Rewrite HTTP response when error code received

Here's another approach that doesn't require HTTP::uri. The rule may or may not work as-is since I pulled out environment-specific stuff and inserted your new pool and host. However, it should still provide a template.

when HTTP_RESPONSE {

    if { [HTTP::status] equals 403 || [HTTP::status] equals 404 } {

         Retrieve the not-found page if the HTTP status is 403, 404
         Note: Retrieving the page is a valid request, which means that the server will respond with a 200 OK.
               This necessitates that we collect the payload so we can later construct a custom response with
               the proper HTTP response code (404) but with the payload (HTML) from this request.

        if { ![info exists retry] } {

            set retry 1

            HTTP::header replace Host "new.domain.com"
            pool /TEST/POOL_FOR_NEW_DOMAIN
            HTTP::retry "GET /error/not-found/ HTTP/1.1\r\nHost:$requested_host\r\n\r\n"

        }

    }

    if { [info exists is404] } {

        if { [HTTP::status] equals 200 } {

             store the payload of the not-found page if the HTTP status is 200
             (this means that the error page was found on the file system)

            HTTP::collect [HTTP::header Content-Length]

        } else {

             No error page was found in the folder. Construct a generic error page
             and send it to the client.

            log local0.debug "is404 true, but there's no error page on the file system. ([HTTP::status])"

            HTTP::respond 404 content {

                
                    
                        
                    
                

            }

        }

    }

}

when HTTP_RESPONSE_DATA {

     Construct the response with the payload of the not-found page that we collected earlier and
     with the HTTP Response Code "404".

    HTTP::respond 404 content [HTTP::payload] "Content-Type" "text/html"

}