C++如何实现DNS域名解析

这片文章介绍了C++如何实现DNS域名解析，还有对相关技术的介绍，代码很详细,需要的朋友可以参考下

一、概述

现在来搞定DNS域名解析，其实这是前面一篇文章C++实现Ping里面的遗留问题，要干的活是ping的过程中画红线的部分：

cmd下域名解析的命令是nslookup，比如“nslookup www.baidu.com”的结果如下：

其中，Address返回的就是www.baidu.com对应的IP地址，这个可能有多个

Alias指别名，也就是说www.baidu.com是www.a.shifen.com的别名，而www.a.shifen.com则是www.baidu.com的规范名(Canonical Name，CName)，具体参考RFC1035 3.2.2 & wikipedia

二、实现结果预览

先看一下最终搞成了什么样子

输入：域名字符串

输出：IP列表、CName列表、DNS查询所用时间

三、相关技术

3.1、UDP or TCP ? (RFC1035 4.2)

UDP：DNS查询和回复采用低开销高性能的UDP，端口号为53。

TCP：辅助DNS服务器从主DNS服务器拉取最新数据时，采用可靠的TCP传输，端口号也为53。

我们这里做DNS查询采用UDP，53端口。

3.2、DNS查询/回复包头部解析 (RFC1035 4.1.1)

重点介绍一下我们关心的部分：

ID(16bits)：标识符，一般填入本进程的标识符

QR(1bits)：标志位，查询包为0，回复包为1

Opcode(4bits)：查询的种类，标准查询为0

QDCOUNT(16bits)：DNS查询/回复包数据部分Question字段的个数

ANCOUNT(16bits)：DNS查询/回复包数据部分Answer字段的个数

3.2、DNS查询/回复包数据部分解析 (RFC1035 4.1.2 & 4.1.3)

查询/回复包的数据部分依次为QDCOUNT个Question字段、ANCOUNT个Answer字段....

对于任意字段，其格式如下：

Name(不定长)：域名，这部分的格式比较复杂，后面单独说。

TYPE(16bits)：查询类型/回复包RDATA类型，比如TYPE=1表示主机IP地址、TYPE=5表示CNAME，详见RFC1035 3.2.2

CLASS(16bits)：类，一般情况下CLASS=1表示Internet，详见RFC1035 3.2.4

TTL(32bits，仅回复包)：生存时间

RDLENGTH(16bits，仅回复包)：RDATA部分的字节数

RDATA(不定长，仅回复包)：资源数据，具体格式取决于TYPE和CLASS，比如TYPE=1、CLASS=1时，RDATA为四个字节的IP地址

3.3、Name解析&消息压缩

3.3.1、一般格式 (RFC1035 4.1.2)

标签内容长度(1个字节) + 标签内容，以标签内容长度0作为Name的结束符，例如：

3.3.2、消息压缩格式 (RFC1035 4.1.4)

如果标签内容长度的二进制前两位是11，则表示消息压缩。

此时，标签内容长度1个字节+后面的1个字节一共16位，后14位表示相对DNS包起始地址的偏移(Byte)，例如：

上述例子中，DNS包起始地址为0x0000，c0 13的二进制为11000000 00010003，即跳转偏移为0x13个字节，对应的数据为03 63 6f 6d 00。

RFC1035中规定，支持的消息压缩规则为：

①以内容长度0结尾的标签序列

②偏移指针

③标签序列+偏移指针

也就是说，Name的消息压缩要求偏移指针必须在Name的尾部，且不支持同一级存在多个偏移指针(偏移指针序列)，

但Name的消息压缩支持嵌套的偏移指针，即指针指向的偏移位置仍然是以偏移指针结尾的数据

四、代码实现

#pragma once//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 <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 = 1000, ULONG *pulTimeSpent = NULL);BOOL DNSLookup(ULONG ulDNSServerIP, char *szDomainName, std::vector<std::string> *pvecstrIPList = NULL, std::vector<std::string> *pvecstrCNameList = NULL, ULONG ulTimeout = 1000, 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);ULONG GetTickCountCalibrate();private:BOOL m_bIsInitOK;SOCKET m_sock;WSAEVENT m_event;USHORT m_usCurrentProcID;char *m_szDNSPacket;
};[DNSLookup.h]#include "DNSLookup.h"
#include <stdio.h>
#include <string.h>CDNSLookup::CDNSLookup() : m_bIsInitOK(FALSE), m_sock(INVALID_SOCKET),m_szDNSPacket(NULL)
{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){return FALSE;}

std::vector<ULONG> veculIPList;
pveculIPList = &veculIPList;

