The OpenBSD host running Pen has IP addresses 192.168.100.12 on em1 and 192.168.101.11 on em2. The client debian2 has IP address 192.168.100.2. and the server debian3 has IP address 192.168.101.3.
OpenBSD takes first price in the easy management department by not requiring any special firewall rules or policy routing whatsoever. Just start Pen exactly the same way as on Linux and FreeBSD:
sudo ./pen -df -O transparent 192.168.100.12:5001 192.168.101.3
The client sees a connection from 192.168.100.2 to 192.168.100.12. The server sees a connection from 192.168.100.2 to 192.168.101.3.