Under Windows7 QUdpSocket::joinMulticastGroup(QHostAddress, QNetworkInterface) does not works correctly on interfaces that has IPv6 and IPv4 addresses simultaneously. It is because of the qnativesocketengine_win.cpp lines 990 - 991. You take only first address and if it IPv6 address that could not be converted to the IPv4 then join will be done to the default windows IPv4 multicast interface. And it could be not that interface that has been specified on function call!