FreeS/WAN: www.freeswan.org

OpenS/WAN: www.openswan.org

Stunnel: www.stunnel.org

OpenVPN: openvpn.sourceforge.net

PoPToP PPTP Server: www.poptop.org

Linux PPTP Client: pptpclient.sourceforge.net

Bruce Schneier 和 Mudge 对 Microsoft PPTP 版本 2 的分析: www.schneier.com/pptp.html

Peter Gutmann 撰写的 Linux 对 MS-PPTP 的回应: www.cs.auckland.ac.nz/~pgut001/pubs/linux_vpn.txt

tinc: www.tinc-vpn.org