When installing NoMachine (either a server or the client package) for the first time on macOS High Sierra 10.13, users will receive the 'Blocked kernel extension' message from the system:
System Extension Blocked
A program tried to load a new system extension(s)
signed by "NoMachine S.à r.l.". If you want to
enable these extensions, go to the Security & Privacy
This is a new feature introduced by macOS High Sierra 10.13 which requires the user to manually enable third-party kernel extensions (KEXTs).
Please see this Trouble Report to see which NoMachine version addresses the problem: https://www.nomachine.com/TR11P09013.
Since NoMachine needs to install KEXTs to manage services like audio support, disk sharing and USB forwarding, accepting to load NoMachine's system etensions is fundamental for the proper working of all services. If KEXTs are not approved, the software will install and work correctly but such services will be not available.
In order to approve the KEXT, access the System Preferences -> Security & Privacy -> General panel on your mac and click on the 'Allow' button to unblock 'System software from developer "NoMachine S.à r.l." blocked from loading'.
the panel to approve KEXTs is available only for 30 minutes after that the system has issued the 'System Extension Blocked' prompt. If the user doesn't approve KEXTs on time, the panel will reappear in the Security & Privacy settings each time there is an attempt to load KEXTs but the 'System Extension Blocked' dialog will be no longer issued.
Please see also: https://developer.apple.com/library/content/technotes/tn2459/_index.html#//apple_ref/doc/uid/DTS40017658 for more technical details about the 'System Extension Blocked' control on Mac and instructions to allow loading the KEXTs.