Tor (network)

Tor

File:Eff tor.png
Developer(s)	The Tor Project
Initial release	September 20, 2003 (2003-09-20)
Stable release	0.4.8.12  (6 June 2024 (6 June 2024))
Repository	gitlab.torproject.org/tpo/core/tor/
Written in	C
Operating system	Cross-platform
Type	Onion routing / Anonymity
License	BSD license
Website	www.torproject.org

Tor (The Onion Router) is a free software implementation of second-generation onion routing – a 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, and the EFF supported Tor financially until November 2005. The Tor software is now developed by the Tor Project, which since December 2006 is a 501(c)(3) research/education non-profit organization based in the United States of America that receives a diverse base of financial support.

Like all current low latency anonymity networks, Tor is vulnerable to traffic analysis from observers who can watch both ends of a user's connection.

Overview

Aiming to protect its users against traffic analysis attacks, volunteers operate an overlay network of onion routers that enable anonymous outgoing connections and anonymous "hidden" services.

Anonymous outgoing connections

File:Vidalia Tor IPChicken.png

Vidalia routing http traffic with a Firefox plugin.

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

Not all Tor nodes are equal. Tor directory authorities describe nodes with several flags: DIR, Running, Guard, Authority, Fast, Exit, Stable, Valid, V2Dir, BadExit, BadDirectory. To start with, there are Tor Authority servers which are usually Stable but do not act as an Exit Node. There are Tor Exit Nodes, and the ones with Stable IP addresses announce themselves as such.

Weaknesses

DNS leaks

As with many anonymous web surfing systems, direct DNS requests are usually still performed by many applications, without using the Tor proxy. Using the previously mentioned Privoxy or the command "torify" included with the Tor distribution is a possible solution to this problem. Additionally, applications using SOCKS5 – which supports name-based proxy requests – can route DNS requests through Tor, having lookups performed at the exit node and thus receiving the same anonymity as other Tor traffic.

As of Tor release 0.2.0.1-alpha, Tor includes its own DNS resolver which will dispatch queries over the mix network. This should close the DNS leak and can interact with Tor's address mapping facilities to provide Tor hidden service (.onion) access to non-SOCKS aware applications.

Traffic analysis

Steven J. Murdoch and George Danezis from University of Cambridge presented an article at the 2005 IEEE Symposium on Security and Privacy. They presented traffic-analysis techniques that allow adversaries with only a partial view of the network to infer which nodes are being used to relay the anonymous streams and therefore greatly reduce the anonymity provided by Tor. They have also shown that otherwise unrelated streams can be linked back to the same initiator. The identity of the original user remains undiscovered by this attack, however. Murdoch has been working with and funded by Tor since 2006.

Eavesdropping by exit nodes

In September 2007, Dan Egerstad, a Swedish security consultant, revealed that by operating and monitoring Tor exit nodes he had intercepted usernames and passwords for a large number of email accounts. As Tor does not, and by design cannot, encrypt the traffic between an exit node and the target server, any exit node is in a position to capture any traffic passing through it which is not encrypted at the application layer, e.g. by SSL. While this does not inherently violate the anonymity of the source, it affords added opportunities for data interception by self-selected third parties, greatly increasing the risk of exposure of sensitive data by users who are careless or who mistake Tor's anonymity for security.

Etiquette and abuse

Because of its inherent anonymity, Tor is subject to abuse. Routers each maintain an exit policy of what traffic is and is not permitted to leave the Tor network through that node. It is possible to combat most major abuses of the Tor network using a combination of addresses and ports. Potential abuses include:

Bandwidth hogging

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.

BitTorrents

Due to the high bandwidth usage caused by the use of this protocol, it is considered impolite and inappropriate to use the tor network for bittorrent transfers.

Illegal uses

Some Tor servers have been used for illegal purposes, including child pornography and p2p sharing of copyrighted files using the bit torrent protocol. Child pornography in particular has attracted international law enforcement investigations. In September 2006, German authorities seized data center equipment running Tor software during a child pornography crackdown.

Hidden services

Tor can also provide anonymity to servers in the form of location-hidden services, which are Tor clients or relays running specially configured server software. Rather than revealing the server's IP address (and therefore its location), hidden services are accessed through the Tor-specific .onion pseudo top-level domain. The Tor network understands this TLD and routes data anonymously both to and from the hidden service. Due to this lack of a public address, hidden services may be hosted behind firewalls or NAT. A Tor client is necessary in order to access a hidden service.

