We have a website which we recently migrated to ASP.NET MVC. All of the URLs are now different from the original website. Google still has all of our old URLs, so if anyone finds us in a search, currently they will get a 404.
I have a catchall route that catches bad URLs, including all of the old ones. In a perfect world I would like to do a 301 redirect to the home page for all urls matching this catchall route, and I do have code for this that works properly on my development machine. However, I finally got someone at our ISP (Network Solutions) to tell me that they block 301 redirections (the web server returns a 404 instead).
So I think my only remaining option is to just accept any bad URL, and point it to the home page.
Here is my question: I know that the search engines (especially Google) are now penalizing duplicate content. If I just point all bad URLs to the home page, how much is this going to hurt us in the search rankings? Do I have any other technical options?