QUdpsocket send and receive broadcast in linux

[danics]

I try to write a QUdpsocket for read and write on braodcast address. following code work in windows with Qt5 library but don't work on my CentOs with Qt4.6, (i haven't any error in CentOs just don't read anything)

configure qudpsocket

iface = "";
 
    QList<QNetworkInterface> ifaces = QNetworkInterface::allInterfaces();
       foreach (QNetworkInterface nf, ifaces) {
           if(nf.name() == iface || iface.isNull() || iface.isEmpty())
           {
               if(nf.flags() & QNetworkInterface::CanBroadcast)
               {
                   foreach (QNetworkAddressEntry na, nf.addressEntries()) {
                       if(na.broadcast().toString() != "") {
                           send_to = na.broadcast().toIPv4Address();
                           bind_to = na.ip().toIPv4Address();
                           hostname = na.ip().toString();
                           if (verbose)
                               qDebug("using address=%s broadcast=%s",
                                          na.ip().toString().toLocal8Bit().data(), na.broadcast().toString().toLocal8Bit().data());
                           break;
                       }
                   }
               }
           }
           if(hostname != "") break;
       }
 
       if(bind_to)
       {
               broadcast.setAddress(send_to);
        #ifdef Q_OS_WIN
               QHostAddress address(bind_to);
        #elif defined(Q_OS_LINUX)
               QHostAddress address(send_to); // dont work on centos Qt 4.7
        #endif
 
               if(!udpsock.bind(address, port_nbr, QUdpSocket::ShareAddress
                         | QUdpSocket::ReuseAddressHint))
                      qFatal("bind udp");
 
               if(verbose)
                      qDebug() << "configured, hostname=" << hostname;
       }

iface = "";

    QList<QNetworkInterface> ifaces = QNetworkInterface::allInterfaces();
       foreach (QNetworkInterface nf, ifaces) {
           if(nf.name() == iface || iface.isNull() || iface.isEmpty())
           {
               if(nf.flags() & QNetworkInterface::CanBroadcast)
               {
                   foreach (QNetworkAddressEntry na, nf.addressEntries()) {
                       if(na.broadcast().toString() != "") {
                           send_to = na.broadcast().toIPv4Address();
                           bind_to = na.ip().toIPv4Address();
                           hostname = na.ip().toString();
                           if (verbose)
                               qDebug("using address=%s broadcast=%s",
                                          na.ip().toString().toLocal8Bit().data(), na.broadcast().toString().toLocal8Bit().data());
                           break;
                       }
                   }
               }
           }
           if(hostname != "") break;
       }

       if(bind_to)
       {
               broadcast.setAddress(send_to);
        #ifdef Q_OS_WIN
               QHostAddress address(bind_to);
        #elif defined(Q_OS_LINUX)
               QHostAddress address(send_to); // dont work on centos Qt 4.7
        #endif

               if(!udpsock.bind(address, port_nbr, QUdpSocket::ShareAddress
                         | QUdpSocket::ReuseAddressHint))
                      qFatal("bind udp");

               if(verbose)
                      qDebug() << "configured, hostname=" << hostname;
       }

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loop for listening

int handleUdp() {
       if(!udpsock.hasPendingDatagrams()) return -1;
       char data[1024];
       QHostAddress peerAddress;
 
       int rsize = udpsock.readDatagram(data, 1024, &peerAddress);
       if(rsize <= 0) return -1;
       .....
       return 0;
    }
 
    while(!terminated) {
        ...
        if(udpsock.waitForReadyRead(2000))
           handleUdp();
        ...
        // end of each loop broadcast data
        udpsock.writeDatagram(data(), broadcast, port_nbr)
    }

int handleUdp() {
       if(!udpsock.hasPendingDatagrams()) return -1;
       char data[1024];
       QHostAddress peerAddress;

       int rsize = udpsock.readDatagram(data, 1024, &peerAddress);
       if(rsize <= 0) return -1;
       .....
       return 0;
    }

    while(!terminated) {
        ...
        if(udpsock.waitForReadyRead(2000))
           handleUdp();
        ...
        // end of each loop broadcast data
        udpsock.writeDatagram(data(), broadcast, port_nbr)
    }

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this is debug information:

using address=123.123.1.14 broadcast=123.255.255.255
configured, hostname=123.123.1.14

[ChrisW67]

Why do you think you should bind(line 34) to a broadcast address (line 31) on the Linux machine?

[anda_skoa]

Check the port number.
Maybe you are accidentally trying to bind to a port < 1024

Cheers,
_

[danics]

Why do you think you should bind(line 34) to a broadcast address (line 31) on the Linux machine?

I read this in different forums for example here and see in other opensource too, however binding to IP address dont resolve problem anyway.

I listen to a port bigger than 5000

[danics]

I test this code in ubuntu with Qt 5 library and it works.

so i think maybe problem is network settings of my centos, actually my centos run in a virtual machine, anybody have a suggestion of changing settings of network for broadcast messages localy?

thanks in advance

[ChrisW67]

If you are using a virtual machine and you want it to talk to the outside world and accept connections/UDP from the outside world then you need to choose an appropriate virtual network interface option: usually "Bridged" or something like it. On the other hand, two virtual machines connected only to a shared VM-only network should require nothing special to talk to each other. Also need to check there is no default network firewalling going on in the OS.