LinuxQuestions.org - SNAT and QUEUE target: iptables - Encapsulation, decapsulation V4 in V6

Hi,
I have the following problem. I am implementing an IPv4 in IPv6 encapsulation solution. For that, I'm using libipq (I know that IPv4 in IPv6 tunnel exists in Linux but I have to do it by myself for specific purposes). So I have a router and a client in my architecture. The client uses private addressing and the routeur has its ingress interface in private addressing and its egress interface in public addressing.

Code:

                  eth0                  eth1

[Client]          [      Router        ]

Private @V4        Ingress Private @V4  Egress: Public @V4 

192.168.1.2        192.168.1.1          192.109.118.27

                                        @V6 x:y:z::1

The goal is to encapsulate Client traffic IPv4 traffic in the @V6 of the router. I need also to do SNAT from the client private address to router public address. My primary idea was to use iptables with snat target.
So for example:

Code:

iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to-source 192.109.118.27

This rule does not work. After browsing on iptables doc, I figured out that it's normal because traffic is forwarded (I activate forwarding in proc/sys/net) from eth0 to eth1 so it never hits POSTROUTING target.

So I use this rules:

Code:

iptables -t nat -A POSTROUTING -p tcp -o eth1 -j SNAT --to-source 192.109.118.27

So OK, my IPv4 traffic is correctly SNATed but the problem is that I still have to do my encapsulation. For that, normally, I add a rule which said, put in the queue traffic which comes from 192.168.1.2 and then I handle with libipq the encapsulation. But POSTROUTING is the last target hit before leaving the interface so if the traffic is SNATed, I can't put SNATed traffic in QUEUE target. And conversely, If I put the rule which said to put the IPv4 traffic in QUEUE, the traffic will be encaspulated but never SNATed because it becomes IPv6 traffic...
The solution will be to SNAT in PREROUTING target but after googling, it seems to be impossible. Does anyone has a clue for this problem?

Cheers,
Tanguy