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<h2><a href="http://torproject.org/">Tor Project</a> - DNSEL Rewrite</h2>
<b>Google Summer of Code Student Application</b> <br>
Harry Bock <hbock AT ele DOT uri DOT edu>
The TorDNSEL project is concerned with identifying individual hosts
as valid and accessible Tor exit relays. Each Tor exit relay has an
associated exit policy governing what traffic may leave the Tor
circuit and go out as requests to the internet. A public database
that can be easily queried or scraped would be of huge benefit to
the Tor community and to services that are interested in whether
clients originate from the Tor network, such as Wikipedia and IRC
What project would you like to work on? Use our ideas lists as a
starting point or make up your own idea. Your proposal should
include high-level descriptions of what you're going to do, with
more details about the parts you expect to be tricky. Your
proposal should also try to break down the project into tasks of a
fairly fine granularity, and convince us you have a plan for
My primary interest is in the TorDNSEL rewrite. Currently
unmaintained and written in Haskell, I would like to rework it
from the ground up using Python and the Torflow interface.
Prior to actually building the new TorDNSEL, some time must be
spent researching and testing strategies for identifying and verifying
recent Tor exit relays.
Much of this information is available straight from the
cached-descriptors file distributed to running Tor relays, but
not all of it is accurate or up-to-date.
To ensure the honesty of advertised exit nodes, the program
must actively build circuits over the Tor network to the
intended exit node and verify the IP address and exit policies
listed in the cached router list. This will be accomplished via
TorFlow, most likely using the NetworkScanners tool SoaT. If
more detailed checking is required than provided by SoaT, it will
be modified and extended to suit the new requirements.
Since new relays entering the Tor network are almost
immediately available for use, it is important that new
relays are checked and added as quickly as possible.
Testing ExitPolicy honesty may be time-consuming for
certain relays, destinations, and services.
To improve exit honesty checking latency, hosts that have
complex exit policies may be checked incrementally; for
example, if popular services such as http/https/domain/ssh
are allowed by the relay's ExitPolicy, these should be
checked first and the relay can be marked as "honest
(preliminary)", so that partial results may be listed
earlier pending more thorough circuit testing.
Once the desired method for compiling and verifying exit relays
is tested, a formal design specification for the following operations
must be compiled:
<li><p>Scraping of all known exit relays and their exit
policies.</p> As noted in the Tor dir-spec v3, in the future
it will not scale to have every Tor client/directory cache know
the IP of every other router. We need to be able to accurately
obtain this data, up-to-date and in bulk, from an authoritative
source. The DNSEL should be able to verify all exit relays,
in the shortest time span allowable.
Active testing for valid exit IP addresses and policies.
Parameters and raw format of the resulting data.
Formal mechanisms and formats for access by consumers of this
data. As a minimal starting point, SQLite and JSON should be
supported for build data sets, as both are well-structured,
standardised formats that have cross-platform open-source
support for the most widely used programming languages. One this
data is readily available, it would be extremely useful to provide
a simple API in Python to improve ease-of-use and integration of
this data with existing services.
Currently, users can check the TorBulkExitList on
check.torproject.org or perform DNS queries against
exitlist.torproject.org, but this is not ideal for all consumers
of this data; currently it is expensive to perform these queries
and they must be done one at a time over the network. While this
is less of an issue for services that need to perform infrequent
exit checks, I propose that this mechanism can actually harm
anonymity, as an adversary that can track these queries (those
with access to the network of the querying service, or those
running rogue DNSEL implementations, should it become more
distributed) can determine, for example, what exit nodes are
currently serving a large amount of IRC connections. This is
more important for services that do frequent queries on a wide
range of IP addresses. By encouraging these queries to be done
locally, we can improve network latency, throughput, and
On the other hand, for some services, requiring services to
download the entire exit set when they only need to query a few
addresses daily would be a waste of valuable resources on both
ends. Thus, both single-exit queries and bulk exit description
lists must be provided; raw queries would be used by services
that are simply doing manual checks of possible Tor addresses,
or infrequent automated checks.
The primary consumers of bulk data are services that need
to do frequent automated checks and benefit strongly from
local caching of data. Some prominent examples would be:
<li>The Tor project itself (see TorBulkExitList.py,
<li>IRC networks such as Freenode and OFTC that
automatically scrub any potentially identifiable
information from WHOIS queries.</li>
<li>Social networks or collaborative communities such as
Wikipedia that would benefit from knowing if a
particular IP address is shared, allowing them to
(possibly) be more lenient towards abuse from
A particularly important goal is to ensure that Tor users
that also happen to run Tor relays are not automatically
blocked by services simply because they are relays or exit
nodes. If a service is able to ascertain that an IP address
corresponds to a Tor relay, but its exit policy would not
allow traffic to access the service from Tor, ideally it
should not block access to its resources. The cheaper and
easier it is for a service operator to validate this kind of
information, the more likely the service is to use it and
collaborate with the Tor community. The more the service is
used, the more likely we are to get feedback, whether
it be able the data format, false positives or negatives,
invalid/incorrect exit policies, etc.