BOOL bRet = DNSLookupCore(ulDNSServerIP, szDomainName, pveculIPList, pvecstrCNameList, ulTimeout, pulTimeSpent);if (bRet){pvecstrIPList->clear();char szIP[16] = {'\0'};for (std::vector<ULONG>::iterator iter = pveculIPList->begin(); iter != pveculIPList->end(); ++iter){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[0], pbyIPSegment[1], pbyIPSegment[2], pbyIPSegment[3]);pvecstrIPList->push_back(szIP);}}return bRet;
}BOOL CDNSLookup::Init()
{WSADATA wsaData;if (WSAStartup(MAKEWORD(2, 2), &wsaData) == SOCKET_ERROR){return FALSE;}if ((m_sock = socket(AF_INET, SOCK_DGRAM, 0)) == INVALID_SOCKET){return FALSE;}m_event = WSACreateEvent();WSAEventSelect(m_sock, m_event, FD_READ);m_szDNSPacket = new (std::nothrow) char[DNS_PACKET_MAX_SIZE];if (m_szDNSPacket == NULL){return FALSE;}m_usCurrentProcID = (USHORT)GetCurrentProcessId();return TRUE;
}BOOL CDNSLookup::UnInit()
{WSACleanup();if (m_szDNSPacket != NULL){delete [] m_szDNSPacket;}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 sockAddrDNSServer; sockAddrDNSServer.sin_family = AF_INET; sockAddrDNSServer.sin_addr.s_addr = ulDNSServerIP;sockAddrDNSServer.sin_port = htons( DNS_PORT );//DNS查询与解析if (!SendDNSRequest(sockAddrDNSServer, szDomainName)|| !RecvDNSResponse(sockAddrDNSServer, ulTimeout, pveculIPList, pvecstrCNameList, pulTimeSpent)){return FALSE;}return TRUE;
}BOOL CDNSLookup::SendDNSRequest(sockaddr_in sockAddrDNSServer, char *szDomainName)
{char *pWriteDNSPacket = m_szDNSPacket;memset(pWriteDNSPacket, 0, DNS_PACKET_MAX_SIZE); //填充DNS查询报文头部DNSHeader *pDNSHeader = (DNSHeader*)pWriteDNSPacket;pDNSHeader->usTransID = m_usCurrentProcID;pDNSHeader->usFlags = htons(0x0100);pDNSHeader->usQuestionCount = htons(0x0001);pDNSHeader->usAnswerCount = 0x0000;pDNSHeader->usAuthorityCount = 0x0000;pDNSHeader->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 + 2);if (szEncodedDomainName == NULL){return FALSE;}if (!EncodeDotStr(szDomainName, szEncodedDomainName, nDomainNameLen + 2)){return FALSE;}//填充DNS查询报文内容USHORT nEncodedDomainNameLen = strlen(szEncodedDomainName) + 1;memcpy(pWriteDNSPacket += sizeof(DNSHeader), szEncodedDomainName, nEncodedDomainNameLen);memcpy(pWriteDNSPacket += nEncodedDomainNameLen, (char*)(&usQType), DNS_TYPE_SIZE);memcpy(pWriteDNSPacket += 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, m_szDNSPacket, nDNSPacketSize, 0, (sockaddr*)&sockAddrDNSServer, sizeof(sockAddrDNSServer)) == SOCKET_ERROR){return FALSE;}return TRUE;
}BOOL CDNSLookup::RecvDNSResponse(sockaddr_in sockAddrDNSServer, ULONG ulTimeout, std::vector<ULONG> *pveculIPList, std::vector<std::string> *pvecstrCNameList, ULONG *pulTimeSpent)
{ULONG ulSendTimestamp = GetTickCountCalibrate();if (pveculIPList != NULL){pveculIPList->clear();}if (pvecstrCNameList != NULL){pvecstrCNameList->clear();}char recvbuf[1024] = {'\0'};char szDotName[128] = {'\0'};USHORT nEncodedNameLen = 0;while (TRUE){if (WSAWaitForMultipleEvents(1, &m_event, FALSE, 100, FALSE) != WSA_WAIT_TIMEOUT){WSANETWORKEVENTS netEvent;WSAEnumNetworkEvents(m_sock, m_event, &netEvent);if (netEvent.lNetworkEvents & FD_READ){ULONG ulRecvTimestamp = GetTickCountCalibrate();int nSockaddrDestSize = sizeof(sockAddrDNSServer);//接收响应报文if (recvfrom(m_sock, recvbuf, 1024, 0, (struct sockaddr*)&sockAddrDNSServer, &nSockaddrDestSize) != SOCKET_ERROR){DNSHeader *pDNSHeader = (DNSHeader*)recvbuf;USHORT usQuestionCount = 0;USHORT usAnswerCount = 0;if (pDNSHeader->usTransID == m_usCurrentProcID&& (ntohs(pDNSHeader->usFlags) & 0xfb7f) == 0x8100 //RFC1035 4.1.1(Header section format)&& (usQuestionCount = ntohs(pDNSHeader->usQuestionCount)) >= 0&& (usAnswerCount = ntohs(pDNSHeader->usAnswerCount)) > 0){char *pDNSData = recvbuf + sizeof(DNSHeader);//解析Question字段for (int q = 0; 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 = 0; a != usAnswerCount; ++a){if (!DecodeDotStr(pDNSData, &nEncodedNameLen, szDotName, sizeof(szDotName), recvbuf)){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), recvbuf)){return FALSE;}pvecstrCNameList->push_back(szDotName);}pDNSData += (usAnswerDataLen);}//计算DNS查询所用时间if (pulTimeSpent != NULL){*pulTimeSpent = ulRecvTimestamp - ulSendTimestamp;}break;}}}}//超时退出if (GetTickCountCalibrate() - ulSendTimestamp > ulTimeout){*pulTimeSpent = ulTimeout + 1;return FALSE;}}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 + 2){return FALSE;}char *szDotStrCopy = new char[nDotStrLen + 1];//strcpy_s(szDotStrCopy, nDotStrLen + 1, szDotStr);
 strcpy(szDotStrCopy, szDotStr);char *pNextToken = NULL;//char *pLabel = strtok_s(szDotStrCopy, ".", &pNextToken);char *pLabel = strtok(szDotStrCopy, ".");USHORT nLabelLen = 0;USHORT nEncodedStrLen = 0;while (pLabel != NULL){if ((nLabelLen = strlen(pLabel)) != 0){//sprintf_s(szEncodedStr + nEncodedStrLen, nEncodedStrSize - nEncodedStrLen, "%c%s", nLabelLen, pLabel);sprintf(szEncodedStr + nEncodedStrLen, "%c%s", nLabelLen, pLabel);nEncodedStrLen += (nLabelLen + 1);}//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 = 0;BYTE nLabelDataLen = 0; *pusEncodedStrLen = 0;while ((nLabelDataLen = *pDecodePos) != 0x00){if ((nLabelDataLen & 0xc0) == 0) //普通格式，LabelDataLen + Label
  {if (usPlainStrLen + nLabelDataLen + 1 > nDotStrSize){return FALSE;}memcpy(szDotStr + usPlainStrLen, pDecodePos + 1, nLabelDataLen);memcpy(szDotStr + usPlainStrLen + nLabelDataLen, ".", 1);pDecodePos += (nLabelDataLen + 1);usPlainStrLen += (nLabelDataLen + 1);*pusEncodedStrLen += (nLabelDataLen + 1);}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 = 0;if (!DecodeDotStr(szPacketStartPos + usJumpPos, &nEncodeStrLen, szDotStr + usPlainStrLen, nDotStrSize - usPlainStrLen, szPacketStartPos)){return FALSE;}else{*pusEncodedStrLen += 2;return TRUE;}}}szDotStr[usPlainStrLen - 1] = '\0';*pusEncodedStrLen += 1;return TRUE;
}ULONG CDNSLookup::GetTickCountCalibrate()
{static ULONG s_ulFirstCallTick = 0;static LONGLONG s_ullFirstCallTickMS = 0;SYSTEMTIME systemtime;FILETIME filetime;GetLocalTime(&systemtime); SystemTimeToFileTime(&systemtime, &filetime);LARGE_INTEGER liCurrentTime;liCurrentTime.HighPart = filetime.dwHighDateTime;liCurrentTime.LowPart = filetime.dwLowDateTime;LONGLONG llCurrentTimeMS = liCurrentTime.QuadPart / 10000;if (s_ulFirstCallTick == 0){s_ulFirstCallTick = GetTickCount();}if (s_ullFirstCallTickMS == 0){s_ullFirstCallTickMS = llCurrentTimeMS;}return s_ulFirstCallTick + (ULONG)(llCurrentTimeMS - s_ullFirstCallTickMS);
}[DNSLookup.cpp]#include <stdio.h>
#include <windows.h>
#include "DNSLookup.h"int main(void)
{char szDomainName[] = "www.baidu.com";std::vector<ULONG> veculIPList;std::vector<std::string> vecstrIPList;std::vector<std::string> vecCNameList;ULONG ulTimeSpent = 0;CDNSLookup dnslookup;BOOL bRet = dnslookup.DNSLookup(inet_addr("114.114.114.114"), szDomainName, &vecstrIPList, &vecCNameList, 1000, &ulTimeSpent);printf("DNSLookup result (%s):\n", szDomainName);if (!bRet){printf("timeout!\n");return -1;}for (int i = 0; i != veculIPList.size(); ++i){printf("IP%d(ULONG) = %u\n", i + 1, veculIPList[i]);}for (int i = 0; i != vecstrIPList.size(); ++i){printf("IP%d(string) = %s\n", i + 1, vecstrIPList[i].c_str());}for (int i = 0; i != vecCNameList.size(); ++i){printf("CName%d = %s\n", i + 1, vecCNameList[i].c_str());}printf("time spent = %ums\n", ulTimeSpent);return 0;
}