What is the value set for the X-Forwarded-For header using the Google App Engine URL?
The documentation says ( http://code.google.com/appengine/docs/java/urlfetch/overview.html#Request_Headers ):
These headers are tuned to the exact App Engine values as appropriate
Does X-Forwarded-For know the value of some app id gae?
a source to share
Currently X-Forwarded-For does not work on requests made by URLFetch.
You can test this yourself by going to http://shell.appspot.com/ and making a URLFetch for a site that echoes HTTP requests - for example, http://www.showhttprequest.com/ . App Engine sets the User Agent string to "AppEngine-Google" (+ http://code.google.com/appengine ) but does not set X-Forwarded- Everyone.
a source to share
If you go to shell.appspot.com and run the following code:
from google.appengine.api import urlfetch
url = 'http://www.showhttprequest.com/'
result = urlfetch.fetch(url)
print result.content
You will see that (around the end of 2009) AppEngine now adds the app to the user-agent header. So your user agent looks like this:
AppEngine-Google; (+http://code.google.com/appengine; appid: shell)
a source to share