HACKING is nearly dead now; long live HACKING.

svn:r4407

doc/HACKING

@@ -1,428 +1,14 @@
-                        Guide to Hacking Tor
 
-(As of 8 October 2003, this was all accurate.  If you're reading this in
+1. Coding conventions
-the distant future, stuff may have changed.)
 
-0. Intro and required reading
+1.1. Details
-
-  Onion Routing is still very much in development stages. This document
-  aims to get you started in the right direction if you want to understand
-  the code, add features, fix bugs, etc.
-
-  Read the README file first, so you can get familiar with the basics of
-  installing and running an onion router.
-
-  Then, skim some of the introductory materials in tor-design.pdf,
-  tor-spec.txt, and the Tor FAQ to learn more about how the Tor protocol
-  is supposed to work.  This document will assume you know about Cells,
-  Circuits, Streams, Connections, Onion Routers, and Onion Proxies.
-
-1. Code organization
-
-1.1. The modules
-
-  The code is divided into two directories: ./src/common and ./src/or.
-  The "common" directory contains general purpose utility functions not
-  specific to onion routing.  The "or" directory implements all
-  onion-routing and onion-proxy specific functionality.
-
-  Files in ./src/common:
-
-     aes.[ch] -- Implements the AES cipher (with 128-bit keys and blocks),
-        and a counter-mode stream cipher on top of AES.  This code is
-        taken from the main Rijndael distribution.  (We include this
-        because many people are running older versions of OpenSSL without
-        AES support.)
-
-     compat.[ch] -- Wrappers to make calls more portable.  This code defines
-        functions such as tor_malloc, tor_snprintf, get/set various data types,
-        renaming, setting socket options, switching user IDs.  It is basically
-        where the non-portable items are conditionally included depending on
-        the platform.
-
-     container.[ch] -- Implements a smart list which is a resizable array along
-        with helper functions to use on these lists.  Also includes a
-        splay-tree implementation of the string-to-void* map.
-
-     crypto.[ch] -- Wrapper functions to present a consistent interface to
-        public-key and symmetric cryptography operations from OpenSSL.
-
-     log.[ch] -- Tor's logging subsystem.
-
-     strlcat.c -- Safer, size-bounded string concatenation.  Use this instead
-        of strncat because it has a safer API.  Included for platforms that
-        that don't already ship this code.
-
-     strlcpy.c -- Safer, size-bounded string copying.  Use this instead of
-        strncpy because it is a safer API which guarantees to NUL terminate.
-        Included for platforms that don't already ship this code.
-
-     test.h -- Macros used by unit tests.
-
-     torgzip.[ch] -- A simple in-memory gzip implementation.
-
-     torint.h -- Provides missing [u]int*_t types for environments that
-        don't have stdint.h.
-
-     tortls.[ch] -- Wrapper functions to present a consistent interface to
-        TLS, SSL, and X.509 functions from OpenSSL.
-
-     util.[ch] -- Miscellaneous portability and convenience functions.
-
-  Files in ./src/or:
-
-   [General-purpose modules]
-
-     or.h -- Common header file: include everything, define everything.
-
-     buffers.c -- Implements a generic buffer interface.  Buffers are
-        fairly opaque string holders that can read to or flush from:
-        memory, file descriptors, or TLS connections.
-
-        Also implements parsing functions to read HTTP and SOCKS commands
-        from buffers.
-
-     tree.h -- A splay tree implementation by Niels Provos.  Used by
-        dns.c for dns caching at exits, and by connection_edge.c for dns
-        caching at clients.
-
-     config.c -- Code to parse and validate the configuration file.
-
-   [Background processing modules]
-
-     cpuworker.c -- Implements a farm of 'CPU worker' processes to perform
-        CPU-intensive tasks in the background, so as not interrupt the
-        onion router.  (OR only)
-
-     dns.c -- Implements a farm of 'DNS worker' processes to perform DNS
-        lookups for onion routers and cache the results.  [This needs to
-        be done in the background because of the lack of a good,
-        ubiquitous asynchronous DNS implementation.] (OR only)
-
-   [Directory-related functionality.]
-
-     directory.c -- Code to send and fetch directories and router
-        descriptors via HTTP.  Directories use dirserv.c to generate the
-        results; clients use routers.c to parse them.
-
-     dirserv.c -- Code to manage directory contents and generate
-        directories. [Directory server only]
-
-     router.c -- Code to parse directories and router descriptors; and to
