irule for 404 redirect

One of the problems is that 200s could be indexed by search engines - not a great thing to have happen.

Regarding 404s, I generally also check for 403s, since a request for a non-existing resource that ends in a "/" can be interpreted by the web server as a directory request and thus result in a 403. Technically they're not the same, of course, but to an end user they're the same.