FreeNATS will do that for you and is available as a pre-built appliance.
The current production one is pretty old based on rPath but there's also a new one up for testing at the moment.
The main site is: http://www.purplepixie.org/freenats/ which has links to download the current version.
Any problems with that you can try the new trial at: http://www.purplepixie.org/freenats/wiki/Virtual_Appliance_6_Trial
Good luck,
Dave.