return FALSE;

    }

    if (!EncodeDotStr(_szDomainName, szEncodedDomainName, nDomainNameLen + ))

    {

        return FALSE;

    }

    //填充DNS查询报文内容

    USHORT nEncodedDomainNameLen = strlen(szEncodedDomainName) + ;

    memcpy(pcSend += sizeof(DNSHeader), szEncodedDomainName, nEncodedDomainNameLen);

    memcpy(pcSend += nEncodedDomainNameLen, (char*)(&usQType), DNS_TYPE_SIZE);

    memcpy(pcSend += DNS_TYPE_SIZE, (char*)(&usQClass), DNS_CLASS_SIZE);

    free(szEncodedDomainName);

    //发送DNS查询报文

    USHORT nDNSPacketSize = sizeof(DNSHeader) + nEncodedDomainNameLen + DNS_TYPE_SIZE + DNS_CLASS_SIZE;

    if (sendto(m_sock, bufSend, nDNSPacketSize, , (sockaddr*)&_addrDnsServer, sizeof(_addrDnsServer)) == SOCKET_ERROR)

    {

        return FALSE;

    }

    return TRUE;

}

BOOL CDNSLookup::RecvDNSResponse(sockaddr_in _addrDnsServer, ULONG ulTimeout,

    std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList, ULONG *pulTimeSpent)

{

    if (pveculIPList != NULL)

        pveculIPList->clear();

    if (pvecstrCNameList != NULL)

        pvecstrCNameList->clear();

    char bufRecv[] = {  };

    char szDotName[] = {  };

    USHORT nEncodedNameLen = ;

    int iAddrDnsServer = sizeof(_addrDnsServer);

    DWORD dw1 = ::GetTickCount();

    //接收响应报文

    if (recvfrom(m_sock, bufRecv, sizeof(bufRecv), , (struct sockaddr*)&_addrDnsServer, &iAddrDnsServer) == SOCKET_ERROR)

    {

        //WSAETIMEDOUT

        printf("DNS recvfrom(...) failed : %d\n", ::GetLastError());

        return FALSE;

    }

    DNSHeader *pDnsHdr = (DNSHeader*)bufRecv;

    USHORT usQuestionCount = ;

    USHORT usAnswerCount = ;

    if (pDnsHdr->usTransID == (USHORT)GetCurrentProcessId()

        && (ntohs(pDnsHdr->usFlags) & 0xfb7f) == 0x8100 //RFC1035 4.1.1(Header section format)

        && (usQuestionCount = ntohs(pDnsHdr->usQuestionCount)) >=

        && (usAnswerCount = ntohs(pDnsHdr->usAnswerCount)) > )

    {

        char *pDNSData = bufRecv + sizeof(DNSHeader);

        //解析Question字段

        for (int q = ; q != usQuestionCount; ++q)

        {

            if (!DecodeDotStr(pDNSData, &nEncodedNameLen, szDotName, sizeof(szDotName)))

            {

                return FALSE;

            }

            pDNSData += (nEncodedNameLen + DNS_TYPE_SIZE + DNS_CLASS_SIZE);

        }

        //解析Answer字段

        for (int a = ; a != usAnswerCount; ++a)

        {

            if (!DecodeDotStr(pDNSData, &nEncodedNameLen, szDotName, sizeof(szDotName), bufRecv))

            {

                return FALSE;

            }

            pDNSData += nEncodedNameLen;

            USHORT usAnswerType = ntohs(*(USHORT*)(pDNSData));

            USHORT usAnswerClass = ntohs(*(USHORT*)(pDNSData + DNS_TYPE_SIZE));

            ULONG usAnswerTTL = ntohl(*(ULONG*)(pDNSData + DNS_TYPE_SIZE + DNS_CLASS_SIZE));

            USHORT usAnswerDataLen = ntohs(*(USHORT*)(pDNSData + DNS_TYPE_SIZE + DNS_CLASS_SIZE + DNS_TTL_SIZE));

            pDNSData += (DNS_TYPE_SIZE + DNS_CLASS_SIZE + DNS_TTL_SIZE + DNS_DATALEN_SIZE);

            if (usAnswerType == DNS_TYPE_A && pveculIPList != NULL)

            {

                ULONG ulIP = *(ULONG*)(pDNSData);

                pveculIPList->push_back(ulIP);

