259 lines
8.8 KiB
ReStructuredText
259 lines
8.8 KiB
ReStructuredText
Configuration
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#############
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Liminix uses the Nix language to provide congruent configuration
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management. This means that to change anything about the way in
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which a Liminix system works, you make that change in
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your :file:`configuration.nix` (or one of the other files it references),
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and rerun :command:`nix-build` or :command:`liminix-rebuild` to action
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the change. It is not possible (at least, without shenanigans) to make
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changes by logging into the device and running imperative commands
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whose effects may later be overridden: :file:`configuration.nix`
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always describes the entire system and can be used to recreate that
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system at any time. You can usefully keep it under version control.
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If you are familiar with NixOS, you will notice some similarities
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between NixOS and Liminix configuration, and also some
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differences. Sometimes the differences are due to the
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resource-constrained devices we deploy onto, sometimes due to
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differences in the uses these devices are put to.
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Configuration taxonomy
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**********************
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There are many things you can specify in a configuration, but these
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are the ones you most commonly need to change:
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* which services (processes) to run
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* what packages to install
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* permitted users and groups
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* Linux kernel configuration options
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* Busybox applets
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* filesystem layout
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Modules
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*******
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**Modules** are a means of abstraction which allow "bundling"
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of configuration options related to a common purpose or theme. For
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example, the ``dnsmasq`` module defines a template for a dnsmasq
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service, ensures that the dnsmasq package is installed, and provides a
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dnsmasq user and group for the service to run as. The ``ppp`` module
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defines a service template and also enables various PPP-related kernel
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configuration.
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Not all modules are included in the configuration by default, because
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that would mean that the kernel (and the Busybox binary providing
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common CLI tools) was compiled with many unnecessary bells and whistles
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and therefore be bigger than needed. (This is not purely an academic concern
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if your device has little flash storage). Therefore, specifying a
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service is usually a two-step process. For example, to add an NTP
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service you first add :file:`modules/ntp` to your ``imports`` list,
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then you create a service by calling
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:code:`config.system.service.ntp.build { .... }` with the appropriate
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service-dependent configuration parameters.
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.. code-block:: nix
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let svc = config.system.service;
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in {
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# ...
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imports = [
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./modules/ntp
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# ....
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];
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config.services.ntp = svc.ntp.build {
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pools = { "pool.ntp.org" = ["iburst"]; };
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makestep = { threshold = 1.0; limit = 3; };
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};
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Merely including the module won't define the service on its own: it
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only creates the template in ``config.system.service.foo`` and you
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have to create an actual service using the template. This is an
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intentional choice to allow the creation of multiple
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differently-configured services based on the same template - perhaps
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e.g. when you have multiple networks (VPNs etc) in different trust
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domains, or you want to run two SSH daemons on different ports.
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(For the background to this, please refer to the :doc:`architecture decision record <adr/module-system>`)
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.. tip:: Liminix modules should be quite familiar (but also different)
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if you already know how to use NixOS modules. We use the
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NixOS module infrastructure code, meaning that you should
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recognise the syntax, the type system, the rules for
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combining configuration values from different sources. We
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don't use the NixOS modules themselves, because the
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underlying system is not similar enough for them to work.
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Services
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********
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We use the `s6-rc service manager <https://www.skarnet.org/software/s6-rc/overview.html>`_ to start/stop/restart services and handle
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service dependencies. Any attribute in `config.services` will become
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part of the default set of services that s6-rc will try to bring up on
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boot.
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For the most part, for common use cases, hopefully the services you
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need will be defined by modules and you will only have to pass the
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right parameters to ``build``.
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Should you need to create a custom service of your own devising, use
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the `oneshot` or `longrun` functions:
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* a "longrun" service is the "normal" service concept: it has a
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``run`` action which describes the process to start, and it watches
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that process to restart it if it exits. The process should not
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attempt to daemonize or "background" itself, otherwise s6-rc will think
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it died. Whatever it prints to standard output/standard error
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will be logged.
