How to configure proxy arp?

From: Erkki Huhta (no.spam_at_dlc.fi)
Date: 07/09/03 

Date: 9 Jul 2003 00:17:47 -0700

My current network looks like this:

 internet
    |
adsl router
    |
  (eth0)
linux-firewall/nat
  (eth1)
    |
  switch
    |
----------
| |
win98 WinXP

All windows machines are connected to internet via
linux firewall/nat (linux machines use dhcp to get private
ip addresses from linux). So from the internet it looks
like there is only one machine (192.168.0.1). This
works fine.

I would like to connect my laptop "directly" into the internet.
So it would get it's public ip address from my ISP's dhcp server
and same time it could use the services which are available only
for machines which are located in the internal network
(currently eth1 interface). The picture would look like this:

 internet
    |
adsl router
    |
  (eth0)
    |
  (eth0)
linux-firewall/nat (Redhat 9)
  (eth1) (eth2)
    | |
  switch laptop
    |
----------
| |
win98 WinXP

I thinks I have to configure proxy arp, but how is it done?
I don't want to just go and try different things, because I'm
affraid it affects the current configuration and my NAT stops
working :I.

This is what I would do, is it correct:

1. Install third NIC (eth2)
The problem is how do I configure it? eth0 gets it's configuration
from my ISP via dhcp and eth1 is configured:
IP address: 192.168.0.1
broadcast: 192.168.0.255
Mask: 255.255.255.0

Could I use e.g. following configuration:
IP address: 192.168.1.0
broadcast: 192.168.1.255
Mask: 255.255.255.0

2. Turn on proxy arp:

echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
echo 1 > /proc/sys/net/ipv4/conf/eth2/proxy_arp

Do I need it for both interfaces?

3. Configure linux to accept packets for the laptop

?

4. Create route from internet (eth0) to laptop (eth2)
and vice versa

?

5. Configure laptop?
Does the laptop (running win98) need some configuration,
routes, gateways?

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