[Bug 165059] vtnet(4): Networking breaks with a router using virtio net driver on KVM host

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=165059

Aleksandr Fedorov <afedorov@FreeBSD.org> changed:

           What    |Removed                     |Added
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                 CC|                            |afedorov@FreeBSD.org

--- Comment #37 from Aleksandr Fedorov <afedorov@FreeBSD.org> ---
This is simply because FreeBSD does not know how to forward and NAT packets
with offload.
KVM forwards packets to a virtual machine that can be up to 64 kilobytes in
size and without checksums.

This is optimization!!!

Just read the virtio specification:

struct virtio_net_hdr {
#define VIRTIO_NET_HDR_F_NEEDS_CSUM 1
#define VIRTIO_NET_HDR_F_DATA_VALID 2
#define VIRTIO_NET_HDR_F_RSC_INFO 4
u8 flags;
#define VIRTIO_NET_HDR_GSO_NONE 0
#define VIRTIO_NET_HDR_GSO_TCPV4 1
#define VIRTIO_NET_HDR_GSO_UDP 3
#define VIRTIO_NET_HDR_GSO_TCPV6 4
#define VIRTIO_NET_HDR_GSO_UDP_L4 5
#define VIRTIO_NET_HDR_GSO_ECN 0x80
 u8 gso_type;
 le16 hdr_len;
 le16 gso_size;
 le16 csum_start;
 le16 csum_offset;
 le16 num_buffers;
 le32 hash_value; (Only if VIRTIO_NET_F_HASH_REPORT negotiated)
 le16 hash_report; (Only if VIRTIO_NET_F_HASH_REPORT negotiated)
 le16 padding_reserved; (Only if VIRTIO_NET_F_HASH_REPORT negotiated)
};

https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html#x1-2170001

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