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By default, Tor uses an onion-routing path with three hops: entry, relay (middle) and exit. Is it possible to configure (or, if necessary, recompile) Tor to use more than three hops? This would lessen the probability that all the hops would be under control of an attacker, right?

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Yes, it is possible (through a source-code change), but it is a bad idea. If an attacker is observing (or controls) the first and last hop of your circuit they will very likely be able to de-anonymize you. Changing to four (or more) hop paths doesn't affect the probability of this occurring but it does slow down your Tor connection and increases the load on the Tor network.

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  • Since you're using 4 and not 3 nodes, isn't it more possible to pick a malicious node in the path?
    – alaf
    Commented Sep 26, 2013 at 19:34
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    @alaf Yes, but it doesn't increase the chance that you pick bad first and last hops. (Unfortunately, my statement is not quite true: see freehaven.net/anonbib/#ccs07-doa for why longer paths can hurt your anonymity more.)
    – Roger Dingledine
    Commented Sep 26, 2013 at 19:50
  • One of the mechanisms that an attacker may employ to figures out which relay is the first hop and which relay is the last hop of a client is by compromising the middle node. Once an attacker compromised the middle node, then she knows which hop is the first node and which hop is the last node the client is using, and now the attacker can compromise these hops to de-anonymise the client. The Guard is specially vulnerable now because it is stationary for weeks and months. If there were several middle nodes, then the attacker was not able to obtain this information from any of the middle nodes
    – Roya
    Commented Dec 28, 2014 at 17:52
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Increasing the number of hops in a Tor circuit has various impacts :

  • Performance is decreased since the path is longer and latency is bigger. A bigger path is also less reliable and more failures will occur.
  • The same goes for Throughput of the circuit. Throughput is better when having three hops.
  • Anonymity is not enhanced as a matter of fact. Increasing the number of nodes you're using to establish a circuit, if we assume that total number of possible relays is fixed, increases the possibility of picking some node controlled by an adversary.

So, all in all, increasing the number of hops will not profit you much and may harm your anonymity and performance. It's feasible but not advisable.

It seems that the three hops circuits initially picked from the authors of Tor is the wise choice for anonymity and performance.

You may read more on path selection strategies and related topics in this paper : http://cseweb.ucsd.edu/users/pasquale/Papers/globecom10c.pdf . Some graphs are also included.

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  • One of the mechanisms that an attacker may employs to figure out which relay is the first hop and which relay is the last hop of a client is by compromising the middle node. Once an attacker compromised the middle node, then she knows which hop is the first node and which hop is the last node the client is using, and now the attacker can compromise these hops to de-anonymise the client. The Guard is specially vulnerable now because it is stationary for weeks and months. If there were several middle nodes, then the attacker was not able to obtain this information from any of the middle nodes
    – Roya
    Commented Dec 31, 2014 at 4:28
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    @Roya I don't believe this is true anymore.
    – james-see
    Commented Dec 5, 2015 at 1:16
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The question was ¨Is it possible to use more nodes (then the default 3 nodes)¨ I see that it is a bad idea, so the answer is YES!..it is possible!

I have seen it in the many many lines on the worldmap in vidalia.

I still don't know how to configure torrc between the EntryNodes and ExitNodes so that would be nice to give as an answer.

So the answer is yes, it is possible to have many nodes between the EntryNodes and ExitNodes.

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Actually the answer is yes you can. The only downfall is your tor network slows down. But the upside is the security and anonymity is enhanced greatly. Don't believe the ones who say if you use beyond three it gets worse for you. That is not true at all. If you truly understand technology then you understand that if 3 bodes is hard to track then more than three is harder. One reason this is true is the 4th node and beyond will not know the up of the second node and so on. If you have a 5th node it will not know the 1st 2nd and 3rd node. And this is so on the more nodes you add . And to make it more difficult you need to customize your exit node, it should be picked in places where data retention does not apply. Ex Sweden, Netherlands, Bulgaria. Search for them. Also know that laws can change so you should keep up with which nation does and doesn't have data retention laws. Cheers and yes I did read all the tor documents. I also can't believe what you guys wrote about it make tor less secure. The recommended length of tor nodes is 5 .

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  • You still only need the first and last node and you win, no matter how many hops in the middle.
    – cacahuatl
    Commented Aug 27, 2017 at 13:59

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