Qt/C++ - Lesson 065. The correspondence of HTTP errors to server response errors in QNetworkAccessManager

QNetworkReply, QNetworkError, QNetworkAccessManager

In one of the lessons, I worked with QNetworkAccessManager to get the content of the page from the site via the http protocol. There was made a check for errors, but no explanation was given as to what errors might be. For this, the QNetworkReply class provides an enum NetworkError , which lists the possible error codes.

If successful, NoError is returned, which is 0.

Otherwise, an error code is returned from the codes below:

QNetworkReply::ConnectionRefusedError

1

QNetworkReply::RemoteHostClosedError

2

QNetworkReply::HostNotFoundError

3

QNetworkReply::TimeoutError

4

QNetworkReply::OperationCanceledError

5

QNetworkReply::SslHandshakeFailedError

6

QNetworkReply::TemporaryNetworkFailureError

7

QNetworkReply::NetworkSessionFailedError

8

QNetworkReply::BackgroundRequestNotAllowedError

9

QNetworkReply::TooManyRedirectsError

10

QNetworkReply::InsecureRedirectError

11

QNetworkReply::ProxyConnectionRefusedError

101

QNetworkReply::ProxyConnectionClosedError

102

QNetworkReply::ProxyNotFoundError

103

QNetworkReply::ProxyTimeoutError

104

QNetworkReply::ProxyAuthenticationRequiredError

105

QNetworkReply::ContentAccessDenied

201

QNetworkReply::ContentOperationNotPermittedError

202

QNetworkReply::ContentNotFoundError

203

QNetworkReply::AuthenticationRequiredError

204

QNetworkReply::ContentReSendError

205

QNetworkReply::ContentConflictError

206

QNetworkReply::ContentGoneError

207

QNetworkReply::InternalServerError

401

QNetworkReply::OperationNotImplementedError

402

QNetworkReply::ServiceUnavailableError

403

QNetworkReply::ProtocolUnknownError

301

QNetworkReply::ProtocolInvalidOperationError

302

QNetworkReply::UnknownNetworkError

99

QNetworkReply::UnknownProxyError

199

QNetworkReply::UnknownContentError

299

QNetworkReply::ProtocolFailure

399

QNetworkReply::UnknownServerError

499

After studying this list of errors, we can conclude that this list combines a much larger list of errors than, for example, the list of error codes that the http server can return. Let's talk a little about possible errors and because of what they can arise.

QNetworkReply::ConnectionRefusedError and QNetworkReply::RemoteHostClosedError

If we talk about the interpretation of these errors, then the most striking example for me is connecting via SSH to another host. The meaning of the errors is that in the first case the server does not allow us, and in the second case the connection could be closed after a certain period of time.

Also, the ConnectionRefusedError error can occur when there is simply no service on a particular PC port that could process the request.

QNetworkReply::HostNotFoundError

Perhaps a fairly understandable error, which means that you just typed the wrong ip address or domain from which you are trying to get data. Also, this error can occur if the domain has ceased to be available on the network. This error is similar to error number 105 - ERR_NAME_NOT_RESOLVED (HTTP).

QNetworkReply::TimeoutError

The waiting interval is exceeded. The closest comparison is the use of the ICMP protocol, checking the availability of the node on the network when we make it ping. That is, there is a node in the network, but it does not respond.

QNetworkReply::OperationCanceledError

And here is an error that already applies to the actions of the program itself on Qt. In this case, it will occur when the program logic stops receiving data through the QNetworkAccessManager . That is, this error should not occur if the data receiving operation was interrupted from the outside. Therefore, if you catch such a mistake, then look for a problem inside your own program.

QNetworkReply::SslHandshakeFailedError

To establish encrypted communication channels that use SSL encryption, it is required to establish a connection through acknowledgment operations, that is, through acknowledgment of the reception / transmission of information. In this case, the encryption parameters are negotiated, the session key is transmitted, as well as optional server authentication operations by the client and the client by the server. If any of the above goes wrong, then this error will be thrown out.

QNetworkReply::TemporaryNetworkFailureError and QNetworkReply::NetworkSessionFailedError

The occurrence of these errors can be caused by any network problem until the physical connection to the network falls. You can check connections through the QNetworkInterface class.

The QNetworkInterface class has a static QList QNetworkInterface::allInterfaces() method that returns a list of all interfaces on your PC. This will take into account absolutely all connections, up to the connections that were created for the virtual machine, for example, for the Virtual Box . Here, by the way, there is a very interesting moment when software is being developed that should work on the network, and at the same time the developer has a virtual machine on board, especially if it is running, then this should be taken into account, because the program can try to get data through the connection With a virtual machine, that is, it will assume that the network is available, although the developer will test the User Case with the connection disconnected. However, even a disabled virtual machine will cause some problems, since the connection will be active, although Link will not be raised.

Therefore, to understand what's wrong, when you receive QNetworkReply::TemporaryNetworkFailureError and QNetworkReply::NetworkSessionFailedError errors, you will have to test the PC interfaces for activity: QNetworkInterface::IsUp and QNetworkInterface::IsRunning .

QNetworkReply::BackgroundRequestNotAllowedError

But this error depends directly on the policies of the platform for which the application is developed. For example, if some abstract Android device prohibits the exchange of information on the network in an energy-saving mode, then in this mode we will receive this error when trying to exchange information.

