What Node.js modules are essential for implementing the server and client in the article's Downloader?

The essential modules are `http` for creating the server and making client requests, `querystring` for parsing and stringifying POST data, and `child_process` for executing system commands on the client side. The article also uses `fs` for file operations and `zlib` for compression. No third-party packages are used—only the built-in Node.js modules. For a deeper dive into this technique, refer to the original article: Node.js in Penetration Testing - Implementation of a Downloader.