The Basics of Basics — Introduction to FTP

Introduction

When it comes to file transfer over a network, the first thing that comes to mind is FTP (File Transfer Protocol) — one of the oldest and most fundamental internet protocols. Despite its age, it’s still in use and serves as a starting point for understanding more complex systems.

What is FTP?

FTP (File Transfer Protocol) is a protocol designed for transferring files between computers on a network. It works on the client–server model:

• FTP server — a computer that stores files and waits for requests from clients.
• FTP client — a program (for example, FileZilla, WinSCP, or even a web browser) that connects to the server to download or upload files.

How does FTP work? Two channels — the key to understanding

The unique feature of FTP is that it uses two separate connections (channels):

1. Control Connection

   • Established when the client connects to port 21 of the server.
   • All commands are transmitted through it: login, password, file listing requests, upload and download commands.
   • Remains active throughout the entire session.

2. Data Connection

   • Created every time a file or a list of files needs to be transferred.
   • Data “flows” through it.
   • Closes after the transfer is complete.

Active vs. Passive Mode

Active Mode

1. The client connects from a random port (e.g., 1030) to the server’s port 21.
2. The client tells the server: “I’ll be listening on my port 1031 for data.”
3. The server, from its port 20, initiates a connection to the client’s port.

Downside: doesn’t work well behind NAT or firewalls, as they block incoming connections.

Passive Mode

1. The client connects to the server’s port 21.
2. The client sends the PASV command.
3. The server replies: “Connect to me on port 5005.”
4. The client then initiates the connection to the specified server port.

Advantage: solves the problem with firewalls and NAT, used by default.

The main drawback of FTP

FTP has a critical vulnerability: all data, including logins and passwords, is transmitted in plain text. Anyone intercepting the traffic can easily read your credentials.

⚠️ Conclusion: FTP is the “grandfather” of modern protocols. It’s important for understanding, but for secure data transfer it’s better to use its successors — FTPS and SFTP, which we’ll cover in the next article.

Resources

• RFC 959 — File Transfer Protocol
• FileZilla — official website