Frox is a server that can intercept and forward FTP connections made by clients that are routed through your server. This allows you to cache commonly downloaded FTP content, enforce access control over which clients are allow to use FTP, and limit the amount of bandwidth that can be consumed. This of it as the equivalent of Squid, but for the FTP protocol instead of HTTP.

It is not particularly useful unless your system is gateway between an internal network and the internet - if you are just looking for a way to cache FTP content on a single system, it is better to use Squid instead.

For Frox to work properly, a firewall rule must be created to redirect all routed traffic on port 21 to the port Frox listens on. If you are running Linux and using the Linux Firewall module to manage your firewall, this module can setup this rule for you easily.