--- name: detecting-network-anomalies-with-zeek-491a76b89e00 description: Deploy and configure Zeek (formerly Bro) to passively analyze network traffic, generate structured connection/DNS/HTTP/SSL/file logs, detect anomalous behavior, and write custom scripts for organization-specific threats. Use for passive monitoring at network choke points, feeding SIEM/threat hunting with protocol metadata, or retrospective log analysis during incident response; not a substitute for inline IDS/IPS or host agents. domain: cybersecurity subdomain: network-security tags: - network-security - zeek - network-monitoring - anomaly-detection - threat-hunting version: '1.0' author: mahipal license: Apache-2.0 nist_csf: - PR.IR-01 - DE.CM-01 - ID.AM-03 - PR.DS-02 mitre_attack: - T1046 - T1040 - T1557 - T1071 --- # Detecting Network Anomalies with Zeek ## When to Use - Deploying passive network security monitoring at key network choke points for continuous visibility - Generating structured connection, DNS, HTTP, SSL, and file transfer logs for SIEM ingestion and threat hunting - Writing custom Zeek scripts to detect organization-specific threats, policy violations, or beaconing behavior - Performing retrospective analysis on network metadata to investigate security incidents - Complementing IDS solutions with protocol-level metadata analysis that signature-based tools may miss **Do not use** as a replacement for inline IDS/IPS that can actively block traffic, for monitoring encrypted payloads without TLS inspection, or on endpoints where host-based agents are more appropriate. ## Prerequisites - Zeek 6.0+ installed from source or package manager (`zeek --version`) - Network interface configured on a span port, network tap, or virtual switch mirror for passive capture - Sufficient disk storage for log files (estimate 1-5 GB/day per 100 Mbps of monitored traffic) - Familiarity with Zeek's scripting language for writing custom detections - Log aggregation system (Splunk, Elastic, Graylog) for centralized analysis ## Workflow ### Step 1: Install and Configure Zeek ```bash # Install Zeek on Ubuntu/Debian sudo apt install -y zeek # Or install from source for latest version git clone --recursive https://github.com/zeek/zeek cd zeek && ./configure --prefix=/opt/zeek && make -j$(nproc) && sudo make install export PATH=/opt/zeek/bin:$PATH # Configure the monitoring interface sudo vi /opt/zeek/etc/node.cfg ``` ```ini # /opt/zeek/etc/node.cfg [zeek] type=standalone host=localhost interface=eth1 ``` ```bash # Configure local network definitions sudo vi /opt/zeek/etc/networks.cfg ``` ``` # /opt/zeek/etc/networks.cfg 10.0.0.0/8 Internal 172.16.0.0/12 Internal 192.168.0.0/16 Internal ``` ```bash # Disable NIC offloading for accurate packet capture sudo ethtool -K eth1 rx off tx off gro off lro off tso off gso off # Deploy Zeek sudo zeekctl deploy # Verify Zeek is running sudo zeekctl status ``` ### Step 2: Understand and Navigate Zeek Logs ```bash # Zeek generates structured log files in /opt/zeek/logs/current/ ls /opt/zeek/logs/current/ # Key log files: # conn.log - All network connections (TCP, UDP, ICMP) # dns.log - DNS queries and responses # http.log - HTTP requests and responses # ssl.log - SSL/TLS handshake details # files.log - File transfers observed on the network # notice.log - Alerts from Zeek detection scripts # weird.log - Protocol anomalies and errors # x509.log - X.509 certificate details # smtp.log - SMTP email transactions # ssh.log - SSH connection details # View connection log with zeek-cut for column selection cat /opt/zeek/logs/current/conn.log | zeek-cut ts id.orig_h id.orig_p id.resp_h id.resp_p proto service duration orig_bytes resp_bytes # View DNS log cat /opt/zeek/logs/current/dns.log | zeek-cut ts id.orig_h query qtype_name answers # View HTTP log cat /opt/zeek/logs/current/http.log | zeek-cut ts id.orig_h host uri method status_code user_agent ``` ### Step 3: Write Custom Detection Scripts ```bash # Create a custom detection script directory sudo mkdir -p /opt/zeek/share/zeek/site/custom-detections ``` Create a script for detecting DNS tunneling: ```zeek # /opt/zeek/share/zeek/site/custom-detections/dns-tunneling.zeek @load base/frameworks/notice module DNSTunneling; export { redef enum Notice::Type += { DNS_Tunneling_Detected, DNS_Long_Query }; # Threshold: number of unique queries per source in time window const query_threshold: count = 200 &redef; const time_window: interval = 5min &redef; const max_query_length: count = 50 &redef; } # Track query counts per source IP global dns_query_counts: table[addr] of count &create_expire=5min &default=0; event dns_request(c: connection, msg: dns_msg, query: string, qtype: count, qclass: count) { local src = c$id$orig_h; # Check for unusually long domain queries (base64-encoded data) if ( |query| > max_query_length ) { NOTICE([ $note=DNS_Long_Query, $msg=fmt("Unusually long DNS query from %s: %s (%d chars)", src, query, |query|), $src=src, $identifier=cat(src, query) ]); } # Track query volume per source dns_query_counts[src] += 1; if ( dns_query_counts[src] == query_threshold ) { NOTICE([ $note=DNS_Tunneling_Detected, $msg=fmt("Possible DNS tunneling: %s sent %d queries in %s", src, query_threshold, time_window), $src=src, $identifier=cat(src) ]); } } ``` Create a script for detecting beaconing: ```zeek # /opt/zeek/share/zeek/site/custom-detections/beacon-detection.zeek @load base/frameworks/notice @load base/frameworks/sumstats module BeaconDetection; export { redef enum Notice::Type += { Possible_Beaconing }; const beacon_threshold: count = 50 &redef; const observation_window: interval = 1hr &redef; } event zeek_init() { local r1 = SumStats::Reducer( $stream="beacon.connections", $apply=set(SumStats::SUM) ); SumStats::create([ $name="detect-beaconing", $epoch=observation_window, $reducers=set(r1), $threshold_val(key: SumStats::Key, result: SumStats::Result) = { return result["beacon.connections"]$sum; }, $threshold=beacon_threshold + 0.0, $threshold_crossed(key: SumStats::Key, result: SumStats::Result) = { NOTICE([ $note=Possible_Beaconing, $msg=fmt("Possible beaconing: %s made %d connections in %s", key$str, result["beacon.connections"]$sum, observation_window), $identifier=key$str ]); } ]); } event connection_state_remove(c: connection) { if ( c$id$resp_h !in Site::local_nets ) { local key = fmt("%s->%s:%d", c$id$orig_h, c$id$resp_h, c$id$resp_p); SumStats::observe("beacon.connections", [$str=key], [$num=1]); } } ``` ### Step 4: Load Custom Scripts and Deploy ```bash # Add custom scripts to local.zeek sudo tee -a /opt/zeek/share/zeek/site/local.zeek << 'EOF' # Custom detection scripts @load custom-detections/dns-tunneling.zeek @load custom-detections/beacon-detection.zeek # Enable additional protocol analyzers @load protocols/ftp/software @load protocols/http/software @load protocols/smtp/software @load protocols/ssh/detect-bruteforcing @load protocols/ssl/validate-certs @load protocols/ssl/log-hostcerts-only @load protocols/dns/detect-external-names # Enable file extraction @load frameworks/files/extract-all-files # Enable Intel framework for threat intelligence @load frameworks/intel/seen @load frameworks/intel/do_notice EOF # Reload Zeek configuration sudo zeekctl deploy # Verify scripts loaded without errors sudo zeekctl diag ``` ### Step 5: Threat Hunting Queries on Zeek Logs ```bash # Find long-duration connections (possible C2) cat /opt/zeek/logs/current/conn.log | zeek-cut ts id.orig_h id.resp_h id.resp_p duration | \ awk '$5 > 3600 {print $0}' | sort -t$'\t' -k5 -rn | head -20 # Find connections with high data transfer volumes cat /opt/zeek/logs/current/conn.log | zeek-cut ts id.orig_h id.resp_h orig_bytes resp_bytes | \ awk '$4 > 100000000 || $5 > 100000000 {print $0}' # Identify rare user agents (potential malware) cat /opt/zeek/logs/current/http.log | zeek-cut user_agent | sort | uniq -c | sort -n | head -20 # Find self-signed or expired certificates cat /opt/zeek/logs/current/ssl.log | zeek-cut ts id.orig_h id.resp_h server_name validation_status | \ grep -v "ok" # Detect DNS queries to newly registered domains (DGA patterns) cat /opt/zeek/logs/current/dns.log | zeek-cut ts id.orig_h query | \ awk -F'\t' '{n=split($3,a,"."); if(length(a[n-1]) > 10) print $0}' # Find SSH brute force attempts cat /opt/zeek/logs/current/ssh.log | zeek-cut ts id.orig_h id.resp_h auth_success | \ grep "F" | awk '{print $2}' | sort | uniq -c | sort -rn | head -10 # Identify unusual port usage cat /opt/zeek/logs/current/conn.log | zeek-cut id.resp_p proto service | \ sort | uniq -c | sort -rn | head -50 ``` ### Step 6: Integrate with SIEM and Set Up Alerting ```bash # Configure JSON log output for SIEM ingestion sudo tee /opt/zeek/share/zeek/site/json-logs.zeek << 'EOF' @load policy/tuning/json-logs.zeek redef LogAscii::use_json = T; EOF # Forward logs to Elastic via Filebeat # /etc/filebeat/filebeat.yml sudo tee /etc/filebeat/filebeat.yml << 'EOF' filebeat.inputs: - type: log enabled: true paths: - /opt/zeek/logs/current/*.log json.keys_under_root: true json.add_error_key: true fields: source: zeek fields_under_root: true output.elasticsearch: hosts: ["https://elastic-siem:9200"] index: "zeek-%{+yyyy.MM.dd}" username: "elastic" password: "${ES_PASSWORD}" EOF sudo systemctl enable --now filebeat # Set up log rotation sudo tee /etc/cron.d/zeek-logrotate << 'EOF' 0 0 * * * root /opt/zeek/bin/zeekctl cron EOF # Monitor Zeek health sudo zeekctl status sudo zeekctl netstats ``` ## Key Concepts | Term | Definition | |------|------------| | **Network Security