[LARTC] Port Forwarding on two external NATed interfaces? (dual route, fwmark)

[Hanno Mueller <sockpuppet@xxxxxxxx>]

Hi there,


I have a little router here that does this:


                                   +---------- ppp0 (no fwmark)
                                   |
internal eth0 ------------o Box o--+           external
                                   |
                                   +---------- eth2 (fwmark 2)



ppp0 is a fast ADSL line at provider A,
eth2 is a slow SDSL line at provider B

I don't use load balancing, the two providers are used for different
purposes depending on the services used by the LAN users.

I have a little fwmark Masq/SNAT configuration with a dual default route
that masquerades the internal LAN onto both external providers. The
fwmark is set depending on the destination port or destination address,
e.g. we send mail over eth2, while web browsing is done over ppp0. This
is transparent for the users and has been working fine for quite some
time now.

The box itself also replies correctly to external requests. It runs a
web server and we can reach it from outside on both ppp0's and eth2's IP
address.

My problem is that I now want to use port forwarding for packets coming
from both of the external networks.

Port forwarding for ppp0 works fine: requests coming in on from ppp0 are
forwarded correctly to the internal IP, the responding packets return
through ppp0.

However, packets coming in from eth2 are not answered on eth2, but on
ppp0, as well. While the incoming packets are forwarded correctly to the
internal IP, the responding packets use the wrong route.

I have tried to set fwmark 2 on all the "mangle" tables for packets
arriving on eth2, yet it still responds on ppp0.

Is there anyone who did something similar? Somehow I think that this
can't really be that obscure to do...


Thanks,

Hanno