Re: software checksuming on IPv6 multicasting is not working

On Sun, Aug 02, 2026 at 11:17:44AM +0200, [email protected] wrote:
> >Description:
> 	In writing IPv6 multicasting code, I have determined after a full day 
> 	of debugging, that software checksumming from udp6_output/ip6_output
> 	does not work.  After examining all my devices there is two that do
> 	hardware offloading of IPv6 UDP packets and this turned out to work.
> 	Conclusion is obvious, software checksumming doesn't work in this
> 	multicast.

I tried to reproduce the issue with regress/sys/netinet6/mcast6 .
For that I disabled UDPv6 checksum offloading in em.  netstat -s
counter went up.

udp:
        6 output packets software-checksummed

On the receiving machine the checksum is correct.  Both are connected
by a direct cable.

22:13:44.842191 fdd7:e83e:66bc:145::70.15971 > ff04::123.12345: [udp sum ok] udp 5 [flowlabel 0x1e8b4] [hlim 1] (len 13)

Please check your setup.  Maybe something is different.  Try to
create a simpler test case.

bluhm