I want to provide a service, intended to make me money, based upon free software.
It consists of a free plugin (client) and a web-service.
The client is published as Open Source, and it is deployed as a plugin for some popular free CMS platforms (e.g. Drupal, Joomla, Wordpress) that requires its plugins to carry a FLOSS license.
All the plugin (client) really does is to provide a GUI to a web-service. The web service is not available unless the user purchase an API key. Without having this API key, the free plugin (client) is useless.
The plugin (client) is written by me, while the web-service is based upon publicly available Free Software written by somebody else, but adapted by me to become a web-service that communicates with my free plugin through an API also designed by me. I do not want to share the adapted source code, and I believe I do not have to, since the web-server part is not shared with anyone (it just runs on my web-server).
Will this always be OK, or are there Free Software licenses (recognized as such by the FSF and OSI) that have restrictions in place that will not allow me to do this this?