Proposal: Suicide descriptors when Tor servers stop
arma at mit.edu
Wed Aug 15 13:38:06 UTC 2007
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
Tor servers should publish a last descriptor whenever they shut down,
to let others know that they are no longer offering service.
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.
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]
TBD but should be pretty straightforward.
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.
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.
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.
tup suggested this idea.
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