Monitor** | Passive analysis tool that observes network traffic and generates structured metadata logs without altering or blocking traffic flow | | **Zeek Script** | Event-driven scripts written in Zeek's domain-specific language that process network events and generate notices, logs, and metrics | | **Connection Log (conn.log)** | Core Zeek log recording every observed connection with source/destination IPs, ports, protocol, duration, and byte counts | | **Notice Framework** | Zeek subsystem for generating alerts when detection scripts identify suspicious activity, outputting to notice.log | | **SumStats Framework** | Statistical analysis framework in Zeek for tracking metrics over time windows, enabling threshold-based detection of anomalies | | **Intel Framework** | Zeek module for matching observed network indicators against threat intelligence feeds and generating alerts on matches | ## Tools & Systems - **Zeek 6.0+**: Open-source network security monitor generating comprehensive protocol-level logs from passive traffic analysis - **zeek-cut**: Zeek utility for extracting specific columns from tab-separated Zeek log files for quick analysis - **zeekctl**: Zeek management tool for deploying, monitoring, and managing Zeek instances across single or clustered deployments - **RITA (Real Intelligence Threat Analytics)**: Open-source tool that analyzes Zeek logs for beaconing, DNS tunneling, and other threat indicators - **Filebeat**: Elastic agent for shipping Zeek JSON logs to Elasticsearch for centralized analysis and visualization ## Common Scenarios ### Scenario: Detecting Command-and-Control Beaconing in Enterprise Traffic **Context**: A threat intelligence report indicates that a specific threat actor uses HTTPS beaconing with 60-second intervals to compromised hosts. The SOC team needs to analyze Zeek logs to identify any hosts exhibiting this pattern across the enterprise network carrying 2 Gbps of traffic. **Approach**: 1. Deploy Zeek on a network tap at the internet egress point with AF_PACKET for high-throughput capture 2. Enable the custom beacon detection script with thresholds tuned for 60-second intervals over 1-hour observation windows 3. Query conn.log for connections to external IPs with consistent duration and inter-connection timing: filter connections where the standard deviation of inter-arrival times is less than 5 seconds 4. Cross-reference suspicious destination IPs against threat intelligence feeds loaded into Zeek's Intel framework 5. Examine ssl.log for the associated TLS certificates -- check for self-signed certificates, unusual issuer names, or certificates with short validity periods 6. Generate a notice for each identified beaconing source and feed into the SIEM for SOC triage **Pitfalls**: - Not tuning beacon detection thresholds for the environment, resulting in false positives from legitimate update services (Windows Update, AV updates) - Failing to exclude CDN and cloud service provider IP ranges that naturally receive many repeat connections - Running Zeek without sufficient CPU cores, causing packet drops on high-throughput links - Not enabling JSON log output, making SIEM integration unnecessarily complex with custom parsers ## Output Format ``` ## Zeek Network Anomaly Detection Report **Sensor**: zeek-sensor-01 (10.10.1.250) **Monitoring Interface**: eth1 (span port from Core-SW1) **Analysis Period**: 2024-03-15 00:00 to 2024-03-16 00:00 UTC **Total Connections Logged**: 2,847,392 ### Anomalies Detected | Notice Type | Source | Destination | Details | |-------------|--------|-------------|---------| | DNS_Tunneling_Detected | 10.10.3.45 | 8.8.8.8 | 847 queries to suspect-domain.xyz in 5 min | | Possible_Beaconing | 10.10.5.12 | 203.0.113.50:443 | 62 connections with 59.8s avg interval | | SSL::Invalid_Server_Cert | 10.10.8.22 | 198.51.100.33:443 | Self-signed cert, CN=localhost | | SSH::Password_Guessing | 45.33.32.156 | 10.10.20.11:22 | 487 failed attempts in 30 min | ### Recommendations 1. Isolate 10.10.3.45 and investigate for DNS tunneling malware 2. Block 203.0.113.50 at firewall and forensically image 10.10.5.12 3. Investigate self-signed TLS certificate on 198.51.100.33 4. Block 45.33.32.156 and enforce SSH key-only authentication ``` --- Source attribution: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/detecting-network-anomalies-with-zeek/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/detecting-network-anomalies-with-zeek/SKILL.md Attribution basis: Source repository record Fields of work: Data & Analytics, Development & Engineering Roles: Data Analyst, ML / AI Engineer, Security Engineer Specialties: API, Backend Tools: Zeek Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: d80468e6bb4d85feb458cceecf7e262459d0447f2626b7cde7b5775a19aec8ae