.onion
| Introduced | 2004 |
|---|---|
| TLD type | Host suffix |
| Status | Not in root, but used by Tor clients, servers, and proxies |
| Registry | Tor |
| Intended use | To designate a hidden service reachable via Tor |
| Actual use | Used by Tor users for services in which both the provider and the user are anonymous and difficult to trace |
| Registration restrictions | Addresses are "registered" automatically by Tor client when a hidden service is set up |
| Structure | Names are opaque strings generated from public keys |
| Documents | |
| Dispute policies | N/A |
| Website | www |
.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.
The "onion" name refers to onion routing, the technique used by Tor to achieve a degree of anonymity.
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 than the official Tor Browser.
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Wikipedia