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:
  • 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.


Remote desktop ‘Exceeded the number of connections’

Posted by Christian Ashby on May 4, 2010

You can optionally connect to the ‘console’ which is a screen 0 (by default screens 1 and 2 are the screens you connect to with a remote desktop client). This option is not in the Windows UI, you need to use the following command from Start->Run:

mstsc -v:{machine address} /f -console

Windows Vista / Windows 7 use the syntax /admin rather than -console.

If you use a mac, you can put /console at the end of the machine name in the machine box.

If you use linux and rdesktop then add -0 to the rdesktop command line.


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.


Blocking Internet Explorer updates

Posted by Christian Ashby on April 2, 2010

If you are developing websites, or simply don’t want to change browsers, you can stop Microsoft automatic update from installing IE 7 or IE 8 by installing the following updates from Microsoft:

Once run, they will extract files to the location of your choice, then you have to use an elevated command prompt and run the command with a /B switch (block) as follows:

IE70Blocker.cmd /B

You can revert to the normal state (allowing Microsoft / Windows Update to replace your browser) as follows:

IE70Blocker.cmd /U

Note that in order to keep IE6 you need to run both 7 and 8 blockers.

Then, you can keep your system completely up to date whilst maintaining the old-school browsers!


Windows: Linking files and folders

Posted by Christian Ashby on March 13, 2010

At first glance it may not be obviously useful, but if you need to link one folder to another, or store a file somewhere else but refer to it within a different folder, you can do so by the very useful mklink command, available in Windows 7 and Vista. There is an alternative for Windows 2000 and XP called ‘junction‘.

To create links, simply open a command prompt (Start->Run->cmd), and type:

  • For files: mklink {destination} {target}
  • For folders: mklink /d {destination} {target}

Note that it may be necessary to become admin to link some files / folders.


Windows XP – how long is there left?

Posted by Christian Ashby on March 3, 2010

It seems to be a common misunderstanding that Microsoft are dropping Windows XP in July this year. This is not the case! They are in fact dropping support for Windows XP Service Pack 2 only – not the entire operating system, so you need to upgrade to Service Pack 3 as soon as possible.

Microsoft in fact withdrew mainstream support for the XP operating system in April 2009, but they continue to support service packs until 24 months after they are released, which means Service Pack 3 will be retired from general support in April 2011 (it was released in 2008). However, Microsoft will continue to release security patches until April 2014, so you have plenty of time to upgrade. For more information see the Microsoft Site.

If you have problems with installing the service packs, or any other questions, please contact us – we’ll happily give you advice over the phone, and if necessary we can fix your machine for you – at a time and price that suits you.


Windows: Becoming admin

Posted by Christian Ashby on March 2, 2010

Windows Vista and 7 introduced a feature called ‘User Account Control’ which restricts users to running user-level commands without somehow confirming that they want to use administrator privileges. Usually applications (such as installation programs) automatically ask the user whether they want to use those additional privileges.

For the command prompt and other programs you can do one of three things to run them as ‘Administrator’:

  1. Use the icon: Right-click on the shortcut to the program and select Run As...
  2. Permanently select run as administrator: Right-click on the shortcut to the program and select Properties then select Run as Administrator in the Compatibility tab.
  3. Use the search box and a shortcut key: Click Start, then type the program name in the search box. Instead of hitting enter when the program is highlighted above, use CTRL+SHIFT+ENTER.

This feature mimics the ‘sudo’ feature in modern Linux operating systems which allows you to run commands ‘as root’.