Re: [Xen-devel] UDP checksums broken in Dom0 -> DomU vif transfer

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2011-12-20 at 15:35 +0000, 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@xxxxxxxxxx> wrote:
> > > On Tue, 2011-12-20 at 09:43 +0000, Jean Guyader wrote:
> > >> On 20 December 2011 10:37, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 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 appears to be using an out of date dhclient.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?
> 
> Ian, would it make sense to add all this awesome details in the Xen FAQ
> Wiki part?

Yes, please!

Ian.



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