How to get local IP address of a computer using QT

Question

I am trying to get the local ip address (IPV4) of a computer in QT. I found the following code:

QNetworkInterface *inter = new QNetworkInterface();
QList<QHostAddress> list;
list=inter->allAddresses();
QString str;

for (int i = 0; i < list.size(); ++i) {
     str = list.at(i).toString();
}

Going through for loop I can see there are a number of values (ip's) in the list, one of them is the actual local ip address that I get by typing ipconfig in a command window.

My question is how to distinguish the ip address from all the ip's that are in list?

Answer 1

PCs often have more than one IP address. There's not really such a thing as "the" local IP address; the IP address which would be used when connecting to some remote host depends at least on the local routing table (which may change drastically at any time, e.g. when starting/stopping VPN software).

It seems to me that it makes more sense to think about IPs as valid only in the context of remote networks, e.g. "this is the local IP address I'd use if I were to connect to this host on the Internet; but this is the local IP address I'd use to connect to this host over my company's VPN".

If you want to find out the local IP address which would be used for general-purpose Internet connectivity, the most accurate way I know is simply to do a connection test to a representative host (and a host with high reliability!)

QTcpSocket socket;
socket.connectToHost("8.8.8.8", 53); // google DNS, or something else reliable
if (socket.waitForConnected()) {
    qDebug()
        << "local IPv4 address for Internet connectivity is"
        << socket.localAddress();
} else {
    qWarning()
        << "could not determine local IPv4 address:"
        << socket.errorString();
}

Note: the above example is blocking, you probably want to rewrite it to use signals and slots if your app has a UI.

Answer 2

I think, several attempts should be tried for increasing the chance of the GUESS (Regardless how clever the software is, it would be still a guess, which won't cover 1% of configurations which will be possibly you case :-)

I've combined and extended both solutions. First I'd check for google DNS, and then for local IPs having a standard gateway. The assumption is: Getaway has the same mask with the addreess ending with ".1". I couldn't find out, how to obtain std. gateway in Qt (which would be more reliable).

Here is the code which works ON MY COMPUTERS:

    QTcpSocket dnsTestSocket;   
    QString localIP="127.0.0.1";    //fall back
    QString googleDns = "8.8.8.83";  //try google DNS or sth. else reliable first
    dnsTestSocket.connectToHost(googleDns, 53);
    if (dnsTestSocket.waitForConnected(3000)) 
    {
        localIP = dnsTestSocket.localAddress().toString();
    } 
    else 
    {
        foreach (const QHostAddress &address, QNetworkInterface::allAddresses())
        {
            QString guessedGatewayAddress = address.toString().section( ".",0,2 ) + ".1";

            if (address.protocol() == QAbstractSocket::IPv4Protocol 
                && address != QHostAddress(QHostAddress::LocalHost)
                )
            {
                dnsTestSocket.connectToHost(guessedGatewayAddress, 53);
                if (dnsTestSocket.waitForConnected(3000))
                {
                    localIP = dnsTestSocket.localAddress().toString();
                    break;
                }
            }   
        }
    }