Re: [Fedora-xen] TCP checksum corruption

[Mike McGrath]

Daniel P. Berrange wrote:
On Tue, May 08, 2007 at 11:39:14AM -0500, Mike McGrath wrote:
  
We're using xen heavily in Fedora's Infrastructure and presently a 
number of the xen domU hosts are experiencing terrible checksum issues.  
I've tried the ethtool -K eth0 tx off fix and it didn't work.
    

What sort of network config have you got with these ?  Briding straight
to physical device, or NAT'd ?

  
Bridge

There are a couple issues at play:

 - There is a general bug in 2.6.20  that breaks checksum offload
   when used with NAT.
 - In 2.6.19 or later Dom0 will transmits to guests using checksum
   offload, so DHCP client in the guest will mistakenly thing it
   has a corrupt checksum.

To address the first bug requires disabling checksum offload in the eth0 in
the guest. ethtool -K eth0 tx off    in the guest should do it.

To address the 2nd is really difficult since the FC6 install images themsves
have a broken DHCP client for example, so we need to workaround it in the
kernel. This can be done by disabling checksums on the device in Dom0 - any
of vifN.0,  xenbr0, phet0 should have ethtook -K <dev> tx off done.

NB, ignore eth0 in Dom0, that's a fake device so turning off tx on that does
not fix things.

So in summary, to get it working in general case requires:

   ethtool -K eth0 tx off    in guest

And

   ethtool -K <dev> tx off   on whatever bridge device the guest is attached to
  
I've actually run that on every interface on every dom[0,U] on the box 
:).  I've also tried it on two other hosts.  One a RHEL5 dom0 and the 
other had different hardware but was also a FC6 dom0.  I can arrange 
access to the box if you're interested.

   -Mike

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