Network Problems and Binary Caches in Nix

1st chunk of `nixos/doc/manual/administration/network-problems.section.md`

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# Network Problems {#sec-nix-network-issues}

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.

```ShellSession
# nixos-rebuild switch --option use-binary-caches false
```

If you have an alternative binary cache at your disposal, you can use it
instead:

```ShellSession
# nixos-rebuild switch --option binary-caches http://my-cache.example.org/
```

Title: Network Problems and Binary Caches in Nix

Summary

Nix uses binary caches to download pre-built binaries instead of building from source, with `https://cache.nixos.org/` as the default. Network issues preventing access to this cache can cause timeouts and slow builds. You can disable binary caches with `--option use-binary-caches false` or use an alternative cache with `--option binary-caches http://my-cache.example.org/`.