            }

            else if (usAnswerType == DNS_TYPE_CNAME && pvecstrCNameList != NULL)

            {

                if (!DecodeDotStr(pDNSData, &nEncodedNameLen, szDotName, sizeof(szDotName), bufRecv))

                {

                    return FALSE;

                }

                pvecstrCNameList->push_back(szDotName);

            }

            pDNSData += (usAnswerDataLen);

        }

    }

    DWORD dw2 = ::GetTickCount();

    if (pulTimeSpent != NULL)

        *pulTimeSpent = dw2 - dw1;

    return TRUE;

}

/*

 * convert "www.baidu.com" to "\x03www\x05baidu\x03com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 03 63 6f 6d 00 ff

 */

BOOL CDNSLookup::EncodeDotStr(char *szDotStr, char *szEncodedStr, USHORT nEncodedStrSize)

{

    USHORT nDotStrLen = strlen(szDotStr);

    if (szDotStr == NULL || szEncodedStr == NULL || nEncodedStrSize < nDotStrLen + )

    {

        return FALSE;

    }

    char *szDotStrCopy = new char[nDotStrLen + ];

    //strcpy_s(szDotStrCopy, nDotStrLen + 1, szDotStr);

    strcpy(szDotStrCopy, szDotStr);

    char *pNextToken = NULL;

    //char *pLabel = strtok_s(szDotStrCopy, ".", &pNextToken);

    char *pLabel = strtok(szDotStrCopy, ".");

    USHORT nLabelLen = ;

    USHORT nEncodedStrLen = ;

    while (pLabel != NULL)

    {

        if ((nLabelLen = strlen(pLabel)) != )

        {

            //sprintf_s(szEncodedStr + nEncodedStrLen, nEncodedStrSize - nEncodedStrLen, "%c%s", nLabelLen, pLabel);

            sprintf(szEncodedStr + nEncodedStrLen, "%c%s", nLabelLen, pLabel);

            nEncodedStrLen += (nLabelLen + );

        }

        //pLabel = strtok_s(NULL, ".", &pNextToken);

        pLabel = strtok(NULL, ".");

    }

    delete[] szDotStrCopy;

    return TRUE;

}

/*

 * convert "\x03www\x05baidu\x03com\x00" to "www.baidu.com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 03 63 6f 6d 00 ff

 * convert "\x03www\x05baidu\xc0\x13" to "www.baidu.com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 c0 13 ff ff ff ff

 * 0x0010 ff ff ff 03 63 6f 6d 00 ff ff ff ff ff ff ff ff

 */

BOOL CDNSLookup::DecodeDotStr(char *szEncodedStr, USHORT *pusEncodedStrLen, char *szDotStr, USHORT nDotStrSize, char *szPacketStartPos)

{

    if (szEncodedStr == NULL || pusEncodedStrLen == NULL || szDotStr == NULL)

    {

        return FALSE;

    }

    char *pDecodePos = szEncodedStr;

    USHORT usPlainStrLen = ;

    BYTE nLabelDataLen = ;

    *pusEncodedStrLen = ;

    while ((nLabelDataLen = *pDecodePos) != 0x00)

    {

        if ((nLabelDataLen & 0xc0) == ) //普通格式，LabelDataLen + Label

        {

            if (usPlainStrLen + nLabelDataLen +  > nDotStrSize)

            {

                return FALSE;

            }

            memcpy(szDotStr + usPlainStrLen, pDecodePos + , nLabelDataLen);

            memcpy(szDotStr + usPlainStrLen + nLabelDataLen, ".", );

            pDecodePos += (nLabelDataLen + );

            usPlainStrLen += (nLabelDataLen + );

            *pusEncodedStrLen += (nLabelDataLen + );

        }

        else //消息压缩格式，11000000 00000000，两个字节，前2位为跳转标志，后14位为跳转的偏移

        {

            if (szPacketStartPos == NULL)

            {

                return FALSE;

            }

            USHORT usJumpPos = ntohs(*(USHORT*)(pDecodePos)) & 0x3fff;

            USHORT nEncodeStrLen = ;

            if (!DecodeDotStr(szPacketStartPos + usJumpPos, &nEncodeStrLen, szDotStr + usPlainStrLen, nDotStrSize - usPlainStrLen, szPacketStartPos))

            {

                return FALSE;

            }

            else

            {

                *pusEncodedStrLen += ;

                return TRUE;

