Get NixOS services on a not-NixOS system

• You still need nix though.
• Also requires that the system runs systemd because NixOS services are based on systemd.
• Also assumes the system uses dpkg (Debian, Ubuntu, ...) but you can probably work around that.

Overview

• You know how a NixOS configuration is a module where you enable a bunch of services and whatever.
• nixos-rebuild combines this configuration module with the rest of the NixOS modules to create a system.
• This system is a derivation, put into a profile somewhere and linked around the filesystem.
  • Remember that profiles are in /nix/var/nix/profiles.
• What if you could control this last linking step? Carefully install only the services you want, without installing a whole NixOS system.
• Basically this is what this is.
• Each "service" directory contains a kind of NixOS configuration, which specifies the services to enable, and build instructions for a package.
• The NixOS configuration is put into a profile.
• The package contains the symlinks to the profile, so that the system's package manager handles the necessary system state.
• After installing a package, updates of the services are done on the profile, so reinstalling the package isn't necessary (unless there are new files).

Usage examples

• nix run .#yggdrasil profile to print the profile where the service will be installed.
• nix run .#yggdrasil update to add/update the yggdrasil service in the current user's profile directory.
• sudo nix run .#yggdrasil update to do this on the root user's profile directory.
  • This is necessary because the packages (see next point) link to the root user's profile directory, but if you're not using a package for a particular module then you can link it to your own user's profile and figure it out yourself.
• sudo nix run .#yggdrasil history to see the profile history, sudo nix run .#yggdrasil rollback --to 123 to switch to another version.
• sudo nix run .#yggdrasil help to see a summary of things you can do.
• nix build .#package-yggdrasil to get a Debian/Ubuntu package containing the symlinks to the service's profile.
  • Install with apt install ./result/share/service-yggdrasil.deb. When the install.sh script is modified, you'll very likely need to rebuild and reinstall the package.
• You'll need to manually start and stop the service, e.g. sudo systemctl start yggdrasil.service.
• The service is automatically enabled though (mainly to reduce the amount of unaccounted-for system state), so you can also just reboot.
• Systemd will complain but probably won't do anything else when you remove the service files of a running service. But unless it's critical networking stuff you probably want to stop them first anyway.
• Remember to restart the service after updates.

Writing a service module yourself

• Good luck.
• Make a directory, say foobar.
• Add a foobar/default.nix file containing the configuration module, a debianControl attribute containing text for a Debian package control file, and an installScript attribute containing the package install script to create the package.
• Add "foobar" to the services in flake.nix (under line 8 at this time).
• 🎉

Thanks

• Me. :P
• Nerath/lun* for help on the profile-setting.
• Dusk for help on how to patch nixpkgs.

Other useful commands

• sudo dpkg-divert --package service-firewall --divert /etc/nftables.conf.divert --rename --add /etc/nftables.conf
  • The firewall service here interferes with nftables in Ubuntu, but the existing nftables service is valuable so, instead of specifying a conflict in the package, the conflict is diverted.
• sudo apt autoremove
  • The ubuntu-advantage-tools-stub """service""" (it's unrelated to NixOS, it only meaningfully makes a package for Ubuntu to remove ads) replaces ubuntu-advantage-tools in Ubuntu, and some dependencies are left dangling as a result.