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

[Martijn Lievaart]

> On 7 January 2013 23:13, Martijn Lievaart <930962 at bugs.launchpad.net> wrote:
>> According to my cursory reading of relevant RFCs and a quick wireshark
>> session, the bug is in KVM virtio. An UDP packet should contain a
>> checksum field of 0 for no checksum, or a correct checksum. I see bogus
>> values coming in.
>>
>> OTOH, wireshark warns that this may be due to offloading, so I may not
>> be correct in this analysis.
>
> This is due to virtio implementing 'offloading' by returning random
> garbage in the checksum field.
>
> Since the code to calculate the checksumis not present this offloading
> cannot be turned off.
>
> Since kernel has interface to inform applications that offloading is
> on it is a bug in the application to interpret the bogus checksum.
> Presumably the card has already checked it and did not bother passing
> it on.

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
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.

Martijn

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to isc-dhcp in Ubuntu.
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

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/930962/+subscriptions