Why OpenVPN?

What distinguishes OpenVPN from other VPN packages?

• OpenVPN's principal strengths include cross-platform portability across most of the known computing universe, excellent stability, scalability to hundreds or thousands of clients, relatively easy installation, and support for dynamic IP addresses and NAT.
• OpenVPN provides an extensible VPN framework which has been designed to ease site-specific customization, such as providing the capability to distribute a customized installation package to clients, or supporting alternative authentication methods via OpenVPN's plugin module interface (For example the openvpn-auth-pam module allows OpenVPN to authenticate clients using any PAM authentication method -- such methods may be used exclusively or combined with X509 certificate-based authentication).
• OpenVPN offers a management interface which can be used to remotely control or centrally manage an OpenVPN daemon. The management interface can also be used to develop a GUI or web-based front-end application for OpenVPN.
• On Windows, OpenVPN can read certificates and private keys from smart cards which support the Windows Crypto API.
• OpenVPN uses an industrial-strength security model designed to protect against both passive and active attacks. OpenVPN's security model is based on using SSL/TLS for session authentication and the IPSec ESP protocol for secure tunnel transport over UDP. OpenVPN supports the X509 PKI (public key infrastructure) for session authentication, the TLS protocol for key exchange, the OpenSSL cipher-independent EVP interface for encrypting tunnel data, and the HMAC-SHA1 algorithm for authenticating tunnel data.
• OpenVPN is built for portability. At the time of this writing, OpenVPN runs on Linux, Solaris, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Windows (2000/XP and later versions). Because OpenVPN is written as a user-space daemon rather than a kernel module or a complex modification to the IP layer, porting efforts are dramatically simplified.
• OpenVPN is easy to use. In general, a tunnel can be created and configured with a single command (and without any required configuration files). OpenVPN's documentation contain examples illustrative of its ease of use.
• OpenVPN has been rigorously designed and tested to operate robustly on unreliable networks. A major design goal of OpenVPN is that it should be as responsive, in terms of both normal operations and error recovery, as the underlying IP layer that it is tunneling over. That means that if the IP layer goes down for 5 minutes, when it comes back up, tunnel traffic will immediately resume even if the outage interfered with a dynamic key exchange which was scheduled during that time.
• OpenVPN has been built with a strongly modular design. All of the crypto is handled by the OpenSSL library, and all of the IP tunneling functionality is provided through the TUN/TAP virtual network driver. The benefits of this modularity can be seen, for example, in the way that OpenVPN can be dynamically linked with a new version of the OpenSSL library and immediately have access to any new functionality provided in the new release. For example, when OpenVPN is built with the latest version of OpenSSL (0.9.7), it automatically has access to new ciphers such as AES-256 (Advanced Encryption Standard with 256 bit key) and the encryption engine capability of OpenSSL that allows utilization of special-purpose hardware accelerators to optimize encryption, decryption, and authentication performance. In the same way, OpenVPN's user-space design allows straightforward porting to any OS which includes a TUN/TAP virtual network driver.
• OpenVPN is fast. Running Redhat 7.2 on a Pentium II 266mhz machine, using TLS-based session authentication, the Blowfish cipher, SHA1 authentication for the tunnel data, and tunneling an FTP session with large, pre-compressed files, OpenVPN achieved a send/receive transfer rate of 1.455 megabytes per second of CPU time (combined kernel and user time).
• While OpenVPN provides many options for controlling the security parameters of the VPN tunnel, it also provides options for protecting the security of the server itself, such as --chroot for restricting the part of the file system the OpenVPN daemon has access to, --user and --group for downgrading daemon privileges after initialization, and --mlock to ensure that key material and tunnel data is never paged to disk where it might later be recovered.