Tor (anonymity network)

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Tor
Image:Tor-logo.png
Tor logo
Maintainer: Roger Dingledine and Nick Mathewson
Latest release: 0.1.0.15 / September 23, 2005
OS: Cross-platform
Genre: Onion routing
License: BSD license
Website: tor.eff.org

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 project in late 2004. As of late 2005, the EFF no longer sponsors the Tor project; they do however continue to host the Tor web site.

Contents

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 secracy 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 - Wikipedia included - 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

External links

Further reading

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