Let say, make it simple, my domain name is abc.com, when people click a hyperlink in the abc.com, it will go to abc.com/123.html. Can I restrict the visitor, ie they can only visit the abc.com/123.html through clicking the hyperlink at abc.com ? I mean, the visitor cannot go directly into abc.com/123.html, just by keying in the abc.com/123.html at their search box (toolbar).

Let me tell you my intention behind. My main page is abc.com, which is an opt-in page to capture visitors' database. There is a link at the abc.com to abc.com/123.html. Then at the abc.com/123.html, there are good stuffs for people to download free. In order to prevent people just copy the link abc.com/123.html and paste it into their emails (send to thier friends), then I have to do something on the protection, ie bar people directly access into abc.com/123.html. if visitors want to access into the abc.com/123.html, they have to key in their database at abc.com.

What is the script (should be pasted at the abc.com/123.html) ? I need it.