Problem binding UDP (server) socket to an interface

  • I am writing a UDP server application that needs to know which interface received a request, so I can know what IP address to deliver in the response.

    To accomplish that, I am creating a separate UDP socket for each interface, and binding each socket to a specific interface address.

    Here is a piece of code so you can see what I am doing to initialize it:

    @PXEResponder::PXEResponder(QObject *parent)
    : QObject(parent)
    bool ok;

    // Get network interface list
    addresses = QNetworkInterface::allAddresses();
    signalMapper = new QSignalMapper(this);
    for (int i = 0; i < addresses.size(); ++i)
        // Only support IPv4
        if (addresses[i].protocol() != QUdpSocket::IPv4Protocol)
        qDebug() << addresses[i];
        // Create a UDP socket and bind it to port 67
        QUdpSocket *listener = new QUdpSocket(this);
        if (!listener->bind(addresses[i], 67, QUdpSocket::DefaultForPlatform))
        //if (!listener->bind(67))
            qDebug() << "Failed to bind DHCP listener";
        // Connect the readyRead signal to our slot
        ok = connect(listener, SIGNAL(readyRead()), signalMapper, SLOT(map()));
        signalMapper->setMapping(listener, i);
        listenerAddress.insert(listener, addresses[i]);
    ok = connect(signalMapper, SIGNAL(mapped(int)), this, SLOT(on_packet(int)));


    Everything completes without error, but I don't get any readyRead signals.


    • My program is running as root, so it is allowed to bind port 67
    • There is no DHCP server running on my machine
    • I am using the signal mapper so I can identify which QUdpSocket causes the event. That part works.
    • The "bind" call that doesn't work (the one that specifies an address) returns true.
    • Removing the "QUdpSocket::DefaultForPlatform" parameter doesn't help.
    • The qDebug() output (the interface address) looks correct.
    • If I comment-out the bind call that I'm trying to use, and uncomment the one that doesn't specify an interface address, everything works.

    Here's the weird part: when I bind to specific interfaces, I see the socket in the "netstat -a" output. Is there a reason I am not receiving broadcasts when bound to a specific interface?

    I found a partial answer:

    Linux won't receive broadcast packets when the socket is bound because it will only receive packets sent (directly) to the bound address. The solution is to use SO_BINDTODEVICE socket option and use INADDR_ANY as the bind address.

    Is there a way to bind to a network interface in Qt (without binding to the address of the interface)?

  • try this:

    #include <sys/socket.h>

    m_udpSocket = new QUdpSocket( this );
    if( m_udpSocket )
    connect( m_udpSocket, SIGNAL( readyRead() ), this, SLOT( SLOT_ondataReceived() ) );
    connect( m_udpSocket, SIGNAL( disconnected() ), this, SLOT( SLOT_disconnected() ) );

     retVal = m_udpSocket->bind( QHostAddress::Any, m_port );

    // set the socket option here.....
    const char dev[] = "eth0";
    if (0 != setsockopt(m_udpSocket->socketDescriptor(), SOL_SOCKET, SO_BINDTODEVICE, dev, sizeof(dev) ))



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