OpenVPN uses OpenSSL to perform all cryptographic operations. This means that the performance of an OpenVPN client or server depends on how fast the incoming traffic can be decrypted and how fast the outgoing traffic can be encrypted. For a client with a single connection to the OpenVPN server, this is almost never an issue, but with an OpenVPN server with hundreds of clients, the cryptographic performance becomes very important. Also, when running OpenVPN over a high-speed network link (Gigabit or higher), the cryptographic performance also plays an important role.
In this recipe, we will show how to measure the performance of the OpenSSL cryptographic routines and how this measurement can be used to improve the performance of an OpenVPN server.