Nix packages can be configured to allow or deny certain options. To apply the configuration edit ~/.nixpkgs/config.nix and set it like { allowUnfree = true; } and will allow the Nix package manager to install unfree licensed packages. The configuration as listed also applies to NixOS under nixpkgs.config set. Allow installing of packages that are distributed under unfree license by setting allowUnfree = true; or deny them by setting it to false. Same can be achieved by setting the environment variable: $ export NIXPKGS_ALLOW_UNFREE=1 Whenever unfree packages are not allowed, single packages can still be allowed by a predicate function that accepts package as an argument and should return a boolean: allowUnfreePredicate = (pkg: ...); Example to allow flash player only: allowUnfreePredicate = (pkg: pkgs.lib.hasPrefix "flashplayer-" pkg.name); Whenever unfree packages are not allowed, packages can still be whitelisted by their license: whitelistedLicenses = with stdenv.lib.licenses; [ amd wtfpl ]; In addition to whitelisting licenses which are denied by the allowUnfree setting, you can also explicitely deny installation of packages which have a certain license: blacklistedLicenses = with stdenv.lib.licenses; [ agpl3 gpl3 ]; A complete list of licenses can be found in the file lib/licenses.nix of the nix package tree.

You can define a function called packageOverrides in your local ~/.nixpkgs/config to overide nix packages. It must be a function that takes pkgs as an argument and return modified set of packages. { packageOverrides = pkgs: rec { foo = pkgs.foo.override { ... }; }; }