Writing Custom Vars Generators

Declare a vars generator, generate a hashed root password, deploy it to a machine, change as needed.

This guide covers the full clan vars workflow:

1. Declare a generator in the machine's NixOS configuration.
2. Inspect the status of variables using the Clan CLI.
3. Generate variables interactively.
4. Observe the changes made to your repository.
5. Update the machine configuration.
6. Change the root password when needed.

For a detailed API reference, see the vars module documentation.

Declare the generator🔗

In this example, a vars generator is used to:

• prompt the user for the password
• run the required mkpasswd command to generate the hash
• store the hash in a file
• expose the file path to the NixOS configuration

Create a new Nix file root-password.nix with the following content and import it into your configuration.nix

{ config, pkgs, ... }:
{

  clan.core.vars.generators.root-password = {
    # prompt the user for a password
    # (`password-input` being an arbitrary name)
    prompts.password-input.description = "the root user's password";
    prompts.password-input.type = "hidden";
    # don't store the prompted password itself
    prompts.password-input.persist = false;
    # define an output file for storing the hash
    files.password-hash.secret = false;
    # define the logic for generating the hash
    script = ''
      cat $prompts/password-input | mkpasswd > $out/password-hash
    '';
    # the tools required by the script
    runtimeInputs = [ pkgs.mkpasswd ];
  };

  # ensure users are immutable (otherwise the following config might be ignored)
  users.mutableUsers = false;
  # set the root password to the file containing the hash
  users.users.root.hashedPasswordFile =
    # clan will make sure, this path exists
    config.clan.core.vars.generators.root-password.files.password-hash.path;
}

Inspect the status🔗

Executing clan vars list, you should see the following:

$ clan vars list my-machine
root-password/password-hash: <not set>

...indicating that the value password-hash for the generator root-password is not set yet.

Generate the values🔗

This step is not strictly necessary, as deploying the machine via clan machines update would trigger the generator as well.

To run the generator, execute clan vars generate for your machine

$ clan vars generate my-machine
Enter the value for root-password/password-input (hidden):

After entering the value, the updated status is reported:

Updated var root-password/password-hash
  old: <not set>
  new: $6$RMats/YMeypFtcYX$DUi...

Observe the changes🔗

With the last step, a new file was created in your repository: vars/per-machine/my-machine/root-password/password-hash/value

If the repository is a git repository, a commit was created automatically:

$ git log -n1
commit ... (HEAD -> main)
Author: ...
Date:   ...

    vars: update via generator root-password (machine: my-machine)

Update the machine🔗

clan machines update my-machine

Change the root password🔗

Changing the password can be done via this command. Replace my-machine with your machine.

$ clan vars generate my-machine --generator root-password --regenerate
...
Enter the value for root-password/password-input (hidden):
Input received. Processing...
...
Updated var root-password/password-hash
  old: $6$tb27m6EOdff.X9TM$19N...

  new: $6$OyoQtDVzeemgh8EQ$zRK...