StrongSWAN VPN tunnel: working, but peers cannot reach remote network

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Hi

I have setup a StrongSWAN VPN IPsec tunnel between two hosts:
[LAN_A] => [HOST_A][PUBLIC_IP_A] <=> [PUBLIC_IP_B][HOST_B][LAN_B]

LAN_A: 192.168.1.0/24
LAN_B: 192.168.6.0/24

HOST_A route:
192.168.6.0/24 gw PUBLIC_IP_A

It's working: every hosts on LAN_A can reach LAN_B hosts and vice-versa.

But, on the hosts running StrongSWAN, I cannot reach remote LAN EXCEPT if I
specify the source address of LAN.
host_A# ping 192.168.6.1
(no answer)
host_A# ping -S 192.168.1.254 192.168.6.1
(works)

That's seems logic to me, because by default packet sent to remote LAN are
using the route LAN_B gateway IP_PUBLIC_A, so kernel is using IP_PUBLIC_A
as source (checks by tcpdump).

What I need to setup to be able to reach the remote LAN from each peer
without specifying source IP address ?

Thanks for you help

--
Clement
SimpleRezo