IPSec BINAT (NAT before IPSec)

Assume company A has local LAN 10.0.1.0/24 and company B has local LAN 10.0.2.0/24. Also we assume that on both sides the other networks are already in use, e.g. in company A the network 10.0.2.0/24 is used for Voice and in company B network 10.0.1.0/24 is used for Guest Wi-Fi.

We have to define new networks for the Phase 2 with unused ones and create NAT entries to reach the final systems.

To make it easier we create a Phase2 with company A using 192.168.1.0/24 as Local Network and 192.168.2.0/24 as Remote Network and with company B using 192.168.2.0/24 as Local network and 192.168.1.0/24 as Remote Network. Now we need to add on each side the local LAN in the field “Manual SPD entries”. So for company A we set 10.0.1.0/24 in the field and for B 10.0.2.0/24. This allows the NAT process to speak with the Security Policy Database.

Finally we have to create NAT entries since a client in LAN A (10.0.1.10) tries to reach 192.168.2.10, but this address has to be rewritten to 10.0.2.10 on Firewall B.

Note

When using multiple phase 2 entries per tunnel, NAT before IPsec is not supported due to the fact that our SP database doesn’t know which entry to send the traffic to. For more context, please refer to setkey and this ticket on GitHub

Create the rule like in the screenshot and vice versa on Firewall A:

../../_images/opnsense_nat_binat_ipsec.png