What is a VPN
A VPN or Virtual Private Network it's a network within another network.
The VPN they are mainly used to transfer data over a private network that travels within a public network, such as the Internet.
The connection goes through an encrypted tunnel. The security of the public network can not be guaranteed, just as access to the intermediate hardware can not be controlled, but the encryption of data should make them inaccessible.
Why can a VPN be useful?
Let's take a practical example.
Imagine having a network of your office covered by a firewall and wanting to connect from home. A VPN allows the connection between the home computer and the office network by encrypting the data transiting from and to these two machines.
In this way, even if you use the Internet, we are taking advantage of another encrypted network within it. It's a bit like going through a tunnel that nobody else has access to.
In order to use one Virtual Private Network no special hardware is needed, we need a server VPN running on the network, of the office in the case we took in the example, and of a proper firewall configuration.
Furthermore we must have a client software VPN that runs locally, then on the home computer.
Although it seems like an expensive implementation, it's not like that at all.
Many routers, even among the cheapest ones, have support for them Virtual Private Network . Just pay attention to the devices marked VPN pass-though.
These devices indeed, even supporting the use of one VPN, I am not able to handle this type of connection and therefore we will need a client.
Ultimately what we need is a properly configured kernel, but we do not have to worry too much about it since most of the distros already use a kernel that is suitable for that purpose.
The software we want to use depends a lot on the variant of Virtual Private Network we want to use at the other end of the link.