149-using-netinfo-data.txt

Filename: 149-using-netinfo-data.txt
Title: Using data from NETINFO cells
Author: Nick Mathewson
Created: 2-Jul-2008
Status: Superseded
Target: 0.2.1.x

[Partially done: we do the anti-MITM part.  Not entirely done: we don't do
the time part.]

Overview

   Current Tor versions send signed IP and timestamp information in
   NETINFO cells, but don't use them to their fullest.  This proposal
   describes how they should start using this info in 0.2.1.x.

Motivation

   Our directory system relies on clients and routers having
   reasonably accurate clocks to detect replayed directory info, and
   to set accurate timestamps on directory info they publish
   themselves.  NETINFO cells contain timestamps.

   Also, the directory system relies on routers having a reasonable
   idea of their own IP addresses, so they can publish correct
   descriptors.  This is also in NETINFO cells.

Learning the time and IP address

   We need to think about attackers here.  Just because a router tells
   us that we have a given IP or a given clock skew doesn't mean that
   it's true.  We believe this information only if we've heard it from
   a majority of the routers we've connected to recently, including at
   least 3 routers.  Routers only believe this information if the
   majority includes at least one authority.

Avoiding MITM attacks

   Current Tors use the IP addresses published in the other router's
   NETINFO cells to see whether the connection is "canonical".  Right
   now, we prefer to extend circuits over "canonical" connections.  In
   0.2.1.x, we should refuse to extend circuits over non-canonical
   connections without first trying to build a canonical one.