If, when logging into minecraft, you see the following error:
error '.io.IOException:server returned HTTP response code:503 for URL:http://www.minecraft.net//game/joinserver.jsp? '
It means that minecraft.net auth servers are down. This is an issue on minecraft's end and has nothing to do with your server. You can try to connect in a little while, however, until minecraft.net is back up, nothing will work.