Abstract This guide documents the process for a minimal installation of NixOS on a Proxmox virtual machine. It leverages the nixos-anywhere tool for remote deployment and disko for declarative disk partitioning. It also covers the essential post-installation steps for integrating the new host with sops-nix for secrets management.

Table of Contents TOC

Prerequisites on the Target VM
Installation Process
- Deploying NixOS
- Note on Hardware Configuration
Key Configuration Details
- Disko Configuration for Proxmox (MBR Boot)
Post-Installation: Secrets Management
- Step 1: Generating the Host AGE Key
- Step 2: Updating SOPS and Re-encrypting Secrets
TODOs

Prerequisites on the Target VM

Before attempting to install NixOS with nixos-anywhere, you must first perform a critical setup step on the target Proxmox VM.

The minimal NixOS installation ISO does not have a default password for the root user. The nixos-anywhere command requires SSH access, which necessitates a password.

Boot the Proxmox VM using the minimal NixOS installation ISO.
Open a terminal on the VM's console.
Set a password for the root user by running the following command:
```
passwd
```
You will be prompted to enter and confirm a new password.

Installation Process

Deploying NixOS

With the root password set on the target VM, you can now run nixos-anywhere from your local machine to deploy your NixOS configuration.

The following command uses nix run to execute nixos-anywhere, pointing it to a specific flake output (.#susano-minimal) and the IP address of the target VM.

nix run github:nix-community/nixos-anywhere -- \
  --flake .#susano-minimal \
  --target-host root@192.168.1.85

Note on Hardware Configuration

While not used in the command above, nixos-anywhere can automatically generate a hardware configuration file from the target machine. This is useful for capturing machine-specific settings.

To do this, include the --generate-hardware-config flag in your command. The following example shows how to generate the file and save it as ./hardware-configuration.nix in your local flake directory.

nix run github:nix-community/nixos-anywhere -- \
  --flake .#your-flake-output \
  --target-host root@192.168.1.85 \
  --generate-hardware-config ./hardware-configuration.nix

Key Configuration Details

Disko Configuration for Proxmox (MBR Boot)

A critical requirement for ensuring a NixOS VM can boot correctly in Proxmox is the disk partition scheme. Proxmox expects a Master Boot Record (MBR) compatible setup.

When using disko for declarative disk management, you must configure it to create a GPT partition table that includes a special 1M BIOS boot partition (type EF02). This partition is specifically used by GRUB for MBR compatibility.

Here is an example snippet for the disko configuration:

{
  disko.devices = {
    disk = {
      main = {
        device = "/dev/sda";
        type = "disk";
        content = {
          type = "gpt";
          partitions = {
            boot = {
              size = "1M";
              type = "EF02"; # for grub MBR
            };
            # ... your other partitions like root, swap, etc.
          };
        };
      };
    };
  };
}

For a complete example, you can refer to the official disko repository: gpt-bios-compat.nix.

Post-Installation: Secrets Management

Step 1: Generating the Host AGE Key

After the initial installation is complete, you will need its host AGE key to manage secrets with tools like sops-nix. This key is derived from the host's SSH key.

SSH into the newly installed NixOS machine.
```
ssh root@192.168.1.85
```
Run the following command. It temporarily installs the ssh-to-age utility and pipes the public SSH host key to it, converting it to an AGE public key.
```
nix-shell -p ssh-to-age --run 'cat /etc/ssh/ssh_host_ed25519_key.pub | ssh-to-age'
```
The command will output the new AGE public key. Copy this key for the next step.

Step 2: Updating SOPS and Re-encrypting Secrets

The new AGE key must be added to your .sops.yaml configuration file. This allows sops to encrypt secrets in a way that the new host (susano) can decrypt them.

Open the .sops.yaml file in the root of your Nix flake.

Replace the old key for the susano host with the new key you generated.

keys:
  - &primary age19wvqtn4ju6k4vs8fxr34unl6xx4cv04jw0lx9ps20xlde927zfssgl4qke
  - &susano age1vkfq9gpqfpyq3s9e79e6vw8kv9485tzna4fm3dy6p0u9uz9feu8qr9sgcf # <--- REPLACE THIS WITH THE NEW KEY
creation_rules:
  - path_regex: secrets/secrets.yaml$
    key_groups:
      - age:
          - *primary
          - *susano

After saving the updated .sops.yaml file, run the updatekeys command. This re-encrypts the specified secrets file with the new set of keys defined in .sops.yaml. For more information, see the official documentation.
```
sops updatekeys secrets/secrets.yaml
```
Your secrets are now encrypted for both the primary key and the new host's key.

TODOs

Refactor the disko configuration to make the disk device name (e.g., /dev/sda) a variable. This will avoid hardcoding the value and make the configuration more portable across different hardware setups.
Investigate and resolve the issue where updating a user's password declaratively using a secret managed by sops failed after the initial installation.