NAT Src Dest Same Subnet

Hi,

I have the following:

172.20.15.160 -> 192.168.10.29:9997 TCP via FortiGate GW 172.20.15.1.

GW has a static route to 192.168.10.0/24 via 172.20.15.150.

OpenVPN AS on 172.20.15.150 will only allow from 172.20.15.1 to 192.168.10.0/24.

Is it possible to do NAT on the FG so that the above will work? So 172.20.15.160 -> 192.168.10.29 wil be NATted to 172.20.15.1.

Even if I enable NAT in the policy it still remains as source 172.20.15.160.

Am I doing something stupid?

FortiOS 5.6.6 on a FortiGate 101E.

Thank you.