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OpenVPN is an open source software application that implements virtual private network (VPN) techniques for creating secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses a custom security protocol[2] that utilizes SSL/TLS for key exchange. It is capable of traversing network address translators (NATs) and firewalls. It was written by James Yonan and is published under the GNU General Public License (GPL).[3]

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 an authentication certificate for every client, using signature and Certificate authority. It uses the OpenSSL encryption library extensively, as well as the SSLv3/TLSv1 protocol, and contains many security and control features.

Architecture 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.[4][5] Support for PolarSSL is coming in version 2.3 Authentication

OpenVPN has several ways to authenticate peers to each another. OpenVPN offers pre-shared keys, certificate-based, and username/password-based authentication. Preshared secret key is the easiest, with certificate based being the most robust and feature-rich. In version 2.0 username/password authentications can be enabled, both with or without certificates. However to make use of username/password authentications, OpenVPN depends on third-party modules. See the Extensibility paragraph for more info. Networking This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2009)

OpenVPN can run over User Datagram Protocol (UDP) or Transmission Control Protocol (TCP) transports, multiplexing created SSL tunnels on a single TCP/UDP port[citation needed] (RFC 3948 for UDP).[6] It has the ability to work through most proxy servers (including HTTP) and is good at working through Network address translation (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, enter a chroot jail after initialization and apply a SELinux context after initialization.

OpenVPN runs a custom security protocol based on SSL and TLS.[2] OpenVPN offers support of smart cards via PKCS#11 based cryptographic tokens. Extensibility

OpenVPN can be extended with third-party plug-ins or scripts which can be called at defined entry points.[7][8] The purpose of this is often to extend OpenVPN with more advanced logging, enhanced authentication with username and passwords, dynamic firewall updates, RADIUS integration and so on. The plug-ins are dynamically loadable modules, usually written in C, while the scripts interface can execute any scripts or binaries available to OpenVPN. In the OpenVPN source code[9] there are some examples of such plug-ins, including a PAM authentication plug-in. There also exists several third party plug-ins to authenticate against LDAP or SQL databases such as SQLite and MySQL. There is an overview over many of these extensions in the related project wiki page for the OpenVPN community. Platforms

It is available on Solaris, Linux, OpenBSD, FreeBSD, NetBSD, QNX, Mac OS X, and Windows 2000/XP/Vista/7. While most mobile phone OSes (iOS, Palm OS, etc.) do not support OpenVPN, it is available for Maemo,[10] Windows Mobile 6.5 and below,[11] Android 4.0+ devices, and Android devices that have had the Cyanogenmod aftermarket firmware flashed[12] or have the correct kernel module installed.[13] It is not a "web-based" VPN, meaning that it is not shown as a web page such as Citrix or TS Web access - the program is installed independently and configured by editing text files manually, rather than through a GUI-based wizard. OpenVPN 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. Client software

While OpenVPN is a command-line utility, it provides a management interface specifically designed to allow for establishment and control of an OpenVPN daemon by external software.[14] This has allowed for the development of a number of third-party clients that provide a GUI for connecting to an OpenVPN server.