netcat doen't see but tcpdump see datagrams sent by qt-client | might be flow in app logic

[ainu]

Hi, i am working on a program which intends to send datagrams to a particular port ( port 8881 ). The program seems to work fine except the fact that datagrams sent can be seen with tcpdump ( exact cmdl: tcpdump -n -vv -x -i eth1 'port 8881' ) and can't be seen with netcat ( exact cmdl: nc -u localhost 8881 ). This question might not relate directly to the topic of this forum but as I suspect that there can be a flow in the code ( which is in Qt ) i by the end decided to place it here. The code of the class relating to sending datagrams looks like this
header

#ifndef TCP_SRC_HPP
#define TCP_SRC_HPP

#include <QScopedPointer>
#include <QUdpSocket>
#include <QFile>
#include <QTimer>

#include "read_config.hpp"
#include "process_data.hpp"

class TcpSrc: public QObject {
    Q_OBJECT
public:
    TcpSrc( const QObject *parent = 0 );
    ~TcpSrc();

    const QScopedPointer< QUdpSocket > &getUdpSocket() const;

    void readFromFile( QFile & );

    void setMsg( const QString & );
    const QString &getMsg() const;


signals:
//    void directoryChanged();
    void fileChanged( QFile & );
    void fileWasNotChanged( QFile & );

private slots:
//    void sessionOpened();
    void parseSrc( QFile & );
    void checkFileChanges( QFile & );
    void checkDirectoryChanges();

private:
    void startSending();
    void filMsgToSnd( const QString &, quint16 );

    QScopedPointer< QUdpSocket > udpSocket;

    ReadConfig *confFile;
    QString msg;
    ProcessData *processData;
    QTimer readTimer;

};

#endif // TCSP_CLIENT_HPP

src:

#include <cstdint>

#include <QDir>
#include <QFile>
#include <QFileInfo>
#include <QDataStream>
#include <QDateTime>
#include <QHostAddress>
#include <QChar>
#include <QTimer>

#include "tcp_src.hpp"

TcpSrc::TcpSrc( const QObject *parent )
    : udpSocket( new QUdpSocket( this ) ) {

    processData = new ProcessData();
    confFile = new ReadConfig();

    connect( &readTimer, &QTimer::timeout, this, &TcpSrc::checkDirectoryChanges );
    connect( this, SIGNAL( fileChanged( QFile & ) ), this, SLOT( checkFileChanges( QFile & ) ) );
    connect( this, SIGNAL( fileWasNotChanged( QFile & ) ), this, SLOT( parseSrc( QFile & ) ) );
    readTimer.start(1000);
}

void TcpSrc::checkDirectoryChanges() {
    QDir crntDir( confFile->getSrcFilePath() );
    if ( !crntDir.exists() )
        qDebug() << "Couldn't open directory";

    if ( !crntDir.isEmpty() ) {
        for ( auto fileN : crntDir.entryList() ) {
            QString fileFullName = crntDir.absoluteFilePath( fileN );

            if ( ( fileFullName.contains( QString( "/." ) ) ) || fileFullName.contains( QString( "/.." ) ) ||
                 ( fileFullName.contains( QString( "\\." ) ) ) || fileFullName.contains( QString( "\\.." ) ) )
                continue;

            QFile file( fileFullName );
                emit fileChanged( file );
        }
    } else
        qDebug() << confFile->getSrcFilePath() << " is empty";
}

void TcpSrc::checkFileChanges( QFile &file ) {
    QFileInfo fi( file );

    if ( fi.lastModified() > QDateTime::currentDateTime().addSecs( -1 ) ) {
        QTimer timer;
        timer.setInterval( 1000 );
        connect( &timer, &QTimer::timeout, this, &TcpSrc::checkDirectoryChanges );
    } else {
        emit fileWasNotChanged( file );
    }
}

const QScopedPointer< QUdpSocket > &TcpSrc::getUdpSocket() const {
    return udpSocket;
}

void TcpSrc::parseSrc( QFile &file ) {
    QDir dir( confFile->getSrcFilePath() );
    QString fileName;
    if ( !file.open( QIODevice::ReadOnly ) )
        qDebug() << "Couldn't open the file";

    processData->setCheckSum( file, processData->defineCheckSumAlg( confFile->getCheckSumAlg() ) );
    QFileInfo fi( file );
    if ( sizeof( ( fi.fileName() ).toStdString().c_str() ) <= FILE_NAME_SIZE ) {
        fileName.insert( 0, fi.fileName() );
        size_t size_diff = FILE_NAME_SIZE - ( fi.fileName() ).size();

        for ( size_t i = 0; i < size_diff; ++i ) {
            fileName.prepend( '0' );
        }
    } else {
        fileName.insert( 0, ( fi.fileName() ).constData(), FILE_NAME_SIZE );
        qDebug() << "Size of the file name exceeds allocated limits";   // write it to log
    }
    quint16 msgN = 1; // chunk pos in seq of chunks sent
    if ( confFile->getDatagramSize() <= DATAGRAM_HEADER ) {
        filMsgToSnd( fileName, msgN );
        processData->setSndMsgsSq( getMsg() );
    } else {
        // actual message size | without header
        size_t chunk_size = confFile->getDatagramSize() - DATAGRAM_HEADER;

        char *temp;  // chunk without header

        // actual size of the message inside datagram
        int size = chunk_size;