            }

        }

    }

    szDotStr[usPlainStrLen - ] = '\0';

    *pusEncodedStrLen += ;

    return TRUE;

}

　　（3）main.cpp

#include <iostream>//#define _CRT_SECURE_NO_WARNINGS

#define _WINSOCK_DEPRECATED_NO_WARNINGS

#include <stdio.h>

#include "CDNSLookup.h"

int main(void)

{

    //char szDomainName[] = "www.baidu.com";

    char szDomainName[] = "hopeex.com";

    std::vector<ULONG> veculIPList;

    std::vector<std::string> vecstrIPList;

    std::vector<std::string> vecCNameList;

    ULONG ulTimeSpent = ;

    CDNSLookup dnslookup;

    BOOL bRet = dnslookup.DNSLookup(inet_addr("114.114.114.114"), szDomainName, &vecstrIPList, &vecCNameList, , &ulTimeSpent);

    //BOOL bRet = dnslookup.DNSLookup(inet_addr("221.228.255.1"), szDomainName, &vecstrIPList, &vecCNameList, 1000, &ulTimeSpent);

    printf("DNSLookup result (%s):\n", szDomainName);

    if (!bRet)

    {

        printf("timeout!\n");

        return -;

    }

    for (int i = ; i != veculIPList.size(); ++i)

    {

        printf("IP%d(ULONG) = %u\n", i + , veculIPList[i]);

    }

    for (int i = ; i != vecstrIPList.size(); ++i)

    {

        printf("IP%d(string) = %s\n", i + , vecstrIPList[i].c_str());

    }

    for (int i = ; i != vecCNameList.size(); ++i)

    {

        printf("CName%d = %s\n", i + , vecCNameList[i].c_str());

    }

    printf("time spent = %ums\n", ulTimeSpent);

    return ;

}

　2.2、只解析IPv4地址的精简版代码

　　（1）dms_z.h

#pragma once

// C++实现DNS域名解析 - Snser - 博客园.html (https://www.cnblogs.com/snser/p/4101729.html)

#define _CRT_SECURE_NO_WARNINGS

#define _WINSOCK_DEPRECATED_NO_WARNINGS

//这里需要导入库 Ws2_32.lib，在不同的IDE下可能不太一样

//#pragma comment(lib, "Ws2_32.lib")

#include <winsock2.h>

#pragma comment(lib, "Ws2_32.lib")

//#include <windows.h>

#include <string>

#include <vector>

#define MAX_DOMAINNAME_LEN  255

#define DNS_PORT            53

#define DNS_TYPE_SIZE       2

#define DNS_CLASS_SIZE      2

#define DNS_TTL_SIZE        4

#define DNS_DATALEN_SIZE    2

#define DNS_TYPE_A          0x0001 //1 a host address

#define DNS_TYPE_CNAME      0x0005 //5 the canonical name for an alias

#define DNS_PACKET_MAX_SIZE (sizeof(DnsHeader) + MAX_DOMAINNAME_LEN + DNS_TYPE_SIZE + DNS_CLASS_SIZE)

struct DnsHeader

{

    USHORT usTransID; //标识符

    USHORT usFlags; //各种标志位

    USHORT usQuestionCount; //Question字段个数

    USHORT usAnswerCount; //Answer字段个数

    USHORT usAuthorityCount; //Authority字段个数

    USHORT usAdditionalCount; //Additional字段个数

};

class DnsLookup

{

public:

    DnsLookup();

    ~DnsLookup();

    ULONG LookupIPv4(const char* pcDnsServerIP, char *szDomainName,

        ULONG ulTimeout =  * );

private:

    BOOL Init();

    BOOL UnInit();

    BOOL LookupCore(ULONG ulDNSServerIP, char *szDomainName,

        std::vector<ULONG> *pveculIP, ULONG ulTimeOut, ULONG *pulTimeSpent);

    BOOL SendDNSRequest(sockaddr_in sockAddrDNSServer, char *szDomainName);

    BOOL RecvDNSResponse(sockaddr_in sockAddrDNSServer,

        std::vector<ULONG> *pveculIP, ULONG ulTimeOut, ULONG *pulTimeSpent);

    BOOL EncodeDotStr(char *szDotStr, char *szEncodedStr, USHORT nEncodedStrSize);