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.. code-block:: nix
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config.services.cowsayd = pkgs.liminix.services.longrun {
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name = "cowsayd";
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run = "${pkgs.cowsayd}/bin/cowsayd --port 3001 --breed hereford";
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# don't start this until the lan interface is ready
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dependencies = [ config.services.lan ];
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}
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* a "oneshot" service doesn't have a process attached. It consists of
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``up`` and ``down`` actions which are bits of shell script that
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are run at the appropriate points in the service lifecycle
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.. code-block:: nix
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config.services.greenled = pkgs.liminix.services.oneshot {
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name = "greenled";
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up = ''
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echo 17 > /sys/class/gpio/export
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echo out > /sys/class/gpio/gpio17/direction
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echo 0 > /sys/class/gpio/gpio17/value
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'';
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down = ''
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echo 0 > /sys/class/gpio/gpio17/value
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'';
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}
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Services may have dependencies: as you see above in the ``cowsayd``
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example, it depends on some service called ``config.services.lan``,
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meaning that it won't be started until that other service is up.
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..
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TODO: explain service outputs
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..
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TODO: outputs that change, and services that poll other services
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Module implementation
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*********************
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Modules in Liminix conventionally live in
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:file:`modules/somename/default.nix`. If you want or need to
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write your own, you may wish to refer to the
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examples there in conjunction with reading this section.
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A module is a function that accepts ``{lib, pkgs, config, ... }`` and
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returns an attrset with keys ``imports, options config``.
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* ``imports`` is a list of paths to the other modules required by this one
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* ``options`` is a nested set of option declarations
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* ``config`` is a nested set of option definitions
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The NixOS manual section `Writing NixOS Modules
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<https://nixos.org/manual/nixos/stable/#sec-writing-modules>`_ is a
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quite comprehensive reference to writing NixOS modules, which is also
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mostly applicable to Liminix except that it doesn't cover
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service templates.
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Service templates
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=================
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To expose a service template in a module, it needs the following:
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* an option declaration for ``system.service.myservicename`` with the
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type of ``liminix.lib.types.serviceDefn``
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.. code-block:: nix
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options = {
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system.service.cowsay = mkOption {
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type = liminix.lib.types.serviceDefn;
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};
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};
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* an option definition for the same key, which specifies where to
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import the service template from (often :file:`./service.nix`)
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and the types of its parameters.
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.. code-block:: nix
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config.system.service.cowsay = liminix.callService ./service.nix {
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address = mkOption {
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type = types.str;
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default = "0.0.0.0";
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description = "Listen on specified address";
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example = "127.0.0.1";
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};
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port = mkOption {
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type = types.port;
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default = 22;
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description = "Listen on specified TCP port";
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};
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breed = mkOption {
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type = types.str;
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default = "British Friesian"
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description = "Breed of the cow";
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};
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};
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Then you need to provide the service template itself, probably in
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:file:`./service.nix`:
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.. code-block:: nix
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{
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# any nixpkgs package can be named here
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liminix
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, cowsayd
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, serviceFns
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, lib
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}:
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# these are the parameters declared in the callService invocation
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{ address, port, breed} :
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let
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inherit (liminix.services) longrun;
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inherit (lib.strings) escapeShellArg;
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in longrun {
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name = "cowsayd";
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run = "${cowsayd}/bin/cowsayd --address ${address} --port ${builtins.toString port} --breed ${escapeShellArg breed}";
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}
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.. tip::
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Not relevant to module-based services specifically, but a common
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gotcha when specifiying services is forgetting to transform "rich"
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parameter values into text when composing a command for the shell
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to execute. Note here that the port number, an integer, is
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stringified with ``toString``, and the name of the breed,
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which may contain spaces, is
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escaped with ``escapeShellArg``
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Types
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=====
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All of the NixOS module types are available in Liminix. These
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Liminix-specific types also exist in ``pkgs.liminix.lib.types``:
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* ``service``: an s6-rc service
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* ``interface``: an s6-rc service which specifies a network
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interface
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* ``serviceDefn``: a service "template" definition
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In the future it is likely that we will extend this to include other
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useful types in the networking domain: for example; IP address,
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network prefix or netmask, protocol family and others as we find them.
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