# Customising Packages {#sec-customising-packages}

The Nixpkgs configuration for a NixOS system is set by the {option}`nixpkgs.config` option.

::::{.example}
# Globally allow unfree packages

```nix
{
  nixpkgs.config = {
    allowUnfree = true;
  };
}
```

:::{.note}
This only allows unfree software in the given NixOS configuration.
For users invoking Nix commands such as [`nix-build`](https://nixos.org/manual/nix/stable/command-ref/nix-build), Nixpkgs is configured independently.
See the [Nixpkgs manual section on global configuration](https://nixos.org/manual/nixpkgs/unstable/#chap-packageconfig) for details.
:::
::::

<!-- TODO(@fricklerhandwerk)
all of the following should go to the Nixpkgs manual, it has nothing to do with NixOS
-->

Some packages in Nixpkgs have options to enable or disable optional functionality, or change other aspects of the package.

::: {.warning}
Unfortunately, Nixpkgs currently lacks a way to query available package configuration options.
:::

::: {.note}
For example, many packages come with extensions one might add.
Examples include:
- [`passExtensions.pass-otp`](https://search.nixos.org/packages?query=passExtensions.pass-otp)
- [`python312Packages.requests`](https://search.nixos.org/packages?query=python312Packages.requests)

You can use them like this:
```nix
{
  environment.systemPackages = with pkgs; [
    sl
    (pass.withExtensions (
      subpkgs: with subpkgs; [
        pass-audit
        pass-otp
        pass-genphrase
      ]
    ))
    (python3.withPackages (subpkgs: with subpkgs; [ requests ]))
    cowsay
  ];
}
```
:::

Apart from high-level options, it's possible to tweak a package in
almost arbitrary ways, such as changing or disabling dependencies of a
package. For instance, the Emacs package in Nixpkgs by default has a
dependency on GTK 2. If you want to build it against GTK 3, you can
specify that as follows:

```nix
{ environment.systemPackages = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ]; }
```

The function `override` performs the call to the Nix function that
produces Emacs, with the original arguments amended by the set of
arguments specified by you. So here the function argument `gtk` gets the