systemd: network: Avoid assigning IP addr from UE IP pool to ogstun i…

…nterface

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 was
being assigned to the interface. This causes routing problems when
traffic from the UE which was assigned that same IP address is routed
to/from the ogstun interface.

As a result, every time that open5gs-upfd systemd service was restarted,
the following changed had to be applied:
ip addr del 10.45.0.1/16 dev ogstun
ip route add 10.45.0.0/16 dev ogstun

The IP address was set in order to get the routing entry for free.
Instead, it should only add the routing entry.

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

configs/systemd/99-open5gs.network

@@ -1,9 +1,13 @@
 [Match]
 Name=ogstun
 
-[Network]
-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