NX technology

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NX accessing a remote Linux desktop from Windows

NX technology is a computer program that handles remote X Window System connections, and attempts to greatly improve on the performance of the native X display protocol to the point that it can be usable over a slow link such as a dial-up modem.

It is developed by Gian Filippo Pinzari at the Italian software company NoMachine.


[edit] History

The NX compression scheme was derived from that of DXPC - the Differential X Protocol Compressor project[1].

[edit] Technical Details

NX compresses the X11 data to minimize the amount of data transmitted. NX also makes extensive use of caching, to make the session as responsive as possible; for example, if a menu is opened, the first time it will take a few seconds, but every subsequent time the menu is opened it will appear almost instantly.

NX is faster than its predecessors, as it eliminates most of the X round-trips, while dxpc and MLView only compress data.

[edit] Other Display Protocols

Although designed primarily to optimize X11 sessions, NX can be configured as a proxy server to tunnel Remote Desktop Protocol (for Windows Terminal Services sessions) and remote Virtual Network Computing sessions (most modern general-purpose operating system platforms), giving them some of the same speed improvements.

[edit] Use of SSH protocol

NX uses the SSH protocol to send its data, making use of public key cryptography for authentication purposes. When an NX client connects to a server, the client program initially logs into the server using the 'nx' username, authenticated with the SSH public key method. 'nx' is a special user account created on the server with its shell set to an NX application, through which client-server communication can take place. In this session, the client requests session data which can then be fed to 'nxproxy' to start the graphical NX session. Once the NX link is established, the real username is authenticated through the pre-established SSH tunnel, and the 'session possession' is handed over to the real user.

In addition to simply allowing users to log in remotely over a slow internet link to a server graphically, NX also allows them to suspend and resume sessions. During suspension, the processes invoked inside the session continue to run, and so many people have come to use NX as a graphical alternative to SSH and the 'screen' application - in order to run applications such as XChat permanently on a computer with a fixed internet link.

SSH was chosen as a base for NX because of its excellent security record.

[edit] License

NoMachine uses the GNU General Public License for the core NX technology, while at the same time offering non-free commercial NX server[2] and client products for Linux, Microsoft Windows, Solaris, Mac OS X and embedded systems.

Due to the free software nature of NX, the FreeNX project was started in order to provide the wrapper scripts for the GPL NX libraries. FreeNX is developed and maintained by Fabian Franz.

2X develops another commercial terminal server for Linux that also uses the NX protocol.[3]

[edit] Clients

The primary client for use with NX is the official NoMachine NX Client, but there are several projects underway to produce an open source client.

The most mature of the projects used to be Lawrence Roufail's nxc client library. This is a full library which can be used for other clients to build upon, and another application, 'nxrun', is provided which makes use of this library. As of 2006, the library does not support suspending or resuming sessions, nor does it support using any compression method other than JPEG for the graphics.

The kNX project was a proof-of-concept application written by Joseph Wenninger. This was meant to eventually become a complete NX client, showing that an open-source client could be written. However, this implementation got stuck in an incomplete stage; to date it lacks many important features. As such, kNX was effectively useless. In late 2005, Fabian Franz and George Wright started to change kNX to use the nxc library, but quickly abandoned the project.

More recent open-source efforts include QtNX, which offers full suspend and resume support. However, this has been reported not to work with the most recent NX libraries.

An update to nxclientlib (which was the core of QtNX) called nxcl has been completed by Seb James in September 2007. nxcl is an update to nxclientlib and works with version 3 of the NX core libraries.[citation needed] It also drops the Qt dependency which prevented nxclientlib from becoming widely used as a cross-platform basis for NX client programs. nxcl provides both a library which can be linked to in a client program (libnxcl), and a self-contained NX client with a D-Bus API (the nxcl binary). nxcl is available from the freenx Subversion server.

[edit] Previous X11 compression schemes

  • lbxproxy - Low Bandwidth X (obsolete and of historical interest only)
  • dxpc The Differential X Protocol Compressor

[edit] See also

[edit] References

[edit] External links

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