Mastering PCAP Review: Essential Guide for Analysts & Engineers
Packet Capture (PCAP) review is a vital skill for network analysts, cybersecurity professionals, and engineers operating within both Information Technology (IT) and Operational Technology (OT) landscapes.
This deep dive explores the critical use cases, powerful tools, and effective techniques necessary to conduct efficient and insightful PCAP reviews, transforming raw packet data into actionable intelligence.
Why Conduct a PCAP Review?
A PCAP review becomes essential when a granular understanding of network traffic is required to address specific issues or achieve particular objectives. It’s the digital equivalent of examining a crime scene for forensic clues, but for network events. Key triggers for a PCAP review include:
Analyzing Captured Network Data: Often, customer-provided PCAPs from diverse environments, including sensitive industrial control systems (ICS), require meticulous examination to diagnose problems or validate operations.
Investigating Suspicious or Anomalous Behavior: When intrusion detection systems (IDS) flag an event, or when network monitoring tools show unusual spikes or drops in traffic, a PCAP can help determine the root cause, nature, and scope of the anomaly. This could range from a misconfigured device spamming the network to an active security breach.
Identifying Communication Protocols and Asset Types: Understanding precisely what protocols are traversing the network (e.g., TCP/IP, UDP, DNS, HTTP, or OT-specific protocols like Modbus, S7comm, DNP3, CIP) is fundamental. This helps in creating an accurate network map, identifying rogue services, and ensuring that only authorized protocols are in use. It also aids in fingerprinting devices to understand their make, model, and function.
Confirming Correct Placement and Configuration of Network Taps: Network taps are crucial for passively capturing traffic. A PCAP review can validate that taps are positioned optimally to capture the desired segments and that they are not inadvertently filtering out important data due to misconfigurations.
Detecting Misconfigurations or Restrictions: This includes identifying issues with firewalls, routers, switches, or the impact of packet filters like Berkeley Packet Filters (BPFs). For instance, a BPF might be unintentionally blocking legitimate traffic essential for an application’s functionality.
Performance Troubleshooting: Slow application response times or network bottlenecks can be diagnosed by analyzing packet loss, retransmissions, latency, and other network-level metrics visible in a PCAP.
Security Incident Response: During or after a security incident, PCAPs are invaluable for understanding the attacker’s actions, identifying compromised systems, and determining the extent of data exfiltration.
Setting a clear objective before diving into any PCAP file cannot be overstated. Much like a focused penetration test or a red teaming engagement, the ultimate value derived from a PCAP review hinges on having well-defined goals. Without clear objectives, analysts risk becoming overwhelmed by a sea of data, leading to inefficient use of time and potentially missed insights.
Common Use Cases for PCAP Analysis
PCAP analysis serves a multitude of purposes across different operational contexts:
Design Partner Engagements & Customer Support: Customers, especially those managing complex industrial or plant environments, often share PCAP files when seeking assistance with network issues or when evaluating new solutions. These files can be enormous, sometimes spanning gigabytes or even terabytes of data captured over extended periods. Targeted goals are essential to efficiently extract meaningful insights, such as identifying communication bottlenecks for a specific critical application or verifying the interaction patterns between new and existing equipment.
Threat Detection and Forensic Analysis: In cybersecurity, PCAP files are goldmines for incident responders and threat hunters. Analysts meticulously examine the data to uncover Indicators of Compromise (IOCs). This could involve searching for traffic to known malicious IP addresses or domains, identifying unusual DNS queries, detecting command-and-control (C2) communication patterns (e.g., beaconing, non-standard protocols over common ports), or spotting data exfiltration attempts. Signatures of known malware or exploit kits can also be identified within packet payloads.
Protocol Identification and Analysis: A primary step in understanding any network is identifying the protocols in use. Wireshark and similar tools can dissect packets and identify hundreds of protocols, from common IT protocols (HTTP, SSL/TLS, SMB, DNS) to specialized OT protocols (Modbus, CIP, S7comm, PROFINET, BACnet). This is key for understanding device interactions, identifying non-standard or unauthorized protocol usage, and ensuring interoperability in heterogeneous environments. Deep analysis of protocol handshakes and data exchange can also reveal misconfigurations or vulnerabilities.
