Configuration

Liminix uses the Nix language to provide congruent configuration management. This means that to change anything about the way in which a Liminix system works, you make that change in your configuration.nix (or one of the other files it references), and rerun nix-build or liminix-rebuild to action the change. It is not possible (at least, without shenanigans) to make changes by logging into the device and running imperative commands whose effects may later be overridden: configuration.nix always describes the entire system and can be used to recreate that system at any time. You can usefully keep it under version control.

If you are familiar with NixOS, you will notice some similarities between NixOS and Liminix configuration, and also some differences. Sometimes the differences are due to the resource-constrained devices we deploy onto, sometimes due to differences in the uses these devices are put to.

Configuration taxonomy

There are many things you can specify in a configuration, but these are the ones you most commonly need to change:

• which services (processes) to run

• what packages to install

• permitted users and groups

• Linux kernel configuration options

• Busybox applets

• filesystem layout

Modules

Modules are a means of abstraction which allow “bundling” of configuration options related to a common purpose or theme. For example, the dnsmasq module defines a template for a dnsmasq service, ensures that the dnsmasq package is installed, and provides a dnsmasq user and group for the service to run as. The ppp module defines a service template and also enables various PPP-related kernel configuration.

Not all modules are included in the configuration by default, because that would mean that the kernel (and the Busybox binary providing common CLI tools) was compiled with many unnecessary bells and whistles and therefore be bigger than needed. (This is not purely an academic concern if your device has little flash storage). Therefore, specifying a service is usually a two-step process. For example, to add an NTP service you first add modules/ntp to your imports list, then you create a service by calling config.system.service.ntp.build { .... } with the appropriate service-dependent configuration parameters.

let svc = config.system.service;
in {
  # ...
  imports = [
    ./modules/ntp
    # ....
  ];
  config.services.ntp = svc.ntp.build {
    pools = { "pool.ntp.org" = ["iburst"]; };
    makestep = { threshold = 1.0; limit = 3; };
  };

Merely including the module won’t define the service on its own: it only creates the template in config.system.service.foo and you have to create an actual service using the template. This is an intentional choice to allow the creation of multiple differently-configured services based on the same template - perhaps e.g. when you have multiple networks (VPNs etc) in different trust domains, or you want to run two SSH daemons on different ports. (For the background to this, please refer to the architecture decision record)

Tip

Liminix modules should be quite familiar (but also different) if you already know how to use NixOS modules. We use the NixOS module infrastructure code, meaning that you should recognise the syntax, the type system, the rules for combining configuration values from different sources. We don’t use the NixOS modules themselves, because the underlying system is not similar enough for them to work.

Services

We use the s6-rc service manager to start/stop/restart services and handle service dependencies. Any attribute in config.services will become part of the default set of services that s6-rc will try to bring up on boot.

For the most part, for common use cases, hopefully the services you need will be defined by modules and you will only have to pass the right parameters to build.

Should you need to create a custom service of your own devising, use the oneshot or longrun functions:

• a “longrun” service is the “normal” service concept: it has a run action which describes the process to start, and it watches that process to restart it if it exits. The process should not attempt to daemonize or “background” itself, otherwise s6-rc will think it died. Whatever it prints to standard output/standard error will be logged.

config.services.cowsayd = pkgs.liminix.services.longrun {
  name = "cowsayd";
  run = "${pkgs.cowsayd}/bin/cowsayd --port 3001 --breed hereford";
  # don't start this until the lan interface is ready
  dependencies = [ config.services.lan ];
}

• a “oneshot” service doesn’t have a process attached. It consists of up and down actions which are bits of shell script that are run at the appropriate points in the service lifecycle

config.services.greenled = pkgs.liminix.services.oneshot {
  name = "greenled";
  up = ''
    echo 17 > /sys/class/gpio/export
    echo out > /sys/class/gpio/gpio17/direction
    echo 0   > /sys/class/gpio/gpio17/value
  '';
  down = ''
    echo 0   > /sys/class/gpio/gpio17/value
  '';
}

Services may have dependencies: as you see above in the cowsayd example, it depends on some service called config.services.lan, meaning that it won’t be started until that other service is up.

Module implementation

Modules in Liminix conventionally live in modules/somename/default.nix. If you want or need to write your own, you may wish to refer to the examples there in conjunction with reading this section.

A module is a function that accepts {lib, pkgs, config, ... } and returns an attrset with keys imports, options config.

• imports is a list of paths to the other modules required by this one

• options is a nested set of option declarations

• config is a nested set of option definitions

The NixOS manual section Writing NixOS Modules is a quite comprehensive reference to writing NixOS modules, which is also mostly applicable to Liminix except that it doesn’t cover service templates.

Service templates

To expose a service template in a module, it needs the following:

• an option declaration for system.service.myservicename with the type of liminix.lib.types.serviceDefn

options = {
  system.service.cowsay = mkOption {
    type = liminix.lib.types.serviceDefn;
  };
};

• an option definition for the same key, which specifies where to import the service template from (often ./service.nix) and the types of its parameters.

config.system.service.cowsay = liminix.callService ./service.nix {
  address = mkOption {
    type = types.str;
    default = "0.0.0.0";
    description = "Listen on specified address";
    example = "127.0.0.1";
  };
  port = mkOption {
    type = types.port;
    default = 22;
    description = "Listen on specified TCP port";
  };
  breed = mkOption {
    type = types.str;
    default = "British Friesian"
    description = "Breed of the cow";
  };
};

Then you need to provide the service template itself, probably in ./service.nix:

{
  # any nixpkgs package can be named here
  liminix
, cowsayd
, serviceFns
, lib
}:
# these are the parameters declared in the callService invocation
{ address, port, breed} :
let
  inherit (liminix.services) longrun;
  inherit (lib.strings) escapeShellArg;
in longrun {
  name = "cowsayd";
  run = "${cowsayd}/bin/cowsayd --address ${address} --port ${builtins.toString port} --breed ${escapeShellArg breed}";
}

Tip

Not relevant to module-based services specifically, but a common gotcha when specifiying services is forgetting to transform “rich” parameter values into text when composing a command for the shell to execute. Note here that the port number, an integer, is stringified with toString, and the name of the breed, which may contain spaces, is escaped with escapeShellArg

Types

All of the NixOS module types are available in Liminix. These Liminix-specific types also exist in pkgs.liminix.lib.types:

• service: an s6-rc service

• interface: an s6-rc service which specifies a network interface

• serviceDefn: a service “template” definition

In the future it is likely that we will extend this to include other useful types in the networking domain: for example; IP address, network prefix or netmask, protocol family and others as we find them.