Em 10-04-2016 14:25, Volker escreveu: > On 09.04.2016 21:40, Marcelo Ricardo Leitner wrote: >> Hi, >> >> Em 07-04-2016 12:19, Volker escreveu: >>> Hi all. >>> >>> I have a strange problem at hand regarding UDP fragmentation on Centos7: >>> Applications are unable to receive UDP packets which have undergone >>> fragmentation UNLESS the netfilter modules are loaded. >>> >>> The problem arose on a application which would run fine on OpenSuse but >>> does not work on Centos7. The application processes UDP data and on >>> Centos only small packets are received and processed, packets below the >>> fragmentation size limit of about 1500 bytes. UDP packets which have >>> undergone fragmentation are not received by the application. >>> >>> The application in question uses Qt, which opens the UDP socket in >>> non-blocking mode - apparently that's an issue because reading from the >>> socket in blocking mode does not cause the problem. >>> >>> By chance I hit on the fact that once the netfilter kernel-modules >>> (nf_nat, iptable_nat, nf_nat ...) are loaded the problem disappears and >>> UDP packets of all sizes are correctly delivered and processed. >>> >>> NOTES: >>> - I'm not using netfilter. My iptables are empty, firewalld is not >>> running. >>> >>> - Other networking applications -at least tcp- are working fine: >>> webbrowsing, ssh, nfs etc even DNS >>> >>> - Does not happen on Opensuse regardless if netfilter modules are loaded >>> or not. >>> >>> - Does not happen on Opensuse on the same machine. Does happen on >>> different machines on Centos7. So it's not HW dependend >>> >>> - There is AFAIK nothing special about my Centos7 installation. Out of >>> the box install, simple network config, latest updates applied. >> >> Which kernel are you using? >> And as you have trimmed it down to a reproducer, can you share it please? > > 3.10.0-327.10.1.el7.x86_64 > > I have put the files on github > https://github.com/volkerp/qtudptest Okay, you can't do that change I mentioned easily because the call is made by Qt instead. When testing with this app, on nstat I'm getting after 2 attempts: UdpInErrors 2 0.0 UdpInCsumErrors 2 0.0 I tried removing the probe for more data, the while condition, but it still peeked the socket. I think it's because of this Qt code: qint64 QNativeSocketEngine::readDatagram(char *data, qint64 maxSize, QHostAddress *address, quint16 *port) { Q_D(QNativeSocketEngine); Q_CHECK_VALID_SOCKETLAYER(QNativeSocketEngine::readDatagram(), -1); Q_CHECK_TYPE(QNativeSocketEngine::readDatagram(), QAbstractSocket::UdpSocket, false); <---- return d->nativeReceiveDatagram(data, maxSize, address, port); } So I'm afraid there is no way around this issue with Qt. Marcelo