The ever-growing need for faster, more flexible cloud applications demands for increasingly complex networking architectures. When a virtual infrastructure is set up, virtual networking overlay must be imposed over the physical network. This is expecially true for those infrastructures where several components are continuously deployed: the ability to connect them without changing the physical configuration allows unprecedented scalability and flexibility. This is the realm of network function virtualization, which is used to build arbitrary networking configurations on top of a physical network. Of course such configurations must be complete and working, and therefore need to provide virtualized versions of the most common networking devices, such as routers, firewalls and nats.