Qt, implementing an SFTP client

[SPlatten]

@jsulm, thank you. As the development laptop is i386, how can I install the arm version of libssh so I can build it on the laptop then deploy to the target device?

[jsulm]

@splatten You have an SDK for your device, right? Maybe it provides the possibility to install additional packages?
If not you can build these libs for your device using the SDK.

[SPlatten]

@jsulm, I build on the laptop then deploy to the target, is the source available for libssh or libssh2 ?

[jsulm]

@splatten said in Qt, implementing an SFTP client:

is the source available for libssh or libssh2

yes, OpenSSH is open source

[SPlatten]

@jsulm , thank you, I downloaded open ssh source and will take a look now.

[SPlatten]

Slightly different approach....using QProcess

I plan to launch sftp and pass it the information for user name, address and port. Is there a way to grab the output from the console so I can look for the text "Password:" ?

[JonB]

@splatten
You are (presumably) using some third-party sftp.exe client? This is what I had to do in another (non-Qt) product. You should not have to parse output for things like host, port, username & password prompting. Look at the documentation for the sftp.exe you intend to use, there should/is likely to be command-line, or possibly take-from-file, for these?

[SPlatten]

@JonB , the sftp I'm using in present in the linux distribution on the device.

[JonB]

@splatten
OK, well I don't know about that! What about man sftp (host, port, user at least on command line)? On, say, Ubuntu I see:

 The final usage format allows for automated sessions using the -b option.
In such cases, it is necessary to configure non-interactive authentica‐
tion to obviate the need to enter a password at connection time (see
sshd(8) and ssh-keygen(1) for details).

Anyway, you use QProcess methods like https://doc.qt.io/qt-5/qprocess.html#readAllStandardOutput or (better) https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput signal to read from/write to the subprocess. You'll want to deal with all of stdin, stdout, & stderr, plus error signals, for proper control of your sub-process.

[SPlatten]

Here is my sample code:

QProcess* pProc = new QProcess(this);
QString strHost = clsMainWnd::strGetHostName()
       ,strPort = clsMainWnd::strGetPort()
       ,strUsername = clsMainWnd::strGetUsername();
QStringList slstCmdArgs;
slstCmdArgs << (strUsername + QString("@") + strHost)
            << (QString("-P") + strPort);
pProc->start("sftp", slstCmdArgs);
while( pProc->waitForStarted() != true ) {
}
QByteArray bytaryIn = pProc->readAllStandardOutput();
qDebug() << bytaryIn;

I've grabbed the content of slstCmdArgs and tried it in a terminal and it works, but in code I never see anything in "bytaryIn".

An example of what the code above does:

    sftp User@HostName -P22

What I am expecting to see is:

    User@HostName's password:

[Edit] I've now re-written this code to:

QString strHost = clsMainWnd::strGetHostName()
       ,strPort = clsMainWnd::strGetPort()
       ,strUsername = clsMainWnd::strGetUsername();
QStringList slstCmdArgs;
slstCmdArgs << (strUsername + QString("@") + strHost)
            << (QString("-P") + strPort);
mpSFTP = new QProcess(this);
connect(mpSFTP, SIGNAL(readyReadStandardOutput())
               ,this, SLOT(readSFTPoutput()));
mpSFTP->start("sftp", slstCmdArgs);

[jsulm]

@splatten said in Qt, implementing an SFTP client:

QByteArray bytaryIn = pProc->readAllStandardOutput();

Don't do it like this - you don't know whether there is already anything to read. Use https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput signal instead.

[SPlatten]

@jsulm, having re-written the code, my slot never gets called, meaning the signal can't be emitted.

[JonB]

@splatten
Which is why I said to use https://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput for better/proper behaviour....

There is a wrinkle you may have to face. It is possible that your command-line sftp executable will not prompt for password if it detects that the user is not in an interactive terminal (stdin). E.g. its code may quite possibly have

if (isatty(0))
    prompt_for_password();
else
   // do not prompt for password

That will be problematic if it is the case. You could try in a terminal sftp < /dev/null or sftp < file: if that does not still prompt for password you have a (potential) problem....

[SPlatten]

@JonB , would this help:

    mpSFTP->setProcessChannelMode(QProcess::MergedChannels);

[JonB]

@splatten
Help what, in what way? If the sftp requires a terminal stdin for password prompt, then no, if, say, you have a purely interactive Qt application. You can try it. But the first thing to test is whether your sftp behaves differently if its stdin is not attached to a terminal?

[SPlatten]

Now the slot gets called and in my slot I have:

    qDebug() << "readAllStandardOutput:";
    qDebug() << mpSFTP->readAllStandardOutput();
    qDebug() << "readChannel:";
    qDebug() << mpSFTP->readChannel();

In the output I see:

    readAllStandardOutput:
    "ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory
    "
    readChannel:
    0

[jsulm]

@splatten What happens if you execute exact same command manually in a terminal?
Also connect a slot to the error signal from QProcess and check whether you get any errors.

[SPlatten]

@jsulm , When I execute "sftp" with the command line arguments, it works without any issues.

[JonB]

@splatten
As I have said before, so last time now:

ssh_askpass: exec(/usr/local/libexec/ssh-askpass): No such file or directory

This is because your sftp is not willing/able to access stdin to get a password. Try what I said about seeing how your sftp behaves with input/output redirected when run from a terminal. I will not comment again, and leave it to you.

[SPlatten]

ssh_askpass is not available on this linux distribution.