Nested Virtualization - VirtualBox inside ESXi

Eric Gustafson


This process describes how to configure an ESXi Linux guest so that the guest can then run VirtualBox and create a nested, 64 bit guest within the ESXi guest – Nested Virtualization.

There is nothing special about the use of Linux as either the ESXi guest, or the nested guest. Any supported operating system should work at either nesting level. This includes nesting ESXi inside of ESXi, which appears to be the most common form of documentation to describe this process.

This process details manually editing an ESXi hosts’s VM configuration file to direct the inclusion of support for virtualization from within the guest. ESXi version 5.5 is specifically addressed. I found a number of blog postings detailing earlier versions of ESXi, unfortunately the process changed in 5.5 and the newer version is much less prevalently documented. I also found blog postings detailing how to use the vSphere Web UI to effect the reconfiguration. Unfortunately, the Web UI is not part of the free license VMWare grants for an ESXi host. As a consequence, this posting details the manual editing to enable nested virtualization – needed if all one has is the free bits from VMWare.

The Process

  1. Enable SSH access to the ESXi server.
  2. Create an ESXi VM for the initial, outer guest.
  3. Shutdown the ESXi VM.
  4. Edit the VM’s .vmx file, adding “vhv.enable = TRUE”.
  5. Done.

1. Enable SSH access to the ESXi Server

If you have not already done so, this process will require direct access to the ESXi instance’s configuration. VMWare often calls this “Tech Support Mode”. This can be done from the ESXi host’s console, or through the vSphere Client. See the appendix in this document titled, Enabling SSH access to ESXi 5.5_, for more detailed instructions on how to enable SSH access.

2. Create an ESXi VM

Create an ESXi VM for the initial, outer guest. VMWare ‘virtual machine version’ 8 or greater should be used; this is the default for ESXi 5.5. Install the operating system, (Linux), nothing special here, just perform a normal install; ensure the installed OS is 64 bit. Do make sure to provision the disk with enough space for the nested OS’s virtual disk. An appropriate amount of CPU and memory will also be needed. The first time you attempt this I recommend over provisioning all three: disk, memory, and CPU. As I was experimenting I found that the nested OS ran notably slower; giving everything extra resources seemed to improve performance. I am still experimenting with fine tuning resource overheads. Generally I’m finding that virtualized components do behave as advertised, but default configurations on install do need some finer tuning – in short, no surprises.

3. Shutdown the ESXi VM

With an ESXi VM configured and installed with an operating system, you are ready to (re)configure the VM to allow nested virtualization. The reconfiguration will happen to the VM’s definition, and as a consequence, the VM needs to be in the stopped state. ESXi, as all others I’m aware of, does not let you (virtually) modify the hardware while the VM is running. Stop the ESXi VM that will be reconfigured to support nested virtualization.

4. Edit the VM’s Configuration

This step is the critical step. The ESXi VM must be shutdown, as noted previously; if it is not the change will be reverted by ESXi. Fundamentally, the change is simple: add the configuration setting vhv.enable = "TRUE" to the configuration. This must be added to the VM’s .vmx file.

SSH into the ESXi server and then locate and edit the VM’s .vmx file:

> find / -name \*.vmx
> echo 'vhv.enable = "TRUE"' >> /path/to/host.vmx

5. Done

Once the change to the VM’s configuration is effected, simply boot the VM and use VirtualBox within the ESXi VM to create a nested VM.

Note: You should be able to create 64 bit, and multiple processor/core VM’s, up to the limits of the ESXi VM’s resources. If the (re)configuration was not successful then it is likely that VirtualBox will allow you to create a VM, but that VM will be restricted to 32 bit and will not support multiple processors - this is a specific pattern I noted prior to discovering the correct process for ESXi 5.5. Earlier versions of ESXi have a different process, not described here, and without my crystal ball, I’m at a loss for future versions.

Extra Credit

In discovering the above process, which I distilled to what I felt was the minimum, for simplicity, I made a few “superfluous” discoveries. So that the above process may be more broadly applicable, an so that the reader is more informed, I have detailed the “extras” here.

Old Wives Tales

Technically not old wives tales, the following details are floating around in blog postings and are no longer accurate, for ESXi 5.5. At one point in time, they were correct; I performed the process detailed here with out them as verification that they were not necessary.

vhv.allow vs. vhv.enable

Earlier versions of ESXi, possibly as late as early 5.x versions, used a slightly different means of enabling nested virtualization. The vhv.allow parameter was applied to the ESXi server’s configuration in /etc/vmware/config. This is not necessary, but does not conflict with the per VM configuration detailed in this posting and required by ESXi 5.5.

