Eliminate redirects

All you'd need to do is re-write the hostname when the client is making a request to the server. The server would then return a Location that includes that modified hostname, and subsequent requests should be sent to the address in the Location header.

So all you should really need to do for this particular scenario is:

when HTTP_REQUEST { 
  if { not ( [HTTP::host] starts_with "www." ) } {
    HTTP::header replace Host "www.[HTTP::host]"
  }
}

Since you want the client to see the updated URL, you shouldn't have to do any further work. The Location header sent back from the server should include the www, and the client should update appropriately. If you wanted to hide the re-write from the client, that's where more work on the response side of things would come in.

Colin