The following howto describes modification to critical system software. If you choose to follow this guide, you do so at your own risk.

The problem

The commercial version of the Citrix XenServer does not allow you to choose the type of ethernet adapter to emulate within your VM. The standard device that is emulated is a Realtek 8139 (RTL8139), which is a 100Mbit/sec Fast Ethernet card.

Citrix themselves do not view this as a major issue, as they expect you to install paravirtualised drivers within your guest operating system. This is usually a very good idea and just fine if you're using Windows, or a major supported OS such as Red Hat, CentOS or Ubuntu. Under these Linux operating systems, your entire kernel must be replaced by a Citrix supplied kernel. The paravirtualised drivers will outperform any emulated device.

However, if you're running a system with a customised non-standard kernel that doesn't support Citrix Xen paravirtualisation, you'll be stuck with a 100Mbit/sec bottleneck in your network. Sure, you can go and rebuild your kernel with the right paravirtualised drivers, but that's not always an option.

Those familiar with the open source version of Xen will know that the underlying QEMU device emulation that Xen uses can emulate an Intel 1Gbit/sec adapter, called "e1000". Apart of the additional speed, this device also supports jumbo ethernet frames. This emulation mode is available under Citrix XenServer, but is a hidden feature, due to hard-coding of the Realtek driver option.

Enabling e1000 emulation

You'll need to ssh into your Citrix server and become root. Then do the following:

First rename /usr/lib/xen/bin/qemu-dm to /usr/lib/xen/bin/qemu-dm.orig

# mv /usr/lib/xen/bin/qemu-dm /usr/lib/xen/bin/qemu-dm.orig

Then make a replacement /usr/lib/xen/bin/qemu-dm file like this

#!/bin/bash
oldstring=$@
newstring=${oldstring//rtl8139/e1000}
exec /usr/lib/xen/bin/qemu-dm.orig $newstring

Then chmod (to make it executable) and chattr it (to stop it being overwritten):

# chmod 755 /usr/lib/xen/bin/qemu-dm
# chattr +i /usr/lib/xen/bin/qemu-dm

If you now shutdown and re-start your Citrix virtual machines, they will have an emulated e1000 device.

Warning

The "chattr" line above makes the replacement file "immutable". This means that the file cannot be overwritten, prevents the loss of this modification in the event of a system update.

However, this may cause updates provided by Citrix to fail at the point of installation. An alternative approach would be leave the file unprotected, and re-applying this modification after Citrix-supplied updates have been applied.

The remove the protection from the file, do the following:

# chattr -i /usr/lib/xen/bin/qemu-dm