Asset Identification and Network Mapping: PCAP analysis helps in creating a dynamic inventory of network assets. By examining source and destination IP and MAC addresses, along with protocol-specific information (e.g., DHCP requests, ARP tables, NetBIOS name announcements, OT vendor-specific communications), analysts can map IPs to physical or virtual assets, identify device types (servers, workstations, PLCs, HMI, sensors), and understand their roles and communication patterns within the network. This is foundational for network management, security hardening, and segmentation strategies.
Tap Validation and Network Visibility Assessment: Ensuring comprehensive network visibility is paramount. PCAPs taken from network taps or SPAN ports can validate that these monitoring tools are correctly placed to capture all relevant traffic for a given segment. It also helps confirm that filters, such as BPFs, are not unintentionally dropping critical packets, thereby ensuring the integrity and completeness of the captured data. For example, if a security tool relies on tap data, incorrect placement could mean blind spots where threats go undetected.
Core Tool: Wireshark
Wireshark stands as the undisputed champion and the de facto open-source tool for PCAP analysis. Its power, versatility, and extensive community support make it indispensable for anyone serious about network traffic investigation. It excels due to:
Deep Packet Inspection: Wireshark decodes and displays the fields of a vast number of protocols, allowing for granular examination of every aspect of a packet.
User-Friendly GUI: Despite its power, Wireshark offers an intuitive graphical interface that allows users to easily navigate through packets, apply filters, and visualize data.
Powerful Filtering Engine: Its display filter language is incredibly flexible, allowing analysts to zero in on specific traffic based on almost any field within a packet.
Cross-Platform Availability: Wireshark runs on Windows, macOS, Linux, and other Unix-like systems.
Extensibility: Wireshark supports plugins for dissecting custom or less common protocols.
Statistical Analysis: It provides a wealth of statistical tools to summarize traffic patterns, identify top talkers, analyze protocol hierarchies, and more.
Identifying Protocols in Real-Time (During Live Capture) or from a File: Its comprehensive dissector library automatically identifies and breaks down known protocols.
Filtering by IPs, Ports, Protocols, or Packet Contents: Precisely isolating traffic of interest.
Analyzing Both IT and OT Network Data: With dissectors for a wide array of industrial protocols, Wireshark is as useful in a factory as it is in a data center.
Reconstructing TCP/UDP Streams: Allowing analysts to view the complete application-level data exchange, such as an HTTP request and response, or a file transfer.
Key Features Used in Wireshark
Leveraging Wireshark’s features effectively can dramatically speed up the analysis process:
Conversations Tab (Statistics > Conversations):
This feature is a great starting point for getting a quick overview of who is talking to whom on the network. It summarizes traffic based on Layer 2 (Ethernet), Layer 3 (IPv4, IPv6), and Layer 4 (TCP, UDP) addresses and ports.
It aids in rapidly identifying the most active hosts (“top talkers”), common services being accessed (e.g., web servers on port 80/443, DNS on port 53), and potentially unusual or unauthorized communication paths. Sorting by packet or byte count can quickly highlight high-volume sessions that may warrant further investigation for performance issues or data exfiltration.
Provides a tree-like view of all protocols present in the capture and their relative prevalence (by packets or bytes).
This is excellent for understanding the overall makeup of the traffic, identifying unexpected or unwanted protocols, and verifying if specific OT protocols are active as anticipated. For example, a high percentage of ARP traffic might indicate a network scan or a misconfigured device.
Endpoints (Statistics > Endpoints):
Similar to Conversations, but focuses on individual endpoints (IP addresses, TCP/UDP ports) and their traffic statistics.
Useful for identifying all unique IP addresses or ports involved in the capture, and the amount of traffic sent/received by each. This can help spot devices that are unusually active or silent.
