.onion is a special-use top level domain suffix designating an anonymous hidden service reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the Internet DNS root, but with the appropriate proxy software installed, Internet programs such as web browsers can access sites with .onion addresses by sending the request through the network of Tor servers. The purpose of using such a system is to make both the information provider and the person accessing the information more difficult to trace, whether by one another, by an intermediate network host, or by an outsider.


Addresses in the .onion TLD are generally opaque, non-mnemonic, 16-character alpha-semi-numeric hashes which are automatically generated based on a public key when a hidden service is configured. These 16-character hashes can be made up of any letter of the alphabet, and decimal digits from 2 to 7, thus representing an 80-bit number in base32. It is possible to set up a human-readable .onion URL (e.g. starting with an organization name) by generating massive numbers of key pairs (a computational process that can be parallelized) until a sufficiently desirable URL is found.[2][3]

The "onion" name refers to onion routing, the technique used by Tor to achieve a degree of anonymity.

WWW to .onion gateways

Proxies into the Tor network like Tor2web allow access to hidden services from non-Tor browsers and for search engines that are not Tor-aware. By using a gateway, users give up their own anonymity and trust the gateway to deliver the correct content. Both the gateway and the hidden service can fingerprint the browser, and access user IP address data. Some proxies use caching techniques to provide better page-loading[4] than the official Tor Browser.[5]


.exit is a pseudo-top-level domain used by Tor users to indicate on the fly to the Tor software the preferred exit node that should be used while connecting to a service such as a web server, without having to edit the configuration file for Tor (torrc).

The syntax used with this domain is hostname + .exitnode + .exit, so that a user wanting to connect to http://www.torproject.org/ through node tor26 would have to enter the URL http://www.torproject.org.tor26.exit.

Example uses for this include accessing a site available only to addresses of a certain country or checking if a certain node is working.

Users can also type exitnode.exit alone to access the IP address of exitnode.

The .exit notation is deprecated as of version[6] It is disabled by default as of version due to potential application-level attacks.[7]

Official designation

The domain used to be pseudo-top-level domain host suffix, similar in concept to such endings as .bitnet and .uucp used in earlier times.

On 9 September 2015 ICANN, IANA and the IETF designated .onion as a 'special use domain', giving the domain an official status following a proposal from Jacob Appelbaum of the Tor Project and Facebook security engineer Alec Muffett.[8][9][10]

HTTPS support

SSL stripping attacks from malicious exit nodes on the Tor network are a risk to users accessing traditional HTTPS clearnet sites. Sites offering dedicated .onion addresses can provide an additional layer of identity assurance via certificates, though the encryption itself is technically redundant, given Tor's native encryption features.[11] Provision of HTTPS certificates also enables browser features which would otherwise be unavailable to users of .onion sites.

Prior to the adoption of CA/Browser Forum Ballot 144, a HTTPS certificate for a .onion name could only be acquired by treating .onion as an Internal Server Name.[12] Per the CA/Browser Forum's Baseline Requirements, these certificates could be issued, but were required to expire before 1 November 2015.[13] Despite these restrictions, four organizations went ahead with certificate authority partnerships to do so; these were DuckDuckGo in July 2013,[14] Facebook in October 2014,[15] Blockchain.info in December 2014,[16] The Intercept in April 2015,[17] and The New York Times in October 2017.[18]

Following the adoption of CA/Browser Forum Ballot 144 and the designation of the domain as 'special use' in September 2015, .onion meets the criteria for RFC 6761.[19] Certificate authorities may issue SSL certificates for HTTPS .onion sites per the process documented in the CA/Browser Forum's Baseline Requirements,[20] introduced in Ballot 144.[12]

As of August 2016, 13 onion domains are https signed across 7 different organisations via DigiCert.[21]

See also


  1. ^ J. Appelbaum, A. Muffett (October 2015). The ".onion" Special-Use Domain Name. doi:10.17487/RFC7686. RFC 7686. https://tools.ietf.org/html/rfc7686. 
  2. ^ "Scallion". GitHub. Retrieved 2014-11-02. 
  3. ^ Muffett, Alec (2014-10-31). "Re: Facebook brute forcing hidden services". tor-talk (Mailing list). Simple End-User Linux. Retrieved 2014-11-02. 
  4. ^ "Onion.cab: Advantages of this TOR2WEB-Proxy". Retrieved 2014-05-21. 
  5. ^ "Tor Browser Bundle". Retrieved 2014-05-21. 
  6. ^ "Tor Release Notes". Retrieved 2017-10-04. 
  7. ^ "Special Hostnames in Tor". Retrieved 2012-06-30. 
  8. ^ Nathan Willis (10 September 2015). "Tor's .onion domain approved by IETF/IANA". LWN.net. 
  9. ^ Franceschi-Bicchierai, Lorenzo (10 September 2015). "Internet Regulators Just Legitimized The Dark Web". Retrieved 10 September 2015. 
  10. ^ "Special-Use Domain Names". Retrieved 10 September 2015. 
  11. ^ Schuhmacher, Sophie (5 December 2014). "Blockchain.Info Launches Darknet Site In Response To Thefts Over TOR". Retrieved 20 September 2015. 
  12. ^ a b "CA/Browser Forum Ballot 144 - Validation rules for .onion names". Retrieved 13 September 2015. 
  13. ^ "Baseline Requirements for the Issuance and Management Publicly-Trusted Certificates, v1.0" (PDF). Retrieved 13 September 2015. 
  14. ^ _zekiel (1 July 2013). "We've updated our Tor hidden service to work over SSL. No solution for the cert. warning, yet!". Reddit. Retrieved 20 December 2016. 
  15. ^ Muffett, Alec (31 October 2014). "Making Connections to Facebook more Secure". Retrieved 11 September 2015. 
  16. ^ Alyson (3 December 2014). "Improved Security for Tor Users". Retrieved 11 September 2015. 
  17. ^ Lee, Micah (8 April 2015). "Our SecureDrop System for Leaks Now Uses HTTPS". Retrieved 10 September 2015. 
  18. ^ Sandvik, Runa (2017-10-27). "The New York Times is Now Available as a Tor Onion Service". The New York Times. Retrieved 2017-11-17. 
  19. ^ Arkko, Jari (10 September 2015). ".onion". Retrieved 13 September 2015. 
  20. ^ "Baseline Requirements Documents". Retrieved 13 September 2015. 
  21. ^ Jamie Lewis, Sarah (7 August 2016). "OnionScan Report: July 2016 - HTTPS Somewhere Sometimes". Retrieved 15 August 2016. 

External links