Overview

Tor (short for "The Onion Router") is a system of software and hundreds to thousands of volunteer-run servers that together enable anonymous communication over the Internet. Traffic from a Tor client is forwarded through a sequence of network nodes so that no single relay knows both the origin and destination. The network and the client software are maintained by the nonprofit The Tor Project and a community of operators. For more technical introductions, see Tor network resources.

How it works

Tor implements a technique called "onion routing," in which data is encrypted in multiple layers. A client builds a circuit through several relays (commonly an entry/guard, one or more middle relays, and an exit relay) and wraps the packet in successive encryption layers so each relay only learns its predecessor and successor. The final node sends the traffic to the public Internet if needed. If the destination uses end-to-end encryption (HTTPS, TLS) the payload remains encrypted beyond the exit node.

Components and characteristics

  • Relays: volunteer machines that forward traffic.
  • Entry/guard nodes: chosen to reduce exposure of the client's IP to the network.
  • Exit nodes: relay traffic to the open Internet; they can see unencrypted data leaving Tor.
  • .onion services: websites and services reachable only within Tor without using the public DNS.

Further details about protocols and design choices are covered in developer and academic material: technical descriptions.

History and development

The ideas behind onion routing began as research into traffic-analysis resistance at government laboratories and in academic settings. The Tor software was later developed and released by early contributors and organized through a nonprofit effort to make the technology widely available. Its design has evolved in response to research, threat models, and operational experience. For project information and documentation, consult project documentation.

Uses, benefits and limitations

People use Tor to protect privacy, resist surveillance, research networked systems, publish anonymously, and access information in censored environments. Journalists, activists, and security-conscious users commonly rely on the Tor Browser, a hardened, privacy-focused web browser built from a modified version of Mozilla Firefox that is configured to reduce tracking and fingerprinting while routing traffic through Tor. You can find the browser and user guidance at Tor Browser resources.

Tor is not a perfect or complete solution: performance can be slower than direct connections; malicious or compromised relays and global traffic-correlation attacks can reduce anonymity; and exit relays can observe unencrypted traffic. Users are advised to combine Tor with good operational security, end-to-end encryption, and an understanding of threat models.