Questions

Why can't my VNC viewer connect to my VNC server?

Can I use VNC to remotely control another computer across the internet?

What's about fullscreen sessions?

Is VNC secure?

How do I use VNC through my firewall?

Answers

Why can't my VNC viewer connect to my VNC server?
VNC requires TCP/IP network connectivity between the viewer and server computers. A simple test is to use the ping utility. If you can't ping from your viewer to your server, and vice-versa, than VNC will not work!

Can I use VNC to remotely control another computer across the internet?
Yes. VNC uses the TCP/IP protocol which is the networking standard of the Internet. You can use VNC over LANs, WANs, broadband ISP, and modem dialup ISP. The performance will vary with the amount of networking bandwidth you have, but there are special encodings and compression to make the most out of the bandwidth available. There is also an automatic linespeed detector, which will dynamically switch in the most suitable compression scheme for the connection you have. You should read the FAQ on security, and about using VNC through firewalls.

What's about fullscreen sessions?
PMVNC doesn't support fullscreen sessions. If you switched to fullscreen and can't return to desktop - select "Send Ctrl-Alt-Del" in your VNC viewer - PMVNC will switch to desktop on receiving CAD sequence (not reboot!).

Is VNC secure?
The only really secure computer is one without a network. VNC requires a password when a viewer tries to connect to a server. This password is encrypted to deter snooping, but the following graphical data, the VNC protocol, is not. In many ways, VNC is more secure than remote login programs such as telnet where the password is and the following data are sent in the clear as ascii characters. Many people find it perfectly acceptable to use VNC like this behind a corporate firewall, across a VPN, or between computers within the home. However, if the computer or network is connected to the internet, we strongly advise the use of additional security. See how to make VNC secure using SSH. You might want to know how to use VNC with a firewall.

How do I use VNC through my firewall?
Many organisations operate firewalls to reduce the risk of intrusion by malicious attackers via the Internet. These firewalls typically operate by only allowing connections in to machines in that organisation on specific ports. Which ports are permitted access depends upon the network protocol that uses the port and the degree of security it provides. VNC servers can accept incoming connections through firewalls in two main ways. Although the first is usually the simplest to arrange, we recommend using the SSH tunnelling method wherever VNC is to be used over an untrusted network such as the Internet.

• Opening Ports - The simplest way to allow VNC connections in through your firewall is to configure your firewalling software to allow connections to the VNC ports. If N is the display number of a particular VNC server then it will accept connections on port 5900+N. Configuring your firewall to allow connections to this port will allow VNC to work. If you wish to use the in-built web server and Java VNC Viewer then you will also need to allow connections to port 5800+N. Unfortunately, because VNC traffic is not encrypted, this approach weakens the security provided by your firewall, and so is not advisable.
• Secure Tunnelling - Most organisations that operate firewalls allow connections to a number of standard ports, that are in principle used only by secure or harmless protocols. While VNC in its present incarnation is not suitably secure for this to be advisable, it can be "tunnelled" through a secure protocol layer to achieve the same effect. The Secure Shell (SSH) protocol is one example of such a wrapper, and is one which most firewalls allow access through. The Secure Shell client is run on the VNC client computer and is made to forward connections to a particular port on that machine to a port on the VNC server machine. The forwarded connection is encrypted by the SSH software, which can provide both encryption and authentication. For more details on how to do this, see here.