-        generate a router descriptor corresponding to this OR's
-        capabilities.  Also presents some high-level interfaces for
-        managing an OR or OP's view of the directory.
-
-   [Circuit-related modules.]
-
-     circuitbuild.c -- Creates circuits.
-
-     circuitlist.c -- Manage the global circuit list.
-
-     circuituse.c -- Launch the right type of circuits and attach streams
-        to them.
-
-     onion.c -- Code to generate and respond to "onion skins".
-
-     relay.c -- Handle relay cell encryption/decryption along with packaging
-        and receiving from circuits.
-
-   [Core protocol implementation.]
-
-     command.c -- Code to handle specific cell types.
-
-     connection.c -- Code used in common by all connection types.  See
-        1.2. below for more general information about connections.
-
-     connection_edge.c -- Code used only by edge connections.
-
-     connection_or.c -- Code to implement cell-speaking connections.
-
-   [Hidden services]
-
-     rendclient.c -- Client code to access location-hidden services.  This
-        allows clients and servers to run services and have people connect
-        without either end knowing who they are connecting to.
-
-     rendcommon.c -- Rendevzous implementation: Shared code between
-        introducers, services, clients, and rendezvous points.
-
-     rendmid.c -- Implement introduction and rendezvous points.
-
-     rendservice.c -- Hidden-service side of rendezvous functionality.
-
-   [Reputation]
-
-     rephist.c -- Basic history functionality for reputation module.
-
-   [Router lists]
-
-     routerlist.c -- Code to maintain and access global list of routerinfos for
-        known servers.
-
-     routerparse.c -- Code to parse and validate router descriptors and
-        directories.
-
-   [Bandwidth and GUI]
-
-     control.c -- Implementation of Tor's control socket interface.  Useful
-        for designing GUIs to interact with Tor.
-
-     hibernate.c -- Functions to close listeners, stop allowing new circuits,
-        and so on in preparation of closing down or going dormant.  Also used
-        to track bandwidth and time intervals to know when to hibernate.
-
-   [Toplevel modules.]
-
-     main.c -- Toplevel module.  Initializes keys, handles signals,
-        multiplexes between connections, implements main loop, and drives
-        scheduled events.
-
-     tor_main.c -- Stub module containing a main() function.  Allows unit
-        test binary to link against main.c
-
-   [Unit tests]
-
-     test.c -- Contains unit tests for many pieces of the lower level Tor
-        modules.
-
-
-1.2. All about connections
-
-  All sockets in Tor are handled as different types of nonblocking
-  'connections'.  (What the Tor spec calls a "Connection", the code refers
-  to as a "Cell-speaking" or "OR" connection.)
-
-  Connections are implemented by the connection_t struct, defined in or.h.
-  Not every kind of connection uses all the fields in connection_t; see
-  the comments in or.h and the assertions in assert_connection_ok() for
-  more information.
-
-  Every connection has a type and a state.  Connections never change their
-  type, but can go through many state changes in their lifetime.
-
-  The connection types break down as follows:
-
-     [Cell-speaking connections]
-       CONN_TYPE_OR -- A bidirectional TLS connection transmitting a
-          sequence of cells.  May be from an OR to an OR, or from an OP to
-          an OR.
-
-     [Edge connections]
-       CONN_TYPE_EXIT -- A TCP connection from an onion router to a
-          Stream's destination. [OR only]
-       CONN_TYPE_AP -- A SOCKS proxy connection from the end user
-          application to the onion proxy.  [OP only]
-
-     [Listeners]
-       CONN_TYPE_OR_LISTENER [OR only]
-       CONN_TYPE_AP_LISTENER [OP only]
-       CONN_TYPE_DIR_LISTENER [Directory server only]
-          -- Bound network sockets, waiting for incoming connections.
-
-     [Internal]
-       CONN_TYPE_DNSWORKER -- Connection from the main process to a DNS
-          worker process. [OR only]
-
-       CONN_TYPE_CPUWORKER -- Connection from the main process to a CPU
-          worker process. [OR only]
-
-   Connection states are documented in or.h.
-
-   Every connection has two associated input and output buffers.
-   Listeners don't use them.  For non-listener connections, incoming
-   data is appended to conn->inbuf, and outgoing data is taken from the
-   front of conn->outbuf.  Connections differ primarily in the functions
-   called to fill and drain these buffers.
-
-1.3. All about circuits.
-
-   A circuit_t structure fills two roles.  First, a circuit_t links two
