Windows DHCP server problem on Xen VM hosts

Posted by Christian Ashby on May 6, 2010

If your XenServer hosted Windows DHCP server(s) are running on the same physical host as linux DHCP clients, then they will not receive a DHCP address.

To work around this problem, turn off checksum offloading on the network adapter. To do this, follow these steps:

  • Click Start, click Run, type regedit, and then click OK.
  • Locate and then click the following registry key: 
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
  • In the right pane, make sure that the DisableTaskOffload registry entry exists. If this entry does not exist, follow these steps to add the entry:
    • On the Edit menu, point to New, and then click DWORD value.
    • Type DisableTaskOffload, and then press ENTER.
    • Click DisableTaskOffload.
    • On the Edit menu, click Modify.
    • Type 1 in the Value data box, and then press ENTER.
    • Exit Registry Editor.

Note that this doesn’t effect Open-Source Xen as it is specifically related to the XenServer-supplied PV drivers.

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Converting open-source Xen Windows VMs to XenServer

Posted by Christian Ashby on April 13, 2010

A number of people are looking to migrate from the open-source Xen to Citrix XenServer itself, and it’s not immediately obvious how to migrate Windows VMs between the two platforms. Citrix have a tool called ‘XenConvert’ designed to move machines between environments, and the way you can use this to move from open-source Xen is as follows:

  • Download XenConvert from the Citrix downloads centre.
  • Add a virtual disk to the guest machine (at least twice the size of the existing disk) to receive the exported VM.
  • Run the original guest machine and partition / format the new virtual disk (NTFS / FAT, as long as you can mount these filesystems in the host).
  • Install & Run XenConvert on the source guest machine.
  • Select ‘XVA’ and select the new virtual disk as the destination for the XVA image.
  • Shut down the source guest machine.
  • Work out where the new virtual partition starts – something like this: 
    root@host:/etc/xen# parted /dev/host-vm/name-XVA
GNU Parted 1.7.1
Using /dev/mapper/host--vm-name--XVA
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) unit B
(parted) print

Disk /dev/mapper/host--vm-name--XVA: 26843545599B
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End           Size          Type     File system  Flags
1      32256B  26839088639B  26839056384B  primary  ntfs
  • Mount the new virtual disk – change ‘offset’ to be the value in the Start column above, replace ntfs with vfat if FAT32 was used: 
    mount -o ro,offset=32256 -t ntfs /dev/host-vm/name-XVA /mnt/t
  • Copy the *.xva folder from this mounted drive to somewhere accessible to the new XenServer host.
  • Run XenCenter on a machine and connect to the new host.
  • Import a new VM, select the ova.xml file from the *.xva folder, and follow the import – you’ll need to setup a new Network Interface.
  • If required, reactivate Windows on the new guest as it starts up, and install XenServer Tools.

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