Do you know book or Ebook about security?

This migth interest you...! • The Web Application Hacker's Handbook: Finding and Exploiting Security Flaws

What does "pf" mean? Packet filtering Personal firewall ...?

pf is the statefull paket filter on OpenBSD, FreeBSD and NetBSD. So good documentation, and source code available.

Good; with that answer I can add this in-context (basically to support reading comprehension). https://www.pfsense.org/ (pfSense, FreeBSD) "...rapidly becoming the world's most trusted open source network security solution." This is an open source firewall / router with unified threat management, load balancing, multi WAN, etc. The name arises because it uses the packet filtering tool "PF". Here's where it generates pf rules, so you can see what the GUI does for you (+debugging): https://www.netgate.com/docs/pfsense/firewall/editing-the-pf-ruleset.html https://www.netgate.com/docs/pfsense/firewall/firewall-rule-troubleshooting.html This firewall is no joke...I hope you have fun with it.