        QDataStream in( &file );
        while( !file.atEnd() ) {
            if ( msgN <= file.size() / chunk_size ) {
                temp = ( char * )calloc( chunk_size + 1, sizeof( char ) );

                size = chunk_size;
            } else {
                temp = ( char * )calloc( ( file.size() % chunk_size ) + 1, sizeof( char ) );

                size = file.size() % chunk_size;
            }

            int dataRead = 0;
            if ( ( dataRead = in.readRawData( temp, size ) ) < 0 )
                qDebug() << "No datum were read";

            qDebug() << "dataRead " << dataRead;
            temp[ dataRead + 1 ] = '\0';

            filMsgToSnd( fileName, msgN );
            setMsg( QString( temp ) );

            processData->setSndMsgsSq( getMsg() );
            ++msgN;    // represent chunk pos in seq
            free( temp );
            temp = NULL;

            msg.clear();
        }
    }
    file.close();

    if ( confFile->getDatagramSize() > DATAGRAM_HEADER ) {
        if ( !dir.remove( fi.fileName()) )
            qDebug() << fi.fileName() << " was not removed";
    }

    startSending();

    dir.refresh();
}

const QString &TcpSrc::getMsg() const {
    return msg;
}

void TcpSrc::setMsg( const QString &msg ) {
    if ( !msg.isNull() )
        this->msg.append( msg );
    else {
        qDebug() << "Message was empty";
        // Add error check
    }
}

void TcpSrc::startSending() {
    udpSocket.data()->connectToHost( confFile->getIpAddress(), confFile->getDstPortNumber(), QIODevice::WriteOnly );

// added for debugging | write it to log
    if ( QAbstractSocket::UnconnectedState == udpSocket.data()->state() )
        qDebug() << "UnconnectedState";
    else if ( QAbstractSocket::HostLookupState == udpSocket.data()->state() )
        qDebug() << "HostLookupState";
    else if ( QAbstractSocket::ConnectingState == udpSocket.data()->state() )
        qDebug() << "ConnectingState";
    else if ( QAbstractSocket::BoundState == udpSocket.data()->state() )
        qDebug() << "BoundState";
    else if ( QAbstractSocket::ClosingState == udpSocket.data()->state() )
        qDebug() << "ClosingState";
    else if ( QAbstractSocket::ListeningState == udpSocket.data()->state() )
        qDebug() << "ListeningState";
    else if ( QAbstractSocket::ConnectedState == udpSocket.data()->state() )
        qDebug() << "ConnectedState";
// ================================================================================ //
    quint64 dataWritten = 0;
    for ( auto sndMsg: processData->getSndMsgsSq() ) {
        for ( size_t i = 0; i < confFile->getNumOfRpSnd(); ++i ) {
            dataWritten = udpSocket.data()->writeDatagram( sndMsg.toStdString().c_str(), qstrlen( sndMsg.toStdString().c_str() ),
                                                           QHostAddress( confFile->getIpAddress() ), confFile->getDstPortNumber() );
        }
    }
    udpSocket.data()->disconnectFromHost();
}

void TcpSrc::filMsgToSnd( const QString &fName, quint16 msgN ) {
    setMsg( fName );
    setMsg( QString( processData->intToStr( msgN ) ) );
    processData->setUuid();
    setMsg( QString( processData->getUuid() ) );
    setMsg( QString( processData->getCheckSum() ) );
}

TcpSrc::~TcpSrc() {}

port and ip adress are predefined in configureation file. Which contain e.g. the following info ip:127.0.0.1 dst_port:8881, i.e. udpSocket in QScopedPointer< QUdpSocket > udpSocket is 8881
and confFile->getIpAddress() returns IP address of the host to which it is intended to send datagrams ( 127.0.0.1 ).

Will be very grateful for any help
Thank you in advance

[SGaist]

Hi,

Might be a silly question but why are you using a QUdpSocket in a class named TcpSrc ? Shouldn't it be QTcpSocket ?

[ainu]

@SGaist Not a silly at alL )) I started from the example on Tcp server from network-exaples and just left the name as it was. Of course it can lead to misunderstanding, which is not a good practice, i understand.

[ainu]

I could solve this problem, not exactly myself though. The thing was that the server to which i was trying to send datagrams
form this client was on a virtual machine and i had wrong network settings there.

[SGaist]

Great !

Glad you found out and thanks for sharing !