lan debugging question

[Karl Cunningham]

On Saturday 05 January 2002 10:44 am, you wrote:
> the problem is annoying, any help, pointers, criticism are all well
> taken.
>
> last night i took your suggestion on-board and, with your brother's
> posts, looked at the routing tables - comparing my setup to his. i tried
> tweaking these, but no dice. i'm still not convinced it is not a routing
> problem however.
>
> this morning, to check my hypothesis that it may not be routing, but
> rather a module-card problem, i swapped the positions of the cards in
> the box, reorderd the loading of the modules to match, and rebooted.
>
> this time, eth0 (now it is the netgear card) failed to get configured
> (i.e., dhcp assignment doesn't happen), while eth1 (now the 3com card)
> got configured . however, i still can't get this box talking to the lan
> through eth1 in this setup.
>
> if i swap the cards back to their original slots, reorder the modules,
> then both of the interfaces show up in ifconfig, and eth0 negotiates the
> dhcp assignment. but again, eth1 can't reach the lan, or vice-versa.

I know this is probably redundant, but could you post the contents of the 
following files:
Firewall box --
    /etc/sysconfig/network
    /etc/sysconfig/network-scripts/ifcfg-eth0
    /etc/sysconfig/network-scripts/ifcfg-eth1

Lan box --
    /etc/sysconfig/network
    /etc/sysconfig/network-scripts/ifcfg-eth0

Then, here's what I would do:

To simplify things, temporarily shut down eth0 on the firewall but leave the 
card in the computer -- just do ifdown eth0.  Then look at the output of 
ifconfig.  It should only show lo and eth1, which should have an ip address 
of 192.168.1.1, mask of 255.255.255.0.  ifconfig on your lan machine should 
show only lo and eth0, which should have an ip address 192.168.1.4 with a 
netmask of 255.255.255.0.

Check the routing tables to see that they are still what they should be.

Can each computer ping its own IP address on the lan?

Can you connect the two boxes with a crossover cable?  That will remove a lot 
of other variables from the mix.  Then see if you can ping each other.

Do you have tcpdump on either machine?  If not, can you install it?  You can 
use it to snoop on the network to see what's going on.  I think ethereal 
works the same way but I haven't used it.    If you have tcpdump installed, 
enter
tcpdump -i eth0 -n -v 
(substitute eth1 if you are using the firewall box).  Then from the other 
computer ( or another console on the first one), try pinging back and forth, 
and see what happens.  You should see one box sending out the ping and the 
other responding.  If you only see arp traffic, then there is probably an IP 
address problem or a bad card.

Karl Cunningham