NixOS:nixos-rebuild build-vm: Difference between revisions

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-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.
+The commands
-<code>
+* <code>nixos-rebuild build-vm</code>
+* <code>nixos-rebuild build-vm-with-bootloader</code>
+will build a virtual machine running NixOS with your system's configuration (usually <code>/etc/nixos/configuration.nix</code>) by using [[Qemu]].
+One of its usages is for testing new configurations without needing to try them on the host.
+= Usage =
+== Create login user ==
+Before calling one of the commands above, you will have to create a user with an initial password first which you can login to because your passwords of your current system are not carried over to the virtual machine.
+Here is an example for a default user which you can simply add to your system config:
+<syntaxhighlight lang="nix">
+users.users.nixosvmtest.isSystemUser = true;
+users.users.nixosvmtest.initialPassword = "test";
+users.groups.nixosvmtest = {};
+users.users.nixosvmtest.group = "nixosvmtest";
+</syntaxhighlight>
+See this [https://discourse.nixos.org/t/default-login-and-password-for-nixos/4683/2 discourse-answer] for more information.
+== Create and run virtual machine ==
+Create the virtual machine by using of the commands above. Afterwards you will find an executable file in <code>./result/bin</code>. By executing the file qemu will open up and you can start testing the system.
+== Examples ==
+<syntaxhighlight lang="bash">
+# build vm of your system config
 nixos-rebuild build-vm
-</code>
-You will that you will not be able to login to this virtual machine, as the passwords are not carried over to the virtual machine you build.
+# e.g. to specify the environment variables / cores used
+nixos-rebuild build-vm\
+  -I nixos-config=./configuration.nix\
+  -I nix_path='<nixpkgs/nixos>'\
+  --max-jobs 4\
+  --show-trace
+# 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.
+nixos-rebuild build-vm-with-bootloader
+</syntaxhighlight>
+= Configure Virtual Machine =
-If you have a user called nixosvmtest for example , you can add
+By default, the virtual machine is configured to have 1 CPU and 1024MiB memory. It may be too small for testing with a desktop environment enabled inside. You can configure the allocated resources with either
+* [https://search.nixos.org/options?channel=25.05&show=virtualisation.vmVariant&query=virtualisation.vmVariant <code>virtualisation.vmVariant</code>]
+* [https://search.nixos.org/options?channel=25.05&show=virtualisation.vmVariantWithBootLoader&query=virtualisation.vmVariant <code>virtualisation.vmVariantWithBootloader</code>]
+by adding the following to your config:
 <syntaxhighlight lang="nix">
-users.users.nixosvmtest.initialHashedPassword = "test";
+# replace `vmVariant` with `vmVariantWithBootLoader` if you are going to use `build-vm-with-bootloder`.
+virtualisation.vmVariant = {
+  # the following configuration is added only when building VM with `build-vm`
+  virtualisation = {
+    memorySize = 2048; # use 2048MiB memory
+    cores = 3;         # use 3 cpu cores
+  };
+}
 </syntaxhighlight>
-to your /etc/nixos/configuration.nix
+= Troubleshooting =
+== Still can't login after updating configuration ==
+When running a virtual machine a file called <code>$hostname.qcow2</code> is created in your current working directory. After changing your <code>/etc/nixos/configuration.nix</code> delete this file, rebuild and then start the new virtual machine. Now you should be able to login.
-you should now be able to login test your system with this user and password.
+= Networking =
+== qemu ==
+To enable connecting from your host to your virtual machine, you'll need to forcefully override the default networking settings to apply those from https://wiki.qemu.org/Documentation/Networking#How_to_get_SSH_access_to_a_guest instead. For example, to expose the VM's port 80 on the (unprivileged) port 8009 of the 'localhost' of the host:
+<syntaxhighlight lang="nix">
+{ config, pkgs, lib, modulesPath, ... }:
+{
+  imports = [
+    (modulesPath + "/virtualisation/qemu-vm.nix")
+  ];
+  virtualisation.qemu.networkingOptions = lib.mkForce [
+    "-device e1000,netdev=net0"
+    "-netdev user,id=net0,hostfwd=tcp:127.0.0.1:8009-:80,\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
+  ];
+  networking.firewall.allowedTCPPorts = [
+  ];
+}
+</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]
-https://discourse.nixos.org/t/default-login-and-password-for-nixos/4683/2
+[[Category:Virtualization]]
+[[Category:NixOS]]