System Installation

Additional configuration

As NixOS configuration is declarative, post-installation tasks, such as user accounts and package selection, can all be done by specifing them in configuration. See NixOS manual for details.

For timezone, hostname, networking, keyboard layout, etc, see /mnt/etc/nixos/configuration.nix.

Set root password

This optional step is an example of declaratively configuring the system.

  1. Generate password hash:

    INST_ROOT_PASSWD=$(mkpasswd -m SHA-512 -s)
  2. Declare initialHashedPassword for root user:

    tee -a /mnt/etc/nixos/${INST_CONFIG_FILE} <<EOF
      users.users.root.initialHashedPassword = "${INST_ROOT_PASSWD}";

System installation

  1. Finalize the config file:

    tee -a /mnt/etc/nixos/${INST_CONFIG_FILE} <<EOF
  2. Take a snapshot of the clean installation, without state for future use:

    zfs snapshot -r rpool_$INST_UUID/$INST_ID@install_start
    zfs snapshot -r bpool_$INST_UUID/$INST_ID@install_start
  3. Apply configuration

    If root password hash is not set:

    nixos-install -v --show-trace --root /mnt

    You will be prompted for a new root password.

    If password hash has been set:

    nixos-install -v --show-trace --no-root-passwd --root /mnt

Finish installation

  1. Take a snapshot of the clean installation for future use:

    zfs snapshot -r rpool_$INST_UUID/$INST_ID@install
    zfs snapshot -r bpool_$INST_UUID/$INST_ID@install
  2. Unmount EFI system partition:

    umount /mnt/boot/efis/*
    umount /mnt/boot/efi
  3. Export pools:

    zpool export bpool_$INST_UUID
    zpool export rpool_$INST_UUID
  4. Reboot:


Immutable root file system

This section is optional.

Often, programs generate mutable files in paths such as /etc and /var/lib. The generated files can be considered a part of the system state.

This generated state is not declaratively managed by NixOS and can not be reproduced from NixOS configuration.

To ensure that the system state is fully managed by NixOS and reproducible, we need to periodically purge the system state and let NixOS regenerate root file system from scratch.

Also see: Erase your darlings: immutable infrastructure for mutable systems.

Save mutable data to alternative path

Before enabling purging on root dataset, we need to back up essential mutable data first, such as host SSH key and network connections. Below are some tips.

  • Some programs support specifying another location for mutable data, such as Wireguard:

    networking.wireguard.interfaces.wg0.privateKeyFile = "/state/etc/wireguard/wg0";
  • For programs without a configurable data path, environment.etc may be used:

    environment.etc = {
      "ssh/ssh_host_rsa_key".source = "/state/etc/ssh/ssh_host_rsa_key";
  • systemd’s tmpfiles.d rules are also an option:

    systemd.tmpfiles.rules = [
      "L /var/lib/bluetooth - - - - /state/var/lib/bluetooth"
  • Bind mount:

    for i in {/etc/nixos,/etc/cryptkey.d}; do
      mkdir -p /state/$i /$i
      mount -o bind /state/$i /$i
    nixos-generate-config --show-hardware-config

Boot from empty root file system

After backing up mutable data, you can try switching to an empty dataset as root file system.

  1. Check current root file system:

    ROOT_FS=$(df --output=source /|tail -n1)
    # rpool/ROOT/default
  2. Set empty file system as root:

    sed -i "s,${ROOT_FS},${ROOT_FS%/*}/empty,g" /etc/nixos/hardware-configuration-zfs.nix
  3. Apply changes and reboot:

    nixos-rebuild boot
  4. If everything went fine, add the output of the following command to configuration:

    ROOT_FS=$(df --output=source /|tail -n1)
    cat <<EOF
      boot.initrd.postDeviceCommands = ''
        zpool import -Nf ${ROOT_FS%%/*}
        zfs rollback -r ${ROOT_FS%/*}/empty@start
  5. Apply and reboot:

    nixos-rebuild boot