Traffic going to unregistered domains

How hard it is for a ISP to update their unix (or otherwise) systems so their customers see a custom web page rather than just a "no page found" error web page when their customers visit an unregistered domain. Their customers would normally be connected either via dial up or a DSL or ADSL line or similar and would be accessing the internet via the ISP's systems. Any thoughts?

I really don't understand what you want to do. An unregistered domain is just that. It has no IP address and can't be directed anywhere. However, if the domain is registered but the web site is not yet up and running, a page saying, this is the future site of whatever.com is possible.

You could do this with a proxy server. Sorry, no other thoughts than that at the moment.

Thanks for the comments. More explanation of what I want to do:

As an ISP I want to be able to customise the page my customers see if they enter a domain that is unregistered in their browser address bar.

eg if they enter http://www.i-do-not-exist.com i would like to be able to control what error page they see.

If a custom error page can be displayed is it possible to get the domain name included in the custom error page.

eg. if they enter http://www.i-do-not-exist.com can I display a html page that says "Hi, you tried to access http://www.i-do-not-exist.com but I am afraid that domain just does not exist just now. Sorry."

Cheers

As far as I know, the browser controls that error message, not the ISP.

Yeah, use squid as a proxy for your clients and you can do that