    BOOL DecodeDotStr(char *szEncodedStr, USHORT *pusEncodedStrLen, char *szDotStr, USHORT nDotStrSize, char *szPacketStartPos = NULL);

private:

    BOOL m_bIsInitOK;

    SOCKET m_sock;

};

　　（2）dns_z.cpp

#include "dns_z.h"

#include <stdio.h>

#include <string.h>

DnsLookup::DnsLookup() :

    m_bIsInitOK(FALSE),

    m_sock(INVALID_SOCKET)

{

    m_bIsInitOK = Init();

}

DnsLookup::~DnsLookup()

{

    UnInit();

}

ULONG DnsLookup::LookupIPv4(const char* pcDnsServerIP, char *szDomainName,

    ULONG ulTimeout)

{

    std::vector<ULONG> veculIP;

    ULONG ulTimeSpent;

    if (LookupCore(inet_addr(pcDnsServerIP), szDomainName, &veculIP, ulTimeout, &ulTimeSpent))

    {

        if (veculIP.size() > )

            return veculIP.front();

    }

    return ;

}

BOOL DnsLookup::Init()

{

    WSADATA wsaData;

    if (WSAStartup(MAKEWORD(, ), &wsaData) == SOCKET_ERROR)

    {

        printf("WSAStartup(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    if ((m_sock = socket(AF_INET, SOCK_DGRAM, )) == INVALID_SOCKET)

    {

        printf("socket(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    return TRUE;

}

BOOL DnsLookup::UnInit()

{

    WSACleanup();

    return TRUE;

}

BOOL DnsLookup::LookupCore(ULONG ulDNSServerIP, char *szDomainName,

    std::vector<ULONG> *pveculIP, ULONG ulTimeOut, ULONG *pulTimeSpent)

{

    if (m_bIsInitOK == FALSE || szDomainName == NULL)

        return FALSE;

    //配置SOCKET

    sockaddr_in addrDnsServer;

    addrDnsServer.sin_family = AF_INET;

    addrDnsServer.sin_addr.s_addr = ulDNSServerIP;

    addrDnsServer.sin_port = htons(DNS_PORT);

    //DNS查询与解析

    if (!SendDNSRequest(addrDnsServer, szDomainName))

        return FALSE;

    if (!RecvDNSResponse(addrDnsServer, pveculIP, ulTimeOut, pulTimeSpent))

        return FALSE;

    return TRUE;

}

BOOL DnsLookup::SendDNSRequest(sockaddr_in _addrDnsServer, char *_szDomainName)

{

    char bufSend[DNS_PACKET_MAX_SIZE] = {  };

    char* pcSend = &bufSend[];

    //填充DNS查询报文头部

    DnsHeader *pDnsHdr = (DnsHeader*)pcSend;

    pDnsHdr->usTransID = (USHORT)GetCurrentProcessId();

    pDnsHdr->usFlags = htons(0x0100);

    pDnsHdr->usQuestionCount = htons(0x0001);

    pDnsHdr->usAnswerCount = 0x0000;

    pDnsHdr->usAuthorityCount = 0x0000;

    pDnsHdr->usAdditionalCount = 0x0000;

    //设置DNS查询报文内容

    USHORT usQType = htons(0x0001);

    USHORT usQClass = htons(0x0001);

    USHORT nDomainNameLen = strlen(_szDomainName);

    char *szEncodedDomainName = (char *)malloc(nDomainNameLen + );

    if (szEncodedDomainName == NULL)

    {

        return FALSE;

    }

    if (!EncodeDotStr(_szDomainName, szEncodedDomainName, nDomainNameLen + ))

    {

        return FALSE;

    }

    //填充DNS查询报文内容

    USHORT nEncodedDomainNameLen = strlen(szEncodedDomainName) + ;

    memcpy(pcSend += sizeof(DnsHeader), szEncodedDomainName, nEncodedDomainNameLen);

    memcpy(pcSend += nEncodedDomainNameLen, (char*)(&usQType), DNS_TYPE_SIZE);

    memcpy(pcSend += DNS_TYPE_SIZE, (char*)(&usQClass), DNS_CLASS_SIZE);

    free(szEncodedDomainName);

    //发送DNS查询报文

    USHORT nDNSPacketSize = sizeof(DnsHeader) + nEncodedDomainNameLen + DNS_TYPE_SIZE + DNS_CLASS_SIZE;

    if (sendto(m_sock, bufSend, nDNSPacketSize, , (sockaddr*)&_addrDnsServer, sizeof(_addrDnsServer)) == SOCKET_ERROR)

