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Despite the manual's declared defaults [below], Onion Circuits shows single hop circuits being "Built" by Tor. So I have edited my torrc to specifically include ExcludeSingleHopRelays 1 and AllowSingleHopCircuits 0. Yet Onion Circuits still shows them being "Built".

Careful observation of the (often four?) initial one-hop circuits that are built at Tails boot (presumably used to "build consensus" as to which of them will continue on to be my "guard" for that session) tells me that the one-hop circuits that Tor will still build later in that particular session ARE from those initial four [at least the ones where I've noticed a later one-hop].

So I guess I can see why Tor builds those initially. While I understand that my IP is known to my "guard", why do those other potential candidates check in again later, and build one-hop circuits?

I am using Tails 2.9.1, and the torrc file at etc/tor/torrc. Other edits to that file function.

The manual states:

ExcludeSingleHopRelays 0|1

This option controls whether circuits built by Tor will include relays with the AllowSingleHopExits flag set to true. If ExcludeSingleHopRelays is set to 0, these relays will be included. Note that these relays might be at higher risk of being seized or observed, so they are not normally included. Also note that relatively few clients turn off this option, so using these relays might make your client stand out. (Default: 1)

AllowSingleHopCircuits 0|1

When this option is set, the attached Tor controller can use relays that have the AllowSingleHopExits option turned on to build one-hop Tor connections. (Default: 0)

[I have not specifcally set the SERVER OPTION AllowSingleHopExits 0|1 to 0, (which is its default anyways), because I am not running a server.]

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  • Dude, all circuits start out at one hop. They don't magically come into being at the correct length. They're "built" hop-by-hop.
    – cacahuatl
    Commented Dec 23, 2016 at 3:48
  • canonizing ironize: I have some understanding of computing. I can even sometimes discern between random events, apparently random events, computer programming, prestidigitation, and magic. And if a "default" appears to be non-functional, I think that I harm NOTHING by specifying it; {and MAY even circumvent a programming error}.
    – agd
    Commented Jan 24, 2017 at 5:26

1 Answer 1

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No, you cannot stop Tor from building single-hop circuits.

  1. All Tor circuits start out at 1 hop in length. Clearly, to build a circuit any circuit needs to start from the first hop to then negotiate with the next one. As such, all Tor circuits will always be, at some point, one hop in length.

  2. When Tor first starts it does not know any relays except those hardcoded into the tor binary. These are the directory authorities and fallback directories. As such, when Tor first starts it has only one place to build a circuit to: one of the directory servers it knows about to fetch the consensus.

    • Since there are a limited set of Tor directory servers in the directory authority and fallback server set and Tor does not intend to hide the fact that users are using Tor there is no advantage for Tor to try to anonymize the directory fetch. With one exception: Bridge users fetch the consensus from their bridges but that is to defeat censorship not to anonymize the directory fetch.
  3. Editing those torrc options does absolutely nothing since those options were already set anyway. As you were told when you last asked this exact same question: they are the defaults.

Tor will not use single hop circuits in cases where it would break user anonymity to do so without the user explicitly asking Tor to do it by, for example, randomly setting options that they do not understand.

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  • Please note that I am differentiating between circuits whose "Status" in Onion Circuits is labelled as "Launched" {first hop}; "Extended" {second hop}, and "Built" {Normally the third hop, BUT SOMETIMES ALSO ONE-HOPS}. [Circuit progress can be viewed by entering systemctl restart tor as root, and watching "Status".] As stated above: those circuits which I find objectionable/questionable are those one-hops whose "Status" is "Built", which appear at intervals during a given session [with no intervening restart].
    – agd
    Commented Jan 24, 2017 at 5:18
  • To repeat {with grammar and highlighting changes for clarification}: WHY DO THOSE OTHER POTENTIAL CANDIDATES CHECK IN AGAIN LATER, AND [create] "Built" ONE-HOP CIRCUITS? [i.e. completed after only one-hop]
    – agd
    Commented Jan 24, 2017 at 5:20
  • Read my answer. Tor uses single hops for specific cases where anonymity is not required and cannot be achieved.
    – cacahuatl
    Commented Jan 26, 2017 at 11:10

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