[tor-relays] Fallback Directories - Upcoming Change

David Goulet dgoulet at torproject.org
Wed Apr 7 19:04:10 UTC 2021


This is to announce that the Tor Project network team will soon change how
fallback directories are selected as we are about to update that list.

As a reminder, here is the fallback directory definition from the tor man

  When tor is unable to connect to any directory cache for directory info
  (usually because it doesn’t know about any yet) it tries a hard-coded
  directory. Relays try one directory authority at a time.  Clients try
  multiple directory authorities and FallbackDirs, to avoid hangs on startup
  if a hard-coded directory is down. Clients wait for a few seconds between
  each attempt, and retry FallbackDirs more often than directory authorities,
  to reduce the load on the directory authorities

Previously, we would ask this list for volunteers and then start a lengthy
process over weeks to add/remove volunteers from an official list and then
make sure the volunteered relays were matching a certain set of criteria.

This process was very time consuming for our small team and so we are making a
change to be more efficient and avoid to slip on the updates like we did in
the previous versions.

We will now select at random relays to become fallback directories that
match those requirements:

  - Fast flag

    "Fast" -- A router is 'Fast' if it is active, and its bandwidth is either
    in the top 7/8ths for known active routers or at least 100KB/s.

  - Stable flag

    "Stable" -- 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.

  - DirCache is set to 1 (default)

  - As been around for at least 90 days.

The above corresponds to more than 4000 relays at the moment in the consensus
and so we'll randomly pick 200 from those at each release so we can have
regular rotation and shift load over time over the entire network.

Over time, we will remove or add more relays at each minor release if the set
of fallback directories not working reaches a 25% threshold or more.

Finally, it is very possible that we'll change those requirements over time as
we assess this change. We'll do our best to inform this list in time.

Tor Network Team

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