288 lines
9.9 KiB
ReStructuredText
288 lines
9.9 KiB
ReStructuredText
.. _configuration:
|
|
|
|
Configuration
|
|
#############
|
|
|
|
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 :file:`modules/ntp` to your ``imports`` list,
|
|
then you create a service by calling
|
|
:code:`config.system.service.ntp.build { .... }` with the appropriate
|
|
service-dependent configuration parameters.
|
|
|
|
.. code-block:: nix
|
|
|
|
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 :doc:`architecture decision record <adr/module-system>`)
|
|
|
|
.. 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.
|
|
|
|
.. _configuration-services:
|
|
|
|
Services
|
|
********
|
|
|
|
In Liminix a service is any kind of long-running task or process on
|
|
the system, that is managed (started, stopped, and monitored) by a
|
|
service supervisor. A typical SOHO router might have services to
|
|
|
|
* answer DHCP and DNS requests from the LAN
|
|
* provide a wireless access point
|
|
* connect using PPPoE or L2TP to an upstream network
|
|
* start/stop the firewall
|
|
* enable/disable IP packet forwarding
|
|
* mount filesystems
|
|
|
|
(Some of these might not be considered services using other
|
|
definitions of the term: for example, this L2TP process would be a
|
|
"client" in the client/server classification; and enabling packet
|
|
forwarding doesn't require any long-lived process - just a setting to
|
|
be toggled. However, there is value in being able to use the same
|
|
abstractions for all the things to manage them and specify their
|
|
dependency relationships - so in Liminix "everything is a service")
|
|
|
|
The service supervision system enables service health monitoring,
|
|
restart of unhealthy services, and failover to "backup" services when
|
|
a primary service fails or its dependencies are unavailable. The
|
|
intention is that you have a framework in which you can specify policy
|
|
requirements like "ethernet wan dhcp-client should be restarted if it
|
|
crashes, but if it can't start because the hardware link is down, then
|
|
4G ppp service should be started instead".
|
|
|
|
Any attribute in `config.services` will become part of the default set
|
|
of services that s6-rc will try to bring up. Services are usually
|
|
started at boot time, but **controlled services** are those that are
|
|
required only in particular contexts. For example, a service to mount
|
|
a USB backup drive should run only when the drive is attached to the
|
|
system. Liminix currently implements three kinds of controlled service:
|
|
|
|
* "uevent-rule" service controllers use sysfs/uevent to identify when
|
|
particular hardware devices are present, and start/stop a controlled
|
|
service appropriately.
|
|
|
|
* the "round-robin" service controller is used for service failover:
|
|
it allows you to specify a list of services and runs each of them
|
|
in turn until it exits, then runs the next.
|
|
|
|
* the "health-check" service wraps another service, and runs a "health
|
|
check" command at regular intervals. When the health check fails,
|
|
indicating that the wrapped service is not working, it is terminated
|
|
and allowed to restart.
|
|
|
|
|
|
Writing services
|
|
================
|
|
|
|
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.
|
|
|
|
.. code-block:: nix
|
|
|
|
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
|
|
|
|
.. code-block:: nix
|
|
|
|
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.
|
|
|
|
..
|
|
TODO: explain service outputs
|
|
|
|
..
|
|
TODO: outputs that change, and services that poll other services
|
|
|
|
Module implementation
|
|
*********************
|
|
|
|
Modules in Liminix conventionally live in
|
|
:file:`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
|
|
<https://nixos.org/manual/nixos/stable/#sec-writing-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``
|
|
|
|
.. code-block:: nix
|
|
|
|
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 :file:`./service.nix`)
|
|
and the types of its parameters.
|
|
|
|
.. code-block:: nix
|
|
|
|
config.system.service.cowsay = config.system.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
|
|
:file:`./service.nix`:
|
|
|
|
.. code-block:: 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.
|