Display Filters (Analyze > Display Filters):
This is arguably Wireshark’s most powerful feature for interactive analysis. Users can apply complex filters to narrow down the displayed packets. Examples:
ip.addr == 192.168.1.10 (Show packets to or from this IP)
tcp.port == 443 (Show HTTPS traffic)
dns.qry.name contains “maliciousdomain” (Show DNS queries for a specific domain)
http.request.method == POST (Show HTTP POST requests)
Coloring Rules can be set up based on display filters to visually highlight interesting packets in the packet list pane, making it easier to spot anomalies or specific types of traffic at a glance.
Follow TCP/UDP/TLS Stream (Right-click on a packet > Follow > TCP Stream):
Reconstructs the data exchanged between two endpoints in a specific session and presents it in a human-readable format.
Extremely useful for understanding application-level conversations, extracting transferred files (if unencrypted), viewing web page content, or analyzing command sequences.
Expert Information (Analyze > Expert Information):
Wireshark’s expert system analyzes the capture and flags potential issues, such as TCP retransmissions, out-of-order packets, window size problems, and protocol errors.
Provides categorized warnings, notes, and chats that can guide the analyst toward areas needing closer inspection. This can be a great starting point for troubleshooting performance issues.
Data-Only Filtering:
To focus on packets that contain actual payload data, filtering out purely acknowledgment (ACK) or control packets can be very useful. The filter tcp.flags.push == 1 helps pinpoint TCP segments where the sender is pushing data to the receiver.
Another common filter for data-carrying packets is data.len > 0 or, for TCP, tcp.len > 0.
This helps in concentrating on the actual information exchange, which is critical when analyzing application behavior or searching for specific data patterns.
Wireshark’s Ability to Detect Specific Protocols: Wireshark’s dissectors are highly sophisticated and can identify a vast array of protocols, even when they run on non-standard ports (though this can sometimes require manual “Decode As…” intervention).
MMS / S7Comm (Typically on TCP Port 102): Critical for Siemens PLC communications. Wireshark can break down S7comm job requests, user data, and diagnostic messages.
SMB / NTP (Often associated with various ports, e.g., TCP 139, 445 for SMB; UDP 123 for NTP): SMB is important for file sharing in Windows environments, and NTP for time synchronization. Anomalies here can indicate misconfigurations or malicious activity (like lateral movement using SMB). Note: Port 135 is more commonly associated with RPC Endpoint Mapper, which SMB might use.
Modbus (Typically TCP Port 502): A widely used OT protocol. Wireshark can display Modbus function codes, register addresses, and data values, making it invaluable for troubleshooting industrial automation.
EtherNet/IP (Utilizes TCP Port 44818 and UDP Port 2222 for Class 1/3 and Class 0/1 respectively): Common in Rockwell/Allen-Bradley environments. Wireshark decodes Common Industrial Protocol (CIP) messages encapsulated within EtherNet/IP.
Other OT Protocols: Including DNP3, BACnet, PROFINET, IEC 61850 (GOOSE, SV), OPC UA, and many more.
Advanced Techniques in PCAP Analysis
Beyond basic filtering and conversation viewing, several advanced techniques can yield deeper insights:
Packet Reassembly: Manually or automatically reassembling fragmented IP packets or segmented TCP streams is essential for analyzing complete messages or file transfers. Wireshark often does this automatically for TCP streams when you “Follow Stream.”
File Extraction: Identifying and extracting files transferred over protocols like HTTP, FTP, SMB, or even embedded in custom protocols. Tools like Wireshark’s “Export Objects” feature can facilitate this. Careful analysis of file types and content is vital, especially in malware investigations.
VoIP Analysis (Statistics > VoIP Calls): Wireshark can detect Voice over IP calls (e.g., SIP, RTP), list them, and even allow for playback of audio streams if the codec is supported and the traffic is unencrypted. This is useful for troubleshooting call quality issues or analyzing voice data.
