limit internet access for vpn clients

[xbipin]

i would like to create the iptables such that the server has full internet access but vpn clients get access to certain sites or lets say whatsapp calls only rather than full access to internet because the last thing i want is clients abusing the server and i mainly need it to unblock whatsapp calls and i know the port and server ip of whatsapp which i need allow through tunnel, in such a scenario how would i modify the firewall rules such that this can be achieved. Any pointers using some sample firewall rules would help