"Hidden services were deployed on the Tor anonymous communication network in 2004." Being decentralized by design, there is no official index of hidden services. There are a number of independent hidden services that serve this purpose.

Because location-hidden services do not use exit nodes, they are not subject to exit node eavesdropping. There are, however, a number of security issues involving Tor hidden services. For example, services that are reachable through Tor hidden services and the public Internet are susceptible to correlation attacks, and consequently are not necessarily hidden. Other pitfalls include "letting your web server reveal identifying information about you, your computer, or your location", uptime and downtime statistics, intersection attacks and user error.

See also

• Tor overview page with basic information Start here
• Tools to use with Tor:
  • Vidalia project – a cross-platform GUI controller for Tor
  • Privoxy – a filtering proxy
  • TorButton Firefox Plug-in for Tor proxy management
  • SOcat Multipurpose relay (SOcket CAT): includes socks4, and socks4a functionality (Linux / Mac)
  • Portable Tor – Repackaged version of Tor + Vidalia + Privoxy for use on a USB flash drive
  • TorK a KDE GUI controller for Tor
  • FoxyProxyFirefox Plug-in for Tor proxy management with more options than torbutton. FoxyProxy is also more reliable than torbutton when using Firefox 3
  • Polipo - a caching web proxy
• Web browsers bundled with Tor
  • Tor Browser Bundle - Lets you use Tor on Windows without needing to install any software.
  • OperaTor OperaTor – Preconfigured bundle with Opera (web browser) + Tor + Privoxy
  • xB Browser - Firefox derivative with integrated Tor; runs from USB flash drive or hard drive
  • e-Capsule™ Private Browser - Gecko based anonymous web browser with integrated Tor (trialware)
• Anonymous chat
  • Tor IM Browser Bundle - Allows instant messaging and chat via Tor on Windows without needing to install any software.
  • ScatterChat – an encrypted instant messaging client based on Gaim
  • TorChat - a serverless instant messaging application based on Tor's hidden services
• Preinstalled environments with Tor enabled:
  • xB Machine - A full visualized OS for anonymous communication, ISO bootable and Live CD
  • JanusVM - A VMware Virtual Machine that clients can VPN to (protecting all TCP traffic) that among other things can provide anonymity though Tor
  • Incognito - A sandboxed, Gentoo based Linux distribution, available as a Live CD or Live USB
• Other anonymity networks
  • Freenet – Censorship-resistant network
  • I2P – Anonymous Internet Protocol layer
  • JAP – Java Anon Proxy
  • Psiphon
• Anonymous remailers
  • Cypherpunk anonymous remailer – Type 1 Cypherpunk remailer
  • Mixmaster anonymous remailer – Type 2 Mixmaster remailer
  • Mixminion – Mixminion anonymous remailer
• Anonymous search
  • FAROO – Peer-to-peer ("P2P") web search engine with encryption
  • Yacy – A P2P web search engine without central server
  • Scroogle - Anonymous Google search through proxy with SSL (https)
• Philosophies incorporating anonymity
  • Crypto-anarchism

References

• Anonymity Bibliography Retrieved: 21 May 2007
• Schneier, Bruce. Applied Cryptography. ISBN 0-471-11709-9.
• Schneier, Bruce. Email Security. ISBN 0-471-05318-X.
• Bacard, Andre. Computer Privacy Handbook. ISBN 1-56609-171-3.

External links

• Official website
• I2P's comparison of anonymity networks
• HowTo Anonymity With Tor and Privoxy for Linux
• Freshmeat project page
• Introduction to Tor
• Security Now! podcast discussing TOR
• Tor design document 18 May 2004
• Tor divided to countries
• Tor Network Status
• Tor Node Status In late 2007, this listed over 2000 nodes, about half acting as exit nodes
• Tor recommendations for hack-proof anonymity
• Tor step by step install and setup guide for Windows XP
• Tor wiki with FAQs & HOWTOs
• Tutorial about Tor

Articles

• AnonWatch, Tor in Depth Security Analysis of the Tor Network
• Goodin, Dan "Tor at heart of embassy passwords leak", 2007-09-10 article on The Register news website. (Accessed 2007-09-20).
• Krebs, Brian, Attacks Prompt Update for 'Tor' Anonymity Network 2007-08-08 (Accessed 2007-10-27)
• "The Hack of the Year". The Syndey Morning Herald. 2007-11-13. Retrieved 2007-11-16.
• Zhelatin.IR (= Storm Worm) 2007-09-07 (Accessed 2007-10-27) Template:Fr icon