If your scanner is behind a strict corporate firewall or sits in an air-gapped network, it won't be able to validate the code against Tenable's cloud. In this scenario, you must perform an Offline Registration . The Nessus UI will provide a "Challenge Code." You must take this challenge code to an internet-connected device, visit Tenable's offline registration page, enter your activation code and challenge code, and download a custom license file ( nessus.license ) to upload back to your scanner.

Users searching that phrase are likely trying to obtain a Nessus activation code (for Tenable's Nessus vulnerability scanner), or looking for the “best” method to get one. That raises legality and security concerns.