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