Facilitator should not give client registrations to Tor exits
Like in #6293 (closed), in order to avoid a Tor-in-Tor situation, the facilitator should not give client registrations to any proxy that is requesting from a Tor exit. This should work similarly to how BridgeDB treats exits specially.
This ideally will use a locally cached database of exits. (Not an on-demand DNS lookup.) It should continue to work (perhaps with some classification errors) even if the database can't be refreshed for some time.
Roger offered this command, which is used to update the exit database for BridgeDB:
cat $HOME/auto-naming/moria1/cached-des* | python $HOME/git/contrib/exitlist <ip>:<port> > exitlist
The facilitator should not return a useful client registration in any case. An additional question is whether it should send a signal requesting that the proxy disable itself. (Knowing that a malicious proxy may ignore it.)