> If you want just stub network based on iface prefix, then just define
> stub interface:
>
> area {
>   interface "ethX" { stub; };
> };

But defining stub on the interface makes it become a passive interface. I still want to exchange OSPF hellos. Another option is with export filter rule, but then it becomes an E1 or E2, and I want to be an OSPF derived prefix. Or can an export filter do other than E1/E2?

The manual says that stub network are generated implicitly by the following statement

"By default, BIRD generates a stub network record for each primary network address on each OSPF interface that does not have any OSPF neighbors, and also for each non-primary network address on each OSPF interface."

Is this true for unnumbered interfaces?