Packet Capture or PCAP (also known as libpcap) is an application programming interface (API) that captures live network packet data from OSI model Layers 2-7. Network analyzers like Wireshark create .pcap files to collect and record packet data from a network. PCAP comes in a range of formats including Libpcap, WinPcap, and PCAPng.

These PCAP files can be used to view TCP/IP and UDP network packets. If you want to record network traffic then you need to create a .pcapfile. You can create a .pcapfile by using a network analyzer or packet sniffing tool like Wireshark or tcpdump. In this article, we’re going to look at what PCAP is, and how it works.

Why do I need to use PCAP? 

PCAP is a valuable resource for file analysis and to monitor your network traffic. Packet collection tools like Wireshark allow you to collect network traffic and translate it into a format that’s human-readable. There are many reasons why PCAP is used to monitor networks. Some of the most common include monitoring bandwidth usage, identifying rogue DHCP servers, detecting malware, DNS resolution, and incident response.

For network administrators and security researchers, packet file analysis is a good way to detect network intrusions and other suspicious activity. For example, if a source is sending the network lots of malicious traffic, you can identify that on the software agent and then take action to remediate the attack.

How Does a Packet Sniffer Work?

Wireshark display filter

To capture PCAP files you need to use a packet sniffer. A packet sniffer captures packets and presents them in a way that’s easy to understand. When using a PCAP sniffer the first thing you need to do is identify what interface you want to sniff on. If you’re on a Linux device these could be eth0 or wlan0. You can select an interface with the ifconfig command.

Once you know what interface you wish to sniff then you can choose what type of traffic you want to monitor. For example, if you only want to monitor TCP/IP packets then you can create rules to do this. Many tools offer filters that allow you to control what traffic you collect.

For example, Wireshark allows you to filter the type of traffic you see with capture filters and display filters. Capture filters allow you to filter what traffic you capture and display filters allow you to filter what traffic you see. For example, you can filter protocols, flows, or hosts.

Once you’ve collected the filtered traffic you can start to look for performance issues. For more targeted analysis you can also filter based on source ports and destination ports to test particular network elements. All of the captured information can then be used to troubleshoot network performance issues.

Versions of PCAP

As mentioned above, there are many different types of PCAP files, including:

  • Libpcap
  • WinPcap
  • PCAPng
  • Npcap

Each version has its own use cases and different types of network monitoring tools support different forms of PCAP files. For instance, Libpcap is a portable open-source c/C++ library designed for Linux and Mac OS users. Libpcap enables administrators to capture and filter packets. Packet sniffing tools like tcpdump use the Libpcap format.

For Windows users, there is the WinPcap format. WinPcap is another portable packet capture library designed for Windows devices. WinpCap can also capture and filter packets collected from the network. Tools like Wireshark, Nmap, and Snort use WinPCap to monitor devices but the protocol itself has been discontinued.

Pcapng or .pcap Next Generation Capture File Format is a more advanced version of PCAP that comes default with Wireshark. Pcapng can capture and store data. The type of data pcapng collects includes extended timestamp precision, user comments, and capture statistics to provide the user with additional information.

Tools like Wireshark are using PCAPng because it can record more information than PCAP. However, the problem with PCAPng is that it isn’t compatible with as many tools as PCAP.

Npcap is a portable packet sniffing library for Windows produced by Nmap, one of the most well-known packet sniffing vendors. The library is faster and more secure than WinpCap. Npcap has support for Windows 10 and loopback packet capture injection so you can send and sniff loopback packets. Npcap is also supported by Wireshark.

Advantages of Packet Capturing and PCAP 

The biggest advantage of packet capturing is that it grants visibility. You can use packet data to pinpoint the root cause of network problems. You can monitor traffic sources and identify the usage data of applications and devices. PCAP data gives you the real-time information you need to find and resolve performance issues to keep the network functioning after a security event.

For example, you can identify where a piece of malware breached the network by tracking the flow of malicious traffic and other malicious communications. Without PCAP and a packet capture tool, it would be more difficult to track packets and manage security risks.

As a simple file format, PCAP has the advantage of being compatible with almost any packet sniffing program you can think of, with a range of versions for Windows, Linux, and Mac OS. Packet capture can be deployed in almost any environment.

Disadvantages of Packet Capturing and PCAP 

Although packet capturing is a valuable monitoring technique it does have its limitations. Packet analysis allows you to monitor network traffic but doesn’t monitor everything. There are many cyberattacks that aren’t launched through network traffic, so you need to have other security measures in place.

For example, some attackers use USBs and other hardware-based attacks. As a consequence, PCAP file analysis should make up part of your network security strategy but it shouldn’t be your only line of defense.

Another significant obstacle to packet capturing is encryption. Many cyber attackers use encrypted communications to launch attacks on networks. Encryption stops your packet sniffer from being able to access traffic data and identify attacks. That means encrypted attacks will slip under the radar if you’re relying on PCAP.

There is also an issue with where the packet sniffer is located. If a packet sniffer is placed at the edge of the network then this will limit the amount of visibility a user has. For example, the user may fail to spot the start of a DDoS attack or malware outbreak. Furthermore, even if you are collecting data in the center of the network it is important to make sure that you are collecting entire conversations rather than summary data.

Open Source Packet Analysis Tool: How does Wireshark Use PCAP Files? 

Wireshark is the most popular traffic analyzer in the world. Wireshark uses .pcap files to record packet data that has been pulled from a network scan. Packet data is recorded In files with the .pcap file extension and can be used to find performance problems and cyberattacks on the network.

In other words, the PCAP file creates a record of network data that you can view through Wireshark. You can then assess the status of the network and identify if there are any service issues that you need to respond to.

It is important to note that Wireshark isn’t the only tool that can open .pcap files. Other widely used alternatives include tcpdump and WinDump, network monitoring tools that also use PCAP to take a magnifying glass to network performance.

Proprietary Packet Analysis Tool Example

SolarWinds Network Performance Monitor (FREE TRIAL)

SolarWinds Network Performance Monitor screenshot

SolarWinds Network Performance Monitor is an example of a network monitoring tool that can capture PCAP data. You can install the software on a device and then monitor packet data pulled from the entire network. The packet data will enable you to measure the response time of the network and diagnose attacks.

The user can view packet data through the Quality of Experience dashboard, which includes a summary of network performance. Graphical displays make it easier to pinpoint spikes in traffic or malicious traffic that could indicate a cyber attack.

The layout of the program also enables the user to differentiate applications by the amount of traffic they are processing. Factors such as Average Network Response Time, Average Application Response Time, Total Data Volume, and Total # of Transactions help the user to keep up to speed with changes to the network as they occur live. There is also a 30-day free trial available for download.

SolarWinds Network Performance Monitor Download 30-day FREE Trial

PCAP File Analysis: Catching Attacks in Network Traffic 

Packet sniffing is a must for any organization that has a network. PCAP files are one of those resources that network administrators can use to take a microscope to performance and discover attacks. Capturing packets will not only help to get to the bottom of the root cause of attacks but also help to troubleshoot sluggish performance.

Open source packet capture tools like Wireshark and tcpdump give network administrators the tools to remediate poor network performance without spending a fortune. There is also a range of proprietary tools for companies that want a more advanced packet analysis experience.

Through the power of PCAP files, a user can log in to a packet sniffer collect traffic data and see where network resources are consumed. Using the right filters will also make it much easier to eliminate the white noise and hone in on the most significant data.