Pings from VPN network to VPN client work; pings into from VPN client to VPN network fail - why?

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http://serverfault.com – We are in the process of setting up an OpenVPN server for some servers running in a cloud. We are stumped with a connectivity problem whereby the hosts on the VPN server's LAN can ping te VPN client, but the reverse is not true. The VPN client can ping the VPN server on its VPN address, but not on its LAN address. tcpdump shows evidence of ping packets from the client reaching the host and replies being issued, but for some reason the replies never reach the tun0 interface on the VPN server or the client. Conversely, traffic for the ping requests from the VPN server's LAN to the VPN client a (HowTos)