Filename: 109-no-sharing-ips.txt
Title: No more than one server per IP address.
Version:
Last-Modified:
Author: Kevin Bauer & Damon McCoy
Created: 9-March-2007
Status: Open

Overview:
  This document describes a solution to a Sybil attack vulnerability in the
  directory servers. Currently, it is possible for a single IP address to
  host an arbitrarily high number of Tor routers. We propose that the
  directory servers limit the number of Tor routers that may be registered at
  a particular IP address to some small (fixed) number, perhaps just one Tor
  router per IP address.

  While Tor never uses more than one server from a given /16 in the same
  circuit, an attacker with multiple servers in the same place is still
  dangerous because he can get around the per-server bandwidth cap that is
  designed to prevent a single server from attracting too much of the overall
  traffic.

Motivation:
  Since it is possible for an attacker to register an arbitrarily large
  number of Tor routers, it is possible for malicious parties to do this
  as part of a traffic analysis attack.

Security implications:
  This countermeasure will increase the number of IP addresses that an
  attacker must control in order to carry out traffic analysis.

Specification:
  We propose that the directory servers check if an incoming Tor router IP
  address is already registered under another router. If this is the case,
  then prevent the new router from joining the network.

Compatibility:

  Upon inspection of a directory server, we found that the following IP
  addresses have more than one Tor router:

  Scruples    68.5.113.81     ip68-5-113-81.oc.oc.cox.net     443
  WiseUp      68.5.113.81     ip68-5-113-81.oc.oc.cox.net     9001
  Unnamed     62.1.196.71     pc01-megabyte-net-arkadiou.megabyte.gr  9001
  Unnamed     62.1.196.71     pc01-megabyte-net-arkadiou.megabyte.gr  9001
  Unnamed     62.1.196.71     pc01-megabyte-net-arkadiou.megabyte.gr  9001
  aurel       85.180.62.138   e180062138.adsl.alicedsl.de     9001
  sokrates    85.180.62.138   e180062138.adsl.alicedsl.de     9001
  moria1      18.244.0.188    moria.mit.edu   9001
  peacetime   18.244.0.188    moria.mit.edu   9100

  There may exist compatibility issues with this proposed fix.  Reasons why
  more than one server would share an IP address include:

  * Testing. moria1, moria2, peacetime, and other morias all run on one
    computer at MIT, because that way we get testing. Moria1 and moria2 are
    run by Roger, and peacetime is run by Nick.
  * NAT. If there are several servers but they port-forward through the same
    IP address, ... we can hope that the operators coordinate with each
    other. Also, we should recognize that while they help the network in
    terms of increased capacity, they don't help as much as they could in
    terms of location diversity. But our approach so far has been to take
    what we can get.
  * People who have more than 1.5MB/s and want to help out more. For
    example, for a while Tonga was offering 10MB/s and its Tor server
    would only make use of a bit of it. So Roger suggested that he run
    two Tor servers, to use more.

Alternatives:

  Roger suggested that instead of capping number of servers per IP to 1, we
  should cap total declared bandwidth per IP to some N, and total declared
  servers to some M.  (He suggested N=5MB/s and M=5.) Directory authorities
  would then always choose to keep the highest-bandwidth running servers
  -- if they pick based on time joining the network we can get into bad
  race conditions.

  Roger also suggested that rather than not listing servers, we mark them as
  not Running. (He originally suggested marking them as Running but not
  Valid, but that would still allow an attacker to control an arbitrary
  number of middle hops, which is still likely to be worrisome.)