Em 11-04-2016 09:10, Marcelo Ricardo Leitner escreveu: > 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); > } http://code.qt.io/cgit/qt/qt.git/tree/src/network/socket/qnativesocketengine.cpp FWIW > So I'm afraid there is no way around this issue with Qt. > > Marcelo >