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- 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 to
- 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 this 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.)
- Roger also suggested that rather than not listing servers, we mark them as
- not Valid.
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