Apple released this Knowledge Base Article on how to enable the adaptive firewall. This firewall automatically creates temporary rules according to certain events. e. g. A number of failed login attempts will cause the adaptive firewall to create a temporary rule to block the IP address attempting to log in.
Run these commands on the server as an admin user to enable the adaptive firewall:
sudo pfctl -f /etc/pf.conf
sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/serverctl enable service=com.apple.afctl
sudo /Applications/Server.app/Contents/ServerRoot/usr/libexec/afctl -f
Edit /System/Library/LaunchDaemons/com.apple.pfctl.plist so that pfctl(8) is invoked with the -e flag. This will automatically enable the packet filter the next time the server boots. This can be accomplished with these commands:
sudo defaults write /System/Library/LaunchDaemons/com.apple.pfctl ProgramArguments '(pfctl, -f, /etc/pf.conf, -e)'
sudo chmod 644 /System/Library/LaunchDaemons/com.apple.pfctl.plist
sudo plutil -convert xml1 /System/Library/LaunchDaemons/com.apple.pfctl.plist