Refactor - use types.nullOr - https://github.com/NixOS/nixpkgs/pull/5058/files#r20830534
Refactor - options should be camelCase - https://github.com/NixOS/nixpkgs/pull/5058/files#r20830605
Add more efficient default values - https://github.com/NixOS/nixpkgs/pull/5058/files#r20830679
Reference the environment variables export inside networking.nix - https://github.com/NixOS/nixpkgs/pull/5058/files#r20830855