Why do scans make my printer activate?

Some printers support an old protocol known as JetDirect or AppSocket. It receives data on port 9100 and prints it, with no authentication or format checking -- the sender is expected to prepare the data in the appropriate format for the printer (such as PCL, PostScript or plain text).

runZero scans port 9100 by default, because that port is also often used by software such as Elasticsearch and Prometheus. While our code attempts to identify printers and avoid triggering them, occasionally you may find that a printer wakes from sleep or prints a page of meaningless text. If this happens, please report the issue to us so that we can improve our printer detection. (Some printers such as HP LaserJets will wake far enough for their displays to light up, but they shouldn't start warming up the print mechanism.)

Note that there are no current commonplace OSs that require port 9100 (JetDirect) support in order to print. Windows, macOS, Linux, iOS, Android, and BSD all use newer protocols by default (typically IPP) and have done since the days of Windows 2000/Mac OS 10.5/RHEL 3. Unless you need to print from legacy software or embedded systems, you can likely turn off JetDirect on the printer and avoid possible security issues.