OpenVPN internals
The design of OpenVPN is not extensively documented, but most of the internals of OpenVPN can be discovered by looking at the source code.
The tun/tap driver
One of the basic building blocks of OpenVPN is the tun/tap driver. The concept of the tun/tap driver comes from the Unix/Linux world, where it is often natively available as part of the operating system. This is a virtual network adapter that is treated by the operating system as either a point-to-point adapter (tun-style) for IP-only traffic or as a full virtual Ethernet adapter for all types of traffic (tap-style). At the backend of this adapter is an application, such as OpenVPN, to process the incoming and outgoing traffic. Linux, Free/Open/NetBSD, Solaris and Mac OS include ...
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