How Tor Works: Onion Routing, Circuits and Onion Services Explained

Tor is one of the most widely used privacy technologies in the world, yet few users understand what happens after they press Connect. This intermediate course opens the hood: you will learn how onion routing separates knowledge across relays, how circuits are built and encrypted, and how onion services let clients and servers meet without revealing their locations.

We start with the foundations, IP routing, what your ISP and websites can see, and an encryption refresher, then move through relays, directory authorities, the consensus, cells and keys. You will study Tor Browser's defenses against fingerprinting, DNS handling, v3 onion services, censorship circumvention with pluggable transports, and the responsibilities of relay operators.

Every lesson ends with a safe, legal hands-on exercise. We also look honestly at Tor's limits: traffic correlation, malicious exits, browser exploits and user mistakes, and compare Tor with VPNs, proxies, I2P and Hyphanet. By the end you will be able to explain how Tor works to others and use it with a clear threat model.

Build a Tor Field Guide: a personal technical reference that explains Tor in your own words and documents your hands-on exercises.

• Your personal threat model and how specific Tor features address each risk.
• An annotated ASCII diagram of a three-hop circuit showing which relay knows what.
• A diagram of the v3 onion service rendezvous process.
• An attack-and-mitigation table covering correlation, malicious exits, exploits and user mistakes.
• A troubleshooting checklist based on Tor's bootstrap log messages.