All users of Nixpkgs are free software users, and many users (and developers) of Nixpkgs want to limit and tightly control their exposure to unfree software. At the same time, many users need (or want) to run some specific pieces of proprietary software. Nixpkgs includes some expressions for unfree software packages. By default unfree software cannot be installed and doesn’t show up in searches. To allow installing unfree software in a single Nix invocation one can export NIXPKGS_ALLOW_UNFREE=1. For a persistent solution, users can set allowUnfree in the Nixpkgs configuration. Fine-grained control is possible by defining allowUnfreePredicate function in config; it takes the mkDerivation parameter attrset and returns true for unfree packages that should be allowed.