Linux Remote Access: VNC, RDP, SSH, and X11

Remote access to Linux in the graphical environment can be done through tools and protocols, such as:

X11

The X11 protocol is the native communication method between the X.Org Server and X applications. 

It uses TCP as the transport protocol, typically between port 6000 and 6063, starting at port 6000, plus the Display number. 

Access control to the protocol can be done through a firewall, restricting access to ports 6000-6063 only to allowed hosts, in addition to application control with the xhost command.

XDMCP

The X.Org architecture is so modular that it allows the Window Manager to run on the same host as the X Server, or remotely on another host.

The XDMCP protocol (acronym for X Display Manager Control Protocol) was created to format the communication between the X Server and the X Window Display Manager.

It uses the UDP transport protocol on port 177 and is insecure, as it does not encrypt its traffic.

XDMCP is analogous to telnet and therefore shares the same security problems, allowing, for example through a wireless network, anyone to "listen" to traffic and easily record keystrokes.

VNC

VNC is a suite of applications that can be used for remote access to the graphical environment. It consists of a server, which listens on TCP port 5900, and a VNC Client, capable of connecting to a server running the VNC Server.

It is widely used for remote access, since it is cross-platform, allowing remote access on Windows, Mac OS, Linux, AIX, HP-UX and Solaris. 

So versatile, the VNC client can run on the Java platform and on Mobile such as Apple iPhone, iPod touch, iPad devices and also on Google Android.

Its authentication is simple, through a password configured on the VNC Server.

SPICE

SPICE (acronym for Simple Protocol for Independent Computing Environments) is a remote display system created for virtual environments that allows viewing Desktop environment on a wide variety of architectures.

Of all the remote connection protocols, SPICE is the most powerful of them, as it provides a complete solution for remote access to virtual machines that allows playing videos, recording audio, sharing USB devices and sharing folders without complications.

SPICE can be divided into 4 different components: protocol, client, server and guest:

• The protocol is the specification in the communication of the other three components;
• A client is the remote viewer responsible for sending data and translating the Virtual Machine (VM) data so that you can interact with it;
• The SPICE server is the library used by the hypervisor (Host Operating System), in order to share the Virtual Machine in SPICE Protocol;
• The guest is all the software that must be run on the Virtual Machine, in order to make SPICE fully functional, such as the QXL driver and SPICE VDAgent.

SPICE is especially designed to provide remote access to virtual machines with the experience of a local machine.

RDP

The RDP (acronym for Remote Desktop Protocol) is a multi-channel protocol created by Microsoft for remote Desktop access.

There are RDP clients for most versions of Windows, and also for Linux.

The RDP server listens by default on TCP port 3389.

FreeRDP is an implementation of RDP under free software terms, licensed under the Apache License. Following Microsoft's Open Specifications, FreeRDP provides a client application for connections through the RDP protocol from Linux to Windows Servers.

The xrdp implementation also provides the server side of RDP for Linux, accepting connections from a variety of clients such as FreeRDP, rdesktop and the Microsoft Desktop Client.

NX

NX created by NoMachine is another remote desktop sharing protocol. Its technology in version 3.5 was open source and available under the GNU GPL2 license. However, from version 4 it became proprietary and closed source.

This caused some open source code variations to be implemented, such as FreeNX and X2Go.

NX technology provides excellent response time, even on low bandwidth connections, being faster than VNC-based products.

NX uses OpenSSH tunneling with encrypted traffic, which is great for remote access over the Internet.

Additionally, NX supports multiple simultaneous users, compresses X11 data and uses caching to provide an enhanced remote desktop experience.

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