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83 lines
3.8 KiB
Plaintext
83 lines
3.8 KiB
Plaintext
Filename: 109-no-sharing-ips.txt
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Title: No more than one server per IP address.
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Version:
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Last-Modified:
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Author: Kevin Bauer & Damon McCoy
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Created: 9-March-2007
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Status: Open
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Overview:
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This document describes a solution to a Sybil attack vulnerability in the
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directory servers. Currently, it is possible for a single IP address to
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host an arbitrarily high number of Tor routers. We propose that the
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directory servers limit the number of Tor routers that may be registered at
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a particular IP address to some small (fixed) number, perhaps just one Tor
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router per IP address.
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While Tor never uses more than one server from a given /16 in the same
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circuit, an attacker with multiple servers in the same place is still
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dangerous because he can get around the per-server bandwidth cap that is
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designed to prevent a single server from attracting too much of the overall
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traffic.
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Motivation:
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Since it is possible for an attacker to register an arbitrarily large
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number of Tor routers, it is possible for malicious parties to do this
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as part of a traffic analysis attack.
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Security implications:
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This countermeasure will increase the number of IP addresses that an
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attacker must control in order to carry out traffic analysis.
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Specification:
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We propose that the directory servers check if an incoming Tor router IP
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address is already registered under another router. If this is the case,
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then prevent the new router from joining the network.
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Compatibility:
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Upon inspection of a directory server, we found that the following IP
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addresses have more than one Tor router:
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Scruples 68.5.113.81 ip68-5-113-81.oc.oc.cox.net 443
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WiseUp 68.5.113.81 ip68-5-113-81.oc.oc.cox.net 9001
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Unnamed 62.1.196.71 pc01-megabyte-net-arkadiou.megabyte.gr 9001
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Unnamed 62.1.196.71 pc01-megabyte-net-arkadiou.megabyte.gr 9001
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Unnamed 62.1.196.71 pc01-megabyte-net-arkadiou.megabyte.gr 9001
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aurel 85.180.62.138 e180062138.adsl.alicedsl.de 9001
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sokrates 85.180.62.138 e180062138.adsl.alicedsl.de 9001
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moria1 18.244.0.188 moria.mit.edu 9001
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peacetime 18.244.0.188 moria.mit.edu 9100
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There may exist compatibility issues with this proposed fix. Reasons why
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more than one server would share an IP address include:
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* Testing. moria1, moria2, peacetime, and other morias all run on one
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computer at MIT, because that way we get testing. Moria1 and moria2 are
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run by Roger, and peacetime is run by Nick.
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* NAT. If there are several servers but they port-forward through the same
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IP address, ... we can hope that the operators coordinate with each
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other. Also, we should recognize that while they help the network in
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terms of increased capacity, they don't help as much as they could in
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terms of location diversity. But our approach so far has been to take
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what we can get.
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* People who have more than 1.5MB/s and want to help out more. For
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example, for a while Tonga was offering 10MB/s and its Tor server
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would only make use of a bit of it. So Roger suggested that he run
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two Tor servers, to use more.
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Alternatives:
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Roger suggested that instead of capping number of servers per IP to 1, we
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should cap total declared bandwidth per IP to some N, and total declared
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servers to some M. (He suggested N=5MB/s and M=5.) Directory authorities
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would then always choose to keep the highest-bandwidth running servers
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-- if they pick based on time joining the network we can get into bad
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race conditions.
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Roger also suggested that rather than not listing servers, we mark them as
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not Running. (He originally suggested marking them as Running but not
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Valid, but that would still allow an attacker to control an arbitrary
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number of middle hops, which is still likely to be worrisome.)
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