How to route ping packet from A to C?

Question

I'm beginning understanding network concepts and tools. I setup three VM's with the following configuration:

A: 192.168.1.3
B: 192.168.1.1 192.168.2.1
C: 192.168.2.2

In A, I ping 192.168.1.1 and it pongs. I think A has no idea where 192.168.2.1 is located. So, when I ping 192.168.2.1, it doesn't know to what interface it should be routed. My first question is that when there's only one interface, why it wonders? :D Anyway, for route to take occur, in A I add the following:

sudo route add default enp0s3

I may use the following commands instead:

sudo route add default gw 192.168.1.1
sudo route add default gw 192.168.1.3

2. Am I right in the commands?
3. The last command also works! Why?! Is it correct?! How?!

With one of these commands A may ping 192.168.2.1 too. Then, I go to ping C from A. I expect the ping packet when received by B, it knows how to route it and route it to C via 192.168.2.x interface, but it doesn't.

4. Why? Should I do something in B?
5. What should I do in B for the ping to get response from C?

Answer 1

In order to achieve what you would like to have you need to do:

On A, you need to configure the route to 192.168.2.X, that is via 192.168.1.1:

ip route add 192.168.2.0/24 via 192.168.1.1

Note: you can specify a device name with dev DEVICE_NAME or you can omit it in this case.

On B, you need to make sure that packets can be forwarded e.g. ip_forwaring is enabled:

sysctl -w net.ipv4.ip_forward=1

or

echo 1 > /proc/sys/net/ipv4/ip_forward

In addition, you need to allow the traffic between the two networks in the firewall. So something like the following command should to the job:

iptables -I FORWARD -s 192.168.1.0/24 -d 192.168.2.0/24 -j ACCEPT
iptables -I FORWARD -s 192.168.2.0/24 -d 192.168.1.0/24 -j ACCEPT

B has two interfaces (192.168.1.1 and 192.168.2.1) and it already knows where to find both subnets. So in this case you don't need to add additional routes on it.

On C, same as on A, you need to tell 192.168.2.2 how to reach 192.168.1.X, that is via 192.168.2.1:

ip route add 192.168.1.0/24 via 192.168.2.1

And finally, you have to make all these changes permanent, because if you now reboot these nodes, the configuration which you have applied will be gone. How to achieve this depends on the Linux distribution you are using and is not in the scope of your current question :)

Answer 2

After some trials and effort and some investigations, I could find a simpler method which omits the requirement of using iptables in B. Although most parts are the same. Let's see what has been the problems of ping packets response which were not received by A. By executing the following command in A:

route add -net 192.168.2.0/24 gw 192.168.1.1

or as you mentioned:

ip route add 192.168.2.0/24 via 192.168.1.1

which are the same, A which has no idea about vmnet 192.168.2.x figures out that 192.168.1.1 has access to this network and it may route packets with destination 192.168.2.x. Now ping packets may be received by 192.168.1.1 which is also 192.168.2.1. Since packet forwarding is enabled in B, it knows that it should send them again and it knows that to what interface this should happen. Because of the destination of the packet this should take place via 192.168.2.x interface. Until now A may get response to ping 192.168.2.1, but not 192.168.2.2, but this doesn't mean that the ping packets are not received by 192.168.2.2. If I use Wireshark, I may see them in C. But the problem is that C has no idea about 192.168.1.x network and doesn't know how to route packets with this destination. So the pong packets which are generated by C, can't be routed back. It's enough to add a similar routing rule in C which we may call it a route-back rule, this way:

route add -net 192.168.1.0/24 gw 192.168.2.1

That's it. There's no need to define any rule in B, in its ip tables or routing table. Although your solution is also correct, I think this may be a simpler method and concentrates on the problem more accurately, although I'm not persisting on my solution and compliment any suggestion or correction if I'm mistaking.