Gateway/forwarding problem between subnetworks

Hi all,

I have a routing problem that I cannot solve :

Topology :
-----------
I have 2 subnetworks :

- a first subnetwork (call it A) with addresses
192.168.1.0/255.255.255.0 that is connected to internet via a gateway
with address 192.168.1.2 (and a dynamic IP address). This network
works fine.
- a second subnetwork (call it B) with addresses
192.168.0.0/255.255.255.0 (in fact a wireless network).

Both networks A and B are interconnected by a machine (call it GW)
having 2 network cards with addresses 192.168.1.1 on the first network
A and 192.168.0.1 on the second network B.

INTERNET
|
+-----+------+
| modem |
+-----+------+
|
+----+------+
|192.168.1.2|
+----+------+
| network A, 192.168.1.0
+----------------------------------------+
| 192.168.1.1
+--+--+
| GW |
+--+--+
network B, 192.168.0.0 | 192.168.0.1
------+------------------+
|
+-------+------+
| 192.168.0.3 |
+--------------+

Problem :
---------
Internet cannot be reached from network B by a machine which has the
address 192.168.0.3 (a laptop).

Configurations :
----------------
- All machines are under linux 2.4.7-10 (RedHat 7.2).

- Of course GW can be reached from 192.168.0.3 perfectly well, and
also GW can reach internet !

- On 192.168.0.3, routes seem ok :
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref
Use Iface
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
0 wlan0
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 wlan0

- On GW, routes seem also ok (and GW accesses the internet perfectly)
:
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref
Use Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0
0 eth0
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0
0 lo
0.0.0.0 192.168.1.2 0.0.0.0 UG 0 0
0 eth0

- GW has the forwarding enabled :
# cat /proc/sys/net/ipv4/ip_forward
1

- GW has no firewall activated.

- But 192.168.0.3 cannot ping an internet address. It seems to know
that the first step is to go to GW, but is then blocked there :
# traceroute -n 216.239.53.100
traceroute to 216.239.53.100 (216.239.53.100), 30 hops max, 38 byte
packets
1 192.168.0.1 3.493 ms 2.657 ms 2.937 ms
2 * * *

Anybody has an idea about how to solve the situation, or even try to
debug/understand what it the problem ?

Thanks.

Aymeric

On 16 Aug 2003 14:48:52 -0700, Aymeric Duclert <(E-Mail Removed)> wrote:
> Hi all,
>
> I have a routing problem that I cannot solve :
>
> Topology :
> -----------
> I have 2 subnetworks :
>
> - a first subnetwork (call it A) with addresses
> 192.168.1.0/255.255.255.0 that is connected to internet via a gateway
> with address 192.168.1.2 (and a dynamic IP address). This network
> works fine.
> - a second subnetwork (call it B) with addresses
> 192.168.0.0/255.255.255.0 (in fact a wireless network).
>
> Both networks A and B are interconnected by a machine (call it GW)
> having 2 network cards with addresses 192.168.1.1 on the first network
> A and 192.168.0.1 on the second network B.
>
> INTERNET
> |
> +-----+------+
> | modem |
> +-----+------+
> |
> +----+------+
> |192.168.1.2|
> +----+------+
> | network A, 192.168.1.0
> +----------------------------------------+
> | 192.168.1.1
> +--+--+
> | GW |
> +--+--+
> network B, 192.168.0.0 | 192.168.0.1
> ------+------------------+
> |
> +-------+------+
> | 192.168.0.3 |
> +--------------+
>
> Problem :
> ---------
> Internet cannot be reached from network B by a machine which has the
> address 192.168.0.3 (a laptop).
>
> Configurations :
> ----------------
> - All machines are under linux 2.4.7-10 (RedHat 7.2).
>
> - Of course GW can be reached from 192.168.0.3 perfectly well, and
> also GW can reach internet !

What kind of device is 192.168.1.2? I suspect its LAN side is set for
192.168.1.0/24 network and only accepts those IPs on its LAN side. So
network B either needs to be a subnet of network A, or the gw needs to
masquerade network B as an IP on network A.

In my case for example using your network gw settings:

eth0 192.168.1.1 (default netmask 255.255.255.0)
eth1 192.168.1.241 bcast 192.168.1.247 netmask 255.255.248
default gw 192.168.1.2
Proxy arp enabled on gw (so eth0 would answer for eth1 IPs):

echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp

Wireless AP 192.168.1.246 bcast 192.168.1.247 netmask 255.255.255.248
Laptop 192.168.1.245 bcast 192.168.1.247 netmask 255.255.255.248
default gw 192.168.1.241

Of course if you have more than a few wireless devices, you could use a
larger subnet for network B.

--
David Efflandt - All spam ignored http://www.de-srv.com/
http://www.autox.chicago.il.us/ http://www.berniesfloral.net/
http://cgi-help.virtualave.net/ http://hammer.prohosting.com/~cgi-wiz/