How to forward the requests to another URL using iRule

If it's a maintenance page you would want to use HTTP response code 503 (Service Unavailable) rather than a 200 (OK), per the RFC standards for HTTP.

Using a 200 (OK) is risky. For instance, there's a chance that search engines will index the maintenance page. If any department is using external monitoring (e.g. Pingdom) those reports would incorrectly show that the site/pages were available.

You can simply change the response code from 200 to 503:

HTTP::respond 503 content [ifile get "/Common/maintenance.html"] noserver "Content-Type" "text/html" "Cache-Control" "no-cache, must-revalidate"