I was trying to setup PPTPD (poptop ) on a CENTOS machine with IPTABLES and it was not working. Iptables blocks VPN connections even if you open the relevant ports. So I googled this problem and tried many solutions, but none worked except the one here
http://www.anindya.com/installing-configuring-pptp-vpn rhel-centos .This one was perfect, i added it to rc.local and now it works like a charm.