Zeek AlternativesNetwork Monitors and other similar apps like Zeek

Open-source utility for security auditing and network exploration, utilizing raw IP packets to scan hosts and services, identify OS and service versions, and assess network infrastructure, compatible across major operating systems with advanced GUI and tool suite.

Suricata is a high performance Network IDS, IPS and Network Security Monitoring engine. Open Source and owned by a community run non-profit foundation, the Open Information Security Foundation (OISF). Suricata is developed by the OISF and its supporting vendors.

Snort® is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Combining the benefits of signature, protocol, and anomaly-based inspection, Snort is the most widely deployed IDS/IPS technology worldwide.

LOKI is a free and simple IOC - "Incidents of Compromise" scanner - that goes beyond AV and malware scans to detect evidence of hacking.

Moloch augments your current security infrastructure to store and index network traffic in standard PCAP format, providing fast, indexed access. An intuitive and simple web interface is provided for PCAP browsing, searching, and exporting.

Maltrail is a malicious traffic detection system, utilizing publicly available (black)lists containing malicious and/or generally suspicious trails, along with static trails compiled from various AV reports and custom user defined lists, where trail can be anything from domain...

redborder is an Open Source, Big Data cybersecurity and traffic analysis platform accompanied by a set of active probes managed in an integrated manner. Can be deployed on-premise or used as a cloud service. Managing SNORT events.