Host-specific modules

Some systems need more attention than others such as your homelab server and your personal desktop. While it is tempting to create a shared module, it’s not suitable to just dump them into their respective modules directory where it is expected to be used by at least more than two hosts. Some of them are applicable for that host and that host only. This is where you could structure host-specific modules.

Simply put, these are just modules that are under the hosts.$HOSTNAME option namespace in your NixOS configuration. For example, here’s how a decently-sized NixOS configuration for our imaginary homelab server is structured.

hosts/$HOSTNAME/
├── modules/
│   ├── services/
│   │   ├── backup.nix
│   │   ├── database.nix
│   │   ├── dns-server.nix
│   │   ├── firewall.nix
│   │   ├── mail-server.nix
│   │   ├── reverse-proxy.nix
│   │   ├── vaultwarden.nix
│   │   └── wireguard.nix
│   └── default.nix
├── secrets/
│   └── secrets.yaml
├── default.nix
└── README.adoc

In this structure, it is expected that…

modules/services/mail-server.nix should have an option hosts.$HOSTNAME.services.mail-server where we can simply set and forget our preferred mailserver service.
modules/services/reverse-proxy.nix should have an option hosts.$HOSTNAME.services.reverse-proxy where we can enable our preferred reverse proxy service.
modules/services/gitea.nix should have an option hosts.$HOSTNAME.services.gitea where we can simply enable parts of Gitea service. In turn, we could also configure our Gitea service relying on certain parts such as when the preferred reverse proxy service or the monitoring service is enabled.
default.nix may contain trivial settings (i.e., timeservers, nameservers) and imports all of the modules (however you structure it which in this case, we assume it’s from modules/default.nix) and simply compose the entire server with our host-specific modules.

Now, we could have our host setup like in the following code.

{ config, lib, pkgs, ... }

{
  imports = [ ./modules ];

  hosts.$HOSTNAME.services = {
    # Essential services.
    backup.enable = true;
    mail-server.enable = true;
    dns-server.enable = true;
    monitoring.enable = true;

    # Self-hosted applications.
    gitea.enable = true;
    vaultwarden.enable = true;
  };
}

You could even structure it further by combining all of the essential services into a module, say in modules/core-services.nix and enable them in a switch with hosts.$HOSTNAME.core-services.enable. The possibilities are only limited in your imagination.

Ezran

Why set it up like this when you could just set conditionals in the config?

foodogsquared

You could do that but it’s just easier for me to think and compose about the state of the host in this way. Why set conditional config for services.dovecot.enable when I could just think about hosts.$HOSTNAME.services.mail-server.enable where I could replace the underlying mailserver service and its related config.

Pretty nitfy, right?

Ezran

If only you could check other hosts' configuration in some way with those modules, it’ll be niftier.