TLS/SSL Decryption (with Pre-Master Secret Log File): If you have access to the SSL/TLS pre-master secret log file (often obtainable from the client or server during development/testing, or by configuring browsers/applications like curl to output it), Wireshark can decrypt TLS encrypted traffic. This allows inspection of otherwise opaque application data. This is highly dependent on having the keys and is not typically possible for arbitrary encrypted traffic you don’t control.
IO Graphs (Statistics > IO Graphs): Creating graphs of traffic rates (packets/sec or bits/sec) over time, often filtered by specific criteria. This helps visualize traffic bursts, identify periods of high or low activity, and correlate network events with other logs or reported issues. You can graph TCP errors, specific protocol traffic, or overall bandwidth utilization.
Using tshark (Command-line Wireshark): For automated analysis, batch processing of large numbers of PCAP files, or extracting specific fields into text formats (like CSV) for further processing with other tools (e.g., scripting languages, SIEMs), tshark is a powerful ally. It allows for robust scripted analysis without the GUI.
Example: tshark -r capture.pcap -T fields -e ip.src -e ip.dst -e tcp.port -Y “dns” to extract specific fields from DNS traffic.
Tips for Efficient PCAP Review
Navigating potentially massive PCAP files requires strategy and efficiency:
Start Broad, Then Narrow: Begin with a high-level overview (Protocol Hierarchy, Conversations, Endpoints) to understand the general traffic landscape before diving into fine-grained details.
Export Conversations and Endpoints to CSV: This allows for easier sorting, filtering, and analysis in spreadsheet software or with scripts. Identifying unique ports or isolating high-volume “talkers” becomes much simpler.
Identify and Investigate Unknown or Unusual Ports/Protocols: Pay close attention to traffic on non-standard ports or protocols you don’t recognize. This could indicate proprietary applications, misconfigured services, or malicious activity attempting to evade detection. Online port encyclopedias can help identify legitimate uses.
Layered Filtering is Key: Don’t rely on a single filter. Combine filters progressively (e.g., start with an IP address, then filter for a specific protocol used by that IP, then look for specific flags or data patterns within that protocol) to reduce noise and highlight the most significant packets.
Use of Ephemeral Port Filtering: Often, client-side connections use high-numbered (ephemeral) ports. While sometimes relevant, if you’re focused on server-side activity or well-known services, you might initially filter out broad ranges of ephemeral ports (e.g., tcp.port > 49151) to reduce noise, then re-introduce them if needed. Be cautious not to filter out legitimate, albeit high-numbered, server ports.
Understand Network Baselines: If possible, compare the captured traffic against a known baseline of normal network activity. Deviations from the baseline are often strong indicators of an issue or an event worth investigating.
Leverage Multiple Wireshark Profiles: Create different Wireshark configuration profiles tailored for specific tasks (e.g., one for general IT troubleshooting, another for OT protocol analysis, one for security investigations). Profiles can save custom coloring rules, display filters, and column layouts.
Time is Your Ally: Correlate timestamps in the PCAP with event logs from other systems (servers, firewalls, IDS). Wireshark’s time display format can be customized (e.g., UTC, local time, seconds since beginning of capture) to aid this. Accurate time synchronization across all network devices (NTP) is paramount for effective correlation.
Look for the “Story”: Try to understand the sequence of events. For example, a DNS query, followed by a TCP handshake, then an HTTP request, and finally a response. Anomalies in these expected sequences can be revealing.
Annotate Your Findings: Use Wireshark’s packet comment feature (right-click on a packet > Packet Comment) or maintain separate notes to document your observations, hypotheses, and the significance of key packets. This practice is highly beneficial when sharing findings or revisiting an analysis later.
Challenges in PCAP Analysis
While powerful, PCAP analysis isn’t without its hurdles:
Massive File Sizes: Capturing all traffic on a busy link can result in terabytes of data very quickly. Analyzing such large files is resource-intensive and time-consuming. Techniques like capture filtering (using BPFs during capture) or using ring buffers to capture only the most recent data can help.
Encrypted Traffic: The increasing prevalence of TLS/SSL encryption means that the payload of much network traffic is opaque. While metadata (IPs, ports, SNI in TLS) is still visible and valuable, deep packet inspection of the application content is often impossible without decryption keys.
