Purbo, Onno Widodo. “Ubuntu Linux Server Security Analysis and Simulation With Port Knocking & Iptable”. International Journal of Basic and Applied Science 8, no. 2 (September 30, 2019): 36–46. Accessed April 22, 2026. https://www.ijobas.pelnus.ac.id/index.php/ijobas/article/view/62.