一、概述
現在來搞定DNS域名解析，其實這是前面一篇文章C++實現Ping里面的遺留問題，要干的活是ping的過程中畫紅線的部分：
cmd下域名解析的命令是nslookup，比如“nslookup www.baidu.com”的結果如下：
其中，Address返回的就是www.baidu.com對應的IP地址，這個可能有多個
Alias指別名，也就是說www.baidu.com是www.a.shifen.com的別名，而www.a.shifen.com則是www.baidu.com的規(guī)范名(Canonical Name，CName)，具體參考RFC1035 3.2.2 & wikipedia
二、實現結果預覽
先看一下最終搞成了什么樣子
輸入：域名字符串
輸出：IP列表、CName列表、DNS查詢所用時間
三、相關技術
3.1、UDP or TCP ? (RFC1035 4.2)
UDP：DNS查詢和回復采用低開銷高性能的UDP，端口號為53。
TCP：輔助DNS服務器從主DNS服務器拉取最新數據時，采用可靠的TCP傳輸，端口號也為53。
我們這里做DNS查詢采用UDP，53端口。
3.2、DNS查詢/回復包頭部解析 (RFC1035 4.1.1)
重點介紹一下我們關心的部分：
ID(16bits)：標識符，一般填入本進程的標識符
QR(1bits)：標志位，查詢包為0，回復包為1
Opcode(4bits)：查詢的種類，標準查詢?yōu)?
QDCOUNT(16bits)：DNS查詢/回復包數據部分Question字段的個數
ANCOUNT(16bits)：DNS查詢/回復包數據部分Answer字段的個數
3.2、DNS查詢/回復包數據部分解析 (RFC1035 4.1.2 & 4.1.3)
查詢/回復包的數據部分依次為QDCOUNT個Question字段、ANCOUNT個Answer字段....
對于任意字段，其格式如下：
Name(不定長)：域名，這部分的格式比較復雜，后面單獨說。
TYPE(16bits)：查詢類型/回復包RDATA類型，比如TYPE=1表示主機IP地址、TYPE=5表示CNAME，詳見RFC1035 3.2.2
CLASS(16bits)：類，一般情況下CLASS=1表示Internet，詳見RFC1035 3.2.4
TTL(32bits，僅回復包)：生存時間
RDLENGTH(16bits，僅回復包)：RDATA部分的字節(jié)數
RDATA(不定長，僅回復包)：資源數據，具體格式取決于TYPE和CLASS，比如TYPE=1、CLASS=1時，RDATA為四個字節(jié)的IP地址
3.3、Name解析&消息壓縮
3.3.1、一般格式 (RFC1035 4.1.2)
標簽內容長度(1個字節(jié)) + 標簽內容，以標簽內容長度0作為Name的結束符，例如：
3.3.2、消息壓縮格式 (RFC1035 4.1.4)
如果標簽內容長度的二進制前兩位是11，則表示消息壓縮。
此時，標簽內容長度1個字節(jié)+后面的1個字節(jié)一共16位，后14位表示相對DNS包起始地址的偏移(Byte)，例如：
上述例子中，DNS包起始地址為0x0000，c0 13的二進制為11000000 00010003，即跳轉偏移為0x13個字節(jié)，對應的數據為03 63 6f 6d 00。
RFC1035中規(guī)定，支持的消息壓縮規(guī)則為：
①以內容長度0結尾的標簽序列
②偏移指針
③標簽序列+偏移指針
也就是說，Name的消息壓縮要求偏移指針必須在Name的尾部，且不支持同一級存在多個偏移指針(偏移指針序列)，
但Name的消息壓縮支持嵌套的偏移指針，即指針指向的偏移位置仍然是以偏移指針結尾的數據
四、代碼實現
#pragma once//這里需要導入庫 Ws2_32.lib，在不同的IDE下可能不太一樣//#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; //標識符 USHORT usFlags; //各種標志位 USHORT usQuestionCount; //Question字段個數 USHORT usAnswerCount; //Answer字段個數 USHORT usAuthorityCount; //Authority字段個數 USHORT usAdditionalCount; //Additional字段個數};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] = {'/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查詢與解析 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查詢報文頭部 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; //設置DNS查詢報文內容 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查詢報文內容 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); //發(fā)送DNS查詢報文 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); //接收響應報文 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); } //計算DNS查詢所用時間 if (pulTimeSpent != NULL) { *pulTimeSpent = ulRecvTimestamp - ulSendTimestamp; } break; } } } } //超時退出 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 //消息壓縮格式，11000000 00000000，兩個字節(jié)，前2位為跳轉標志，后14位為跳轉的偏移 { 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;}以上就是C++實現DNS域名解析的全部內容，希望對大家的學習有所幫助。
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