Understanding BPF (Berkeley Packet Filter) in OT Security with Use-Cases

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Understanding BPF (Berkeley Packet Filter) in OT Security with Use-Cases

1. What is BPF (Berkeley Packet Filter)?

BPF is a highly efficient, low-level packet filtering mechanism originally developed for Unix-like operating systems. It allows programs to capture and filter network packets directly within the kernel before sending them to user space, reducing unnecessary data processing.

BPF works by defining filtering rules in a lightweight virtual machine (VM) within the OS kernel. These rules are written in a specialized bytecode that allows for extremely fast execution compared to traditional user-space filtering techniques.

Modern BPF implementations, such as eBPF (Extended BPF), further enhance functionality by allowing users to attach custom programs to various kernel and networking events.

2. Why is BPF Used for Packet Mirroring in OT Security?

OT security, packet mirroring (or traffic mirroring) is crucial for monitoring industrial networks, detecting anomalies, and preventing cyber threats. However, OT networks often involve high-bandwidth, latency-sensitive traffic, making efficient packet filtering essential to avoid overwhelming security monitoring systems.

BPF is used in packet mirroring for several reasons:

  1. Performance Optimization:

    • Only relevant packets are forwarded, reducing overhead.

    • Filtering happens inside the kernel, avoiding unnecessary copying to user-space applications.

  2. Fine-Grained Packet Selection:

    • Specific protocol-based filtering (e.g., Modbus, DNP3, IEC 104, MQTT).

    • Allows OT security tools to focus only on critical traffic.

  3. Reducing Noise:

    • Industrial networks generate high volumes of repetitive traffic (e.g., sensor updates).

    • BPF ensures only security-relevant packets are mirrored.

  4. Low-Latency Processing:

    • Since OT systems require real-time responses, filtering traffic before mirroring avoids delays.

  5. Integration with Security Tools:

    • Works with tcpdump, Suricata, Zeek, and IDS/IPS systems to enhance OT security monitoring.

3. Practical Examples of BPF Usage in OT Packet Mirroring

Example 1: Filtering Only Industrial Control Protocols (e.g., Modbus)

In an OT environment, Modbus TCP is a widely used protocol for industrial communication. A security analyst may want to mirror only Modbus traffic for intrusion detection.

BPF Filter for Modbus TCP (port 502): tcpdump -i eth0 port 502

Use Case:

  • Used in ICS/SCADA security monitoring to capture only Modbus traffic from PLCs.

  • Helps detect unauthorized Modbus commands that may indicate an attack.

Example 2: Excluding Broadcast and Multicast Traffic

OT networks generate excessive broadcast traffic from devices like PLCs, RTUs, and sensors. To improve efficiency, BPF can be used to exclude broadcast/multicast packets from mirrored traffic.

BPF Filter to Exclude Broadcast and Multicast: tcpdump -i eth0 not broadcast and not multicast

Use Case:

  • Prevents log flooding in security monitoring systems.

  • Ensures that only point-to-point communications are mirrored.

Example 3: Monitoring DNP3 Traffic in Electrical Grid Networks

DNP3 (Distributed Network Protocol 3) is commonly used in electrical substations. A cybersecurity team might want to monitor DNP3 traffic to detect anomalies.

BPF Filter for DNP3 (Port 20000): tcpdump -i eth0 port 20000

Use Case:

  • Used by power grid operators to detect unauthorized DNP3 commands.

  • Helps prevent cyberattacks targeting substations (e.g., Industroyer malware).

Example 4: Capturing Only Traffic to a Specific PLC

In some cases, security teams need to mirror only traffic going to or from a specific PLC (e.g., IP 192.168.1.100).

BPF Filter for a Specific PLC: tcpdump -i eth0 host 192.168.1.100

Use Case:

  • Focuses monitoring on high-value assets like PLCs, avoiding unnecessary traffic.

  • Helps investigate targeted attacks or misconfigurations.

Example 5: Filtering Out Normal OT Traffic and Capturing Only Large Data Transfers

OT protocols typically exchange small packets. Large packets may indicate data exfiltration or command injections.

BPF Filter for Large Packets (Above 1000 Bytes):tcpdump -i eth0 greater 1000

Use Case:

  • Detects unusual large data transfers, which may indicate cyber espionage.

  • Helps identify malicious firmware updates being pushed to devices.

Example 6: Detecting Unauthorized Remote Access in OT Networks

Attackers may use SSH or RDP to gain remote access to OT systems. To detect unauthorized access, security teams can filter SSH and RDP traffic.

BPF Filter for SSH (Port 22) and RDP (Port 3389): tcpdump -i eth0 port 22 or port 3389

Use Case:

  • Monitors for unauthorized remote logins to industrial servers.

  • Helps detect insider threats or lateral movement in OT environments.

Example 7: Capturing Only Traffic from External IPs (Possible Attackers)

If an OT network is supposed to be isolated, any traffic from external IPs may indicate a security breach.

BPF Filter for External Traffic (Non-Private IPs): tcpdump -i eth0 not net 192.168.0.0/16 and not net 10.0.0.0/8

Use Case:

  • Detects unauthorized access from external sources.

  • Useful for ICS perimeter security monitoring.

4. How BPF Enhances OT Security

Security ObjectiveHow BPF Helps
Reduce network trafficFilters unnecessary packets before mirroring
Focus on specific protocolsCaptures only relevant ICS/SCADA traffic
Improve real-time detectionReduces load on IDS/IPS and SIEM solutions
Detect anomaliesFlags unusual data transfers or unauthorized access
Increase efficiencyPrevents unnecessary log storage and processing

Recap: BPF is an essential tool for packet mirroring in OT security because it optimizes network monitoring, improves detection efficiency, and minimizes performance overhead. By applying BPF filters, security teams can focus on critical threats, reduce false positives, and ensure the integrity of industrial control networks.


#CPS #OT #XIoT #IoT #IIoT #IoMT #CPSSecurity #OTSecurity #IoTSecurity #CPS보안 #OT보안 #IoT보안

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