== Configuration

There are many things you can specify in a configuration, but most
commonly you 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.

[source,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 `+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 [.title-ref]#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.

=== Runtime secrets (external vault)

Secrets (such as wifi passphrases, PPP username/password, SSH keys, etc)
that you provide as literal values in `+configuration.nix+` are
processed into into config files and scripts at build time, and
eventually end up in various files in the (world-readable)
`+/nix/store+` before being baked into a flashable image. To change a
secret - whether due to a compromise, or just as part of to a routine
key rotation - you need to rebuild the configuration and potentially
reflash the affected devices.

To avoid this, you may instead use a "secrets service", which is a
mechanism for your device to fetch secrets from a source external to the
Nix store, and create at runtime the configuration files and scripts
that start the services which require them.

Not every possible parameter to every possible service is configurable
using a secrets service. Parameters which can be configured this way are
those with the type `+liminix.lib.types.replacable+`. At the time this
document was written, these include:

* ppp (pppoe and l2tp): `+username+`, `+password+`
* ssh: `+authorizedKeys+`
* hostapd: all parameters (most likely to be useful for
`+wpa_passphrase+`)

To use a runtime secret for any of these parameters:

* create a secrets service to specify the source of truth for secrets
* use the `+outputRef+` function in the service parameter to specify the
secrets service and path

For example, given you had an HTTPS server hosting a JSON file with the
structure

[source,json]
----
"ssh": {
  "authorizedKeys": {
"root": [ "ssh-rsa ....",  "ssh-rsa ....", ... ]
"guest": [ "ssh-rsa ....",  "ssh-rsa ....", ... ]
  }
}
----

you could use a `+configuration.nix+` fragment something like this to
make those keys visible to ssh:

[source,nix]
----
services.secrets = svc.secrets.outboard.build {
  name = "secret-service";
  url = "http://10.0.0.1/secrets.json";
  username = "secrets";
  password = "liminix";
  interval = 30; # minutes
  dependencies = [ config.services.lan ];
};
services.sshd = svc.ssh.build {
  authorizedKeys = outputRef config.services.secrets "ssh/authorizedKeys";
};
----

There are presently two implementations of a secrets service:

===== Outboard secrets (HTTPS)

This service expects a URL to a JSON file containing all the secrets.

You may specify a username and password along with the URL, which are
used if the file is password-protected (HTTP Basic authentication). Note
that this is not a protection against a malicious local user: the
username and password are normal build-time parameters so will be
readable in the Nix store. This is a mitigation against the URL being
accidentally discovered due to e.g. a log file or error message on the
server leaking.

===== Tang secrets (encrypted local file)

Aternatively, secrets may be stored locally on the device, in a file
that has been encrypted using https://github.com/latchset/tang[Tang].

____
Tang is a server for binding data to network presence.

This sounds fancy, but the concept is simple. You have some data, but
you only want it to be available when the system containing the data is
on a certain, usually secure, network.
____

[source,nix]
----
services.secrets = svc.secrets.tang.build {
  name = "secret-service";
  path = "/run/mnt/usbstick/secrets.json.jwe";
  interval = 30; # minutes
  dependencies = [ config.services.mount-usbstick ];
};
----

The encryption uses the same scheme/algorithm as
https://github.com/latchset/clevis[Clevis] : you may use the
https://github.com/latchset/clevis?tab=readme-ov-file#pin-tang[Clevis
instructions] to encrypt the file on another host and then copy it to
your Liminix device, or you can use `+tangc encrypt+` to encrypt
directly on the device. (That latter approach may pose a chicken/egg
problem if the device needs secrets to boot up and run the services you
are relying on in order to login).