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

[Stefan Bader <stefan.bader@canonical.com>]

On 20.12.2011 16:35, Konrad Rzeszutek Wilk wrote:
> On Tue, Dec 20, 2011 at 12:57:48PM +0100, Michal Suchanek wrote:
>> 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 appears to be using an out of date dhclient.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.
> 
> Stefan, is this something you could help us with?

Just returning from being away over the end of year, so not yet followed any
links and such. But yes, ideally there would be a Launchpad bug reported for
this. And the bug number sent to me or here. That report should mention the
release that is used for domU, too.

-Stefan

> Ian, would it make sense to add all this awesome details in the Xen FAQ
> Wiki part?