Firewall (4)

• Home (firewalld) tool
  zone-based dynamic firewall daemon separating runtime from permanent config, with a D-Bus API
  Network Routing Security Hardening #
• OWASP Modsecurity Project (Modsecurity Project) tool
  cross-platform WAF module with a rules language, paired with OWASP CRS; open source
  Security Hardening Web Server #
• UFW Essentials: Common Firewall Rules and Commands for Linux Security (digitalocean.com) tutorial
  UFW rule syntax and command reference: allow, deny, ports, services, logging
  Security Hardening #
• OpenBSD Frequently Asked Questions (openbsd.org) reference
  supplemental admin guide to the man pages: console and serial setup, pf example rulesets, current release
  BSD System Administration #