[Bug 235607] Incorrect checksums with NAT on vtnet with offloading

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235607

Vincenzo Maffione <vmaffione at FreeBSD.org> changed:

           What    |Removed                     |Added
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                 CC|                            |vmaffione at FreeBSD.org

--- Comment #9 from Vincenzo Maffione <vmaffione at FreeBSD.org> ---
Not 100% sure, but I think you are hitting the same problem as me.
If I am not mistaken, you can reproduce a similar problem with a simpler (test)
configuration in your fbsd_guest_fw: no layer 3 processing, but just a bridge0
to bridge vtnet0 and vtnet1 together, e.g.:

   # ifconfig bridge0 create addm vtnet0 addm vtnet1

In this scenario (with csum enabled) packets coming from vtnet0 with "incorrect
checksum" will exit vtnet1 with incorrect checksum, and will be discarded at
the next hop.
The root of the problem is that vtnet cannot really implement a csum offload
(since there is no hw that does that), but it rather delays the TCP checksum
computation hoping that someone will do that later (if necessary).
Unfortunately, the FreeBSD mbuf does not have enough metadata to store and
forward the metadata contained in the per-packet virtio-net header (as defined
in the VirtIO standard). This information is already lost when the packet
arrives on vtnet1 on transmission, so that vtnet1 is not asked to do the
offloads (CSUM_TCP is not set), and therefore the packet does not survive the
next hop.

See also here:
https://reviews.freebsd.org/D21315

About your setup: I see that you have offloads support on vtnet0 and vtnet1
(e.g. TXCSUM, RXCSUM, etc.) --> I guess you are not using bhyve as an
hypervisor, because it does not support offloads in 12.0? I guess you are using
Linux QEMU/KVM with TAP interfaces backing the vtnet* devices?

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