I'm setting up a Linode server and the Getting Started security guide suggests disabling ssh access over either IPv4 or IPv6 so that I only have one of the two enabled.
I understand the general theory of reducing attack surface, but why would I pick one over the other? How do I know which one I need?
Listen on only one internet protocol. The SSH daemon listens for incoming connections over both IPv4 and IPv6 by default. Unless you need to SSH into your Linode using both protocols, disable whichever you do not need. This does not disable the protocol system-wide, it is only for the SSH daemon.