[tor-commits] [tordnsel/master] add the tor exitlist design doc into the repo.
andrew at torproject.org
andrew at torproject.org
Fri Aug 17 02:12:17 UTC 2012
commit f94f6f3bb6c8a8f407f4b187d6cfb536b13e46ce
Author: Andrew Lewman <andrew at torproject.org>
Date: Thu Aug 16 22:11:55 2012 -0400
add the tor exitlist design doc into the repo.
---
doc/torel-design.txt | 181 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 181 insertions(+), 0 deletions(-)
diff --git a/doc/torel-design.txt b/doc/torel-design.txt
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+Design For A Tor DNS-based Exit List
+
+Status:
+
+ This is a suggested design for a DNS Exit List (DNSEL) for Tor exit nodes.
+ See http://exitlist.torproject.org/ for an implementation.
+
+Why?
+
+ It's useful for third parties to be able to tell when a given connection
+ is coming from a Tor exit node. Potential applications range from
+ "anonymous user" cloaks on IRC networks like oftc, to networks like
+ Freenode that apply special authentication rules to users from these
+ IPs, to systems like Wikipedia that may want to make a priority of
+ _unblocking_ shared IPs more liberally than non-shared IPs, since shared
+ IPs presumably have non-abusive users as well as abusive ones.
+
+ Since Tor provides exit policies, not every Tor server will connect to
+ every address:port combination on the Internet. Unless you're trying to
+ penalize hosts for supporting anonymity, it makes more sense to answer
+ the fine-grained question "which Tor servers will connect to _me_?" than
+ the coarse-grained question "which Tor servers exist?" The fine-grained
+ approach also helps Tor server ops who share an IP with their Tor
+ server: if they want to access a site that blocks Tor users, they
+ can exclude that site from their exit policy, and the site can learn
+ that they won't send it anonymous connections.
+
+ Tor already ships with a tool (the "contrib/exitlist" script) to
+ identify which Tor nodes might open anonymous connections to any given
+ exit address. But this is a bit tricky to set up, so only sites like
+ Freenode and OFTC that are dedicated to privacy use it.
+ Conversely, providers of some DNSEL implementations are providing
+ coarse-grained lists of Tor hosts -- sometimes even listing servers that
+ permit no exit connections at all. This is rather a problem, since
+ support for DNSEL is pretty ubiquitous.
+
+
+How?
+
+ Keep a running Tor instance, and parse the cached-routers and
+ cached-routers.new files as new routers arrive. To tell whether a given
+ server allows connections to a certain address:port combo, look at the
+ definitions in dir-spec.txt or follow the logic of the current exitlist
+ script. If bug 405 is still open when you work on this
+ (https://bugs.torproject.org/flyspray/index.php?do=details&id=405), you'll
+ probably want to extend it to look at only the newest descriptor for
+ each server, so you don't use obsolete exit policy data.
+
+ FetchUselessDescriptors would probably be a good torrc option to enable.
+
+ If you're also running a directory cache, you get extra-fresh
+ information.
+
+
+The DNS interface
+
+ Standard DNSEL, if I understand right, looks like this: There's some
+ authoritative name server for foo.example.com. You want to know if
+ 1.2.3.4 is in the list, so you query for an A record for
+ 4.3.2.1.foo.example.com. If the record exists and has the value
+ 127.0.0.2[DNSBL-EMAIL], 1.2.3.4 is in the list. If you get an NXDOMAIN
+ error, 1.2.3.4 is not in the list. If you ask for a domain name outside
+ of the foo.example.com zone, you get a Server Failure error[RFC 1035].
+
+ Assume that the DNSEL answers queries authoritatively for some zone,
+ torhosts.example.com. Below are some queries that could be supported,
+ though some of them are possibly a bad idea.
+
+
+ Query type 1: "General IP:Port"
+
+ Format:
+ {IP1}.{port}.{IP2}.ip-port.torhosts.example.com
+
+ Rule:
+ Iff {IP1} is a Tor server that permits connections to {port} on
+ {IP2}, then there should be an A record with the value 127.0.0.2.
