Qt multicast works from Windows to Mac but not another around

Jerry Hou · wrote on 14 Jan 2016, 19:35

I use an experimental project to test Qt multicast. It's acting like a simple chat program within a local network. I'm able to send messages from Windows to Mac, but not from Mac to Windows. Please see the comment in Multicast.cpp.

Both of them join a predefined group. I used some hard coded IP to find the corresponding interface to bind to. I have also checked the multicast groups on both platforms using netstat and netsh and it show the predefined group on both sides.

It's not a firewall issue, since I turned it off and the issue persisted.

Qt version:5.5

Any suggestion what I should do next to diagnose this issue?

netsh output:

C:\Users\Jerry>netsh interface ip show joins

Interface 1: Loopback Pseudo-Interface 1

Scope       References  Last  Address
----------  ----------  ----  ---------------------------------
0                    1  Yes   224.0.0.251
0                    4  Yes   239.255.255.250

Interface 4: Ethernet

Scope       References  Last  Address
----------  ----------  ----  ---------------------------------
0                    0  Yes   224.0.0.1
0                    2  Yes   224.0.0.251
0                    1  Yes   224.0.0.252
0                    0  Yes   239.255.43.21
0                    4  Yes   239.255.255.250

Interface 8: Local Area Connection

Scope       References  Last  Address
----------  ----------  ----  ---------------------------------
0                    0  Yes   224.0.0.1
0                    2  Yes   224.0.0.251
0                    1  Yes   224.0.0.252
0                    4  Yes   239.255.255.250

netstat output

eve:~ Jerry$ netstat -g
Link-layer Multicast Group Memberships
Group               	Link-layer Address	Netif
1:0:5e:7f:2b:15     	<none>          	en0
1:0:5e:0:0:fb       	<none>          	en0
1:0:5e:0:0:1        	<none>          	en0
33:33:ff:c9:32:14   	<none>          	en0
33:33:0:0:0:fb      	<none>          	en0
33:33:ff:41:27:e    	<none>          	en0
33:33:0:0:0:1       	<none>          	en0
33:33:ff:33:5b:7a   	<none>          	en0
1:80:c2:0:0:3       	<none>          	en0
33:33:0:0:0:fb      	<none>          	en1
1:3:93:df:b:92      	<none>          	en1
33:33:0:0:0:fb      	<none>          	awdl0
33:33:80:0:0:fb     	<none>          	awdl0

IPv4 Multicast Group Memberships
Group               	Link-layer Address	Netif
224.0.0.251         	<none>          	lo0
224.0.0.1           	<none>          	lo0
239.255.43.21       	1:0:5e:7f:2b:15 	en0
224.0.0.251         	1:0:5e:0:0:fb   	en0
224.0.0.1           	1:0:5e:0:0:1    	en0

IPv6 Multicast Group Memberships
Group               	Link-layer Address	Netif
ff02::fb%lo0        	<none>          	lo0
ff02::2:ff33:9cc0%lo0	<none>          	lo0
ff01::1%lo0         	<none>          	lo0
ff02::1%lo0         	<none>          	lo0
ff02::1:ff00:1%lo0  	<none>          	lo0
ff02::1:ffc9:3214%en0	33:33:ff:c9:32:14	en0
ff02::fb%en0        	33:33:0:0:0:fb  	en0
ff01::1%en0         	33:33:0:0:0:1   	en0
ff02::2:ff41:270e%en0	33:33:ff:41:27:e	en0
ff02::1%en0         	33:33:0:0:0:1   	en0
ff02::1:ff33:5b7a%en0	33:33:ff:33:5b:7a	en0
ff02::fb%en1        	33:33:0:0:0:fb  	en1
ff02::fb%awdl0      	33:33:0:0:0:fb  	awdl0

Here is the source code

Multicast.h

#ifndef MULTICAST_H
#define MULTICAST_H

#include <QQuickItem>
#include <QHostAddress>
#include <QNetworkInterface>

class QUdpSocket;

class Multicast : public QQuickItem
{
	Q_OBJECT
public:
	Multicast();

	Q_INVOKABLE void multicast(QString s);

signals:
	void messageAdded(const QString msg);

private slots:
	void processPendingDatagrams();

private:
	QNetworkInterface getNetworkInterfaceByAddress(QString adr);
	void printNetworkInterfaceInfo(QNetworkInterface ni);
private:
	QHostAddress groupAddress;
	QUdpSocket *udpSocket;
};

#endif // MULTICAST_H

Multicast.cpp

#include "multicast.h"