The TorDNSEL project itself will likely benefit from being broken
into several components that interact with each other and operate on
exit list data sets.
<li>Data scraper and honesty checking daemon: constantly runs in
the background, fetching new information about Tor relays and
their exit policies and updating the raw journal. As new relays
come in, they are accessed and verified for honesty and
correctness. As relay data becomes stale according to a
configurable TTL, they are re-checked.
Data compiler: Manages, updates, and merges exit lists. Can
import and export data from bulk formats and provide
statistics about data sets.
Single-query engine: Allows applications and external services
to query about a single IP address and exit policy at a time.
Can interface via several convenient methods, starting out
simple (e.g., Python API requests or HTTP requests) and
eventually working up to RBL-style DNS queries, like the
original DNSEL, should time permit.
Data distribution: How do clients actually access bulk data?
This doesn't necessarily have to be its own application, but
it must be feasible for clients to easily scrape and update
their local caches. The data may be served by a static web
server, like lighttpd, or via other mechanisms (sftp, or dns
zone transfer as a more convoluted example).
As an aside, there are several open and accepted Tor proposals
that are relevant to the work I will complete:
<li>140 <b>Provide diffs between consensuses</b> (ACCEPTED)</li>
<li>146 <b>Add new flag to reflect long-term stability</b> (OPEN)</li>
<li>147 <b>Eliminate the need for v2 directories in generating v3 directories</b> (ACCEPTED)</li>
<li>159 <b>Exit Scanning</b> (OPEN)</li>
From what I have researched and discussed so far, the tricky parts
will be verifying that listed exit nodes and their exit policies
are correct (as advertised) and presenting this data in a useful
way to dnsel consumers. It was originally assumed that consumers
of dnsel records would want to use a DNS RBL-style interface, and
while this interface has been useful to (for example) IRC server
operators, these direct queries end up revealing more about the
usage of exit nodes than is necessary.
<p>For each weekly milestone, appropriate documentation should be written to
coincide with the completed work. For example, end-user tools should have
manual pages at the very least, and preferably include a LaTeX manual. Milestones
that are primarily experimental in nature should include complete descriptions and
proposals in plain-text where appropriate. All source code will be thoroughly
commented and include documentation useful to developers.
<b>April 26 - May 24 (Pre-SoC)</b>: Get up to speed with Tor
directory and caching architecture, pick apart existing Haskell
implementation of TorDNSEL, and master TorFlow.
<b>May 31 (end of week 1)</b>: Have a working mechanism for
compiling as much testable information about exit relays as
possible. This data must be easily accessible for subsequent work.
This may be taken, adapted, or abstracted from existing data directory crawling
<b>June 14 (week 3)</b>: Working implementation of tests using TorFlow, especially
ExitAuthority tools. This will probably be the most time-consuming period; may take
up to a week more than anticipated.
<b>June 21 (week 4)</b>: Be able to produce consistent, constantly updating exit lists
with tested and untested exit policies listed. Find Tor developer guinea pigs to
test and hunt for glaring holes in exit relay honesty testing and verification. :)
<b>June 28 (week 5)</b>: Begin proof-of-concept production of bulk
data formats (raw, SQLite and JSON), all of which should be similar
in format. Consultations should be made with consumers of such data
(Freenode, Wikipedia, etc.) to ensure the current data presentation
is not overreaching or missing information that would be useful to
<b>July 12 (week 7)</b>: Integrate existing functionality and data access methods into
a Python API that is usable for consumers and the DNSEL application itself. Style should
be similar to TorFlow where possible.
<b>July 16 (midterm evals):</b> Completed specifications of TorDNSEL
operation, basic data formats, and delivery methods. Completed
first proof-of-concept implementation. First major review with
mentors and as much of the Tor developer community as time permits.
<b>July 19 (week 8)</b>: Work on designing and testing exit list cache update mechanisms.
Start with something similar to cached-descriptors.new journaling, and work up
to something for useful for other data formats. Integrate mechanism into API.
<b>July 26 (week 9)</b>: Solidify main scrape/check application and
perform as much real-world testing as time permits, adjusting for
major setbacks, if any.
<b>August 2 (week 10)</b>: Make adjustments based on feedback from
(hopefully) several real-world consumers of TorDNSEL data.
Generally polish and improve usability of core application(s).
<b>August 9 ("pencils down"):</b>: Start pumping out documentation and comprehensive
code and review.
