Syncthing

Syncthing is available as a standalone package: nix-env -iA nixos.syncthing

It can also be enabled as a service. Example:

services = {
    syncthing = {
        enable = true;
        group = "mygroupname";
        user = "myusername";
        dataDir = "/home/myusername/Documents";    # Default folder for new synced folders
        configDir = "/home/myusername/.config/syncthing";   # Folder for Syncthing's settings and keys
    };
};

You can confirm Syncthing runs by visiting http://127.0.0.1:8384/ and following the official Getting Started guide: https://docs.syncthing.net/intro/getting-started.html

Note: Be sure to set the group, otherwise all your files will be set to 'syncthing' as the group.

Declarative configuration

You can declaratively set your Syncthing folders by using the services.syncthing.settings.devices and services.syncthing.settings.folders options:

(Note: Before NixOS 21.11, declarative configuration was done in the services.syncthing.declarative option, such as services.syncthing.declarative.folders = {};)

services = {
  syncthing = {
    enable = true;
    group = "mygroupname";
    user = "myusername";
    dataDir = "/home/myusername/Documents";
    configDir = "/home/myusername/.config/syncthing";
    overrideDevices = true;     # overrides any devices added or deleted through the WebUI
    overrideFolders = true;     # overrides any folders added or deleted through the WebUI
    settings = {
      devices = {
        "device1" = { id = "DEVICE-ID-GOES-HERE"; };
        "device2" = { id = "DEVICE-ID-GOES-HERE"; };
      };
      folders = {
        "Documents" = {         # Name of folder in Syncthing, also the folder ID
          path = "/home/myusername/Documents";    # Which folder to add to Syncthing
          devices = [ "device1" "device2" ];      # Which devices to share the folder with
        };
        "Example" = {
          path = "/home/myusername/Example";
          devices = [ "device1" ];
          ignorePerms = false;  # By default, Syncthing doesn't sync file permissions. This line enables it for this folder.
        };
      };
    };
  };
};

If running a headless server, you should also change guiAddress to a publicly visible one (or just 0.0.0.0:8384, for example).

You will also probably have to open a few ports in the firewall:

   # Syncthing ports: 8384 for remote access to GUI
   # 22000 TCP and/or UDP for sync traffic
   # 21027/UDP for discovery
   # source: https://docs.syncthing.net/users/firewall.html
   networking.firewall.allowedTCPPorts = [ 8384 22000 ];
   networking.firewall.allowedUDPPorts = [ 22000 21027 ];

It is also a good idea to protect the web GUI with a username and password combination:

services.syncthing.settings.gui = {
    user = "username";
    password = "password";
};

Home-manager service

https://github.com/nix-community/home-manager/blob/master/modules/services/syncthing.nix