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Libvirt Backend

The default backend. VMs run locally on libvirt/KVM, using QCOW2 backing files for thin provisioning. See INSTALL.md for Nix and just setup first.

Additional Requirements

  • libvirt and QEMU
  • OVMF (UEFI firmware)

Fedora

sudo dnf install git libvirt qemu-kvm virt-manager guestfs-tools edk2-ovmf
sudo systemctl enable --now libvirtd

Debian / Ubuntu

sudo apt install git libvirt-daemon-system qemu-kvm virt-manager libguestfs-tools ovmf
sudo systemctl enable --now libvirtd

Arch Linux

sudo pacman -S git libvirt qemu-full virt-manager guestfs-tools edk2-ovmf dnsmasq
sudo systemctl enable --now libvirtd

Host Firewall (UFW)

If your host has UFW enabled, it will block DHCP and NAT traffic on the libvirt bridge. Allow traffic on virbr0:

sudo ufw allow in on virbr0
sudo ufw allow out on virbr0
sudo ufw route allow in on virbr0
sudo ufw route allow out on virbr0

Quick Start (Libvirt)

git clone https://github.com/EnigmaCurry/nixos-vm-template ~/nixos-vm-template
cd ~/nixos-vm-template

# Create a VM interactively (prompts for name, profile, resources, etc.)
just create

# Check VM status and get IP address
just status myvm

# SSH into the VM
just ssh myvm              # As 'user' (no sudo)
just ssh admin@myvm        # As 'admin' (has sudo)

Libvirt Storage

The libvirt backend uses QCOW2 backing files for thin provisioning. Multiple VMs sharing the same profile share a single base image, with each VM's boot disk storing only the delta (copy-on-write). This means ten VMs built from the same profile consume roughly the space of one full image plus their individual deltas.

The Proxmox backend does not use backing files — each VM receives a full (flattened) copy of the boot disk. Storage-level deduplication (e.g., ZFS) can offset this, but there is no per-profile thin provisioning at the QCOW2 layer.

Bridged Networking (Libvirt)

By default, VMs use NAT networking via libvirt's virbr0 bridge. This gives VMs internet access but they're not directly accessible from other machines on your LAN.

For bridged networking, VMs connect directly to your physical network and get IP addresses from your network's DHCP server (or use a static IP). This requires a bridge interface on the host.

Interactive Bridge Setup

When you select "Bridge" during just create, an interactive picker shows available bridges with their state, IP address, and physical interfaces:

Select network bridge:
  br0 (up, 10.13.14.1/24, enp6s0)
  br1 (down)
  Create new bridge

The wizard will guide you through:

  • Creating a bridge if none exist (or choose "Create new bridge")
  • Adding a physical interface if the bridge has no ports
  • Bringing the bridge up if it's currently down

All bridge management uses NetworkManager (nmcli) so connections persist across reboots.

Manual Bridge Setup

You can also set up bridges manually:

# Create the bridge
sudo nmcli connection add type bridge ifname br0 con-name br0 stp no

# Add your physical interface (replace enp6s0)
sudo nmcli connection add type bridge-slave ifname enp6s0 master br0

# Bring up the bridge (WARNING: briefly disconnects network!)
sudo nmcli connection down "Wired connection 1" && sudo nmcli connection up br0

Static IP Configuration

When using bridged networking, you can assign a static IP instead of relying on DHCP. The interactive wizard prompts for DHCP vs Static after bridge selection:

IP address configuration:
  DHCP (automatic)
  Static IP

If you choose Static IP, you'll be prompted for:

  • IP address with CIDR notation (e.g., 10.13.14.5/24) — the bridge subnet is auto-detected and shown as the example
  • Gateway — discovered from the host routing table, or defaults to .1 on the subnet
  • DNS servers — choose from gateway, Cloudflare, Google, or custom

The CIDR mask (/24) is automatically appended if you enter a bare IP address.

Static IP config is saved in machines/<name>/static_ip and applied at boot. Remove this file and run just upgrade to switch back to DHCP.

Changing Network Mode

just network-config myvm bridge
just upgrade myvm             # Apply the change