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? Marcelo