NixOS Modded Minecraft Servers

This is a Nix flake with a module services.modded-minecraft-servers that allows you to run multiple instances of modded Minecraft servers.

Usage

In your flake.nix

{
  inputs.mms.url = "github:mkaito/nixos-modded-minecraft-servers";
}

In your server config:

{ inputs, ... }:
{
  imports = [ inputs.mms.module ];

  services.modded-minecraft-servers = {
    # This is mandatory, sorry.
    eula = true;

    # The name will be used for the state folder and system user.
    # In this case, the folder is `/var/lib/mc-e2es`
    # and the user `mc-e2es`.
    e2es = {
      enable = true;

      # Keys that can access the state of this instance (read/write!) over an rsync module
      # Leave empty to disable
      rsyncSSHKeys = [
        "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGguJRLNBsQJ80dEemxeUjBcpF5N7iylGLW4ZMP0eSP8"
        "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBim0Y5S0CXBsRWQsYbEVMrjXUe3l5lLp2gBiZ5mWMO0"
      ];

      serverConfig = {
        # Port must be unique
        server-port = 25566;
        motd = "Welcome to Enigmatica 2: Expert Skyblock";
      };
    };
  };
}

For a more elaborate example, you can have a look at my actual server configuration.

Server configuration

Server configuration is done by setting options under the serverConfig attribute. This is a typed attrset that supports all vanilla settings. If your modpack adds custom settings, you may define these as key-value pairs in serverConfig.extraConfig.

This will overwrite any server.properties file found in the state folder. This module does not support stateful configuration.

For most modpacks, you can use all the defaults with very minor alterations, like a worldgen preset.

The server won't run without accepting the EULA. But we also can't just assume that you've accepted it. This could potentially get me into legal trouble. For this reason, you have to specify eula = true at the top level.

The modules makes no attempt at managing server state.

Ports

Each defined instance will need its own ports. By default, rcon and query are disabled, so you only need to make sure the serverConfig.server-port is unique.

The defined ports will automatically be opened in the system firewall.

If you run multiple instances on the same machine, you might want to look into defining CNAME and SRV records for each of them, to make it easier for your friends to connect.

Assuming you have friends. I wouldn't know what that's like.

Vanilla

I don't run vanilla servers. There's a module in nixpkgs for that though.

Modded

I have yet to see two modpacks that use the same procedure for installation and invocation. The latter only got worse with the introduction of Fabric. The module makes no real attempt at guessing how to package or run your modpack. It just provides the necessary options to define a folder for you to dump the files in, and it will call a script start.sh in this folder. An environment variable $JVMOPTS will be set based on the nix settings for the instance. Using this option is up to you in your start.sh script.

Download and installation

There are no plans to handle this with Nix. Downloading things from Curse Forge or similar is easy, but every modpack does something slightly different. The result of this would be a completely separate derivation for each modpack, which is a fool's errand. And that's before we consider that you almost certainly will want to override some mod settings.

It is far easier to just download the server pack to your computer, do whatever it is the modpack author wants you to do to populate files and folders, adjust things to your liking, and wrap it all up in a start.sh script that you either write yourself or adapt from one of the scripts provided with the modpack.

Bear in mind that if the modpack has mods with native dependencies, such as OpenComputers, the download script will likely fetch these native dependencies for your current platform. If your platform is not the same as your servers (eg Windows vs Linux), then you will want to run the installation process on a computer with the same platform.

I suggest you just do it in your home folder on the same server you are going to use.

Writing a start script

The package jre8_headless is available in $PATH, so you may simply call java in your script. The environment variable $JVMOPTS will be set based on your instance configuration. This allows you to handle your flags and memory with nix, but still let modpack authors do their thing with regard to server launch.

A simple example:

exec java -server "${JVMOPTS[@]}" -jar forge-1.12.2-14.23.5.2847-universal.jar nogui"

Depending on what exactly the modpack uses to launch, you may or may not be able to pass the $JVMOPTS variable along. In that case, you'll be stuck setting the flags manually in whatever format they use.

Rsync module

If you set rsyncSSHKeys, an rsync module will be available at ssh://mc-${name}@yourserver:${name}. This rsync module has read/write access, so you should only allow people that you trust.

The SSH keys defined here will all have a forced command that invokes rsync with a specific configuration, defining only a single module with access to the appropriate minecraft state folder.

Under the hood

Each defined instance runs as its own user mc-${name} and has its own state folder in /var/lib/mc-${name}.

In order to "install" a modpack, you put all the files for the pack in this folder.

Whether you do this using the configured rsync module or any other way is irrelevant. Just make sure the entire folder is owned by the correct user.

The server will be run as a systemd unit with the name mc-${name}.