Can a NAT close out VPN connections?

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http://serverfault.com – I have a client running on a VPN behind a NAT. The client becomes inaccessible after a while if it is not "active". The VPN software is PPTP and the OS of the client if Debian. I'm unsure about the exact cause of what makes it inaccessible or if it is another piece of software that is causing the problem. But my current thing I am trying to figure out is if the NAT is healing and causing the client VPN to become inaccessible. Could this be causing the problem or should I look elsewhere? (HowTos)