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

Tor's modules

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.