tor/doc/HACKING/design/03-modules.md

Tor's modules

Generic modules

buffers.c

Implements the buf_t buffered data type for connections, and several low-level data handling functions to handle network protocols on it.

channel.c

Generic channel implementation. Channels handle sending and receiving cells among tor nodes.

channeltls.c

Channel implementation for TLS-based OR connections. Uses connection_or.c.

circuitbuild.c

Code for constructing circuits and choosing their paths. (Note: this module could plausibly be split into handling the client side, the server side, and the path generation aspects of circuit building.)

circuitlist.c

Code for maintaining and navigating the global list of circuits.

circuitmux.c

Generic circuitmux implementation. A circuitmux handles deciding, for a particular channel, which circuit should write next.

circuitmux_ewma.c

A circuitmux implementation based on the EWMA (exponentially weighted moving average) algorithm.

circuituse.c

Code to actually send and receive data on circuits.

command.c

Handles incoming cells on channels.

config.c

Parses options from torrc, and uses them to configure the rest of Tor.

confparse.c

Generic torrc-style parser. Used to parse torrc and state files.

connection.c

Generic and common connection tools, and implementation for the simpler connection types.

connection_edge.c

Implementation for entry and exit connections.

connection_or.c

Implementation for OR connections (the ones that send cells over TLS).

main.c

Principal entry point, main loops, scheduled events, and network management for Tor.

ntmain.c

Implements Tor as a Windows service. (Not very well.)

onion.c

Generic code for generating and responding to CREATE and CREATED cells, and performing the appropriate onion handshakes. Also contains code to manage the server-side onion queue.

onion_fast.c

Implements the old SHA1-based CREATE_FAST/CREATED_FAST circuit creation handshake. (Now deprecated.)

onion_ntor.c

Implements the Curve25519-based NTOR circuit creation handshake.

onion_tap.c

Implements the old RSA1024/DH1024-based TAP circuit creation handshake. (Now deprecated.)

relay.c

Handles particular types of relay cells, and provides code to receive, encrypt, route, and interpret relay cells.

scheduler.c

Decides which channel/circuit pair is ready to receive the next cell.

statefile.c

Handles loading and storing Tor's state file.

tor_main.c

Contains the actual main() function. (This is placed in a separate file so that the unit tests can have their own main().)

Node-status modules

directory.c

Implements the HTTP-based directory protocol, including sending, receiving, and handling most request types. (Note: The client parts of this, and the generic-HTTP parts of this, could plausibly be split off.)

microdesc.c

Implements the compact "microdescriptor" format for keeping track of what we know about a router.

networkstatus.c

Code for fetching, storing, and interpreting consensus vote documents.

nodelist.c

Higher-level view of our knowledge of which Tor servers exist. Each node_t corresponds to a router we know about.

routerlist.c

Code for storing and retrieving router descriptors and extrainfo documents.

routerparse.c

Generic and specific code for parsing all Tor directory information types.

routerset.c

Parses and interprets a specification for a set of routers (by IP range, fingerprint, nickname (deprecated), or country).

Client modules

addressmap.c

Handles client-side associations between one address and another. These are used to implement client-side DNS caching (NOT RECOMMENDED), MapAddress directives, Automapping, and more.

circpathbias.c

Path bias attack detection for circuits: tracks whether connections made through a particular guard have an unusually high failure rate.

circuitstats.c

Code to track circuit performance statistics in order to adapt our behavior. Notably includes an algorithm to track circuit build times.

dnsserv.c

Implements DNSPort for clients. (Note that in spite of the word "server" in this module's name, it is used for Tor clients. It implements a DNS server, not DNS for servers.)

entrynodes.c

Chooses, monitors, and remembers guard nodes. Also contains some bridge-related code.

torcert.c

Code to interpret and generate Ed25519-based certificates.

Server modules

dns.c

Server-side DNS code. Handles sending and receiving DNS requests on exit nodes, and implements the server-side DNS cache.

dirserv.c

Implements part of directory caches that handles responding to client requests.

ext_orport.c

Implements the extended ORPort protocol for communication between server-side pluggable transports and Tor servers.

hibernate.c

Performs bandwidth accounting, and puts Tor relays into hibernation when their bandwidth is exhausted.

router.c

Management code for running a Tor server. In charge of RSA key maintenance, descriptor generation and uploading.

routerkeys.c

Key handling code for a Tor server. (Currently handles only the Ed25519 keys, but the RSA keys could be moved here too.)

Onion service modules

rendcache.c

Stores onion service descriptors.

rendclient.c

Client-side implementation of the onion service protocol.

rendcommon.c

Parts of the onion service protocol that are shared by clients, services, and/or Tor servers.

rendmid.c

Tor-server-side implementation of the onion service protocol. (Handles acting as an introduction point or a rendezvous point.)

rendservice.c

Service-side implementation of the onion service protocol.

replaycache.c

Backend to check introduce2 requests for replay attempts.

Authority modules

dircollate.c

Helper for dirvote.c: Given a set of votes, each containing a list of Tor nodes, determines which entries across all the votes correspond to the same nodes, and yields them in a useful order.

dirvote.c

Implements the directory voting algorithms that authorities use.

keypin.c

Implements a persistent key-pinning mechanism to tie RSA1024 identities to ed25519 identities.

Miscellaneous modules

control.c

Implements the Tor controller protocol.

cpuworker.c

Implements the inner work queue function. We use this to move the work of circuit creation (on server-side) to other CPUs.

fp_pair.c

Types for handling 2-tuples of 20-byte fingerprints.

geoip.c

Parses geoip files (which map IP addresses to country codes), and performs lookups on the internal geoip table. Also stores some geoip-related statistics.

policies.c

Parses and implements Tor exit policies.

reasons.c

Maps internal reason-codes to human-readable strings.

rephist.c

Tracks Tor servers' performance over time.

status.c

Writes periodic "heartbeat" status messages about the state of the Tor process.

transports.c

Implements management for the pluggable transports subsystem.