Linux Routing/IP Masquerading w/ Cable Modem & ISDN

1. Problems with routing and ip masquerading with cable modem in linux

Hi, I have some problems with getting masquerading to work, properly. Funny
thing is it works perfectly in windows98 :( which really ticks me off...

Sorry for crossposting, but I am getting to be desperate by now....

Here is the problem:
Not always, but most of the time when my internal computers are in linux they
start working fine, then external net dissapears, when I look at route -Cn I
see that many eoutes going outside have the cablemodem set as gateway, even
though my defaultroute is the laptop.... removing and reinstating the default
route clears the cache, but same thing happens soon. arp -a shows me the mac
of the laptop as well as the cable modem. Traceroute does weird things, like
trying to go straight to the modem, and then comes back to my host with !H,
no default route anywhere in there as far as traceroute goes....
Often some sites work some do not, and it degrades over time. Reboot of
everything does not always help, but usually few pings, traceroutes just
after restart seem to work.

Here is my setup:

A cable modem - motorola's surfboard - that's my net connection

Then all plugged into 1 hub (8 port)
  (so all traffic is intermingled)
1 linux laptop, the gateway with ONLY 1 NIC, with 2 ips.
  (eth0 - internet, eth0:! local net)
2 Linux/windows boxes.
On these boxes and internal on the laptop I got static IPs from 192.168
subnet (same c segment) and my laptop internal IP is the gateway for these 2.

Linux is RedHat 7.1 with kernel 2.4.5

On the laptop I got ip forward enabled (/proc/sys/net/ipv4/ip_forward set to
ipchains are set to ACCEPT and
ipchains -A forward -s -j MASQ
  (I'tried with /24 and with a specific IP as well)
(I also tried the DENY policy on forward, based on one of the faqs or howto's
I read, no difference)

Most of the time my problems do not seem to happen when my local boxes are in
win98, and I know linux is better than windows in networking, so I am totally

Could this be in any way caused by the isp? It all works fine if I got 2 NICs
and modem goes to one of them and not the hub, but.. I cannot do that for
good, not enough pcmcia slots.

I realize it may be something simple, but I am not seing it, even though I
looked around quite a bit.... so any help will be appreciated.

 Greg Tokarski                   Grzegorz Tokarski
 Denver, CO, U.S.A.                 Poznan, Polska


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