There are three main types Virtual Private Networks (VPN), but all three have the same goal of making the user feel as if they were plugged into a different network.

Site-To-Site VPN

Both the client and server are Network Devices, typically either Routers or Firewalls, and share entire network ranges. This is most commonly used to join company networks together over the Internet, allowing multiple locations to communicate over the Internet as if they were local.

Remote Access VPN

This involves the client's computer creating a virtual interface that behaves as if it is on a client's network. Hack The Box utilizes OpenVPN, which makes a TUN Adapter letting us access the labs. When analyzing these VPNs, an important piece to consider is the routing table that is created when joining the VPN. If the VPN only creates routes for specific networks (ex: 10.10.10.0/24), this is called a Split-Tunnel VPN, meaning the Internet connection is not going out of the VPN. This is great for Hack The Box because it provides access to the Lab without the privacy concern of monitoring your internet connection. However, for a company, split-tunnel VPN's are typically not ideal because if the machine is infected with malware, network-based detection methods will most likely not work as that traffic goes out the Internet.

SSL VPN

This is essentially a VPN that is done within our web browser and is becoming increasingly common as web browsers are becoming capable of doing anything. Typically these will stream applications or entire desktop sessions to your web browser.