This document describes the general structure of the Tor codebase, how it fits together, what functionality is available for extending Tor, and gives some notes on how Tor got that way.

Tor remains a work in progress: We’ve been working on it for more than a decade, and we’ve learned a lot about good coding since we first started. This means, however, that some of the older pieces of Tor will have some “code smell” in them that could sure stand a brisk refactoring. So when I describe a piece of code, I’ll sometimes give a note on how it got that way, and whether I still think that’s a good idea.

The first drafts of this document were written in the Summer and Fall of 2015, when Tor 0.2.6 was the most recent stable version, and Tor 0.2.7 was under development. If you’re reading this far in the future, some things may have changed. Caveat haxxor!

This document is not an overview of the Tor protocol. For that, see the design paper and the specifications at https://spec.torproject.org/ .

For more information about Tor’s coding standards and some helpful development tools, see doc/HACKING in the Tor repository.

For more information about writing tests, see doc/HACKING/WritingTests.txt in the Tor repository.

The very high level

Ultimately, Tor runs as an event-driven network daemon: it responds to network events, signals, and timers by sending and receiving things over the network. Clients, relays, and directory authorities all use the same codebase: the Tor process will run as a client, relay, or authority depending on its configuration.

Tor has a few major dependencies, including Libevent (used to tell which sockets are readable and writable), OpenSSL (used for many encryption functions, and to implement the TLS protocol), and zlib (used to compress and uncompress directory information).

Most of Tor’s work today is done in a single event-driven main thread. Tor also spawns one or more worker threads to handle CPU-intensive tasks. (Right now, this only includes circuit encryption.)

On startup, Tor initializes its libraries, reads and responds to its configuration files, and launches a main event loop. At first, the only events that Tor listens for are a few signals (like TERM and HUP), and one or more listener sockets (for different kinds of incoming connections). Tor also configures a timer function to run once per second to handle periodic events. As Tor runs over time, other events will open, and new events will be scheduled.

The codebase is divided into a few main subdirectories:

src/common – utility functions, not necessarily tor-specific.

src/or – implements the Tor protocols.

src/test – unit and regression tests

src/ext – Code maintained elsewhere that we include in the Tor source distribution.

src/trunnel – automatically generated code (from the Trunnel) tool: used to parse and encode binary formats.

Some key high-level abstractions

The most important abstractions at Tor’s high-level are Connections, Channels, Circuits, and Nodes.

A ‘Connection’ represents a stream-based information flow. Most connections are TCP connections to remote Tor servers and clients. (But as a shortcut, a relay will sometimes make a connection to itself without actually using a TCP connection. More details later on.) Connections exist in different varieties, depending on what functionality they provide. The principle types of connection are “edge” (eg a socks connection or a connection from an exit relay to a destination), “OR” (a TLS stream connecting to a relay), “Directory” (an HTTP connection to learn about the network), and “Control” (a connection from a controller).

A ‘Circuit’ is persistent tunnel through the Tor network, established with public-key cryptography, and used to send cells one or more hops. Clients keep track of multi-hop circuits, and the cryptography associated with each hop. Relays, on the other hand, keep track only of their hop of each circuit.

A ‘Channel’ is an abstract view of sending cells to and from a Tor relay. Currently, all channels are implemented using OR connections. If we switch to other strategies in the future, we’ll have more connection types.

A ‘Node’ is a view of a Tor instance’s current knowledge and opinions about a Tor relay orbridge.

The rest of this document.

Note: This section describes the eventual organization of this document, which is not yet complete.

We’ll begin with an overview of the various utility functions available in Tor’s ‘common’ directory. Knowing about these is key to writing portable, simple code in Tor.

Then we’ll go on and talk about the main data-flow of the Tor network: how Tor generates and responds to network traffic. This will occupy a chapter for the main overview, with other chapters for special topics.

After that, we’ll mention the main modules in Tor, and describe the function of each.

We’ll cover the directory subsystem next: how Tor learns about other relays, and how relays advertise themselves.

Then we’ll cover a few specialized modules, such as hidden services, sandboxing, hibernation, accounting, statistics, guards, path generation, pluggable transports, and how they integrate with the rest of Tor.

We’ll close with a meandering overview of important pending issues in the Tor codebase, and how they affect the future of the Tor software.