The original Cisco PIX firewall used modified Plan 9 as the operating system. In fact, despite the official history, the name PIX came from Plan 9, or Plan IX.

The original Cisco PIX firewall used modified Plan 9 as the operating system. In fact, despite the official history, the name PIX came from Plan 9, or Plan IX. I know this because the person who wrote the entire PIX software stack, Brantley Coile, told me.