Install a VPN Server on Ubuntu or CentOS which is router compatible

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http://stackoverflow.com – I am new to this networking stuff and for the last three days I have been trying to install PPTP VPN Server on my Ubuntu. Computers running XP can connect to it without a single problem, but when I try to connect the VPN Router (D-Link DIR-100) to it, it hangs on connecting. At first, I thought it was a problem in my router, so I tried to connect UnblockVPNs VPN Network; this network was connected to the router successfully. My question here is: If the router can connect to UnblockVPN, then why can't connect to mine ubuntu? I followed this guys guide to setup a VPN on Ubuntu: http://blog.rio (HowTos)