Extend external app with a reverse proxy option to make internal sites available within owncloud

[e-alfred]

Affected apps

external

Expected behaviour

The external app should display an internal page like as if it is a reverse proxy. The internal page is not reachable from outside (maybe even runs on a different port internally), but only within Owncloud configured as an external site/reverse proxied site. This would allow to easily access an internal resource from within Owncloud.

Steps to reproduce

1. Add a page to an internal resource with the external app.
2. Access Owncloud from the internet and access this site like it was behind a reverse proxy.