[SLUG] Routing / forwarding prob

[Paul Robinson <p_d_robinson@xxxxxxxxxxxxxxxx>]

Okies, time for a bit of a newbie question from someone who had considered 
himself fairly ok at linux. I wouldn't ask except I've been bashing / 
researching this problem for a day or two now and I'm starting to go over old 
methods again which isn't a recipe for success.

Problem:

Until I can afford another hub I'm bridging an old coax network and a rj45 
network via my mandrake machine which has 2 network cards, 1 for coax and one 
for the rj45. (ideally I'd try to find one of those old hubs with a coax 
connector on it. Trouble is, windows can connect to the mandrake box ok, but 
it can't connect to anything past the mandrake box.

Synopsis:

Main gateway to the internet is a slackware box on the coax network.

Trying to connect my win2k box to the net via the mandrake box (mandrake 7.2)
(/proc/sys/net/ipv4/ip_forward is set to 1 btw)

I've set up the route table in the mandrake box as :

[root@starbug paul]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     0.0.0.0         255.255.255.255 UH    0      0        0 eth0
1.2.3.0         0.0.0.0         255.255.255.255 UH    0      0        0 eth1
1.2.3.0         0.0.0.0         255.255.255.224 U     0      0        0 eth1
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
0.0.0.0         192.168.0.1     0.0.0.0         UG    0      0        0 eth0
[root@starbug paul]# 

the slackware firewall box is the 192.168.0.1 default gw at the bottom

192.168.0.3 is the coax address of the mandrake box
1.2.3.1 is the rj45 address of the mandrake box
1.2.3.2 is the windows box

windows can talk to both 1.2.3.1 and 192.168.0.3 without any problems.
it can ping 192.168.0.1 and it is detected by the slackware box :

Jul 31 00:53:41 holly kernel: IP fw-in acc eth0 ICMP/8 1.2.3.2 192.168.0.1 
L=60 S=0x00 I=12039 F=0x0000 T=127 
Jul 31 00:53:43 holly kernel: IP fw-in acc eth0 ICMP/8 1.2.3.2 192.168.0.1 
L=60 S=0x00 I=12043 F=0x0000 T=127 

however windows gets Request timed out error messages.. ie the response 
message from 192.168.0.1 isn't getting back to windows.

This seems like a masq / forwarding problem. Does Mandrake 7.2 come with masq 
support pre-compiled into the kernel or do I have to recompile? I've tried 
using ipchains and iptables but it berks at me saying:
[root@starbug ipv4]# ipchains -P forward DENY
ipchains: Protocol not available

>From my scouring of man pages / how-to's and other web sites / LUG archives 
the common message seems to be that this is one of the hardest things to set 
up.. I'm beginning to agree with them :)

Any help (including "all you need to do is compile with masq support u dolt", 
which I have a sneaky suspicion is the cause) would be most appreciated.

Thanks in advance,
Paul