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 != NULL) { std::vector<ULONG> veculIPList; pveculIPList = &veculIPList; } BOOL bRet = DNSLookupCore(ulDNSServerIP, szDomainName, pveculIPList, pvecstrCNameList, ulTimeout, pulTimeSpent); if (bRet) { pvecstrIPList->clear(); char szIP[16] = {''}; 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] = {''}; char szDotName[128] = {''}; 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 "x03wwwx05baidux03com" * 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 "x03wwwx05baidux03comx00" to "www.baidu.com" * 0x0000 03 77 77 77 05 62 61 69 64 75 03 63 6f 6d 00 ff * convert "x03wwwx05baiduxc0x13" 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] = ''; *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) = %un", i + 1, veculIPList[i]); } for (int i = 0; i != vecstrIPList.size(); ++i) { printf("IP%d(string) = %sn", i + 1, vecstrIPList[i].c_str()); } for (int i = 0; i != vecCNameList.size(); ++i) { printf("CName%d = %sn", i + 1, vecCNameList[i].c_str()); } printf("time spent = %umsn", ulTimeSpent); return 0; }

以上就是C++实现DNS域名解析的全部内容，希望对大家的学习有所帮助。