QNetworkReply::TooManyRedirectsError and QNetworkReply::InsecureRedirectError

But these errors from the category of new in Qt 5.6. Now you can monitor the excess of the number of redirects from site pages, as well as redirects from pages with https encryption to pages without encryption. What can be useful for developing software for website analysis.

Without additional settings, these errors usually do not occur. The point is that when QNetworkRequest is set and configuration flags are not set, then when you try to request a page from the site from which the redirect is done, there will be no transition to the new url, and QNetworkReply will not have any useful content. Therefore, you will need to set the QNetworkRequest flag in the QNetworkRequest::FollowRedirectsAttribute , then QNetworkAccessManager will go through the redirects until it receives the summary page or exceeds the limit on the number of transitions. By default, the number of transitions is limited to 50th.

The attribute can be set as follows:

QNetworkRequest request;  
request.setAttribute(QNetworkRequest::FollowRedirectsAttribute, QVariant(true));

As for QNetworkReply::InsecureRedirectError , the point is to track the redirect to an unsafe protocol.

Errors of QNetworkReply::Proxy*

The following five errors are similar in nature to server connection errors without proxying.

QNetworkReply::ContentAccessDenied

The documentation says that this error is similar to the 401 error in HTTP, that is, authorization is required, but I believe that this error can also occur in cases of receiving codes 403 and 407. Error 407 is similar to error 401, but is used for a proxy server. And error 403 is generative for errors 401 and 407.

QNetworkReply::AuthenticationRequiredError

This error is specific to the HTTP protocol. The fact is that the http protocol supports authentication schemes. In this case it is necessary to send a request to the server with the setting of login and password. Note that this directly means that you can write an application on Qt that will be authenticated on sites and perform certain actions on the site already in authorized mode, that is, you can write and bot to work on the site.

The solution to this problem is to set the user's login and password in QAuthenticator, which will mix the credentials to the request. And this will be done in automatic mode at the signal QNetworkAccessManager::authenticationRequired , and the login and password will be cached, so the signal will not be emitted at each request.

connect(&m_manager,&QNetworkAccessManager::authenticationRequired,
        [this](QNetworkReply *rep, QAuthenticator* auth){
            auth->setUser("username");
            auth->setPassword("passwordd");
        });

QNetworkReply::ContentReSendError

It is difficult to make any exact analogy with the HTTP codes for this error, since it can be caused by various reasons.

QNetworkReply::ContentConflictError

This error corresponds to a 409 Conflict error in the HTTP protocol.

QNetworkReply::ContentGoneError

This error corresponds to a 410 Gone error in the HTTP protocol.

QNetworkReply::InternalServerError

This error corresponds to 500 Internal Server Error in the http protocol.

QNetworkReply::OperationNotImplementedError

This error corresponds to a 501 Not Implemented error in the http protocol. It occurs when you try to execute, for example, a POST request to a URL where the server processes only GET requests.

This behavior can be found, for example, in the Django project. If the query is not implemented, the server with Django will send a blank page to the POST request with this code.

QNetworkReply::ProtocolUnknownError

An error such as QNetworkReply::ProtocolUnknownError can occur because a url was passed to the request without specifying the protocol type, that is, instead of

http://www.example.com

Was transferred

//www.example.com

This problem can arise if you parse the pages and extract the url of the images. For example, on this site the addresses of all images in the articles are as follows

/media/uploads/2017/02/03/testquickwidget.jpg

That is, the protocol and domain of the site are not specified. Therefore, if you extract all image URLs from a page, you will not be able to download images without additional processing, namely without the domain and protocol substitution at the beginning of the line, having received this error.

QNetworkReply::ServiceUnavailableError

Corresponds to the error 503 Service Unavailable in the http protocol

QNetworkReply::ProtocolInvalidOperationError

In this case, the protocol is defined, but the exact correspondence of the error from the http protocol is not traced, with respect to other protocols, it can also be anything.

QNetworkReply::UnknownNetworkError

The most common variant of what this error may correspond to is the 502 Bad Gateway error  in the http protocol

QNetworkReply::UnknownProxyError

There may also be a 502 error, but already in the version with a proxy server

Errors QNetworkReply::UnknownContentError,QNetworkReply::ProtocolFailure, QNetworkReply::UnknownServerError

With these errors, I find it difficult to find a working correspondence, I will be glad to help if anyone has encountered.

Example with error output in qDebug

You can check for an error when the request is terminated from the QNetworkAccessManager in the slot that is connected to process the result of the request.

manager = new QNetworkAccessManager(this);
connect(manager, &QNetworkAccessManager::finished, this, &Downloader::onResult);

In this case, there is a wrapper class over the QNetworkAccessManager class named Downloader . This class has an onResult slot, which in turn is responsible for processing the result of the QNetworkAccessManager response.

void Downloader::onResult(QNetworkReply *reply)
{
    // If an error occurred while receiving the data
    if(reply->error()){
        // We inform you about this and show information about errors
        qDebug() << "ERROR";
        // Here we get one of the enum NetworkError, that is, the error code
        qDebug() << reply->error();
    } else {
        // ToDo something
    }
}