OpenVPN

OpenVPN

Original author(s)	James Yonan
Developer(s)	OpenVPN Technologies, Inc.
Stable release	2.0.9 / October 1, 2006
Preview release	2.1_rc19 / July 16, 2009
Repository	github.com/OpenVPN
Platform	Cross-platform
Type	VPN
License	GNU GPL
Website	openvpn.net

OpenVPN is a free and open source virtual private network (VPN) program for creating point-to-point or server-to-multiclient encrypted tunnels between host computers. It is capable of establishing direct links between computers across network address translators (NATs) and firewalls. It was written by James Yonan and is published under the GNU General Public License (GPL).

Introduction

OpenVPN allows peers to authenticate each other using a pre-shared secret key, certificates, or username/password. When used in a multiclient-server configuration, it allows the server to release authentication certificate for every client, using signature and Certificate authority. It uses the OpenSSL encryption library extensively, as well as the SSLv3/TLSv1 protocol. It is available on Solaris, Linux, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Windows 2000/XP/Vista. It contains many security and control features. It is not a "web-based" VPN, and is not compatible with IPsec or any other VPN package. The entire package consists of one binary for both client and server connections, an optional configuration file, and one or more key files depending on the authentication method used. It is sometimes used by computer gamers as a way of accessing LAN games over the internet.

Encryption

OpenVPN uses the OpenSSL library to provide encryption of both the data and control channels. It lets OpenSSL do all the encryption and authentication work, allowing OpenVPN to use all the ciphers available in the OpenSSL package. It can also use the HMAC packet authentication feature to add an additional layer of security to the connection (referred to as an "HMAC Firewall" by the creator). It can also use hardware acceleration to get better encryption performance.

Authentication

OpenVPN has several ways to authenticate peers to one another. OpenVPN offers pre-shared secret key, certificate-based, and username/password-based authentication. Preshared secret key is the easiest, with certificate based being the most robust and feature-rich. The username/password is a new feature (version 2.0) that can be used with or without a client certificate (the server still needs a certificate). The source tarball includes a sample Perl script to verify the username/password with PAM and a C auth-pam plugin.

Networking

OpenVPN can run over UDP (preferred, and default) or TCP. It multiplexes all communications over a single TCP/UDP port. It has the ability to work through most proxy servers (including HTTP) and is good at working through NAT and getting out through firewalls. The server configuration has the ability to "push" certain network configuration options to the clients. These include IP addresses, routing commands, and a few connection options. OpenVPN offers two types of interfaces for networking via the Universal TUN/TAP driver. It can create either a layer-3 based IP tunnel (TUN), or a layer-2 based Ethernet TAP that can carry any type of Ethernet traffic. OpenVPN can optionally use the LZO compression library to compress the data stream. Port 1194 is the official IANA assigned port number for OpenVPN. Newer versions of the program now default to that port. A feature in the 2.0 version allows for one process to manage several simultaneous tunnels, as opposed to the original "one tunnel per process" restriction on the 1.x series.

OpenVPN's use of common network protocols (TCP and UDP) makes it a desirable alternative to IPsec in situations where an ISP may block specific VPN protocols in order to force users to subscribe to a higher-priced, "business grade," service tier.

Security

OpenVPN offers several internal security features. It runs in userspace, instead of requiring IP stack (and therefore kernel) operation. OpenVPN has the ability to drop root privileges, use mlockall to prevent swapping sensitive data to disk, and enter a chroot jail after initialization.

OpenVPN offers support of smart cards via PKCS#11 based cryptographic tokens.

Community

There are many support options for OpenVPN. The primary method for community support is through the OpenVPN mailing lists. Other sources of support, not directly affiliated with OpenVPN include:

Support Source	Description
OpenVPN Documentation	2.0 Manual 2.1 Manual
IRC	##openvpn on irc.freenode.net
Forum	OpenVPN Forum

Client Software (third-party)

Client	Operating System	Link
OpenVPN GUI	Microsoft Windows	http://openvpn.se
Tunnelblick	Mac OS X	http://code.google.com/p/tunnelblick
Viscosity	Mac OS X	http://viscosityvpn.com
Shimo	Mac OS X	http://www.shimoapp.com
OpenVPN	DD-WRT	DD-WRT
TomatoVPN	Tomato (firmware)	TomatoVPN

Implementations

OpenVPN has been integrated into routing firmwares like DD-WRT and Tomato (firmware), allowing users to run OpenVPN in client or server mode from their network routers. A router running OpenVPN in client mode, for example, facilitates users within that network to access their VPN without having to install OpenVPN on each computer on that network.

See also

• eurephia, a flexible authentication and access controll plug-in for OpenVPN
• OpenSSH, which also implements a level-2/3 "tun"-based VPN
• stunnel encrypt any TCP connection (single port service) over SSL
• UDP hole punching, a technique for establishing UDP "connections" between firewalled/NATed network nodes

External links

• OpenVPN project homepage
• openvpn presentation and demonstration video