Open Source Firewall
A firewall only protects you if its rules actually say what you think they say, and the gap between intent and effect is where most breaches quietly live - a deny that never matched, an allow nobody remembers adding. The open source tools here let you read the filtering logic line by line and run it on hardware you control, so the boundary of your network is something you can inspect rather than infer.
Fail2Ban
Daemon that scans logs and bans IPs with repeated authentication failures
CrowdSec
Security engine for crowd-sourced IP detection, active remediation, and shared blocklists
pfSense CE
FreeBSD-based firewall and router distribution managed entirely from a web interface
OPNsense
FreeBSD-based firewall and routing platform with a web GUI, VPN, IDS/IPS, and high availability
FireHOL
Linux firewall and traffic shaping built on iptables and tc, configured in plain text files
VyOS
Debian-based network OS combining routing, firewall, and VPN with a unified CLI configuration
firewalld
Dynamic Linux firewall daemon with zones, a D-Bus interface, and separate runtime and permanent rules