I want to know how many preemptively circuits Tor can builds. I got confused about it when I read "Tor Path Specification".

According to this article, at the startup Tor builds 3 clean circuits, but somewhere else it says Tor builds 2 circuits for each recently seen ports (If there are 12 or more clean circuits open, it doesn't open more even if it has more predictions.)

What does Tor actually do when preemptively building circuits?

1 Answer 1


It's complicated. There are many reasons Tor builds circuits preemtively: access hidden services, provide them, exit to any ports that you may want to use (based on recent behavior) and also measuring the network to determine what latency/circuit build times to expect.

circuit_predict_and_launch_new() has all the gory details.

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