    {

        return FALSE;

    }

    return TRUE;

}

BOOL DnsLookup::RecvDNSResponse(sockaddr_in _addrDnsServer,

    std::vector<ULONG> *_pveculIP, ULONG _ulTimeOut, ULONG *_pulTimeSpent)

{

    if (_pveculIP != NULL)

        _pveculIP->clear();

    if (setsockopt(m_sock, SOL_SOCKET, SO_RCVTIMEO, (char*)&_ulTimeOut, sizeof(_ulTimeOut)) == SOCKET_ERROR)

    {

        printf("setsockopt(...) err : %d\n", ::GetLastError());

        return FALSE;

    }

    char bufRecv[] = {  };

    char szDotName[] = {  };

    USHORT nEncodedNameLen = ;

    int iAddrDnsServer = sizeof(_addrDnsServer);

    DWORD dw1 = ::GetTickCount();

    //接收响应报文

    if (recvfrom(m_sock, bufRecv, sizeof(bufRecv), , (struct sockaddr*)&_addrDnsServer, &iAddrDnsServer) == SOCKET_ERROR)

    {

        //WSAETIMEDOUT

        printf("DNS recvfrom(...) failed : %d\n", ::GetLastError());

        return FALSE;

    }

    DnsHeader *pDnsHdr = (DnsHeader*)bufRecv;

    USHORT usQuestionCount = ;

    USHORT usAnswerCount = ;

    if (pDnsHdr->usTransID == (USHORT)GetCurrentProcessId()

        && (ntohs(pDnsHdr->usFlags) & 0xfb7f) == 0x8100 //RFC1035 4.1.1(Header section format)

        && (usQuestionCount = ntohs(pDnsHdr->usQuestionCount)) >=

        && (usAnswerCount = ntohs(pDnsHdr->usAnswerCount)) > )

    {

        char *pDnsData = bufRecv + sizeof(DnsHeader);

        //解析Question字段

        for (int q = ; q != usQuestionCount; ++q)

        {

            if (!DecodeDotStr(pDnsData, &nEncodedNameLen, szDotName, sizeof(szDotName)))

            {

                return FALSE;

            }

            pDnsData += (nEncodedNameLen + DNS_TYPE_SIZE + DNS_CLASS_SIZE);

        }

        //解析Answer字段

        for (int a = ; a != usAnswerCount; ++a)

        {

            if (!DecodeDotStr(pDnsData, &nEncodedNameLen, szDotName, sizeof(szDotName), bufRecv))

            {

                return FALSE;

            }

            pDnsData += nEncodedNameLen;

            USHORT usAnswerType = ntohs(*(USHORT*)(pDnsData));

            USHORT usAnswerClass = ntohs(*(USHORT*)(pDnsData + DNS_TYPE_SIZE));

            ULONG usAnswerTTL = ntohl(*(ULONG*)(pDnsData + DNS_TYPE_SIZE + DNS_CLASS_SIZE));

            USHORT usAnswerDataLen = ntohs(*(USHORT*)(pDnsData + DNS_TYPE_SIZE + DNS_CLASS_SIZE + DNS_TTL_SIZE));

            pDnsData += (DNS_TYPE_SIZE + DNS_CLASS_SIZE + DNS_TTL_SIZE + DNS_DATALEN_SIZE);

            if (usAnswerType == DNS_TYPE_A && _pveculIP != NULL)

            {

                ULONG ulIP = *(ULONG*)(pDnsData);

                _pveculIP->push_back(ulIP);

            }

            pDnsData += (usAnswerDataLen);

        }

    }

    DWORD dw2 = ::GetTickCount();

    if (_pulTimeSpent != NULL)

        *_pulTimeSpent = dw2 - dw1;

    return TRUE;

}

/*

 * convert "www.baidu.com" to "\x03www\x05baidu\x03com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 03 63 6f 6d 00 ff

 */

BOOL DnsLookup::EncodeDotStr(char *szDotStr, char *szEncodedStr, USHORT nEncodedStrSize)

{

    USHORT nDotStrLen = strlen(szDotStr);

    if (szDotStr == NULL || szEncodedStr == NULL || nEncodedStrSize < nDotStrLen + )

    {

        return FALSE;

