Proposal: Suicide descriptors when Tor servers stop

[Roger Dingledine]

Filename: 120-suicide-descriptors.txt
Title: Suicide descriptors when Tor servers stop
Version: $Revision: 11115 $
Last-Modified: $Date: 2007-08-15 09:37:33 -0400 (Wed, 15 Aug 2007) $
Author: Roger Dingledine
Created: 15-Aug-2007
Status: Open

Overview:

  Tor servers should publish a last descriptor whenever they shut down,
  to let others know that they are no longer offering service.

The Problem:

  The main reason for this is in reaction to Internet services that want
  to treat connections from the Tor network differently. Right now,
  if a user experiments with turning on the "relay" functionality, he
  is punished by being locked out of some websites, some IRC networks,
  etc --- and this lockout persists for several days even after he turns
  the server off.

Design:

  During the "slow shutdown" period if exiting, or shortly after the
  user sets his ORPort back to 0 if not exiting, Tor should publish a
  final descriptor with the following characteristics:

  1) Exit policy is listed as "reject *:*"
  2) It includes a new entry called "opt shutdown 1"

  The first step is so current blacklists will no longer list this node
  as exiting to whatever the service is.

  The second step is so directory authorities can avoid wasting time
  doing reachability testing. Authorities should automatically not list
  as Running any router whose latest descriptor says it shut down.

  [I originally had in mind a third step --- Advertised bandwidth capacity
  is listed as "0" --- so current Tor clients will skip over this node
  when building most circuits. But since clients won't fetch descriptors
  from nodes not listed as Running, this step seems pointless. -RD]

Spec:

  TBD but should be pretty straightforward.

Security issues:

  Now external people can learn exactly when a node stopped offering
  relay service. How bad is this? I can see a few minor attacks based
  on this knowledge, but on the other hand as it is we don't really take
  any steps to keep this information secret.

Overhead issues:

  We are creating more descriptors that want to be remembered. However,
  since the router won't be marked as Running, ordinary clients won't
  fetch the suicide descriptors. Caches will, though. I hope this is ok.

Implementation:

  To make things easy, we should publish the suicide descriptor only
  on controlled shutdown (SIGINT as opposed to SIGTERM). That would
  leave enough time for publishing that we probably wouldn't need any
  extra synchronization code.

  If that turns out to be too unintuitive for users, I could imagine doing
  it on SIGTERMs too, and just delaying exit until we had successfully
  published to at least one authority, at which point we'd hope that it
  propagated from there.

Acknowledgements:

  tup suggested this idea.