Problem with checksum offloading on RPi3 (PF + Jails involved)

[Kristof Provost]

On 3 Nov 2020, at 20:14, John-Mark Gurney wrote:
> Kristof Provost wrote this message on Sun, Nov 01, 2020 at 19:48 
> +0100:
>> On 31 Oct 2020, at 16:06, Carsten Bäcker wrote:
>>> Packet-capture from the router is attached.
>>>
>> Okay??? but that doesn???t actually demonstrate anything at all.
>> It shows an outbound packet with an incorrect checksum. See what 
>> I???ve
>> mentioned before about checksums potentially being calculated by
>> hardware.
>> Given that there???s been a reply to that DNS query the checksum 
>> clearly
>> got set correctly after the packet was captured.
>
> Actually, isn't that odd that the outbound checksum is wrong?  Since
> the chip doesn't advertise (or it shouldn't) tx checksum handling,
> FreeBSD's stack should have computed the checksum.
>
Bear in mind that this capture was made with pflog, not the usual 
BTF_MTAP() in the driver.
That means it was captured before the stack does checksum calculations.

Just before it, in fact. See ip_output(). The checksum calculation is 
pretty much immediately after the pfil hook.

Best regards,
Kristof