Notes on using VNC over a VPN connection
In certain circumstances a user may be required to connect over a VPN (ie, access to an enterprise network). Using VNC over a VPN connection may suffer from certain problems, which are generally underlying configurations with the VPN hardware/software.1) VPN's reduce the MTU (maximum transmission unit) of the network. A normal IP network will have an MTU of 1500 bytes, usually. All network data will be split up by the sending computer into chunks of at most 1500 bytes. The VPN works by making a connection between the client computer and the VPN server, and data sent from a client to LAN is "tunnelled" through this connection to the VPN server, which then sends it out as as normal network packet on the LAN.
In the other direction, LAN data destined for the VPN client computer hits the router, which wraps it up and passes it over the VPN connection. The VPN connection is just a normal network connection, which requires adding some network headers to the original data, which makes it slightly larger, effectively making the MTU for the original data smaller. VPNs should deal with this intelligently, but lots don't, and the result is normally that you can connect but get a black screen from the VNC server and eventually a timeout.
2) Some users connect via VNC to a computer, and then dial-in to a VPN from the VNC Server computer that they've just connected to. Unfortunately, VPNs (and dial-up networks, typically) usually set themselves as the "default router" for data destined for machines outside the LAN. This can mean that the moment you connect to the VPN, all traffic to the VNC Viewer goes via that link, rather than via the VNC Server computer's Internet connection (even though that's still there, and in the VPN case the VPN itself is actually running over that connection). The VNC Viewer can't then access the VNC Server computer, because it's no longer accessible via its normal IP address - it thinks it's on the VPN's LAN instead.
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Last Updated
13th of December, 2011
