I currently need to join a Windows Server 2012 that I have in Amazon to my Active Directory Local (It is only for user authentication). What is the most advisable in this case? I thought of a VPN, but should I be connected all the time for replication? Or should only connect from time to time? thank you very much!
The requirement to make this work is that the machine needs to be able to communicate with your Domain Controller. First to join the domain, second to validate credentials as different users log in. This means you need continuous connectivity over time. You should consider a site-to-site VPN to connect your AWS environment to your local network.
You need some kind of VPN gateway to hook AWS up to. This is a typical capability of most business class routers and firewalls.