Information Overload: Even in moderately sized PCAPs, the sheer volume of packets can be overwhelming. Effective filtering and a clear methodology are essential to avoid getting lost in the noise.
Time Synchronization Issues: If packet timestamps are inaccurate (due to the capturing machine’s clock being off or inconsistent timestamps from multiple capture points), correlating events accurately becomes difficult.
High-Speed Networks: Capturing every packet on very high-speed links (10Gbps, 40Gbps, 100Gbps+) can be challenging and may require specialized capture hardware to avoid packet drops.
Understanding “Normal”: For analysts new to a specific environment, it can be difficult to distinguish between normal, benign traffic and genuinely anomalous or malicious activity without established baselines.
PCAP in IT vs. OT Environments: Key Distinctions
While the fundamental principles of PCAP analysis apply to both IT and OT, there are crucial differences:
Protocols:
IT: Dominated by protocols like HTTP/S, DNS, SMB, SMTP, RDP, SSH.
OT: Features specialized industrial protocols like Modbus, DNP3, S7comm, EtherNet/IP, PROFINET, IEC 61850, BACnet. Many OT protocols are unencrypted and lack robust authentication.
Traffic Patterns:
IT: Often bursty, client-server oriented, with significant internet-facing traffic.
OT: Can be more deterministic, cyclical, and time-sensitive. Communication is often machine-to-machine (M2M) within isolated or semi-isolated segments. Any internet-facing traffic from an OT segment is usually highly scrutinized.
Impact of Downtime:
IT: Downtime can lead to financial loss, reputational damage, and loss of productivity.
OT: Downtime can lead to production halts, equipment damage, environmental incidents, and even risks to human safety. This elevates the criticality of understanding and securing OT network communications.
Device Lifecycles & Security:
IT: Devices are typically patched regularly and have shorter lifecycles.
OT: Devices (PLCs, RTUs, HMIs) can have lifecycles of 15-20 years or more and may not be patchable or may run legacy operating systems. Security was often not a primary design consideration.
Analyst Skillset:
Analyzing OT PCAPs often requires familiarity with industrial processes and the specific OT protocols in use, in addition to strong core networking skills.
PCAP review in OT environments is critical for understanding control system interactions, identifying misconfigurations that could impact production, and detecting cyber threats that could have physical consequences.
Real-World Takeaways & The Continuous Learning Curve
PCAP review is undeniably as much an art as it is a science. It blends technical knowledge with an investigative mindset. A solid, repeatable workflow typically includes:
Defining Clear Goals: Know what questions you’re trying to answer.
Strategic Capture: Ensure you capture the right data at the right point.
High-Level Orientation: Use statistics (Conversations, Endpoints, Protocol Hierarchy) to get the lay of the land.
Iterative, Layered Filtering: Systematically drill down to the traffic of interest.
Deep Dive Analysis: Follow streams, examine payloads (if visible), and scrutinize protocol behavior.
Correlation: Compare PCAP findings with other logs and event data.
Documentation: Thoroughly document your analysis steps and conclusions.
During team discussions and collaborative analysis sessions, engineers consistently emphasize the value of identifying subtle cryptographic patterns (even in encrypted traffic, by observing handshake anomalies or certificate issues) and meticulously verifying network tap positioning and filter configurations – insights often achievable only through diligent, hands-on review.
For junior analysts, embarking on PCAP review can be a profound introduction to practical network analysis, solidifying theoretical knowledge with real-world data. As one seasoned teammate humorously concluded after a particularly complex analysis session: “Did we all just become junior analysts again?” Perhaps so, in the sense that every complex PCAP presents new learning opportunities.
But with the right tools like Wireshark, a structured approach, and a curious mindset, even the most daunting packet captures become manageable and rich sources of information. The field is ever-evolving with new protocols and evasion techniques, making continuous learning a cornerstone of mastering PCAP review.
Let us handle PCAP review for you. Interested in learning how our experts can manage IT and OT convergence for you? Schedule time with our team today.
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