Edubuntu routing question -- from external to internal (fwd)

[Gavin McCullagh]

Hi,

On Tue, 20 Nov 2007, Tom Wolfe wrote:

> Thanks for the tips, but I already had followed the NAT FAQ to no avail. 
> /proc/sys/net/ipv4/ip_forward is therefore already set to "1"
> 
> So I'm stumped. I don't have any firewall rules at all right now. Any more 
> suggestions?

On reflection, you shouldn't need NAT for this and you shouldn't need
iptables.

> The Edubuntu/Gutsy server is 192.168.192.18 on eth1 as you might gather.
>
> pinging 192.168.100.254 (the Edubuntu server on eth0) from the
> 192.168.192.0 subnet gets no reply

That's a bit of a surprise.  Are you sure you have the route set up
correctly on the 192.168.192.0 box?

> tracert 192.168.100.254 from the 192.168.192.0 subnet gets:
> 
> Tracing route to 192.168.100.2 over a maximum of 30 hops
> 
>   1    <1 ms    <1 ms    <1 ms  EDUBUNTU [192.168.192.18]
>   2     *

This looks to be a different experiment.  Now you're tracerouting to
192.168.100.2 (a machine on the thin client net) and it's getting to the
edubuntu router, but not further.  Bear in mind that 192.168.100.2 would
have to have an explicit (or default) route back through 192.168.100.254
for the ping to get back to you.

Can you traceroute 192.168.100.254 from 192.168.192.0?

If you want to get a better picture for what's going on, I'd suggest using
tcpdump.  You can run 

	tcpdump -i eth0 icmp 

to see all ping packets passing a particular interface.  You can then see
them arrive at the router eth0, pass through eth1, arrive at the far end,
and see the reply all the way back (or however far it gets).

Gavin