SSH relay client/server solution

I have a certain number of Linux out there that I need to reach in SSH. Each Linux box is installed on a customer network that NAT the box on a local network that avoid the box to be contacted directly. Most of the time the customer wan has also a dynamic IP, so I also do not have the public IP. Number of remote boxes is also variable, the solution should manage automatically the connection of new remote boxes. What I'm looking for is a software solution that should be similar to the following: - an internet connected linux server with public fixed IP host a service that allow remote boxes to register - remote boxes and server recognise eachother with certificates - when a box boot up the system register itself on the server - the server logs into a text file the list of available remote boxes with all the related informations - when I want to connect to a remore box I can run a command on the server to SSH into the remote box - each remote box will be identified by its MAC address of the first network interface