A few day ago when I was upgrading some of my servers, I notice that Zentyal was showing as being up to date in the dashboard, but there was also an alert in the dashboard that said that the server was missing an security update. When I logged into it over SSH and tried to manually install updated, I encountered the following error message.
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY
wget -q http://keys.zentyal.org/zentyal-6.2-packages.asc -O- | sudo apt-key add -
Note: That’s the key for version 6.2, if you are running a different version of Zentyal, then you’ll need to find the correct public key for the packages.