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Filename: 109-no-sharing-ips.txt
Title: No more than one server per IP address
Author: Kevin Bauer & Damon McCoy
Created: 9-March-2007
Status: Closed
Implemented-In: 0.2.0.x
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:
For each IP address, each directory authority tracks the number of routers
using that IP address, along with their total observed bandwidth. If there
are more than MAX_SERVERS_PER_IP servers at some IP, the authority should
"disable" all but MAX_SERVERS_PER_IP servers. When choosing which servers
to disable, the authority should first disable non-Running servers in
increasing order of observed bandwidth, and then should disable Running
servers in increasing order of bandwidth.
[[ We don't actually do this part here. -NM
If the total observed
bandwidth of the remaining non-"disabled" servers exceeds MAX_BW_PER_IP,
the authority should "disable" some of the remaining servers until only one
server remains, or until the remaining observed bandwidth of non-"disabled"
servers is under MAX_BW_PER_IP.
]]
Servers that are "disabled" MUST be marked as non-Valid and non-Running.
MAX_SERVERS_PER_IP is 3.
MAX_BW_PER_IP is 8 MB per s.
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.
[Note Roger's tweak to this behavior, in
http://archives.seul.org/or/cvs/Oct-2007/msg00118.html]
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