OpenVPN is the spearhead for a fairly recent category of VPN: SSL VPNs. It has been around since 2002 and was written by James Yonan.
SSL VPNs reuse SSL encryption mechanisms to authenticate and encrypt their connections. OpenVPN is based on the product OpenSSL, the main open source implementation of the SSL protocol in terms of both quality and usage levels, and utilises its encryption and identity verification routines to provide excellent data security.
OpenVPN is distributed under the GPL v2.
Available for Solaris, Linux, OpenBSD, FreeBSD, NetBSD, Mac OS X, Windows 2000, XP, Vista and 7, it also offers a good number of additional security and control functions.
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