We use Nginx server to download our files from. It supports all those cache control headers (etag, last-modified, if-modified-since, etc.)

We use QNetworkDiskCache.

For QNetworkRequest, we use this (just to make sure):

request.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferNetwork);

Steps to reproduce bug:

1. Download some resource. It will be cached.
2. Change resource on server.
3. Try to download it again. Bug: Qt does not even try to make any request to server to check if the resource is modified or not. It just uses cache as if PreferCache was specified.