problem solved by using this "up" script :
#!/bin/sh
echo "$dev : $ifconfig_local -> $ifconfig_remote gw: $route_vpn_gateway"
ip route add default via $route_vpn_gateway dev $dev table 20
ip rule add from $ifconfig_local table 20
ip rule add to $route_vpn_gateway table 20
ip route flush cache
exit 0
and this client conf :
client
dev tun0
proto udp
remote fr.*.com 443
resolv-retry infinite
nobind
persist-key
persist-tun
ca CACertificate.crt
cert UserCertificate.crt
key PrivateKey.key
verb 5
route-method exe
route-delay 2
tun-mtu 1500
tun-mtu-extra 32
mssfix 1450
cipher AES-256-CBC
auth SHA256
keysize 256
comp-lzo
auth-user-pass login.key
script-security 2
route-noexec
route-up vpn_up.sh
now if i set the interface to tun0 in curl it use openvpn and if not the classic network interface