Port forwarding with iptables on tunnel interface

[Guido Trentalancia <guido@trentalancia.com>]

Hello !

I have the following problem. I need to forward/redirect all connections
to port 25 (smtp) from one host (192.168.3.64) to another host on the
same network (192.168.3.69).

I am using the following iptables rules:

-A PREROUTING -p tcp --dport 25 -j DNAT --to-destination 192.168.3.69
-A POSTROUTING -p tcp -d 192.168.1.65 --dport 25 -j MASQUERADE

and I have also tried the following alternative rules:

-A PREROUTING -p tcp --dport 25 -j DNAT --to-destination 192.168.3.69
-A POSTROUTING -p tcp -d 192.168.1.65 --dport 25 -j SNAT --to-source
192.168.3.64

The connections come from an IP-IP tunnel and they are allowed by the
following iptables rules:

-A INPUT -p 4 -i eth0 -j ACCEPT
-A OUTPUT -p 4 -o eth0 -j ACCEPT

It used to work until a few days ago. But now it only works from within
the LAN and not from the tunnel. I have not changed any iptables rules
and I have only upgraded the system to the latest kernel (2.6.32.8).

The version of iptables that I am using is 1.4.3.1.

Any idea on how to get it back working ?

Regards,

Guido