#include <QtNetwork>

Multicast::Multicast()
{
    // hard coded group
	groupAddress = QHostAddress("239.255.43.21");
	udpSocket = new QUdpSocket(this);

	udpSocket->bind(QHostAddress::AnyIPv4, 45454, QUdpSocket::ShareAddress);
	// hard coded IP to find network interface. 10.0.1.40 for the other devices
    // if I don't manually set this interface, and set loopback to 1, 
    // this can receive the message from itself. Otherwise, it receives
    // nothing.
    udpSocket->setMulticastInterface(getNetworkInterfaceByAddress("10.0.1.39"));
	bool r = udpSocket->joinMulticastGroup(groupAddress);
	udpSocket->setSocketOption(QAbstractSocket::MulticastLoopbackOption, QVariant(1));
	QNetworkInterface intf(udpSocket->multicastInterface());
	printNetworkInterfaceInfo(intf);

	connect(udpSocket, SIGNAL(readyRead()),
			this, SLOT(processPendingDatagrams()));
}

QNetworkInterface Multicast::getNetworkInterfaceByAddress(QString adr)
{
	QList<QNetworkInterface> il(QNetworkInterface::allInterfaces());
	for (int i = 0; i < il.size(); i++)
	{
		QList<QNetworkAddressEntry> ade(il[i].addressEntries());
		for (int j = 0; j < ade.size(); j++)
		{
			if (ade[j].ip().toString() == adr)
				return il[i];
		}
	}

	return QNetworkInterface();
}

void Multicast::printNetworkInterfaceInfo(QNetworkInterface ni)
{
	qDebug() << ni.index() << ni.humanReadableName();
	QList<QNetworkAddressEntry> ade(ni.addressEntries());
	for (int j = 0; j < ade.size(); j++)
		qDebug() << ade[j].ip();
}

void Multicast::multicast(QString msg)
{
	QByteArray datagram = msg.toUtf8();
	udpSocket->writeDatagram(datagram.data(), datagram.size(),
							 groupAddress, 45454);
}

void Multicast::processPendingDatagrams()
{
	while (udpSocket->hasPendingDatagrams()) {
		QByteArray datagram;
		datagram.resize(udpSocket->pendingDatagramSize());
		udpSocket->readDatagram(datagram.data(), datagram.size());
		QString data(datagram);
		emit messageAdded(data);
	}
}

main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>

#include "multicast.h"

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);

	qmlRegisterType<Multicast>("com.multicast.test", 1, 0, "Multicast");

	QQmlApplicationEngine engine;
	engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

	return app.exec();
}

main.qml

import QtQuick 2.3
import QtQuick.Controls 1.2


import com.multicast.test 1.0

ApplicationWindow {
	id: applicationWindow1
	visible: true
	width: 640
	height: 480
	title: qsTr("Hello World")

	Multicast {
		id : multicast
	}

	menuBar: MenuBar {
		Menu {
			title: qsTr("File")
			MenuItem {
				text: qsTr("&Open")
				onTriggered: console.log("Open action triggered");
			}
			MenuItem {
				text: qsTr("Exit")
				onTriggered: Qt.quit();
			}
		}
	}

	 TextInput {
		 id: textInput1
		 y: 57
		 width: 450
		 height: 20
		 text: qsTr("Text Input")
		 anchors.left: parent.left
		 anchors.leftMargin: 50
		 font.pixelSize: 12
	 }

	 Button {
		 id: button1
		 y: 57
		 text: qsTr("Button")
		 anchors.left: textInput1.right
		 anchors.leftMargin: 20

		 onClicked: {
			textEdit1.append(textInput1.text)
			multicast.multicast(textInput1.text)
			textInput1.text = ""
		 }
	 }

	 TextEdit {
		 id: textEdit1
		 y: 106
		 height: 327
		 readOnly: true
		 anchors.left: parent.left
		 anchors.leftMargin: 50
		 anchors.right: parent.right
		 anchors.rightMargin: 50
		 font.pixelSize: 12

		 Connections {
			target: multicast
			onMessageAdded: {
				textEdit1.append(msg)
			}
		}
	}
}

Jerry Hou · wrote on 14 Jan 2016, 20:42

Using wireshark to monitor the traffic, the message from Mac dose arrive on Windows.

Jerry Hou · wrote on 14 Jan 2016, 22:28

Just found on Windows the multicast group in bound to interface 4 Ethernet. But my program multicastInterface output 8 Local Area Connection.

Do I need 2 separate sockets for receiving and sendin individually?

Jerry Hou · wrote on 14 Jan 2016, 23:40

The cause is the VirtualBox network card steals multicasting.This is a known issue in VirtualBox.

Disable the network interface or set a high Metic in settings solves the problem.

Now I need to find out if it's feasible to bind a socket on a specific interface.