Kernel IPsec implementation

Assuming there's a VPN server to which clients connect which utilizes some internal network 'through' a set of ESP tunnels and also acts as NAT gateway to the outside world for clients, the ESP/ ESP-in-UDP traffic, the incoming traffic on the internal network and the incoming and outgoing traffic from the external address of the gateway can be captured but not the reply traffic on the internal network (logically) originating from the internal address of the gateway and destined for the internal address of some client as that's transformed according to the rules in the SPD before anything outside gets a chance to look at it (AIUI).