forwarding? routing? dvbd filters? or what?

[Sasha Sotnicov <sandworm77@yahoo.com>]

I have two hosts , both hosts connected with Ethernet.

One host is Linux 2.0.36 with ppp link to common
provider with ip I recive from my sat provider.

second is Linux 2.2.16 with dvb modem.
default gateway on second host is point to first host
Ethernet ip.
/proc/sys/net/ipv4/ip_forwarding set 1
/proc/sys/net/ipv4/conf/*/forward set 1
/proc/sys/net/ipv4/conf/*/rp_filter set 0

on second host tcpdump shows that from dvb0 is comming
packets and his destination is ppp link ip on first
host.

hosts CAN ping each other.

BUT when I ping something in Internet from first host,
I see outgoing ICMP_REQ on ppp link , see incoming
ICMP_REPLY on dbv0 
but WHY I don't see ICMP_REPLY routed to first host
from second by Ethernet?

The interesting part of that is if replace second host
with Win NT all work fine (exept that I want to see
Linux instead of Win NT :)
So the problem is somewhere in Linux tuning but where?
I run out of ideas week ago , and don't know what to
do next.
 Any help whould be great.
Thanks.

PS: What difference of using filter_0 , filter_1, ...? 

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