    }

    char *szDotStrCopy = new char[nDotStrLen + ];

    //strcpy_s(szDotStrCopy, nDotStrLen + 1, szDotStr);

    strcpy(szDotStrCopy, szDotStr);

    char *pNextToken = NULL;

    //char *pLabel = strtok_s(szDotStrCopy, ".", &pNextToken);

    char *pLabel = strtok(szDotStrCopy, ".");

    USHORT nLabelLen = ;

    USHORT nEncodedStrLen = ;

    while (pLabel != NULL)

    {

        if ((nLabelLen = strlen(pLabel)) != )

        {

            //sprintf_s(szEncodedStr + nEncodedStrLen, nEncodedStrSize - nEncodedStrLen, "%c%s", nLabelLen, pLabel);

            sprintf(szEncodedStr + nEncodedStrLen, "%c%s", nLabelLen, pLabel);

            nEncodedStrLen += (nLabelLen + );

        }

        //pLabel = strtok_s(NULL, ".", &pNextToken);

        pLabel = strtok(NULL, ".");

    }

    delete[] szDotStrCopy;

    return TRUE;

}

/*

 * convert "\x03www\x05baidu\x03com\x00" to "www.baidu.com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 03 63 6f 6d 00 ff

 * convert "\x03www\x05baidu\xc0\x13" to "www.baidu.com"

 * 0x0000 03 77 77 77 05 62 61 69 64 75 c0 13 ff ff ff ff

 * 0x0010 ff ff ff 03 63 6f 6d 00 ff ff ff ff ff ff ff ff

 */

BOOL DnsLookup::DecodeDotStr(char *szEncodedStr, USHORT *pusEncodedStrLen, char *szDotStr, USHORT nDotStrSize, char *szPacketStartPos)

{

    if (szEncodedStr == NULL || pusEncodedStrLen == NULL || szDotStr == NULL)

    {

        return FALSE;

    }

    char *pDecodePos = szEncodedStr;

    USHORT usPlainStrLen = ;

    BYTE nLabelDataLen = ;

    *pusEncodedStrLen = ;

    while ((nLabelDataLen = *pDecodePos) != 0x00)

    {

        if ((nLabelDataLen & 0xc0) == ) //普通格式，LabelDataLen + Label

        {

            if (usPlainStrLen + nLabelDataLen +  > nDotStrSize)

            {

                return FALSE;

            }

            memcpy(szDotStr + usPlainStrLen, pDecodePos + , nLabelDataLen);

            memcpy(szDotStr + usPlainStrLen + nLabelDataLen, ".", );

            pDecodePos += (nLabelDataLen + );

            usPlainStrLen += (nLabelDataLen + );

            *pusEncodedStrLen += (nLabelDataLen + );

        }

        else //消息压缩格式，11000000 00000000，两个字节，前2位为跳转标志，后14位为跳转的偏移

        {

            if (szPacketStartPos == NULL)

            {

                return FALSE;

            }

            USHORT usJumpPos = ntohs(*(USHORT*)(pDecodePos)) & 0x3fff;

            USHORT nEncodeStrLen = ;

            if (!DecodeDotStr(szPacketStartPos + usJumpPos, &nEncodeStrLen, szDotStr + usPlainStrLen, nDotStrSize - usPlainStrLen, szPacketStartPos))

            {

                return FALSE;

            }

            else

            {

                *pusEncodedStrLen += ;

                return TRUE;

            }

        }

    }

    szDotStr[usPlainStrLen - ] = '\0';

    *pusEncodedStrLen += ;

    return TRUE;

}

　　（3）main.cpp

//#define _CRT_SECURE_NO_WARNINGS

#define _WINSOCK_DEPRECATED_NO_WARNINGS

#include <stdio.h>

#include "dns_z.h"

int main(void)

{

    //char szDomainName[] = "www.baidu.com";

    char szDomainName[] = "hopeex.com";

    DnsLookup dnslookup;

    ULONG ul = dnslookup.LookupIPv4("114.114.114.114", szDomainName, );

    //BOOL bRet = dnslookup.DNSLookup(inet_addr("221.228.255.1"), szDomainName, &vecstrIPList, &vecCNameList, 1000, &ulTimeSpent);

    in_addr* pAddr = (in_addr*)&ul;

    printf("%s\n", inet_ntoa(*pAddr));

    return ;

}

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