Virtual Private Networks
by Guus Sliepen <guus@sliepen.warande.net>
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Virtual Private Networking is an interesting phenomenon that appears at a
lot of places in one form or another. VPNs have a lot of applications,
from tunnelling through firewalls, getting a fixed IP address in a dynamic
IP environment or in one where there aren't many IP addresses, to
connecting private networks spread all over the world together.

Although the concept is quite simple, the principles and implementations
of VPNs can be very complex. Next to the fact that maintaining a VPN gives
the same problems as maintaining a real network, there are some problems
specific to VPNs that will be treated in-depth:

Authorisation, key-exchange algorithms, encryption, performance impact of
the used tunnelling protocol, kernel or userspace approach, scalability,
routing, redundancy and coherence.

An explanation will be given for the principles behind the most
well-known VPN standards like PPtP, L2TP and IPsec. The advantages and
disadvantages of these protocols will be discussed.

Using examples from the source code and the implemented algorithms, an
explanation will be given about how the VPN program "tinc" (written for
Linux) works and how it tries to solve problems most other VPN
implementations encounter. At the end an example of how to set up a
real-life VPN using "tinc" will be given.