Problem downloading file from FTP using QNetworkAccessManager::get() function

Question

This is my first request.. :)

I am having some problem using the QT framework and downloading a single file (12kb long) from a FTP server. Some info regarding the problem:

• the environment is linux embedded (yocto) using QT 5.10
• the network connection of the device is ethernet, via LTE connection(quectel modem mc25)
• the ftp server is with no tlc and no ssl protection (customer's ftp server..)
• the server port is 2121
• the same code works using a normal ethernet (RJ cable) connection
• the same LTE connection via modem , works using "wget" linux tool for downloading.
  
• the ftp client mode is passive (I have read wget output)
• trying on different ftp server (address and server are different) I have never experienced problem, the same code works in other ftp server (for sure with tlc and ssl protection), also using LTE modem.
• the problem is only on that ftp , also uploading the file.

What I experience is that I have no error message from QNetworkAccessManager, nor from QNetworkRequest object. Simply no anser, no error. Capturing QmetaObjectChange signal, I see only the code 200 from the server and the size of the file. Stop.

Any Ideas that can help me to find the reason why I experiment this strange behaviour? Thanks for your help.

Alberto.

Answer 1

I finally understood the problem.
Openssl library has intentionally disabled the use of obsolete and dangerous protocols like sslv2.That's why QT cannot download the file from that particular ftp server: the server uses sslv2.
Using obsolete protocols like sslv2 introduce the possibility that hackers can obtain sensible data intercepting your network session.
I think the best solution is to avoid my software to connect to such kind of ftp servers, I do not want to allow security problem also on my system.I will prompt an error message like : "error: the ftp server use an osolete and dangerous protocol(sslv2). Connection not allowed."
You can connect a slot to signal in order to get ssl warnings..

void DownloadManager::doDownload(const QUrl &url)
{
    QNetworkRequest request(url);
    QNetworkReply *reply = manager.get(request);

#if QT_CONFIG(ssl)
    connect(reply, SIGNAL(sslErrors(QList<QSslError>)),
            SLOT(sslErrors(QList<QSslError>)));
#endif

    currentDownloads.append(reply);
}

https://code.qt.io/cgit/qt/qtbase.git/tree/examples/network/download/main.cpp?h=5.14

BR! : )