How can I get the default gateway IP using QT ?

[aabc]

How can I get the default gateway IP using -QT- Qt ?

[ZapB]

I don't think Qt provides this functionality so you'll need to add some platform-specific code to do it yourself. "Here":http://www.linuxquestions.org/questions/linux-networking-3/howto-find-gateway-address-through-code-397078/#post2023303 is one way to do it on Linux.

[aabc]

Is there a way to write this linux code in QT (so it will be cross-platform)?

[kkrzewniak]

Use #ifdef-s to make sure your one platform code is compiled for that platform only.

[aabc]

But I wants it to also work on Windows

[ZapB]

Then you will need to google to find equivalent code for Windows and use platform specific #ifdef's to make sure only the relevent code gets compiled for your target platforms. The Windows code is bound ot be different since Windows and Posix systems have very different architectures.

This is essentially what Qt does. If you get something useful then I would recommend that you submit a merge request to get it included in Qt.

[Raketten]

I found the following code somewhere, it works for me

@int getDefaultGatewayCC(char *gatewayip, socklen_t size)
{
int found_gatewayip = 0;

struct nlmsghdr *nlMsg;
struct route_info *rtInfo;
char msgBuf[BUFSIZE]; // pretty large buffer

int sock,  msgSeq = 0;
int len;

/* Create Socket */
if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
{
    perror("Socket Creation: ");
    return(-1);
}

/* Initialize the buffer */
memset(msgBuf, 0, BUFSIZE);

/* point the header and the msg structure pointers into the buffer */
nlMsg = (struct nlmsghdr *)msgBuf;

/* Fill in the nlmsg header*/
nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); // Length of message.
nlMsg->nlmsg_type = RTM_GETROUTE; // Get the routes from kernel routing table .

nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST; // The message is a request for dump.
nlMsg->nlmsg_seq = msgSeq++; // Sequence of the message packet.
nlMsg->nlmsg_pid = getpid(); // PID of process sending the request.

/* Send the request */
if(send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0)
{
    fprintf(stderr, "Write To Socket Failed...\n");
    return -1;
}

/* Read the response */
if((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0)
{
    fprintf(stderr, "Read From Socket Failed...\n");
    return -1;
}

/* Parse and print the response */
rtInfo = (struct route_info *)malloc(sizeof(struct route_info));

for(;NLMSG_OK(nlMsg,len);nlMsg = NLMSG_NEXT(nlMsg,len))
{
    memset(rtInfo, 0, sizeof(struct route_info));
    parseRoutes(nlMsg, rtInfo);

    // Check if default gateway
    if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0")  && !strstr((char *)inet_ntoa(rtInfo->gateWay), "0.0.0.0"))
    {
        // copy it over
        inet_ntop(AF_INET, &rtInfo->gateWay, gatewayip, size);
        found_gatewayip = 1;
        break;
    }
}

free(rtInfo);
close(sock);

return found_gatewayip;

}@

[aabc]

What libraries did you use?
Do you have Win32 example also ?

[ZapB]

For a Win32 API implementation see MSDN and the "GetAdaptersInfo":http://msdn.microsoft.com/en-us/library/Aa365917 function.

[Anticross]

Another one platform-dependent solution for Windows using WinAPI:
@ char strHost[50];
WSADATA wsaData;
SOCKADDR_IN SockAddr;

    memset(strHost, 0 , 50);

    if(WSAStartup(0x0101, &wsaData) != 0)
        return m_myIP;

    gethostname(strHost, 50);

    hostent* HostInfo = gethostbyname(strHost);
    memcpy(&(SockAddr.sin_addr),HostInfo->h_addr,HostInfo->h_length);

    char* pIP = inet_ntoa(SockAddr.sin_addr);@

Now pIP holds your default IP address.