-   connections together: either an edge connection and an OR connection,
-   or two OR connections.  (When joined to an OR connection, a circuit_t
-   affects only cells sent to a particular circID on that connection.  When
-   joined to an edge connection, a circuit_t affects all data.)
-
-   Second, a circuit_t holds the cipher keys and state for sending data
-   along a given circuit.  At the OP, it has a sequence of ciphers, each
-   of which is shared with a single OR along the circuit.  Separate
-   ciphers are used for data going "forward" (away from the OP) and
-   "backward" (towards the OP).  At the OR, a circuit has only two stream
-   ciphers: one for data going forward, and one for data going backward.
-
-1.4. Asynchronous IO and the main loop.
-
-   Tor uses the poll(2) system call (or it wraps select(2) to act like
-   poll, if poll is not available) to handle nonblocking (asynchronous)
-   IO.  If you're not familiar with nonblocking IO, check out the links
-   at the end of this document.
-
-   All asynchronous logic is handled in main.c.  The functions
-   'connection_add', 'connection_set_poll_socket', and 'connection_remove'
-   manage an array of connection_t*, and keep in synch with the array of
-   struct pollfd required by poll(2).  (This array of connection_t* is
-   accessible via get_connection_array, but users should generally call
-   one of the 'connection_get_by_*' functions in connection.c to look up
-   individual connections.)
-
-   To trap read and write events, connections call the functions
-   'connection_{is|stop|start}_{reading|writing}'. If you want
-   to completely reset the events you're watching for, use
-   'connection_watch_events'.
-
-   Every time poll() finishes, main.c calls conn_read and conn_write on
-   every connection. These functions dispatch events that have something
-   to read to connection_handle_read, and events that have something to
-   write to connection_handle_write, respectively.
-
-   When connections need to be closed, they can respond in two ways.  Most
-   simply, they can make connection_handle_* return an error (-1),
-   which will make conn_{read|write} close them.  But if it's not
-   convenient to return -1 (for example, processing one connection causes
-   you to realize that a second one should close), then you can also
-   mark a connection to close by setting conn->marked_for_close. Marked
-   connections will be closed at the end of the current iteration of
-   the main loop.
-
-   The main loop handles several other operations: First, it checks
-   whether any signals have been received that require a response (HUP,
-   KILL, USR1, CHLD).  Second, it calls prepare_for_poll to handle recurring
-   tasks and compute the necessary poll timeout.  These recurring tasks
-   include periodically fetching the directory, timing out unused
-   circuits, incrementing flow control windows and re-enabling connections
-   that were blocking for more bandwidth, and maintaining statistics.
-
-   A word about TLS: Using TLS on OR connections complicates matters in
-   two ways.
-   First, a TLS stream has its own read buffer independent of the
-   connection's read buffer.  (TLS needs to read an entire frame from
-   the network before it can decrypt any data.  Thus, trying to read 1
-   byte from TLS can require that several KB be read from the network
-   and decrypted.  The extra data is stored in TLS's decrypt buffer.)
-   Because the data hasn't been read by tor (it's still inside the TLS),
-   this means that sometimes a connection "has stuff to read" even when
-   poll() didn't return POLLIN. The tor_tls_get_pending_bytes function is
-   used in main.c to detect TLS objects with non-empty internal buffers.
-   Second, the TLS stream's events do not correspond directly to network
-   events: sometimes, before a TLS stream can read, the network must be
-   ready to write -- or vice versa.
-
-1.5. How data flows (An illustration.)
-
-   Suppose an OR receives 256 bytes along an OR connection.  These 256
-   bytes turn out to be a data relay cell, which gets decrypted and
-   delivered to an edge connection.  Here we give a possible call sequence
-   for the delivery of this data.
-
-   (This may be outdated quickly.)
-
-   do_main_loop -- Calls poll(2), receives a POLLIN event on a struct
-                 pollfd, then calls:
-    conn_read -- Looks up the corresponding connection_t, and calls:
-     connection_handle_read -- Calls:
-      connection_read_to_buf -- Notices that it has an OR connection so:
-       read_to_buf_tls -- Pulls data from the TLS stream onto conn->inbuf.
-      connection_process_inbuf -- Notices that it has an OR connection so:
-       connection_or_process_inbuf -- Checks whether conn is open, and calls:
