Add VXLAN backend

[stevefan1999-personal]

According to bridge https://man7.org/linux/man-pages/man8/bridge.8.html

We can try to do

ip link add <ifaceName> type vxlan \
  id <randomID> \
  dstport <bindPort> \
  local <listenAddr>
bridge fdb append 00:00:00:00:00:00 dev vxlan100 <peer1addr> port <peer1port>
bridge fdb append 00:00:00:00:00:00 dev vxlan100 dst <peer1addr> port <peer2port>

In fact I believe this is what Calico and Flannel does behind the scene

[neocturne]

Hmm, I assume you mean to use this for connections to multiple peers using a single VXLAN interface? Would MAC address learning be handled in the kernel? It is not clear to me from the docs that 00:00:00:00:00:00 can be passed like that.

In addition, there are some design decisions of fastd that are incompatible with VXLANs:

• Connections to multiple peers will work rather poorly behind very broken NAT implementations (fastd deliberately uses a separate UDP socket for each peer, as some NAT routers will assume UDP is 1:1 rather than 1:N; unless configured to a static port, it will also choose a new random port whenever the connection is lost, also as a workaround for some NAT routers we've seen in the wild)
• fastd runs periodic handshakes, payload data and keepalive packets on the same UDP socket (using the control message flag in the case of L2TP). This allows fastd to detect when a connection-tracking router/firewall has dropped the flow.