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

# 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

# /opt/zeek/etc/node.cfg
[zeek]
type=standalone
host=localhost
interface=eth1

# 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

# 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

# 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

# Create a custom detection script directory
sudo mkdir -p /opt/zeek/share/zeek/site/custom-detections

Create a script for detecting DNS tunneling:

# /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;


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