nixpkgs/nixos/doc/manual/administration/network-problems.xml

<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        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 <command>nixos-rebuild</command> 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
  <uri>https://cache.nixos.org/</uri>. 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>--option
  use-binary-caches false</option>, e.g.
<screen>
# nixos-rebuild switch --option use-binary-caches false
</screen>
  If you have an alternative binary cache at your disposal, you can use it
  instead:
<screen>
# nixos-rebuild switch --option binary-caches http://my-cache.example.org/
</screen>
 </para>
</section>