“IPsec is an addition to IP protocol that allows authentication
and encryption of IP datagrams. It is defined in detail in IETF
RFCs 2401, RFC 2402, RFC 2406 and RFC 2407… IPsec can be used to
secure a rather wide range of scenarios; one of its best-known
usages is creating virtual private networks (VPNs). A VPN is a
secure, private tunnel between two sub-networks using encrypted
communication over the Internet.“FreeS/WAN has been the main IPsec implementation for Linux for
a long time. Unfortunately, FreeS/WAN has never been integrated
into the Linux kernel itself. Instead, the new native kernel IPsec
implementation is based on the KAME project, a part of the UNIX/BSD
family…”