Tunneling protocols are increasingly important in modern networking setups. By tying distant networks together, they enable the creation of virtual private networks, access to otherwise-firewalled ports, and more. Tunneling can happen at multiple levels in the networking stack; SSH tunnels are implemented over TCP, while protocols like GRE and IPIP work directly at the IP level. Increasingly, though, there is interest in implementing tunneling inside the UDP protocol. The "foo over UDP" (FOU) patch set from Tom Herbert, which has been pulled into the net-next tree for 3.18, implements UDP-level tunneling in a generic manner. via @lea_linux