[CentOS] UDP de-fragmentation problem

Sun Apr 10 17:25:46 UTC 2016
Volker <volker at openbios.org>

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.
>> - 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?


I have put the files on github