<b>August 16 ("okay really, pencils down"):</b>Major remaining kinks
should be ironed out; polish specification and documentation and
begin writing final evaluations. Plan for future maintenance of
<strong> Point us to a code sample: something good and clean to
demonstrate that you know what you're doing, ideally from an
Code from almost any project I've worked on is available at
http://git.spanning-tree.org/. Some of my better code:
<li><a href="http://git.spanning-tree.org/index.cgi/grizzlor/">libgrizzlor</a> [C, Common Lisp], an abstraction layer for the
SILC client library focused on bots.
<li><a href="http://git.spanning-tree.org/index.cgi/rigel/tree/">rigel</a> [C], a UNIX PIC16/PIC18 program loader for use with the FIRST robotics competition.</li>
<li><a href="http://git.spanning-tree.org/index.cgi/nis/">Network Subsystem Inventory</a> [Python], a Django web application for keeping an inventory of network resources on a large university network. </li>
[Common Lisp], a network monitoring application inspired by IP-Audit,
designed from the ground up to work with the Argus netflow application.</li>
<li><strong> Why do you want to work with The Tor Project / EFF in particular?</strong>
The Tor Project interests me primarily from architectural and
information security perspectives; my primary focus in information
security has always been authentication and authorization - verifying
the identity of a user to explicitly or implicitly control access to
machine and network resources. The goal of all forms of public-key
and secure hash cryptography is the authentication of a third party or
data, essentially pinning their identity down.
Tor greatly interests me because it has the opposite goal; it tries to
ensure that pinning down the identity of any particular user is
(ideally) impossible or at least greatly hindered for any non-global
adversary. Protecting the rights of network users by preserving their
anonymity is an incredibly important and complicated goal, and Tor's
role in increasing anonymity of internet access in the face of many
types of adversaries is extremely valuable. To this end, I hope that
my contributions will be found useful by the Tor project, its users,
and those working to protect these end users.
Tell us about your experiences in free software development
environments. We especially want to hear examples of how you have
collaborated with others rather than just working on a project by
<p>While nearly all of the projects I've worked on have been free
software, my experience working directly with the free software
community at large is minimal. I have contributed briefly to the
KDE project, working on their display configuration application,
and submitted patches to other open source projects (QoSient's
Argus netflow tools and Google's ipaddr-py, for example). I have
collaborated with various universities in New England on
development of the Nautilus project (http://nautilus.oshean.org/)
and its main subproject, Periscope
(http://nautilus.oshean.org/wiki/Periscope), while working at the
OSHEAN non-profit consortium. </p>
I sincerely look forward to working with the vibrant development
community of the Tor project and hope to gain more experience in
collaborating with an experienced group of developers.
Will you be working full-time on the project for the summer, or
will you have other commitments too (a second job, classes, etc)?
If you won't be available full-time, please explain, and list
timing if you know them for other major deadlines
(e.g. exams). Having other activities isn't a deal-breaker, but we
don't want to be surprised.
I will be working part-time at the University of Rhode Island
Information Security Office, and will have one summer class for five
weeks starting in late May. I don't anticipate either will
significantly affect my involvement with the Tor project.
Will your project need more work and/or maintenance after the
summer ends? What are the chances you will stick around and help
out with that and other related projects?
While I am confident I can produce a working initial implementation of
dnsel in the time allotted, I anticipate it will need more work at the
end of summer. One of my primary goals for the dnsel project is to
make it easier to maintain, as its operation will have to be adjusted
to fit with changes in the Tor architecture. Making the project more
accessible to other maintainers will allow for greater collaboration
and improvements to dnsel where development on the current
implementation has stagnated.
What is your ideal approach to keeping everybody informed of your
progress, problems, and questions over the course of the project? Said
another way, how much of a "manager" will you need your mentor to be?
I will do my best to communicate with my mentors and the Tor developer
community at large as frequently and directly as possible, via
#tor-dev and the mailing lists. I also hope to inform others of more
major milestones in the project via a blog or web page, and keep
detailed documentation and progress updates on the Tor wiki.
<strong>What school are you attending? What year are you, and what's
your major/degree/focus? If you're part of a research group, which one?</strong>
I am currently attending the University of Rhode Island. This is my
fourth year in college and second at URI; I am a Computer Engineering
major, intending to graduate next year and obtain my masters degree the
following year. My primary interests are low-level software development
and systems programming, networking, information security, and signal
<strong>How can we contact you to ask you further questions? Google
doesn't share your contact details with us automatically, so you should
include that in your application. In addition, what's your IRC nickname?
Interacting with us on IRC will help us get to know you, and help you
get to know our community.</strong>
You can contact me at firstname.lastname@example.org; my nickname on IRC is <b>hbock</b>.