Linux FreeS/WAN provides IPSEC (IP Security, which is both encryption and authentication) kernel extensions and an IKE (Internet Key Exchange, keying and encrypted routing daemon) as well as various rc scripts and documentation. It is known to interoperate with other IPSEC and IKE system already deployed by other vendors such as OpenBSD, Cisco, or CheckPoint. It also features Opportunistic Encryption, subnet extrusion, and with the appropriate patches interops nicely with Microsoft Windows XP/2000 using X.509 certificates.