-        connection_process_cell_from_inbuf -- Notices it has enough data for
-                 a cell, then calls:
-         connection_fetch_from_buf -- Pulls the cell from the buffer.
-         cell_unpack -- Decodes the raw cell into a cell_t
-         command_process_cell -- Notices it is a relay cell, so calls:
-          command_process_relay_cell -- Looks up the circuit for the cell,
-                 makes sure the circuit is live, then passes the cell to:
-           circuit_deliver_relay_cell -- Passes the cell to each of:
-            relay_crypt -- Strips a layer of encryption from the cell and
-                 notices that the cell is for local delivery.
-            connection_edge_process_relay_cell -- extracts the cell's
-                 relay command, and makes sure the edge connection is
-                 open.  Since it has a DATA cell and an open connection,
-                 calls:
-             circuit_consider_sending_sendme -- check if the total number
-                 of cells received by all streams on this circuit is
-                 enough that we should send back an acknowledgement
-                 (requesting that more cells be sent to any stream).
-             connection_write_to_buf -- To place the data on the outgoing
-                 buffer of the correct edge connection, by calling:
-              connection_start_writing -- To tell the main poll loop about
-                 the pending data.
-              write_to_buf -- To actually place the outgoing data on the
-                 edge connection.
-             connection_consider_sending_sendme -- if the outbuf waiting
-                 to flush to the exit connection is not too full, check
-                 if the total number of cells received on this stream
-                 is enough that we should send back an acknowledgement
-                 (requesting that more cells be sent to this stream).
-
-   In a subsequent iteration, main notices that the edge connection is
-   ready for writing:
-
-   do_main_loop -- Calls poll(2), receives a POLLOUT event on a struct
-                 pollfd, then calls:
-    conn_write -- Looks up the corresponding connection_t, and calls:
-     connection_handle_write -- This isn't a TLS connection, so calls:
-      flush_buf -- Delivers data from the edge connection's outbuf to the
-                 network.
-      connection_wants_to_flush -- Reports that all data has been flushed.
-      connection_finished_flushing -- Notices the connection is an exit,
-                 and calls:
-       connection_edge_finished_flushing -- The connection is open, so it
-                 calls:
-        connection_stop_writing -- Tells the main poll loop that this
-                 connection has no more data to write.
-        connection_consider_sending_sendme -- now that the outbuf
-                 is empty, check again if the total number of cells
-                 received on this stream is enough that we should send
-                 back an acknowledgement (requesting that more cells be
-                 sent to this stream).
-
-1.6. Routers, descriptors, and directories
-
-   All Tor processes need to keep track of a list of onion routers, for
-   several reasons:
-       - OPs need to establish connections and circuits to ORs.
-       - ORs need to establish connections to other ORs.
-       - OPs and ORs need to fetch directories from a directory server.
-       - ORs need to upload their descriptors to directory servers.
-       - Directory servers need to know which ORs are allowed onto the
-         network, what the descriptors are for those ORs, and which of
-         those ORs are currently live.
-
-   Thus, every Tor process keeps track of a list of all the ORs it knows
-   in a static variable 'directory' in the routers.c module.  This
-   variable contains a routerinfo_t object for each known OR. On startup,
-   the directory is initialized to a list of known directory servers (via
-   router_get_list_from_file()).  Later, the directory is updated via
-   router_get_dir_from_string().  (OPs and ORs retrieve fresh directories
-   from directory servers; directory servers generate their own.)
-
-   Every OR must periodically regenerate a router descriptor for itself.
-   The descriptor and the corresponding routerinfo_t are stored in the
-   'desc_routerinfo' and 'descriptor' static variables in routers.c.
-
-   Additionally, a directory server keeps track of a list of the
-   router descriptors it knows in a separate list in dirserv.c.  It
-   uses this list, checking which OR connections are open, to build
-   directories.
-
-1.7. Data model
-
-  [XXX]
-
-1.8. Flow control
-
-  [XXX]
-
-2. Coding conventions
-
-2.1. Details
 
