This article applies to NoMachine v. 6 or later running on macOS 10.13 High Sierra or later (10.14 Mojave, 10.15 Catalina, 11.x Big Sur).
When installing NoMachine (either a server or the client package) for the first time on macOS High Sierra 10.13 or later, users will receive the 'System Extension Blocked' message from the system.
This is a new feature introduced by macOS High Sierra 10.13 which requires the user to manually enable third-party kernel extensions (KEXTs):
"macOS High Sierra 10.13 introduces a new feature that requires user approval before loading new third-party kernel extensions." (Ref. https://developer.apple.com/library/archive/technotes/tn2459/_index.html#//apple_ref/doc/uid/DTS40017658)
Since NoMachine needs to install kernel extensions to manage services like audio and disk sharing accepting to load NoMachine's system extensions is fundamental for the proper working of all these services.
If system extensions are not approved, the NoMachine application will install and work correctly but such services will not be available.
Fig. 1 - Click on Open Security Preferences to approve the loading of the NoMachine system extension
In order to approve the NoMachine system extension:
1) Open on your macOS: System Preferences -> Security & Privacy -> General
2) Click the lock at the bottom left to be able to make changes.
Fig. 2 - Go to Security & Privacy S-> Security & Privacy -> General and unlock the lockpad to make changes
3) In section 'Allow apps downloaded from', check option 'App Store and identified developers'
4) Click on 'Allow' to unblock the loading of system software from developer 'NoMachine S.a.r.l)
Fig. 3 - Click on 'Allow' to un-block the loading of the NoMachine system extensions
5) Then, restart your macOS as requested.
Fig. 4 - Click on ''Restart' to reboot your macOS
The possibility to unblock the loading of NoMachine system extensions (Fig. 3) is available only for 30 minutes after the system has issued the 'System Extension Blocked' dialog (Fig. 1).
NoMachine system extention for USB forwarding is not loaded during the installation, but when you go to the 'Connect a USB device' panel in the NoMachine menu inside the session (click on the page peel in the top right corner of the session window to open it, then click on 'Devices').
When accessing the 'Connect a USB device' panel for the first time, you will be prompted with a new the 'System Extension Blocked' message from the system. Follow the procedure above and approve the loading of the system extension, in this case the USB KEXT.