Re: [tor-dev] Time interval for changing Introduction Points in Tor Hidden Services

[Piyush Kumar Sharma <piyushs@xxxxxxxxxxx>]

Thank you David for the prompt reply. The information you provide is helpful.

I would also like to ask one more question related to HSDIRs.

From what I understand from the onion service v3 specification, the HSDIRs corresponding to a particular hidden service change every 24 hours.

This is achieved with the help of a shared random variable (SRV) advertised in the network consensus, which is changed after every 24 hours.

So my question is: If all the HSDIRs corresponding to a hidden service go down (for some reason), can the hidden service somehow change it's HSDIRs before the 24 hours epoch? Or does it mean that as soon as the HSDIRs are inaccessible, the hidden service corresponding to them cannot be accessed before the next epoch?

Thanks in advance for your response!

On Wed, Mar 16, 2022 at 1:37 PM David Goulet <dgoulet@xxxxxxxxxxxxxx> wrote:

On 16 Mar (00:16:19), Piyush Kumar Sharma wrote:
> Hi all,
>
> I have been investigating the Tor hidden services for some research work.
>
> I am finding it hard to obtain information about how often do Introduction
> Points of hidden service change.
>
> More specifically, I am looking for answers to the following questions:
>
> 1.) If the tor relays selected as Introduction Points for a hidden service
> go down (e.g., due to DoS), then in what time duration does the hidden
> service select new Introduction Points automatically?

Almost immediately. Basically, if the introduction circuit collapses, the
service will select a new one, update its descriptor and upload them.

>
> 2.) Even when the Introduction Points are not down, is there a duration
> after which they are automatically changed by the hidden service? If so,
> what is this duration?

Yes, it is a random value between 18h and 24h.

https://gitlab.torproject.org/tpo/core/tor/-/blob/main/src/feature/hs/hs_service.c#L481

>
> I have already tried to find this information in the Tor-spec about onion
> v3 services. But could not find answers to these specific questions.

Yeah, these are mostly implementation details rather than specific to the
protocol.

And so unfortunately, these details can only be found in the source code.

David

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