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This prevents them knowing both the service and client IP addresses, and therefore being targets for network traffic logging, sybil, or hacking attacks.

We need to implement the following checks:

• if an introduction point was made using a direct connection (single onion services), refuse direct client connections,
• for v3 intro points, always refuse direct client connections
• for v2 intro points, refuse direct client connections based on a consensus parameter
• if the rend point was made using a direct connection (custom client, no tor2web for HSv3), refuse direct service connections (single onion services).

See legacy/trac#22688 (moved) for how this is done for HSDir3s using channel_is_client(). The comments in that patch explain why it works.

We could even refactor the common code out of connection_dir_is_anonymous() into connection_is_anonymous(), and avoid including channel[tls].h into directory.c.

I'm not sure if I will get time to do this, so please feel free to take this ticket.