<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-nix-network-issues">
  <title>Network Problems</title>
  <para>
    Nix uses a so-called <emphasis>binary cache</emphasis> to optimise
    building a package from source into downloading it as a pre-built
    binary. That is, whenever a command like
    <literal>nixos-rebuild</literal> 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
    <literal>https://cache.nixos.org/</literal>. 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 <literal>--option use-binary-caches false</literal>, e.g.
  </para>
  <programlisting>
# nixos-rebuild switch --option use-binary-caches false
</programlisting>
  <para>
    If you have an alternative binary cache at your disposal, you can
    use it instead:
  </para>
  <programlisting>
# nixos-rebuild switch --option binary-caches http://my-cache.example.org/
</programlisting>
</section>
