More and more vendors of various computing devices, especially network-related appliances such as Routers, NAT-Gateways and 802.11 Access Points are using Linux and other GPL licensed free software in their products. While the Linux community can look at this as a big success, there is a back side of that coin: A large number of those vendors have no idea about the GPL license terms, and as a result do not fulfill their obligations under the GPL. The gpl-violations.org project has started legal proceedngs against a number of companies in violation of the GPL since December 2003. Those legal proceedings were quite successful so far, resulting in some 30+ amicable agreements and three granted preliminary injunction. The list of companies includes large corporations such as Siemens, Asus or Belkin. The speaker will present an overview about his recent successful enforcement of the GNU GPL within German jurisdiction. He will go on speaking about what exactly is neccessarry to fully comply with the GPL, including his legal position on corner cases such as cryptographic signing. Resulting from his experience in dealing with the german legal system, he will give some hints to software authors about what they can do in order to make eventual later license enforcement easier. In the end, it seems like the idea of the founding fathers of the GNU GPL works: Guaranteeing Copyleft by using Copyright.