pfSense
pfSense is a free and open source firewall and router that also features unified threat management, load balancing, multi WAN, and more.
What is pfSense?
pfSense is a free, open source customized distribution of 
FreeBSD tailored for use as a firewall, and router. In addition to being a powerful, flexible firewalling and routing platform, it includes a long list of related features and a package system allowing further expandability without adding bloat and potential security vulnerabilities to the base distribution, making it a fully qualified UTM device. pfSense is a popular project with more than 1 million downloads since its inception, and proven in countless installations ranging from small home networks protecting a PC and an Xbox to large corporations, universities and other organizations protecting thousands of network devices.
FreeBSD tailored for use as a firewall, and router. In addition to being a powerful, flexible firewalling and routing platform, it includes a long list of related features and a package system allowing further expandability without adding bloat and potential security vulnerabilities to the base distribution, making it a fully qualified UTM device. pfSense is a popular project with more than 1 million downloads since its inception, and proven in countless installations ranging from small home networks protecting a PC and an Xbox to large corporations, universities and other organizations protecting thousands of network devices.
Used pc & a pair of network cards
+
$0 pfsense
If you're a masochist add wifi, but at least employ Ubiquiti UniFi.
its Free | But i Prefer OPNSense
It's simple, and it just work.
I'd say the best firewall software around. Full feature and runs, runs, runs. The fitting netgate hardware is rock solid.
Full feature FIrewall Solution. No Bullshit.
To me the best firewall software / solution there is.
pfSense runs on FreeBSD not Linux