Apache Virtual Hosts and Load balancing

 from: http://devcentral.f5.com/wiki/iRules.http_request_send.ashx
when HTTP_REQUEST_SEND {

    Need to force the host header replacement and HTTP:: commands into the clientside context
    as the HTTP_REQUEST_SEND event is in the serverside context
   clientside {

       Replace the HTTP host header with the connected server IP and port
      HTTP::header replace Host "[IP::server_addr]:[TCP::server_port]"
   }
}