Hi friends,
i am confused with IP forwarding and masqerading and port forwarding... i have a setup as below.. in which of these categories this falls in and please gimme a solution in solving this problem...

SETUP:

PC A (IP addr is 192.168.178.182 )with windows XP-> connected to eth0 of a board running linux (LinPC =eth0 IP is 192.168.178.244) Gateway of PC A is eth0 IP

PC B (IP addr is 192.168.174.111 )with windows XP-> connected to eth1 LinPC
=eth1 IP is 192.168.174.119) Gateway of PC B is eth1 IP

and

CASE 1: echo 0 > /proc/sys/net/ipv4/ip_forward
Now ping from PC A to PC B NOT POSSIBLE but ping from PCA to eth0 and eth1 is both possible (should this ping both eth0 and eth1 IP ??? i doubt )

CASE 2: echo 1 > /proc/sys/net/ipv4/ip_forward
Now ping from PC A to PC B is POSSIBLE also ping from PCA to eth0 and eth1 is both possible.

Now the problem is when i connect eth1 to LAN instead of PC B with echo 1 > /proc/sys/net/ipv4/ip_forward
i am not able to ping from PC A to any IP in the network to which the eth1 is connected.
Please help me solving this problem...or gimme some alternate solution to solve this problem...
thanks...

I assume that the netmask in both subnets is 255.255.255.0.

Check the modules that are running in the Linux machine (with eth0 & eth1). I once used my Laptop to route internet traffic (for my Desktop) via the laptop's wireless interface. I found that I needed to load a kernel module to do it. ( At the time it was either called ip_conntrack or tcp_conntrack ). I think you need the nf_conntrack kernel module loaded. My router was a NAT router so I didn't need to configure my firewall for NAT translation.

Look at the network configurations on the XP machines. What default gateway address do you have for them? If they don't have their own internet connections on another interface, use the Linux box's ip address on that subnet. If an XP does have an internet connection then add a route for the other subnet as well.

Also, you should look at the route configuration on the Linux host. Do you have a route for each subnet? You probably do, but you will want to check anyway.