Linux, Unix, and *nix like Operating Systems Forum

To make any half-decent security solution you would need to augment the referrer-based system somehow and to be honest I think you'll find that the amount of work involved in this is prohibitive - essentially you'd be re-inventing the wheel (where the wheel in this case is user-authentication).

However if they are going from a site you control to another site you control and then you could probably have some fun with dynamic pages and a database - ie you generate a "launch" page which uses a unique URL, this URL is entered into the database and stays valid for x minutes.

When the user goes to the other site they will pass that unique URL in their referrer, the other site spots this and authenticates them for access to the site and at the same time removes the "launch" URL as a valid authenticator.

Since the unique URL would only be shown to valid in users (I presume they would have logged in at this point) and would only work once you have something which is pretty hard to spoof and pointless in linking :)