UDP Communication issue

[Thixomag]

Hello

I’m making an application with an UDP client/server communication.
The client has to open the 3301 port on his router to receive the socket. (and the server has to open the 3300, but that is not importante)
And I would like to know how I could receive the socket on the PC's client without this opening port.

Server Side :
-Listening

m_udpSocket = new QUdpSocket;
m_udpSocket->bind(QHostAddress::Any, 3300, QUdpSocket::ShareAddress);
connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));

m_udpSocket = new QUdpSocket;
m_udpSocket->bind(QHostAddress::Any, 3300, QUdpSocket::ShareAddress);
connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));

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-Sending

QUdpSocket udpSocket;
udpSocket.bind(3301); // I don't know if that is important
udpSocket.writeDatagram(datagram, ip, 3301);

QUdpSocket udpSocket;
udpSocket.bind(3301); // I don't know if that is important
udpSocket.writeDatagram(datagram, ip, 3301);

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Client Side :
-Listening

m_serverAdress = new QHostAddress("**.**.***.**");
 
m_udpSocket = new QUdpSocket;
m_udpSocket->bind(QHostAddress::Any, 3301, QUdpSocket::ShareAddress); /// if I put *m_serverAdress instead of QHostAddress::Any, I receive nothing even with the port openning
connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));

m_serverAdress = new QHostAddress("**.**.***.**");

m_udpSocket = new QUdpSocket;
m_udpSocket->bind(QHostAddress::Any, 3301, QUdpSocket::ShareAddress); /// if I put *m_serverAdress instead of QHostAddress::Any, I receive nothing even with the port openning
connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));

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-Sending

m_udpSocket->writeDatagram(datagram, *m_serverAdress, 3300);

m_udpSocket->writeDatagram(datagram, *m_serverAdress, 3300);

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Thanks You

[wysota]

You can't. If it was possible, using firewalls wouldn't make sense because any application could have overriden it. The only thing you could do is to use a port that the firewall passes through like the HTTP port (TCP/80) but you need to have some mechanism that will proxy the traffic or you can set up a VPN connection but one way or another this requires a manual intervention somewhere.

[Thixomag]

Are you really sure ? Because I heard that it could work with some router.
And if I analyse with Wireshark (a network protocol analyzer) some application like Skype, I see that my PC can receive UDP socket without openning port.

Thanks you for your help

[Thixomag]

This method works with me and my co-developer :
Client side :

//Ecoute
    m_udpSocket = new QUdpSocket;
    m_udpSocket->bind(QHostAddress::Any, m_port, QUdpSocket::ShareAddress);
    connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));
 
// ...
 
//Envoi
    m_udpSocket->writeDatagram(datagram, *m_adresseServeur, m_port);

//Ecoute
    m_udpSocket = new QUdpSocket;
    m_udpSocket->bind(QHostAddress::Any, m_port, QUdpSocket::ShareAddress);
    connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));
 
// ...
 
//Envoi
    m_udpSocket->writeDatagram(datagram, *m_adresseServeur, m_port);

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Serveur side:

//Ecoute
    m_udpSocket = new QUdpSocket;
    m_udpSocket->bind(QHostAddress::Any, m_port, QUdpSocket::ShareAddress);
    connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));
 
// ...
 
//Envoi
    m_udpSocket->writeDatagram(datagram, ipCLient, m_port);

//Ecoute
    m_udpSocket = new QUdpSocket;
    m_udpSocket->bind(QHostAddress::Any, m_port, QUdpSocket::ShareAddress);
    connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(get()));
 
// ...
 
//Envoi
    m_udpSocket->writeDatagram(datagram, ipCLient, m_port);

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You have to use the same port and the same socket to receive and to send.
Ok, problem is clear.

[wysota]

Hmm? No, you don't have to use the same port, that's a false assumption

[Thixomag]

Ok, but I don't succeed with a lot of other method and test, with two ports. Maybe you know what I have to do in the code to succeed.

[wysota]

In my opinion this is not a matter of your code but of the router that drops the transmission.

[Thixomag]

But is it better to use two ports ?

[wysota]

And this proves... what? Apart from the fact that you didn't bind the socket anywhere, of course...

[Thixomag]

Yes sorry, I saw and I edited my post ^^

[wysota]

It's not that it's better or worse. You can do as many ports as you wish and this won't make your transmission to be accepted or dropped by the router (unless of course it is configured to do so, but this only works on a case by case basis).

[Thixomag]

Ok thanks.

Until now I do my test on simple application. Now I do the modification on my real application (a FPS) :

remplace

m_udpSocket->writeDatagram(datagram, ipCLient, m_port);

m_udpSocket->writeDatagram(datagram, ipCLient, m_port);

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instead of

QUdpSocket udpSocket;
udpSocket.writeDatagram(datagram, ipCLient, m_port);

QUdpSocket udpSocket;
udpSocket.writeDatagram(datagram, ipCLient, m_port);

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and when the server or the client run the line m_udpSocket->writeDatagram(datagram, ipCLient, m_port); the application stop working.
Maybe you have an idea ? I tested to do waitForBytesWritten but it change nothing

[wysota]

I bet dollars against nuts that your socket object goes out of scope before any data has a chance to leave it (in the second case).