Difference between revisions of "Tor (anonymity network)"

Revision as of 12:06, 20 February 2006

Tor is an implementation of second-generation onion routing - an anonymity system enabling its users to communicate anonymously on the Internet. Originally sponsored by the US Naval Research Laboratory, Tor became an Electronic Frontier Foundation (EFF) project in late 2004. The EFF supported Tor financially until November 2005 [1], and continues to provide web hosting for the project.

Overview

Tor aims to protect its users against traffic analysis attacks. Tor operates an overlay network of onion routers that enables two things: anonymous outgoing connections and anonymous hidden services.

Anonymous outgoing connections

Users of the Tor network run an onion proxy on their machine. This software connects out to Tor, periodically negotiating a virtual circuit through the Tor network. Tor employs cryptography in a layered manner (hence the 'onion' analogy), ensuring perfect forward secrecy between routers. At the same time, the onion proxy software presents a SOCKS interface to its clients. SOCKS-aware applications may be pointed at Tor, which then multiplexes the traffic through a Tor virtual circuit.

Once inside the Tor network, the traffic is sent from router to router, ultimately reaching an exit node at which point the cleartext data is available and is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at the Tor exit node.

Tor's application independence separates it apart from most other anonymity networks: it works at the TCP stream level. Applications commonly anonymised using Tor include IRC, instant messaging and browsing the Web. When browsing the Web, Tor is often coupled with Privoxy - a filtering proxy server - that aims to add privacy at the application layer.

Etiquette and abuse

Because Tor is capable of anonymising arbitrary TCP traffic, it attracts its fair share of abuse. Routers maintain an exit policy of what traffic is and is not permitted to leave the Tor network through that node. Using a combination of addresses and ports, it is possible to combat most major abuses of the Tor network. Potential abuses include:

P2P 

Legalities aside, it is considered impolite to transfer massive amounts of data across the Tor network - the onion routers are run by volunteers using their own bandwidth at their own cost.

E-mail 

Anonymity and SMTP usually results in spam. Consequently the default exit policy of Tor nodes rejects outgoing connections to port 25.

Vandalism 

Safe in the knowledge that they cannot be traced, users are occasionally tempted to vandalise collaborative web sites. As a result, many sites take the decision to block Tor traffic.

Anonymous hidden services

Although Tor's most popular feature is its provision of anonymity to clients, it can also provide anonymity to servers. By using the Tor network, it is possible to host servers in such a way that their network location is unknown. In order to access a hidden service, Tor must also be used by the client.

Hidden services are accessed through the Tor-specific .onion top level domain. The Tor network understands this TLD and routes, anonymously, to the hidden service. The hidden service then hands over to standard server software, which should be configured to listen only on non-public interfaces. Services that are reachable through Tor hidden services and the public Internet are susceptible to correlation attacks, and consequently are not really hidden.

An added advantage of Tor hidden services is that, because no public IP address is required, services may be hosted behind Firewalls and NAT.

See also

• JAP - Java Anon Proxy
• Mixmaster anonymous remailer - Mixmaster anonymous remailer
• Mixminion - Mixminion anonymous remailer
• Freenet, Entropy (network) - Censorship-resistant network

External links

• Tor homepage
• Tor wiki with FAQs & HOWTOs
  • Comparison of Tor with other anonymity networks
• Tor design document
• Tor network statistics
• I2P's comparison of anonymity networks
• Hidden Tor Wiki (Tor required)
• VPM - Running Tor in a secure sandbox
• TorPark - Software suite for anonymous web surfing from a USB flash drive
• MetroPipe.net ISP that supports Freenet, JAP, and Tor transparently