systemd: network: Add explicit routes towards tundev for UE pool of a…

…ddresses

When running the open5gs package with systemd network config, the 1st IP address
of the UE pool configured in open5gs-upfd config file for ogstun is
being assigned to the interface through this file.
That was discussed as being a desirable default setup.

However, in the event a user wants a setup where no IP address is
assigned to the tundev, then it's not enough removing the IP address,
because then the implicit routing rules regarding the subnet of the IP
address added automatically by the kernel are also removed.

This patch adds config sections to set up the routing explicitly, with
the aim to get the routing still applied if the user decides to comment
out the IP address, so that packets are still forwarded properly in that
case.

Related: https://osmocom.org/issues/6361

configs/systemd/99-open5gs.network

@@ -5,6 +5,14 @@ Name=ogstun
 Address=10.45.0.1/16
 Address=2001:db8:cafe::1/48
 
+[Route]
+Gateway=0.0.0.0
+Destination=10.45.0.0/16
+
+[Route]
+Gateway=::
+Destination=2001:db8:cafe::0/48
+
 [Link]
 MTUBytes=1400
 RequiredForOnline=false