Non-Microsoft Implementations
There are numerous non-Microsoft implementations of RDP clients and servers that implement subsets of the Microsoft functionality. For instance, the open-source command-line client rdesktop is available for Linux/Unix and Microsoft Windows operating systems. There are many GUI clients, like tsclient and KRDC, that are built on top of rdesktop. In 2009, rdesktop was forked as FreeRDP, a new project aiming at modularizing the code, addressing various issues, and implementing new features. The most popular front-end to FreeRDP is Remmina. An open-source implementation of a Remote Desktop Protocol server on Linux is xrdp. The Windows' Remote Desktop Connection client can be used to connect to a server running xrdp. Proprietary RDP client solutions such as rdpclient are available as a stand alone application or embedded with client hardware.
There is also a so-called VRDP protocol used in VirtualBox virtual machine implementation by Oracle. This protocol is compatible with all RDP clients, such as a default provided with Windows but, unlike the original RDP protocol, can be configured to accept unencrypted and password unprotected connections, which may be useful in secure and trusted networks, such as home or office LAN. By default, Microsoft's RDP server refuses connections to user accounts with empty passwords (but this can be changed with Group Policy Editor). External and guest authorization options are provided by VRDP as well. It does not matter which operating system is installed as a guest because VRDP is implemented on the virtual machine level, not in the guest system. The proprietary VirtualBox Extension Pack is required.
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