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Setting DNS using iphelp and register, DhcpNotifyConfigChange and DnsFlushResolverCache

, 8 Oct 2007 CPOL
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Setting DNS using iphelp and register, DhcpNotifyConfigChange and DnsFlushResolverCache


This is one way of changing DNS in C++ using Iphlpapi.h, register, and two undocumented Windows APIs. It's a simple program, no UI for you, sorry...

  1. Use GetAdaptersInfo to get adapters information.
  2. Then use GetPerAdapterInfo to get information for every ethernet network card. The information contains DNS list.
  3. Use register to set the DNS by open HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\Tcpip\\Parameters\\
    , the name is got from GetAdaptersInfo.
  4. Use DhcpNotifyConfigChange to notify the IP change.
  5. Use DnsFlushResolverCache to flush the DNS.

Using the Code

  1. Use GetAdaptersInfo:
    if( GetAdaptersInfo(pAdapterInfo, &ulAdapterInfoSize) == 
    			ERROR_BUFFER_OVERFLOW ) // out of buff
      delete pAdapterInfo;
      pAdapterInfo = (IP_ADAPTER_INFO*)new char[ulAdapterInfoSize];
      pAdapterInfoBkp = pAdapterInfo;
    if( GetAdaptersInfo(pAdapterInfo, &ulAdapterInfoSize) == ERROR_SUCCESS )
        if(pAdapterInfo->Type == MIB_IF_TYPE_ETHERNET) // If type is etherent
            //Here to use GetPerAdapterInfo 
        pAdapterInfo = pAdapterInfo->Next;
  2. Use GetPerAdapterInfo:
    if( GetPerAdapterInfo(
        &ulPerAdapterInfoSize) == ERROR_BUFFER_OVERFLOW ) // out of buff
        delete pPerAdapterInfo;
        pPerAdapterInfo = (IP_PER_ADAPTER_INFO*)new char[ulPerAdapterInfoSize];
        pPerAdapterInfoBkp = pPerAdapterInfo;
    DWORD dwRet;
    if((dwRet = GetPerAdapterInfo(
        &ulPerAdapterInfoSize)) == ERROR_SUCCESS)
        AdaptInfo tmpadpif;
        pAddrStr = pPerAdapterInfo->DnsServerList.Next;
        strncpy(tmpadpif.AdapterName, pAdapterInfo->AdapterName, 
        tmpadpif.NameSever = pPerAdapterInfo->DnsServerList.IpAddress.String;
        char tmp[16];
        strcpy( tmp, "");
        if ( strcmp(pPerAdapterInfo->DnsServerList.IpAddress.String, tmp) == 0 )
            return 0;
            tmpadpif.NameSever += ",";
            tmpadpif.NameSever += pAddrStr->IpAddress.String;
            pAddrStr = pAddrStr->Next; 
        vecAdpif.push_back( tmpadpif );
        RegSetDNS( tmpadpif.AdapterName, tmp);
  3. Open registry to set DNS or IP addr. For this part, please refer to the download code.
  4. We have something fresh here:
    typedef int (CALLBACK* DNSFLUSHPROC)();
  5. After setting DNS in registy, we need to inform the system about the change. Now we use Windows undocumented API: DhcpNotifyConfigChange. It is located in dhcpcsvc.dll.
    BOOL DhcpNotifyConfigChange(
    LPWSTR lpwszServerName, 	// local machine should be NULL
    LPWSTR lpwszAdapterName, 	// Adapt name
    BOOL bNewIpAddress, 	// TRUE indicates changing ip
    DWORD dwIpIndex, 		// which IP addr, if only 1, it's 0
    DWORD dwIpAddress, 	// IP addr
    DWORD dwSubNetMask, 	// mask
    int nDhcpAction ); // DHCP, 0 for not change, 1 for enable, 2 for disable DHCP

    Please refer to the code.

  6. Flush DNS, use another Windows undocumented API: DnsFlushResolverCache, located in dnsapi.dll.
  7. It's ok now.

Points of Interest

I have not learned how to use WMI in VC++. Sigh, it's not a good way to change Ipaddr and DNS in registy. netsh is not good too. If anyone has a better idea, please share with us.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralMy vote of 5 PinmemberHarly24-Sep-11 3:19 
QuestionVisual C++ 2010 Express PinmemberMember 215751130-Jul-10 12:55 
AnswerRe: Visual C++ 2010 Express Pinmemberarabcoder28-Jan-11 9:05 
QuestionHi all.. does the "DhcpNotifyConfigChange" function work for ipv6 also? PinmemberG Haranadh27-Jul-10 4:08 
GeneralUm... PinmemberSnuff Daddy6-Oct-07 22:07 
GeneralRe: Um... PinmemberNescot8-Oct-07 16:54 

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