   Use tor_malloc, tor_free, tor_snprintf, tor_strdup, and tor_gettimeofday
   instead of their generic equivalents.  (They always succeed or exit.)
 
   Use INLINE instead of 'inline', so that we work properly on windows.
 
-2.2. Calling and naming conventions
+1.2. Calling and naming conventions
 
   Whenever possible, functions should return -1 on error and and 0 on
   success.
@@ -441,13 +27,13 @@ the distant future, stuff may have changed.)
   (e.g. buffer_clear, buffer_resize); functions that return booleans should
   have predicate names (e.g. buffer_is_empty, buffer_needs_resizing).
 
-2.3. What To Optimize
+1.3. What To Optimize
 
   Don't optimize anything if it's not in the critical path.  Right now,
   the critical path seems to be AES, logging, and the network itself.
   Feel free to do your own profiling to determine otherwise.
 
-2.4. Log conventions
+1.4. Log conventions
 
   Log convention: use only these four log severities.
 
@@ -472,7 +58,7 @@ the distant future, stuff may have changed.)
   the message (perhaps with a string like "internal error").  Option (A) is
   to be preferred to option (B). -NM]
 
-2.5. Doxygen
+1.5. Doxygen
 
   We use the 'doxygen' utility to generate documentation from our source code.
   Here's how to use it:
@@ -525,56 +111,3 @@ the distant future, stuff may have changed.)
   6. See the Doxygen manual for more information; this summary just scratches
      the surface.
 
-3. References
-
-  About Tor
-
-     See http://tor.eff.org/
-         http://tor.eff.org/cvs/doc/tor-spec.txt
-         http://tor.eff.org/cvs/doc/tor-design.tex
-         http://tor.eff.org/cvs/doc/FAQ
-
-  About anonymity
-
-     See http://freehaven.net/anonbib/
-
-  About nonblocking IO
-
-     [XXX insert references]
-
-# ======================================================================
-# Old HACKING document; merge into the above, move into tor-design.tex,
-# or delete.
-# ======================================================================
-The pieces.
-
-  Routers. Onion routers, as far as the 'tor' program is concerned,
-  are a bunch of data items that are loaded into the router_array when
-  the program starts. Periodically it downloads a new set of routers
-  from a directory server, and updates the router_array. When a new OR
-  connection is started (see below), the relevant information is copied
-  from the router struct to the connection struct.
-
-  Connections. A connection is a long-standing tcp socket between
-  nodes. A connection is named based on what it's connected to -- an "OR
-  connection" has an onion router on the other end, an "OP connection" has
-  an onion proxy on the other end, an "exit connection" has a website or
-  other server on the other end, and an "AP connection" has an application
-  proxy (and thus a user) on the other end.
-
-  Circuits. A circuit is a path over the onion routing
-  network. Applications can connect to one end of the circuit, and can
-  create exit connections at the other end of the circuit. AP and exit
-  connections have only one circuit associated with them (and thus these
-  connection types are closed when the circuit is closed), whereas OP and
-  OR connections multiplex many circuits at once, and stay standing even
-  when there are no circuits running over them.
-
-  Streams. Streams are specific conversations between an AP and an exit.
-  Streams are multiplexed over circuits.
-
-  Cells. Some connections, specifically OR and OP connections, speak
-  "cells". This means that data over that connection is bundled into 512
-  byte packets (14 bytes of header and 498 bytes of payload). Each cell has
-  a type, or "command", which indicates what it's for.
-