Duqu was essentially a remote-access Trojan, or RAT, which operated as a simple back door to give the attackers a persistent foothold on infected machines. Once the back door was installed, however, Duqu contacted a command-and-control server, from which the attackers could download additional modules to give their attack code more functionality, such as the keystroke logger/infostealer the Hungarians had found on one of their systems.
