THT's den

THT's den

How to install encrypted root-on-tmpfs nixos


  • This guide assumes that you have already booted into NixOS live.
  • Basic Linux knowledge is assumed.

This guide is mainly based off of this blog post but tweaked for Btrfs and encryption

Partitioning the drives

cfdisk /dev/sda

Creating filesystems

Firstly you need to create a 32-bit vFAT for the ESP partition

mkfs.vfat -F32 /dev/sda1

Then we can create a LUKS parition for /nix

# Format the drive to LUKS
cryptsetup luksFormat /dev/sda2
# Open the LUKS drive
cryptsetup luksOpen /dev/sda2 cryptroot
mkfs.btrfs /dev/mapper/cryptroot

Creating subvolumes

# Temporally mount the partition to /mnt
mount /dev/mapper/cryptroot /mnt
# Create a subvolume for /nix
btrfs subvolume create /mnt/@nix
# Create a subvolume for /home
btrfs subvolume create /mnt/@home
# Unmount it
umount /mnt

Setting up the filesystems

# Mount the root file system
mount -t tmpfs none /mnt
# Create directories
mkdir -p /mnt/{boot,nix,home}

# Mount /boot and /nix
mount -t btrfs -o compress=zstd,subvol=@nix /dev/mapper/cryptroot /mnt/nix
mount -t btrfs -o compress=zstd,subvol=@home /dev/mapper/cryptroot /mnt/home
mount /dev/sda1 /mnt/boot


Now first create a nixos config using

nixos-generate-config --root /mnt
fileSystems."/" = {
  fsType = "tmpfs";
  options = [ "defaults" "size=2G" "mode=755" ];


When you have a system with a tmpfs root you have to configure all users and passwords in configuration.nix, otherwise you won’t have any user or a password on the next boot.

You probably want to have immutable users as well since it doesn’t make any sense to have mutability of users if it’s going to reset anyways.

Note: Don’t use the options password or hashedPassword for users because it won’t work. It has to be the options named initialPassword or initialHashedPassword.

  # …

  # Don't allow mutation of users outside of the config.
  users.mutableUsers = false;

  # Set a root password, consider using initialHashedPassword instead.
  # To generate a hash to put in initialHashedPassword
  # you can do this:
  # $ nix-shell --run 'mkpasswd -m SHA-512 -s' -p mkpasswd
  users.users.root.initialPassword = "hunter2";

  # …


You can now tweak anything else you want in the config file(Timezones, users, etc) Just make sure to copy /etc/nixos to /nix to preserve your configs


Now to install the system just run

nixos-install --no-root-passwd