Editing the VM’s CPU Preferences

In other postings on this subject I noted that an additional step involving editing the VM’s CPU Preferences was detailed. Changing the “CPU/MMU Virtualization” from ‘Automatic’ to forced settings for Intel VT-x and hardware MMU virtualization appears to no longer be required. I have successfully nested VirtualBox 64 bit VMs inside ESXi 5.5 VMs with the ‘Automatic’ setting. Caveat: I have found no official documentation discussing this for 5.5 either way.

ESXi Networking - Promiscuous Mode

Almost all of the writings I encountered that discuss nesting virtualization with ESXi cite setting “networking” into promiscuous mode. This is not a requirement for nesting virtualization. The basic example of nesting VirtualBox, (using NAT), inside an ESXi Linux instance was executed with out adjusting any ESXi network parameters.

Enabling promiscuous mode is required if your ESXi VM will nest virtual machines that require bridge mode, sometimes evidenced by the fact that the nested VM has a new ethernet MAC address. Promiscuous mode, be it on ESXi, or other technologies, is the means by which a network interface can receive packets for hardware addresses, (MAC addresses), other than the one the interface is assigned.

The most common nesting example I observed was ESXi inside ESXi. In this case, promiscuous mode would be necessary with the default ESXi networking configuration because that model is a bridged model in which each VM is given a newly allocated, and different, MAC address.

Disk Performance

During my initial experimentation with nested VM’s I observed a clear decrease in performance of the nested VM. My initial experimentation mostly only went as far as installing the OS on the nested VM. Installing an OS is generally a disk intensive activity.

Disk virtualization is more expensive than most. Nesting virtualized disks will accumulate “virtualization debt” quicker than other virtualized components. The short, but rambling explanation goes something like this:

In my inner VM I write a block to “disk”. This traverses the inner OS’s file system code and is mapped to a sector on the inner VM’s virtual block device. Writing is the passed to the outer VM, traverses the file system code, and is mapped to the outer VM’s virtual block device. Finally, the block is passed to the host, (physical), file system, mapped through to a sector, and finally placed on the actual physical device. – If your head is spinning now, it should be. That’s three times the block is passed through file system code on it’s eventual path to a physical write.

This problem is understood in the virtualization community, and there are methods for avoiding differing degrees of the penalty based on the requirements of an installation. I will not cover these here. My point: if your nested VM’s strike you as slow, this may be a significant part of the why.

Armed with the above understanding, I set out to make things run a bit smoother, i.e. faster. Here are a few ideas I had; some I carried out successfully, some are on my “to try” list:

  1. Enabling “Host I/O Caching”. This seemed to help and was quick to try.
  2. Giving each virtualized device some ‘room to breath’ eases the pressure. Some extra CPU and memory felt like it helped.
  3. Using alternative “devices” for disk should help as well, I haven’t tried this yet.
    • Raw disk device, by passing the virtualization of the device completely.
    • iSCSI


Enabling SSH access to ESXi 5.5

ESXi supports direct SSH access to the server running ESXi. This mechanism is referred to in VMWare documentation as “Tech Support Mode”. VMWare has a Knowledge Base article elaborating the process for a range of versions: KB article 1017910

Here is the verbal description to enable SSH to the ESXi server from the Windows vSphere (thick) Client:

  1. Select the server’s Configuration tab. Select the server, not a child VM, from the left hand side and then select the ‘Configuration’ tab across the top.
  2. Select the ‘Security Profile’ from the Software section on the left hand side of the Configuration tab.
  3. Open the ‘Services Properties’ dialog window. Click on the ‘Properties…’ link along the right hand side near the top in the ‘Services’ section of the Security Profile table.
  4. Open the ‘SSH Options’ dialog. In the ‘Services Properties’ dialog scroll down to the ‘SSH’ label and select it, (one click). Then press the ‘Options…’ button in the lower right corner of the dialog.
  5. Use the ‘SSH Options’ dialog to enable SSH by choosing “Start and stop with host”. The SSH service can also be started immediately with the ‘Start’ button.
  6. Access the host by ssh’ing to the IP or hostname of the ESXi server and logging in with an appropriate account. The ‘root’ user and password created during ESXi initial installation will work.