View source for NixOS:nixos-rebuild build-vm

A virtual machine can be created , it will use your /etc/nixos/configuration.nix to make a 'clone' of your system. Useful for testing new configurations. 

<code>
nixos-rebuild build-vm

nixos-rebuild build-vm -I nixos-config=./configuration.nix -I nix_path='<nixpkgs/nixos>' --max-jobs 4  --show-trace # e.g. to specify the environment variables / cores used
</code>

You can also use <code>build-vm-with-bootloader</code>. From the man page: this boots using the regular boot loader of your configuration rather than booting directly into the kernel and initial ramdisk of the system.

You will not be able to login to this virtual machine, as the passwords are not carried over to the virtual machine you build.

You should have user nixosvmtest (isSystemUser or isNormalUser)
<syntaxhighlight lang="nix">
users.users.nixosvmtest.isSystemUser = true ;
users.users.nixosvmtest.initialPassword = "test";
</syntaxhighlight>

If you have a user called nixosvmtest for example, you can add

<syntaxhighlight lang="nix">
users.users.nixosvmtest.group = "nixosvmtest";
users.groups.nixosvmtest = {};
</syntaxhighlight>

to your <code>/etc/nixos/configuration.nix</code>

you should now be able to login and test your system with this user and password.

https://discourse.nixos.org/t/default-login-and-password-for-nixos/4683/2

By default, the virtual machine is configured to have 1 CPU and 1024MiB memory. It may be too small for testing with desktop environment enabled inside. You can set options <code>virtualisation.vmVariant.virtualisation.cores</code> and <code>virtualisation.vmVariant.virtualisation.memorySize</code> to enlarge the CPU cores and memory size for the virtual machine. 

<syntaxhighlight lang="nix">
{
virtualisation.vmVariant = {
  # following configuration is added only when building VM with build-vm
  virtualisation = {
    memorySize =  2048; # Use 2048MiB memory.
    cores = 3;         
  };
}
</syntaxhighlight>

Use <code>virtualisation.vmVariantWithBootLoader</code> option if you used `build-vm-with-bootloader` earlier.

== Troubleshooting ==
=== Still can't login after updating configuration ===
When running a virtual machine a file called <code>$hostname.qcow2</code> is created in your current working directory. After changing your <code>/etc/nixos/configuration.nix</code> delete this file, rebuild and then start the new virtual machine. Now you should be able to login.

== Networking ==

=== qemu ===

To enable connecting from your host to your virtual machine, you'll need to forcefully override the default networking settings to apply those from https://wiki.qemu.org/Documentation/Networking#How_to_get_SSH_access_to_a_guest instead. For example, to expose the VM's port 80 on the (unprivileged) port 8009 of the 'localhost' of the host:

<syntaxhighlight lang="nix">
{ config, pkgs, lib, modulesPath, ... }:

{
  imports = [
    (modulesPath + "/virtualisation/qemu-vm.nix")
  ];

  virtualisation.qemu.networkingOptions = lib.mkForce [
    "-device e1000,netdev=net0"
    "-netdev user,id=net0,hostfwd=tcp:127.0.0.1:8009-:80,\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
  ];

  networking.firewall.allowedTCPPorts = [
    80
  ];
}
</syntaxhighlight>

== Alternatives ==

===  Bootable ISO ===
Build it as a [https://nix.dev/tutorials/nixos/building-bootable-iso-image bootable ISO image]:

=== VM ===
<syntaxhighlight lang="nix">
{ 
...
  imports = [ 
    <nixos/nixos/modules/virtualisation/virtualbox-image.nix> ]
...
</syntaxhighlight>

<code>
nix build -f '<nixpkgs/nixos>' -I nixos-config=./configuration.nix config.system.build.virtualBoxOVA
</code>
[https://discourse.nixos.org/t/nixos-rebuild-build-vm-not-portable-across-linux-distributions/28564/4 Source]