[Bug 930962] Re: dhcp3-server reports many bad udp checksums to syslog using virtio NIC

[Michal Suchanek]

Hello,

On 8 January 2013 15:01, Martijn Lievaart <930962 at bugs.launchpad.net>
wrote:

> Hi Michal,
>
> I don't quite understand what you are saying here. Are you saying that
> because Linux has a kernel interface to check for offloading, any and
> all application (that cares about udp checksums) should use this? It

Yes, reading several discussion about this issue it seems that
application that wants to work correctly on Linux has to do this. This
issue pops up on actual hardware with actual hardware checksum
offloading reportedly.

> would seem much simpler if the virtio driver zero'd the checksum field,
> any application already should be able to handle that. No Linux specific
> code needed.

Except virtio would have to do the work then. Also it does not solve
the issue for other hardware with offloading.

Also does the zero checksum not change the meaning of the message? It
would seem that setting the checksum to zero in driver and application
ignoring the offload status would imply interpretation that the sender
did not include any checksum at all whereas the correct interpretation
would be that the checksum was verified and stripped by network
hardware.

Thanks

Michal

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https://bugs.launchpad.net/bugs/930962

Title:
  dhcp3-server reports many bad udp checksums to syslog using virtio NIC

Status in “isc-dhcp” package in Ubuntu:
  Confirmed

Bug description:
  Tested with  dhcp3-server ver. 3.1.3-2ubuntu3.3  in Ubuntu 10.04.3
  Server, x64.

  The DHCP server reports that UDP packets sent to it have bad
  checksums:

  Feb 11 06:57:18 ... dhcpd: 5 bad udp checksums in 5 packets
  Feb 11 06:58:22 ... dhcpd: last message repeated 7 times
  Feb 11 06:59:17 ... dhcpd: last message repeated 7 times

  The DHCP server host is a KVM virtual machine using a virtio-based
  virtual NIC.

  This problem has been reported for other distros using KVM and virtio:

  https://bugs.mageia.org/show_bug.cgi?id=1243

  http://www.mail-archive.com/kvm@vger.kernel.org/msg41958.html
   - suggests using iptables to write in a checksum

  http://pkgs.fedoraproject.org/gitweb/?p=dhcp.git;a=blob;f=dhcp-4.2.2-xen-checksum.patch;h=038d346d726e131f1ab2579fe015a72b49733a0d;hb=HEAD
   - Fedora patch to dhcp to avoid this

  The simplest workaround is to change the virtual NIC type from virtio
  to Intel e1000 in KVM. Apparently this driver calculates checksums.
  But virtio is the default driver type.

  Thanks,
  Tim Miller Dyck

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