Re: [PATCH (RFC and a half?)] network: add rule to nftables backend that zeroes checksum of DHCP responses
On 10/24/24 12:36 PM, Daniel P. Berrangé wrote:
> [...]
AFAIR, it isn't actually a bug with virtio-net usage as this last
bit suggests. Rather it is a result of feature negotiation with QEMU
on the host, whereby the guest & QEMU mutually agree to turn off
checksums because they are redundant when the "link" is just local
memory not a physical cable.
IOW, packets don't arrive in the guest with a bad checksum. They
arrive in the guest with no checksum *as requested* by the guest.
The DHCP client decides this is a bad checksum, as it wasn't
aware of the checksum offload usage.
[...]
....and in Fedora/RHEL context it was fixed 18 years ago, as we
first hit this when working on Xen integration in 2006 :-)
> A few quick tests proved that it was the same old "bad checksum"
> problem from 2010 come back to haunt us.
2006 :-)
Interesting - so my origin story is at least partially a false memory :-/
But if you had this problem with Xen in 2006, how was it fixed then (and
why was it a surprised when it came up again in 2010 after vhost
processing was turned on? Or maybe it wasn't a surprise, and I just
thought so because I wasn't around in 2006 :-)
(The commit adding --checksum-fill rules to libvirt was fd5b15ff1, from
July 2010)