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

From: <bugzilla-noreply_at_freebsd.org>
Date: Sat, 10 Sep 2022 20:04:28 UTC 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235607

--- Comment #15 from Eugene Grosbein <eugen@freebsd.org> ---
FreeBSD's vtnet(4) driver upto 12.3 version implements checksum offload for
transmit path ONLY.

For receive path, it blindly assumes that ALL traffic comes from another
virtual machine running on same hypervisor that just did checksumming already,
so it just skips its own checksumming. The solution is to disable non-working
rxcsum "offload" with ifconfig(8):

ifconfig vtnet0 -rxcsum

In case of NAT, this solves the problem.

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