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Tor
The Onion Router, onion services, hidden services
An anonymity network that routes traffic through three-hop circuits to hide the link between sender and destination.
Tor is a volunteer-run network that carries encrypted traffic through a chain of three relays, using a technique called onion routing. Each relay only knows the previous hop and the next hop, so no single node can tie a user to a destination. Bitcoin software uses Tor to connect to peers, fetch fee data, and publish onion services that accept inbound connections without a public IP.
Onion services (sometimes called hidden services) give a server a .onion address instead of a regular DNS name. The server is reachable without exposing its IP, and the connection is end-to-end encrypted between the two .onion peers.
The project lives at torproject.org. Bitcoin Core ships built-in support for Tor v3 onion services; Lightning nodes typically do the same. See also the Tor project page for OpenSats grants in this area.