Penguin
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Acronym for Point-to-Point Tunneling Protocol

A technology for creating Virtual Private Networks (VPNs) , developed jointly by MicrosoftCorporation, U.S. Robotics, and several remote access vendor companies, known collectively as the PPTP Forum. A VPN is a private network of computers that uses the public Internet to connect some nodes. Because the Internet is essentially an open network, the Point-to-Point Tunneling Protocol (PPTP) is used to ensure that messages transmitted from one VPN node to another are "secure". With PPTP, users can dial in to their corporate network via the Internet.

See PPTPServerHowto.

If you want to support this under Linux, get PoPToP. It starts a pppd in the correct place; you might be interested in the MPPE patches.

PPTP is a great way to get onto the MetaNet (or indeed, any local network) if you're away from it and all you have is a Windows machine.

For firewalling interests. PPTP uses GRE packets (protol 47) and a TCP connection on port 1723 for control. Most FireWall/NAT implementations don't understand the GRE connection identifier and thus will only support one PPTP connection, to a single PPTP server, when your connection is over NAT. Linux 2.4 doesn't seem to have this problem. See PPTPConnectionTracking for details.