Free software remote assistance tool for double NAT

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http://serverfault.com – I need a remote assistance tool (i.e. tool that makes it possible to control another persons screen) that: works between two machines that are both NAT'ed behind different NAT-walls does not require modification of NAT-wall (i.e. port forwarding is no-go) works as both client and server on both Windows 7+XP and GNU/Linux is free software It should preferably survive if any of the machines change its internal IP address (e.g. as part of a DHCP change). TeamViewer delivers 1, 2, and 3, but fails because of its license. The different VNC tools deliver 3 and 4, but fails for either 1 or 2. (HowTos)