[Bug 1192464] Re: udp checksum computed as 0 not converted to 0xffff, from guest os that share a common linux bridge among multiple guest os

[Michal Suchanek]

Question is where is this zero checksum observed which is not clear from
the report.

If in the guest it is certainly correct.

If in the host it is correct so long as the bridge appears to have
checksum offloading as well. If whatever interface the guest packets
appear to come from is not set up with checksum offloading this is a bug
which should be fixed by setting the offload flags to match the guest.

If outside the host this is a problem.

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https://bugs.launchpad.net/bugs/1192464

Title:
  udp checksum computed as 0 not converted to 0xffff, from guest os that
  share a common linux bridge among multiple guest os

Status in QEMU:
  Incomplete

Bug description:
  UDP checksum computed as '0' during transmission of packets that uses
  e1000 NIC in the Guest as well as emulated h/w in the qemu layer, That
  needs to be converted to 0xffff, This occurs only when Hardware
  checksum offload is been set in the guest OS NIC and made it as a
  transmitter. The guest O.S use the N/W interface that is been shared
  to the linux brige created in the host (used source=<bridge>) in the
  xml tags of libvirt.

  As per RFC768(http://tools.ietf.org/html/rfc768 [^]), If the computed
  checksum is zero, it is transmitted as all ones (the equivalent in
  one's complement arithmetic). An all zero transmitted checksum value
  means that the transmitter generated no checksum (for debugging or for
  higher level protocols that don't care).

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