[Adrian_Eddy]

Tested on Windows 7, Debian 7 and Android:

#ifdef Q_OS_WIN
#include <wbemidl.h>
#include <comdef.h>
#include <windows.h>
#pragma comment(lib, "ws2_32.lib")
#pragma comment(lib, "Wbemuuid.lib")
#else
#define BUFSIZE 8192
#include <arpa/inet.h>
#include <net/if.h>
#include <sys/socket.h>
#include <linux/rtnetlink.h>
#include <unistd.h>

struct route_info {
  in_addr dstAddr;
  in_addr srcAddr;
  in_addr gateWay;
  char ifName[IF_NAMESIZE];
};

int readNlSock(int sockFd, char *bufPtr, unsigned int seqNum, int pId) {
  struct nlmsghdr *nlHdr;
  int readLen = 0, msgLen = 0;
  do {
    if ((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0) {
      perror("SOCK READ: ");
      return -1;
    }
    nlHdr = (struct nlmsghdr *)bufPtr;
    if ((NLMSG_OK(nlHdr, readLen) == 0) || (nlHdr->nlmsg_type == NLMSG_ERROR)) {
      perror("Error in received packet");
      return -1;
    }
    if (nlHdr->nlmsg_type == NLMSG_DONE)
      break;

    bufPtr += readLen;
    msgLen += readLen;

    if ((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0)
      break;

  } while ((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != (unsigned)pId));
  return msgLen;
}

void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo) {
  struct rtmsg *rtMsg = (struct rtmsg *)NLMSG_DATA(nlHdr);

  if ((rtMsg->rtm_family != AF_INET) || (rtMsg->rtm_table != RT_TABLE_MAIN))
    return;
  
  struct rtattr *rtAttr = (struct rtattr *)RTM_RTA(rtMsg);
  int rtLen = RTM_PAYLOAD(nlHdr);
  for (; RTA_OK(rtAttr, rtLen); rtAttr = RTA_NEXT(rtAttr, rtLen)) {
    switch (rtAttr->rta_type) {
      case RTA_OIF:
        if_indextoname(*(int *)RTA_DATA(rtAttr), rtInfo->ifName);
        break;
      case RTA_GATEWAY:
        rtInfo->gateWay = *(in_addr *)RTA_DATA(rtAttr);
        break;
      case RTA_PREFSRC:
        rtInfo->srcAddr = *(in_addr *)RTA_DATA(rtAttr);
        break;
      case RTA_DST:
        rtInfo->dstAddr = *(in_addr *)RTA_DATA(rtAttr);
        break;
    }
  }
}
#endif

class NetworkHelper {
public:
  static QString DefaultGateway() {
    QString ret;
#ifdef Q_OS_WIN
    CoInitialize(NULL);
    if (CoInitializeSecurity(NULL, -1, NULL, NULL, RPC_C_AUTHN_LEVEL_PKT, RPC_C_IMP_LEVEL_IMPERSONATE, NULL, EOAC_NONE, 0) != S_OK)
      return ret;

    IWbemLocator *pLoc = NULL;
    if (CoCreateInstance(CLSID_WbemAdministrativeLocator, NULL, CLSCTX_INPROC_SERVER | CLSCTX_LOCAL_SERVER, IID_IUnknown, (void **)&pLoc) != S_OK)
      return ret;

    IWbemServices *pSvc = NULL;
    if (pLoc->ConnectServer(L"root\\cimv2", NULL, NULL, NULL, 0, NULL, NULL, &pSvc) != S_OK)
      return ret;

    IEnumWbemClassObject *pEnumerator = NULL;
    HRESULT hr = pSvc->ExecQuery(L"WQL", L"SELECT * FROM Win32_NetworkAdapterConfiguration WHERE IPEnabled = 'TRUE'", WBEM_FLAG_FORWARD_ONLY | WBEM_FLAG_RETURN_IMMEDIATELY, NULL, &pEnumerator);
    if (FAILED(hr)) {
      pSvc->Release();
      pLoc->Release();
      CoUninitialize();
      return ret;
    }

    IWbemClassObject *pclsObj = NULL;
    while (pEnumerator && ret.isEmpty()) {
      _variant_t vaDefaultIPGateway;
      ULONG uReturn;
      hr = pEnumerator->Next(WBEM_INFINITE, 1, &pclsObj, &uReturn);
      if (!uReturn)
        break;

