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Guide to Hacking Tor
(As of 8 October 2003, this was all accurate. If you're reading this in
the distant future, stuff may have changed.)
0. Intro and required reading
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.)
crypto.[ch] -- Wrapper functions to present a consistent interface to
public-key and symmetric cryptography operations from OpenSSL.
fakepoll.[ch] -- Used on systems that don't have a poll() system call;
reimplements() poll using the select() system call.
log.[ch] -- Tor's logging subsystem.
test.h -- Macros used by unit tests.
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]
routers.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.]
circuit.c -- Code to create circuits, manage circuits, and route
relay cells along circuits.
onion.c -- Code to generate and respond to "onion skins".
[Core protocol implementation.]
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.
command.c -- Code to handle specific cell types.
connection_or.c -- Code to implement cell-speaking connections.
[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_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
Whenever possible, functions should return -1 on error and and 0 on
success.
For multi-word identifiers, use lowercase words combined with
underscores. (e.g., "multi_word_identifier"). Use ALL_CAPS for macros and
constants.
Typenames should end with "_t".
Function names should be prefixed with a module name or object name. (In
general, code to manipulate an object should be a module with the same
name as the object, so it's hard to tell which convention is used.)
Functions that do things should have imperative-verb names
(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
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
Log convention: use only these four log severities.
ERR is if something fatal just happened.
WARN if something bad happened, but we're still running. The
bad thing is either a bug in the code, an attack or buggy
protocol/implementation of the remote peer, etc. The operator should
examine the bad thing and try to correct it.
NOTICE if it's something the operator will want to know about.
(No error or warning messages should be expected during normal OR or OP
operation. I expect most people to run on -l notice eventually. If a
library function is currently called such that failure always means
ERR, then the library function should log WARN and let the caller
log ERR.)
INFO means something happened (maybe bad, maybe ok), but there's nothing
you need to (or can) do about it.
DEBUG is for everything louder than INFO.
[XXX Proposed convention: every messages of severity INFO or higher should
either (A) be intelligible to end-users who don't know the Tor source; or
(B) somehow inform the end-users that they aren't expected to understand
the message (perhaps with a string like "internal error"). Option (A) is
to be preferred to option (B). -NM]
3. References
About Tor
See http://freehaven.net/tor/
http://freehaven.net/tor/cvs/doc/tor-spec.txt
http://freehaven.net/tor/cvs/doc/tor-design.tex
http://freehaven.net/tor/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 256
byte packets (8 bytes of header and 248 bytes of payload). Each cell has
a type, or "command", which indicates what it's for.
Robustness features.
[XXX no longer up to date]
Bandwidth throttling. Each cell-speaking connection has a maximum
bandwidth it can use, as specified in the routers.or file. Bandwidth
throttling can occur on both the sender side and the receiving side. If
the LinkPadding option is on, the sending side sends cells at regularly
spaced intervals (e.g., a connection with a bandwidth of 25600B/s would
queue a cell every 10ms). The receiving side protects against misbehaving
servers that send cells more frequently, by using a simple token bucket:
Each connection has a token bucket with a specified capacity. Tokens are
added to the bucket each second (when the bucket is full, new tokens
are discarded.) Each token represents permission to receive one byte
from the network --- to receive a byte, the connection must remove a
token from the bucket. Thus if the bucket is empty, that connection must
wait until more tokens arrive. The number of tokens we add enforces a
longterm average rate of incoming bytes, yet we still permit short-term
bursts above the allowed bandwidth. Currently bucket sizes are set to
ten seconds worth of traffic.
The bandwidth throttling uses TCP to push back when we stop reading.
We extend it with token buckets to allow more flexibility for traffic
bursts.
Data congestion control. Even with the above bandwidth throttling,
we still need to worry about congestion, either accidental or intentional.
If a lot of people make circuits into same node, and they all come out
through the same connection, then that connection may become saturated
(be unable to send out data cells as quickly as it wants to). An adversary
can make a 'put' request through the onion routing network to a webserver
he owns, and then refuse to read any of the bytes at the webserver end
of the circuit. These bottlenecks can propagate back through the entire
network, mucking up everything.
(See the tor-spec.txt document for details of how congestion control
works.)
In practice, all the nodes in the circuit maintain a receive window
close to maximum except the exit node, which stays around 0, periodically
receiving a sendme and reading more data cells from the webserver.
In this way we can use pretty much all of the available bandwidth for
data, but gracefully back off when faced with multiple circuits (a new
sendme arrives only after some cells have traversed the entire network),
stalled network connections, or attacks.
We don't need to reimplement full tcp windows, with sequence numbers,
the ability to drop cells when we're full etc, because the tcp streams
already guarantee in-order delivery of each cell. Rather than trying
to build some sort of tcp-on-tcp scheme, we implement this minimal data
congestion control; so far it's enough.
Router twins. In many cases when we ask for a router with a given
address and port, we really mean a router who knows a given key. Router
twins are two or more routers that share the same private key. We thus
give routers extra flexibility in choosing the next hop in the circuit: if
some of the twins are down or slow, it can choose the more available ones.
Currently the code tries for the primary router first, and if it's down,
chooses the first available twin.
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