Recently, I wanted to share my Syncthing config across multiple NixOS hosts, and I couldn’t find any good information on it online. I hope this helps someone. I have the following config that’s parameterized only by the username of the syncthing user - everything else stays the same:

services.syncthing = {
  enable = true;
  user = user;
  configDir = "/home/${user}/.config/syncthing";
  dataDir = "/home/${user}/.config/syncthing/db";

  overrideDevices = true;
  overrideFolders = true;

  settings = {
    devices = {
      # map from device name to ID
    };

    folders = {
      # generic sync folder
      "cccjw-5fcyz" = {
        path = "/home/${user}/sync";
        devices = [
          # list of device names to share with
        ];
      };
    };
  };
};

To modularize this, I created a Nix function with the user parameter:

# ./custom/syncthing.nix
{ user }: {
  # same config from above
  ...
}

Then, in my individual NixOS configuration files, I can just import it in my imports line and pass in the right param:

# ./systems/asahi/default.nix
{ config, pkgs, ... }:

{
  imports = [
    ./asahi-hardwarecfg.nix
    ( import ../../custom/syncthing.nix { user = "peyton"; } )
  ];
...

And voilà! I’ve now DRYed up my configuration files. Here’s the config for the curious.