How to forward the requests to another URL using iRule

hello,

The best way is to apply the irule for maintenance on the VS directly. for example :

when HTTP_REQUEST {
  if { [active_members $default_pool] < 1 } { 
    HTTP::respond 200 content [ifile get "/Common/maintenance.html"] noserver "Content-Type" "text/html" "Cache-Control" "no-cache, must-revalidate" 
  }
} 
when LB_FAILED { 
      HTTP::respond 200 content [ifile get "/Common/maintenance.html"] noserver "Content-Type" "text/html" "Cache-Control" "no-cache, must-revalidate" 
    }

Otherwise, you can use the virtual command (e.g. virtual /Common/my_VS_Name)