Fortigate: NAT + ipsec tunnel mode

I had an interesting case regarding a Fortinet firewall, the scenario goes like this

We have a client with a Fortigate Firewall who needs to establish a VPN tunnel to another network,

A simple task, but the second site has another client with the same subnet as ours so we have to use NAT.

Problem was – the peer is not a Foti which means interface mode will not work smoothly, so this makes the NAT configuration different than usual..

The final config looks like this

phase1 "peer-ph1" :
        set interface "WAN"
        set nattraversal enable
        set dhgrp 2
        set proposal aes256-sha1
        set keylife 86400
        set remote-gw Peer
        set psksecret ENC *********
    next
end
phase2 "peer-ph2":
        set keepalive enable
        set phase1name "peer-ph1"
        set proposal aes256-sha1
        set replay enable
        set use-natip disable
        set dst-subnet 192.168.1.0 255.255.255.0 /* Other side's subnet
        set keylifeseconds 3600
    next
end
 firewall policy:
        set srcintf "LAN"
        set dstintf "WAN"
            set srcaddr "LAN-Pool"  /* subnet is192.168.1.0/24
            set dstaddr "Peer-LAN"
        set action ipsec
        set schedule "always"
            set service "ANY"
        set natip 10.1.81.0 255.255.255.0 . 

        set inbound enable
        set outbound enable
        set natinbound enable
        set natoutbound enable
        set vpntunnel "peer-ph1"
    next
end

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