Drupal multi-site configuration has been very convenient in the past, and has even gotten better in drupal7. However, it requires that the web server from which drupal is served, is called with distinguishable domain names. Most of the time, this is obvious. However, what happens when the Webserver is located behind a firewall, or because of various reasons is accessed via a reverse proxy?
Nginx provides a very flexible way to be used as a reverse proxy. It also can be used to solve the stated problem.