IDS/IPS—Zeek (NIDS)

Zeek's key capabilities: IDS, SIEM, or packet analyzer?

Zeek (formerly Bro) is a powerful network analysis framework, widely used in research and enterprises. Zeek can be categorized as a Network Security Monitoring (NSM) tool that functions primarily as a packet analyzer and Intrusion Detection System (IDS) in its most common applications. Here’s a breakdown of how it fits into these categories:

1. Packet Analyzer (Primary Role)

• Zeek operates by deeply inspecting network traffic (like Wireshark/Tcpdump but at scale).

• It reconstructs sessions, extracts files, and logs high-level protocol transactions (HTTP, DNS, SSL, etc.) rather than just raw packets.

• Unlike simple packet captures, Zeek generates structured logs (.log files) that are easier to analyze.

2. Intrusion Detection System (IDS)

• Zeek is often used as a passive, signature-less IDS (though it can integrate signatures via frameworks like Suricata).

• It detects anomalies by analyzing protocol behavior, not just pattern matching (unlike traditional IDS/IPS).

• It doesn’t actively block traffic (not an IPS by default), but it can trigger alerts or external actions.

3. Network Security Monitoring (NSM)

• Zeek excels in NSM by providing visibility into network activity for threat hunting, forensics, and anomaly detection.

• It’s a core tool in NSM alongside tools like Suricata, Elasticsearch, and Splunk.

What Zeek is NOT:

• Not a SIEM: While Zeek generates logs that can feed into a SIEM (like Splunk, ELK), it lacks SIEM features like centralized correlation, dashboards, or long-term storage.

• Not an EDR: Zeek monitors network traffic, not endpoint processes/memory (unlike CrowdStrike, Carbon Black).

• Not an IPS: By default, it doesn’t block traffic (though it can integrate with firewalls/IPS systems for response actions).

Conclusion

Zeek is best classified as a hybrid of a packet analyzer and network IDS, with a strong emphasis on NSM. If forced to choose between packet analyzer and network IDS, Zeek can be categorized as a network IDS—but with a critical caveat:

Why Network IDS?

• Primary Purpose: Zeek is designed for security monitoring, not just packet inspection. It focuses on detecting malicious activity (e.g., C2 traffic, protocol anomalies, data exfiltration) rather than simply dissecting packets.

• Output: It generates security-relevant logs (e.g., http.log, conn.log, dns.log) tailored for threat analysis, unlike raw packet tools (e.g., Wireshark).

• Behavioral Detection: It can identify suspicious patterns (e.g., unusual HTTP headers, DNS tunneling) without relying solely on signatures (a hallmark of IDS).

Why "Packet Analyzer" Doesn’t Fully Fit

• Beyond Packets: Zeek doesn’t just capture/display packets—it reconstructs sessions, normalizes protocols, and enriches data with context (e.g., extracting files from SMTP).

• Higher Abstraction: Tools like Wireshark/Tcpdump are for network engineers; Zeek is for security analysts.

Key Distinction

• Wireshark = "What’s in this packet?"

• Zeek = "Is this traffic malicious or abnormal?"

Zeek vs. traditional monitoring tools

Zeek is dominant in network visibility—it has the capability to provide deep, protocol-aware monitoring and logging of network activity. Zeek focuses on analyzing network traffic at the application layer, generating structured logs that answer:

• Who is communicating? (IPs, devices, users)

• What are they doing? (HTTP requests, DNS queries, SSL certificates, SSH logins, etc.)

• When/Where/How is it happening? (Timestamps, geolocation, protocol behavior)

Unlike tools that just alert on threats (e.g., Snort/Suricata), Zeek records everything in a way that’s useful for:

• Forensics (e.g., "What files were downloaded over HTTP?")

• Threat hunting (e.g., "Find all DNS queries to known malicious domains")

• Behavioral analysis (e.g., "Detect unusual SSH login patterns")

Zeek vs. Traditional Monitoring Tools

Aspect	Zeek	Traditional Network Monitoring (e.g., Nagios, PRTG, Zabbix)
Primary Focus	Security-relevant network activity	Availability, bandwidth, latency, uptime
Data Output	Logs (e.g., conn.log, http.log)	Metrics (e.g., throughput, packet loss, jitter)
Use Case	Threat detection, forensics, compliance	Network health, SLA monitoring
Protocols	Deep parsing (HTTP, DNS, SSL, etc.)	SNMP, NetFlow, ICMP
Tool Examples	Suricata (for comparison)	Cacti, SmokePing, SolarWinds

Why Zeek Isn’t a Traditional IPS

• No Real-Time Blocking: Zeek logs and alerts but doesn’t actively drop packets (unlike Suricata in IPS mode).

• Passive Analysis: It reconstructs sessions and extracts metadata but doesn’t manipulate traffic.

• Flexible, Not Prescriptive: You define what to log (e.g., "all HTTP User-Agent strings"), rather than relying on fixed rules.

Example of Zeek’s Visibility

A single HTTPS connection generates logs with:

• SSL/TLS details (certificate issuer, cipher suites)

• Timing/duration of the session

• Associated DNS query that resolved the domain

• Linked files (e.g., downloaded executables)

This is invaluable for detecting:

• Malware C2 channels (e.g., unusual SSL certs)

• Data exfiltration (e.g., large, unexpected uploads)

• Policy violations (e.g., unauthorized cloud services).

When to Pair Zeek with Other Tools

• For IPS: Combine Zeek with Suricata (blocking) + Wazuh (host-level correlation).

• For Dashboards: Pipe Zeek logs to Elasticsearch + Kibana or Splunk.

• For Performance Monitoring: Use NetFlow (pmacct) or SNMP.

Zeek offers deep protocol-level visibility by analyzing raw network traffic and generating structured logs (e.g., HTTP requests, DNS queries, SSL certificates), enabling detailed forensic investigations and behavioral analysis—unlike traditional monitoring tools that focus only on performance metrics (bandwidth, uptime). Zeek passively reconstructs network activity into actionable security data without blocking traffic.