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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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