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[Andrew Woodward <xarses@xxxxxxxxx>]

Ok, so here is the breakdown of what i was able to find.

DHCP server using dnsmasq (2.65.5.el6) on CentoOS 6.4 a kvm guest with virtio 
+ Also running cobbler with some preseed building scripts based on 12.04 LTS

node-1 a bare (guest) to PXE boot from DHCP server, on its own linux bridge (node_1_net) in routed mode, guest nic is virtio
node-2  a bare (guest) to PXE boot from DHCP server, on its own linux bridge (node_2_net) in routed mode, guest nic is e1000
node-3 a bare (guest) to PXE from DHCP server on the same linux bridge as DHCP server with virtio nic

In the kvm host I have stopped dnsmasq (which starts automatically) on default, node_1_net, node_2_net 
In the kvm host I have started dhcp-helper -i node_1_net -i node_2_net -s <DHCP server IP> (dhcp-helper 1.1)

when booting node-1 it will PXE just fine, but when running the
installer, the node aborts attempting to request and address from DHCP,
running dhclient by hand yields the magical "bad udp checksum" message.

when booting node-2 it will boot and install just fine, the only change being the nic driver
when booting node-3 it will boot and install just fine but in this case, the DHCP requests aren't relayed over dhcp-helper.

So it appears that some drivers won't perform the udp checksum when some
dhcp servers / relays are involved in the conversation.

I will try to test the same using a 13.10 loader and report back

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https://bugs.launchpad.net/bugs/610391

Title:
  UDP checksum offload breaks DHCP on virtual machines

Status in The Linux Kernel:
  Unknown
Status in “isc-dhcp” package in Ubuntu:
  Invalid
Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  When running an Ubuntu server on RHEL6 the dhclient on Ubuntu will
  report 'bad udp checksum' in response to the DHCP Offer packets coming
  from the Host.

  This is on a libvirt controlled KVM system using libvirt controlled
  dnsmasq processes on the Host.

  Optimisations within the kernel appear to be upsetting dhclient.

  http://www.pubbs.net/201004/dhcp/30492-dhcpd-problem-on-virtual-
  machines.html

  ProblemType: Bug
  DistroRelease: Ubuntu 10.04
  Package: linux-image-2.6.32-24-generic-pae (not installed)
  Regression: Yes
  Reproducible: Yes
  ProcVersionSignature: User Name 2.6.32-24.38-generic-pae 2.6.32.15+drm33.5
  Uname: Linux 2.6.32-24-generic-pae i686
  AlsaDevices: Error: command ['ls', '-l', '/dev/snd/'] failed with exit code 2: ls: cannot access /dev/snd/: No such file or directory
  AplayDevices: Error: [Errno 2] No such file or directory
  Architecture: i386
  ArecordDevices: Error: [Errno 2] No such file or directory
  CurrentDmesg:
   [   13.240032] eth0: no IPv6 routers present
   [ 2868.734972] psmouse.c: Explorer Mouse at isa0060/serio1/input0 lost synchronization, throwing 1 bytes away.
  Date: Tue Jul 27 09:33:31 2010
  Lspci: Error: [Errno 2] No such file or directory
  Lsusb: Error: [Errno 2] No such file or directory
  MachineType: Red Hat KVM
  PciMultimedia:
   
  ProcCmdLine: root=UUID=129537b3-af8e-4e06-bd9d-06626fa40823 ro quiet splash
  ProcEnviron:
   LANG=en_GB.UTF-8
   SHELL=/bin/bash
  SourcePackage: linux
  dmi.bios.date: 01/01/2007
  dmi.bios.vendor: Seabios
  dmi.bios.version: 0.5.1
  dmi.chassis.type: 1
  dmi.chassis.vendor: Red Hat
  dmi.modalias: dmi:bvnSeabios:bvr0.5.1:bd01/01/2007:svnRedHat:pnKVM:pvrRHEL6.0.0PC:cvnRedHat:ct1:cvr:
  dmi.product.name: KVM
  dmi.product.version: RHEL 6.0.0 PC
  dmi.sys.vendor: Red Hat

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