When a proy is configured in Windows and should not be used by Qt:

QNetworkProxyFactory::setUseSystemConfiguration (false);

QTcpSocket fails to connect.

The problem is that in qnativesocketengine.cpp line 398 the system proxy is queried, regardless if the system configuration should be used:

        proxy = QNetworkProxyFactory::systemProxyForQuery(query).constFirst();

To make it work in Qt 5.9.0 I have to set the QNetworkProxy::NoProxy for every QTcpSocket:

    tcpSocket = new QTcpSocket(this);
    tcpSocket->setProxy(QNetworkProxy::NoProxy);    // new line inserted!

Seems to be triggered by patch

https://codereview.qt-project.org/#/c/186124/