VirtualBox on FreeBSD


Windows and Ubunutu running inside a FreeBSD Host.


Virtualisation, running more than one instance of (potentially) more than one Operating System on a single physical computer, is growing in popularity and power.

FreeBSD offers two different types of virtualisation, which can be characterised as single-kernel virtualisation and multiple-kernel virtualisation.

The first, implemented natively to FreeBSD, is Jails, allowing one FreeBSD kernel to run many instances of a compatible FreeBSD system. This approach is simple, and fast and is probably the best choice if you wish to run several non-GUI instances (ie a mail machine, a web server and a database). The author has successfully used this approach for some years commercially and at home. However if the author wished to run Windows or any other operating system on the Jail Host it would not work - every instance in a Jail uses the same kernel, so not even different versions of FreeBSD will run on a jail. In order to run different operating systems one turns to multi-kernel virtualisation.

This second approach, often called “true” virtualisation, (but usually only by the vendors of this class of software), is where a Virtual Machine Manager (VMM) is created that acts as a “interpretation layer” between the Host OS (in our case FreeBSD) and the Guest OS (for example a Windows XP instance).

VirtualBox, currently owned by Oracle, is a dual-licensed VMM, with a commercial version and and OSE (Open Source Edition). This Open Source Edition has been ported natively to FreeBSD, and this chapter shall demonstrate the steps necessary to successful implementation.

It is worth noting that VirtualBox OSE has been intentionally crippled - it will not pass through RDP requests (you cannot “remote desktop” into the box) and it will not allow USB devices plugged into the Host OS to be accessible to the Guest OS. The commerical version does allow these functions.

Hardware support

Until recently virtualised machines were noticeably slow - because the “interpretation layer” was implemented solely in software. However as virtualisation has grown in popularity, the major chip vendors have worked to develop hardware extensions to improve speed.

There have been two major innovations in hardware-based virtualisation. VMX extensions and nested paging. The first has been around for a couple of years, and in essence performs the marking of each process in the host as belonging to the correct Guest OS.

The second is most recent, found in the Nehalem-range on Intel CPUs and their AMD competitor (?)- without this, a VM must pretend it holds a paging cache - so any cache misses end up being processed twice, once in the “real” cache, once in the VM version.

Anecdotally, a CPU without VMX flags and Nehalem, can be around 3-5 times slower in virtualisation, and for a desktop machine this is very noticeable.

You can detect if your CPU supports the VMX extensions by looking at dmesg for CPU flags, notable VMX and SSE flags

CPU: Intel(R) Core(TM) i7 CPU       M 620  @ 2.67GHz (2660.02-MHz 686-class CPU) Origin = "GenuineIntel"  Id = 0x20652  Family = 6  Model = 25  Stepping = 2


VMX (almost certainly a requirement for decent Desktop vitualisation) and SSE4 extensions are shown above.


You will need a properly configured X-Windows installation (see Handbook) and sufficient RAM and Hard Disk space. For each desktop-style Guest OS you would expect to need between 256-384MB of RAM and 6+GB of space, and the more especially of RAM the better the experience.

Installing VirtualBox

NB. These instructions deal with port at or after OSE 3.1.2/

To build VirtualBox use the port found at

cd /usr/ports/emulators/virtualbox; make install clean

You are strongly recommended to select the GuestAdditions option when configuring the port. This will install the port virtualbox-ose-additions, an .iso of dirvers and other useful features. If you find Windows is unable to exceed 800x600 then Guest additions is not installed.

This port has been rewritten recently to make VirtualBox install simpler and easier. Once you have compiled it, you will need to adjust /boot/loader.conf as below

echo vboxdrv_load="YES" >> /boot/loader.conf

If you enter “VirtualBox” from a terminal you should see a start up screen like this:


Preparing for first Guest OS

The principle behind the setup is simple. Allocate a certain amount of your harddisk to become a single file. That single file will pretend to be a real Hard Disk to the VM, which will also have access to all your peripherals, network card and a pre-determined slice of RAM.

The VMM has a simple to use wizard and it is recommended to use this for the first VM. You will need to experiment a little on the optimum Hard disk and RAM allocation for your machine, but 384MB and 8GB has been found to be a workable minimum for Windows XP and ubuntu installs.

After the wizard has run you should now have a “virtual machine” ready to start - 8GB of Harddrive (actually a single file on your real harddrive) and 384MB of RAM ready to go. At this point we can boot up the instance, and you should see a complaint that there is no bootable image. We shall now install a OS onto the vitual machine.


Setting up first Guest OS

We want the VM to boot up, and then look for a bootable image (ie the VM boots and is given a install CD). This can be done in two ways - by allowing the VM access to our own CD/DVD drive, or by mounting an ISO for it. THe second is a lot more convenient - especially for ubuntu.

Obtain ISO

You can just download the latest Ubuntu ISO, or you can extract your licensed copy of XP by placing the CD in your CD drive and

# dd if=/dev/acd0 of=/home/pbrian/downloads/xp.iso bs=2048

(NB the block size setting is very important - without it you will not copy anything from a CD drive)

Now visit the CD tab in the VMM GUI. tick the ‘Mount CD Drive’ and then tick ‘Mount from ISO’. Simply find the iso image on your HDD, and now the Virtual Machine you selected will be able to “see” the CD as if it was in a normal CD drive.

Install From ISO

Start up the VM instance, now you will be able to install the chosen OS as you would expect, perhaps a little faster than you are used to.

After installing your chosen OS, you will be able to “start” the VM from the VMM control panel - do so now, and you will see the usual boot up screens and then a working instance of another Operating System in your FreeBSD machine.


VirtualBox offers a NAT-based networking address for your VM out-of-the-box. This enables you to initiate connections from the VM, but to have connections initiated to the VM, you will need to set up “Ethernet Bridging”.

Bridged Networking

Netgraph is one such popular way of setting up Bridged Networking, creating a driver that is able to bypass the usual networking stack.

With this the VM can take a packet off the real NIC, read it and then put its own reply back onto the real NIC at the same layer - it looks to the Host OS as if right next door on the Ethernet network is another NIC reading the packets it reads and putting more on the network.

The Virtual machine on the other hand just sets up its stack as normal, thinking it has access to a genuine real NIC.

Its a bit like this:

Host  VM
 \    /
  \  /
   --  Netgraph

You will need to re-compile your kernel (see Handbook for instructions). The following can be used as a kernel configuration file:

include GENERIC
ident NGRAPH

options NETGRAPH

You will then need to switch the current virtual NIC over to Bridged mode in the VMM GUI and select the appropriate driver - PCNet III works well the Netgraph driver.



[1]Well, attitudes to this might be changing. ARM-based blade servers can actually deliver more CPU cycles per Watt, actually being more green, but other issues start to dominate, ranging from deciding if your work is IO bound or CPU bound, or handling the logistics required.