# Network Problems

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 `https://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/