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
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#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] = {'\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;
}
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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。
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