Table of Contents

1. OpenVPN (OSS)
2. OpenVPN Access Server (OAS)
3. What can you do with OpenVPN?
4. What distinguishes OpenVPN from other VPN packages?

OpenVPN (OSS)

OpenVPN is a full-featured open source SSL VPN solution that accommodates a wide range of configurations, including remote access, site-to-site VPNs, Wi-Fi security, and enterprise-scale remote access solutions with load balancing, failover, and fine-grained access-controls. Starting with the fundamental premise that complexity is the enemy of security, OpenVPN offers a cost-effective, lightweight alternative to other VPN technologies that is well-adapted for the SME and enterprise markets.

The OpenVPN security model is based on SSL, the industry standard for secure communications via the internet. OpenVPN implements OSI layer 2 or 3 secure network extension using the SSL/TLS protocol, supports flexible client authentication methods based on certificates, smart cards, and/or 2-factor authentication, and allows user or group-specific access control policies using firewall rules applied to the VPN virtual interface. OpenVPN is not a web application proxy and does not operate through a web browser.

For a good conceptual introduction to OpenVPN, see the ​program notes for James Yonan's talk at Linux Fest Northwest 2004. You may also wish to read ​OpenVPN and the SSL VPN Revolution by Charlie Hosner.

OpenVPN Access Server (OAS)

OpenVPN Technologies, Inc. business model is to build products on top of OpenVPN. One of these products is the Access Server, which integrates OpenVPN server with a management GUI, enterprise management capabilities and simplified OpenVPN Connect clients for Windows, Mac, Android and iOS. OpenVPN Access Server supports a wide range of configurations, including secure and granular remote access to internal network and/or private cloud network resources.

What can you do with OpenVPN?

• Tunnel any IP subnetwork or virtual ethernet adapter over a single UDP or TCP port,
• Configure a scalable, load-balanced VPN server farm using one or more machines which can handle thousands of dynamic connections from incoming VPN clients,
• Use all of the encryption, authentication, and certification features of the SSL library to protect your private network traffic as it transits the internet,
• Use any cipher, key size, or HMAC digest (for datagram integrity checking) supported by the SSL library,
• Choose between static-key based conventional encryption or certificate-based public key encryption,
• Use static, pre-shared keys or TLS-based dynamic key exchange,
• Use real-time adaptive link compression and traffic-shaping to manage link bandwidth utilization,
• Tunnel networks whose public endpoints are dynamic such as DHCP or dial-in clients,
• Tunnel networks through connection-oriented stateful firewalls without having to use explicit firewall rules,
• Tunnel networks over NAT,
• Create secure ethernet bridges using virtual tap devices, and
• Control OpenVPN using a GUI on Windows or Mac OS X.

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 supports IPv6 transport (OpenVPN over IPv6 network) and IPv6 payload (IPv6 inside IPv4)
• OpenPVN can make use of either OpenSSL or PolarSSL
• 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 (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 SSL 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 SSL library and immediately have access to any new functionality provided in the new release. 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