Keeping folders synced between several machines
Is there any software that allows you to sync the files arbitrary folders on four computers over the LAN and keep it going over the internet when LAN is not available? Internet syncing is too slow for two of the computers which will always be on the LAN and the other two are laptops. I'd like them to synchronize away from home.
I would have called it the holy grail of home networking, but for some reason this seems to be a very unique request. All the software I can find either uses only LAN only or only internet, or, like DropBox, uses cloud storage. DropBox is cool and does sync over LAN, but I don't need cloud storage and I can't afford DropBox's price just to keep my computers in sync. Also DropBox won't LAN sync without an internet connection, which the kind of setup I have in mind should be able to do.
Free and open is preferred. If there is no such software, free or otherwise, can I string it together using free utilities? The computers are all running Windows 7 and one Windows Server 2008. About 150 GB is in question and we have plenty of room on each computer.
EDIT: I set up OpenVPN, so I have what amounts to a LAN at all times. Now I just need software that syncs four ways instead of two.