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>