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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.