Need a little help: VPN into Network without default gateway - NAT?

[noppingen]

Hi there.

I'd need a little help or a pointer to the right direction.

Setup:

Opnsense here
LAN 192.168.40.0/22
VLAN33 192.168.110.0/24
Opensense in VLAN33: 192.168.110.100

VLAN33 is set up for our companys production air conditioning systems: A large amount of AC compressors, valve controls, Siemens HMI/control panels etc.

The majority of these AC network devices does not have a default gateway set and we currently do not want ot set one. One Windows server in that network has the GW set to 192.168.110.100, however.

A 3rd party company managing these AC devices used to VPN into that VLAN 33 usting a Lancom/NCP client and an old Lancom 1711 router at our site.

That Lancom router and it's DSL line are gone, I set up an IPsec Tunnel that works perfectly using their Lancom/NCP client. They can reach VLAN 33 and RDP to the Windows Server.

The problem is:

They can reach the Windows server which has a default gateway set, but none of the gateway-less devices without a gateway, of course.

This is where I could need someone pointing me to the right direction.

Can I NAT the VPN client (coming from virtual IP 192.168.169.0/24) as if it came from OPNsense (192.168.110.100)?
Some kind of Proxy ARP?

I have no clue how the Lancom VPN router did this, I guess Proxy ARP.

Thanks in advance!

[bartjsmit]

yes, NAT 

Either that, or put their tools on the Windows server and control the devices through the RDP sessions.

Bart...