How can I get the default gateway IP using 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.