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.
    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
  • 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 unofficial pluggable transports wiki page for more pluggable transport information.

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 Bundle

As of Tor Browser 3.6-beta-1, pluggable transports are now included in the official Tor Browser packages.