Problem binding UDP (server) socket to an interface

doug65536

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);
listeners.reserve(interfaces.size());

for (int i = 0; i < addresses.size(); ++i)
{
    // Only support IPv4
    if (addresses[i].protocol() != QUdpSocket::IPv4Protocol)
        continue;

    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]);
    listeners.push_back(listener);
}

ok = connect(signalMapper, SIGNAL(mapped(int)), this, SLOT(on_packet(int)));

}
@

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

NOTE:

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)?

bagpiper

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) ))
{

}@