Registration over App Engine

It apparently is possible to use Google App Engine apps if you can access !https://www.google.com/. We can use this for rendezvous.

As an example of doing it manually, you can run flashproxy-reg-url and paste the URL you get into an existing proxy app like https://g-proxy.appspot.com/ or https://bingproxy.appspot.com/, and that is sufficient for rendezvous.

One way of doing it automatically with a custom App Engine app is to have the app figure out the client's IP address from the request, and insert it along with the client's given port number in a new registration to the facilitator. (This is pretty much what flashproxy-reg-http and facilitator.cgi do now, except it's like having facilitator.cgi run on a different host than the facilitator.) The downside of this approach is that the IP:port information becomes known to the app and to Google. (Though we can't hide the IP anyway, because it's part of the HTTP request to the app.)

A better way would be to have the app forward encrypted registration blobs, like Gmail does with the [[ticket:6383|email rendezvous]]. The problem here is that the client needs to know its own IP address. I propose having the App Engine app interpret requests for /ip as a request for the requestor's IP address. It should return the IP address as a text/plain document in a single line. The other path pattern understood by the app will be /reg/<blob>, which it will simply forward by making a new HTTP request for !https://fp-facilitator.org/.

Two parts to this project:

1. App Engine app handling /ip and /reg as above.
2. A client program flashproxy-reg-appspot. The client program makes a request for /ip to find out its IP, then generates a base64 blob from the IP and port, the same way flashproxy-reg-url does. It then makes a second request to /reg/<blob> to effect the registration. The App Engine app does nothing but a URL fetch of !https://fp-facilitator.org/reg/. The client program should have -4 and -6 options.