Recently I was playing with SL Server and wanted to enable VPN just to gain the experience. I was using a Mac at my house as the server and although I’d configured the SL Server firewall to open the correct ports for VPN and I had forwarded the correct ports from my server to the router I still couldn’t authenticate or connect to the server from a remote location.

After some hair-pulling and some frustrated Googling I ran across this fact: the Back-to-my-Mac feature in MobileMe uses one of the same ports that VPN requires and for whatever reason Apple has chosen (even in SL Server) to give priority to MM/BTMM communication. So if you have BTMM enabled on the server for some reason (I had enabled MM syncing in order to move bookmarks and contact info onto the server and hadn’t realized that BTMM was enabled by default) VPN access to the server will be blocked. Solution is just to disable BTMM; the other MM syncing services continue to work and don’t interfere with VPN.

(Port in question is UDP 4500)