Inlining the proposal written for #8243. Feedback is welcome. Thanks!

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Filename: 243-hsdir-flag-need-stable.txt Title: Give out HSDir flag only to relays with Stable flag Author: George Kadianakis Created: 2015-03-23 Status: Open

1. Introduction

The descriptors of hidden services are stored by hidden service directories. Those are chosen by directory authorities who assign the "HSDir" flag to those relays according to their uptime.

It's important for new relays to not be able to get the HSDir flag too easily, because a few correctly placed HSDirs can launch a denial of service attack on a hidden service. We should make sure that a naive Sybil attacker that injects thousands of new Tor relays to the network cannot position herself like this.

2. Motivation

Currently, directory authorities give out the HSDir flag to relays that volunteer to be hidden service directories by sending a "hidden-service-dir" line in their relay descriptor, which is the default relay behavior. Furthermore, the HSDir flag is only given to relays that have been up for more than MinUptimeHidServDirectoryV2 hours. MinUptimeHidServDirectoryV2 is a parameter locally set at the directory authorities and it's somewhere between 25 to 96 hours.

We propose changing that last requirement, and instead giving the HSDir flag only to relays that have the Stable flag. We believe that this will result in a few benefits:

- We stop using the ad-hoc uptime calculation that we are currently doing (see dirserv_thinks_router_is_hs_dir()). Instead, we use the MTBF uptime calculation that is performed for the Stable flag which is more robust.

- We increase the time required to get the HSDir flag, making it harder for naive adversaries that flood the network with relays to actually get the HSDir flag.

- After implementing non-deterministic HSDir picks (#8244) we also make it harder for sophisticated adversaries to DoS a hidden service, since at that point their main attack strategy is to flood the network with relays.

- By increasing the stability of HSDirs, we reduce the misses during descriptor fetching that get caused by natural churn of relays on the list of HSDirs.

3. Specification

We are suggesting changing the criteria that directory authorities use to vote for HSDirs to the following:

- The relay has included the "hidden-service-dir\n" line in its descriptor.

- The relay is eligible for having the "Stable" flag.

4. Security considerations

As it currently is, a router is 'Stable' if it is active, and either its Weighted MTBF is at least the median for known active routers or its Weighted MTBF corresponds to at least 7 days. This is stricter criteria than what's required for HSDir, which means that the number of HSDirs will decrease after the suggested changes.

Currently there are about 2400 HSDirs in the consensus, and about 2300 of them are Stable, which means that we will lose about 100 HSDirs. We believe that this is an acceptable temporary loss. In the short-term future, the number of HSDirs will greatly improve as more directory authorities upgrade to #14202 and more relays upgrade to #12538.

5. Future

Should we give out the HSDir flag only to relays that are Fast? Is being an HSDir a demanding job bandwidth-wise?

With the upcoming keyblinding scheme (#8106) and non-deterministic HSDir selection (#8244), are there any other criteria that we should use when assigning HSDir flags?