Remote access from an Ubuntu desktop on internal LAN over internet to Ubuntu machine on another LAN

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http://askubuntu.com – This question is prompted by another question here. Now I use teamviewer to connect to a Ubuntu desktop in a customer's LAN from my LAN. I get a password and access ID from him and connect, then use it to login to other ubuntu servers/machines using ssh or whatever commands/packages I need. I am looking for an open source alternatives where I can do NAT Pass through on both ends (my router and customers router). If this can be accomplished by Free-NX, Remmina or VNC, can some one point to documentation on how to set this up? Note: Port-forwarding from router at customer's router is not an op (HowTos)