[Nix-dev] Better firewalling in NixOS (PR: 12940)

[Arnold Krille]

Hey all,

I updated my PR for better firewalling once again and you can find it at https://github.com/NixOS/nixpkgs/pull/12940. Please give reviews, comments and merge if appropriate.

I can't believe its been almost a year since I started that PR. Needless to say these changes are part of the growing stack of custom changes I keep rebasing onto my personal stable branch.

Have fun,

Arnold