Google App Engine

Connecting to an app engine development server from remote clients

OK, I'm writing this down just in case I run into again the future, because I just spent HOURS tearing my hair out before finding the solution.  Maybe it will help someone else someday, too. I recently messed around with a whole bunch of configuration-related stuff for my Google App Engine app, requiring me to reestablish my my Eclipse run configuration parameters for executing my app in the development server.  In the past, I had set up port forwarding rules in my router, so that I could hit my app running in my dev server from remote clients like browsers running on other machines, from my client Android app, etc., and it had been working fine.

As is so often the case, after making configuration changes, there is a hill to be re-climbed in order to get everything working again.  In my case, although I could access my app in the dev server from clients running on the same machine, my remote clients could not connect.  I checked my port forwarding rules in my router -- they were still there.  I tested them using PFPortCheck tool, and it verified the ports were forwarding correctly.

I turned off my firewall; nope, that's not the problem.

I rebooted.  Nope.

I power cycled my router and my cable modem (obviously getting desperate here, since I'd already verified that port forwarding wasn't the issue, but I've also found that sometimes challenging your assumptions helps).  But nope.

I tried different ports.  I checked to make sure there weren't other apps listening on the same port and stealing the requests.  Nope, nope.

I set up Fiddler in reverse-proxy mode, and get this -- it started working.  That is, with my dev server listening to port 8080 (the forwarded port) and Fiddler listening on 8888 (also forwarded), my outside requests to 8888 were being picked up by Fiddler, rerouted to, and successfully received by my app.  Damn, a Heisenbug. OK, interesting, but still frustrating -- why did that work, and what does it tell me about the problem?

After much Googling and cursing, I eventually stumbled across the culprit.  I needed to provide an "--address=..." argument in the run configuration's command line, to tell the server to listen for connections to addresses other than  After setting this argument to the value of my externally-visible IP address (see WhatsMyIP), I was back in business!


[follow-up: if you specify the --address argument as, you can connect from remote clients using the externally-visible IP address, and still connect locally using]

Capturing incoming requests to your Google App Engine development server, using Fiddler

I'm working on an app the leverages Google App Engine, and I recently was stumped trying to figure out how to use Fiddler to capture incoming HTTP request to my server when running in development mode, when those requests were coming from another machine.  I'm jotting this down to save others from the hair-pulling I went through.

  1. Change the run/debug configurations in Google Plugin for Eclipse so that the development server uses some port other than 8888.  In my case, I changed it to 8080.  The problem with 8888 is that Fiddler wants to use that port, too.  I tried changing the port that Fiddler listens on, in its options dialog, but it didn't work for me.
  2. Set up port forwarding on your router, so that the port your development server is listening to is being forwarded to your development machine.
  3. Follow the instructions at to set up Fiddler as a remote proxy, but be sure to use option #2 (setting up a custom rule).  The registry approach doesn't seem to work for requests coming in from another machine.
  4. Ensure the external machine making requests to your development server is using the externally-visible IP address for your machine (see and port 8888.  If it uses port 8080, the request will still go to your server, but Fiddler won't capture it.

Note: I have found that there can be some flakiness in getting this setup to take effect.  I haven't figured out exactly what the trick is, whether it's restarting Fiddler multiple times, or simply waiting for some cached value somewhere to time out, ...  If somebody solves that part of the puzzle, please let me know.