Learn OpenBSD - Virtualization: vmm & vmd

#Introduction

In episode 17 you ran a mail server with OpenSMTPD. Now we return to the foundation of your lab from episode 0 — except this time, you're the hypervisor. OpenBSD brings virtualization directly into the base system: vmm as the type 1 hypervisor and vmd as its managing daemon.

Imagine: everything you've learned in this series can be tested on top of OpenBSD itself. You can run a second OpenBSD as a client, back it up, snapshot it, destroy it, and start again — without any extra machine. This is why vmm is a favorite for labs and testing.

#Architecture: vmm and vmd

OpenBSD's virtualization consists of two parts:

• vmm(4): the hypervisor running in the kernel — the lower layer that executes VMs.
• vmd(8): the userland daemon managing the VM lifecycle.
• vmctl(8): the command to control vmd.

The type 1 nature means the hypervisor runs directly on the hardware, without a host OS in between. It's efficient and — in OpenBSD style — its code surface is far smaller than the big hypervisors.

#Enabling vmm

First check whether your hardware supports virtualization:

dmesg | grep -i vmm
sysctl hw.vmm

If VMX/SVM is supported, enable the services:

rcctl enable vmm
rcctl start vmm
rcctl enable vmd
rcctl start vmd

vmm and vmd are two separate services; both need to be active.

#vmctl: The VM Management Command

vmctl is the main tool. Some basic operations:

vmctl show
vmctl start vm1 -m 512M -d /vm/obsd.img -L
vmctl stop vm1
vmctl console vm1

• vmctl show lists the running VMs.
• vmctl start runs a VM with specified memory and disk; -L connects to the console.
• vmctl stop stops a VM.
• vmctl console connects to a VM's console.

#Setting Up a VM Disk

A VM disk is an image file on the host. Create it with vmctl create:

mkdir -p /vm
vmctl create /vm/obsd.img -s 10G
ls -la /vm

A 10 GB image is ready to use. To install from an ISO, attach the ISO to the CD-ROM interface:

vmctl start vm1 -m 1G -d /vm/obsd.img -c /tmp/install74.iso -L

The VM boots from the ISO, and you go through an OpenBSD installation like in episode 3 — but now inside a VM. After the installation finishes, run it without the ISO.

#Tap-Based Networking

For VMs to communicate, connect them via a tap interface. For a VM connected to a bridge with the host:

add bridge0 up

Then attach the interface at start time:

vmctl start vm1 -m 1G -d /vm/obsd.img -i 1 -L

The VM gets a network interface connected to the same bridge as the host — the same pattern you know from episode 8. Other VMs can join the same bridge and communicate with each other.

#Running Linux on vmm

vmm isn't only for OpenBSD — it also runs Linux and NetBSD. The key is using the right kernel. Many distributions provide a bzImage kernel that can be run directly:

vmctl start linux -m 1G -b bzImage -d /vm/linux.img -i 1 -L

-b specifies the bzImage kernel. Great for quickly testing Linux on top of OpenBSD — useful when comparing ecosystems (episode 22).

#Snapshots for Safe Testing

One of the most useful features for a lab is snapshots — saving and restoring a VM's state:

vmctl stop vm1
vmctl snapshot vm1 /vm/vm1.before-pf.snap
vmctl start vm1 -r /vm/vm1.before-pf.snap -d /vm/obsd.img

• vmctl snapshot saves a VM's memory state.
• vmctl start -r restores from a snapshot.

This pattern is ideal for episodes 12-16: experiment with pf or relayd, and if you break something, revert to a clean snapshot in seconds.

#Monitoring and Managing Resources

Check VM resource consumption:

vmctl show
ps aux | grep vmd
vmctl stop -a

vmctl show displays the status, memory, and uptime of each VM. vmctl stop -a stops all VMs — useful when the lab needs to be clean. For more granular resource settings, check the vm.conf(5) man page for stored VM configurations.

#Closing

In episode 18 you ran virtualization on OpenBSD: getting to know vmm(4) and vmd(8), managing VMs with vmctl, installing and running OpenBSD and Linux VMs, building tap- and bridge-based networking, and using snapshots for safe testing.

Key takeaways:

• vmm is the type 1 hypervisor in the kernel; vmd manages VMs; vmctl controls them.
• A VM disk is an image file created with vmctl create.
• VM networking goes through a tap connected to a bridge.
• VM snapshots are the most valuable testing tool in this series.

In the next episode, episode 19, we'll optimize performance & system optimization — tuning sysctl, using loader.conf, monitoring with systat, top, and netstat, and adjusting buffers and pf performance.