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