+
+ Example:
+ "1.0.0.10.80.4.3.2.1.ip-port.torhosts.example.com" should have the
+ value 127.0.0.2 if and only if there is a Tor server at 10.0.0.1
+ that allows connections to port 80 on 1.2.3.4.
+
+ Example use:
+ I'm running an IRC server at w.x.y.z:9999, and I want to tell
+ whether an incoming connection is from a Tor server. I set
+ up my IRC server to give a special mask to any user coming from
+ an IP listed in 9999.z.y.x.w.ip-port.torhosts.example.com.
+
+ Later, when I get a connection from a.b.c.d, my ircd looks up
+ "d.c.b.a.9999.z.y.x.w.ip-port.torhosts.example.com" to see
+ if it's a Tor server that allows connections to my ircd.
+
+
+ Query type 2: "IP-port group"
+
+ Format:
+ {IP}.{listname}.list.torhosts.example.com
+
+ Rule:
+ Iff this Tor server is configured with an IP:Port list named
+ {listname}, and {IP} is a Tor server that permits connections to
+ any member of {listname}, then there exists an A record.
+
+ Example:
+ Suppose torhosts.example.com has a list of IP:Port called "foo".
+ There is an A record for 4.3.2.1.foo.list.torhosts.example.com
+ if and only if 1.2.3.4 is a Tor server that permits connections
+ to one of the addresses in list "foo".
+
+ Example use:
+ Suppose torhosts.example.com has a list of hosts in "examplenet",
+ a popular IRC network. Rather than having them each set up to
+ query the appropriate "ip-port" list, they could instead all be
+ set to query a central examplenet.list.torhosts.example.com.
+
+ Problems:
+ We'd be better off if each individual server queried about hosts
+ that allowed connections to itself. That way, if I wanted to
+ allow anonymous connections to foonet, but I wanted to be able to
+ connect to foonet from my own IP without being marked, I could add
+ just a few foonet addresses to my exit policy.
+
+
+ Query type 3: "My IP, with port"
+
+ Format:
+ {IP}.{port}.me.torhosts.example.com
+
+ Rule:
+ An A record exists iff there is a tor server at {IP} that permits
+ connections to {port} on the host that requested the lookup.
+
+ Example:
+ "4.3.2.1.80.me.torhosts.example.com" should have an A record if
+ and only if there is a Tor server at 1.2.3.4 that allows
+ connections to port 80 of the querying host.
+
+ Example use:
+ Somebody wants to set up a quick-and-dirty Tor detector for a
+ single webserver: just point them at 80.me.torhosts.example.com.
+
+ Problem:
+ This would be easiest to use, but DNS gets in the way. If you
+ create DNS records that give different results depending on who is
+ asking, you mess up caching. There could be a fix here, but might
+ not.
+
+
+ RECOMMENDATION: Just build ip-port for now, and see what demand is
+ like. There's no point in building mechanisms nobody wants.
+
+Web interface:
+
+ Should provide the same data as the dns interface.
+
+Other issues:
+
+ After a Tor server op turns off their server, it stops publishing server
+ descriptors. We should consider that server's IP address to still
+ represent a Tor node until 48 hours after its last descriptor was
+ published.
+
+ 30-60 minutes is not an unreasonable TTL.
+
+ There could be some demand for address masks and port lists. Address
+ masks wider than /8 make me nervous here, as do port ranges.
+
+ We need an answer for what to do about hosts which exit from different
+ IPs than their advertised IP. One approach would be for the DNSEL
+ to launch periodic requests to itself through all exit servers whose
+ policies allow it -- and then see where the requests actually come from.
+
+References:
+
+ [DNSBL-EMAIL] Levine, J., "DNS Based Blacklists and Whitelists for
+ E-Mail", http://tools.ietf.org/html/draft-irtf-asrg-dnsbl-02, November
+ 2005.
+
+ [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
+ Specification", RFC 1035, November 1987.
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