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I'm planning to run a service that will rely on hidden-services for anonymous data routing. Hence I will need to host a large number of them, sometimes in the same place. I have ideas about how to scale the application that will be hosted, but I am not sure about how to scale the number of hosted hidden services. I already saw the hard failover and load balancing properties we could have by running multiple Tor instances with the same hidden service keys.

But still, I have a few questions :

  • Is there a hard limit of how many hidden services can be ran by a single Tor instance?
  • If not, is there a known formula or number of hidden services beyond which the Tor instance will lose performance even though the server didn't run out of resources?
  • If not, how much memory do we need for each hidden service hosted (inside a Tor instance where other hidden services are hosted as well)?
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I discussed your question in the #tor IRC channel. phobos and velope helped with this answer.

When I fiddled around with your question I tried to create a large number of hidden services (HS). It was quite easy to create 100 HS, but when I started Tor, the process run for some minutes on 100% CPU utilization. There was no impact on the memory. I looked it up with free -m. The numbers before the creation and afterwards were nearly the same.

phobos reported that he created 5000 HS once. It tool Tor quite some time to start, because it has to create lots of circuits. In the end all services ran on one computer and caused no performance issues.

So in general there is no limit for a number of hidden services. You can run as many as you want. Also there seems no performance degradation, besides the fact, that Tor needs some time for start-up.

Youi can run a quite larger number of hidden services on one machine. However if you plan to start a new Freedom Hosting, here is one hint: Don't do it! ;-)

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    Thanks a lot for your comprehensive answer! And don't worry, I'm not planning to start a new Freedom Hosting ;)
    – Syrian Watermelon
    Commented Jan 29, 2014 at 9:09
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It is like when somebody offers webhosting behind Tor network, I think you can have as many as you want, depending on strength and bandwidth of your server.

If I remember correctly, freedom hosting had thousands of websites, everything at one server with 32GB RAM. and every site had its own onion domain.

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