Westermo MRD-3x0 - Manual (Page 105)

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6.1 Secure Sockets Layer (SSL) VPN.
Secure Sockets Layer (SSL), are cryptographic protocols that
provide secure communications over a communications net-
work. SSL operates at the transport layer, layer 4 of the OSI
model, this means that it is can be used to create a tunnel
through which other layer 4 protocols such as TCP & UDP
can pass.
An example of an SSL VPN is OpenVPN which is a free and
open source virtual private network (VPN) program for creat-
ing point-to-point or server-to-multiclient encrypted tunnels.
It is capable of establishing direct links between computers
that are behind NAT firewalls. For information on installing
and configuring OpenVPN refer to the OpenVPN website:
http://openvpn.net/