using iptables to route thru specific interface?

From: Tobias Skytte (tobias_at_itservices.co.mz)
Date: 12/22/03 

Date: 22 Dec 2003 07:33:43 -0800

Hi,

I have recently discovered that my ISP, who provides me with a vpn
tunnel for reception, is filtering out smtp, dns, telnet, ping and
other packets. Thus I can not telnet into my machine from the outside
even though I also have a DSL line to that server (that I use for
sending out thru).
This is set up on RH 7.1, using PPTP for the tunnel.
So what happens in a ping for example is this:
I ping from outside and it enters on eth0 which is the DSL connection.
The server sends a pong-reply but the reply is sent to the default
route which points to the VPN server thru ppp0.
The reply gets dropped in the VPN server.

So what I need is something that makes the reply packet bypass the
normal default route and go straight to the sender.
Someone told me I should be able to do this with iptables, but as I am
new to it I am not sure how.
Should I use DNAT? or mark the packages or what? I have read up on it
but am still confused.
Can this be done with ipchains?
I am sure someone has done the same before, maybe someone knows of a
good description on exactly what to do?

Best regards,
Tobias Skytte

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