Re: UDP checksums broken in Dom0 -> DomU vif transfer

[Michal Suchanek <hramrach@centrum.cz>]

On 20 December 2011 10:55, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> On Tue, 2011-12-20 at 09:43 +0000, Jean Guyader wrote:
>> On 20 December 2011 10:37, Ian Campbell <Ian.Campbell@citrix.com> wrote:
>> > The issue is that dom0 does checksum offload which means the checksum is
>> > not valid on the domU end, although we know the packet is intact because
>> > it never hit the wire.
>> >
>> > The network stack deals with this because skb->ip_summed is set
>> > appropriately but when e.g. raw sockets are used it can ends up exposing
>> > getting exposed to userspace.
>> >
>> > We don't want to do the checksum by default since there are performance
>> > gains from avoiding it in the general case. Note that "tx off" turns of
>> > TCP and UDP checksum offload.
>> >
>> > It's not clear where the bug is here, it could be a bug in dhclinet for
>> > dropping the packet or perhaps this is something that raw socket driver
>> > should be correcting (based on ip_summed) as the packet passes through
>> > to userspace?
>> >
>> > I'm not sure but this might impact native hardware too -- depends on the
>> > H/W's handling of the checksum field on RX, you'd hope they mostly just
>> > leave it alone, but I'm not sure how e.g. LRO/GRO effects things?
>> >
>>
>> I've seen this issue in the past. I belive the bug is in dhclient.
>> dhclient checks the checksum on the packets and drop them if it's wrong.
>
> Indeed, googling around a bit shows that this dhclient bug affects more
> than just Xen, e.g. virtio and even some native hardware appear to be
> effected.
>
> http://marc.info/?l=kvm&m=121882968407525&w=2
> https://qa.mandriva.com/show_bug.cgi?id=63320
> https://bugs.mageia.org/show_bug.cgi?id=1243
> http://pkgs.fedoraproject.org/gitweb/?p=dhcp.git;a=blob_plain;f=dhcp-4.2.2-xen-checksum.patch;hb=HEAD
>
> The OP didn't say what distro or version of dhclient he was using but I
> think the right place to report this would be to the distro since it
> appears to be using an out of date dhclient. In the meantime disabling
> offload seems like a reasonable local workaround but it is not a fix we
> should apply by default.

Thanks for the links

I am running Ubuntu in the DomU.

Michal