Snort

Snort is a legendary, profoundly historic, and globally ubiquitous Open Source intrusion detection system. Originally created in 1998 by Martin Roesch and now maintained by Cisco, Snort is the foundational bedrock of the entire IDPS industry. Almost every modern commercial firewall or security appliance on earth either directly uses the Snort engine under the hood or heavily relies on the "Snort Rules" syntax. The absolute core superpower of Snort is its "Lightweight Packet Sniffing and Universal Rule Language." Snort can be installed on a cheap Linux server in 10 minutes to act as a highly effective network wiretap, alerting administrators if suspicious traffic flows by. Its rule syntax (e.g., 'alert tcp any any -> any 80 (msg:"HTTP Attack";)') is the absolute universal language of cybersecurity analysts globally. Because it is completely free, profoundly customizable, and actively maintained by Cisco Talos, Snort is the absolute go-to for academic researchers, lean IT departments, independent security analysts, and massive managed security service providers (MSSPs) building custom security stacks.