hsdir

an hsdir is a tor relay that volunteers to store signed descriptors for onion services, so visitors can find them without learning where the server really is.

hsdir stands for hidden service directory - a normal tor relay that opts in to storing tiny documents called descriptors on behalf of onion services. think of it as a distributed, replicated phone book with no names in it, just pointers.

when an onion service comes online it uploads its descriptor (which lists its current introduction points and their keys) to a small set of hsdirs determined by hashing the service's public key onto a logical ring of all hsdirs. the design spreads each descriptor across multiple relays, so no single relay failure or compromise hides a service or reveals it.

clients do the same lookup: to reach exampleaddress.onion, tor hashes the address, figures out which relays are responsible for that slice of the ring, and fetches the descriptor from them - learning only that "someone wants address x", not who hosts it.

in practice hsdirs matter because they are load-bearing infrastructure: when large batches of relays flap or leave the network, descriptor storage churns and previously reachable services suddenly time out for everyone. several of the mysterious "whole market offline" waves tracked on this site trace back to directory stress rather than seizures.