clean up the torbl-design draft

Why?

  It's useful for third parties to be able to tell when they've got a

  connection from a Tor exit node.  Potential aplications range from

  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 want to make a priority of

  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 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

  add that site to their exit policy, and the site can learn that they

  won't send it anonymous connections.

  Tor already ships with a tool (the "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, and isn't seeing much use.

  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 DNSBL 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

  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.

  script. If bug 405 is still open when you work on this

  (http://bugs.noreply.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 option to enable.

  FetchUselessDescriptors would probably be a good torrc option to enable.

  If you're also running a directory cache, you get extra-fresh

  information.

  possibly a bad idea.

  "General IP:Port"

  Query type 1: "General IP:Port"

    Format:

        {IP1}.{port}.{IP2}.ip-port.torhosts.example.com

    Example use:

        I'm running an IRC server at w.x.y.z:9999, and I want to tell

        whether an incoming connections are from Tor servers.  I set

        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.

        if it's a Tor server that allows connections to my ircd.

  "IP-port group."

  Query type 2: "IP-port group"

    Format:

        {IP}.{listname}.list.torhosts.example.com

        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|.

        to one of the addresses in list "foo".

    Example use:

        Suppose torhosts.example.com has a list of hosts in "examplenet",

        just a few foonet addresses to my exit policy.

  "My IP, with port."

  Query type 3: "My IP, with port"

    Format:

        {IP}.{port}.me.torhosts.example.com

        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

        now.

        here.

        not.

  RECOMMENDATION: Just build ip-port for now, and see what demand is

  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.

  IPs than their advertised IP. One approach would be for the DNSBL

  to launch periodic requests to itself through all exit servers whose

  policies allow it -- and then see where the requests actually come from.