So, your server creates the page that contains a JAVA applet (you might find it from the plenty of JAVA stuff in the web). And then the user loads that html page and the applet starts working connected with your server (just sending pings or some other thing, don't know).

From the applet's JAVA side I could inform you just what you need to know: that it works with some parameters that your server writes in the html page. The parameters that the applet needs (as any perl script). Then, with this parameters it starts working when the user loads the html page. JAVA works inside a 'sandbox' that doesn't compromise any other part of the users machine. So it is safe and should provide you with what you need at the server side.

Hope that it helps!