Otherwise sandbox builds can fail, e.g. $ NIX_REMOTE=local?root=/tmp/nix nix-build '<nixpkgs>' -A hello --option build-use-substitutes false ... downloading ‘http://ftpmirror.gnu.org/bash/bash-4.3-patches/bash43-047’... error: unable to download ‘http://ftpmirror.gnu.org/bash/bash-4.3-patches/bash43-047’: Couldn't resolve host name (6)