This paper introduces the NetFed network federation approach, which enables inter-cloud operations on the basis of shared overlay networks. This allows to match the particular application needs to the most suitable infrastructure provider. The agent-based federation approach utilizes WireGuard and GRE to deliver a flexible and transparent layer 2 or layer 3 overlay network, while maintaining data integrity and confidentiality. The evaluation shows that our prototype can be deployed to arbitrary cloud platforms and benefits from a low traffic and processing overhead.