After that last post on enabling SSH back to my iMac, I realized I should do a little more research into security precautions. I stumbled onto information about the macOS Server Adaptive Firewall.
Enabling it couldn’t be much easier. It’s two commands; first to self-configure, and then to start the firewall:
sudo /Applications/Server.app/Contents/ServerRoot/usr/libexec/afctl -c sudo /Applications/Server.app/Contents/ServerRoot/usr/libexec/afctl -f
kyrpted.com has a more thorough explanation of how to use the Adaptive Firewall, but I feel a little better knowing it’s running.