NixOS within NixOS?

Question

I'm starting to play around with NixOS deployments. To that end, I have a repo with some packages defined, and a configuration.nix for the server.

It seems like I should then be able to test this configuration locally (I'm also running NixOS). I imagine it's a bad idea to change my global configuration.nix to point to the deployment server's configuration.nix (who knows what that will break); but is there a safe and convenient way to "try out" the server locally - i.e. build it and either boot into it or, better, start it as a separate process?

I can see docker being one way, of course; maybe there's nothing else. But I have this vague sense Nix could be capable of doing it alone.

Answer 1

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

Answer 2

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.