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Linux Router Configuration for 2 subnets ??????????????????????????????????????????????????????????????????????? LOST

Posted on 2004-09-10
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Last Modified: 2010-03-18
Hello all,

I currently have two subnets at my house, connected by a Linux router.  Looks like the following:
     
     _________192.175.0.11                                                                                                                          __________192.175.0.19
     |                                                                    -------     _______192.175.0.17                                        |
     |                         192.175.0.10                        |L  R|    /                                                                        |
--------                    ___________                         |I   O| /                        ----------------                            -------
| PC A|___________|D-Link 624 A|_____________|N  U|/______________|D-Link 624 B |_______________| PC B|
--------                    ----------------                       /|U  T|                         -----------------                           -------
                                                                        / |X  E|                          192.175.0.18
                                             192.175.0.9____ /   |    R|
                                                                          --------
192.175.0.8 Network         <------------------------------|------------------------------->           192.175.0.16 Network
                                                                               
                                                                               

I am using the 'D-Link 624' router as a switch to connect multiple computers at each subnet and then to the Linux router.  I am not using the WAN port on the D-Link routers.
The Linux Router includes two nics.

For the 192.175.0.8 Network ---
------------------------------------
D-Link A
IP -- 192.175.0.10
SNM -- 255.255.255.248

eth0 (on the linux machine)
IP -- 192.175.0.9
SNM -- 255.255.255.248

PC A
IP -- 192.175.0.11
SNM -- 255.255.255.248


For the 192.175.0.8 Network ---
------------------------------------
D-Link B
IP -- 192.175.0.18
SNM -- 255.255.255.248

eth1 (on the linux machine)
IP -- 192.175.0.17
SNM -- 255.255.255.248

PC B
IP -- 192.175.0.11
SNM -- 255.255.255.248

For the linux router I simply installed Red Hat 9 without a firewall, and turned on ip forwarding.  I don't need a firewall at the moment.
Now, from any machine on the network, I can ping all devices on its network and the nic on the linux router that is on the other network.
EX:   I can ping from 192.175.0.11 (computer) to 192.175.0.17(eth1 linux router).

My problem is that I can't ping any thing else in the other side of the linux router. EX   192.175.0.11 (computer) cannot ping any device past 192.175.0.17(eth1 linux router).

I think that the IP Forwarding on the linux box is working because eth0 and eth1 are on different subnets, and i can ping both cards from any device on the network.
I just cant ping past the D-Link routers from the other subnet.

Any help on my configuration with the Linux box or the D-Links would be appreciated.
Thanks
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Question by:jneely
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6 Comments
 
LVL 40

Expert Comment

by:jlevie
ID: 12031576
IP forwarding is working since you can ping the other Linux NIC. What you haven't done is to give the nodes on each of the subnets a route to the other subnet. So when 192.175.0.11 tries to ping 192.175.0.18 the target node doesn't know that the reply should use 192.175.0.17 as a gateway to the 192.175.0.0/28 network.

> PC B
> IP -- 192.175.0.11

I assume that's a typo and it should be "IP -- 192.175.0.19"
0
 

Author Comment

by:jneely
ID: 12035067
oops,

PC B
IP -- 192.175.0.19
SNM -- 255.255.255.248
0
 

Author Comment

by:jneely
ID: 12035168
So, you're saying that I should create an entry in the routing tables for each computer on the network.  If so, this is something I've already tried.  Each computer on the network is pointing to the linux router as its gateway already.  i.e.  PC A -- 192.175.0.11 gateway -- 192.175.0.9       and
                                                                                                PC B -- 192.175.0.19 gateway -- 192.175.0.17

What I haven't figured out is how to point the D-links to the linux router.  They have the correct ip address and subnet mask.  But there is no place for me to enter a gateway to the linux router, without using the WAN ports.

I really think that this is where my problem lies.
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Accepted Solution

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jlevie earned 500 total points
ID: 12039183
If everything is plugged in to the LAN ports of the D-Link it doesn't play a role in routing. You then are simply using the D-Link as a switch. And the fact that you can touch the RH box's NICs indicates that to be true. So it is something else.

How did you enable routing on the RedHat box, via an edit to /etc/sysctl.conf or an 'echo 1 >/proc/sys/net/ipv4/ip_forward'? If the later has there been a reboot since? The echo method won't survive a reboot.

What we need to do now is to look at a tcpdump when you try a ping to an IP of the far side of the Linux router. You should see the packet come in on one NIC and go out on the other. If there's no return packet the problem lies with the far end machine.
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