Tor: Pluggable Transports
An increasing number of censoring countries are using Deep Packet Inspection (DPI) to classify Internet traffic flows by protocol. While Tor uses bridge relays to get around a censor that blocks by IP address, the censor can use DPI to recognize and filter Tor traffic flows even when they connect to unexpected IP addresses.
Pluggable transports transform the Tor traffic flow between the client and the bridge. This way, censors who monitor traffic between the client and the bridge will see innocent-looking transformed traffic instead of the actual Tor traffic. External programs can talk to Tor clients and Tor bridges using the pluggable transport API, to make it easier to build interoperable programs.
- Obfsproxy is a Python framework for implementing new
pluggable transports. It uses Twisted for its networking needs, and
pyptlib
for some pluggable transport-related features. It supports the
obfs2
and
obfs3
pluggable transports. Maintained by asn.
Status: Deployed - Flashproxy turns ordinary web browsers into bridges using
websockets, and has a little python stub to hook Tor clients to the
websocket connection. See its
git repository,
and
design paper.
Maintained by David Fifield.
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Status: Deployed - Format-Transforming
Encryption (FTE) transforms Tor traffic to arbitrary
formats using their language descriptions. See the research
paper.
Status: Deployed - ScrambleSuit
is a pluggable transport that protects
against follow-up probing attacks and is also capable of changing
its network fingerprint (packet length distribution,
inter-arrival times, etc.). It's part of the Obfsproxy framework.
Maintained by Philipp Winter.
Status: To be deployed - Meek
is a transport that uses HTTP for carrying bytes and TLS for
obfuscation. Traffic is relayed through a third-party server
(​Google App Engine). It uses a trick to talk to the third party so
that it looks like it is talking to an unblocked server.
Maintained by David Fifield.
Status:Coming soon - obfs4
is a transport with the same features as ScrambleSuit
but utilizing Dan Bernstein's elligator2
technique for public key obfuscation, and the
ntor protocol
for one-way authentication. This results in a faster protocol. Written in Go.
Maintained by Yawning Angel.
Status:Coming soon - obfsclient
is a multi-transport pluggable transport proxy (like obfsproxy),
written in C++ that implements the client-side of obfs2,
obfs3 and scramblesuit. It's used by
Orbot on
Android because of the difficulties of using Python applications.
Maintained by Yawning Angel.
Status: Deployed - StegoTorus is an Obfsproxy fork that extends it to a)
split Tor streams across multiple connections to avoid packet size
signatures, and b) embed the traffic flows in traces that look like
html, javascript, or pdf. See its
git repository.
Maintained by Zack Weinberg.
Status: Undeployed - SkypeMorph transforms Tor traffic flows so they look like
Skype Video. See its
source code
and
design paper.
Maintained by Ian Goldberg.
Status: Undeployed - Dust aims to provide a packet-based (rather than
connection-based) DPI-resistant protocol. See its
git repository.
Maintained by Brandon Wiley.
Status: Undeployed
Also see the
Our goal is to have a wide variety of pluggable transport designs. Many are at the research phase now, so it's a perfect time to play with them or suggest new designs. Please let us know if you find or start other projects that could be useful for making Tor's traffic flows more DPI-resistant!
Download the Pluggable Transports Tor Browser
As of Tor Browser 3.6-beta-1, pluggable transports are now included in the official Tor Browser packages.