Subject: Re: [PATCH net-next] lwt: fix rx checksum setting for lwt devices tunneling over ipv6

From: Jiri Benc <[email-masked]>
Date: Thu, 11 Feb 2016 11:41:24 +0100

On Wed, 10 Feb 2016 16:47:21 +0100, Paolo Abeni wrote:
> --- a/drivers/net/geneve.c
> +++ b/drivers/net/geneve.c
> @@ -1441,7 +1441,8 @@ struct net_device *geneve_dev_create_fb(struct net *net, const char *name,
>  		return dev;
>  
>  	err = geneve_configure(net, dev, &geneve_remote_unspec,
> -			       0, 0, 0, htons(dst_port), true, 0);
> +			       0, 0, 0, htons(dst_port), true,
> +			       GENEVE_F_UDP_ZERO_CSUM6_RX);
>  	if (err) {
>  		free_netdev(dev);
>  		return ERR_PTR(err);
> diff --git a/net/openvswitch/vport-vxlan.c b/net/openvswitch/vport-vxlan.c
> index 1605691..d933cb8 100644
> --- a/net/openvswitch/vport-vxlan.c
> +++ b/net/openvswitch/vport-vxlan.c
> @@ -90,7 +90,7 @@ static struct vport *vxlan_tnl_create(const struct vport_parms *parms)
>  	int err;
>  	struct vxlan_config conf = {
>  		.no_share = true,
> -		.flags = VXLAN_F_COLLECT_METADATA,
> +		.flags = VXLAN_F_COLLECT_METADATA | VXLAN_F_UDP_ZERO_CSUM6_RX,

I'm afraid this looks wrong, we should not default to zero UDP checksum
over IPv6. See RFC 2460, section 8.1:

      o  Unlike IPv4, when UDP packets are originated by an IPv6 node,
         the UDP checksum is not optional.  That is, whenever
         originating a UDP packet, an IPv6 node must compute a UDP
         checksum over the packet and the pseudo-header, and, if that
         computation yields a result of zero, it must be changed to hex
         FFFF for placement in the UDP header.  IPv6 receivers must
         discard UDP packets containing a zero checksum, and should log
         the error.

One may argue that with tunneling, the situation is different but
that's the reason why we have the IPv6 checksum flag and why it has
opposite meaning to the IPv4 one. We shouldn't default to non-RFC
behavior by default.

What is the problem with the current code? Is the UDP checksum not
calculated correctly?

 Jiri

-- 
Jiri Benc