nixos

this approach is one of the easiest and most declarative ways to get a fully configured NixOS VM (with K3s baked in) running on Proxmox. The nixos-generate -f proxmox command does exactly what you described: it turns a configuration.nix into a ready-to-import Proxmox .vma.zst backup file that you can restore as a VM. This is a supported format in nixos-generators and works great for repeatable, infrastructure-as-code style deployments.

You don’t even need to run the generator inside Proxmox — booting the NixOS live ISO (on any machine or as a temporary Proxmox VM) is perfect because it gives you a clean Nix environment with everything you need.

Quick step-by-step (the easy path)

Boot the NixOS ISO
Download the latest NixOS minimal ISO and boot it (you can create a tiny throwaway VM in Proxmox just to run the generator if you want).
Generate the Proxmox VM image
```
nixos-generate -f proxmox -c ./configuration.nix
```
Or if you prefer the flake version (always up-to-date):
```
nix run github:nix-community/nixos-generators -- --format proxmox -c ./configuration.nix
```
The first run can take a while (it builds a patched QEMU for the .vma format).
Output: a file like nixos-*.vma.zst in your current directory (or /nix/store).

Copy to Proxmox

scp nixos-*.vma.zst root@your-proxmox-host:/var/lib/vz/dump/

Import & create the VM on Proxmox
On the Proxmox host:

# Option A: CLI (recommended)
qmrestore /var/lib/vz/dump/nixos-*.vma.zst <VMID> --unique true

# Option B: GUI
Go to the storage where you put the .vma.zst → Content → select the file → Restore

Pick a new unused VMID. The VM will boot with your full NixOS config already applied (users, SSH, networking, K3s, etc.).

Start the VM and enjoy
You now have a fully declarative NixOS machine. Later updates are just nixos-rebuild switch (or remotely with flakes).

Make it include K3s from day one

Just add the following to your configuration.nix (this is a minimal single-node K3s setup — adjust as needed for a multi-node cluster):

{ config, pkgs, ... }: {
  # Basic system stuff (add your SSH keys, user, etc.)
  users.users.admin = {
    isNormalUser = true;
    extraGroups = [ "wheel" "k3s" ];
    openssh.authorizedKeys.keys = [ "ssh-ed25519 YOUR-KEY-HERE" ];
  };
  security.sudo.wheelNeedsPassword = false;

  # Networking / firewall for K3s
  networking.firewall.allowedTCPPorts = [ 6443 ];  # kube-apiserver
  networking.firewall.allowedUDPPorts = [ 8472 ]; # flannel

  # K3s (single-node server)
  services.k3s = {
    enable = true;
    role = "server";           # use "agent" for worker nodes
    # clusterInit = true;      # only needed on the very first server node
    # tokenFile = /run/secrets/k3s-token;  # optional for HA

    # Extra flags (common useful ones)
    extraFlags = toString [
      "--disable=traefik"      # use your own ingress if you want
      "--write-kubeconfig-mode=644"
    ];
  };

  # Optional: auto-deploy some manifests (e.g. nginx test pod)
  # services.k3s.manifests.nginx = { ... };  # see NixOS docs

  system.stateVersion = "24.11";  # or whatever release you're on
}

You can also set Proxmox VM hardware specs directly in the same file (cores, RAM, disk size, etc.) using the proxmox module options — no need to tweak them in the Proxmox GUI after restore.

Why this is the easy way

Fully declarative — one configuration.nix (or flake) defines everything.
Reproducible — spin up identical nodes in seconds.
No manual post-install — K3s, users, SSH, firewall, etc. are already there when the VM boots.
After the first deploy you can manage the VM with plain nixos-rebuild (even remotely).

If you want an even more automated workflow later, you can combine this with flakes + nixos-anywhere or deploy-rs, but for getting started the method you described is already excellent.

{ config, pkgs, lib, … }: {

imports = [ <nixpkgs/nixos/modules/virtualisation/proxmox-image.nix> # Important! # or if using flakes: “${inputs.nixpkgs}/nixos/modules/virtualisation/proxmox-image.nix” ]; users.users.root.initialPassword = “your-password”;

Proxmox VM hardware settings (example)

proxmox.qemuConf = { cores = 4; memory = 8192; # in MiB # bios = “ovmf”; # for UEFI if you prefer };

=== Basic system setup ===

networking.hostName = “k3s-nixos”;

=== K3s ===

services.k3s = { enable = true; role = “server”; extraFlags = “–disable=traefik”; # remove if you want Traefik };

system.stateVersion = “25.11”; }

nix –extra-experimental-features “nix-command flakes”
run github:nix-community/nixos-generators –
–format proxmox -c ./configuration.nix

scp nixos-*.vma.zst root@192.168.0.143:/var/lib/vz/dump/

on proxmox host qmrestore /var/lib/vz/dump/nixos-*.vma.zst 9000 –unique true