Setting up Linux routing tables between networks

Question

I have the following topology:

                     VM A                         VM C
                 192.168.56.102               192.168.57.102
                      |                            |
                      |                            |
          (eth0) 192.168.56.103 --[ VM B ] -- 192.168.57.101 (eth1)

I'm trying to make B act as a router between the two networks. In trying to send a datagram from A to C, I can see the packet on the 192.168.56.103 side (using tcpdump), but I don't see the packet going outbound on the 192.168.57.101 side.

I have verified that the datagram's destination address is 192.168.57.102, and the forwarding table in B looks like:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.56.0    *               255.255.255.0   U     1      0        0 eth0
192.168.57.0    *               255.255.255.0   U     1      0        0 eth1

I've tried adding this forwarding rule as well:

route add -net 192.168.57.0 netmask 255.255.255.0 gw 192.168.57.102 dev eth1

Which yields this routing table:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.56.0    *               255.255.255.0   U     1      0        0 eth0
192.168.57.0    C.local         255.255.255.0   UG    0      0        0 eth1
192.168.57.0    *               255.255.255.0   U     1      0        0 eth1

I'm pretty sure this rule is just redundant, and I don't get any better results with it.

If it helps, here is the ARP table from B:

C.local (192.168.57.102) at 08:00:27:12:6b:c9 [ether] PERM on eth1
A.local (192.168.56.102) at 08:00:27:0a:86:a2 [ether] PERM on eth0

Is there a step I'm missing that can connect these two networks? Thanks for any help!

Answer 1

You usually needs to enable ipv4 routing on the machine.

You can do this by issuing the following command:

sysctl -w net.ipv4.ip_forward=1

Or:

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