Wrong UDP Checksum when using IP Fragmentation

[alorbach]

I am working on https://github.com/rsyslog/rsyslog/blob/master/plugins/omudpspoof/omudpspoof.c and trying to fix a problem with IP Fragmentation. The project is using libnet to build IP/UDP Packages and sending syslog data.

The problem is, that if we try to fragment packages, the UDP Checksum is not correctly calculated if we set the correct UDP Packet length here: https://github.com/rsyslog/rsyslog/blob/master/plugins/omudpspoof/omudpspoof.c#L422

And If I am using the IP Packet length, the UDP Packet is truncated.

So I am looking for a solution to do propper IP fragementation with UDP Packages, thanks for help.

[alorbach]

I have workaround the issue by disabling checksum calculation for the UDP Header for now with:
libnet_toggle_checksum

I guess there are some library changes needed in order to generated a corrrect UDP Checksum if you do IP fragmentation on your own.

[sam-github]

Perhaps you have the time and interest to work on this feature @alorbach ?

[troglobit]

Any suggestions you have, API ideas: naming, pseudo code, etc. are welcome, @alorbach :)