      hr = pclsObj->Get(L"DefaultIPGateway", 0, &vaDefaultIPGateway, NULL, NULL);
      if (hr == WBEM_S_NO_ERROR && vaDefaultIPGateway.vt != VT_NULL) {
        LONG lLow = 0;
        LONG lUp = 0;
        ::SafeArrayGetLBound(vaDefaultIPGateway.parray, 1, &lLow);
        ::SafeArrayGetUBound(vaDefaultIPGateway.parray, 1, &lUp);
        for (LONG i = lLow; i <= lUp; i++) {
          BSTR bsDefaultIPGateway;
          if (::SafeArrayGetElement(vaDefaultIPGateway.parray, &i, &bsDefaultIPGateway) == S_OK) {
            ret = QString::fromUtf16(reinterpret_cast<const ushort *>(bsDefaultIPGateway));
            ::SysFreeString(bsDefaultIPGateway);
            break;
          }
        }
      }
      VariantClear(&vaDefaultIPGateway);
    }

    if (pclsObj)
      pclsObj->Release();
    if (pEnumerator)
      pEnumerator->Release();
    if (pSvc)
      pSvc->Release();
    if (pLoc)
      pLoc->Release();
    CoUninitialize();
#else
    int sock, msgSeq = 0;
    if ((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0) {
      perror("Socket Creation: ");
      return ret;
    }

    char msgBuf[BUFSIZE];
    memset(msgBuf, 0, BUFSIZE);
    struct nlmsghdr *nlMsg = (struct nlmsghdr *)msgBuf;

    nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
    nlMsg->nlmsg_type = RTM_GETROUTE;

    nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
    nlMsg->nlmsg_seq = msgSeq++;
    nlMsg->nlmsg_pid = getpid();

    if (send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0)
      return ret;

    int len;
    if ((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0)
      return ret;

    struct route_info *rtInfo = (struct route_info *)malloc(sizeof(struct route_info));

    for (; NLMSG_OK(nlMsg, len); nlMsg = NLMSG_NEXT(nlMsg, len)) {
      memset(rtInfo, 0, sizeof(struct route_info));
      parseRoutes(nlMsg, rtInfo);

      if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0") && !strstr((char *)inet_ntoa(rtInfo->gateWay), "0.0.0.0")) {
        char buf[64];
        inet_ntop(AF_INET, &rtInfo->gateWay, buf, sizeof(buf));
        ret = QString(buf);
        break;
      }
    }

    free(rtInfo);
    close(sock);
#endif
    return ret;
  }
};

Usage:

QString addr = NetworkHelper::DefaultGateway();

[SGaist]

Hi and welcome to devnet,

Thanks for sharing your code. You should consider making it a wiki entry. Forum posts will get buried time passing by.

[saqefy]

@Mechan
on windows 8 doesn't work
for comipling replace:
#include <wbemidl.h>

to

Wbemcli.h

CLSID_WbemAdministrativeLocator to CLSID_WbemLocator
if (pLoc->ConnectServer(L"root\cimv2", NULL, NULL, NULL, 0, NULL, NULL, &pSvc) != S_OK)

to

if (pLoc->ConnectServer(BSTR (L"root\cimv2"), NULL, NULL, NULL, 0, NULL, NULL, &pSvc) != S_OK)

HRESULT hr = pSvc->ExecQuery(L"WQL", L"SELECT * FROM Win32_NetworkAdapterConfiguration WHERE IPEnabled = 'TRUE'", WBEM_FLAG_FORWARD_ONLY | WBEM_FLAG_RETURN_IMMEDIATELY, NULL, &pEnumerator);

to

HRESULT hr = pSvc->ExecQuery(BSTR (L"WQL"),BSTR ( L"SELECT * FROM Win32_NetworkAdapterConfiguration WHERE IPEnabled = 'TRUE'"), WBEM_FLAG_FORWARD_ONLY | WBEM_FLAG_RETURN_IMMEDIATELY, NULL, &pEnumerator);

after launching:
CoInitializeSecurity(NULL, -1, NULL, NULL, RPC_C_AUTHN_LEVEL_PKT, RPC_C_IMP_LEVEL_IMPERSONATE, NULL, EOAC_NONE, 0) never OK

if delete this check then uReturn is bad

[Taytoo]

@Adrian_Eddy said in How can I get the default gateway IP using QT ?:

Tested on Windows 7, Debian 7 and Android:

Does this work on OSX and Ubuntu?

[JonB]

@Taytoo
Since that post is from 2014 and he last posted in 2016, he probably will not be answering you....

[mrjj]

@Taytoo
well it used to work on Debian so high chance it will work
on Ubuntu too but time moved on and changes might be needed.
I have no idea about macOs but my guess would be no.
also the last post using
CoInitializeSecurity seems very windows focused.