Nix uses a so-called binary cache to optimise building a package from source into downloading it as a pre-built binary. That is, whenever a command like nixos-rebuild needs a path in the Nix store, Nix will try to download that path from the Internet rather than build it from source. The default binary cache is http://cache.nixos.org/. If this cache is unreachable, Nix operations may take a long time due to HTTP connection timeouts. You can disable the use of the binary cache by adding --option use-binary-caches false, e.g. $ nixos-rebuild switch --option use-binary-caches false If you have an alternative binary cache at your disposal, you can use it instead: $ nixos-rebuild switch --option binary-caches http://my-cache.example.org/