I have an http server running on the linux terminal VM in chromeos on port 5555. I have the port forwarding set up so that TCP on port 5555 can go to the linux terminal. When I visit localhost, 127.0.0.1, or the chromebook's private IP address from the chromebook, nothing happens and no request is sent. When I visit the chromebook's ip from another device, I see the request show up in the server logs, but no data is sent back.
It seems for accessing the local linux environment from the chromeos browser, you should use "penguin.local" (where penguin is the name of the linux container) rather than "localhost". So, http://penguin.local:5555 should get a connection to your listener in the linux environment, assuming it's listening on 5555. You can see what ports have listeners by running "netstat -na |grep tcp" . If you have a listener on 5555, it should show "0.0.0.0:5555 0.0.0.0:*" in the 4th and 5th columns.