Similar to the virtual ethernet driver veth, vxcan implements a
local CAN traffic tunnel between two virtual CAN network devices.
When creating a vxcan, two vxcan devices are created as pair
When one end receives the packet it appears on its pair and vice
versa. The vxcan can be used for cross namespace communication.
Now we don't support tunnels to be created without a .network file
that is we need a interface index.
This work allows tunnel to be created without a ifindex.
Closes#6695
netdev to bond.
There are situations where a link can be in an "UP" state when
systemd-networkd attempts to add the link to a bond device.
This is a problem because the bonding driver will refuse to
enslave a link if it is in the "UP" state.
This check ensures systemd-networkd sets the link to "DOWN"
before attempting to add the link to the bond.
Fixes#5838.