Traffic disappears from one server to another when routed through VPN

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http://serverfault.com – In the following setup when I try to send a ping from host 'a' over VPN to host 'c', I see it being received on host b's tun0 interface and forwarded on eth0, but then it disappears. I don't see anything coming into host c's eth0 interface. The net result is that pings are not answered back. I can ping host c from host b directly(where source ip address shows up within the subnet where host b and c are located, host a has different range ( How do I debug this? What could be the issues? How do I detect whether the traffic was send out by the hardware or not? (Could it be blocked (HowTos)