NixOS within NixOS?

There is a fairly standard way of doing this that is built into the default system.

Namely nixos-rebuild build-vm. This will take your current configuration file (by default /etc/nixos/configuration.nix, build it and create a script allowing you to boot the configuration into a virtualmachine.

once the script has finished, it will leave a symlink in the current directory. You can then boot by running ./result/bin/run-$HOSTNAME-vm which will start a boot of your virtualmachine for you to play around with.

TLDR;

1. nixos-rebuild build-vm
2. ./result/bin/run-$HOSTNAME-vm

As you may already know, system configurations can coexist without any problems in the Nix store. The problem here is running more than one system at once. For this, you need an isolation or virtualization tools like Docker, VirtualBox, etc.

NixOS Containers

NixOS provides an efficient implementation of the container concept, backed by systemd-nspawn instead of an image-based container runtime.

These can be specified declaratively in configuration.nix or imperatively with the nixos-container command if you need more flexibility.

Docker

Docker was not designed to run an entire operating system inside a container, so it may not be the best fit for testing NixOS-based deployments, which expect and provide systemd and some services inside their units of deployment. While you won't get a good NixOS experience with Docker, Nix and Docker are a good fit. UPDATE: Both 'raw' Nix packages and NixOS run in Docker. For example, Arion supports images from plain Nix, NixOS modules and 'normal' Docker images.

NixOps

To deploy NixOS inside NixOS it is best to use a technology that is designed to run a full Linux system inside.

It helps to have a program that manages the integration for you. In the Nix ecosystem, NixOps is the first candidate for this. You can use NixOps with its VirtualBox backend or the (currently experimental) NixOS container backend, or you can use the none backend to deploy to machines that you have created using another tool.

Tests

If the your goal is to run automated integration tests, you can make use of the NixOS VM testing framework. This uses Linux KVM virtualization (expose /dev/kvm in sandbox) to run integrations test on networks of virtual machines, and it runs them as a derivation. It is quite efficient because it does not have to create virtual machine images because it mounts the Nix store in the VM. These tests are "built" like any other derivation, making them easy to run.

Nix store optimization

A unique feature of Nix is that you can often reuse the host Nix store, so being able to mount a host filesystem in the container/vm is a nice feature to have in your solution. If you are creating your own solutions, depending on you needs, you may want to postpone this optimization, because it becomes a bit more involved if you want the container/vm to be able to modify the store. NixOS tests solve this with an overlay file system in the VM. Another approach may be to bind mount the Nix store forward the Nix daemon socket.

nixos-rebuild build-vm is the easiest way to do this, however; you could also import the configuration into a NixOS container (see Chapter 47. Container Management in the NixOS manual and the nixos-container command).

This would be done with something like:

containers.mydeploy = {
  privateNetwork = true;
  config = import ../mydeploy-configuration.nix;
};

Note that you would not want to specify the network configuration in mydeploy-configuration.nix if it's static as that could cause conflicts with the network subnet created for the container.